A Comprehensive Review of Water-Based Nanolubricants

Applying nanomaterials and nanotechnology in lubrication has become increasingly popular and important to further reduce the friction and wear in engineering applications. To achieve green manufacturing and its sustainable development, water-based nanolubricants are emerging as promising alternatives to the traditional oil-containing lubricants that inevitably pose environmental issues when burnt and discharged. This review presents an overview of recent advances in water-based nanolubricants, starting from the preparation of the lubricants using different types of nanoadditives, followed by the techniques to evaluate and enhance their dispersion stability, and the commonly used tribo-testing methods. The lubrication mechanisms and models are discussed with special attention given to the roles of the nanoadditives. Finally, the applications of water-based nanolubricants in metal rolling are summarised, and the outlook for future research directions is proposed.


Introduction
Friction and wear occur between moving materials in contact, the study of which is of fundamental importance in many applied sciences [1]. Lubricants, such as neat oils [2][3][4] and oil-in-water emulsions [5,6], have been extensively used to reduce the friction and wear, and satisfactory results have been obtained. To further enhance the friction-reduction and anti-wear properties of the oil-containing lubricants, great efforts have been directed towards incorporating different types of nanoadditives into the base lubricants [7][8][9]. These nanoadditives include metals, metal oxides, metal sulphides, non-metallic oxides, carbon materials, composites, and others such as nitrides [10,11]. The use of these oil-containing lubricants, however, unavoidably leads to adverse effects on the environment, especially when burnt and discharged, and regular maintenance of oil nozzles is always a laborious task. It is thus desirable to use eco-friendly lubricants as alternatives to the oil-containing ones without compromising on the lubrication performance in terms of the decreases in both friction and wear.
Over the past decade, water-based nanolubricants have been emerging as promising eco-friendly lubricants by dispersing nanoadditives into water [12][13][14][15][16][17][18][19][20][21][22][23], which integrates superb cooling capacity of water with excellent lubricity contributed by the nanoadditives. The use of water-based nanolubricants not only provides protection against friction and wear between the tool and the workpiece, but also improves overall quality of the product, demonstrating a great potential in engineering applications, such as metal forming [24][25][26][27][28][29]. Despite an increasing number of experimental studies on various nanomaterials as nanoadditives in water, several aspects including dispersion stability, tribological behaviour, and lubrication mechanism have not yet been fully understood. Most importantly, a review of the advances in water-based nanolubricants is still in its infancy [30,31], which brings an urgent need for comprehensive summary of the fundamental knowledge in the field of water-based nanolubrication.
In this review, the recent advances in water-based nanolubricants will be systematically summarised. To begin with, preparation of water-based nanolubricants will be introduced, including the preparation methods and the nanoadditives used in water. Based on the classified lubricants, we will then discuss the dispersion stability of the nanoadditives and the enhancement techniques. Subsequently, the tribo-testing methods used for tribological characterisation of the water-based lubricants will be compared, with a focus on the roles of nanoadditives, to propose the lubrication mechanism. We finally present the applications of the water-based nanolubricants in metal rolling, followed by proposing the outlook for future research directions.

Preparation Methods
Preparation of industrial lubricants usually requires the consideration of using various additives, such as antioxidants, corrosion inhibitors, defoamers, emulsifiers, extreme pressure (EP) agents, pour point depressants, and viscosity index improvers for different purposes in practical applications [32]. In particular, water-based nanolubricants are basically prepared by dispersing nano-scale solid particles into base water with the aid of a dispersant or surfactant, followed by mechanical agitation and ultrasonic treatment. In general, two primary techniques including one-step and two-step methods are adopted by most researchers to prepare nanolubricants. These two methods are subdivided in Figure 1.
bricants 2021, 9, x FOR PEER REVIEW 2 of 62 In this review, the recent advances in water-based nanolubricants will be systematically summarised. To begin with, preparation of water-based nanolubricants will be introduced, including the preparation methods and the nanoadditives used in water. Based on the classified lubricants, we will then discuss the dispersion stability of the nanoadditives and the enhancement techniques. Subsequently, the tribo-testing methods used for tribological characterisation of the water-based lubricants will be compared, with a focus on the roles of nanoadditives, to propose the lubrication mechanism. We finally present the applications of the water-based nanolubricants in metal rolling, followed by proposing the outlook for future research directions.

Preparation Methods
Preparation of industrial lubricants usually requires the consideration of using various additives, such as antioxidants, corrosion inhibitors, defoamers, emulsifiers, extreme pressure (EP) agents, pour point depressants, and viscosity index improvers for different purposes in practical applications [32]. In particular, water-based nanolubricants are basically prepared by dispersing nano-scale solid particles into base water with the aid of a dispersant or surfactant, followed by mechanical agitation and ultrasonic treatment. In general, two primary techniques including one-step and two-step methods are adopted by most researchers to prepare nanolubricants. These two methods are subdivided in  One-step method is a procedure that simultaneously combines the production of nanoparticles (NPs) with dispersion of NPs in base lubricant. One of the most commonly used methods is named vapour deposition, which was developed by Choi   One-step method is a procedure that simultaneously combines the production of nanoparticles (NPs) with dispersion of NPs in base lubricant. One of the most commonly Lubricants 2021, 9,89 3 of 62 used methods is named vapour deposition, which was developed by Choi and Eastman [33]. The schematic of this method is shown in Figure 2a. First, a flowing thin film made of base liquid is formed on the vessel wall under centrifugal force of the rotating disk. Then the raw material is placed in the resistively heated crucible with heating for evaporation. The produced vapour is condensed into nano-sized particles when contacting the cold base liquid film, and nanolubricant is thus obtained. Another one-step direct evaporation method named vacuum evaporation onto a running oil substrate (VEROS) was developed by Akoh et al. [34], which aimed to produce ultrafine NPs with an average size of around 0.25 nm. Additionally, other techniques in the one-step method include microwave irradiation [35], chemical reduction [36], laser ablation [37], polyol process [38], physical vapour condensation method [39], plasma discharging technique [40], and submerged arc NP synthesis system [41]. However, the disadvantage in the one-step method is that there may be residual reactants such as impurities left in the nanolubricants due to the incomplete reaction which is difficult to avoid. ration. The produced vapour is condensed into nano-sized particles when contacting the cold base liquid film, and nanolubricant is thus obtained. Another one-step direct evaporation method named vacuum evaporation onto a running oil substrate (VEROS) was developed by Akoh et al. [34], which aimed to produce ultrafine NPs with an average size of around 0.25 nm. Additionally, other techniques in the one-step method include microwave irradiation [35], chemical reduction [36], laser ablation [37], polyol process [38], physical vapour condensation method [39], plasma discharging technique [40], and submerged arc NP synthesis system [41]. However, the disadvantage in the one-step method is that there may be residual reactants such as impurities left in the nanolubricants due to the incomplete reaction which is difficult to avoid.
In contrast, the two-step method is more widely used in the preparation and synthesis of nanolubricants with consideration of raw materials provided by manufacturing companies due to large scalability and cost effectiveness. The schematic of this method involves two procedures (see Figure 2b). The first step is to directly mix NPs with base fluid, followed by subsequent addition of dispersant or surfactant with ultrasonication. If necessary, extra treatments such as magnetic force agitation, mechanical stirring, highshear mixing, and homogenising at certain temperatures should be combined with the second step to enhance the dispersion stability of the final nanolubricant. Two typical representatives in the two-step methods comprise microwave assisted synthesis and direct mixing technique [42].

Nanoadditives in Water
Nanomaterials have emerged as one of the most interesting materials in the areas of chemistry, physics, and materials science, and they have been widely applied in many engineering fields. Several varieties of nanoadditives can be dispersed in water, including pure metals, metal and non-metal oxides, metal sulphides, carbon-based materials, composites, and some others such as metal salts, nitrides, and carbides. Figure 3 reveals a summary of different types of nanoadditives used in water-based lubricants in the past decade. Among all the nanoadditives the most used one is carbonbased materials, accounting for 35%, followed by metal and non-metal oxides (18%) and composites (16%), whereas the least used ones are pure metals (3%) and carbides (2%). More details about various nanoadditives used in water-based lubricants are discussed below. Figure 2. Schematics of preparation methods of nanolubricants: (a) one-step method (vapour deposition) [33], and (b) two-step method (direct mixing technique) [42].
In contrast, the two-step method is more widely used in the preparation and synthesis of nanolubricants with consideration of raw materials provided by manufacturing companies due to large scalability and cost effectiveness. The schematic of this method involves two procedures (see Figure 2b). The first step is to directly mix NPs with base fluid, followed by subsequent addition of dispersant or surfactant with ultrasonication. If necessary, extra treatments such as magnetic force agitation, mechanical stirring, highshear mixing, and homogenising at certain temperatures should be combined with the second step to enhance the dispersion stability of the final nanolubricant. Two typical representatives in the two-step methods comprise microwave assisted synthesis and direct mixing technique [42].

Nanoadditives in Water
Nanomaterials have emerged as one of the most interesting materials in the areas of chemistry, physics, and materials science, and they have been widely applied in many engineering fields. Several varieties of nanoadditives can be dispersed in water, including pure metals, metal and non-metal oxides, metal sulphides, carbon-based materials, composites, and some others such as metal salts, nitrides, and carbides. Figure 3 reveals a summary of different types of nanoadditives used in water-based lubricants in the past decade. Among all the nanoadditives the most used one is carbonbased materials, accounting for 35%, followed by metal and non-metal oxides (18%) and composites (16%), whereas the least used ones are pure metals (3%) and carbides (2%). More details about various nanoadditives used in water-based lubricants are discussed below.

Pure Metals
Metallic NPs such as Au, Ag, and Cu have been considered to be important additives in base water due to their unique optical, electrical, and photo thermal properties in the fields of physics, chemistry, and biology [43]. These NPs, however, are only weakly compatible with base lubricant because of their high surface activity. This issue can be resolved by surface modification techniques [44]. Among all the candidates, Cu NPs have been suitably used in aqueous lubricants due to their low cost and superb tribological characteristics by acting as nano-bearings and forming metallic and/or tribo-sintered film with low shear stress on rubbed surfaces [45]. For example, Zhao et al. [46] fabricated Cu NPs through in situ surface modification, and then uniformly dispersed varying concentrations of Cu NPs (0.1-2.0 wt.%) into distilled water using prepared water-soluble bis (2hydroxyethyl) dithiocarbamic acid (HDA) as a capping ligand. A reddish-brown uniform water-based nanolubricant with superior dispersion stability was thus synthesised. Zhang et al. [47] also prepared water-based nanolubricant containing nano-Cu surfacecapped with methoxyl-polyethylene-glycol xanthate potassium. The surface-capped Cu NPs had an average diameter of 2 nm and showed no sign of apparent agglomeration in water.

Metal and Non-Metal Oxides
Metallic oxide NPs including Al2O3, TiO2, and ZnO have been widely used as possible lubricating additives in base water. Radice et al. [48] investigated the lubrication behaviour of globular-shaped Al2O3 NPs (AKP50) with approximately 0.197 µm in diameter. The lubricant was prepared by diluting AKP50 with an acetate buffer solution (0.1 M NaCH3CO2 + 0.1 M CH3CO2H) in deionised water (DW) under 15 min ultrasound stirring prior to each tribo-test to avoid sedimentation. He et al. [16] used spherical Al2O3 NPs of 30, 150, and 500 nm in water to prepare water-based nanolubricants containing 0.2-8 wt.% Al2O3 and 10 wt.% glycerol through 400 W ultrasonic agitation for 10 min, and no evident agglomeration was observed until 3 days. It is also of vital importance to report several

Pure Metals
Metallic NPs such as Au, Ag, and Cu have been considered to be important additives in base water due to their unique optical, electrical, and photo thermal properties in the fields of physics, chemistry, and biology [43]. These NPs, however, are only weakly compatible with base lubricant because of their high surface activity. This issue can be resolved by surface modification techniques [44]. Among all the candidates, Cu NPs have been suitably used in aqueous lubricants due to their low cost and superb tribological characteristics by acting as nano-bearings and forming metallic and/or tribo-sintered film with low shear stress on rubbed surfaces [45]. For example, Zhao et al. [46] fabricated Cu NPs through in situ surface modification, and then uniformly dispersed varying concentrations of Cu NPs (0.1-2.0 wt.%) into distilled water using prepared water-soluble bis (2-hydroxyethyl) dithiocarbamic acid (HDA) as a capping ligand. A reddish-brown uniform water-based nanolubricant with superior dispersion stability was thus synthesised. Zhang et al. [47] also prepared water-based nanolubricant containing nano-Cu surface-capped with methoxylpolyethylene-glycol xanthate potassium. The surface-capped Cu NPs had an average diameter of 2 nm and showed no sign of apparent agglomeration in water.

Metal and Non-Metal Oxides
Metallic oxide NPs including Al 2 O 3 , TiO 2 , and ZnO have been widely used as possible lubricating additives in base water. Radice et al. [48] investigated the lubrication behaviour of globular-shaped Al 2 O 3 NPs (AKP50) with approximately 0.197 µm in diameter. The lubricant was prepared by diluting AKP50 with an acetate buffer solution (0.1 M NaCH 3 CO 2 + 0.1 M CH 3 CO 2 H) in deionised water (DW) under 15 min ultrasound stirring prior to each tribo-test to avoid sedimentation. He et al. [16] used spherical Al 2 O 3 NPs of 30, 150, and 500 nm in water to prepare water-based nanolubricants containing 0.2-8 wt.% Al 2 O 3 and 10 wt.% glycerol through 400 W ultrasonic agitation for 10 min, and no evident agglomeration was observed until 3 days. It is also of vital importance to report several examples for TiO 2 NPs as water-based nanoadditives. For example, Wu et al. [13][14][15]25,27] prepared the lubricant by mixing 0.2-8.0 wt.% TiO 2 NPs (20 nm in diameter) into DW by mechanical stirring, stepwise adding 0.002-0.08 wt.% polyethyleneimine (PEI) and 10% glycerol under 30 min high-speed centrifuge at 2000 rpm followed by 10 min ultrasonication. The as-prepared lubricants had no significant sedimentation even after one week. In their recent studies, they prepared industrial-scale water-based nanolubricants using coarse TiO 2 NPs of 300 nm, with the aid of 0.1-0.2 wt.% of sodium dodecyl benzene sulfonate (SDBS) and 1 wt.% Snailcool by mechanical stirring only. The as-prepared lubricants could be stably dispersed for 24 h [24]. Sun et al. [49][50][51][52] also prepared 0.1-5.0 wt.% TiO 2 water-based nanolubricants by adding 20-90 nm TiO 2 particles and different additives, including sodium hexametaphosphate (SHMP) and SDBS which exhibited good stability for 7 days. Additionally, there has been some work conducted on the preparation of water-based nanolubricants using other metal oxide nanoadditives such as CeO 2 [53], CuO [54], γ-Fe 2 O 3 [55], MoO 3 [56], and WO 3 [57], and all these nanoadditives can also be well dispersed in water under mechanical agitation and ultrasonic processing.
Non-metallic oxides have also been used extensively as nanoadditives in base water. Silica (SiO 2 ), one of the typical representatives, is a widely used ceramic material both as a precursor to the fabrication of other ceramic products and as a material on its own. Silica has good abrasion resistance, electrical insulation, and high thermal stability [43]. Ding et al. [58] followed four steps of preparation, including synthesis, modification, purification, and dispersion, to obtain ceramic water-based lubricant with 100 nm SiO 2 , which showed no apparent sedimentation for 1 h. Bao et al. [59] also prepared SiO 2 water-based lubricants using 0.1-1 wt.% surface-modified spherical SiO 2 nanoparticles (30 nm in diameter) in 15% ammonia solution under 5 min stirring at room temperature, followed by 30 min stirring at 60 • C with addition of polyethylene glycol-200. The solution was finally mixed using 20 kHz ultrasonic disperser for 5 min, and finally a stably dispersed SiO 2 water-based lubricant was obtained.
To date, many experimental studies have reported the use of metal and non-metal oxides as nanoadditives, as listed in Table 1.

Metal Sulphides
In recent years, lubricant additives based on metal sulphides have received considerable interest in the lubricant industry. It is generally accepted that metal sulphides such as Ag 2 S, CuS, and MoS 2 present outstanding lubrication performance when used both as solid lubricants and as additives in liquid lubricants. These materials offer a low-shear resistance to an external shear stress due to their layered structure with strong-interlayer and weak-interlayer bonds [60]. It was reported that Kuznetsova et al. [61] synthesised MPS-capped Ag 2 S NPs (2-10 nm) by a simple one-step process as per the following reaction: 2AgNO 3 + Na 2 S = Ag 2 S↓ + 2NaNO 3 , eventually adding MPS (3-mercaptopropyl trimethoxysilane) and ethanol in water by sonication in an ultrasonic bath for 10 min to avoid sedimentation in the solution, and the solution remained stable for up to several months at room temperature. Zhao et al. [62] used HAD-CuS nanoparticles as nanoadditives in water. The nanolubricant was prepared by pouring PEG-400 and HAD in a solution of CTAB, Cu(NO 3 ) 2 ·3H 2 O and distilled water under 10 min stirring, followed by 170 • C heating under 1 h vigorous stirring. The final black homogeneous solution was centrifuged, cleaned, and dried to obtain HAD-CuS NPs that could be uniformly dispersed in water for at least 2 days within the concentration of 0.1-2.0 wt.%.
In the last few years, special attention has been focused on using MoS 2 NPs in water due to their excellent chemical and thermal stability. For example, Meng et al. [63] added multilayer-MoS 2 of 100 nm in water under 10 min magnetic stirring to prepare 0.3-0.5% MoS 2 nanolubricants, and the as-prepared nanolubricants presented good stability for 16 h. To further enhance the dispersion stability of nano-MoS 2 in water, Wang et al. [64] conducted the exfoliation and modification processes on bulk MoS 2 (15 µm in thickness) to prepare functional MoS 2 nanosheets (3.5 nm in thickness) which can be stably dispersed in water for 10 days after ultrasonication for 1 h. In contrast, the unfunctional MoS 2 nanosheets would aggregate in water within a very short time.

Carbon-Based Materials
When compared with metal sulphides, carbon-based nanomaterials have higher chemical stability and superior mechanical properties, which provides outstanding tribological In the case of pH-dependent GO as water-based nanoadditives, two studies were reported by He et al. [17] and Meng et al. [76], which proposed two different preparation processes. A mechanical de-agglomeration method was used by He et al. [17] to prepare GO suspension with 0.06 wt.% GO and 0.1 M NaOH in DW under high intensity ultrasonic agitation (400 W) for 10 min, resulting in good dispersion stability for a week. The pH value was adjusted by NaOH and varied from 3.1 to 9.7, which had insignificant influence on agglomeration of GO sheets. In contrast, Meng et al. [76] mixed triethanolamine (TEA) with 0.1 wt.% GO in DIW to adjust the pH value from acidic (pH 2.8) to alkaline (pH 9). They found that the lubricant with pH 9 was the most homogeneous with no precipitation for 50 days.
It has been documented that some researchers have investigated the carbon-based nanocomposites in the formulation of water-based lubricants, including GO/graphene [77], GO/nanodiamond [78,79], GO/carbon [80], and GO/g-C 3 N 4 [18]. The detailed information regarding their preparation parameters as well as stability duration is listed in Table 1.

Composites
Nanocomposite which comprises two or more different nano-sized particles is becoming a significant part of nanotechnology and one of the fastest growing research areas in materials science and engineering [81]. The use of nanocomposite is to simultaneously combine physical and chemical properties of the constituent nanomaterials in an attempt to produce a homogeneous phase for better performance than single-component NPs [82]. The composite water-based nanolubricant, therefore, can be prepared by dispersing two or more nanoadditives in the base water.
Over the past few years, much research has been conducted on graphene-based composites including GO/SiO 2 [19,83], GO/TiO 2 [84], and GO/Al 2 O 3 [21]. For example, Huang et al. [19] synthesised GO/SiO 2 water-based lubricants by mechanical stirring the aqueous suspension at 25 • C for 30 min, followed by ultrasonic processing (450 W) for 60 min (on/off interval of 5 s) in a chilled water bath. Similar preparation method was used to synthesis the water-based lubricants containing GO/Al 2 O 3 . Both results indicated that the formation of hybrid nanostructure enabled smaller particle size distribution in water, compared to that of constituent nanoadditives. Some other nanocomposites such as Cu/SiO 2 , ZnO/Al 2 O 3 , Fe 3 O 4 /MoS 2 , TiO 2 /Ag, and Ag/C have also been used as nanoadditives in water. For example, Li et al. [85] used a two-step method to prepare 0.05-0.3 wt.% TiO 2 /Ag in cooling water by 2 h magnetic stirring, followed by 12 h ultrasonication with 40 kHz frequency and 110 W power, which showed no sedimentation within a month. In contrast, Fe 3

Nitrides
Among all the nitrides, hexagonal boron nitride (h-BN), as a promising and an ideal alternative to other nanoadditives dispersed in water, has attracted extensive attention due to its exceptional performance, such as high-temperature stability, high thermal conductivity, high electrical resistivity, low coefficient of friction, strong inertness in a wide variety of chemical environments, and environmental friendliness [87]. Cho et al. [88] revealed superb stability of h-BN nanosheets in water without any precipitation within 1 month. The authors synthesised 30 nm thick and 300 nm wide h-BN nanosheets with 0.01, 0.05 and 1 wt.% concentration under 20 h bath sonication excluding any additional surfactants. Moreover, Bai et al. [89] recommended the use of thin hydroxylated BN nanosheets (HO-

Carbides
Recently some research works have been conducted on carbides such as Nb 2 C and Ti 3 C 2 . Cheng and Zhao [90] prepared Nb 2 C nanofluid using three different degrees of oxidised Nb 2 C nanosheets with the mass ratio of 0.25-1.0 mg/mL of pure water. The Nb 2 C nanosheets were obtained by mixing 1 mg/mL accordion shaped Nb 2 C powder into 100 mL aqueous solution, followed by 10 mL/50 wt.% TBAOH as intercalation agent, and 20 mg ascorbic acid as anti-oxidant under 12 h magnetic stirring in an ice bath. The pH level of the mixture was balanced by adding hydrochloric acid and 10 min centrifugation at 3000 rpm. The final Nb 2 C solution was acquired by freeze-drying method and divided into three parts: Nb 2 C of 20 nm (black coloured), after 6 h magnetic stirring at 60 • C water bath; moderately oxidised Nb 2 C (MO-Nb 2 C) of 12 nm (yellow-coloured), after 7 days magnetic stirring at room temperature; and completely oxidised Nb 2 C (CO-Nb 2 C) of 6 nm (transparent). The authors also mixed benzalkonium chloride (surfactant) to enhance stability and, among the three groups, MO-Nb 2 C showed the best stability even after 1 month. Nguyen and Chung [91] prepared five solutions with 1 wt.%, 2 wt.%, 3 wt.%, 5 wt.%, and 7 wt.% Ti 3 C 2 concentrations by adding 0.01, 0.02, 0.03, 0.05, and 0.07 g of Ti 3 C 2 to 1 mL of DW respectively. Each solution was mixed under 1 h magnetic stirring at room temperature to ensure uniform dispersion of the NPs in water.

Others
The literature demonstrates that very little research has been conducted on some rare NPs, such as black phosphorus and hydroxides. Tang et al. [92] synthesised 3.9 nm black phosphorus quantum dots (BPQDs) by adding N-methyl-2-pyrrolidone (NMP) under ultrasonic treatment for 8 h. The as-prepared BPQDs with 0.1 wt.% concentration were dispersed into 2.0 wt.% Triethanolamine (TEA) aqueous solution by another 10 min ultrasonic treatment. The BPQDs dispersion showed good stability with no precipitates even after 2 weeks. Metal hydroxides have been demonstrated to perform well as lubricant additives, and layered double hydroxides (LDHs) stand out as particularly impressive examples of this. Wang et al. [93] presented the preparation of 19.42 nm wide and 8.59 nm thick oleylamine-modified Ni-Al LDH (NiAl-LDH/OAm) nanoplatelets as water-based lubricant additives with a concentration of 0.1-1.0 wt.%. The lubricants were synthesised by a microemulsion method under both stirring and ultrasonication in DW followed by 10 h drying at 80 • C. A transparent and stable solution was obtained without additional dispersion or surfactants. The authors also prepared aqueous polyalkylene glycol (PAG) solution by dispersing two different LDHs with 0.5 wt.% concentration under only ultrasonication, including ultrathin LDH nanosheets (ULDH-NS, 60 nm wide and 1 nm thick), and LDH NPs (19.73 nm wide and 8.68 nm thick) [94]. The ULDH-NS (0.5 wt.%) were uniformly dispersed in water, showing no agglomeration. While 0.5 wt.% LDH-NPs were dispersed in water, a transparent solution was obtained with no precipitates due to the addition of a surfactant 1-butanol and oleylamine under the reverse microemulsion reaction process.
In addition to this, some exceptional NPs such as chitosan [95] and stearic acid [96] were also explored recently. Li et al. [95] used 0.5 wt.% nanoscale liquid metal droplets wrapped by the chitosan (NLMWC) as water-based nanoadditives and compared with 0.5 wt.% gallium based liquid metal Ga 76 In 24 . The NLMWC was dispersed in water under 15 min ultrasonication in water bath, and the as-prepared aqueous solution remained stable for 1 month without any precipitation and agglomeration. In contrast, the precipitates were observed for Ga 76 In 24 in water within 2 h.
The details of different types of nanoadditives used in water-based lubricants are listed in Table 1.

Evaluation Methods
Research on dispersion stability of nanoadditives in water is of vital importance for the development of water-based nanolubricants, and it has become one of the key challenges in restricting their widespread practical application. Stable nanosuspension is usually considered as a prerequisite for the successful preparation of water-based nanolubricants. In general, there are some methods that can be used to effectively evaluate the dispersion stability, including microscopy, zeta potential, UV-vis spectral analysis, dynamic light scattering, and sedimentation.

Microscopy
Microscopic methods using optical microscope (OM), transmission electron microscope (TEM), and scanning electron microscope (SEM) are very useful to distinguish the size and shape of NPs, and the dispersion stability of NPs in water can be evaluated as per the distribution and agglomeration of NPs under microscopic observation. Among all the microscopic methods, the use of OM is the easiest and quickest technique to examine the agglomeration behaviour and trend of the nanolubricants even at micrometre scale. However, the limited resolution of OM is the main disadvantage of viewing the size and shape of NPs at nanometre scale.
Currently, electron microscopy is the most commonly used method to evaluate the NPs stability in a base lubricant due to its high resolution. TEM and SEM are two of the most popular methods for observing the morphology and size distribution of NPs. TEM samples are prepared by placing a nanolubricant drop on a carbon-coated copper grid until complete evaporation of the base lubricant. SEM samples are prepared by dropping a small amount of nanolubricant onto a tape that is attached to the top of a sample holder before heating and drying under vacuum [164,165]. During the sample preparation processes for both TEM and SEM, NPs may somewhat agglomerate, leading to inaccurate evaluation of NPs stability in base lubricant. In spite of this, it is still acceptable to evaluate the dispersion stability of NPs between different concentrations or formulations by comparing the NP size difference. Figure 4 shows TEM images of TiO 2 NPs with 2.0 wt.% and 4.0 wt.% concentrations in water. It can be found that the NPs were uniform and well dispersed in water, and no visible agglomeration was observed even at 4.0 wt.% concentration. With the increase of concentration, however, there is a trend of few NPs agglomeration.

Evaluation Methods
Research on dispersion stability of nanoadditives in water is of vital importance for the development of water-based nanolubricants, and it has become one of the key chal lenges in restricting their widespread practical application. Stable nanosuspension is usu ally considered as a prerequisite for the successful preparation of water-based nanolubri cants. In general, there are some methods that can be used to effectively evaluate the dis persion stability, including microscopy, zeta potential, UV-vis spectral analysis, dynamic light scattering, and sedimentation.

Microscopy
Microscopic methods using optical microscope (OM), transmission electron micro scope (TEM), and scanning electron microscope (SEM) are very useful to distinguish the size and shape of NPs, and the dispersion stability of NPs in water can be evaluated as per the distribution and agglomeration of NPs under microscopic observation. Among al the microscopic methods, the use of OM is the easiest and quickest technique to examine the agglomeration behaviour and trend of the nanolubricants even at micrometre scale However, the limited resolution of OM is the main disadvantage of viewing the size and shape of NPs at nanometre scale.
Currently, electron microscopy is the most commonly used method to evaluate the NPs stability in a base lubricant due to its high resolution. TEM and SEM are two of the most popular methods for observing the morphology and size distribution of NPs. TEM samples are prepared by placing a nanolubricant drop on a carbon-coated copper grid until complete evaporation of the base lubricant. SEM samples are prepared by dropping a small amount of nanolubricant onto a tape that is attached to the top of a sample holder before heating and drying under vacuum [164,165]. During the sample preparation pro cesses for both TEM and SEM, NPs may somewhat agglomerate, leading to inaccurate evaluation of NPs stability in base lubricant. In spite of this, it is still acceptable to evaluate the dispersion stability of NPs between different concentrations or formulations by com paring the NP size difference. Figure 4 shows TEM images of TiO2 NPs with 2.0 wt.% and 4.0 wt.% concentrations in water. It can be found that the NPs were uniform and wel dispersed in water, and no visible agglomeration was observed even at 4.0 wt.% concen tration. With the increase of concentration, however, there is a trend of few NPs agglom eration.   Given the shortcomings of using TEM and SEM, scientists recommend the use of freeze etching replication TEM (FERTEM) or cryogenic electron microscopy (cryo-TEM) [166][167][168] to observe the nanolubricants because each of them is appropriate to characterise wet samples, and the size and morphology of NPs can be kept the same as those in the original nanolubricants.

Zeta Potential Test
Zeta potential (ZP) test shows the potential difference between the static fluid layers adhered to the dispersed particles and the dispersion medium [166,169]. The nanofluid stability is evaluated by observing the fluid's electrophoretic behaviour, as a layer of charged particles is formed when the free charge in the base fluid is attracted by the surface opposite charges of dispersed particles [166]. ZP values range from a positive value at low pH to a negative value at high pH in any nanofluid. A nanofluid with relatively high ZP absolute value (>30 mV) is electrically stable due to a strong repulsive force between NPs. Instead, a nanofluid with relatively low ZP absolute value (<15 mV) tends to have NPs agglomerate because an attractive force dominates. In particular, a ZP absolute value above 60 mV indicates excellent dispersion stability of a nanofluid.
ZP test has been used in many studies to determine the size distribution and stability of water-based lubricants [63,90,93,95,111,170]. Figure 5 demonstrates a bar graph of the ZP (mV) vs. nanolubricant with 0.12 wt.% GO, 0.12 wt.% Al 2 O 3 , and 0.12 wt.% GO-Al 2 O 3 (1:1) suspension [21]. It can be seen that the GO-Al 2 O 3 lubricant exhibits comparatively higher ZP absolute value (40 mV) than Al 2 O 3 (30 mV) and GO lubricants (35 mV), signifying a greater level of stability.
Lubricants 2021, 9, x FOR PEER REVIEW 18 of 6 [166][167][168] to observe the nanolubricants because each of them is appropriate to character ise wet samples, and the size and morphology of NPs can be kept the same as those in th original nanolubricants.

Zeta Potential Test
Zeta potential (ZP) test shows the potential difference between the static fluid layer adhered to the dispersed particles and the dispersion medium [166,169]. The nanofluid stability is evaluated by observing the fluid's electrophoretic behaviour, as a layer o charged particles is formed when the free charge in the base fluid is attracted by the sur face opposite charges of dispersed particles [166]. ZP values range from a positive value at low pH to a negative value at high pH in any nanofluid. A nanofluid with relatively high ZP absolute value (>30 mV) is electrically stable due to a strong repulsive force be tween NPs. Instead, a nanofluid with relatively low ZP absolute value (<15 mV) tends to have NPs agglomerate because an attractive force dominates. In particular, a ZP absolut value above 60 mV indicates excellent dispersion stability of a nanofluid.

UV-Vis Spectral Analysis
One of the reliable methods to measure the dispersion stability and durability o nanofluids is spectral absorbency analysis against NPs concentration in nanofluids utilis ing an ultraviolet-visible (UV-vis) spectrophotometer which follows the Beer-Lambert law [171,172]. The stability of suspension is determined by calculating the volume of sedimen relative to the time of sediment. The intensity of light different from the scattering and absorption of light passing through the fluid is used in the UV-vis spectrophotometer. I evaluates the absorbance of a fluid within a wavelength of 200-900 nm to analyse variou dispersions in the fluid [173]. One of the unique aspects of this method is that it is capable of obtaining quantitative data of NP concentration in nanofluid and is applicable for al boundary fluids [174].
Research has been undertaken using UV-vis spectrometer to analyse the liquid ab sorbance, physical properties, and chemical state of NPs in water-based lubricants such

UV-Vis Spectral Analysis
One of the reliable methods to measure the dispersion stability and durability of nanofluids is spectral absorbency analysis against NPs concentration in nanofluids utilising an ultraviolet-visible (UV-vis) spectrophotometer which follows the Beer-Lambert law [171,172]. The stability of suspension is determined by calculating the volume of sediment relative to the time of sediment. The intensity of light different from the scattering and absorption of light passing through the fluid is used in the UV-vis spectrophotometer. It evaluates the absorbance of a fluid within a wavelength of 200-900 nm to analyse various dispersions in the fluid [173]. One of the unique aspects of this method is that it is capable of obtaining quantitative data of NP concentration in nanofluid and is applicable for all boundary fluids [174].
Research has been undertaken using UV-vis spectrometer to analyse the liquid absorbance, physical properties, and chemical state of NPs in water-based lubricants such as graphene [17,74,135], boron nitride [89], and copper [46,116]. For example, the absorbance of 0.05 wt.% hydroxylated boron nitride nanosheets (HO-BNNS) in water was measured within 5 days, as shown in Figure 6. A good dispersion of HO-BNNS in water can be observed from the slight drop of absorbance, probably because of the hydrogen bond existence between the hydroxyl groups in HO-BNNS and water-glycol. The NPs absorbency ratio is directly proportional to the nanofluid concentration. However, the use of UV-vis is limited by the dispersion of high concentration nanofluids, mainly for carbon nanotube CNT solutions [175] or when the colour of the base lubricant is too dark to distinguish the deposits of NPs.
as graphene [17,74,135], boron nitride [89], and copper [46,116]. For examp ance of 0.05 wt.% hydroxylated boron nitride nanosheets (HO-BNNS) in wa ured within 5 days, as shown in Figure 6. A good dispersion of HO-BNN be observed from the slight drop of absorbance, probably because of the h existence between the hydroxyl groups in HO-BNNS and water-glycol. T bency ratio is directly proportional to the nanofluid concentration. Howe UV-vis is limited by the dispersion of high concentration nanofluids, mai nanotube CNT solutions [175] or when the colour of the base lubricant is t tinguish the deposits of NPs. Generally, a linear relationship exists between the NP concentration a tion intensity [166,176,177]. For instance, Liang et al. [123] assessed NP sta absorbance method for 48 h and noted that the dispersion stability wa Mn3B7O13Cl-WO3:Eu 3+ from a linear graph (absorbance vs. time), as shown The results also showed that WO3: Eu 3+ and Mn3B7O13Cl exhibited a decre absorbance over time, indicating that NPs settled more rapidly in acidity that in alkalinity solution. Compared to Mn3B7O13Cl, Mn3B7O13Cl-WO3:Eu 3+ relative absorbance, thus presenting better stability. Similarly, Figure 7b rev relative absorbance of the BN, BNNS, and HO-BNNS decreased with the i tling time [89]. The settling rate was different for these three additives. To settling rate of BN was higher because of its larger mass. BNNS showed a compared to BN due to fewer hydroxyl groups on its surface. While HO-BN the best dispersion stability due to the presence of more hydroxyl groups o is worth noting that at higher nanofluid concentration this method might n rately because the absorption range is outside the system's highest limit [17 Generally, a linear relationship exists between the NP concentration and the absorption intensity [166,176,177]. For instance, Liang et al. [123] assessed NP stability using an absorbance method for 48 h and noted that the dispersion stability was obtained for Mn 3 B 7 O 13 Cl-WO 3 :Eu 3+ from a linear graph (absorbance vs. time), as shown in Figure 7a. The results also showed that WO 3 : Eu 3+ and Mn 3 B 7 O 13 Cl exhibited a decrease in relative absorbance over time, indicating that NPs settled more rapidly in acidity solution than that in alkalinity solution. Compared to Mn 3 B 7 O 13 Cl, Mn 3 B 7 O 13 Cl-WO 3 :Eu 3+ showed lower relative absorbance, thus presenting better stability. Similarly, Figure 7b revealed that the relative absorbance of the BN, BNNS, and HO-BNNS decreased with the increase in settling time [89]. The settling rate was different for these three additives. To be specific, the settling rate of BN was higher because of its larger mass. BNNS showed a better stability compared to BN due to fewer hydroxyl groups on its surface. While HO-BNNS exhibited the best dispersion stability due to the presence of more hydroxyl groups on its surface. It is worth noting that at higher nanofluid concentration this method might not work accurately because the absorption range is outside the system's highest limit [178].

Dynamic Light Scattering
The dispersion stability of lubricants over time can also be evaluated by measuring the NPs size distribution using the dynamic light scattering (DLS) method through analysing the autocorrelation index function [177,179,180]. This method evaluates the particle diffusion moving in Brownian motion and uses the Stokes-Einstein ratio to convert them into particle size ranging between 0.3 nm to 10 µm [181]. Numerous researchers use the DLS method in order to determine the diameter of NPs, although this method is also promising for estimating the size distribution of water-based nanoadditives such as graphene [132,141], ceria [53], and copper [65,117,118,182]. The more stable dispersion of NPs in base lubricant, the greater the impact is on the scattered light intensity, and vice versa. DLS was used to monitor colloidal stability of Cu@SiO2 NPs in DW over time, as illustrated in Figure 8. After 30 days of storage at ambient conditions, very limited size variation was observed, and the hydrodynamic diameter (HD) stayed constant at 230 nm, demonstrating no agglomeration of Cu@SiO2 NPs in DW.
Basically, before measurement, thick nanolubricants must be diluted, which may change the microstructure nature of the NPs [168,171]. Therefore, a suitable method is needed to determine the particle size of nanolubricant without changing the particle's microstructure.

Dynamic Light Scattering
The dispersion stability of lubricants over time can also be evaluated by measuring the NPs size distribution using the dynamic light scattering (DLS) method through analysing the autocorrelation index function [177,179,180]. This method evaluates the particle diffusion moving in Brownian motion and uses the Stokes-Einstein ratio to convert them into particle size ranging between 0.3 nm to 10 µm [181]. Numerous researchers use the DLS method in order to determine the diameter of NPs, although this method is also promising for estimating the size distribution of water-based nanoadditives such as graphene [132,141], ceria [53], and copper [65,117,118,182]. The more stable dispersion of NPs in base lubricant, the greater the impact is on the scattered light intensity, and vice versa. DLS was used to monitor colloidal stability of Cu@SiO 2 NPs in DW over time, as illustrated in Figure 8. After 30 days of storage at ambient conditions, very limited size variation was observed, and the hydrodynamic diameter (HD) stayed constant at 230 nm, demonstrating no agglomeration of Cu@SiO 2 NPs in DW.

Sedimentation
The basic and low-cost method to analyse the dispersion stability of nanolubricants is the sedimentation method. It is a process in which NPs settle or precipitate as sludge at the lubricant base. Additionally, it is related to the balance of friction force and buoyancy Basically, before measurement, thick nanolubricants must be diluted, which may change the microstructure nature of the NPs [168,171]. Therefore, a suitable method is needed to determine the particle size of nanolubricant without changing the particle's microstructure.

Sedimentation
The basic and low-cost method to analyse the dispersion stability of nanolubricants is the sedimentation method. It is a process in which NPs settle or precipitate as sludge at the lubricant base. Additionally, it is related to the balance of friction force and buoyancy [183]. In nanofluid, the NPs sediment volume or weight is an indicator of nanofluid stability under an external force field.
According to Stokes' law [171], the rate of NP sedimentation rate is calculated: where d is the diameter (cm) of NP. δ is the density (g/cm 3 ) of NP. ρ is the density (g/cm 3 ) of base lubricant and µ is the viscosity (Pa·s) of base lubricant. Photography of nanofluid sedimentation using a camera over a time period is the simplest method to monitor the nanofluid stability and sedimentation coefficient [166,184]. Comparing the photos is widely used in nanofluid research, and many examples were mentioned for water-based nanoadditives such as TiO 2 [15,24,29,51]. The stability analysis using sedimentation is usually conducted immediately after the dispersion of NPs in the base lubricant, and the disturbance or movement should be avoided to ensure its reliability. Wu et al. [24] carried out the sedimentation experiment to compare the precipitation of TiO 2 NPs in different as-prepared water-based nanolubricants, as shown in Figure 9. They found that all the lubricants remained stable without apparent particle sedimentation at the bottom within 120 min as long as the dispersants such as sodium dodecyl benzene sulfonate and Snailcool were added into water. After standing the lubricants for 24 h, the TiO 2 NPs began to settle down slightly, and a shallow supernatant appeared. Moreover, the sedimentation rate of NPs in a certain time is measured by the change in the mass/volume concentration in the base fluid using a highly sensitive analytical balance. According to the following formula, the NPs suspension fraction at a specific time is calculated: Moreover, the sedimentation rate of NPs in a certain time is measured by the change in the mass/volume concentration in the base fluid using a highly sensitive analytical balance. According to the following formula, the NPs suspension fraction F at a specific time is calculated: where W o is the total mass of all NPs in the base fluid; W is the mass of the sediment NPs at a certain time [185]. The nanofluid is considered to be stable if the concentration of NPs in the base fluid stays constant over time, and vice versa [186]. This is a simple method that just requires an accurate and sensitive balance to quantify the volume of the deposited NPs. The only limitation of this method is that it cannot detect small NP aggregates that are not considered to settle at a reasonable amount [166,187].

Other Methods
There are some other methods applicable for evaluating dispersion stability of nanofluids, including three-omega, centrifugation, and density measurement, each of which has its own characteristics. For example, a densitometer can be used in the density measurement method to monitor NP concentration varying with time due to NPs aggregation and precipitation [188]. The three-omega method determines colloidal stability with large volume fractions of NPs through thermal conductivity [187,189]. In centrifugation method, analysis is carried out using the centrifugal force acting on the nanofluids at various speeds and time [164,177,180]. It is acknowledged that centrifugation is a faster way to determine nanofluids stability compared to sedimentation photography [164].

pH Control
Altering the pH of nanofluids affects the surface of NPs and enhances the stability of dispersed NPs [190]. Nanofluids are characterised by their electrokinetic properties, which influence their stability. Therefore, an increase or decrease in nanofluid pH is accompanied by an increase or decrease in zeta potential. As previously noted, NPs repel one another when their zeta potentials increase above +30 mV or decrease below −30 mV. With the objective to change the pH of a nanofluid, suitable acidic or alkaline solutions should be added [191].
In order to enhance the nanofluids' stability, it is necessary to modify its pH level. The pH of nanofluids must be equal to a specific critical value in order for them to have zero net charges. As the electrostatic force is diminished, the NPs begin to break apart. At this point, the system is at the point of zero charge (PZC) or an isoelectric point (IEP). Aggregation and sedimentation of NPs peaks at this stage [166]. At an IEP, both surface charge and zeta potential are zero. By enhancing repulsive forces among particles and zeta potential, a pH value far from the IEP demonstrates better durability. When a pH is far away from IEP and the zeta potential is high, it leads to a stable suspension with almost no particle coagulation [191]. The effect of pH on the stability of water-based lubricants has been investigated by many researchers using NPs such as graphene oxide (GO). For instance, Meng et al. [76] discovered that alkaline GO water-based lubricant (pH 9.0) presented the most effective lubricity, and an increase in lubricant pH value can modify the GO structure, improving the dispersibility of GO in water.

Ultrasonication
Generally, chemical or physical methods are used to generate NPs as dry powders that are then distributed into water for synthesising aqueous lubricants [192]. Every chemical treatment-based procedure changes the surface chemistry of the NPs distributed in boundary fluids, protecting them from agglomeration and sedimentation, and improving their long-term stability [166]. In contrast, physical methods such as magnetic stirring, shear homogenizer, and probe and bath ultrasonication are also widely used for producing stable nanofluids with a two-step method [188]. In comparison to bath ultrasonication, probe ultrasonication is expected to produce a nanofluid with better dispersion stability. Furthermore, it has been demonstrated that ultrasonication is more effective than mechanical stirring in reducing the agglomeration of NPs in water. In ultrasonication, an ultrasonic wave is spread through a periodic motion, leading to unstable cavitation bubbles bursting. Implosion occurs when unstable cavitation bubbles break up aggregates [193,194]. The produced ultrasonic waves can break up the attractive force between NPs to increase the stability of the nanofluid [195]. It should be noted that an excessive ultrasonication time may result in rapid sedimentation of NPs.

Surface Modification
In the surface modification method, the surface-modified NPs are straightforwardly added into water to attain stable water-based nanolubricants without the aid of surfactants. Surface modifiers are necessary to modify NPs' surface activity. Table 2 demonstrates a list of surface modifiers used for different types of NPs. For example, Tang and Cheng [196] modified ZnO NPs to enhance the stability in an aqueous solution using polymethacrylic acid (PMAA). Nano-ZnO has hydroxyl groups that interact with carboxyl groups of PMAA and develop zinc methacrylate complexes on nano-ZnO surfaces, which enables ZnO NPs to be stably dispersed in water. Zhao et al. [46] implemented an in situ modification of water soluble Cu NPs, which involves both the preparation and surface modification simultaneously. The two polar groups of Bis (2-hydroxyethyl) dithiocarbamic acid (HDA) that act as capping agents enhance the dispersibility of Cu NPs in water. Through in situ surface modification, they also synthesised water soluble CuO with different morphologies (nanorods, nanobelts, and spindle shapes) using polyethylene glycol (PEG, a guiding agent) and polyvinyl pyrrolidone (PVP, a stabilising agent), showing improved stability [54].
It is possible to obtain long-term nanofluid stability by functionalising the NP surface. Therefore, in order to prepare self-stabilised nanofluid, functionalised NPs are added to the base fluid. In this context, appropriate functional organic groups are selected that will most likely adhere to the NP surface, thereby assisting self-stabilisation. Functional groupings can be presented in two ways. The first technique requires bifunctional organic compounds to introduce all of the functional ligands in one step. The NP surface is attached with one functionality, and the other group is used to functionalise the NP. In the second method, bifunctional compounds are combined through reaction, in which a group acts as a bonding point, and can later be transformed to an ultimate functionality [197]. Kayhani et al. [106] functionalised spherical TiO 2 NPs (15 nm) by mixing with hexamethyldisilazane (C 6 H 19 NSi 2 ) at 2:1 mass fraction ratio to produce stable TiO 2 /water nanofluid. The mixture was sonicated for 1 h at 30 • C to obtain soaked NPs which were dried and dispersed in distilled water for 3-5 h by ultrasonic vibration. No agglomeration was observed in the nanofluids for several days. The stable behaviour of TiO 2 NPs resulted from the hydrophilic ammonium group. In addition to this, Yang and Liu [198] succeeded in maintaining the stability of 10 wt.% SiO 2 water-based nanolubricant by functionalising with silanes of (3-glycidoxylpropyl) trimethoxysilane, and found no sedimentation existed for a year.

Surfactant Addition
In general, NPs will not agglomerate as long as the nanofluid surface tension is low. It is reported that surfactants can be used to reduce the surface tension of nanofluids [199]. The presence of surfactants decreases the interfacial tension between two liquids or between a liquid and a solid, supporting the spread of liquids [200]. In aqueous solutions, surfactants addition can significantly improve NP stability. It is not only necessary to investigate the influence of surfactants on base nanofluids, but also important to discover new surfactant candidates with the potential to enhance the stability while minimising the damage to native nanofluid properties [188].
Popular surfactants used in literature to maximise the dispersion stability have been listed in Table 2. It is essential to select the right surfactant, which is either cationic, anionic or non-ionic. For instance, the performance of sodium dodecyl sulfate (SDS) and polyvinylpyrrolidone (PVP) was compared by Xia and Jiang [201] to present the dispersion stability of Al 2 O 3 NPs in DW. Based on this study, PVP demonstrated better dispersibility than SDS by improving stability at 0.5, 1.0, and 2.0 wt.% surfactant concentration. The nonionic PVP performed comparatively better because of its extended alkyl chain. Kakati et al. [202] prepared 0.1-0.8 wt.% Al 2 O 3 and ZnO water-based nanolubricants with the aid of 0.03 wt.% SDS. The results showed that the nanofluid with SDS had 4-5 days stability while without SDS the NPs agglomerated within 1 h after preparation. Table 2. A list of surface modifiers and surfactants used in various water-based nanolubricants.

Theories of Dispersion Stability
In addition to the above discussion, the dispersion stability of nanofluids can be explained by a number of theories including DLVO (Derjaguin and Landau 1941, Verwey and Overbeek 1948), depletion and steric stability theory. The DLVO theory describes the dispersion stability by electrostatic repulsive forces and van der Waals forces, which is only applicable for spherical NPs. When the van der Waals force is less than electrostatic repulsion, molecule agglomeration and collision are reduced significantly, resulting in more stable suspension [176,205]. The NPs tend to reassemble while the van der Waals force plays a main role. A DLVO potential is calculated as follows: where G e ( h) denotes the repulsive potential and G A ( h) denotes the attractive potential. G T ( h) is the total potential at varying values of h. Both are influenced by the distance between NPs [206], as shown in Figure 10.
repulsion, molecule agglomeration and collision are reduced significantl more stable suspension [176,205]. The NPs tend to reassemble while the force plays a main role. A DLVO potential is calculated as follows: where (ℎ) denotes the repulsive potential and (ℎ) denotes the attra (ℎ) is the total potential at varying values of ℎ. Both are influenced b between NPs [206], as shown in Figure 10. Additionally, the electrostatic repulsive potential and van der Waals tential can be determined using the following equations [207]: van der Waals attractive potential (ℎ) = − ln where is particle radius; is particle's surface potential; is medium к is inverse Debye length that is influenced by the thickness of electrical d is the distance between NPs; and is Hamaker constant. The electrostat influenced by three factors: size, distance, and surface potential of NPs, wh Waals force is influenced by the distance and radius of NPs. NPs aggreg distance is below a certain value due to the molecular attraction [207].
However, for NPs with platelet, rod, ellipse, or other shapes, both the steric stability theories have to be taken into account. The steric stabilit plained by van der Waals force and elastic steric force in nanofluids with ionic/non-ionic surfactants or absorbed polymers. The steric force is infl chemical composition of the suspension and thickness, and the density of mer layers [208][209][210][211]. The depletion theory contributes to nanofluids with additives, and depletion force is present between the NPs and non-abso which results in depletion layer formation [212]. For depletion force, the co free polymers is the influencer. When the polymer concentration is low, the due to the attractive potential energy, whereas repulsive potential energy more free polymers developing more stable nanofluids. Additionally, the electrostatic repulsive potential and van der Waals attraction potential can be determined using the following equations [207]: van der Waals attractive potential G A ( h) = − A 6 where a is particle radius; ψ o is particle's surface potential; ε is medium's permittivity; к is inverse Debye length that is influenced by the thickness of electrical double layer; h is the distance between NPs; and A is Hamaker constant. The electrostatic repulsion is influenced by three factors: size, distance, and surface potential of NPs, while the van der Waals force is influenced by the distance and radius of NPs. NPs aggregate when their distance is below a certain value due to the molecular attraction [207]. However, for NPs with platelet, rod, ellipse, or other shapes, both the depletion and steric stability theories have to be taken into account. The steric stability theory is explained by van der Waals force and elastic steric force in nanofluids with different NPs, ionic/non-ionic surfactants or absorbed polymers. The steric force is influenced by the chemical composition of the suspension and thickness, and the density of absorbed polymer layers [208][209][210][211]. The depletion theory contributes to nanofluids with free polymer additives, and depletion force is present between the NPs and non-absorbed polymers which results in depletion layer formation [212]. For depletion force, the concentration of free polymers is the influencer. When the polymer concentration is low, the NPs aggregate due to the attractive potential energy, whereas repulsive potential energy is strong with more free polymers developing more stable nanofluids.
Apart from the van der Waals, depletion, and steric stability forces, the Brownian force [213,214], buoyancy force [215], hydration force [216], and interphase resistance [217] may also contribute to the dispersion stability of nanofluids under specific conditions.

Tribo-Testing Methods
The American Society of Lubrication Engineers (ASLE) has compiled 234 apparatus for tribo-testing which are classified by their geometries. Tribometers that have been designed with advanced instrumentation are equipped with instruments that enable the measurement of coefficients of friction, friction forces, wear rates, noise, vibrations, and temperature of a system [218]. Several tribo-testing methods, including four-ball, pin-ondisk, ball-on-disk, ball-on-plate, ball-on-three-plates, and block-on-ring have been primarily used to evaluate the tribological performance of as-prepared water-based lubricants.

Four-Ball
A four-ball tribometer is one of the most common type of tribometers for analysing lubricant performance. Figure 11 shows a diagram of the four-ball tribometer. This structure consists of three balls fixed in a lubricant bath. The rotating ball is placed above the three fixed balls [218,219]. During the tribo-testing, a normal force is exerted pneumatically to the rotating ball at a constant speed, and the force is increased until the balls are welded together under the frictional heat. This tribometer can not only be used for measurement of COF and wear scar diameter (WSD), but also be applicable for assessment of load-carrying capacity and extreme pressure (EP) property. ASTM standard (D2783) is followed while conducting the tests. The EP characteristics can be evaluated by measurement of P B (last non-seizure load, the last load at which the measured scar diameter is not more than 5% greater than the compensation value at that load) and P D (weld point, the lowest applied load at which sliding surfaces seize and then weld). In principle, higher P B and P D values indicate better EP properties. It should be noted that the feasibility of evaluating EP properties is the unique feature of the four-ball test, which is independent of other types of tribo-testing configurations. measurement of coefficients of friction, friction forces, wear rates, noise, v temperature of a system [218]. Several tribo-testing methods, including fou disk, ball-on-disk, ball-on-plate, ball-on-three-plates, and block-on-ring hav ily used to evaluate the tribological performance of as-prepared water-base

Four-Ball
A four-ball tribometer is one of the most common type of tribometers lubricant performance. Figure 11 shows a diagram of the four-ball tribome ture consists of three balls fixed in a lubricant bath. The rotating ball is pla three fixed balls [218,219]. During the tribo-testing, a normal force is exer cally to the rotating ball at a constant speed, and the force is increased unt welded together under the frictional heat. This tribometer can not only be urement of COF and wear scar diameter (WSD), but also be applicable for load-carrying capacity and extreme pressure (EP) property. ASTM stand followed while conducting the tests. The EP characteristics can be evaluate ment of PB (last non-seizure load, the last load at which the measured scar d more than 5% greater than the compensation value at that load) and PD (w lowest applied load at which sliding surfaces seize and then weld). In princ and PD values indicate better EP properties. It should be noted that the feas uating EP properties is the unique feature of the four-ball test, which is i other types of tribo-testing configurations. The studies found in the literature using four-ball tribometer to eva properties are listed in Table 3. For instance, Zhang et al. used this tribom gate the tribological properties of surface capped Cu [47] and Cu/SiO2 [116 site as additives in DW, and found that the water-based nanolubricants exhi load carrying capacity and reduced wear and friction. Furthermore, multip been conducted with TiO2 NPs using four-ball tribometer to evaluate the tr formance [49,52,102,103]. For example, Sun et al. [50] investigated the tribo iour of nano-TiO2 water-based lubricant using the four-ball tribometer und The studies found in the literature using four-ball tribometer to evaluate lubricant properties are listed in Table 3. For instance, Zhang et al. used this tribometer to investigate the tribological properties of surface capped Cu [47] and Cu/SiO 2 [116] nanocomposite as additives in DW, and found that the water-based nanolubricants exhibited excellent load carrying capacity and reduced wear and friction. Furthermore, multiple studies have been conducted with TiO 2 NPs using four-ball tribometer to evaluate the tribological performance [49,52,102,103]. For example, Sun et al. [50] investigated the tribological behaviour of nano-TiO 2 water-based lubricant using the four-ball tribometer under 196 N load at a speed of 60 rpm for 30 min, which resulted in a significant reduction of wear by 30.6%, COF by 64%, with an optimal concentration of 0.7 wt.%. Additionally, with the increase of nano-TiO 2 concentration, the P B increased. To be specific, for 0.1 wt.% nano-TiO 2 the P B raised by 6.5%, ultimately reached 784 N at 5.0 wt.%. Meng et al. [52] also conducted tribo-testing with nano-TiO 2 using four-ball tribometer for 30 min at 1200-1760 r/min under 392 N. The test result indicated that adding nano-TiO 2 in lubricant assisted with the increase in P B by 62.5%, along with the reduction in COF and wear by 33.8% and 47.4%, respectively. Many factors such as load, speed, optimal concentration, and temperature can be modified for testing tribological characteristics with various simulated circumstances.

Pin-on-Disk
The pin-on-disk tribometer shown in Figure 12a is used according to standard testing procedures. The arrangement consists of a fixed pin and a rotating disk. A circular sliding path is established on the rotating disk, while the load cells and sensors measure the frictional and tangential force generated by the fixed pin. The loading force is applied on the pin which is placed on the disk surface with a distance away from the disk centre. The disk is driven by a servo motor with a certain rotation speed (rpm). The applied pins are generally cylindrical in form, with flat, truncated, or spherical conical ends. The wear of pin can be measured by monitoring its dimensional changes, such as the length of pin, the WSD of the contact face, or by measuring weight loss. If the weight loss is too little to be weighed, it can be calculated or evaluated using wear area obtained from surface profile under a 3D microscope.
of nano-TiO2 concentration, the PB increased. To be specific, for 0.1 wt.% nano-TiO2 the raised by 6.5%, ultimately reached 784 N at 5.0 wt.%. Meng et al. [52] also conducted tri testing with nano-TiO2 using four-ball tribometer for 30 min at 1200-1760 r/min under N. The test result indicated that adding nano-TiO2 in lubricant assisted with the incre in PB by 62.5%, along with the reduction in COF and wear by 33.8% and 47.4%, resp tively. Many factors such as load, speed, optimal concentration, and temperature can modified for testing tribological characteristics with various simulated circumstances.

Pin-on-Disk
The pin-on-disk tribometer shown in Figure 12a is used according to standard test procedures. The arrangement consists of a fixed pin and a rotating disk. A circular slid path is established on the rotating disk, while the load cells and sensors measure the f tional and tangential force generated by the fixed pin. The loading force is applied on pin which is placed on the disk surface with a distance away from the disk centre. T disk is driven by a servo motor with a certain rotation speed (rpm). The applied pins generally cylindrical in form, with flat, truncated, or spherical conical ends. The wear pin can be measured by monitoring its dimensional changes, such as the length of pin, WSD of the contact face, or by measuring weight loss. If the weight loss is too little to weighed, it can be calculated or evaluated using wear area obtained from surface pro under a 3D microscope. In the field of tribology, this type of tribometers has been widely used in simulat practical working conditions. The pin-on-disk tribo-testing can be used for research bearing systems, brake systems, train wheel systems, and manufacturing industries. T mechanism can be carried out under both the dry and lubricating conditions [218,21 The pin-on-disk tribometer was used by He et al. [159] to evaluate the tribological pr erties of hexagonal boron nitride (h-BN) in base water at 300 rpm for 30 min under 4 600 N loads, which revealed an excellent reduction in WSD by 14.6% and COF by 29. with an optimal concentration of 0.7 wt.% h-BN. Zhao et al. [53] also used pin-on-d In the field of tribology, this type of tribometers has been widely used in simulating practical working conditions. The pin-on-disk tribo-testing can be used for research on bearing systems, brake systems, train wheel systems, and manufacturing industries. This mechanism can be carried out under both the dry and lubricating conditions [218,219]. The pin-on-disk tribometer was used by He et al. [159] to evaluate the tribological properties of hexagonal boron nitride (h-BN) in base water at 300 rpm for 30 min under 400-600 N loads, which revealed an excellent reduction in WSD by 14.6% and COF by 29.1% with an optimal concentration of 0.7 wt.% h-BN. Zhao et al. [53] also used pin-on-disk tribometer to conduct 30 min tribological test at 50 mm/s using 0.05-0.2 wt.% cerium dioxide (CeO 2 ) water-based nanolubricant. The results indicated 20% reduction in COF and 49% reduction in wear track depth. In a similar way, Elomaa et al. [139] conducted tribo-testing with graphene oxide (GO) water-based nanolubricant and found that the optimal parameters for significant friction and wear reduction can be achieved using 1 wt.% GO under 10 N.

Ball-on-Disk
The term pin-on-disk is substituted by ball-on-disk in the presence of the spherical cap end, which is also used extensively in tribo-testing configurations due to massive application of ball bearings in engineering. In this case, the parameters of the ball including diameter, Poisson's ratio, and elastic modulus are supposed to influence the Hertzian contact stress when contacting the disk. The initial experimental conditions, therefore, are quite different, which affects the final results of the tribological test. Normally, the working principles of the pin-on-disk and ball-on-disk arrangement are quite similar. The only difference is that the contact type is set by pin or ball. The ball-on-disk tribometer includes a ball and a circular flat disk, and a contact is established between these two surfaces of contact. The ball usually remains stationary when the flat disk rotates around its central axis, as shown in Figure 12b. A pre-set normal load is applied on the fixed ball, and a circular sliding path is generated on the disk after testing. Load cells and sensors can be used to measure the friction or tangential force generated between the fixed ball and the rotating disk. Usually, the tests are conducted in accordance with the standard test procedure (ASTM G99) [219].
There is a great demand for ball-on-disk tribometers to investigate the performance of water-based nanolubricants. For example, ball-on-disk tribometer was used by Radice and Mischler [48] to inspect the effect of Al 2 O 3 NPs in aqueous solutions on the tribocorrosion behaviour of steel/alumina sliding surface under 4-10 N and 10-40 mm/s, which resulted in COF and wear rate reduction by 40-50%. Ball-on-disk tribometer has also drawn the attention of many researchers to analyse the tribological properties of nanoadditives including nanocomposite, graphene based, and ionic liquids. For example, tribological properties of carbon dots (CDs) [66] and poly ethylene glycol-graphene (PEG-G) [147] nanoadditives were evaluated under 10 N and a sliding velocity of 5 Hz. Superior friction-reduction and anti-wear properties were obtained with 0.05 wt.% PEG-G in water, indicating 39.04% wear rate reduction and 81.23% COF reduction. The use of CDs in water-based lubricants revealed outstanding tribological behaviour compared to pure water, with 39.66% COF reduction and 38% wear rate reduction. Table 3 contains a list of studies related to the use of ball-on-disk tribometers.

Ball-on-Plate
A ball-on-plate configuration is designed for sliding reciprocating motions (Figure 12c). Under certain conditions, the ball slides linearly along the plate. When compared to a ball-on-disk configuration, where the ball moves unidirectionally over a circular track, a ball-on-plate apparatus performs an alternating linear movement of the ball back and forth over the stationary plate at a constant speed [219]. The normal load is applied on the ball, and COF can be recorded against time during the sliding process. The wear track produced by reciprocating motion is much shorter than that produced by rotating motion for the same dimension of specimen under the same linear speed and testing duration. Therefore, ball-on-disk test is used more often than ball-on-plate test due to its higher efficiency for wear loss generation and more spots yielded for wear track observation.
There have been quite a few researchers who have used a ball-on-plate tribometer to characterise the tribological properties of graphene-based NPs. A reciprocating ball-on-plate tribometer was used by Xie et al. to evaluate the tribological properties of graphene/GO [77] and SiO 2 /graphene [112] as nanoadditives in water-based lubricant for magnesium alloy sheets under 3 N normal load at a speed of 0.08 m/s for 30 min. Compared to pure water, 0.5 wt.% GO enabled 77.5% COF reduction and 90% wear rate reduction, while 0.5 wt.% graphene showed only 21.9% COF reduction and 13.5% wear rate reduction. Furthermore, the best tribological properties were obtained using SiO 2 /graphene (0.1:0.4) combination in water compared to only 0.5 wt.% graphene in water, presenting decreases in COF and wear volume by 48.5% and 79%, respectively. Moreover, black phosphorus quantum dots served as a high-efficient nanoadditive in water-based lubricant with an ultra-low concentration of 0.005 wt.%, not only exhibiting remarkable wear reduction by 56.4%, COF reduction by 32.3%, but also indicating an increment in load-carrying capacity from 120N to 300 N [92]. In recent years, ball-on-plate tribometer has been used consistently for various nanoadditivies including GO, nanodiamond, copper, ionic liquids, and naphthalene (listed in Table 3).

Ball-on-Three-Plates
The ball-on-three-plates tribometer is another device that researchers can use to examine the tribological properties of lubricants. Such a configuration is also called ballon-pyramid. This device is composed of a spherical shaped ball and three plates that move independently in all directions. In order to efficiently distribute a normal load on the upper ball's three points of contact, the bottom plates need to be placed at 45 • along the loaded axis. Results can be inaccurate when normal loads are unevenly distributed on three plates. It is possible to adapt the system to the desired material combinations since the balls and the plates are interchangeable [218,219]. Figure 13a shows a ball-on-three-plates tribometer that is commonly used for testing lubricants. He et al. [16] conducted studies to investigate the tribological behaviour of water-based Al 2 O 3 nanosuspensions using a ball-on-three-plates tribometer under 10-40 N load at 20-100 mm/s sliding speed. Results indicated that, compared to water/glycerol solution, 1-2 wt.% Al 2 O 3 (30 nm) nanolubricant presented the highest reductions in COF and wear mark by 27% and 22%, respectively. The authors also used this tribometer at a sliding speed of 50 mm/s under a normal load of 20 N to assess the tribological characteristics of GO sheets in water. By using 0.06 wt.% GO nanolubricant, vibration and noise in tribo-testing were minimised simultaneously, and the COF and WSD were reduced by 44.4% and 17.1%, respectively [17]. He et al. [18] also examined the tribological properties of pure GO and g-C 3 N 4 nanosheets in water applying 10-35 N normal load with varying sliding speeds between 25-125 mm/s at 25 • C. With optimal concentration of 0.06 wt.% GO, g-C 3 N 4 and g-C 3 N 4 /GO (1:1), reductions in COF by 37%, 26%, and 37% and WSD by 19.1%, 16.0%, and 19.6%, respectively, were observed. Thus, g-C 3 N 4 /GO presented better tribological performance than only GO and g-C 3 N 4 in water under varying loads and speeds. magnesium alloy sheets under 3 N normal load at a speed of 0.08 m/s for 30 min. Compared to pure water, 0.5 wt.% GO enabled 77.5% COF reduction and 90% wear rate reduction, while 0.5 wt.% graphene showed only 21.9% COF reduction and 13.5% wear rate reduction. Furthermore, the best tribological properties were obtained using SiO2/graphene (0.1:0.4) combination in water compared to only 0.5 wt.% graphene in water, presenting decreases in COF and wear volume by 48.5% and 79%, respectively. Moreover, black phosphorus quantum dots served as a high-efficient nanoadditive in water-based lubricant with an ultra-low concentration of 0.005 wt.%, not only exhibiting remarkable wear reduction by 56.4%, COF reduction by 32.3%, but also indicating an increment in load-carrying capacity from 120N to 300 N [92]. In recent years, ball-on-plate tribometer has been used consistently for various nanoadditivies including GO, nanodiamond, copper, ionic liquids, and naphthalene (listed in Table 3).

Ball-on-Three-Plates
The ball-on-three-plates tribometer is another device that researchers can use to examine the tribological properties of lubricants. Such a configuration is also called ball-onpyramid. This device is composed of a spherical shaped ball and three plates that move independently in all directions. In order to efficiently distribute a normal load on the upper ball's three points of contact, the bottom plates need to be placed at 45° along the loaded axis. Results can be inaccurate when normal loads are unevenly distributed on three plates. It is possible to adapt the system to the desired material combinations since the balls and the plates are interchangeable [218,219]. Figure 13a shows a ball-on-threeplates tribometer that is commonly used for testing lubricants. He et al. [16] conducted studies to investigate the tribological behaviour of water-based Al2O3 nanosuspensions using a ball-on-three-plates tribometer under 10-40 N load at 20-100 mm/s sliding speed. Results indicated that, compared to water/glycerol solution, 1-2 wt.% Al2O3 (30 nm) nanolubricant presented the highest reductions in COF and wear mark by 27% and 22%, respectively. The authors also used this tribometer at a sliding speed of 50 mm/s under a normal load of 20 N to assess the tribological characteristics of GO sheets in water. By using 0.06 wt.% GO nanolubricant, vibration and noise in tribo-testing were minimised simultaneously, and the COF and WSD were reduced by 44.4% and 17.1%, respectively [17]. He et al. [18] also examined the tribological properties of pure GO and g-C3N4 nanosheets in water applying 10-35 N normal load with varying sliding speeds between 25-125 mm/s at 25 °C. With optimal concentration of 0.06 wt.% GO, g-C3N4 and g-C3N4/GO (1:1), reductions in COF by 37%, 26%, and 37% and WSD by 19.1%, 16.0%, and 19.6%, respectively, were observed. Thus, g-C3N4/GO presented better tribological performance than only GO and g-C3N4 in water under varying loads and speeds.

Block-on-Ring
In the block-on-ring setup, as shown in Figure 13b, the block is placed against the rotating ring under a predetermined load. The COF between the block and the ring is recorded when the ring rotates at a certain rate. Block-on-ring tribometers have been extensively used to study nanolubricants, coatings, and polymers. This type of tribometer configuration can evaluate the wear and COF between sliding surfaces in contact such as lubricating films, rings, and bearings.
Huang et al. [21] used a block-on-ring tribometer to study the role of GO, Al 2 O 3 , and GO-Al 2 O 3 as nanoadditives in water-based lubricants under varying loads from 10 to 30 N and varying sliding speeds from 100 to 400 mm/s at 20-25 • C. Compared to individual 0.06 wt.% GO and 0.06 wt.% Al 2 O 3 solutions, 0.12 wt.% GO-Al 2 O 3 (1:1) lubricant showed significant decreases in COF by 47% and 64%, respectively, and surface roughness was improved by 60% and 63%, respectively. The authors conducted tribotesting with 0.16 wt.% GO/SiO 2 water-based slurry under 20 N and 109 rpm, leading to a surface roughness (R a ) reduction by 35% and an increase in material removal rate (MRR) by 28% [19]. Huang et al. [12] conducted another study using a block-on-ring tribometer for testing water-based nanosuspension containing ZrO 2 /TiO 2 NPs and GO with a normal load of 100 N and a sliding speed of 400 mm/s. The results demonstrated that the use of GO-ZrO 2 /TiO 2 hybrid nanosuspension resulted in 65% surface roughness improvement and 25% reduction in COF. The studies using block-on-ring tribometers are summarised in Table 3.

Others
There are several tribometers with a wide range of testing capabilities used for different applications. For example, piston ring-on-cylinder line is used to analyse wear and friction phenomena in ring-piston pairs in combustion engines, compressors, and pumps, as shown in Figure 14a. Two-disk tribometers are used to measure relative displacement among two cylindrical surfaces to determine the wear on gears, rollers, bearings, and wheel systems [218,220], as shown in Figure 14b. Furthermore, a ring-on-ring tribometer is used to study the wear and friction of cylindrical tribological pairs, such as camshafts, clutches, and bearings, as shown in Figure 14c. Contact areas between rings typically vary based on their topology, which can be either tangential or concentric [218].
In recent months, a novel ultrahigh speed ball-on-disk tribometer with a sliding speed up to 50 m/s has been developed [221], which may be used to characterise the lubricants in a large speed range. This ultrahigh sliding speed can be achieved through a combined solution of on-line precision cutting and in situ dynamic.
In the tribological studies related to water-based lubricants, some researchers used other tribometers including ring-on-plate [222], ball-on-block, pin-on-cylinder [148,149], and 2-ball-plate tribometers [126] according to the testing parameters. In recent years, Ye et al. used ball-on-block tribometer to evaluate the tribological properties of multi-walled carbon nanotubes [129], stearic acid [96], and urea-modified fluorinated graphene [134] as water-based nanoadditives by analysing their anti-wear and friction-reduction properties. Table 3 demonstrates a list of tribological studies summarised in the field of waterbased nanolubrication using different tribometers.  Table 3 demonstrates a list of tribological studies summarised in the field of waterbased nanolubrication using different tribometers.

Lubrication Mechanism
Due to small sizes and high surface areas, NPs have unique properties that are different from bulk materials. Hence, research groups have focused on the addition of NPs to lubricant dispersions for enhancing the thermophysical and tribological properties [230,231]. Scientists have been committed to understanding the roles of nanoadditives in base lubricants and the lubrication mechanism of synthesised nanolubricants in terms of friction-reduction and anti-wear properties. Numerous mechanisms, including rolling effect, protection film, mending effect, polishing effect, and synergistic effect [232], have been proposed to explain the enhancement of the lubricity.

Rolling/Ball Bearing Effect
According to the theory of rolling/ball-bearing effect, nanolubricants have outstanding lubricity due to two factors. First, NPs are spherical and they can act as ball bearings under friction. Second, the NPs flatten and create a sliding system between the two friction surfaces, eventually reducing friction and wear [233]. Figure 15a illustrates the rolling/ball bearing mechanism. NPs with a sphere, quasi-spherical, or hemispherical shape play this role. Generally, spherical NPs reduce the COF by converting sliding friction to rolling friction as a result of their morphological properties [233][234][235]. Some scholars have revealed that these NPs themselves might roll or embed into surfaces [236][237][238]. Moreover, many scholars have mentioned the ball bearing effect in lubrication mechanism analysis of the water-based nanoadditives including metal and non-metal oxide [54,59,100], composite [86,119], and many more [68,78,92,124,137,159]. In the past few years, it has been noted that due to their spherical shape, metal and non-metal oxides such as SiO 2 [59,100] and TiO 2 [15,25,29] generate a ball bearing effect between sliding surfaces, converting the friction mechanism from sliding to rolling and thus causing friction reduction. NPs will maintain their shape and stiffness under mild conditions [239][240][241].

Lubrication Mechanism
Due to small sizes and high surface areas, NPs have unique properties that are different from bulk materials. Hence, research groups have focused on the addition of NPs to lubricant dispersions for enhancing the thermophysical and tribological properties [230,231]. Scientists have been committed to understanding the roles of nanoadditives in base lubricants and the lubrication mechanism of synthesised nanolubricants in terms of friction-reduction and anti-wear properties. Numerous mechanisms, including rolling effect, protection film, mending effect, polishing effect, and synergistic effect [232], have been proposed to explain the enhancement of the lubricity.

Rolling/Ball Bearing Effect
According to the theory of rolling/ball-bearing effect, nanolubricants have outstanding lubricity due to two factors. First, NPs are spherical and they can act as ball bearings under friction. Second, the NPs flatten and create a sliding system between the two friction surfaces, eventually reducing friction and wear [233]. Figure 15a illustrates the rolling/ball bearing mechanism. NPs with a sphere, quasi-spherical, or hemispherical shape play this role. Generally, spherical NPs reduce the COF by converting sliding friction to rolling friction as a result of their morphological properties [233][234][235]. Some scholars have revealed that these NPs themselves might roll or embed into surfaces [236][237][238]. Moreover, many scholars have mentioned the ball bearing effect in lubrication mechanism analysis of the water-based nanoadditives including metal and non-metal oxide [54,59,100], composite [86,119], and many more [68,78,92,124,137,159]. In the past few years, it has been noted that due to their spherical shape, metal and non-metal oxides such as SiO2 [59,100] and TiO2 [15,25,29] generate a ball bearing effect between sliding surfaces, converting the friction mechanism from sliding to rolling and thus causing friction reduction. NPs will maintain their shape and stiffness under mild conditions [239][240][241]. On the other hand, stable spherical NPs can enhance bearing capacity and performance under maximum pressure [242,243]. The ball bearing effect of spherical NPs is influenced by the film thickness, and NPs with a diameter close to the film thickness will retain their shape. A transfer film is generated when the film's thickness is less than the On the other hand, stable spherical NPs can enhance bearing capacity and performance under maximum pressure [242,243]. The ball bearing effect of spherical NPs is influenced by the film thickness, and NPs with a diameter close to the film thickness will retain their shape. A transfer film is generated when the film's thickness is less than the NPs' diameter [59]. During the hot rolling test, Wu et al. [25,29] found that the rolling effect of spherical shaped nano-TiO 2 assisted in reducing wear and friction to a large extent, thereby decreasing the roll roughness and rolling force.

Protective Film/Tribo-Film
The idea behind this method is the formation of a thin protective film consisting of NPs on the area of frictional contact [236]. The protective film refers to coating the friction surfaces by separating them from direct contact, and it is derived from three aspects. First, the NPs are able to be deposited or adsorbed on the rubbing surfaces to form a boundary film. Second, the NPs are melted as a protective film under friction heat to cover the friction surfaces. Third, the formation of the protective film is produced by tribo-sintering of the NPs, and the compacted and smooth tribo-film is conducive to the decreases in friction and wear. Figure 15b shows the schematics of protective film/tribo-film formation.
It is believed by scientists that the nanofluids have better lubricity than the base lubricant because the NPs can form a protective film over friction surfaces and thereby prevent the friction surfaces from sliding [244][245][246]. Multiple studies have mentioned the significance of protective film formation using NPs such as graphene based [73,75,77,134,145,147], ionic liquids [155][156][157], and phosphorus [162]. Some other NPs such as metal oxides [27][28][29][56][57][58], diamond [137], and composites [19,113,115,123] are also able to form a protective film on friction surfaces. Based on previous studies, it is concluded that the protective film not only reduces the friction and wear between friction pairs but also helps in isolating air, thus leading to reduced oxide scale thickness during hot steel rolling [27][28][29]. It is also found that the enhanced wettability of nanofluids caused by the addition of NPs promotes the formation of protective film.

Mending Effect
Mending effect is very important in moving mechanical parts that are repeatedly loaded and might fail from the crack, as there is the possibility that cracks can spread outside the contact area to other parts of the metal body. Using this technology, the metal surface can become hardened by sintering and filling NPs into macroscopic cracks [236], as shown in Figure 15c. A mending effect has been observed in numerous studies [16,21,27,66,92,117,121]. Many results reveal that NPs act with a mending effect by filling in the surface defects, thereby improving the surface quality [247][248][249]. Bao et al. [59,100] used SiO 2 NPs to improve the tribological performance of water-based lubricants through the mending effect, which contributed to the decreases in scratches and pits on friction surface with enhancement of anti-wear and friction-reduction properties. To compensate for weight loss, it is particularly important to deposit NPs on the interaction surface [11,59,171,234,249].
Scientists proposed the roles of NPs to self-heal or repair surfaces by depositing them in grooves or scars, which can prevent further wear [250]. Tribo-sintering may occur in the mending process when the frictional heat generated is strong enough to melt the scattered NPs, thus causing permanent deposition of the NPs on the worn surface [219,233]. It is reported that soft metals with a face-centred cubic structure are generally capable of self-repairing [233]. However, the mending effect does not simply include the deposition of the NPs on the friction surface. The melting point of NPs falls significantly as their size decreases. These NPs are thus easy to melt at a high friction surface temperature, thus forming a uniform filler and tightly bonding to the friction surface [233].

Polishing/Smoothing Effect
The polishing effect can be observed prior to and after using lubricants that contain nanoadditives. The surface roughness is decreased because of NPs deposition on the surface profile [251]. In the polishing/smoothing effect, NPs are used to minimise the roughness of the lubricated surface by abrasion treatment [252], as shown in Figure 15d. NPs deposited on the hollow contour provides smoothness to the metal surface. A number of studies demonstrated that the polishing effect can be contributed by some types of NPs such as phosphorus [92], metal oxide [27][28][29]100], and nitride [159]. Under high pressure and speed, NPs flatten the surface peaks to make a smooth tool surface, which in turn greatly improves the surface quality of workpiece.
Nanofluid is an ideal polishing substance that mechanically polishes the friction surface. The tribo-pair contact area increases when harder nanoparticles are used to polish a smoother surface, which reduces friction and meanwhile increases the load-carrying capacity of the nanolubricant. However, the mechanical properties of NPs are poor on a very rough surface. Due to their small size, NPs can only restore rough surfaces at an atomic scale. In other words, a mechanical polishing effect is more evident when the surface of tribo-pair is relatively smooth. In addition to the polishing effect, NPs can be deposited easily in microcracks and valleys as per the mending effect, which also produces smooth surfaces [233].

Synergistic Effect
Lubricants and modified friction surfaces are often combined with nanoadditives to generate a synergistic effect. Most of the time, NPs perform their lubrication effects through not just one mechanism, but rather a combination of various mechanisms. Furthermore, lubrication conditions can change when various mechanisms are modified. Research on lubrication mechanism analysis has always been focused on the roles of NPs in base lubricants. It is also important to consider the interaction among the NPs, the dispersant, and the base lubricant [233].
Synergistic lubrication effect may improve the tribological performance of nanolubricants by integrating two or more mechanisms, and this effect is usually contributed by composite nanoadditives [21,78,84,115,118,121,225]. A synergistic effect is also reported for some other nanoadditives such as metal carbide [90], nitrides [159,160], and oxides [73,96,108,130,146,147,154], and these nanoadditives themselves play multiple roles in combining two or more lubrication effects together for enhanced lubricity.

Exfoliation
For the NPs with layered structure, the lubrication mechanisms are different from those of spherical NPs. Tevet et al. [253] pointed out three main lubrication mechanisms in terms of fullerene-like (IF) NPs which have layered hollow polyhedral structure. First, the IF NPs may act as a spacer between the friction pairs, which supplies low frictional force during sliding process, as shown in Figure 16a. Second, the IF NPs can also behave as ball bearings that roll on the friction surface, as shown in Figure 16b. Third, the IF NPs can be exfoliated into nanosheets under certain shear force. The evolved nanosheets are deposited on the asperities of friction surface and thus supply an easy shearing for the subsequent friction process, as shown in Figure 16c. The "exfoliation" is typical lubrication mechanism for layer-structure NPs dispersed in base lubricant. deposited on the hollow contour provides smoothness to the metal surface. A number of studies demonstrated that the polishing effect can be contributed by some types of NPs such as phosphorus [92], metal oxide [27][28][29]100], and nitride [159]. Under high pressure and speed, NPs flatten the surface peaks to make a smooth tool surface, which in turn greatly improves the surface quality of workpiece.
Nanofluid is an ideal polishing substance that mechanically polishes the friction surface. The tribo-pair contact area increases when harder nanoparticles are used to polish a smoother surface, which reduces friction and meanwhile increases the load-carrying capacity of the nanolubricant. However, the mechanical properties of NPs are poor on a very rough surface. Due to their small size, NPs can only restore rough surfaces at an atomic scale. In other words, a mechanical polishing effect is more evident when the surface of tribo-pair is relatively smooth. In addition to the polishing effect, NPs can be deposited easily in microcracks and valleys as per the mending effect, which also produces smooth surfaces [233].

Synergistic Effect
Lubricants and modified friction surfaces are often combined with nanoadditives to generate a synergistic effect. Most of the time, NPs perform their lubrication effects through not just one mechanism, but rather a combination of various mechanisms. Furthermore, lubrication conditions can change when various mechanisms are modified. Research on lubrication mechanism analysis has always been focused on the roles of NPs in base lubricants. It is also important to consider the interaction among the NPs, the dispersant, and the base lubricant [233].
Synergistic lubrication effect may improve the tribological performance of nanolubricants by integrating two or more mechanisms, and this effect is usually contributed by composite nanoadditives [21,78,84,115,118,121,225]. A synergistic effect is also reported for some other nanoadditives such as metal carbide [90], nitrides [159,160], and oxides [73,96,108,130,146,147,154], and these nanoadditives themselves play multiple roles in combining two or more lubrication effects together for enhanced lubricity.

Exfoliation
For the NPs with layered structure, the lubrication mechanisms are different from those of spherical NPs. Tevet et al. [253] pointed out three main lubrication mechanisms in terms of fullerene-like (IF) NPs which have layered hollow polyhedral structure. First, the IF NPs may act as a spacer between the friction pairs, which supplies low frictional force during sliding process, as shown in Figure 16a. Second, the IF NPs can also behave as ball bearings that roll on the friction surface, as shown in Figure 16b. Third, the IF NPs can be exfoliated into nanosheets under certain shear force. The evolved nanosheets are deposited on the asperities of friction surface and thus supply an easy shearing for the subsequent friction process, as shown in Figure 16c. The "exfoliation" is typical lubrication mechanism for layer-structure NPs dispersed in base lubricant. IF NPs are inorganic and layered compounds such as fullerenes and carbon nanotubes, with spherical or tubular structures. These concentric layered materials are often solid, though hollow sometimes, and are arranged in an onion shape [254]. This onion morphology is spherical outside and lamellar inside. It is more likely that the onion may have spherical morphology if it is stable, otherwise it may exfoliate and turn into a sheet [255]. One of the benefits of onion structure is the sphere-like shape and the lack of dangling bonds [256]. Su et al. [80] evaluated the lubrication performance of onion-like carbon (OLC) NPs and graphene oxide (GO) sheets, and reported that due to its onion-like structure, OLC may reduce friction and wear in water, producing rolling motion and tribo-film formation during sliding. Due to the protective film formation between adjacent wear surfaces, the 2D structure of GO provided better sliding and shear between adjacent wear surfaces, exhibiting excellent lubricity. Another layer-lattice-structured NPs, hexagonal boron nitride, has also exhibited superb friction-reduction properties, probably through a similar mechanism [246,257].

Hydration Lubrication
In water-based lubricants, regardless of whether NPs are added or not, water is absolutely the largest component. The water molecule, due to its positive (H atoms) and negative charges (O atom), interacts strongly with charged ions or zwitterions to form stable hydration layers (thin water films) in aqueous media [258]. The hydration layers form hydrated charges between sliding surfaces, which can sustain large pressures due to the reluctance of the hydration water to be squeezed out and meanwhile generate a fluid response to shear [259]. The hydration lubrication mechanism enables a significant friction reduction between surfaces which expose or slide across such hydrated layers under low shear stresses. This striking lubrication mechanism is expected to provide new insight into the boundary lubrication processes in water-based lubricant.
Although these mechanisms are well established, it has always been a matter of debate among researchers. In fact, it has been difficult for any single theory to fully explain the lubrication mechanism in water-based nanolubricants. In this case, further research is needed to establish a more accurate theory of nanolubrication. IF NPs are inorganic and layered compounds such as fullerenes and carbon nanotubes, with spherical or tubular structures. These concentric layered materials are often solid, though hollow sometimes, and are arranged in an onion shape [254]. This onion morphology is spherical outside and lamellar inside. It is more likely that the onion may have spherical morphology if it is stable, otherwise it may exfoliate and turn into a sheet [255]. One of the benefits of onion structure is the sphere-like shape and the lack of dangling bonds [256]. Su et al. [80] evaluated the lubrication performance of onion-like carbon (OLC) NPs and graphene oxide (GO) sheets, and reported that due to its onion-like structure, OLC may reduce friction and wear in water, producing rolling motion and tribo-film formation during sliding. Due to the protective film formation between adjacent wear surfaces, the 2D structure of GO provided better sliding and shear between adjacent wear surfaces, exhibiting excellent lubricity. Another layer-lattice-structured NPs, hexagonal boron nitride, has also exhibited superb friction-reduction properties, probably through a similar mechanism [246,257].

Hydration Lubrication
In water-based lubricants, regardless of whether NPs are added or not, water is absolutely the largest component. The water molecule, due to its positive (H atoms) and negative charges (O atom), interacts strongly with charged ions or zwitterions to form stable hydration layers (thin water films) in aqueous media [258]. The hydration layers form hydrated charges between sliding surfaces, which can sustain large pressures due to the reluctance of the hydration water to be squeezed out and meanwhile generate a fluid response to shear [259]. The hydration lubrication mechanism enables a significant friction reduction between surfaces which expose or slide across such hydrated layers under low shear stresses. This striking lubrication mechanism is expected to provide new insight into the boundary lubrication processes in water-based lubricant.
Although these mechanisms are well established, it has always been a matter of debate among researchers. In fact, it has been difficult for any single theory to fully explain the lubrication mechanism in water-based nanolubricants. In this case, further research is needed to establish a more accurate theory of nanolubrication.

Physicochemical Properties of Applied Lubricants
Traditional water-based lubricants are often restricted in practical metal rolling due to their low viscosity, poor wettability on roll surface, and harsh corrosion of base water, despite their eco-friendliness, low-cost, noninflammability, and outstanding cooling performance. The physicochemical properties of applied lubricants including viscosity, wettability, and corrosivity are crucial to the lubrication performance.
Viscosity is a measurement of the tendency of liquid to resist flow, and it is defined as the ratio of the shear stress to the shear rate [82]. The liquid is known to be Newtonian or non-Newtonian when the viscosity is constant or varies, respectively, at different val-ues of shear rate [260]. The viscosity of lubricant is dependent on many factors, and it influences lubrication performance in most cases. It has been reported that the viscosity of base lubricant increases with the addition of NPs [25,261,262], and it also increases with the increase of their mass fraction [173]. In addition, the viscosity decreases with the increase of temperature [263], and it is a function of pressure throughout the inlet zone in metal rolling [264]. Most importantly, a higher viscosity leads to a lower COF due to a transition of lubrication regime from boundary lubrication into mixed or hydrodynamic lubrication [241], which indicates an increase in the thickness of lubricating film as per the well-known Stribeck curve [4]. This increased film thickness restrains the work rolls and workpiece from direct contact, leading to decreased friction in the contact zone.
Wettability is one of the most important lubricant characteristics, which reveals how well a lubricant can wet a solid surface [265]. Wetting of surfaces in contact is of vital importance to decrease friction and wear, which relates directly to the decreases of rolling force and roll wear in metal rolling [29,266]. This result is ascribed to the phenomena that excellent wettability facilitates the formation of a lubricating film [112], and also helps retain the effective lubricants on work roll surfaces [28]. In general, wettability can be characterised by the measurement of contact angle on a solid surface, and a smaller contact angle indicates a better wettability [267]. It has been reported that the wettability of base lubricant can be significantly improved by adding NPs, and the addition of dispersant or surfactant also enables improved wettability due to their functional groups [25].
Corrosivity of water-based lubricants often plays a negative role in aggravating the surface quality of rolled products, especially for ferrous materials. Water itself and pH values of the lubricants both determine the corrosion effect of as-prepared water-based lubricants on metal surfaces. It has been proved that the corrosion resistance of water can be enhanced by adding water-soluble additives. For example, nanoadditives such as fluorinated graphene [72], surface-modified CuS NPs [62], and GO-TiO 2 [84] are able to retard the corrosion of steel by forming a protective film on the steel surface. Corrosion inhibitors such as Triethanolamine (TEA) [92] and sodium dodecyl benzene sulfonate (SDBS) [63], have also been used to minimise the corrosivity of water-based lubricants. On the other hand, water-based lubricant with a pH value of 7 and above leads to insignificant corrosion on the rolled surface [76]. Nevertheless, the optimal pH values in the lubricants should be determined with the consideration of their effects on the dispersion stability of the lubricants. In some cases, the corrosion resistance of water-based lubricants may be negligible, especially in hot rolling of steels and cold rolling of non-ferrous metals such as aluminium and magnesium. However, special attention should be given to the corrosion effect of water-based lubricants on cold rolling of ferrous metals.

Hot Rolling of Steels
Hot rolling of steels is applied to obtain not only the required dimensions and mechanical properties, but also satisfying surface finish [268]. Friction and wear are unavoidably generated between the work rolls and the workpiece, which results in increased consumption of energy and damage of work rolls [269,270]. Water-based nanolubricants are emerging to substitute the traditional oil-containing lubricants to resolve these issues with environmental concern. The use of water-based nanolubricants in hot steel rolling also exhibits a great potential in reducing the thickness of oxide scale, improving the surface quality of rolled products, and refining the grains in rolled steel. The water-based nanolubricants can also act as better coolants of work rolls than conventional cooling water due to their enhanced thermal conductivity [271], which further prolongs the roll service life by replacing the cooling water using a lubricant supply system. In view of these, energy consumption of the rolling mill can be lowered, and rolling high-strength steels with heavy reduction can thus be readily achieved within the limits of mill load. Meanwhile, the roll changing frequency would be reduced, and the mill configuration would be simplified, thus leading to increased productivity and decreased operation cost. The overall quality and yield together with the properties of rolled products are therefore greatly enhanced.
As the hot rolling process involves harsh working conditions such as high temperature and pressure, special requirements are placed on the physicochemical properties of the nanoadditives and other chemical additives. The applications of different types of water-based nanolubricants in hot rolling are comprehensively reviewed in the subsequent sub-sections in terms of their effects on rolling force, surface morphology, oxidation behaviour of steel, and microstructure obtained after rolling.

Rolling Force
In general, rolling force data during hot steel rolling are measured through two individual transducers placed at both the drive and the operation sides over the bearing blocks of the top work roll. The presence of lubricants between the work rolls and the workpiece enables a decrease in friction and hence the rolling force is lowered by up to 25% [272]. The value of rolling force is one of the key indicators that evaluates the energy expenditure during hot steel rolling.
TiO 2 water-based nanolubricants used in hot steel rolling were first reported in [273]. The rolling force obtained using the lubricant with 2% anatase TiO 2 decreased in each pass, and the total decrease reached up to 20%, in comparison to that obtained under dry condition. However, there was a lack of experimental evidence to underpin the analysis of lubrication performance, and the role of TiO 2 NPs was not well understood. The effects of TiO 2 concentration and rolling parameters on the rolling force were not discussed either. In light of this, single-pass hot steel rolling tests were conducted to investigate the influences of TiO 2 concentration and rolling temperature on the rolling force [29]. It was found that the use of 4% TiO 2 lubricant led to the lowest rolling force at rolling temperatures of 850 and 950 • C, while the rolling force obtained under 1050 • C did not vary significantly under all the lubrication conditions (see Figure 17a). The mechanisms of the decrease in rolling force were dominated by rolling and mending effects together with the formation of protective film, and they were demonstrated through cross-sectional SEM-EDS analysis. The hot rolled steel samples observed under SEM are, in fact, inevitably involved in grinding and polishing processes, which may affect the distribution of TiO 2 NPs and therefore the understanding of corresponding lubrication mechanisms. To overcome this drawback, a focused ion beam (FIB) foil was cut from the surface of rolled steel, and then observed under TEM to identify the NP distribution through EDS mapping [28]. The synergistic effect of lubricating film, rolling, polishing, and mending was thus confirmed to contribute to the predominant lubrication mechanisms. The effect of work roll roughness on the rolling force was also examined, which has been neglected by the majority of researchers in the field of hot steel rolling [25]. This revealed a significant research finding that the continuous use of TiO 2 water-based nanolubricants was inclined to enable a successive decrease in rolling force up to 8.3% due to the polishing on work roll surface (see Figure 17b). Some other researchers applied composite nanomaterials such as MoS 2 -Al 2 O 3 as the nanoadditives in water, and the average rolling force within five rolling passes was reduced by 26.9% compared with the base fluid [118]. Another notable finding was that the effect of MoS 2 -Al 2 O 3 lubricant on the decrease in rolling force was superior to that of the lubricant with individual nanoadditive, owing to the synergistic effect of the composite nanoadditives.
Although significant decrease in rolling force has been achieved in [25,28,29,118,273], the nanoadditives used in these lubricants all had particle sizes less than 100 nm, which brought forth extremely high material cost especially when large quantities of lubricants were applied, let alone the combination of two or more nanoadditives. Beyond this, the nanolubricants have always been prepared using complex chemical agents, followed by subsequent processes including ultrasonic treatment, which led to extra production cost. All these disadvantages have greatly restricted the application and popularisation of waterbased nanolubricants in industrial-scale hot steel rolling. Accordingly, relatively coarse TiO 2 NPs (~300 nm in diameter) were adopted to replace the expensive nanosized particles in water-based lubrication formula [24]. Novel dispersant (SDBS with a linear structure) and extreme pressure agent (Snailcool) were added to compensate the degradation of lubrication performance caused by coarsened TiO 2 NPs. These TiO 2 water-based nanolubricants were prepared using mechanical agitation without applying ultrasonic treatment. Nevertheless, the rolling force could be decreased up to 8.1% in a single-pass hot rolling at 850 • C when using 4% TiO 2 lubricant.
An overview of the typical representatives of the use of water-based nanolubricants in decreasing rolling force is presented in Table 4. The total decrease in rolling force is expected to be more significant upon the use of lubricants in multi-pass hot steel rolling.  Although significant decrease in rolling force has been achieved in [25,28,29,118,273], the nanoadditives used in these lubricants all had particle sizes less than 100 nm, which brought forth extremely high material cost especially when large quantities of lubricants were applied, let alone the combination of two or more nanoadditives. Beyond this, the nanolubricants have always been prepared using complex chemical agents, followed by subsequent processes including ultrasonic treatment, which led to extra production cost. All these disadvantages have greatly restricted the application and popularisation of water-based nanolubricants in industrial-scale hot steel rolling. Accordingly, relatively coarse TiO2 NPs (~300 nm in diameter) were adopted to replace the expensive nanosized particles in water-based lubrication formula [24]. Novel dispersant (SDBS with a linear structure) and extreme pressure agent (Snailcool) were added to compensate the degradation of lubrication performance caused by coarsened TiO2 NPs. These TiO2 water-based nanolubricants were prepared using mechanical agitation without applying ultrasonic treatment. Nevertheless, the rolling force could be decreased up to 8.1% in a single-pass hot rolling at 850 °C when using 4% TiO2 lubricant.
An overview of the typical representatives of the use of water-based nanolubricants in decreasing rolling force is presented in Table 4. The total decrease in rolling force is expected to be more significant upon the use of lubricants in multi-pass hot steel rolling.   Surface morphology of hot rolled steel strips, i.e., the surface topography of oxide scale, can be characterised in terms of surface roughness measurement and microscopic observation, which is directly related to the assessment of surface quality. In the case of pickle-free as-hot-rolled steel strip, in particular, the surface roughness plays an important role in the downstream processing such as sheet metal forming, coating, and stamping [274,275]. It has been of great interest in recent years to use TiO 2 water-based lubricants to improve the surface morphology of as-hot-rolled steel strips [24,25,28,29,52]. Among these studies, a typical example is the use of 4% TiO 2 lubricant dispersed with PEI and glycerol, which yielded the smoothest strip surface after rolling according to the 3D surface morphologies (see Figure 18) [28]. The surface roughness of the rolled strip under dry condition can thus be improved by up to 19.5% (see Figure 19). Some other researchers used SiO 2 NPs (<0.5%) as nanoadditives in the base lubricant to improve the surface morphology of hot rolled strips [100]. The main mechanisms of the decrease in surface roughness were derived from mending and polishing effects of SiO 2 NPs. These SiO 2 NPs not only filled the surface defects such as pores and cracks, but also removed the peaks protruded from the surface [29]. The other mechanism that contributed to the decrease in surface roughness was the formation of tribofilm, and hence the direct contact between the work roll and the workpiece was relieved [276]. When the concentration of nanoadditive exceeded the optimal one, the agglomeration of NPs might aggravate the friction and wear in the contact zone, thereby increasing the surface roughness. On the contrary, a nanoadditive concentration lower than the optimal one resulted in insufficient lubrication, and therefore an insignificant effect on the decrease in surface roughness. Surface morphology of hot rolled steel strips, i.e., the surface topography of oxide scale, can be characterised in terms of surface roughness measurement and microscopic observation, which is directly related to the assessment of surface quality. In the case of pickle-free as-hot-rolled steel strip, in particular, the surface roughness plays an important role in the downstream processing such as sheet metal forming, coating, and stamping [274,275]. It has been of great interest in recent years to use TiO2 water-based lubricants to improve the surface morphology of as-hot-rolled steel strips [24,25,28,29,52]. Among these studies, a typical example is the use of 4% TiO2 lubricant dispersed with PEI and glycerol, which yielded the smoothest strip surface after rolling according to the 3D surface morphologies (see Figure 18) [28]. The surface roughness of the rolled strip under dry condition can thus be improved by up to 19.5% (see Figure 19). Some other researchers used SiO2 NPs (<0.5%) as nanoadditives in the base lubricant to improve the surface morphology of hot rolled strips [100]. The main mechanisms of the decrease in surface roughness were derived from mending and polishing effects of SiO2 NPs. These SiO2 NPs not only filled the surface defects such as pores and cracks, but also removed the peaks protruded from the surface [29]. The other mechanism that contributed to the decrease in surface roughness was the formation of tribofilm, and hence the direct contact between the work roll and the workpiece was relieved [276]. When the concentration of nanoadditive exceeded the optimal one, the agglomeration of NPs might aggravate the friction and wear in the contact zone, thereby increasing the surface roughness. On the contrary, a nanoadditive concentration lower than the optimal one resulted in insufficient lubrication, and therefore an insignificant effect on the decrease in surface roughness.

Oxidation Behaviour of Steel
As hot steel rolling is normally conducted at the temperatures ranging from 800 to 1050 °C, significant oxidation occurs instantaneously in air [268]. The oxide scale formed on a hot rolled steel strip plays a prominent role in the subsequent industrial processes, including pickling, cold rolling, heat treatment, and electrolytic tinning [277]. In most cases, as-hot-rolled steel strip needs to suffer pickling, during which the oxide scale is removed by the use of acid treatment to obtain a high-quality surface for upcoming cold rolling. The oxides descaled as such have a dramatic impact on the consumption of acid and the yield of finished products. Decreasing oxide scale thickness is thus a highly desirable target in practical hot rolling production line. In some other cases, it is required to produce pickle-free as-hot-rolled steel strip that has 'tight oxide scale' formed on the strip surface prior to downstream forming [278]. Besides the roughness of the strip surface, the constitution of oxide phases in the scale also has a significant effect on the tribological feature during metal working [279]. Taking the oxide scale formed in mild steel as an example, hematite increases friction and wear as abrasive behaviour due to its high hardness, while magnetite and wustite are more ductile and hence resistant to wear [280]. As a whole, thin and tight oxide scale with considerable amounts of magnetite and wustite is always preferred after hot steel rolling.
Several studies were devoted to decreasing the oxide scale thickness during hot steel rolling by the use of various nanoadditives in water, including Eu-doped CaWO4 NPs [276], SiO2 NPs [100], MoS2-Al2O3 nanocomposite [118], and TiO2 NPs [24,28,29,52]. Some possible mechanisms of the decrease in oxide scale thickness have been proposed. First, the increase in strip deformation leads to the decrease in oxide scale thickness [275]. Second, the NPs fill in the voids of oxide scales and then prevent oxygen from penetrating into steel matrix for further oxidation [52]. Third, the NPs deposit on the strip surface to form a protective film that isolates oxygen and thus reduces the diffusion of O 2-into oxide layers [118]. These so-called 'mending effect' and 'protective film' can be characterised using SEM or TEM, in which the distribution of NPs can be clearly identified in the oxide layers [28,29]. The effects of using water-based nanolubricants on steel oxidation in these studies, however, were only confined to the entire thickness of oxide scale. It is also very Figure 19. Surface roughness of rolled steels at 850 • C under different lubrication conditions as shown in Figure 18 [28]. (Rolling reduction of 30% in each single pass; rolling speed of 0.35 m/s).

Oxidation Behaviour of Steel
As hot steel rolling is normally conducted at the temperatures ranging from 800 to 1050 • C, significant oxidation occurs instantaneously in air [268]. The oxide scale formed on a hot rolled steel strip plays a prominent role in the subsequent industrial processes, including pickling, cold rolling, heat treatment, and electrolytic tinning [277]. In most cases, as-hot-rolled steel strip needs to suffer pickling, during which the oxide scale is removed by the use of acid treatment to obtain a high-quality surface for upcoming cold rolling. The oxides descaled as such have a dramatic impact on the consumption of acid and the yield of finished products. Decreasing oxide scale thickness is thus a highly desirable target in practical hot rolling production line. In some other cases, it is required to produce picklefree as-hot-rolled steel strip that has 'tight oxide scale' formed on the strip surface prior to downstream forming [278]. Besides the roughness of the strip surface, the constitution of oxide phases in the scale also has a significant effect on the tribological feature during metal working [279]. Taking the oxide scale formed in mild steel as an example, hematite increases friction and wear as abrasive behaviour due to its high hardness, while magnetite and wustite are more ductile and hence resistant to wear [280]. As a whole, thin and tight oxide scale with considerable amounts of magnetite and wustite is always preferred after hot steel rolling.
Several studies were devoted to decreasing the oxide scale thickness during hot steel rolling by the use of various nanoadditives in water, including Eu-doped CaWO 4 NPs [276], SiO 2 NPs [100], MoS 2 -Al 2 O 3 nanocomposite [118], and TiO 2 NPs [24,28,29,52]. Some possible mechanisms of the decrease in oxide scale thickness have been proposed. First, the increase in strip deformation leads to the decrease in oxide scale thickness [275]. Second, the NPs fill in the voids of oxide scales and then prevent oxygen from penetrating into steel matrix for further oxidation [52]. Third, the NPs deposit on the strip surface to form a protective film that isolates oxygen and thus reduces the diffusion of O 2− into oxide layers [118]. These so-called 'mending effect' and 'protective film' can be characterised using SEM or TEM, in which the distribution of NPs can be clearly identified in the oxide layers [28,29]. The effects of using water-based nanolubricants on steel oxidation in these studies, however, were only confined to the entire thickness of oxide scale. It is also very important to systematically examine the formation and evolution mechanisms of different oxide phases during hot steel rolling. Given this point, Wu et al. [27] detailed the effect of water-based nanolubrication on the oxidation behaviour of steel through quantitative analyses of the oxide phases by the use of Raman microscope and SEM along with image processing software Image J. The schematic illustration of oxide scale formed on steel surface under no deformation (sampled from the tapered edge of steel workpiece), dry or water condition, and TiO 2 nanolubrication is presented in Figure 20. For non-deformed steels (see Figure 20a), a typical three-layered oxide scale was formed with a dominant inner layer of wustite, intermediate layer of magnetite, and top layer of hematite, as well as the magnetite seam at the scale/substrate interface and proeutectoid magnetite precipitated inside the wustite layer. For the steel under dry or water condition (see Figure 20b), considerable amounts of cracks and pores were generated, thereby causing fast conversion of magnetite to hematite near the scale surface and wustite to magnetite close to the scale/substrate interface. When TiO 2 water-based nanolubricants were used (see Figure 20c), TiO 2 NPs not only reduced the friction and the rolling force due to rolling effect, but also enabled the formation of protective film which was a barrier to inhibit oxygen diffusion. These effects decreased the extent of oxide deformation and therefore the porosity and cracks in oxide scale. As a result, the channels for oxygen penetration were reduced, which thus slowed down the conversions of wustite to magnetite and magnetite to hematite.

Microstructure of Rolled Steel
It has been widely accepted that the steel's microstructure largely determines its mechanical properties which can be greatly enhanced by grain refinement and phase transformation [283]. For a steel with specified chemical compositions, controls of deformation during rolling and cooling rate after rolling are the dominating strategies to attain desired microstructure and targeted mechanical properties [284]. During hot steel rolling, heavy For the steel experiencing hot rolling, in general, the conversions between different oxide phases follow the reactions below [27]: Equations (6) and (7) exist at a temperature above 570 • C, while Equation (8) exists during cooling process at temperatures below 570 • C. The formation of proeutectoid magnetite and magnetite seam is related to the eutectoid reaction in Equation (8) [281,282].

Microstructure of Rolled Steel
It has been widely accepted that the steel's microstructure largely determines its mechanical properties which can be greatly enhanced by grain refinement and phase transformation [283]. For a steel with specified chemical compositions, controls of deformation during rolling and cooling rate after rolling are the dominating strategies to attain desired microstructure and targeted mechanical properties [284]. During hot steel rolling, heavy reduction and accelerated cooling are two of the most effective ways to refine the grains in steels [285]. In recent years, some researchers have been committed to applying water-based nanolubricants during hot steel rolling, instead of conventional ways, to achieve grain refinement. Nanolubricants containing 0.1-0.5 wt.% SiO 2 , for instance, were used to refine the grain size in microstructure during hot rolling of ASTM 1045 steel within 5 passes from 1000 to 750 • C [100]. Nano-TiO 2 lubricants were also used to refine the grain size of rolled strips by around 50% after 5-pass hot rolling [52]. In particular, Wu et al. [28] conducted single-pass hot steel rolling at 850 • C using TiO 2 water-based nanolubricant, and found that the grain size of ferrite was refined up to 50.5% through statistical analysis of size distribution in the surface microstructure of rolled steels (see Figure 21). The possible factors that contribute to the decrease in grain size during hot steel rolling include the cooling rate of nanolubricant and the actual rolling deformation. On one hand, adding NPs into water increases the lubricant's thermal conductivity, and therefore the enhancement in cooling rate [185,261,286,287]. At an instant contact of work rolls with the workpiece, the lubricant in between acts as a coolant that offers a higher cooling rate than that of non-lubrication conditions. In this case, the increase in cooling rate leads to the increase in the number of nucleation sites for ferrite grains, and meanwhile a higher cooling rate enables the ferrite grains to pass through the high temperature region faster, thus shortening the time for ferrite growth [288]. Therefore, both the increased nucleation of ferrite and the retarded grain growth prompt the grain refinement due to enhanced cooling rate. On the other hand, the decrease in rolling force enabled by the use of nanolubricants gives rise to the decrease in exit thickness due to reduced spring-back value according to the Gaugemeter equation [289], where the exit thickness is the sum of the height of roll gap and the spring-back value. Given the increase in rolling deformation, the lattice distortion energy increases, and the dislocation density also increases, which respectively provides the driving force and nucleation sites for grain nucleation and therefore the grain refinement [290]. In some other cases, however, the grain size of surface microstructure would not change significantly after hot rolling with water-based nanolubricants [118,276], which might be ascribed to the slight differences in the deformation rate and the cooling rate of the lubricants being used. and the spring-back value. Given the increase in rolling deformation, the lattice distortion energy increases, and the dislocation density also increases, which respectively provides the driving force and nucleation sites for grain nucleation and therefore the grain refinement [290]. In some other cases, however, the grain size of surface microstructure would not change significantly after hot rolling with water-based nanolubricants [118,276], which might be ascribed to the slight differences in the deformation rate and the cooling rate of the lubricants being used.

Cold Rolling of Steels
In most cases, hot rolled steel strips experience subsequent cold rolling which is conducted at ambient temperature to obtain higher surface quality, special surface textures, or dimensional accuracy [291]. Due to the plastic deformation of workpiece in cold rolling, similar to hot rolling, friction exists between the work roll and the workpiece, and therefore the wear of work rolls is generated. To relieve the friction and wear occurred in the contact zone, rolling lubricants are also extensively applied in cold steel rolling. The requirements for cold rolling lubricants are a bit different from those for hot rolling ones in terms of rolling temperature and corrosive property of the lubricants. Nevertheless, the lubrication mechanisms, in general, are at least similar for both rolling processes.

Cold Rolling of Steels
In most cases, hot rolled steel strips experience subsequent cold rolling which is conducted at ambient temperature to obtain higher surface quality, special surface textures, or dimensional accuracy [291]. Due to the plastic deformation of workpiece in cold rolling, similar to hot rolling, friction exists between the work roll and the workpiece, and therefore the wear of work rolls is generated. To relieve the friction and wear occurred in the contact zone, rolling lubricants are also extensively applied in cold steel rolling. The requirements for cold rolling lubricants are a bit different from those for hot rolling ones in terms of rolling temperature and corrosive property of the lubricants. Nevertheless, the lubrication mechanisms, in general, are at least similar for both rolling processes.
It has been reported that the use of 0.7 wt.% nano-TiO 2 water-based rolling lubricant would largely decrease the surface scratches and adhesion defects on rolled steel surface [50]. However, the corresponding lubrication mechanisms were not well understood. Furthermore, 2D materials such as MoS 2 , MoO 3 , and graphene have also presented great potential in cold steel rolling due to their weak interlayer interaction and easy sliding nature between neighbouring atomic layers [292]. For example, water-based MoS 2 nanolubricant was applied in cold rolling of a low-carbon steel, and the results revealed that both the rolling force and the minimum rolling thickness obtained under base lubricant were decreased even using the recycled MoS 2 nanolubricant [63]. As nano-MoS 2 used as a lubricant additive would be readily oxidised and then converted to nano-MoO 3 under cold rolling conditions, they also applied MoO 3 water-based lubricant in a four-high cold rolling mill [56]. The rolling force and power were found to be significantly reduced in each rolling pass, and the minimum rolling thickness was apparently thinned, owing to a lubricating film composed of MoO 3 , MoO 2 , and Fe(OH) 3 formed on strip surface. Graphene, as another promising nanoadditive in water, has also been extensively studied over the past five years [67,132]. As a matter of fact, graphene oxide (GO) instead of graphene has been more commonly used in water due to the presence of hydrophilic groups such as hydroxyl, carboxyl, and epoxy which relieve the agglomeration caused by π-π stacking interaction [142,145]. However, the pristine GO suspension without pH modification is acidic, which is detrimental to the strip quality [17]. In light of this, Meng et al. [76] used tri-ethanolamine (TEA) to prepare alkaline GO aqueous lubricants that were feasibly applied in cold steel rolling. The results demonstrated that the use of alkaline lubricant with pH 9 resulted in the best lubrication performance by distinctly reducing the rolling force in each pass, and the minimum rolling thickness after 7-pass rolling was 21.36% lower than that without using lubricant. The lubrication mechanisms were proposed as the combination of mending effect and formation of physically adsorbed layers.
Given the excellent tribological characteristics of 0D and 2D materials as individual additives in water, GO-based composites are attracting considerable research interest in the area of water-based nanolubrication, which exhibits improved tribological properties by combining two or more components through a synergistic effect [12,[19][20][21]. In light of this, Du et al. [84] conducted cold steel rolling by means of 0.5 wt.% GO-TiO 2 nanolubricants in comparison with 0.5 wt.% GO and 0.5 wt.% TiO 2 nanolubricants. The results indicated that the hybrid nanolubricant exhibited better tribological performance than the lubricant with individual nanoadditive. Beyond that, the strip rolled with GO-TiO 2 nanolubricant had the minimum rolling thickness and surface roughness, and meanwhile the rolling force was the lowest among all the lubricants being used. The exceptional lubricating properties of GO-TiO 2 nanocomposite were caused by the excellent dispersion stability and the formation of absorption films, carbonaceous protective films and transfer films.

Cold Rolling of Non-Ferrous Metals
Magnesium (Mg) alloys, owing to their exceptional physicochemical properties such as low density, high strength/weight ratio, good damping performance, superb biocompatibility, excellent machinability, and castability, have been widely used in many engineering applications including transportation, electronics, aerospace, biomedical, and energy sectors [293]. During the forming of Mg alloys, generating severe friction and wear between the tool and the workpiece is inevitable, which may result in inhomogeneous deformations, shortened tool life, and poor surface quality of workpiece [294]. To overcome these disadvantages, the use of suitable lubricants is essential in the forming process. Although some eco-friendly lubricant additives, for instance, N-containing compounds [295], borates [296], and ionic liquids [297] have been developed for forming Mg alloys, they still raise some issues such as poor extreme pressure property, complex synthesis process, and high material cost. To date, there have been few choices of lubricants for the forming process of Mg alloys, while water-based nanolubricants are becoming increasingly popular due to their minimum impact on the environment, low cost, desirable recyclability, and exceptional lubrication performance. For example, graphene and graphene oxide (GO) have been adopted as water-based lubricant additives for steel-Mg alloy contacts through ball-on-plate tribo-testing, and 0.5 wt.% GO lubricant has been proposed for the forming of Mg alloys due to its prominent tribological performance in the reduction of COF and wear rate of Mg alloy plate [77]. It has been always anticipated that hybrid nanolubricants provide possibilities to further improve the tribological performance of the lubricant with individual nanoadditive through a synergistic effect of two or more components. Based on this, SiO 2 /graphene composite was dispersed in water to evaluate their tribological performance for Mg alloy rolling [112]. It was found that the combinations of 0.1 wt.% nano-SiO 2 and 0.4 wt.% graphene displayed the best friction-reduction and anti-wear properties among all the SiO 2 /graphene mixing ratios. During the cold rolling of Mg alloy under a single-pass reduction of 10%, the rolling force and rolled surface roughness obtained with dry rolling were decreased by up to 12% and 42.4%, respectively, when using the nanolubricant with the optimal SiO 2 /graphene combinations. It has also been documented that SiO 2 /nanotube hybrid water-based nanolubricant with a mass ratio of 0.3:0.2 can be used for Mg alloy forming according to its excellent tribological properties and load-carrying capacity as well as formation of stable lubricating film on the surface of Mg alloy [113]. These research findings have provided new insights into the potential of applying carbon-based aqueous nanolubricants in Mg alloy forming.

Conclusions and Outlook
In this paper, we provide a comprehensive overview of recent advances in waterbased nanolubricants with particular emphasis on the preparation of different types of nanolubricants, the methods to evaluate their dispersion stability and tribological properties, the discussion on lubrication mechanisms, and their application in metal rolling. Among all the candidate nanoadditives used in water, carbon-based nanomaterials have attracted the most attention due to their exceptional physicochemical properties and unique chemical structures. Nanocomposites with carbon-based nanomaterials as constituent materials have shown better performance and greater potential than individual component in water-based nanolubricants. Although some water-based nanolubricants have presented remarkable performance in metal rolling, great efforts are still needed to well understand the behaviours of the nanoadditives and reduce their material and preparation costs for accelerating their applications on an industrial scale. Some research directions that deserve further exploration are provided as follows.

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Ensuring long-term dispersion stability of nanoadditives in water is still a big challenge. The interaction among different lubricant components needs to be investigated for the perfection of the theories of dispersion stability.

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For the application in metal rolling, the formulation of water-based nanolubricants needs to be optimised to further enhance their physicochemical properties in terms of dispersion stability, wettability, and extreme pressure property. Special attention should be given to the strategies for reducing material and preparation costs of the applied nanolubricants.

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The application of water-based nanolubricants in hot steel rolling has exhibited positive effects on the decreases in rolling force, rolled surface roughness, and oxide scale thickness, and also enabled refined grains in microstructure. However, the lubrication effects on controls of profile, flatness, and texture have been rarely involved. More studies are also needed to examine the grain refinement mechanism and attain maximally refined grains, which is a promising and economical technique to significantly promote the overall properties of hot rolled steels.

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For the case of application of cold steel rolling, it is of vital importance to have more focus on the study of the corrosive property of applied water-based nanolubricants. In addition to the lubrication effects on rolling force and surface quality, extra attention should be paid to those on rolling texture and shape control. • Although certain water-based nanolubrication mechanisms in rolling of steels have been proposed through analysis of post-rolling specimen by means of electron microscopy, in situ observation of NPs and demonstration of their motion behaviour have not been specifically conducted. To have a systematic and comprehensive understanding of the lubrication mechanisms, varying rolling parameters such as rolling temperature, rolling reduction, and speed should be employed, and corresponding multi-scale numerical simulation can be carried out.

•
As pointed out earlier, work roll service life can be prolonged using water-based nanolubricants, which largely reduces the roll changing frequency and thus enhances the productivity of rolling mill. However, no research has been conducted to quantitatively evaluate the wear of work rolls under water-based nanolubrication conditions. • The use of green lubricant is becoming mainstream in sustainable manufacturing. It is of vital importance to develop a cost-effective recycling technology for waste water-based nanolubricants.