Advancements in Environmentally Friendly Lubricant Technologies: Towards Sustainable Performance and Efficiency

Abstract

The advancement of next-generation lubricants is pivotal for enhancing energy efficiency and mitigating environmental impacts across diverse industrial applications. This review systematically examines recent developments in lubricant technologies, with a particular focus on sustainable strategies incorporating bio-based feedstocks, nanostructured additives, and hybrid formulations. These innovations are designed to reduce friction and wear, decrease energy consumption, and prolong the operational lifespan of mechanical systems. A critical assessment of tribological behavior, environmental compatibility, and functional performance is presented. Furthermore, the integration of artificial intelligence (AI) into lubricant formulation and performance prediction is explored, highlighting its potential to accelerate development cycles and enable application-specific optimization through data-driven approaches. The findings emphasize the strategic role of eco-innovative lubricants in supporting low-carbon technologies and facilitating the transition toward sustainable energy systems.

1. Introduction

Friction is an inherent phenomenon in mechanical systems, leading to heat generation, material degradation, and significant energy losses [1,2]. Effective lubrication is therefore essential to ensure the proper functioning, energy efficiency, and durability of machines [3,4]. Lubricants not only reduce friction and wear but also serve auxiliary roles such as dissipating heat, removing contaminants, and protecting surfaces against corrosion.

Historically, lubricants were derived from animal fats and vegetable oils [5,6]. However, with industrialization, petroleum-based lubricants became dominant due to their performance advantages and large-scale availability [2,7,8,9]. Today, the environmental consequences of mineral oils—such as toxicity, bioaccumulation, and non-biodegradability—have reignited interest in sustainable alternatives [5].

Modern research in lubricant technology increasingly focuses on eco-friendly solutions, particularly those based on renewable resources and advanced functional additives. Bio-based lubricants derived from vegetable oils, waste cooking oil (WCO), or microbial lipids offer biodegradability, low toxicity, and reduced carbon footprint. At the same time, nanotechnology enables the design of nano-additives that significantly improve tribological performance, thermal stability, and load-bearing capacity. The development of nanolubricants and biolubricants, as well as their hybrids (nano-biolubricants), aligns with global priorities of decarbonization and circular economy.

Furthermore, artificial intelligence (AI) offers transformative capabilities in lubricant formulation by predicting performance metrics, optimizing compositions, and accelerating development cycles. AI-driven tools such as artificial neural networks and hybrid optimization algorithms can model complex relationships between formulation parameters and output properties, reducing the reliance on trial-and-error experimentation.

The aim of this review paper is to systematically examine recent progress in sustainable lubrication technologies. The reader is first introduced to the principles and mechanisms of nanolubrication, followed by a critical overview of bio-based raw materials and waste valorization strategies. Finally, emerging trends in AI-assisted lubricant design are discussed, highlighting the intersection of materials science, environmental engineering, and intelligent systems. Emphasis is placed on integrating performance, sustainability, and economic viability in next-generation lubricants.

To ensure comprehensiveness and relevance, this review was based on a systematic literature search covering the period 2019–2025. The primary objective was to gather peer-reviewed studies that report on the synthesis, characterization, tribological evaluation, or AI-supported development of environmentally friendly lubricants. Scientific databases including Scopus, Web of Science, and ScienceDirect were queried using Boolean combinations of the following keywords: “biolubricants”, “nano-lubricants”, “AI in tribology”, “waste oil-based lubricants”, and “sustainable lubricants”. The search was limited to articles published in English. Additional manual screening was performed to include seminal older works if they were cited in more recent reviews or high-impact studies.

2. Nanolubricants

The development of nanolubricants represents a pivotal innovation in the quest for sustainable, high-performance lubrication technologies. These advanced lubricants, enriched with nanoparticles, aim to mitigate friction and wear, extend service life, and enhance the operational reliability of mechanical systems. Thus, the addition of nanoparticles to lubricants serves to improve their anti-wear and friction-reduction capabilities [5,10,11,12,13]. Furthermore, nano-additives enable lubricant functionality under elevated temperature conditions [11]. Other advantages associated with nanolubricants, which contribute to substantial cost reductions, include decreased energy and fuel consumption, improved heat dissipation, corrosion mitigation, optimization of mechanical efficiency, prolonged lubricant lifespan, and extended operational durability of mechanical components [12,14]. The use of nano-fluid reduces cutting force and results in improving the surface integrity of the workpiece; thus it is beneficial in machining [15].

Despite the numerous benefits of introducing nanoparticles into lubricants, this solution also has certain limitations, for example, the need to maintain a stable dispersion of the nano-additive in the base oil. The following sections discuss the benefits and challenges associated with the use of various nanolubricants, their mechanisms of action, and directions for recent and future research.

2.1. Key Parameters Influencing the Tribological Performance of Nanolubricants

A wide range of nanomaterials, including metal oxides, sulfides, and carbon-based allotropes, can be employed to enhance the lubricating properties of base oils, as illustrated in Figure 1 and Table 1. The effectiveness of nanoparticles in improving the performance of nanolubricants is influenced by the chemical nature of the nanomaterial and the base oil, and also by several physical parameters, such as the concentration of the additive, particle size, and particle morphology [5,6,10,16]. Some nanomaterials are subjected to functionalization, i.e., a chemical process by which desired functional groups are attached to the surface of nanoparticles. This practice is intended to improve compatibility, stability, and effectiveness [8].

One of the primary design parameters influencing the behavior of nanolubricants is particle concentration. Studies consistently show that tribological properties such as the coefficient of friction (COF) and wear scar diameter (WSD) are highly sensitive to the dosage of nanoparticles. In general, in the low concentration range, the performance characteristics of nanofluids improve with increasing concentration of nano-additive. However, a continuous increase in the concentration of nanoparticles can contribute to the decrease in the stability of the lubricant and increase abrasive wear and surface roughness [6,15]. Therefore, determining the optimal concentration for a specific tribological system is essential. Most studies cited in the literature report nanoparticle concentrations ranging from 0.025 to 2.0 wt% [6]. The authors of [17] inform that a concentration of nanoparticles below 2 wt% is sufficient to enhance the tribological performance, and the optimal concentration for the majority of nano-oxides is 5 wt%. Thampi et al. [13] conclude that the properties of lubricating oils can be better when the addition of nanoparticles is 0.1–0.5 wt%. The data presented in Table 1 confirm the above findings, and the optimal concentration in some cases may be below 0.5 wt%. However, this value is influenced by various factors, including the type and properties of the additive and the base oil, and the working conditions of the nanolubricant. Thus, the optimal concentration of the nano-additive in the lubricant is highly system-specific and should be determined experimentally, considering the planned application of the nanolubricant.

Particle size is equally important. Smaller particles generally offer superior tribological performance due to their ability to penetrate surface asperities, fill micro-defects, and form uniform boundary layers than larger particles [13]. Pawar et al. [17] state that the introduction of nanomaterials in the particle size range from 2 to 120 nm into the lubrication system reduces friction and wear. Nano-sized particles outperform micro-sized ones, and within the nanoscale range, smaller particles are more effective than their larger counterparts [18]. A synergistic effect may also be achieved by combining nanoparticles of different sizes. Liu et al. [19] found that lubricants containing a mixture of nanoparticles with varying grain sizes exhibited improved anti-wear and friction-reducing properties compared to those with uniformly sized additives.

The tribological performance of nanolubricants can be further optimized by tailoring the grain size of the nanoparticles to the operating conditions. For instance, in a study involving nano-SiO₂ particles of different sizes [19], smaller particles were more effective at reducing friction under low-frequency conditions, while larger particles performed better at high frequencies.

When selecting a nano-additive for a given application, both the grain size and the hardness of the nano-additive particles must be considered, because scratches and dents can occur if the hardness of the friction surface is lower than the hardness of the nano-additive [17].

Surface morphology also plays a significant role. Two extremes can be identified: smooth (mirror-like) and rough surfaces. The literature suggests that smoother nanoparticles offer better tribological performance due to their higher resistance to scuffing under elevated hydrodynamic pressures [18]. Furthermore, smoother particles may require lower concentrations to achieve effective lubrication compared to rougher particles, as noted by Raina et al. [18].

Nanoparticles used in lubricants exhibit a variety of shapes—spherical, nanosheets, nanoplatelets, nanotubes, and onion-like structures—each closely related to their dimensional classification. Based on their geometry, nanoparticles can be categorized as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) [20,21]. Zero-dimensional nanoparticles, such as spherical particles, have all three dimensions within the nanoscale (typically 1–50 nm) [20,21]. In contrast, 1D nanomaterials—such as nanorods, nanofibers, and nanotubes—have one dimension in the macroscale and two in the nanoscale [20,21]. Two-dimensional materials, including nanosheets and nanolayers, possess two macroscale dimensions and one nanoscale thickness. Meanwhile, 3D nanomaterials are bulk structures composed of nanoscale or larger constituents [20,21].

Spherical nanoparticles, which are the most commonly used, provide point contact, high load-carrying capacity, extreme pressure characteristics, and good colloidal stability [6,16,17]. In contrast, nanosheets and platelets form line or planar contacts, respectively, which can alter the lubrication mechanism.

The type of base oil and its compatibility with the nanoparticle additive are also critical factors [11]. It should be emphasized that effective lubrication with nano-enhanced oils necessitates the stable dispersion of nanoparticles within the base oil. Due to their extremely small size, nanoparticles exhibit a tendency to agglomerate. Undesirable phenomena such as sedimentation and agglomeration can impair performance by decreasing friction-reducing and anti-wear capabilities [11,17]. These issues may arise over time, particularly under static conditions [17]. Thus, the colloidal stability of nanolubricants is an essential parameter, and improving nanoparticle dispersion within lubrication systems represents a primary research focus for scientists and engineers in the field.

Achieving and maintaining a stable dispersion of nanoparticles in oil presents several challenges. Initial dispersion can be facilitated through techniques such as magnetic stirring, ultrasonication, or combined methods, with ultrasonication being particularly effective [6,11]. However, optimizing sonication time is crucial [22]. Surface modification using surfactants is another common approach to enhance dispersion stability [22,23]. Oleic acid, for example, has been frequently employed to enhance dispersibility [24]. Both methods of improving nanolubricant dispersion are often used together—the addition of a surfactant and mixing with ultrasound, what can be seen in Table 1. Nevertheless, surfactants may degrade under high shear stress and elevated temperatures typical of tribological systems [24].

A comprehensive overview of dispersion techniques and strategies to improve nanoparticle stability in lubricants is provided in [16]. Recent advancements also include innovative methods such as microwave plasma treatment, which enhances the surface energy and wettability of nanoparticles, thereby improving their dispersion and stability in lubricating oils [25]. Also, proper modification of the nanoparticle surface by functionalization helps maintain stable dispersion of the nano-additive in the lubricating liquid [8].

2.2. Mechanisms of Nanoparticle Action in Tribological Systems

The enhancement of lubricating properties in oils containing nanoparticles is primarily attributed to the adhesion of nanoparticles to solid surfaces and the subsequent formation of a protective layer at the interface between the lubricant and the frictional surfaces. A more detailed examination reveals that nanoparticles can influence tribological behavior in multiple ways.

Four main mechanisms of interaction of nanoparticles in the lubricant have been identified: rolling effect, mending/filling effect, polishing, and formation of tribofilm [7,8,17]. A synergistic effect between nanoparticles and modified surfaces, as well as a transformation of the nanoparticle microstructure, can also take place [17].

Under low-load conditions, spherical nanoparticles may act as rolling elements—like miniature ball bearings—or form a thin lubricating film between contact surfaces. These actions contribute to reduced friction and wear, representing the primary lubrication mechanisms [1,6,10,16]. Additional mechanisms include the filling of surface asperities and the polishing effect of hard nanoparticles (e.g., diamond or alumina), which smoothens surface roughness [1,6,10,16]. These are considered secondary lubrication mechanisms. Notably, some nanoparticles may operate through a combination of these mechanisms [1,10].

Metallic nanoparticles exhibit excellent tribological performance, partly due to their self-healing capabilities. This effect arises from the compaction of nanoparticles under elevated temperature and pressure within the frictional interface. Furthermore, metallic nanoparticles contribute to lubrication through (1) the formation of tribofilms and (2) rolling between contact surfaces. Similar mechanisms are observed for metal oxide nanoparticles [26].

In the case of metal sulfide nanoparticles, lubrication is primarily achieved through the formation of a robust adsorption layer on the contact surface. In some instances, sulfur atoms may diffuse into the friction surface, enhancing the protective effect [26]. Carbon-based nanoparticles also exhibit multiple mechanisms of action, including the formation of protective films, surface repair, and rolling effects, sometimes occurring simultaneously.

Two-dimensional (2D) nanomaterials, characterized by their layered structures and sheet-like morphology, interact with tribological systems in several distinct ways. These include penetration into surface gaps, formation of tribofilms at contact interfaces, filling of surface imperfections (e.g., cavities and scratches), and influence on lubricant viscosity [27]. In the case of machining, using a lubricant containing nanosheets may be beneficial because of the possibility of effectively filling the pits on the surface of the workpiece. In this way the surface becomes smoother, and lubrication performance is improved [15].

Lubrication mechanisms may involve not only physical interactions but also tribochemical reactions. This is particularly relevant for nano-additives containing chemically active elements such as sulfur (S), chlorine (Cl), or phosphorus (P). In such cases, tribochemical films may form through reactions between the additive, surfactants, and the frictional surfaces [14].

2.3. Examples of Nanoparticles Used in Lubricating Fluids

Metallic nanoparticles are widely utilized as lubricant additives due to their excellent thermal conductivity, which is a desirable property for dissipating heat generated during friction. Additionally, these nanoparticles can settle into surface irregularities such as cavities and asperities, thereby protecting the surface from wear and damage [28]. However, the chemical nature of certain nanoparticles may pose challenges. For instance, while metal sulfides are commonly employed to enhance lubrication, they pose ecological problems and their use is limited [6]. Notably, molybdenum disulfide (nano-MoS₂) exhibits high chemical and thermal stability, making it a particularly effective and environmentally acceptable lubricant additive [29]. This nanomaterial has a layered structure that allows particles to slide over each other. Due to its properties, nano-MoS₂ is particularly helpful as a nanolubricant component in high-stress operating conditions, such as in heavy machinery [8].

Although most nano-additives used in lubricants are free from sulfur and phosphorus, some may still exhibit toxicological concerns, which must be considered in practical applications [1]. Fortunately, the majority of nanomaterials currently employed in lubricants are regarded as environmentally benign [26].

Two-dimensional (2D) materials have garnered increasing attention for tribological applications. This category includes metal dichalcogenides (e.g., WS₂, MoS₂), graphene, and related structures [27,30]. As mentioned earlier, these materials offer superior friction and wear resistance due to their ability to penetrate surface defects and their sheet-like morphology, which enhances lubrication efficiency [30]. The unique structure of 2D materials—characterized by strong covalent bonding within layers and weak van der Waals forces between layers—enables high mechanical strength and elasticity while allowing interlayer sliding. This combination facilitates the formation of a lubricating film on contact surfaces, significantly improving tribological performance [27].

Some nanomaterials, such as zinc oxide (ZnO), can form various nanostructures, including nanoparticles, nanoballs, nanorods, and nanowires. Nano-ZnO is non-toxic and possesses several advantageous properties for tribological applications, including high chemical stability, large specific surface area, high adsorption capacity, and the ability to reduce friction and wear when used as a lubricant additive [31].

Titanium dioxide (TiO₂) and aluminum oxide (Al₂O₃) nanoparticles also exhibit excellent chemical and thermal stability, making them suitable for a wide range of tribological applications [14]. TiO₂, in particular, is known for its high suspension stability, low toxicity, and ease of synthesis [6].

Carbon-based nanomaterials, particularly those from the graphene family, have attracted considerable attention in recent years due to their exceptional tribological properties. Graphene, a two-dimensional (2D) nanomaterial, consists of carbon atoms arranged in a hexagonal lattice, forming a monolayer with nanoscale thickness [27]. The layers can slide, which is important when using graphene as a lubricant. Moreover, the characteristic graphene structure is difficult to distort under large loads [15]. This structure imparts graphene with outstanding thermal, electrical, mechanical, and tribological characteristics [27]. Excellent mechanical properties are also demonstrated by graphene derivatives, e.g., graphene oxide [32]. A certain limitation of the use of graphene and its derivatives in lubricants may be poor dispersive ability and stability in the dispersion medium, especially over a long period of time. Graphene nanoparticles tend to aggregate in the fluid [32,33,34]. Surfactants can be used to counteract this phenomenon. The preferred method is chemical functionalization (covalent or non-covalent), which can reduce the need for surfactant and improve nanoparticle stability in an oil medium, including long-term stability [32]. Favorable results were also obtained with the use of functionalized graphene oxides and the addition of ionic liquid in the base oil [35].

Carbon nanotubes (CNTs), which are one-dimensional (1D) nanomaterials, possess a cylindrical structure composed of hexagonally arranged carbon atoms bonded covalently. These nanomaterials are characterized by their exceptional load-bearing capacity, which makes them useful, for example, as nano-reinforced materials. Moreover, they exhibit excellent thermal and electrical properties [4] and also excellent chemical stability [8]. Properties of CNTs can be further improved by functionalization—the introduction of chemical groups that facilitate maintaining a stable dispersion and compatibility with the base oil [8].

One of the more significant innovations in recent years is the use of hybrid nanomaterials, examples of which are shown in Table 1. These systems combine the unique advantages of multiple materials—for instance, CNTs and nano-ZnO [31], or CNTs embedded in graphene matrices [3]—resulting in synergistic enhancements of lubricant properties. Such combinations also allow tailoring of additive behavior across different lubrication regimes, from boundary to hydrodynamic.

The data collected in Table 1, as well as those presented previously, indicate important directions for further action regarding nanolubricants. Nano-additives for lubricants constitute a very diverse group of materials with different chemical compositions and particle morphologies. This means that they can interact in the lubrication system in different ways and with different effectiveness. Therefore, although some generalizations can be made, the type of nanoparticles, their concentration, and possible modifications should be determined individually for a given system, considering different lubrication regimes. The mechanism of nanoparticle interaction is directly related to the properties of the nanolubricant in the friction system; therefore, research on these issues should be continued. The challenge is to develop new hybrid nano-additives with improved effectiveness, as well as new effective techniques for maintaining a long-term stable dispersion of nanoparticles in the lubricant. The research is particularly important when considering a transition from laboratory-scale synthesis to industrial applications. In practice, the final performance of a nanolubricant depends on the interplay between nanoparticle characteristics, base oil properties, and operating parameters such as load, temperature, and contact geometry. Colloidal stability remains a bottleneck for commercial deployment. Finally, environmental and health considerations, life cycle assessment (LCA), and cost-efficiency should also be considered. In summary, the use of nanoparticles in lubricants offers a versatile and effective pathway to improving tribological performance while potentially reducing dependency on conventional chemical additives. As evidenced by the diversity of the kinds of nanoparticles illustrated in Figure 1, the different mechanisms of their action in the lubricating system, and the data presented in Table 1 and discussed previously, the design of next-generation nanolubricants must be informed by a holistic understanding of materials science, fluid mechanics, and surface chemistry. Future developments will likely emphasize hybrid systems, in situ tribofilm characterization, and AI-guided optimization of nanoparticle–oil interactions.

Table 1. Comparison of selected nano-additives and their influence on the properties of the nanolubricant in the friction system.

Nanoparticle	Average Particle Size, nm	Base Oil, Nano-Additive Concentration	Way to Improve Dispersion	Results	References
TiO2	10	commercial engine oil, 0.05 to 0.50 wt%	oleic acid (surface modifier), magnetic stirring	0.25 wt% (optimal concentration); rolling effect. ↓ COF, ↓ power losses, ↑ anti-wear performance	[14]
TiO2	5	low-viscosity base oil, 0.10 to 0.50 wt%	oleic acid (surface modifier), ultrasonic homogenization	0.35 wt% (optimal concentration), adsorbed tribofilm, polishing and mending effects; ↓ COF	[23]
Al2O3	8–12	commercial engine oil, 0.05 to 0.5 wt%	oleic acid (surface modifier), magnetic stirring	0.25 wt% (optimal concentration); formation of self-laminating protective layers; ↓ COF, ↓ power losses, ↑ anti-wear performance	[14]
SiO2	18–35	engine oil, 0.25 to 1.00 wt%	silane coupling agent, magnetic stirring, and then ultrasonic homogenization	surface modification improved the physicochemical properties and stability of the lubricant	[22]
CNTs	size and length: 2 and 8 (single-walled CNTs); 20 and 8 (multi-walled CNTs)	commercial engine oil, 0.1 to 2.0 wt%	mechanical mixing and then ultrasonic homogenization	accumulation of CNTs on the surface; ↓ COF, ↓ wear scar diameter, kinematic viscosity increased with concentration; multi-walled CNTs yielded more favorable results	[36]
ZnO + CNTs (hybrid formulation with different mass ratios)	ZnO: 40, CNTs: 20 (outer diameter) and 5–20 (length)	commercial engine oil, 0.25 to 1.0 wt%	oleic acid (surfactant), ultrasonic homogenization	synergistic effect (particularly at 0.25 wt%); ↓ COF, ↓ wear volume	[31]
Chemically modified Graphene + CNTs (hybrid formulation with different mass ratios)		n-Hexadecane, 0.1 to 2.0 wt%	ultrasonic homogenization	0.3 wt% (optimal concentration); some CNTs were put into the interlayer of graphene; ↓ COF, ↓ wear of the friction surfaces	[3]
Graphene oxide		white oil, 0.02 to 0.12 wt%	dodecanethiol (surface modifier) and two-component gelators, magnetic stirring and then ultrasonic homogenization	stable dispersion for over a year; two kinds of protective layers: tribochemical reaction film and physical adsorption film; ↓ COF, ↓ wear scar diameter	[34]
Nanographite	~50	engine oil, 0.3 wt%	surfactant, ultrasonic homogenization	superior tribological and thermophysical properties, improving engine performance	[37]

↑ and ↓ indicate an increase or reduction, respectively, of a given characteristic for the nano-lubricant compared to the base oil.

Table 1. Comparison of selected nano-additives and their influence on the properties of the nanolubricant in the friction system.

Nanoparticle	Average Particle Size, nm	Base Oil, Nano-Additive Concentration	Way to Improve Dispersion	Results	References
TiO2	10	commercial engine oil, 0.05 to 0.50 wt%	oleic acid (surface modifier), magnetic stirring	0.25 wt% (optimal concentration); rolling effect. ↓ COF, ↓ power losses, ↑ anti-wear performance	[14]
TiO2	5	low-viscosity base oil, 0.10 to 0.50 wt%	oleic acid (surface modifier), ultrasonic homogenization	0.35 wt% (optimal concentration), adsorbed tribofilm, polishing and mending effects; ↓ COF	[23]
Al2O3	8–12	commercial engine oil, 0.05 to 0.5 wt%	oleic acid (surface modifier), magnetic stirring	0.25 wt% (optimal concentration); formation of self-laminating protective layers; ↓ COF, ↓ power losses, ↑ anti-wear performance	[14]
SiO2	18–35	engine oil, 0.25 to 1.00 wt%	silane coupling agent, magnetic stirring, and then ultrasonic homogenization	surface modification improved the physicochemical properties and stability of the lubricant	[22]
CNTs	size and length: 2 and 8 (single-walled CNTs); 20 and 8 (multi-walled CNTs)	commercial engine oil, 0.1 to 2.0 wt%	mechanical mixing and then ultrasonic homogenization	accumulation of CNTs on the surface; ↓ COF, ↓ wear scar diameter, kinematic viscosity increased with concentration; multi-walled CNTs yielded more favorable results	[36]
ZnO + CNTs (hybrid formulation with different mass ratios)	ZnO: 40, CNTs: 20 (outer diameter) and 5–20 (length)	commercial engine oil, 0.25 to 1.0 wt%	oleic acid (surfactant), ultrasonic homogenization	synergistic effect (particularly at 0.25 wt%); ↓ COF, ↓ wear volume	[31]
Chemically modified Graphene + CNTs (hybrid formulation with different mass ratios)		n-Hexadecane, 0.1 to 2.0 wt%	ultrasonic homogenization	0.3 wt% (optimal concentration); some CNTs were put into the interlayer of graphene; ↓ COF, ↓ wear of the friction surfaces	[3]
Graphene oxide		white oil, 0.02 to 0.12 wt%	dodecanethiol (surface modifier) and two-component gelators, magnetic stirring and then ultrasonic homogenization	stable dispersion for over a year; two kinds of protective layers: tribochemical reaction film and physical adsorption film; ↓ COF, ↓ wear scar diameter	[34]
Nanographite	~50	engine oil, 0.3 wt%	surfactant, ultrasonic homogenization	superior tribological and thermophysical properties, improving engine performance	[37]

↑ and ↓ indicate an increase or reduction, respectively, of a given characteristic for the nano-lubricant compared to the base oil.

3. Biolubricants

The growing demand for sustainable and environmentally benign lubricants has driven significant advancements in the development of biolubricants derived from renewable resources and waste materials. This section reviews recent progress in the field, with a particular focus on vegetable oils, biodegradable polymers, polyols, and other promising materials—especially those sourced from industrial and agricultural waste streams. Emphasis is placed on innovative strategies for valorizing waste resources to produce biolubricants, highlighting their tribological, physicochemical, and environmental performance [38,39,40].

Biolubricants are increasingly recognized as viable, eco-friendly alternatives to conventional petroleum-based lubricants. Sourced from renewable and biodegradable feedstocks, they address critical concerns related to environmental pollution, sustainability, and the depletion of fossil resources. Recent research has emphasized the utilization of underexploited materials, particularly waste-derived feedstocks, to enhance resource efficiency and support circular economy principles by converting industrial and agricultural residues into high-value lubricants [41,42,43].

The global transition toward environmentally sustainable industrial practices has accelerated the development and adoption of biolubricants. These lubricants are primarily derived from vegetable oils and waste cooking oils (WCOs), offering renewable, biodegradable, and non-toxic alternatives to mineral-based lubricants. However, the inherent physicochemical limitations of raw oils, such as low oxidative stability, poor cold flow behavior, and limited thermal resistance, necessitate targeted chemical modifications to meet the performance standards required for industrial applications [38,40,44].

Vegetable oils and WCOs are predominantly composed of triacylglycerols (TAGs), in which three fatty acid chains are esterified to a glycerol backbone. The performance characteristics of these oils—such as viscosity, pour point, and oxidative stability—are largely determined by the degree of saturation and the molecular structure of the constituent fatty acids [38,45,46].

To enhance these properties, three primary chemical modification strategies are commonly employed: transesterification, epoxidation, and ring-opening reactions, as illustrated in Figure 2. These methods aim to tailor the molecular structure of the oils to improve their physicochemical and tribological behavior while maintaining their biodegradability and environmental compatibility [38,47].

Transesterification is a fundamental step in biolubricant synthesis and other useful and eco-friendly products, like biofuel or biosolvents from waste materials [48,49,50]. This process involves the reaction of triglycerides with short-chain alcohols (typically methanol) in the presence of a base catalyst (e.g., NaOH), resulting in the formation of fatty acid methyl esters (FAMEs) and glycerol. For instance, waste cooking oil methyl esters (WCOMEs) are typically synthesized using a methanol-to-oil molar ratio of 6:1, with 1% NaOH catalyst at 60 °C for 90 min [43,44]. The resulting esters exhibit improved cold flow properties and reduced viscosity, making them suitable intermediates for further chemical modification [38].

The epoxidation of FAMEs targets the unsaturated carbon–carbon double bonds (C=C) within the fatty acid chains, converting them into more stable oxirane (epoxy) rings. This transformation is typically achieved through the in situ generation of peracids—commonly from hydrogen peroxide and glacial acetic acid. Epoxidation significantly enhances the oxidative stability of the esters and introduces reactive sites for subsequent chemical modifications, particularly ring-opening reactions [51,52].

Analytical validation using, e.g., Fourier-transform infrared spectroscopy (FTIR) confirms successful epoxidation by the disappearance of olefinic C–H stretching peaks (~3004 cm⁻¹) and the emergence of characteristic epoxy bands in the range of ~823–842 cm⁻¹, indicating the formation of epoxidized waste cooking oil methyl esters (EWCOMEs) [53,54].

The final modification step involves ring-opening of the epoxidized esters using long-chain alcohols such as 1-heptanol or 2-ethylhexanol in the presence of an acid catalyst, typically p-toluenesulfonic acid. This reaction introduces hydroxyl-terminated side chains and results in the formation of branched diesters. These structural modifications enhance the viscosity, reduce the pour point, and improve the overall lubrication performance of the resulting biolubricants [55,56].

FTIR spectra of the ring-opened products exhibit broad O–H stretching bands (~3450–3477 cm⁻¹), while ^1HNMR analysis confirms the disappearance of epoxy proton signals, indicating successful ring-opening and conversion to hydroxylated esters [53,57].

Physicochemical and tribological evaluations of the synthesized biolubricants demonstrate significant performance improvements. The viscosity index (VI) increased to values between 137 and 156, the pour point was reduced to as low as −18 °C, and the flash point exceeded 220 °C. Tribological testing revealed a COF as low as 0.02—substantially lower than that of conventional mineral lubricants (typically ~0.07)—and a wear scar diameter (WSD) comparable to commercial benchmarks [53,58]. These enhancements render the modified WCO-derived esters suitable for a wide range of applications, including hydraulic systems, industrial machinery, and automotive lubrication under both mild and heavy-duty operating conditions [53,59].

Utilizing WCO as a feedstock aligns with circular economy principles, mitigates waste disposal challenges, and prevents the unauthorized reuse of spent oils. The valorization of WCO into high-performance biolubricants not only supports global sustainability goals but also offers a cost-effective alternative to lubricants derived from edible oils or synthetic sources [53,60].

3.1. Vegetable Oils

The transition toward environmentally sustainable lubricants has intensified research into bio-based alternatives to conventional mineral oils. Among these, vegetable oils have garnered significant attention due to their renewable origin, high lubricity, biodegradability, and low toxicity (as illustrated in Figure 3). These attributes make them promising candidates for biolubricant applications. However, as mentioned in the earlier parts of this paper, their direct use is often constrained by inherent limitations such as oxidative instability and poor low-temperature performance.

Non-edible vegetable oils offer the dual benefits of sustainability and reduced competition with food resources. Through targeted chemical modifications—such as transesterification, epoxidation, and functionalization—vegetable oils can be tailored to meet the performance requirements of modern lubrication systems while maintaining their ecological benefits [38,53,61].

Vegetable oils have long been recognized for their lubricating properties, with historical use dating back to ancient civilizations. In the modern context, increasing environmental concerns and the depletion of petroleum reserves have renewed interest in plant-derived oils as sustainable alternatives to mineral-based lubricants [62,63]. Their molecular structure—primarily composed of triglycerides with long-chain fatty acids—confers several inherent advantages, including excellent lubricity, a high viscosity index (VI), and low volatility (presented in

Table 2. Physical properties and thermo-oxidative performance data of various vegetable oils and their modified derivatives used in lubricant applications.

Vegetable Oil	Modification	Biolubricant	Viscosity of Pure Oil (cSt)		Viscosity of Modified Oil (cSt)		Viscosity Index		Pour Point of Modified Oil (°C)	Flash Point of Modified Oil (°C)	Oxidation Onset Temperature (°C)		Oxidation Induction Time (min)		Reference
			40 °C	100 °C	40 °C	100 °C	Pure Oil	Modified Oil			Pure Oil	Modified Oil	Pure Oil	Modified Oil
Edible oils
Canola	Two-stage transesterification	Canola FAME biolubricant (AV 0.39 mg KOH/g)	34.9	8.1	10.04	4.09	219	377	−10	222	164 339	ND	60.28–67.05, 3.4 h, 37.41 (140 °C)	0.94 (h)	[61,65,66,67,68]
	Two-stage transesterification	Canola TMP ester			154.50	18.03		121	−9	98		401		ND
Coconut	Two-stage transesterification	Coconut TMP ester	27.6	5.9	12.87	4.12	166	259	7	140	241,257	345	44.86 (140 °C)	ND	[66,69,70]
Grapeseed	Transesterification	FAEE	26.92	ND	4.32	ND	ND	ND	−8	ND	ND	ND	7.53 (140 °C)	2.01 h	[71,72]
Olive	Two-stage transesterification	Canola TMP ester	ND	ND	72.29	13.85	ND	199	-8	116	178.65–192.28 377	384	13.94 (140 °C)	ND
Palm	Hydrolysis	CPOFAs	ND	ND	52.9	9.2	ND	130	10	240	179	ND	14 (RPVOT) 82.36 (140°C)	ND	[73,74,75,76]
	Hydrolysis + esterification	CPOFAs-NPG diester			50.1	15.5		190	7	245		ND		ND
	Enzymatic esterification	HO-PME:TMP triester			43.7	9.3		203	−37	ND		ND		41 (RPVOT)
	Enzymatic esterification	HO-PME:TMP triester			46.2	9.5		195	−48	ND		ND		ND
	Two-stage transesterification	Palm TMP ester			22.95	5.54		188	11	152		362		ND
Rice bran	Epoxidation + ring opening	Epoxidized RBO Ring opened RBO	41.13	5.19	89.28 174.8	ND	ND	ND	−4 −7	ND	ND	ND	10.11 (RPVOT)	ND	[77,78,79]
	Two-stage transesterification	Rice bran TMP ester			32.25	4.55		ND	−6	ND		ND		14.21 (RPVOT)
	Epoxidation	Epoxidized rice bran oil			93.51	14.94		ND	−3	ND		ND		22.47
Sesame	Transesterification	SEOTMPE	25.78	ND	35.43	7.93	ND	206	−12	ND	246 370	ND	69.55 (140 °C)	378 (110 °C)	[66,69,80,81,82,83]
Soybean	Epoxidation + transesterification+ oxirane ring opening (hydroxylation) + acetylation	Crude	32.93	8.06	ND	ND	233.608	230	−9	177	371 173.1	ND	20 (140 °C)	ND	[43,66,81,84,85]
		SOY1 (epoxidized)			ND	ND		147	−12	183		ND		ND
		SOY2 (tranesterified)			ND	ND		153	−9	186		ND		ND
		SOY3 (oxirane ring opened)			ND	ND		183	−15	83		ND		ND
		SOY4 (acetylated)			ND	ND		194	−21	192		350		ND
	Two-stage transesterification	EDGE			21.30	6.31		281	−5	ND		200		ND
Sunflower	Transesterification	EDGE	36.8	8.5	11.24	3.35	218	196	−4	ND	165.81 342	200	1.1 h 19.96 (140 °C)	ND	[43,65,66,85,86]
Non-edible oils
Castor	Two-stage transesterification	Castor FAME biolubricant (AV 0.45 mg KHO/g)	281.8	72.53	208.25	26.74	321	163	−16	271	ND	ND	ND	ND	[67,87,88]
	Esterification + epoxidation + oxirane ring opening	COFA			130.7	12.76		83	−24	ND		ND		0.31 h
		2E1H			28.25	5.03		104	−3.9	ND		ND		3.6 h
		E2E1H			57.49	7.79		99	−36	ND		ND		4.3 h
		BIOWAT			472.78	62.25		51	−9	ND		251.5-297.1		3.27 h
		BIOBUT			23.53	8.12		97	−48	ND		233-265.7		4.22 h
Cottonseed	Hydrolysis + esterification + ring opening and branching	Crude (AV 5.88 mg/g)	34.3	8.0	32.55	7.25	216	197	−3	320	343 159	370.01 303.11	1.9 h	13	[61,65,89]
		Isodecyl ester (esterified, AV 1.02 mg/g)			8.74	2.75		175	−59	ND		ND		ND
		Epoxide isodecyl ester (AV 10.64 mg/g)			22.56	4.77		136	10	ND		ND		ND
		Nonyl branched isodecyl ester (ring opening and branching, AV 0.65 mg/g)			200.3	23		141	−47	254		310.11		21
Jatropha	Transesterification	EDGE	34.6	8.0	14.60	4.73	213	311	−12	ND	322 169	200	2.6 h	ND	[43,65,90]
	Esterification	TMPJO			51	8.3		136	−6.5	ND		ND		ND
	Esterification + epoxidation	ETMPJO			160	21.6		160	−2.1	ND		ND		ND
Karanja	Two-stage transesterification	Karanja TMP ester	43.39	6.93	33.33	4.76	117.176	ND	3	ND	ND	ND	13.4 (RPVOT)	19.32 (RPVOT)	[61,78]
	Epoxidation	Epoxidized Karanja oil			78.79	10.17		ND	5	ND		ND		21.17 (RPVOT)
Neem	Epoxidation	Epoxidized neem oil	54.37	10.39	78.51	11.3	134	135	ND	239	ND	ND	ND	ND	[91,92,93]

ND—Not Determined.

). However, as shown in Table 3, practical application in tribological systems is often limited by poor oxidative stability and suboptimal cold flow behavior [62,64].

Castor oil, extracted from the seeds of Ricinus communis L., is notable for its high content of ricinoleic acid, which imparts unique physicochemical properties such as high polarity and superior lubricity. Brazil is a major producer of castor oil, making it an economically viable and sustainable feedstock for biolubricant production [94]. Studies have shown that castor oil-based biolubricants exhibit high VI, excellent biodegradability, and superior lubricating performance [63,94]. Recent research has focused on synthesizing acylglycerol-estolides from neat castor oil via alkoxide-catalyzed transesterification and interesterification. These estolides demonstrate high kinematic viscosity (up to 635 mm²/s), a pour point of −30 °C, and a VI of 173—comparable to or exceeding commercial mineral oils [63]. Their oxidative and thermo-oxidative stability make them suitable for demanding applications such as gear oils and hydraulic fluids. Gao et al. evaluated a castor oil-based cutting fluid in oil-in-water emulsions for machining operations, reporting oxidative stability of 19.13 h, viscosity of 1.3550 cSt at 40 °C, and microbiological resistance of two days [94]. The formulation performed comparably to commercial mineral oil-based fluids, with added environmental benefits.

Brassica carinata oil, rich in erucic acid, has emerged as a promising non-edible feedstock for biolubricant synthesis. Naik et al. [64] produced biolubricants from this oil via a two-step transesterification process, yielding ethyl hexyl esters with a conversion efficiency of 84.3%. The resulting lubricants exhibited excellent tribological properties, including a pour point of −30 °C, a COF of 0.06, and a wear scar diameter (WSD) of 0.862 mm. Degaga et al. [95] further enhanced the oil’s properties through epoxidation using a bifunctional CaO–sulfated SnO₂ catalyst. This green synthesis approach improved oxidative stability, making the oil suitable for high-performance biolubricant applications.

Palm stearin, a by-product of palm oil refining, has also been explored for biolubricant production through transesterification and epoxidation. Afifah et al. [96] evaluated the rheological, thermal, and tribological properties of palm stearin methyl ester (PSME) and epoxidized palm stearin methyl ester (EPSME). EPSME exhibited the lowest COF and wear rate among the tested samples, with oxidation onset temperatures reaching up to 239.5 °C. Attanatho et al. [97] developed a hydrothermally synthesized calcium methoxide nanocatalyst for the transesterification of palm oil methyl esters, achieving conversion rates of 78–89% and selectivity of 62–80% toward biolubricant production. The resulting biolubricants demonstrated physicochemical properties comparable to ISO VG 46 mineral oils, making them suitable for use in tropical climates.

Table 3. Characteristics and applications of edible and non-edible vegetable oils for the formulation of biolubricants, showing oxidation stability, viscosity, and thermal behavior.

Vegetable Oil	Characteristics	Applications	References
Edible oils
Canola	Rich in oleic acid, offers good biodegradability and lubricity, moderate oxidative stability. Chemical modifications like double bond hydrogenation, epoxidation, and incorporation of antioxidant additives are applied to enhance stability.	Grease, diesel fuel substitutes, biodegradable grease	[38,98]
Coconut	Rich in medium-chain saturated fatty acids, primarily lauric acid. High oxidative stability and low viscosity—suitable for high-temperature lubrication systems. Poor low-temperature properties limit its standalone use. To improve performance, transesterification and epoxidation are applied.	Gas engine oil	[38,98]
Grapeseed	High proportion of linoleic acid—good lubricity but leads to low oxidative stability. Chemical modifications like epoxidation and subsequent ring-opening reactions are used to enhance thermal and oxidative performance.	Diesel fuel substitutes, automotive lubricants	[61,99]
Olive	High oleic acid content—high oxidative stability and lubricity. Limited cold-flow behavior. Typically, chemically modified via esterification and epoxidation to improve its performance under a broader temperature range.	Automotive lubricants	[38,98,100]
Palm	Abundant and cost-effective. Contains both saturated and unsaturated fatty acids—a balance of oxidative stability and lubricity. Frequently modified via epoxidation, transesterification, and branching reactions to yield biolubricants with improved pour point and viscosity indices.	Grease, rolling lubricant	[38,98]
Rice bran	High oryzanol and tocopherol content—natural antioxidant properties. It has been used in biolubricant formulations after undergoing transesterification and epoxidation, enhancing its viscosity and wear-resistance properties.	Cutting fluid	[38]
Sesame	Contains sesamol and sesamin, natural antioxidants that increase its oxidative resistance. Cold-flow properties are suboptimal. Modifications such as blending with synthetic esters or epoxidation are commonly used to improve its overall performance	Biodiesel, soap, cosmetics, lubricants	[101]
Soybean	One of the most studied oils for biolubricant development due to its availability and high content of unsaturated fatty acids. Chemically modified via epoxidation, maleation, and hydroxylation to enhance oxidative stability, viscosity index, and tribological properties.	Plasticizers, hydraulic oil, printing inks, pesticides, disinfectants	[98]
Sunflower	High linoleic acid content—good lubricity but low oxidative stability. Chemical alterations like epoxidation and esterification are employed to overcome this limitation, creating biolubricants suitable for general-purpose lubrication.	Grease, diesel fuel substitutes	[98,102]
Non-edible oils
Castor	Unique due to the presence of ricinoleic acid—imparts hydroxyl functionality directly in the triglyceride structure. Excellent lubricity and polarity. Its derivatives, such as estolides and polyesters, are widely used in biolubricant formulations without needing drastic modifications	Gear lubricants, greaces, fuel, and biodiesel	[98,103]
Cottonseed	Blend of saturated and unsaturated fatty acids. It is a by-product of cotton farming, making it cost-effective. High linoleic acid content requires epoxidation or blending to improve oxidative stability and cold-flow behavior.	Synthetic resins, inks, pastes, metal soaps, waxes, insecticides	[104,105]
Jatropha	Suitable for industrial biolubricant applications. It undergoes extensive modifications such as transesterification, epoxidation, and polymerization to develop high-performance lubricants, especially for heavy-duty applications.	Biodiesel, producing soap, and biocides	[106,107]
Karanja	Good saponification value and fatty acid composition. Modified via processes like transesterification and esterification to enhance viscosity, oxidation stability, and biodegradability.	Biodiesel, automotive lubricant	[78,108]
Neem	Antimicrobial properties and unsaturated fatty acid content. Despite being underutilized, it has potential in specialty biolubricants, especially where anti-corrosive and antifungal properties are desired. It is typically subjected to epoxidation and subsequent modifications.	Machining mild steel	[109]

Table 3. Characteristics and applications of edible and non-edible vegetable oils for the formulation of biolubricants, showing oxidation stability, viscosity, and thermal behavior.

Vegetable Oil	Characteristics	Applications	References
Edible oils
Canola	Rich in oleic acid, offers good biodegradability and lubricity, moderate oxidative stability. Chemical modifications like double bond hydrogenation, epoxidation, and incorporation of antioxidant additives are applied to enhance stability.	Grease, diesel fuel substitutes, biodegradable grease	[38,98]
Coconut	Rich in medium-chain saturated fatty acids, primarily lauric acid. High oxidative stability and low viscosity—suitable for high-temperature lubrication systems. Poor low-temperature properties limit its standalone use. To improve performance, transesterification and epoxidation are applied.	Gas engine oil	[38,98]
Grapeseed	High proportion of linoleic acid—good lubricity but leads to low oxidative stability. Chemical modifications like epoxidation and subsequent ring-opening reactions are used to enhance thermal and oxidative performance.	Diesel fuel substitutes, automotive lubricants	[61,99]
Olive	High oleic acid content—high oxidative stability and lubricity. Limited cold-flow behavior. Typically, chemically modified via esterification and epoxidation to improve its performance under a broader temperature range.	Automotive lubricants	[38,98,100]
Palm	Abundant and cost-effective. Contains both saturated and unsaturated fatty acids—a balance of oxidative stability and lubricity. Frequently modified via epoxidation, transesterification, and branching reactions to yield biolubricants with improved pour point and viscosity indices.	Grease, rolling lubricant	[38,98]
Rice bran	High oryzanol and tocopherol content—natural antioxidant properties. It has been used in biolubricant formulations after undergoing transesterification and epoxidation, enhancing its viscosity and wear-resistance properties.	Cutting fluid	[38]
Sesame	Contains sesamol and sesamin, natural antioxidants that increase its oxidative resistance. Cold-flow properties are suboptimal. Modifications such as blending with synthetic esters or epoxidation are commonly used to improve its overall performance	Biodiesel, soap, cosmetics, lubricants	[101]
Soybean	One of the most studied oils for biolubricant development due to its availability and high content of unsaturated fatty acids. Chemically modified via epoxidation, maleation, and hydroxylation to enhance oxidative stability, viscosity index, and tribological properties.	Plasticizers, hydraulic oil, printing inks, pesticides, disinfectants	[98]
Sunflower	High linoleic acid content—good lubricity but low oxidative stability. Chemical alterations like epoxidation and esterification are employed to overcome this limitation, creating biolubricants suitable for general-purpose lubrication.	Grease, diesel fuel substitutes	[98,102]
Non-edible oils
Castor	Unique due to the presence of ricinoleic acid—imparts hydroxyl functionality directly in the triglyceride structure. Excellent lubricity and polarity. Its derivatives, such as estolides and polyesters, are widely used in biolubricant formulations without needing drastic modifications	Gear lubricants, greaces, fuel, and biodiesel	[98,103]
Cottonseed	Blend of saturated and unsaturated fatty acids. It is a by-product of cotton farming, making it cost-effective. High linoleic acid content requires epoxidation or blending to improve oxidative stability and cold-flow behavior.	Synthetic resins, inks, pastes, metal soaps, waxes, insecticides	[104,105]
Jatropha	Suitable for industrial biolubricant applications. It undergoes extensive modifications such as transesterification, epoxidation, and polymerization to develop high-performance lubricants, especially for heavy-duty applications.	Biodiesel, producing soap, and biocides	[106,107]
Karanja	Good saponification value and fatty acid composition. Modified via processes like transesterification and esterification to enhance viscosity, oxidation stability, and biodegradability.	Biodiesel, automotive lubricant	[78,108]
Neem	Antimicrobial properties and unsaturated fatty acid content. Despite being underutilized, it has potential in specialty biolubricants, especially where anti-corrosive and antifungal properties are desired. It is typically subjected to epoxidation and subsequent modifications.	Machining mild steel	[109]

Neat vegetable oils such as avocado, peanut, rapeseed, and olive oil have long been recognized for their excellent lubricity, primarily due to their high content of mono- and polyunsaturated fatty acids. These oils form stable adsorbed films on metal surfaces through polar interactions, effectively reducing friction and wear [110,111]. However, the presence of unsaturated bonds renders them susceptible to oxidative degradation, limiting their thermal stability and long-term performance in demanding tribological environments [62,64].

Studies by Reeves et al. [81,112,113] demonstrated that oils with a high oleic acid content—such as avocado and rapeseed oils—exhibit superior tribological performance in pin-on-disk configurations. These oils showed lower COF and reduced wear volumes compared to oils rich in polyunsaturated fatty acids, underscoring the critical role of fatty acid composition in determining lubrication behavior.

To address oxidative limitations, epoxidized vegetable oils such as epoxidized avocado oil (EAv) and epoxidized peanut oil (EPn) have been extensively studied. Epoxidation enhances thermal-oxidative stability and increases viscosity, making these oils suitable for high-temperature applications [81]. For example, EAv exhibited an oxidation onset temperature of 199.2 °C, compared to 181.9 °C for unmodified avocado oil, indicating a significant improvement in thermal resistance. Similarly, EPn demonstrated enhanced viscosity and oxidative stability, contributing to improved load-carrying capacity and reduced friction under tribological testing [81,112].

Residual fatty acids (RFAs), by-products of vegetable oil refining, are increasingly being valorized for biolubricant production. Melo Neta et al. [62] synthesized biolubricants from RFAs primarily composed of palmitic and oleic acids. These biolubricants achieved a 54% reduction in friction coefficient compared to commercial mineral oils and exhibited excellent thermal stability, with 50% mass loss temperatures reaching 341.68 °C under inert conditions. The integration of such waste streams into biolubricant synthesis not only addresses waste management challenges but also yields high-value, environmentally friendly products.

The transesterification of methyl oleate with trimethylolpropane (TMP) produces trimethylolpropane fatty acid triesters (TFATEs), which are known for their superior oxidative stability and low-temperature performance. Gao et al. [114] utilized a K₂CO₃/activated carbon catalyst to achieve a TFATE selectivity of 93.7% under optimized conditions, with microwave heating employed to enhance reaction kinetics and reduce processing time. de Brito et al. [115] synthesized novel polyols analogous to neopentyl glycol and TMP, which were subsequently esterified with oleic acid. The resulting biolubricants exhibited high viscosity indices (189–222), excellent thermo-oxidative stability (onset temperatures between 258 and 285 °C), and low coefficients of friction (0.021–0.041), making them suitable for industrial tribological applications.

Considering the challenges of modern tribology, it is worth emphasizing once again that the utilization of vegetable oils and associated waste streams for biolubricant production offers substantial environmental benefits, including biodegradability, low toxicity, and reduced carbon footprint. These materials align with circular economy principles and support the development of sustainable industrial practices [62]. The economic feasibility of biolubricants depends on factors such as raw material availability, production costs, and market demand. Continued advancements in catalyst development and process optimization are expected to enhance the commercial viability of these renewable lubricants [95,97].

The comparative table above (Table 3) provides insights into how different plant- and insect-derived oils perform as lubricant base stocks. While all feedstocks are renewable and biodegradable, their tribological and physicochemical performance varies significantly. Canola and soybean oils are abundant but require significant oxidative stabilization. Castor oil has excellent film strength due to hydroxyl functionality but suffers from poor low-temperature performance. Palm oil offers a balanced profile but faces scrutiny over land use. Black soldier fly (BSF) larvae oil presents an emerging alternative with high oxidative stability and favorable viscosity behavior, particularly when optimized using AI-based models such as ANN-GWO (described in more detail in Section 5).

Table 4. Vegetable oils’ key properties, limitations, and common chemical modification comparison.

Feedstock	Key Properties	Main Limitations	Chemical Modifications	Reference
Canola oil	Moderate VI, good film strength	Low oxidation resistance	Epoxidation, antioxidant addition	[67,68,69,70]
Soybean oil	Good lubricity	Prone to oxidation	TMP esters, hydroxylation	[83,84,85,86,87]
Castor oil	High VI, strong polarity	High viscosity	Estolide formation	[88,89,90]
Palm oil	Balanced fatty acid profile	Requires thermal stability boost	Transesterification	[75,76,77,78]
BSF larvae oil	High oxidative stability, unique lipids	Processing complexity	ANN-GWO optimized blending	[116,117,118]

further underscores the importance of chemical tailoring, as raw feedstocks often fail to meet industrial lubricant standards without modification.

In summary, vegetable oils—particularly those derived from non-edible sources and industrial waste—represent a sustainable and environmentally responsible alternative to conventional lubricants. Recent progress in chemical modification techniques, catalytic processes, and tribological performance evaluation has demonstrated their potential for a wide range of industrial applications. Future research should prioritize scaling production, improving oxidative stability, and conducting comprehensive life cycle assessments to support the broader adoption of vegetable oil-based biolubricants.

3.2. Waste Materials

The increasing demand for sustainable and environmentally benign alternatives to petroleum-based lubricants has intensified research into renewable feedstocks. Among these, waste-derived materials have emerged as promising candidates for biolubricant production due to their abundance, low cost, and environmental advantages. The potential of these materials to serve as base oils or functional additives is critically examined, with emphasis on their physicochemical properties, tribological performance, and environmental impact. The lubrication industry is undergoing a paradigm shift toward sustainable and circular economy models, where waste valorization plays a central role. Materials such as cashew nut shell liquid (CNSL), BSF larvae oil, and WCO are being increasingly explored for their potential in tribological applications [61,116,119,120].

CNSL, a by-product of cashew processing, is rich in cardanol—a phenolic lipid with high reactivity. Tianjiao Li et al. [119] reported the synthesis of cardanyl acetate (CA) via esterification of cardanol extracted from CNSL. The resulting biolubricant exhibited a high VI, excellent thermal stability, and superior tribological performance compared to synthetic basestocks such as polyalphaolefin (PAO2) and coal-to-liquid (CTL3) oils. Notably, CA achieved a wear scar diameter (WSD) of 0.54 mm—significantly lower than PAO2 (0.85 mm) and CTL3 (0.90 mm)—and demonstrated the highest last non-seizure load capacity of 510 N in extreme pressure tests, confirming its suitability for industrial lubrication [119].

BSF larvae oil, derived from organic waste bioconversion, represents a novel and sustainable feedstock. Silitonga et al. [120] synthesized biolubricants from BSF oil via transesterification followed by esterification with TMP. Using artificial neural network—gray wolf optimizer (ANN-GWO) algorithms, the study optimized tribological parameters such as load, time, and temperature to minimize the COF. The resulting biolubricants exhibited performance comparable to commercial SAE 15W-40 lubricants, validating the potential of insect-based oils in tribological systems [120].

WCO is one of the most widely studied waste feedstocks due to its availability, low cost, and high conversion efficiency. Álvarez et al. [116] demonstrated a two-step transesterification process using methanol and TMP, achieving biolubricant yields exceeding 98%. The resulting products exhibited high viscosity (127 cSt at 40 °C) and oxidative stability (6 h by Rancimat (Metrohm, AG, Herisau, Switzerland)) [116], meeting industrial standards. Joshi et al. [53] further enhanced WCO-derived esters through epoxidation and ring-opening reactions, significantly improving their oxidative and thermal stability and yielding tribological properties comparable to commercial lubricants.

Riayatsyah et al. [117] optimized the synthesis of TMP esters from WCO methyl esters using response surface methodology (RSM). The optimized process achieved a biolubricant yield of 96.12%, with a kinematic viscosity of 41.55 mm²/s at 40 °C, a COF of 0.045 (lower than SAE 15W-40 at 0.062), and a viscosity index of 125.30, indicating excellent temperature stability and tribological performance [117].

Biju et al. [118] tested coconut oil subjected to multi-stage chemical modification, i.e., epoxidation and then trans-esterification with methanol. They found that the properties of chemically modified waste oils, important when they are considered as biolubricants, have been improved, especially in the case of trans-esterified oil.

Biolubricants derived from waste materials consistently demonstrate favorable physicochemical properties, including high viscosity indices, ensuring performance across a wide temperature range; low pour points and high flash points, enhancing safety and versatility; superior tribological behavior, evidenced by reduced COF and WSD compared to conventional mineral oils [53,61,116,117,119,120]. These findings underscore the viability of waste-derived materials as sustainable, high-performance alternatives in the lubricant industry.

The utilization of waste-derived materials in biolubricant production aligns closely with global sustainable development goals by addressing waste disposal challenges, reducing environmental pollution, and offering biodegradable, non-toxic alternatives to petroleum-based lubricants. Moreover, the valorization of industrial and agricultural residues contributes to cost-effective production and supports circular economy principles [53,61].

Agro-waste, animal by-products, and WCO have demonstrated considerable potential as renewable feedstocks for high-performance biolubricants. These materials not only reduce reliance on fossil resources but also exhibit excellent tribological properties, making them suitable for a wide range of industrial applications. Future research should prioritize scaling up production, conducting life cycle assessments (LCAs), and developing standardized testing protocols to facilitate broader industrial adoption.

The growing demand for eco-friendly lubricants has accelerated the development of biolubricants derived from renewable and waste-based feedstocks. Conventional petroleum-based lubricants, while effective, pose significant environmental risks due to their poor biodegradability and toxicity [38]. In contrast, waste-derived oils—such as WCO, agro-industrial by-products, and animal fats—offer a sustainable alternative, contributing to circular economy models by transforming waste into high-value products [121,122].

WCO is particularly attractive due to its abundance and low cost. Composed primarily of long-chain triglycerides, WCO can be chemically modified to improve oxidative stability, viscosity, and pour point. Singh et al. [122] developed biolubricants by blending WCO with garlic essential oil (GO), leveraging its antioxidant properties to enhance thermo-oxidative stability. Formulations containing 10%, 20%, and 30% v/v GO showed improved viscosity indices and reduced pour points, with the GL-30 formulation achieving a pour point of −29 °C and a VI of 148. Tribological testing revealed a 13% reduction in the COF and a 5% reduction in wear scar diameter compared to conventional mineral oils [122].

Oliveira et al. [121] optimized the esterification of oleic acid (from WCO) with trimethylolpropane (TMP) using heterogeneous catalysts based on zirconium and sulfate-modified KIT-6 mesoporous silica. Using Response Surface Methodology (RSM), they achieved an 86% conversion yield. The resulting biolubricants exhibited high oxidative stability, viscosity indices exceeding 150, and favorable friction-reducing properties.

Jatropha oil, derived from non-edible seeds, has also been explored as a sustainable feedstock. Reséndiz-Calderón et al. [123] evaluated its application in minimum quantity lubrication (MQL) for sheet metal forming. The study showed that Jatropha oil reduced mass loss in stainless steel and aluminum pins by 50% and 70%, respectively, and significantly lowered the COF compared to mineral oils.

Table 5 presents a summarized comparison of the most popular waste materials with biolubricant application in terms of source, environmental benefits, challenges in processing system, and performance indicators.

Table 5. Performance of waste-derived feedstocks for biolubricants.

Waste Feedstock	Source	Performance Indicators	Environmental Benefit	Processing Challenges	Ref.
Waste Cooking Oil (WCO)	Households, restaurants	COF ↓ to 0.02, VI ↑ to 150+	Circular economy, prevents improper disposal	Requires purification, multi-step synthesis	[53,116,117,122]
Cashew Nutshell Liquid (CNSL)	Agro-industrial waste	WSD ↓, excellent pressure resistance	High value from waste	Toxicity control, reactive handling	[119]
BSF Larvae Oil	Bioconverted organic waste	Comparable to SAE 15W-40	Sustainable insect farming	Public acceptance, oil extraction yield	[117,120]
Residual Fatty Acids (RFAs)	Vegetable oil refining	Low COF, high oxidative resistance	Reduction in industrial waste	Highly variable composition	[62]

↑ and ↓ indicate an increase or reduction, respectively, of a given characteristic for the biolubricant.

Table 5. Performance of waste-derived feedstocks for biolubricants.

Waste Feedstock	Source	Performance Indicators	Environmental Benefit	Processing Challenges	Ref.
Waste Cooking Oil (WCO)	Households, restaurants	COF ↓ to 0.02, VI ↑ to 150+	Circular economy, prevents improper disposal	Requires purification, multi-step synthesis	[53,116,117,122]
Cashew Nutshell Liquid (CNSL)	Agro-industrial waste	WSD ↓, excellent pressure resistance	High value from waste	Toxicity control, reactive handling	[119]
BSF Larvae Oil	Bioconverted organic waste	Comparable to SAE 15W-40	Sustainable insect farming	Public acceptance, oil extraction yield	[117,120]
Residual Fatty Acids (RFAs)	Vegetable oil refining	Low COF, high oxidative resistance	Reduction in industrial waste	Highly variable composition	[62]

↑ and ↓ indicate an increase or reduction, respectively, of a given characteristic for the biolubricant.

The biodiesel industry generates large quantities of glycerol as a by-product. Almeida de Brito et al. [115] utilized diglycerol and triglycerol to synthesize long-chain esters with oleic acid and its derivatives. The resulting polyesters—such as diglycerol tetraoleate (DGMO) and triglycerol pentaoleate (TGOA)—exhibited high viscosity indices (up to 222), excellent thermo-oxidative stability, and biodegradability. When used as additives in mineral oils, these polyesters enhanced viscosity and melting points, demonstrating their potential in high-performance lubricant formulations.

3.3. Other Biomaterials Considered for Biolubricant Applications

Ionic liquids (ILs) are emerging as effective, eco-friendly additives in biolubricant technology due to their unique properties: high thermal and chemical stability, low volatility, and strong polarity. Their ability to form durable boundary films on metal surfaces significantly improves lubrication under extreme conditions, aligning with the principles of green tribology [110,111]. By tailoring the combination of organic cations and anions, ILs can be optimized for specific tribological applications.

Vegetable oils, though renewable and biodegradable, suffer from limited oxidative and thermal stability due to unsaturated fatty acids. Epoxidation and IL incorporation offer a synergistic improvement. Epoxidized oils gain oxirane rings that enhance oxidative resistance and molecular interaction with ILs, improving viscosity and film-forming ability [110].

Recent studies by Avilés et al. investigated epoxidized avocado (EAv) and peanut oils (EPn) with 1 wt% diethylmethylammonium methanesulfonate IL. The modified oils exhibited significantly higher oxidative onset temperatures—195.7 °C for EAv + IL and 192.0 °C for EPn + IL—compared to their unmodified versions [110,111]. These formulations also demonstrated markedly reduced friction and wear. EAv + IL decreased wear by 98.8% and COF by 74.5%, while EPn + IL achieved a 96.3% wear reduction [110,111].

X-ray photoelectron spectroscopy (XPS) and Raman analysis revealed the presence of IL- and oil-derived elements in tribofilms, contributing to improved surface protection and load-bearing capacity [111]. Viscosity measurements showed substantial increases compared to neat oils (e.g., 0.153 Pa·s for EAv + IL), and contact angle analysis confirmed better wettability on steel surfaces (43.7° for EAv + IL), promoting continuous lubricant films and reducing depletion during operation [110,111].

Overall, incorporating ILs into epoxidized vegetable oils enhances tribological and thermal performance through synergistic molecular interactions and the formation of stable, protective tribofilms [110,111].

In parallel, the development of microbially derived biolubricants has gained momentum as a sustainable alternative to petroleum-based lubricants. Microorganisms such as yeasts, fungi, and bacteria possess the metabolic capability to synthesize fatty acids, esters, and other compounds that can be further processed into high-performance lubricants [124,125]. One notable example is the use of Lipase B from Candida antarctica (CALB), a highly efficient biocatalyst for ester synthesis. CALB catalyzes the esterification of fatty acids with alcohols, producing esters such as 2-ethylhexyl oleate (2EHO), known for its excellent lubrication properties [124]. Levy et al. [124] demonstrated the biosynthesis of 2EHO using CALB immobilized on a lignin-based nanocomposite derived from cashew apple bagasse (CAB). The immobilized enzyme system achieved up to 90% conversion under optimized conditions (1:5 molar ratio of oleic acid to 2-ethyl-1-hexanol at 40 °C for 36 h), yielding a biolubricant with >90% purity, a viscosity index of 146.6, and high thermal stability. The immobilization of CALB on lignin–magnetite composites (Lig-MNP-CALB) enhanced enzyme reusability via magnetic separation and improved catalytic efficiency by reducing mass transfer limitations. The biocatalyst retained activity over multiple cycles, offering both economic and environmental advantages in biolubricant production [124]. The 2EHO produced through CALB catalysis demonstrated kinematic viscosities within industrial standards and high viscosity indices, ensuring consistent performance across temperature ranges. Thermogravimetric analysis (TGA) revealed decomposition onset temperatures above 249 °C, indicating suitability for high-temperature applications [124]. Although direct tribological testing data remain limited, the chemical composition and physicochemical properties of these esters suggest strong lubrication potential. Long-chain fatty acid esters such as 2EHO are associated with low friction coefficients and reduced wear in boundary lubrication regimes, making them promising candidates for industrial and automotive applications.

Further innovations include the synthesis of antimicrobial biolubricants through the integration of microbial enzymes with plant-based substrates. Gebreyes et al. [125] synthesized a multifunctional biolubricant from Ocimum lamiifolium leaf oil and lactic acid via graft copolymerization. The resulting product combined antimicrobial activity with favorable spreadability, stability, and diffusion characteristics, making it suitable for biomedical and personal care applications. The antimicrobial efficacy was validated against Staphylococcus aureus and Escherichia coli, demonstrating the potential for dual-function biolubricants. The use of cashew apple bagasse (CAB) lignin as a support for enzyme immobilization exemplifies the valorization of agro-industrial residues. CAB, typically considered waste, serves as a low-cost, sustainable resource for biocatalyst systems, aligning with circular economy principles and waste minimization strategies [124].

3.4. Applications, Environmental Impact, and Economic Considerations of Biolubricants

Some directions of applications of vegetable oils are presented in Table 3. The automotive sector represents a major application area for biolubricants, particularly in engine and transmission systems. Akanksha et al. [38] reviewed the modification and deployment of vegetable oil-based biolubricants in automotive applications, emphasizing the necessity of chemical modifications—such as epoxidation and transesterification—to enhance cold flow properties and oxidative stability. Biolubricants have also shown promise in metal forming operations. Reséndiz-Calderón et al. [123] demonstrated that Jatropha oil, when used in minimum quantity lubrication (MQL) systems, significantly improved the tribological performance of borided tool steels during sheet metal forming. The study reported reduced friction and wear, contributing to extended tool life and lower environmental impact.

The use of waste-derived and renewable resources for biolubricant production aligns with global sustainability goals. Environmental benefits include lower greenhouse gas emissions, enhanced biodegradability, and reduced ecotoxicity [38,121,122]. Economically, the valorization of waste streams—such as WCO and glycerol—reduces raw material costs and minimizes reliance on virgin agricultural resources, thereby mitigating competition with food production [115].

Advancements in chemical modification techniques, catalytic systems, and process optimization have enabled the development of biolubricants with superior tribological and physicochemical properties. However, further research is required to scale these technologies, assess their long-term industrial performance, and conduct comprehensive LCAs to validate their environmental advantages.

Epoxidized vegetable oils combined with ionic liquid (IL) additives have emerged as promising candidates for use in automotive engine oils, industrial gear oils, and metal-forming lubricants. These formulations exhibit enhanced thermal stability, oxidative resistance, and tribological performance, meeting the stringent demands of modern lubrication systems [110,111]. The ILs used in the studies [110,111]—specifically protic ammonium sulfonates—are free from halides, phosphorus, and aromatic groups, reducing their environmental footprint. The combination of renewable vegetable oils and biodegradable ILs supports green tribology initiatives by minimizing ecotoxicity, improving biodegradability, and reinforcing circular economy principles in lubricant design.

Biolubricants derived from microbial processes and plant-based feedstocks offer additional environmental benefits, including superior biodegradability and lower toxicity compared to mineral oil-based lubricants. The use of naturally occurring enzymes and renewable biomass substrates ensures minimal environmental impact during both production and application phases [124,125].

From an economic perspective, the use of low-cost substrates such as agro-industrial waste, combined with enzyme immobilization strategies for biocatalyst reuse, enhances the commercial viability of microbial biolubricant production. This approach is particularly attractive in regions with abundant agricultural residues, offering a scalable and sustainable solution for lubricant manufacturing [124].

4. Nano-Biolubricants

The integration of nanoparticles into biodiesel fuels and biolubricants has emerged as a promising strategy to enhance their performance and mitigate inherent limitations such as low oxidative stability [13,126]. As discussed in previous sections, lubricants derived from natural oils are environmentally friendly and exhibit excellent lubricating properties. However, their practical application is often constrained by physicochemical drawbacks, which can be addressed through chemical modification and the incorporation of nanoparticles. Similar to their use in mineral oils, nanoparticles serve as effective additives in biolubricants, improving their tribological performance. Current research focuses on the incorporation of various nanoparticles into biolubricant formulations to enhance properties such as thermal and oxidative stability, lubricity, friction reduction, wear resistance, and mechanical durability [2,7]. Commonly studied nanoparticles include metal oxides (e.g., CuO, TiO₂, MgO, Al₂O₃), silicon dioxide (SiO₂), and graphene oxide. The latest literature reports also include information on attempts to use other nano-additives in biolubricants, previously rarely considered for applications in this type of biomaterial, e.g., nano-kaolin [127].

As with traditional lubricants enriched with nanoparticles, the effectiveness of nano-enhanced biolubricants depends on several factors, including the type of nanoparticle, its concentration, dispersion stability, and the mechanism of interaction within the tribological system. Among the physicochemical characteristics, particle shape plays a critical role. Many nanoparticles used in biolubricants are spherical, such as nano-Al₂O₃, nano-TiO₂, nano-SiO₂, and nano-CuO, which contribute to specific lubrication mechanisms [128].

The primary mechanism by which biolubricants function in a frictional system involves the formation of a protective film that adheres strongly to metal surfaces. This is facilitated by the chemical bonding of the oil’s carboxyl groups to the metal surface and the formation of a monolayer by the alkyl chains [1]. The tribological enhancement mechanisms of nano-biolubricants are analogous to those observed in mineral oil-based systems and can be categorized into primary and secondary mechanisms [1]. As presented on a Figure 4, primary mechanisms include protective film formation, which prevents direct metal-to-metal contact and significantly reduces friction, and microbearing effect, where spherical or quasi-spherical nanoparticles act as rolling elements between surfaces. Secondary mechanisms include polishing effect, where nanoparticles smoothen surface roughness and repair effect, where nanoparticles fill in surface defects such as cracks and pits [128].

These synergistic effects contribute to the overall enhancement of biolubricant performance, making nano-biolubricants a promising class of materials for sustainable and high-performance lubrication in industrial and automotive applications.

As previously discussed, various types of nanoparticles can be incorporated into biolubricants to enhance their tribological and thermophysical properties. Among these, sulfide-based nanoparticles, particularly molybdenum disulfide (MoS₂), are widely recognized for their ability to form protective films on metal surfaces, thereby reducing friction and wear [6].

In a study [28], nano-biolubricants were formulated using castor oil and olive oil as base fluids, with nano-MoS₂ (particle diameter < 100 nm) added at concentrations of 0.2, 0.5, and 0.7 wt%. No surfactants or property modifiers were used; instead, the oils were ultrasonicated to ensure dispersion. Compared to commercial mineral oils, the nano-MoS₂-enhanced vegetable oils exhibited improved thermal conductivity, higher flash and fire points, and enhanced anti-wear performance, attributed to the formation of a thin MoS₂-enriched lubricating film.

Koshy et al. [29] investigated coconut oil-based nano-biolubricants containing nano-MoS₂ (average particle size: 90 nm), with and without surfactant modification. The nanoparticle concentrations ranged from 0.25 to 1 wt%, and the mixtures were homogenized using ultrasonic agitation. The study confirmed that nano-MoS₂ significantly reduced friction and wear rates, with optimal concentrations identified as 0.53 wt% for coconut oil and 0.58 wt% for paraffin oil. Surfactant-modified MoS₂ nanoparticles yielded superior tribological and thermophysical performance.

In addition to sulfides, oxide nanoparticles such as titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), zirconium dioxide (ZrO₂), and graphene oxide have attracted considerable attention due to their developed specific surface area and chemical stability [128]. These nanoparticles are particularly relevant for applications in automotive systems, including electric vehicles, where lubrication under high pressure and temperature is critical.

Palm oil, due to its high availability, renewability, and superior oxidative stability compared to oils like rapeseed and sunflower, is a preferred base oil for nano-biolubricants [30]. The incorporation of nanoparticles into palm oil enhances its lubricity, thermal conductivity, oxidative stability, and wear resistance, making it suitable for extreme operating conditions [2,7].

Among oxide nanoparticles, nano-TiO₂ has demonstrated excellent lubrication and anti-wear properties when dispersed in base oils such as palm, rapeseed, and castor oil [6]. Similarly, copper oxide (CuO) nanoparticles have proven effective in improving the tribological behavior of vegetable oils. According to Silva-Alvarez et al. [6], CuO nanoparticles have been successfully used in rapeseed, palm, and sunflower oils (including chemically modified variants). Their mechanisms of action include surface repair and protective film formation, which contribute to enhanced anti-wear performance and, in some cases, improved anti-explosion properties. In [139], the lubrication efficiency and dispersion stability of a blended nanobio-oil consisting of rice bran oil and sunflower oil with the addition of nano-CuO with a grain size below 80 nm and used in different concentrations were investigated. Surfactant was also used to prepare the nanolubricants. The authors observed an improvement in dispersion stability for an optimal nano-CuO to surfactant ratio of 1:3 by weight. A reduction in COF and wear scar was also observed compared to the blended oil without the nano-additive.

In recent years, there has been increasing interest in the use of carbon-based nanoparticles, particularly allotropic forms such as graphene, for tribological applications. When incorporated in small amounts into biolubricants, graphene significantly enhances their tribological performance due to its layered structure and unique physicochemical and mechanical properties. However, a major limitation arises from the tendency of these nanoparticles to aggregate and sediment, which prevents full utilization of their beneficial characteristics. To address this issue, oleic acid has been proposed as a surfactant. Zulhanafi et al. [24] investigated the effect of oleic acid in a system composed of trimethylolpropane ester and graphene oxide (in concentrations ranging from 0.05 to 0.5 wt%). Their study assessed parameters such as dispersion stability, COF, wear scar diameter, surface roughness, and physical wear. The results demonstrated that the inclusion of oleic acid improved the dispersion stability and further reduced the COF compared to the system containing only graphene oxide. However, a decline in anti-wear properties was observed, which the authors attributed to the acidic nature of oleic acid and its potential to corrode metal components [24].

Kamarapu et al. [140] tested a bio-based nanolubricant composed of two kinds of oils, mineral and palm, enhanced with multiwall carbon nanotubes. Ultrasound was used to distribute the nano-additive in the liquid. Before being added to the oil, the nano-additive was chemically treated to achieve functionalized multiwall carbon nanotubes. The authors observed that the blended nano-lubricant, intended for use as a lubricant for industrial bearings, showed better properties compared to commercial oil lubricant, such as improved heat distribution and reduction in surface wear. Furthermore, it was found that in the case of functionalized multiwall carbon nanotubes, the steric hindrance effect prevents agglomeration of nanoparticles, which improves their stability in lubricants.

Özakın et al. [127] showed that the use of nano-kaolin additive to palm oil beneficially improves tribological performance and specific properties: improvement of oxidation stability, increasing hydrolytic stability, and reduction in COF and wear. Before the introduction of palm oil, the surface of kaolin nanoparticles was modified with oleic acid to improve the dispersion stability of the additive in the biolubricant.

In the context of sustainable development, the use of waste materials in the formulation of nano-biolubricants has gained attention. This approach not only enhances the ecological profile of biolubricants but also contributes to waste valorization. One notable example involves the synthesis of nano-calcium oxide (nano-CaO) from waste eggshells. The process begins with the conversion of eggshells into calcium carbonate (CaCO₃), followed by its transformation into nano-CaO particles, which typically range in size from 30 to 80 nm and exhibit a nearly spherical morphology. These nanoparticles are then dispersed into mustard oil using a combination of hot plate agitation, sonication, and surfactant application. Studies have shown that the resulting nano-biolubricant exhibits tribological properties that are comparable to or even superior to those of conventional commercial oils [141].

The lubricating properties of biofuels also warrant attention, particularly in engine injection systems where diesel fuel serves a dual role as both fuel and lubricant. Biodiesel generally offers better lubricity than petroleum-based diesel [142]. In efforts to enhance sustainability, researchers have explored the use of waste-derived materials in biofuel production. One such material is oil obtained from the pyrolysis of used car tires. Although this oil requires desulfurization and distillation before use, it represents a renewable alternative fuel. Loo et al. [126] examined the tribological performance of a blended fuel composed of tire pyrolysis oil, palm biodiesel, and diesel fuel. They introduced various nano-additives—namely nano-MgO, graphene, and nano-Al₂O₃—into the fuel blends using ultrasonic dispersion at concentrations of 0.1 and 0.2 wt%. Their findings revealed that the presence of nanoparticles significantly influenced the tribological behavior of the fuels. Specifically, nano-MgO provided the best surface performance, while nano-Al₂O₃ at a concentration of 0.1 wt% exhibited the most effective anti-wear properties. Interestingly, lower concentrations of nanoparticles generally resulted in better anti-wear performance [126].

In summary, enriching biolubricants with nano-additives and their development are very promising. The properties of such new-generation lubricants can be effectively improved compared to traditional ones. Furthermore, the use of waste materials offers additional ecological benefits. In general, the problems and challenges facing the development of nano-biolubricants are like those discussed in Section 2. It is important that appropriate formulations, including the choice of nano-additive type, its concentration, and the method for achieving a stable dispersion, should be designed and tailored to the given application, considering the specific lubrication conditions. Therefore, research on further improvement of nano-biolubricant properties and the search for new solutions should be continued.

5. AI Involvement in Ecological Lubricants Development

The rapid advancement of artificial intelligence (AI) has significantly influenced the development of ecological lubricants, offering new methods for optimizing their performance and sustainability. Bio- and nanolubricants, which are derived from renewable sources such as vegetable oils and animal fats, are increasingly being considered as alternatives to traditional petroleum-based lubricants. However, optimizing their tribological properties—such as friction reduction, wear resistance, and thermal stability—remains a complex challenge due to the variability in feedstocks, molecular interactions, and additive compatibility.

To address these challenges, AI techniques, particularly artificial neural networks (ANNs) and hybrid optimization algorithms like genetic algorithms (GAs) and gray wolf optimizer (GWO), have been employed to predict and enhance lubricant performance with high accuracy and efficiency [117,120,143]. These tools allow researchers to model and forecast key performance indicators such as the COF, WSD, and oxidative stability based on various compositional and experimental parameters.

One notable application of AI in this field involved the use of BSF oil as a lipid-rich feedstock. A hybrid ANN-GWO model was used to simulate experimental conditions such as load, temperature, and duration, predicting an optimal COF of 0.0145. This prediction was experimentally validated with only a 3.57% deviation, demonstrating the model’s effectiveness in reducing the need for extensive empirical testing and in identifying optimal blending conditions for BSF-derived esters with commercial oils [117,120,143,144].

In another study, AI was used to design vegetable oil-based nanolubricants by blending castor, coconut, and palm oils with nanoparticles like multi-walled carbon nanotubes (MWCNTs) and graphene. A comprehensive database of tribological properties was compiled from experimental data, and ANN models were trained to predict COF under various compositions and operating conditions. Sensitivity analysis revealed that sliding speed and temperature were among the most influential factors affecting performance [120,143].

These ANN models were further integrated with a genetic algorithm to optimize lubricant formulations. Two optimized blends were experimentally validated: Lube A, consisting of 40% castor oil, 40% palm oil, 20% coconut oil, and 0.7 wt% MWCNT; and Lube B, composed of equal parts of each oil with 1 wt% MWCNT and 1 wt% graphene. Both formulations showed improved tribological performance, with Lube A achieving the lowest COF of 0.037, confirming the effectiveness of the ANN-GA optimization based on the four-ball test model [117,120,145,146].

AI-driven simulations also provided mechanistic insights into the behavior of bio- and nanolubricants. For example, increasing the concentration of graphene at moderate temperatures consistently reduced COF, highlighting the film-forming and anti-wear properties of carbon-based nanoparticles. These simulations also showed that vegetable oil blends without nanoparticles generally exhibited poorer wear characteristics, emphasizing the importance of additive synergy [120,143]. Further validation came from Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) analyses, which confirmed the deposition of nanoparticles—such as carbon from MWCNTs—on wear surfaces. These findings aligned with AI predictions and demonstrated the formation of protective tribofilms that enhanced wear resistance [120,143,147].

The integration of AI into the development of bio- and nanolubricants accelerates innovation by reducing the number of experimental cycles required, enabling real-time multi-variable optimization, and facilitating the design of lubricants tailored to specific applications such as automotive, marine, or heavy machinery environments [120,148]. Looking ahead, future research may involve coupling AI models with LCAs and techno-economic analyses (TEAs) to further improve the environmental and economic sustainability of bio-based lubricants. Additionally, AI could support the genetic optimization of lipid-producing organisms like BSF and the incorporation of green nanoparticles such as cellulose and silica [120,143,146].

The ability of AI to integrate large datasets, including chemical structures, physicochemical properties, operational conditions, and even cost data, allows for a holistic evaluation of lubricant formulations. In addition, coupling AI predictions with LCA tools can enhance the environmental performance of lubricant systems, aligning product development with sustainability metrics. Table 6 presents a comparison of the most popular and suitable AI techniques for the lubricant industry in terms of applications, advantages, and predicted outcomes.

Table 6. Summary of artificial intelligence (AI) techniques applied in eco-lubricant research, with input variables, predicted outputs, and practical applications.

AI Technique	Application Area	Input Variables	Predicted Outcomes	Advantages	Ref.
ANN	Biolubricant formulation	Oil type, NP concentration, load, speed	COF, WSD, VI	High accuracy, handles nonlinearity	[120,143]
GA	Optimization of blends	Blend ratios, modifier type	Minimum friction, max viscosity	Global optimum search	[117,120,145]
ANN-GWO	BSF oil nano-biolubricant	Temp, time, NP dose	COF, wear loss	Hybrid model improves convergence	[117,120,143]
Random Forest	Nano-additive screening	Material properties, tribological history	Additive ranking	Robust, interpretable	[143]
SVM	Tribofilm prediction	Surface data, oil chemistry	Film thickness, durability	Effective with small datasets	[143]

Table 6. Summary of artificial intelligence (AI) techniques applied in eco-lubricant research, with input variables, predicted outputs, and practical applications.

AI Technique	Application Area	Input Variables	Predicted Outcomes	Advantages	Ref.
ANN	Biolubricant formulation	Oil type, NP concentration, load, speed	COF, WSD, VI	High accuracy, handles nonlinearity	[120,143]
GA	Optimization of blends	Blend ratios, modifier type	Minimum friction, max viscosity	Global optimum search	[117,120,145]
ANN-GWO	BSF oil nano-biolubricant	Temp, time, NP dose	COF, wear loss	Hybrid model improves convergence	[117,120,143]
Random Forest	Nano-additive screening	Material properties, tribological history	Additive ranking	Robust, interpretable	[143]
SVM	Tribofilm prediction	Surface data, oil chemistry	Film thickness, durability	Effective with small datasets	[143]

Overall, artificial intelligence provides a powerful framework for advancing the design and deployment of high-performance ecological lubricants. By leveraging ANN, GA, and GWO algorithms, researchers can navigate the complex design space of multicomponent systems with precision, supporting the transition away from fossil-derived lubricants and aligning with broader goals of sustainable manufacturing and the circular economy [120,143].

6. Conclusions

This review has presented an in-depth exploration of current advancements in environmentally friendly lubricant technologies, with a particular focus on nanolubricants, biolubricants, and the emerging role of artificial intelligence in lubricant design. The findings confirm that sustainable lubrication is not only a realistic goal but also a rapidly evolving field driven by multidisciplinary innovations in chemistry, materials science, tribology, and data science.

Nanoparticle-enhanced lubricants (nanolubricants) have been shown to offer significant improvements in tribological performance, primarily through mechanisms such as tribofilm formation (i.e., formation of a protective film on metal surfaces under friction, reducing wear and corrosion), rolling action, surface repair, and load-bearing effects. A wide spectrum of nanomaterials—ranging from metal oxides (TiO₂, Al₂O₃, ZnO) and metal sulphides (MoS₂, WS₂) to carbon-based structures like graphene and carbon nanotubes—have been investigated for their capacity to reduce friction and wear, improve thermal conductivity, and extend operational lifespan. Despite their potential, challenges remain in achieving stable dispersions, ensuring long-term compatibility with diverse base oils, and preventing environmental or health hazards associated with certain nanoparticle types. Progress in surface functionalization and colloidal stabilization methods continues to enhance the practical viability of nanolubricants.

Biolubricants derived from renewable resources—especially vegetable oils, non-edible plant feedstocks, waste cooking oils, and microbial oils—have emerged as viable alternatives to petroleum-based lubricants. Chemical modifications such as transesterification, epoxidation, ring-opening reactions, and blending with polyols or nano-additives have significantly improved the oxidative stability, viscosity index, and low-temperature behavior of these renewable fluids. The valorization of agro-industrial and food-processing waste streams aligns with the principles of the circular economy and offers promising pathways toward sustainable lubrication solutions. However, further work is needed to scale up production processes, reduce the cost of chemical modifications, and ensure consistent physicochemical performance under diverse operating conditions.

Notably, the integration of nanotechnology with bio-based oils has led to the emergence of nano-biolubricants, combining the environmental advantages of biodegradability with the performance benefits of nanoparticle reinforcement. These hybrid formulations are particularly suited for applications where high thermal loads, low toxicity, and biodegradability are critical.

Artificial intelligence has introduced a transformative dimension to lubricant formulation. Advanced AI models—such as artificial neural networks (ANNs), genetic algorithms (GAs), support vector machines (SVMs), and hybrid systems like ANN-GWO—have demonstrated the ability to predict key lubricant properties, optimize component ratios, and reduce the need for costly and time-intensive empirical testing. The use of AI facilitates multi-objective optimization, incorporating tribological performance, environmental metrics, and economic factors into the decision-making process. AI-driven frameworks have been particularly effective in designing bio-based lubricants enhanced with nano-additives, identifying performance optima, and discovering hidden patterns in large experimental datasets.

Looking forward, the future of environmentally friendly lubricants will depend on several converging research and industrial trends. These include the systematic evaluation of underexplored feedstocks, such as insect-derived lipids, lignocellulosic residues, and industrial fermentation by-products; the development of standard protocols for evaluating tribological and oxidative performance, particularly in the context of nano-bio formulations; increased integration of life cycle assessment (LCA) and techno-economic analysis (TEA) tools into early-stage lubricant design; and the coupling of AI prediction models with experimental validation pipelines, enabling real-time feedback and accelerated optimization cycles.

Furthermore, regulatory developments, consumer awareness, and the global shift toward decarbonization will continue to drive demand for lubricants that are safe, sustainable, and effective. Innovations in green chemistry—particularly the design of functional, non-toxic additives and bio-derived base stocks—will complement advancements in data science and tribological testing to deliver next-generation lubricant technologies.

In conclusion, this review underscores the strategic importance of combining nanotechnology, bioresource valorization, and artificial intelligence to meet the dual goals of performance and environmental stewardship. Continued interdisciplinary research, industrial partnerships, and global policy support will be essential to realize the full potential of sustainable lubrication across diverse sectors of the economy.