The Effect of Addition of Nanoparticles, Especially ZrO2-Based, on Tribological Behavior of Lubricants

The aim of the paper was to discuss different effects, such as, among others, agglomeration of selected nanoparticles, particularly those from zirconia, on the tribological behavior of lubricants. The explanation of the difference between the concepts of ‘aggregation’ and ‘agglomeration’ for ZrO2 nanoparticles is included. The factors that influence such an agglomeration are considered. Classification and thickeners of grease, the role of additives therein, and characteristics of the lithium grease with and without ZrO2 additive are discussed in the paper. The role of nanoparticles, including those from ZrO2 utilized as additives to lubricants, particularly to the lithium grease, is also discussed. The methods of preparation of ZrO2 nanoparticles are described in the paper. The agglomeration of ZrO2 nanoparticles and methods to prevent it and the lubrication mechanism of the lithium nanogrease and its tribological evaluation are also discussed. Sample preparation and a ball-on disc tester for investigating of spinning friction are described. The effect of ZrO2 nanoparticles agglomeration on the frictional properties of the lithium grease is shown. The addition of 1 wt.% ZrO2 nanoparticles to pure lithium grease can decrease the friction coefficient to 50%. On the other hand, the agglomeration of ZrO2 nanoparticles in the lithium grease can increase twice the friction coefficient relative to that for the pure grease.


Introduction
Grease is a largely used semi-solid lubricant composed of base oil, thickeners and, optionally, of improvers. It is often used in roller bearing systems when the supply of the indispensable oil through the central lubrication system is difficult or even impossible, or when the lubricant must remain in a closed space [1]. The presence of grease supports the sealing process of bearings, as it is resistant to leakage, prevents to some extent the occurrence of corrosion of mating surfaces, and requires uncomplicated maintenance. The grease has a complex microstructure that is subjected to degradation during prolonged use. Grease is a thixotropic body, whose properties change over time under the influence of shear stresses [2], in a partially reversible manner [3]. Paszkowski [4] investigated lithium grease using a rotary rheometer. He found that both the thickener fraction and shear rate significantly affected reconstruction of the lithium thickener microstructure after friction.
Greases, especially the lithium ones, are largely used for lubrication of rolling bearings and mobile connections in automotive and other industries. Such friction associations can be highly loaded, however, under the condition that they operate under not very high values of slip speed and temperature. This is mainly due to overwhelming necessity of cost reduction applied to proven structures and materials or practices related to their maintenance, such as selection of proper lubricants. This approach requires changing composition of grease used so far, adapting its viscosity and stability to given operating conditions. One of the possible solutions to the problem is to disperse some amount of metal or non-metal nanoparticles in the base grease. It turned out that a barrier for wide application of this solution was occurrence of agglomerations of the dispersed nanoparticles. The term 'agglomeration' is frequently interchangeably called 'aggregation' and has led to confusion based on the phenomenological ground [26].
Aggregation and agglomeration are two different concepts used to describe a given set of particles or nanoparticles in a material. Both concepts have different meanings. Clustering of particles by aggregation is a reversible process, while clustering of particles through agglomeration is an irreversible process [26]. Both processes are difficult to distinguish using conventional techniques such as dynamic light scattering (DLS) [27], nanoparticle tracking analysis (NTA) [28], or electron microscopy imaging (EMI) [24]. Such techniques reveal large aggregates of particles in the investigated material only in general. Aggregations and agglomerations of particles in a material sample were created due to different mechanisms. One of these mechanisms consists of collisions and gluing of particles as a result of their random Brownian motion (Brownian agglomerations). Another mechanism called 'gravitational agglomeration' depends strictly on the particles' size and their final velocities. Slowly settling particles are captured by faster settling ones, which results in the formation of clusters of particles. Particle clustering affects the degree of acidity pH and ionic bonding strength. Therefore, methods for proper determination of the degree of aggregation and agglomeration of nanoparticles in a material sample are very important.
Smoluchowski [38] first approached the colloid aggregation and agglomeration with a mathematical particle size-dependent model. That so-called Diffusion-Limited Colloid Agglomeration (DLCA) model described the interparticle collision rate driven by Brownian motion. According to Mersmann [39], particle agglomeration induced by Brownian motion is known as 'perikinetic' one, whereas agglomeration caused by hydrodynamic forces is called 'orthokinetic'.
Zhang et al. [40] described the Reaction-Limited Colloid (perikinetic) Agglomeration (RLCA) model, where agglomeration was limited by the binding reaction between particles. This limitation is described by the attachment efficiency (α) corresponding to the probability of interparticle adherence after collision. According to Reference [41], the abovementioned limitation can be described with the Fuchs stability ratio (W), referring to the colloidal stability (the inverse of the attachment efficiency W = 1/α).
Hotze et al. [42] presented the Derjaguin-Landau-Verwey-Overbeek theory describing nanoparticle agglomeration in aqueous media. The theory allowed the formulation of the attachment efficiency and Fuchs ratio. Such factors representing the difference between DLCA and RLCA resulted from the colloid integration reaction or stabilizing mechanisms hindering interparticle adherence.
Stolzenburg et al. [43] studied the secondary particle formation processes during the nonaqueous synthesis of ZrO 2 and TiO 2 nanoparticles and described mechanisms driving aggregation and agglomeration ( Figure 1). They found that densely packed aggregates composed of primary nanoparticles were formed at the beginning, and next agglomerated to so-called 'superstructures'. The development of particle size over the course of a standard ZrO2 and TiO2 nanoparticle synthesis is shown in Figure 2. The development shows an almost exponential trend.   Rizzuti et al. [44] studied a hydrothermal zirconia nanoparticle synthesis and observed spherical aggregate structures composed of 8 nm-sized particles. These structures further agglomerated to superstructures.
Rodriguez-Devecchis et al. [45] reported that the nanoparticles added to oil lubricant tended to agglomeration.
Jazaa et al. [46] found that once agglomeration occurs, the particle size changes. In order to better estimate the nominal particle size in the solution, the size distribution of the dispersed nanoparticles in PAO should be measured with the use of dynamic light scattering (DLS) technique. The theory Figure 1. Particle formation mechanisms during the nonaqueous synthesis with an emphasis on the aggregation and agglomeration processes [43]. Reprinted with permission from ( The development of particle size over the course of a standard ZrO 2 and TiO 2 nanoparticle synthesis is shown in Figure 2. The development shows an almost exponential trend.

Figure 1.
Particle formation mechanisms during the nonaqueous synthesis with an emphasis on the aggregation and agglomeration processes [43]. Reprinted with permission from ( The development of particle size over the course of a standard ZrO2 and TiO2 nanoparticle synthesis is shown in Figure 2. The development shows an almost exponential trend.   Rizzuti et al. [44] studied a hydrothermal zirconia nanoparticle synthesis and observed spherical aggregate structures composed of 8 nm-sized particles. These structures further agglomerated to superstructures.
Rodriguez-Devecchis et al. [45] reported that the nanoparticles added to oil lubricant tended to agglomeration.
Jazaa et al. [46] found that once agglomeration occurs, the particle size changes. In order to better estimate the nominal particle size in the solution, the size distribution of the dispersed nanoparticles in PAO should be measured with the use of dynamic light scattering (DLS) technique. The theory   Rizzuti et al. [44] studied a hydrothermal zirconia nanoparticle synthesis and observed spherical aggregate structures composed of 8 nm-sized particles. These structures further agglomerated to superstructures.
Rodriguez-Devecchis et al. [45] reported that the nanoparticles added to oil lubricant tended to agglomeration. Jazaa et al. [46] found that once agglomeration occurs, the particle size changes. In order to better estimate the nominal particle size in the solution, the size distribution of the dispersed nanoparticles in PAO should be measured with the use of dynamic light scattering (DLS) technique. The theory underlying the measurements with this method is based on the effects of Brownian motion of nanoparticles on Rayleigh light scattering data. This allows determination of the size of a particle or molecule in the solution [47]. Jazaa et al. [46] explained that the primary result obtained from a DLS measurement represented the intensity-weighted mean diameter derived from the cumulations. Such a diameter was very sensitive to the existence of nanoparticle agglomeration due to the inherent intensity weighting.

Factors Affecting the Agglomeration
According to Reference [48], the agglomeration of the nanoparticles is important for lubricants as it affects the sedimentation rate and there is a loss of wear and ability to decrease the coefficient of friction (COF). In the case of poor dispersion stability, sedimentation and clogging may occur. The stable suspension in which particles do not form agglomerates at a significant rate allows an effective nanolubrication.
The agglomeration of nanoparticles can be undesirable not only during the preparation of nanolubricants but also during sintering. According to References [49,50], parameters of granulometry, such as shape and size of particles and their agglomerates or aggregates, both chemical and phase composition of initial powders, and the agglomerate or aggregate type affect compaction level and sintering process. The so-called 'soft' agglomerates are characterized by weak van der Waals forces between particles that can be easily separated in a liquid medium by ultrasonic treatment or/and addition of dispersants. The so-called 'hard' agglomerates are characterized by strong forces between particles due to high-temperature calcinations or inappropriate chemical additions.
According to References [51,52], the agglomeration kinetics of TiO 2 nanoparticles synthesized by aqueous sol-gel processes highly depended on pH level, temperature, and ionic bond strength.
Shih et al. [53] studied the stability of TiO 2 nanoparticle suspensions and found that the system destabilized and nanoparticles began to agglomerate at a certain salt concentration (at so-called 'critical coagulation concentration').
Padovini et al. [54] analyzed a hydrothermal zirconia nanoparticle synthesis and found that ZrO 2 nanoparticles directly formed amorphous microspheres before the primary particles crystallized within these structures.
Focusing on the effect of NH 4 OH water solution concentration, the authors of Reference [55] studied the effect of coprecipitation conditions during the synthesis of ultrafine yttria-stabilized zirconia (YSZ) nanoparticles and gadolinium-doped YSZ. The use of 1 M or 5 M of NH 4 OH led to a similar grain size of less than 6 nm. However, greater concentration led to a much greater fraction of agglomerated nanoparticles. This was due to an increased ionic bond force between particles caused by the high-temperature calcination or inappropriate chemical additions.
Some papers showed a clear relationship between tribological properties of the lubricant containing nanoparticles and their agglomerations.
During studies on graphite nanoparticles dispersed in palm oil [56] the COF value of nanolubricants increased after using the magnetic stirrer and the overhead stirrer. It was due to the agglomeration of nanoparticles that adversely affected the friction performance.
Moshkovith et al. [57] found that decreasing the agglomeration size increased the frequency of the nanoparticle penetrations into the contact area, thus reducing the COF value. The size of agglomerates of the nanoparticles decreases with the long-term mixing process.
Xie et al. [58] reported that poor dispersion stability increased the tendency of nanoparticles to agglomerate and, in turn, to a lesser friction performance.
According to References [56,59], the agglomeration of nanoparticles was conducive to a greater wear rate. The tribological performance of nanolubricants depended on the dispersion stability.
According to Reference [60], the ZrO 2 nanoparticles make an easier sintering process. The ordered agglomeration of zirconia primary crystallites into secondary particle assemblies ensures their homogeneous packing. However, size reduction of ZrO 2 nanoparticles also promotes undesirable disordered agglomeration. The assembled crystallites obtained during ordered and disordered agglomeration exposed to intense electromagnetic radiation can agglomerate even further.
The preliminary analysis of the nanopowder market, particularly in Poland, has shown that zirconia nanopowders are available on the market and are relatively cheap. The number of available publications on the use of ZrO 2 nanoparticles as an additive to lubricants is not great; nevertheless, the results of preliminary studies carried out by Wozniak et al. [61] pointed such nanoparticles as a candidate improving the tribological properties of some lubricants. It has been noted that the problem of agglomeration may also occur during preparation of nanolubricants with zirconia nanoparticles.

Classification and Thickeners of Greases
There are several classifications of greases. According to Reference [62], greases are classified according to their thickener composition, and on their consistency, according to the NLGI (National Lubricating Grease Institute) classification. This classification is characterized by 9 grades, depending on the so-called Worked Penetration Range. There are two major families of greases containing soap and non-soap thickeners. More than 90% of the thickeners used worldwide are soap-based.
There are three types of soap-based thickeners [62]: • Simple soap-obtained from the reaction of one fatty acid, such as 12-hydroxystearic acid (12 HSA), and a metallic hydroxide, such as lithium hydroxide. The metallic hydroxide defines the thickener type. In the described case, the grease is called 'simple lithium soap'.

•
Mixed soap-obtained from the reaction of a fatty acid with two metallic hydroxides. For example, if the 12 HSA reacted with lithium and calcium hydroxide, it resulted in a mixed Ca/Li soap.

•
Complex soap-obtained from the reaction of a fatty acid, such as 12 HSA, with a short chain complexing acid, such azelaic one, conducive to a complex soap. If lithium hydroxide was used, the result was a lithium complex grease. This thickener type has much better high-temperature properties than a simple soap one.
Complex lithium soaps and simple lithium soaps are the most widely used [63] (their share in the world market exceeds 60% [64]).

Characteristics of Lithium Grease
A very important feature of the lithium grease is its load-carrying capacity. The lithium grease with the CaF 2 addition has the load-carrying capacity up to 48% greater than the base lithium one [66].
Zhao et al. [67] reported that addition of nano-calcium borate (NCB) to the lithium grease resulted in a greater load-carrying capacity as compared to the simple lithium one. The pure lithium grease did not provide effective lubrication under a load exceeding 300 N. Contrary to that, the grease with 6 wt.% of NCB exhibited good performance under the load up to 600 N.
It turned out as well during the study of the lithium grease with addition of CeF 3 nanoclusters surface capped with oleic acid (OA) that its load-bearing capacity was higher than that for the simple lithium one. The OA-CeF 3 nanocluster as the additive improved the resistance to wear and to extreme pressure of the synthetic lithium grease. However, the OA-CeF 3 nanoclusters had only a little effect on the friction-reducing behavior of the lithium grease [68].
It was shown in another investigation of lithium grease with CaCO 3 nanoparticles [69] that the load-carrying capacity was greater than that of the simple lithium one. Moreover, Prakash et al. [70] reported that addition of CaCO 3 nanoparticles was conducive to lower wear and friction, greater load-carrying capacity, and resistance to extreme pressures in comparison to those of simple lithium grease.
Liu et al. [71] studied tribological behavior of amorphous overbased calcium sulfonate (AOBCS) and crystalline overbased calcium sulfonate (COBCS, transformed from the AOBCS) added to lithium complex grease. Addition of COBCS to the grease was conducive to lower friction and wear and greater load-carrying capacity in comparison to simple lithium grease. Addition of AOBCS particles improved tribological properties of complex lithium grease to a greater extent than the COBCS ones at similar concentration and load in contact zone.
It follows from the results mentioned above that rheology of the lithium grease is affected by use of nano-additives [72]. The rheological properties of the lithium grease were investigated in either static conditions or under steady-state (dynamic) conditions.
Addition of nano-calcium borate [67] to the lithium grease caused a slight decrease of the unworked penetration (measured according to ASTM D217 and D1403 standards).
Mohamed et al. [72] studied the rheological behavior of lithium grease with addition of carbon nanotubes. As a result, its shear stress and apparent viscosity were increased up to 67% and 82%, respectively.

Additives to Greases
According to Reference [73], lithium-complex greases are usually stable and resistant to water and high temperature. Appropriately selected additives can also improve such characteristics as extreme pressure, anti-wear, rust, and corrosion. These greases are also in agreement with the NLGI's GC-LB specification requirements (where GC-wheel bearing greases performance class chosen from existing GA, GB, and GC ones, and LB-chassis greases performance class chosen from existing LA and LB ones). The lithium-complex greases need a significant amount of antimony-zinc or other types of additives to have good resistance to extreme pressure and to wear properties. In addition, lithium-complex greases should be accomplished with rust-inhibiting additives. To improve impermeability, lithium-complex greases usually require tackifiers, which are prone to deplete quickly in the presence of water.
According to Reference [74], the conventional additives to greases are, among others, antioxidants, anti-wear agents, corrosion inhibitors, extreme pressure agents, friction modifiers, and metal deactivators.
The authors of the paper [75] found that additives enhanced performance of grease and protected the grease and lubricated surfaces when it was composed of mineral oil blended with a soap thickener.
According to Reference [76], greases cannot satisfy the requirements of high-performance lubricants without using the benefit of modern additives, such as corrosion inhibitors, as well as those increasing resistance to wear (so-called antiwear agents) and to extreme pressure (so-called extreme pressure agents).
According to Reference [78], basic calcium sulfonate, neutral calcium sulfonate, and alkoxylated amine particles can be used as rust inhibitors in lithium greases.
According to Reference [79], the additives resistant to wear, also called antiwear (AW) ones, can be based on various P-, N-, and S-based chemical compounds. Dinonyldiphenylamines, octyl/butyldiphenylamines, 2,6 ditertbutylphenol derivatives, or methylene-bisdibutyldithiocarbamates can be utilized as the primary antioxidants in lithium and calcium greases. The Li-, Ca-, Ba-, Mg-, or Zn-based dinonylnaphthalenesulfonates can be utilized as the rust and oxidation inhibitors in such greases. Heterocyclic derivatives and also S-and N-based chemical compounds can be used as the yellow metal deactivators in the calcium and lithium greases. Amino acid derivatives with or without amines can be utilized as the rust inhibitors and they are synergistic with the phosphate esters, sulfurized olefins, triphenyl phosphorothionates (TPPT), and ashless dialkyldithiocarbamates used as the extremal pressure/antiwear (EP/AW) additives. The organic acid/ester compounds and zinc-based ones like zinc naphthenates can be used as rust inhibitors. Some of them can enhance the properties of EP/AW additives in the calcium and lithium greases. Furthermore, fatty acid esters, sulfurized fatty acid esters/triglycerides, and sulfurized α-olefins/triglycerides can be the EP/AW additives used in calcium and lithium greases.
Greases composed of mineral oil blended with soap thickener and additives have enhanced performance and protect well-lubricated surfaces [72].
Singh [80] showed that the blends of bis (1,5-diaryl-2,4-dithiomalonamido) dioxomolybdenum complexes with base lithium grease are greatly appreciated due to their resistance to extreme pressures. The greases reinforced with additives prevent rusting and corrosion of bearings and have better oxidation protection as compared to the greases without any additive.
Samy and Ali [81] studied contact of steel samples lubricated with lithium grease, contaminated with white cement grains and with addition of different tin content. The tests were carried out using a cross pin wear tester. The effect of the main components of white cement, such as lithium sand, kaolin, and limestone, on friction coefficient and wear of the tested samples was examined. It was found that tin as a solid lubricant dispersed in lithium grease reduces the abrasive wear of mating surfaces. Formation by the tin of a protective tribofilm on surfaces of steel samples and surfaces of particles of contaminants allowed to reduce the rate of the abrasive wear in the contact zone.
Friction coefficient was relatively greater when the lithium grease was contaminated with sand particles compared to that observed for the one contaminated with kaolin and limestone particles. The addition of sand particles was conducive to the greatest wear rates. The beneficial effect of tin was more pronounced at 5 wt.% content of sand particles. Addition of tin reduced the COF value and promoted the sliding of sand particles instead of abrasion. The decrease of the abrasive wear after addition of tin was more pronounced with increasing tin content up to 3 wt.%.

The role of Nanoparticles as Additive to the Oil Lubricant
According to Reference [82], lubricating oil containing nanoparticles allows: • micro-ball bearing-like operation between the mating surfaces; • generation of protective film averting friction and smoothing the rough surface; • repairing lubricated surface through fulfilling the wear tracks or apertures caused by loss of mass; • polishing of mating surfaces via nanoparticles-assisted abrasion.
Hernández et al. [83] studied the resistance to wear of nanoparticles' suspensions in a polyalphaolefin (PAO 6) using a block-on-ring tribotester. ZrO 2 nanoparticles were separately dispersed at 0.5, 1.0, and 2.0 wt.% in PAO 6 using an ultrasonic bath for 2 min. As a result, friction and wear were reduced for all nanoparticle suspensions compared to the corresponding values registered for the base oil. The best result was obtained at 0.5 wt.% of ZrO 2 . Neither agglomerations nor uniform depositions of ZrO 2 nanoparticles were found on the worn surfaces.
Battez et al. [84] studied the resistance to extreme pressure of CuO, ZnO, and ZrO 2 nanoparticles suspended in a polyalphaolefin (PAO 6) at amounts of 0.5, 1.0, and 2.0 wt.%. The dispersion of nanoparticles was obtained through an ultrasonic bath for 2 min. The resistance to extreme pressure (EP) was investigated with a four-ball (steel) EP lubricant tester. For all different contents of nanoparticles in PAO 6, the EP properties were better relative to the simple PAO 6. The resistance to EP was the best for CuO nanoparticles and the worst for ZrO 2 ones. The resistance to EP of ZrO 2 nanoparticles was same regardless of their content in PAO 6.
The authors of the paper [85] modified the surface of ZrO 2 nanoparticles with silane coupling agent KH-560 and dispersed them in 20# machine oil. They studied the tribological properties of such nano-lubricant with a four-ball tribotester and with a thrust-ring one. Addition of ZrO 2 nanoparticles to 20# machine oil was conducive to lack of wear and to COF value reduced up to 27% in comparison to the simple oil. The best result was obtained for the 0.5 wt.% of ZrO 2 nanoparticles.
Casado et al. [86] elaborated a procedure to chemically cap the surface of ZrO 2 nanoparticles (ZrO 2 NPs) with long hydrocarbon chains (saturated C-8, C-10, and C-16) in order to obtain stable dispersions of ZrO 2 NPs in lubricating oils without reducing their attributes as lubricant additives. During tribological tests with such nanolubricants, they found that dispersions of the long-chain capped ZrO 2 NPs in base oils were conducive to low COF and wear rate values of base oils in comparison to those for raw lubricating oils.
Kumar et al. [87] studied tribological behavior of TiO 2 nanoparticles suspended in servo system lubricating oil through the sonication without adding any surfactant. They found that with an increase of volume concentrations (VC) of the nanoparticles, calorific value and flashpoint of the nano-lubricant decreased, whereas its viscosity remained unchanged. The wear rate and COF values increased due to agglomeration of TiO 2 nanoparticles caused by the lack of any surfactant during the preparation of the nano-lubricant.
Using the ball-on-disc tribotester, Yu et al. [88] studied the tribological properties of the Castor oil with different concentrations of MoS 2 nanoparticles. Addition of MoS 2 nanoparticles to the oil was conducive to low COF and adhesive wear rate values. However, MoS 2 nanoparticles in excessive concentration could agglomerate into large particles reducing the beneficial effects mentioned above.
Jendrzej et al. [89] dispersed in engine oil gold nanoparticles synthesized by pulsed laser ablation. After nine months, almost no agglomeration occurred. It was due to the attachment of engine oil additives or pyrolyzed/oxidized molecules to the nanoparticles limiting attractive interactions between nanoparticles.
Using a disc-on-disc tribotester, Hwang et al. [90] studied tribological properties of mineral oil with dispersed graphite and carbon black nanoparticles, graphite nanofibers, and carbon nanotubes. The nano-sized spherical particles suspended in mineral oil prevented direct contact between mating surfaces and significantly reduced the friction coefficient. Contrary to that, the fibrous nanoparticles with high aspect ratios deteriorated the lubrication between mating surfaces due to a higher degree of agglomeration.
Joly-Pottuz et al. [91] studied the effect of IF-WS 2 nanoparticles (IF-WS 2 NPs, where IF means Inorganic Fullerene) concentration in PAO base oil at room temperature under the boundary lubrication regime. They found that the increase in the nanoparticles' concentration was conducive to low COF value compared to that for simple PAO oil even at very low concentration of IF-WS 2 NPs as 0.1 wt.%. Above 1 wt.% of the NPs the COF value decreases. The same behavior was observed for IF-MoS 2 nanoparticles dispersed in base oil [92].
The authors of Reference [93] studied the effect of temperature on tribological performance of PAO blended with 1 wt.% of IF-MoS 2 nanoparticles at 20 • C and at 70 • C. They found a high and unstable COF value at 70 • C, whereas it was lower and more stable at 20 • C. It was due to an increased tendency of particles to agglomeration at a higher temperature.
Hu et al. [94] investigated the tribological behavior of hollow MoS 2 nanoparticles dispersed in liquid paraffin. At 1.5 wt.% of the nanoparticles, the COF and wear rate value were the lowest.
At higher concentrations, the probability for interparticle collisions was greater, which was conducive to an increased agglomeration of NPs.
Results of other tribological tests [95,96] showed that the Re-doped IF-MoS 2 nanoparticles were conducive to reduced agglomeration and produced stable suspensions in PAO base oil. Addition of undoped IF and 2H NPs was conducive to decrease of the COF value compared to that for the simple PAO oil. The Re-doped IF-MoS 2 NPs showed a two-fold lower value. It was due to improved dispersion of the Re-doped NPs and to increased conductivity of the tribofilm formed by those NPs, which inhibited tribocharging at the surface.
Aralihalli et al. [97] studied the tribological behavior of MoS 2 nanoparticles added to base oil with different surfactants, such as Aminopropyl trimethoxy silane (ATS), Sorbital monooleate (SPAN 80), Octadecyltrimethoxysilane (OTS), Perfluorodecyltriethoxysilane (PTS), and Perfluorooctyltrichlorosilane (PTCS). The lowest COF values were obtained for the mixtures containing ATS and SPAN. The particle size of MoS 2 NPs was smaller for these additives in comparison to the others. The molecules of the ATS and SPAN remained strongly grafted to the particles preventing agglomeration even under shear stresses, in contrast to other additives. The amine group bound covalently the molecules of ATS to MoS 2 while the hydrogen of the -OH groups combined with S atoms of MoS 2 .

The Role of Nanoparticles as Additives in the Grease
Various aspects of nanoparticles used as additives and creating the so-called nanogrease were discussed in the review [98]. Nanogreases improve tribological properties, increase bearing capacity, and reduce wear, as compared to base oil grease. The enhanced performance of the nanogreases is due (among others) to their increased surface area to volume ratio.
Zhornik et al. [99] described the mechanism underlying the structure formation of the dispersed phase of greases when using the chemical principle of modification of complex calcium sulfonate grease by addition of nanocalcite particles.
Mohamed et al. [100] prepared calcium grease with different ratios of TiO 2 particles to carbon nanotubes TiO 2 /CNTs (0.5, 1, 2, 3, 4 wt.%). They studied the tribological and rheological characteristics of nanogreases at different temperatures. Addition of TiO 2 /CNTs at an optimal ratio of 3 wt.% was conducive to lower wear rate and COF values by 73% and 60%, respectively, compared to those for simple grease. The apparent viscosity and shear stress were increased by 48% and 74%, respectively.
Using a pin-on-disc tribotester, Mobasher et al. [101] studied tribological behavior of calcium grease with different concentration of multiwalled carbon nanotubes (MWCNTs) and Talc powder (at 1, 2, 3, 3 and 5 wt.%). The ratio of MWCNTs to Talc powder nanoparticles was equal to 1:1. Addition of 4 wt.% of MWCNTs/Talc NPs to calcium was conducive to lower wear rate and COF values by 81% and 64%, respectively, in comparison to simple grease. The obtained boundary film composed of MWCNTs, Talc powder nanoparticles, and other inorganic nanoparticle compounds was formed on the worn surface during friction tests. The thermal conductivity of nanogrease increased by 52%.
Younes et al. [102] obtained a stable and homogeneous nanogrease with PAO as base oil and carbon nanotubes as sole thickener. They found that carbon nanofiber (CNF), graphene, fullerene (C60), and Ni-coated single-wall carbon nanotube (SWNT) could not form stable greases as nanotubes did. The CNF/SWNT mixture did not form a stable grease, while Ni-coated SWNT/SWNT mixture did, due to the effect of intermolecular van der Waals forces. They stated that appropriate tube size (of nanotube and nanofiber), stereo structure (nanotube, graphene, and fullerene), and surface energy (nanotube and Ni-coated nanotube) were critical factors determining if van der Waals forces could have an effect or not.
Hu et al. [103] carried out comparative studies on tribological behavior among micro-MoS 2 (m-MoS 2 ), platelet-like nano-MoS 2 , and spherical nano-MoS 2 particles added to lithium grease, lithium-calcium one and polyurea one, respectively. The m-MoS 2 slightly improved the lubrication properties of all studied greases. The nano-MoS 2 samples gave better results relative to the micro-MoS 2 particles. Tribofilms produced by nano-MoS 2 contained MoS 2 and MoO 3 . Tribofilms produced by m-MoS 2 contained no Mo or only the MoO 3 form. The lack of MoS 2 form in tribofilms was conducive to poor lubrication of m-MoS 2 greases. The improved lubrication of the spherical nano-MoS2 greases as compared with platelet-like nano-MoS 2 ones resulted from the lubricating superiority of the spherical structure in comparison to the platelet-like one.

Nanoparticles as Additive to Lithium Grease
Recently, some studies on the effect of different nanoparticles addition to the lithium grease on its tribological behavior were reported. SiO 2 , ZrO 2 , and TiO 2 nanoparticles were used.
He et al. [104] tested the lithium grease without and with an amount of SiO 2 nanoparticles using a four-ball tribotester. For the grease with 0.3 wt.% of SiO 2 nanoparticles, the friction coefficient was lowered by 26%-39% as compared to the case of the base grease, depending on loads in the contact zone.
Zhao et al. [67] prepared NCB addition with an average particle size of about 70 nm. They studied the friction and wear behavior of the NCB as additive to lithium grease using oscillating friction and wear tester. They found that the addition of NCB to the grease significantly improved its resistance to wear and the load-carrying capacity and, simultaneously, decreased its friction coefficient. The good tribological performance of NCB is affected by formation of a boundary film composed of deposited NCB and different products of tribochemical reactions, such as B 2 O 3 , CaO, and iron oxides on the rubbing surface.
The authors of the paper [105] studied the effect of addition of CNTs on tribological properties of the lithium grease. The CNTs of 10 nm average diameter and 5 µm in length were synthesized by electric arc discharge.
Xia et al. [106] studied changes of tribological properties of the lithium grease containing ZrO 2 nanoparticles as an additive. The friction coefficient decreased by over 30% after addition of 0.25% and 0.5% mass fractions of the ZrO 2 nanoparticles and the width of the wear scar decreased by 49% and 46%, respectively.
The authors of the paper [107] used eco-friendly carbon spheres (CS) of the size from 600 nm to 1 µm as an additive to lithium grease. The spheres were synthesized by hydrothermal synthesis. The obtained composite greases were evaluated with the use of HSR-2M ball-on-disk tester to explore the friction and wear at different mass fractions of the additive. When the content of the CS in the grease was 1 wt.% and the load 60 N, the friction coefficient decreased by more than 20% compared with that for the base grease. When the content of the CS additive was 1 wt.%, the worn surface was quite smooth with few tiny furrows and the wear volume decreased by more than 40% in comparison with that for the base grease. The lubricating mechanisms at CS additive consisted of 'micro-roller' effects, repairing, and forming the protective film.
Ashour et al. [108] using the Falex four-ball tester studied the effect of hybridized nanocomposite additive composed of multiwalled carbon nanotubes and multilayer nanographene plateles (MWCNTs)/(MLNGPs) onto tribological characteristics of lithium based-grease. The optimum concentration of MLNGPs was equal to 1 wt.%. Wear scar diameter (WSD) was reduced by 66%, friction coefficient was reduced by 91%, while maximum non-seizer load was increased by 90 kg over that for ordinary lithium grease. Studies of hybrid MWCNTs/MLNGPs additive showed that the optimum ratio of MLNGPs to MWCNTs was equal to 4:1.
Wang et al. [109] using the SRV-IV tribotester investigated tribological properties of lithium grease with additive from graphene-copper nanoparticles (GN/Cu NPs) composites. Such a composite was prepared by in situ chemical reduction in liquid phase. Grease samples with additives were prepared according to the procedure described in Reference [110]. The GN/Cu NPs composites allowed reduction of the wear loss of the disk by 85% and of the average COF value by 15% compared to the corresponding values for the base grease.
Ji et al. [69] obtained 45 nm-sized CaCO 3 nanoparticles through the carbonation process. They studied tribological properties of lithium grease containing the CaCO 3 nanoparticles using a four-ball tribotester. The CaCO 3 nanoparticles were conducive to lower wear and friction, good load-carrying capacity, and resisted to extreme pressures. During friction process, the worn surfaces were covered by the boundary films containing CaCO 3 , CaO, iron oxide, and other inorganic compounds.
Wang et al. [68] prepared CeF 3 nanocluster surface-capped with oleic acid (coded as OA-CeF 3 ) using extraction method. The effect of the OA-CeF 3 nanocluster as an additive to synthetic lithium grease was studied using a four-ball friction and wear tester. The as-prepared OA-CeF 3 nanocluster had a broad size distribution, with an average diameter value of approximately 20 nm. The oleic acid as the capping agent was chemically bonded to the CeF 3 nanocores via strong chemical bonds.
Zheng et al. [111] studied the friction and wear properties of lithium grease with addition of copper oxide (CuO) nanoparticles using a four-ball friction tester. The oil films were formed on wear surface providing decrease of friction and wear. When the content of CuO nanoparticles was 0.6 wt.%, only a small number of furrows were created on the wear surface. The lithium grease with CuO nanoparticles showed the lowest average friction coefficient and wear scar diameter, which were 30% and 13%, respectively, lower than those for pure lithium grease. CuO-containing oil films were detected on the wear surface. Appearance of the film lowered number of furrows and the roughness of wear surface, demonstrating the great role of CuO nanoparticles for improving tribological properties of the lithium grease.
A lithium grease with interesting additive of WS 2 nanoparticles of the size 50-200 nm was elaborated. The NanoLub ® GH-X Lithium EP Grease Additive [112] is a surface-reconditioning nano-additive based on tungsten disulfide (WS 2 ) multilayered nano-fullerene particles. The grease containing such an additive has an exclusive resistance to Extreme Pressure (EP) and to wear and is characterized by low value of friction coefficient.
According to Reference [113], four complexes of rare earth hexadecylates (La, Pr, Sm, Gd) and copper hexadecylate were synthesized. Their tribological properties as additives to lithium grease were evaluated using a four-ball machine and a ring-block tester. These rare earth hexadecylates possessed antiwear and friction reduction properties in lithium grease. Their antiwear and friction reduction properties in lithium grease were better than that of copper hexadecylate. The tribochemical reaction happened during rubbing. The products of the tribochemical reaction improved the friction and wear behaviors of the lithium grease.
The authors of the paper [114] synthesized nanocrystallites of calcium fluoride (CaF 2 ) of the size of 60 nm by precipitation. The tribological properties of lithium grease with additive of such CaF 2 nanocrystallites were studied with a four-ball tribotester. The additive with such nanocrystallites exhibits excellent resistance to wear and to extreme pressure (EP) as well as a low value of friction coefficient. The EP and antiwear properties of the grease are non-proportionally affected by the content of CaF 2 nanocrystals. During tribological tests, the mating surfaces were covered by up to 12 nm thick boundary film. It was composed mainly of CaF 2 , CaO, iron oxide, and some organic compounds. Moreover, tribochemical reactions of CaF 2 nanocrystallites occurred on the worn steel surfaces at severe tribological conditions. According to Reference [115], in order to improve the wear resistance of the mating surfaces, they were coated by strengthened brush plating through deposition of Bi nanoparticles. The latter were obtained via DC arc plasma evaporation. The tribological properties of lithium greases containing Bi nanoparticles were studied with a four-ball tribotester. The lithium grease containing as-prepared Bi nanoparticles showed significantly improved tribological properties and an increased load-carrying capacity in comparison with corresponding values for the base lithium grease.
According to Reference [116], tribological properties of lithium grease with different amounts of the layered compound of ZnS(NH 2 CH 2 CH 2 NH 2 ) 0.5 was studied with a four-ball tribotester. Such grease showed good load-bearing capacity and resistance to wear. ZnS(NH 2 CH 2 CH 2 NH 2 ) 0.5 additive to lithium grease showed better wear resistance than that for simple ZnS additive under similar test conditions. It was due to that the two-dimensional structure of ZnS(NH 2 CH 2 CH 2 NH 2 ) 0.5 with larger interface distances made the sliding process easier, and thus it improved the resistance to wear. The arisen tribofilm contained mainly FeS, ZnS, ZnO, Fe x O y , Fe u (SO 4 ) v , and ZnSO 4 compounds.
Using a ball-on-disc sliding wear tester, Chang et al. [117] studied the friction in the contact zone between a ball and a flat surface lubricated with TiO 2 /CuO nanogrease. The tested lithium grease was synthesized with 0.5, 1.0, 1.5, and 2.0 wt.% of TiO 2 /CuO nanoparticles dispersed therein. The TiO 2 /CuO nanoparticle additives improved wear resistance of the lithium grease and decreased the COF value. It turned out that the greatest reduction of the friction and wear and significant reduction of the COF value by approximately 40% was obtained at 1.0 wt.% of TiO 2 nanoparticles in the grease. The wear of the lithium grease with a 2.0 wt.% content of CuO nanoparticles was lower by about 60% in comparison with that for pure lithium grease.
Rapoport et al. [118] obtained lithium grease (H3-Delkol) by mixing a mineral oil (30 cSt at 40 • C) with the lithium thickener. Such a base grease with 5 wt.% of layered 2H-MoS 2 particles (HM3-Delkol) was used as the reference one. In order to prepare grease containing the IF-WS 2 , the base grease was heated up to 60 • C and mixed carefully with 5 wt.% of the IF nanoparticles for 5 h. The effect of the IF-additive to grease was studied using a ring-block tribotester. The lithium grease containing the IF-additive showed better tribological properties than those of grease with 2H-MoS 2 particles. Preservation of the IF nanoparticles on the mating surfaces under friction at a high load provided improved tribological properties of greases with the IF-additives.
Wozniak et al. [61] studied lithium grease with the TiO 2 and ZrO 2 nanoparticles lubricating the mating surfaces in tie rod ends. The steel ball-polymer insert contact zone was lubricated with the pure lithium grease or containing 1 wt.% of TiO 2 or ZrO 2 nanoparticles. Studies on friction were carried out using the tribotester, allowing reversal motion at different contact loads.
The authors of the paper [119] studied the spinning friction in contact zone between steel ball and plate lubricated by lithium grease without and with addition of SiO 2 NPs at constant speed and stepwise increasing contact load. The COF value decreased with an increase of contact load for simple lithium grease. Addition of 6 wt.% of SiO 2 NPs to the grease was conducive to the same trend. However, addition of other concentrations of SiO 2 NPs was conducive to decrease of COF value with an increase of the contact load.
The stability of zirconia nanoparticles can be increased by addition of trivalent ions (e.g., yttrium ions or aluminum ones) [127,128]. These oxides are commonly called "YSZ" (Yttria Stabilized Zirconia).
According to Reference [129], zirconium oxide (ZrO 2 ) can be prepared by various techniques such as the sol-gel method, co-precipitation, or hydrothermal processes.
According to Reference [50], there are numerous technologies based on wet chemical methods for synthesis of ZrO 2 nanopowders, such as precipitation methods, mechano-chemical methods, emulsion methods, hydrothermal ones, and others.
The hydrothermal route allows a good stabilization of YSZ due to its ability to create crystalline phases at low temperature [132].
Rosa et al. [133] synthesized YSZ nanoparticles by the Continuous Hydrothermal Synthesis. Nanorods from YSZ can be grown by metal-organic chemical vapor deposition (MOCVD) method [134] or using hydrothermal route [135].
The use of sol-gel technology allowed improvement in material processing. Such a process covers organic, inorganic hybrid, and metallic materials [136].
According to Reference [137], the sol-gel technique allowed obtaining nanostructured coatings with enhanced functional or mechanical properties as well as high purity, homogeneity, and improved microstructure.
The sol-gel method does not require vacuum and allows fabricating a large area with low cost and at low processing temperature [138,139].
Bashir et al. [140] synthesized ZnO-doped ZrO 2 nanoparticles using the sol-gel method with different concentrations of ZnO in acidic and basic medium. Zirconyl chloride octahydrate (ZrOCl 2 ·8H 2 O) Ethanol and Deionized (DI) water were used as precursors, whereas NaOH was used as a gelation agent. All materials were of chemical grade and used without further purification. Zirconyl chloride octahydrate (ZrOCl 2 ·8H 2 O) was added to DI water to form solution of molarity 0.1 M and pH = 2. Solution was stirred and heated at 50 • C for 1 h before and after addition of ethanol. Then ZnO was added dropwise from 1 to 5 wt.% into the synthesized sol. All sols were subjected to gel formation by heating at 60 • C to form powder of ZrO 2 nanoparticles.
Ashkarran et al. [141] synthesized ZrO 2 nanoparticles by high current electrical arc discharge of zirconium electrodes in deionized (DI) water. A mixture of nanocrystalline ZrO 2 monoclinic and tetragonal phase was formed. The size of the particles increased with increase of the arc current.
The sol-gel method was used in Reference [142] for preparation of Fe 3 O 4 -doped ZrO 2 nanoparticles (NPs). The ZrO 2 sol was doped with pre-synthesized Fe 3 O 4 NPs resulting in a mixture of monoclinic and tetragonal zirconia. An increase in crystallite size was observed with increase of the dopant concentration. The lattice contraction was observed for the Fe 3 O 4 -doped ZrO 2 samples. This contraction resulted in volumetric change conducive to formation of tetragonal phase. Relatively high density (~6 g/cm 3 ) was observed.
Arantes et al. [143] obtained nanocrystalline ZrO 2 particles by hydrothermal treatment of 0.25 M ZrO(NO 3 ) 2 and 0.5 M ZrOCl 2 aqueous solutions at different values of the temperature and time in the presence of hydrogen peroxide. Nanocrystalline monoclinic ZrO 2 powders with a narrow distribution of the 3.5 nm-sized crystallites were obtained. As the crystallization depended strongly on the temperature, an amorphous material was synthesized at 100 • C, and crystalline one with monoclinic phase was obtained at 110 • C, independently of the salt used. ZrO 2 colloidal nanoparticles were obtained only by hydrothermal treatment longer than 24 h.
Vasylkiv and Sakka [49] prepared tetragonal 3 mol% Y 2 O 3 -doped ZrO 2 nanopowder by hydrothermal precipitation from metal chlorides and urea sol followed by a washing-drying treatment and calcination.
According to Colbea et al. [145], the wet impregnation process of ZrO 2 nanoparticles with 10% and 20% Eu oxide followed by thermal annealing in the air at a temperature above 500 • C allowed obtaining the fully-stable tetragonal phase of ZrO 2 . The bare ZrO 2 nanoparticles were obtained through the oil in water microemulsion, rapid hydrothermal treatment, and citrate complexation method.

Agglomeration of ZrO 2 Nanoparticles and Methods to Prevent It
Danilenko et al. [50] stated that the agglomeration degree depends on surface properties of nanoparticles, type of dispersant, and other conditions. Some reports [146][147][148] pointed that the precipitation methods with microwave drying, pulse magnetic field treatment, and ultrasonication of Zr hydroxide resulted in formation of small size "soft" Zr hydroxide agglomerates. Subsequent processes of calcination under low temperature allowed obtaining ZrO 2 nanoparticles with predetermined particle size of narrow size distribution and "soft" agglomerates.
Danilenko et al. [50] studied the influence of different surfactants (Trilon B, Polyammoniumacrylate and Neonol) and mineralizers (HCl, KOH, CH 3 COOH, etc.) on agglomeration degree of ZrO 2 nanopowders. By adding surfactants and mineralizers, different phase composition, particle size, and agglomeration degree of zirconia nanopowders were obtained. Increasing the specific surface area did not always result in a decrease in the particle size. Additions of KOH and KNO 3 led to an increase in particle size and specific surface area simultaneously. It was affected by a decrease in the agglomeration degree of nanoparticles due to changes in the interaction forces between particles influenced by surfactants and mineralizers. Low calcination temperatures (less than 700 • C) led to the formation of the zirconia particles with cylindrical form. The addition of KNO 3 led to the formation of non-agglomerated cylindrical zirconia particles with a diameter near to 12 nm and a length near to 2-3 nm.
According to Palanisamy et al. [144], the ball-milling process allowed disaggregation of the aggregated ZrO2 nanoparticles obtained by precipitation. The ball-milling process provided fully crystalline monoclinic ZrO 2 nanoparticles of a size of 64 nm and a specific surface area of 126 m 2 /g. Furthermore, the sodium-stabilized cubic ZrO 2 nanoparticles with a size of 39 nm and the specific surface area of 227 m 2 /g were obtained through the ball-milling process.
Vasylkiv and Sakka [49] studied the effects of nanopowder washing by water and ethanol with subsequent centrifuging, with possible deagglomeration using microtip ultrasonication on the properties of ZrO 2 nanopowders. The ultrasonic irradiation induced the pressure waves generating the violently collapsing cavities, producing intense stress. Such stress allowed minimization of secondary particle size, deagglomerating the powder, redispersing the ZrO 2 after all the washing-centrifuging cycles, and minimizing mean aggregate size after final calcination. A uniformly aggregated tetragonal ZrO 2 nanopowder without hard agglomerates was prepared.
The stable dispersion and the reduced agglomeration of the nanoparticles can be achieved via ultrasonication, homogenizing, magnetic force agitation, probe sonication, ball milling, and high shear mixing [149]. Achieving proper dispersion of the nanoparticles needs the reduction of the chance for agglomeration strongly affected by the method and the time duration of dispersion.
According to Reference [48], the surface modification of nanoparticles or using the surfactants can enhance dispersion stability of nanoparticles.
Sui et al. [150] explained that the surface modification of nanoparticles allows tailoring of their surface properties to the use of organic modifying agents. The most used representative of the latter is oleic acid. It absorbs around the nanoparticles reducing their surface energy and, as a result, blocks the agglomeration.
The dispersion stability of nanolubricants can be enhanced by adding surfactants also called dispersants. Such a surfactant can be oleic acid, sodium dodecyl sulfate (SDS), TRITON TMX102, benzalkonium chloride, or benzethonium chloride [151].
According to Reference [48], the addition of the surfactants enhances the dispersion stability of nanoparticles and allows reduction of friction and wear.
According to De Monredon et al. [152], the significant agglomeration of nanoparticles can occur during the sol-gel process involving hydrolysis and condensation, which are generally fast and need to be inhibited to avoid precipitation and allow sol or gel formation.

Lubrication Mechanism of Lithium Nanogrease
According to Reference [66], the formation of boundary lubrication film highly affects the tribological performance of the lithium nanogrease. The lubricant film contains nanoparticles on the mating surfaces. The mechanism takes place in three stages: Stage-I: rubbing surfaces come in contact through asperities. The gap between asperities is filled by nanogrease, providing a lubricating effect.
Stage-II: CaF 2 nanoparticles are deposited on the rubbing surfaces. The low shear strength of CaF 2 due to its low size decreases the friction coefficient. The micro-point contact has a high surface pressure and high temperature, facilitating a complex tribochemical reaction.
Stage-III: The reaction product together with CaF 2 nanoparticles form complicated wear-resistant film on the rubbing surface resulting in excellent lubrication at the contact zone.
The mechanism of film formation on the rubbed surfaces was verified and reported by several authors [66][67][68][69]118]. The boundary lubricating film was composed of deposited NCB and reaction products such as B 2 O 3 , CaO, and iron oxide [67].
According to Reference [153], the layered structured material, such as graphene, as additives to lithium grease, can form conformal protective layer on rubbing surfaces.
According to Reference [154], the planar structure and nano-spaced layer facilitate easy shearing between contacts due to weak van der Waals forces between layers.
Petrone [155] carried out the rheological tests of the lithium greases with the MoS 2 additive obtaining the same results for the grease samples with 2 wt.% nanoparticles and for one with 4 wt.% of MoS 2 . The different types of inorganic nanoparticles, with different concentrations in weight, did not affect the rheological behavior of the lubricating grease. He also carried out tribological tests for steel-steel contact zone using the rotational tribometer. He found a decrease of the friction coefficient both in boundary and mixed lubrication regimes achievable through MoS 2 nanosheets dispersion, with average reduction for lithium grease around 35%. The best performance in terms of wear scar diameter (WSD) was provided by the lithium grease with 2 wt.% of MoS 2 .
The anti-wear property of MoS 2 nanosheets as an additive to the lithium grease was clearly visible in both the abovementioned lubrication regimes, leading to an average decrease level of 30% of the scar diameter measured on the worn surface of the steel ball. The lithium grease with 2 wt.% of MoS 2 performed better than that with 4 wt.% formulation.
The micro ball-bearing rolling of spherical IF-nanoparticles reducing contact area also improves tribological properties of nanogreases [118].

Tribological Evaluation of Nanogrease
Wang et al. [66] studied the tribological properties of lithium grease containing CaF 2 nanocrystallites with a four-ball tribotester. They obtained a reduction in wear scar diameter by 29% and a decrease of friction by 19% in comparison to those for the base lithium grease. The improvement in tribological properties of CaF 2 -added grease was not proportional to the concentration of CaF 2 and was limited.
Zhao et al. [67] studied tribological properties of lithium grease containing nano-calcium borate (NCB) on an Optimol SRV-IV oscillating reciprocating friction and wear (SRV) tester. The excellent wear-resistant and friction-reducing properties corresponded to an optimal concentration of 6 wt.% of NCB in lithium grease. For the worn steel surface lubricated with lithium grease, a wide wear scar with a deep groove was observed. For worn steel surface lubricated by lithium grease with 6 wt.% of NCB, the wear debris on rubbing surfaces created relatively small, smooth, and shallow wear scar.
During studies on lithium grease with addition of Graphene, Graphene Oxide, and Graphite described in [153], it was found a decrease in coefficient of friction by 30%, 40%, and 60% for grease based on graphene oxide, graphite, and graphene, respectively.
A 63% decrease was reported in wear scar diameter during studies on the lithium grease with addition of 1 wt.% of carbon nanotubes using a four-ball tester [105]. Furthermore, a decrease of COF value by 81% and an increase of the resistance to EP properties and load-carrying capacity by 52% was achieved. During friction process, the mating surfaces were covered by the boundary film containing CNTs, Cr, iron oxide, and other compounds.
Studies described in Reference [117] on lithium grease with addition of TiO 2 and CuO showed a decrease of friction by approximately 40% and 60% for TiO 2 and CuO, respectively.

Samples Preparation for Tribological Tests
During friction tests, a polished steel ball of 18 mm diameter rotated with constant speed relative to the fixed counterpart. The ball mates with the polyethylene counterpart of 30 mm inner diameter and fixed in the steel ring. The contact zone was loaded with a constant force and lubricated with lithium grease without or with 1 wt.% amount of ZrO 2 nanoparticles of a size 10-20 nm (for details see Reference [61]). The lithium grease was mixed with the ZrO 2 nanoparticles by the High-Speed Planetary Mixer THINKY SR-5000 [156] followed by treatment with GT Sonic VGT-800 Ultrasonic Cleaner for approximately 120 s in order to avoid agglomeration or the structural break of the nanoparticles. To indicate the occurrence of agglomeration of ZrO 2 nanoparticles, the lithium grease with 1 wt.% of ZrO 2 nanoparticles were left at rest for two weeks from preparation to the beginning of tests in a closed container at 20 • C.

Investigation of Friction Coefficient
The friction in the steel ball/polymer insert contact zone lubricated by the lithium grease without or with ZrO 2 nanoparticles was studied with the use of a tribotester [61]. The tribotester allowed obtaining values of the spinning friction torque in the contact zone between ball and its counterpart. Measurements were performed under constant rotating speed of 36 rpm of the ball and under constant load in the contact zone that was increased stepwise from 7.2 to 27.2 N. The COF values were calculated using measured values of the friction torque, diameter of contact zone, and an average contact pressure in the contact zone between the ball and its counterpart. The average contact pressure and diameter of the contact zone were estimated using the FEM model of such a zone [61]. The values of the contact pressure did not exceed 38 MPa.

Results
The obtained graphs of the friction coefficient as a function of load in the contact zone between steel ball and polyamide inner sphere are presented in Figure 3 for 3 different lubrication systems: with pure lithium grease, with fresh lithium grease containing 1 wt.% of ZrO 2 nanoparticles as the additive without agglomerations of ZrO 2 nanoparticles, and with lithium grease with 1 wt.% of ZrO 2 nanoparticles left at rest for two weeks from preparation to the beginning of the tests in a closed container at 20 • C that provoked occurrence of agglomerations of ZrO 2 nanoparticles. constant load in the contact zone that was increased stepwise from 7.2 to 27.2 N. The COF values were calculated using measured values of the friction torque, diameter of contact zone, and an average contact pressure in the contact zone between the ball and its counterpart. The average contact pressure and diameter of the contact zone were estimated using the FEM model of such a zone [61]. The values of the contact pressure did not exceed 38 MPa.

Results
The obtained graphs of the friction coefficient as a function of load in the contact zone between steel ball and polyamide inner sphere are presented in Figure 3 for 3 different lubrication systems: with pure lithium grease, with fresh lithium grease containing 1 wt.% of ZrO2 nanoparticles as the additive without agglomerations of ZrO2 nanoparticles, and with lithium grease with 1 wt.% of ZrO2 nanoparticles left at rest for two weeks from preparation to the beginning of the tests in a closed container at 20 °C that provoked occurrence of agglomerations of ZrO2 nanoparticles.
For the cases of pure grease and the grease with 1 wt.% of non-agglomerated ZrO2 nanoparticles, the increase of the load in the contact zone resulted in decrease of the average value of the friction coefficient (COF). The standard deviations of the COF values also decreased with an increase in the load. For grease with 1 wt.% of agglomerated ZrO2 nanoparticles, the increase of load in the contact zone was conducive to other changes in average values of COF. For smaller values of the load, the average COF value at first decreased and next increased. Further increase of the load resulted in a decrease of the COF value. The standard deviations of the COF values for such a grease were higher than those for pure grease and the grease with non-agglomerated ZrO2 nanoparticles and varied with increase of the load in the contact zone. For smallest and the highest values, the load and the standard deviations of the COF values were the lowest. For the other values of the load, the standard deviations of the COF values were clearly higher. Figure 3. Friction coefficient vs. load in the contact zone between steel ball and polyamide inner sphere for three different lubricating systems: with pure lithium grease, with fresh lithium grease containing 1 wt.% amount of ZrO2 nanoparticles as the additive without agglomeration, as well as with lithium grease containing 1 wt.% of ZrO2 nanoparticles left at rest for two weeks from its preparation to the beginning of the tests in a closed container at 20 °C due to which a partial agglomeration of ZrO2 nanoparticles took place.

Discussion and Conclusions
It follows from the survey of lithium grease properties an important effect of additives, particularly of ZrO2-based nanoparticles on tribological properties of the grease. The agglomeration of such nanoparticles is an unwanted phenomenon, not only during preparation of the lubricant but . Friction coefficient vs. load in the contact zone between steel ball and polyamide inner sphere for three different lubricating systems: with pure lithium grease, with fresh lithium grease containing 1 wt.% amount of ZrO 2 nanoparticles as the additive without agglomeration, as well as with lithium grease containing 1 wt.% of ZrO 2 nanoparticles left at rest for two weeks from its preparation to the beginning of the tests in a closed container at 20 • C due to which a partial agglomeration of ZrO 2 nanoparticles took place.
For the cases of pure grease and the grease with 1 wt.% of non-agglomerated ZrO 2 nanoparticles, the increase of the load in the contact zone resulted in decrease of the average value of the friction coefficient (COF). The standard deviations of the COF values also decreased with an increase in the load. For grease with 1 wt.% of agglomerated ZrO 2 nanoparticles, the increase of load in the contact zone was conducive to other changes in average values of COF. For smaller values of the load, the average COF value at first decreased and next increased. Further increase of the load resulted in a decrease of the COF value. The standard deviations of the COF values for such a grease were higher than those for pure grease and the grease with non-agglomerated ZrO 2 nanoparticles and varied with increase of the load in the contact zone. For smallest and the highest values, the load and the standard deviations of the COF values were the lowest. For the other values of the load, the standard deviations of the COF values were clearly higher.

Discussion and Conclusions
It follows from the survey of lithium grease properties an important effect of additives, particularly of ZrO 2 -based nanoparticles on tribological properties of the grease. The agglomeration of such nanoparticles is an unwanted phenomenon, not only during preparation of the lubricant but also during subsequent sintering process of the ZrO 2 nanoparticles. There are several methods of producing ZrO 2 nanoparticles, however, a part of them cannot prevent nanoparticles' agglomeration. For example, the commonly used precipitation method leads to aggregated ZrO 2 nanoparticles, which are next disaggregated by ball-milling. In this context, the method based on ZrO 2 nanopowder washing by water and ethanol with subsequent centrifuging looks interesting, with possible deagglomeration with the use of microtip ultrasonication. The dispersion stability of ZrO 2 nanoparticles in oil or in grease can be enhanced either by surface modification of such nanoparticles or by using the surfactants. The choice of the method of surface modification or of surfactant application is difficult and further investigations are needed for ZrO 2 nanopowders.
There is a clear relationship between tribological properties of lubricants containing nanoparticles and agglomeration of such nanoparticles.
The addition of 1 wt.% ZrO 2 nanoparticles to pure lithium grease decreased COF value by up to 50%, especially for lower values of the load in the contact zone. The occurrence of agglomerations of ZrO 2 nanoparticles in the lithium grease increased COF value twice as compared to that for the pure grease. Moreover, standard deviations of the COF values are significantly higher than those for the lithium grease without agglomerated nanoparticles.