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Review

Applications of Phyllosilicates Mineral Powder as Anti-Wear Lubricating Materials in Lubricating Oil and Grease: A Review

by
Nan Jiang
1,2,3 and
Feng Nan
1,2,3,*
1
Hubei Provincial Key Laboratory of Chemical Equipment Intensification and Intrinsic Safety, School of Mechanical and Electrical Engineering, Wuhan Institute of Technology, Wuhan 430205, China
2
Hubei Provincial Engineering Technology Research Center of Green Chemical Equipment, School of Mechanical and Electrical Engineering, Wuhan Institute of Technology, Wuhan 430205, China
3
School of Mechanical and Electrical Engineering, Wuhan Institute of Technology, Wuhan 430205, China
*
Author to whom correspondence should be addressed.
Lubricants 2024, 12(9), 324; https://doi.org/10.3390/lubricants12090324
Submission received: 12 August 2024 / Revised: 12 September 2024 / Accepted: 19 September 2024 / Published: 20 September 2024
(This article belongs to the Special Issue Anti-wear Lubricating Materials)
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Abstract

:
Wear caused by friction is one of the main causes of most mechanical component failures. The application of powders as anti-wear lubricating materials has achieved great advances, which are of great significance in reducing friction and wear. This review focuses on the applications of phyllosilicates mineral powders as anti-wear lubricating materials in lubricating oil. The tribological behaviors of phyllosilicates mineral powders and the combination of phyllosilicates mineral powders with other materials as lubricant additives are provided. Moreover, the fundamental mechanism are systematically reviewed and concluded based on tribology data and surface, and interface analysis. Finally, current unsolved issues and suggestions for future research on phyllosilicates mineral powders as lubricant additives are proposed.

Graphical Abstract

1. Introduction

Friction and wear are among the main causes of most mechanical component failures. The friction of mechanical equipment leads to a large amount of material waste and energy loss. Therefore, reducing friction and wear is of great significance. There are several measures to reduce friction and wear, among which using lubricating oil and grease is the most widely applied. As is well known, the lubrication performance of traditional lubricating oils and greases is extremely limited. In recent years, it has been found that some micro- and nanoparticles can be used as anti-wear lubricating materials to improve the lubricating properties of oils and greases. The micro- and nanoparticles mainly include metals [1,2,3,4,5], metallic disulfides [6,7,8,9,10,11], metallic oxides [12,13,14,15,16], carbon materials [17,18,19,20,21], and so on. In recent years, it was found that micro-nano phyllosilicates mineral powder can also be used as a lubricating additive.
Compared with other lubricating additives, phyllosilicates mineral powder shows different tribological effects and mechanisms. Under the effect of phyllosilicates mineral powder, a special metal-ceramic protective layer can form on the friction surface, which can not only effectively reduce friction and wear but also give the friction pair a unique self-healing ability. This unique property enables mechanical equipment to achieve in-situ repair without stopping the machine, thereby improving efficiency of the machine while extending its service life. This technology can not only achieve energy conservation and emission reduction but also is an innovation and breakthrough in the field of tribology. This technology has great research potential and application space.
Phyllosilicates mineral materials are a combination of silicon, oxygen and other chemical elements (mainly aluminum, iron, calcium, magnesium, potassium, sodium, etc.). In phyllosilicates, each Si4+ exists in the center of a tetrahedron with four O2− vertices, forming a Si-O tetrahedron, which is the basis of the crystal structure. In addition, there are other structures present in the crystal structure, such as the Al-O octahedron. Phyllosilicates mineral powder is widely distributed in the Earth’s crust and is an important part of rock and soil. Although phyllosilicates mineral materials exhibit unique self-healing properties and have abundant reserves, which indicates an innovative breakthrough in tribology with considerable research and application prospects, it is important to recognize the current challenges. These include doubts about the long-term stability of the materials in oil, high friction coefficients that are not yet ideal for low-friction applications, and the costly synthesis processes.
The application of phyllosilicates mineral powder as a lubricating additive has been extensively reported. In this work, the authors aim to review the recent achievements that have been realized with phyllosilicates mineral powder as a lubricating additive. In addition, the research challenges and possible research outlooks for phyllosilicates mineral powder as a lubricating additive are also stated.

2. Tribological Behaviors of Phyllosilicates Mineral Powder

Phyllosilicates mineral powder possesses great potential as a lubricating additive due to its unique crystal structure and wear self-repair function. Researchers have developed various kinds of phyllosilicates mineral powders, including muscovite, talc, montmorillonite, serpentine, and attapulgite as additives, proving that these phyllosilicates mineral powders can effectively enhance the lubrication and wear-resistance performances of lubricating oils and greases. However, the tribological behavior and mechanisms of these materials are discrepant.

2.1. Muscovite

Muscovite (KAl2[AlSl3O10](OH)2) is a potassium aluminum silicate mineral belonging to the monoclinic system, with a layered structure. It exhibits insulation, high-temperature resistance, stable physical and chemical properties, good insulation, high elasticity and toughness. There are a few reports on its application as a lubricating additive. Du et al. [22] investigated the tribological behavior of muscovite as a lubricating additive in lithium grease using a four-ball tribotester. It was found that the friction-reducing and anti-wear properties of lithium grease can be remarkably improved by moderate muscovite additives. The excellent reinforcement effect of muscovite can be ascribed to its layered structure and the formation of a tribofilm composed of FeOx, FeOOH, carbon, silicate, and organic carbides. Furthermore, it was found that some rare earth oxides can further enhance the friction-reduction and anti-wear properties of muscovite. Wang et al. [23] prepared a muscovite/La2O3 composite powder by ball-milling solid-state chemical reactions and investigated its tribological properties as an additive in 500 SN base oil. The results of friction and wear tests showed that moderate amounts of La2O3 could significantly enhance the friction-reduction and anti-wear properties of muscovite, which could be attributed to the catalytic role of La2O3 in physicochemical process reactions on the wear surface. In particular, the microscopic morphology of the wear surface was significantly improved by adding 5 wt% of muscovite/La2O3 composite powder to 500 SN base oil. The positive effect of this composite powder on enhancing the tribological properties of the lubricant is illustrated in Figure 1, where (a) shows the wear surface of the base oil, while (b) shows the wear surface after the addition of the muscovite/La2O3 composite powder with 5 wt% La2O3, showing a smoother surface with fewer and shallower scratches.
Du et al. [24] prepared muscovite/CeO2 composite particles and added them into lithium grease. The grease containing muscovite/CeO2 composite particles exhibited better friction-reduction and anti-wear ability than grease containing single particles. CeO2 probably acts as a catalyst to accelerate physicochemical reactions on the worn surface. A tribofilm mainly consisting of muscovite/CeO2 adsorption film and a chemical reaction film of SiO2 and Fe2O3 generated on the worn surface.

2.2. Talc

Talc belongs to the trioctahedral minerals, with a structural formula of Mg6Si8O20(OH)4. Talc possesses excellent physical and chemical properties such as lubricity, fire resistance, acid resistance, insulation, a high melting point, chemical inactivity, good hiding power, softness, good gloss, and strong adsorption. Talc and its composite materials have also been found to improve the lubricity of lubricants. Rudenko et al. [25] conducted a comprehensive evaluation of talc as an extreme pressure additive in 5W-30 engine oil, revealing that the optimal conditions for lubricity were achieved at 100 °C with a concentration of 0.15 wt%, which resulted in a significant reduction in the friction coefficient by over 30%. This reduction is theorized to be due to the pressure- and temperature-induced lamellar dehydration mechanism of talc, which forms oxide transfer films on the friction surface, thereby preventing metal-to-metal adhesion. Figure 2 in their study encapsulates the dynamic and static friction coefficients’ progression over time and their variation with sliding distance for various concentrations, demonstrating the impact of talc concentration on frictional behavior. The study concludes that ultrafine talc powder can be effectively utilized as an environmentally friendly additive for formulating hybrid lubricant compositions with enhanced extreme pressure and friction-reducing properties.
In addition, talc particles were found to improve the wear response of the leaded-tin bronze bush sample in terms of reduced wear loss, frictional heating, and friction coefficient [26]. Furthermore, some studies have found that talc and carbon materials have a synergistic lubrication effect. Fei et al. [27] fabricated talc/carbon spheres (TC) composites and evaluated their tribological properties as additives in 85W-90 using a ball-on-disc (BOD) tribometer. Compared with Ref-Oil, the COFs of oil containing 0.5 wt% TC composite (0.5 TC-Oil) decreased by about 33.6%, 35.3%, and 23.8% at 3 N, 5 N, and 8 N, respectively. Moreover, severe abrasive wear between friction pairs changed into slight plastic deformation when lubricated with 0.5 TC-Oil. These results reflect a notable synergy between the self-repair performance effect of talc and the ball-bearing effect of carbon spheres. Mobasher et al. [28] prepared a homogeneous and stable MWCNTs/talc nanoparticle mixture and studied its effect on the tribological properties of calcium grease. When adding 4 wt% nanoparticles to the base grease, the COF was reduced by 80.62%, the wear rate decreased by 63.44%, and the thermal conductivity increased by 51.72%.

2.3. Montmorillonite

Montmorillonite ((Na,Ca)0.33(Al,Mg)2[Si4O10](OH)2·nH2O) has a sandwich structure with upper and lower layers of silicon oxide tetrahedra and a middle layer of aluminum oxygen octahedron. It possesses unique performance, such as a large special surface area, high strength, superior insulating properties, and outstanding adsorption capacity. In addition, montmorillonite also own novel tribological properties. Cao et al. [29] investigated the effect of inorganic modified montmorillonite (IOMMT) on the tribological properties of insulating greases. It was shown that the introduction of IOMMT significantly increased the number of electron traps in the base grease, resulting in superior insulating properties. In addition, IOMMT effectively reduces friction between steel/steel contact pairs through its unique layered structure and generates a protective friction film produced by physical adsorption and chemical reactions, significantly improving friction reduction and wear resistance. Figure 3 shows the wear surface morphology after lubrication with different greases at a 200 N load, 5 Hz frequency, and room temperature. The graph compares the effect of OMMT grease, IOMMT grease, MMT grease, and SiO2 grease. The results show that the wear surfaces treated with IOMMT grease (Figure 3b,b’) have the narrowest wear width and the smoothest surface morphology, with only a few shallow grooves. In contrast, the OMMT, MMT, and SiO2 grease-treated surfaces exhibit denser grooves and larger pits, which are mainly caused by abrasive and adhesive wear. These images clearly demonstrate that IOMMT grease has better anti-wear properties than the other greases. In addition, Li et al. [30] synthesized functionalized graphene/montmorillonite (FG/MTT) nanosheets and evaluated their lubricating performance as additives in 15W-40 engine oil using a four-ball tribometer. The study found that the average friction coefficient and wear scar diameter were reduced by 50.4% and 13.2%, respectively, when the concentration of FG/MTT was 0.4 mg/mL. FG/MTT was observed to react with the surface of the friction pair, forming a repair layer composed of Fe2O3, SiC, SiO2, and aluminosilicates, which contributed to the improvement of tribological properties.

2.4. Serpentine

Serpentine (Mg6[Si4O10](OH)8) is a type of hydrated magnesium-rich silicate mineral, which includes varieties such as antigorite, lizardite, and chrysotile. Owing to its excellent physical and chemical properties, such as heat resistance, corrosion resistance, wear resistance, thermal insulation, sound insulation, and good craftsmanship, serpentine is widely used in construction, transportation, metallurgy, mechanical engineering, aerospace, and the military industry. In recent years, materials scientists and engineers have found that serpentine powders may significantly reduce the friction coefficient and wear loss of friction pairs when used as lubricating additives. Zhang et al. [31] investigated the friction and wear properties of surface-coated natural serpentine powders (SP) suspended in diesel engine oil (CD 15W-40) using an Optimal SRV oscillating friction and wear tester (Optimol, Munich, Germany). The study evaluated the impact of various loads on the tribological properties of the lubricants at 10 N, 50 N, 100 N, and 200 N, which correspond to Hertzian mean contact stresses of 1.02 GPa, 1.15 GPa, 2.21 GPa, and 2.78 GPa. The results indicated that the 0.5 wt% content of the serpentine powders in oil was most efficient in decreasing friction and wear at a load of 50 N (Figure 4). A tribofilm composed of iron oxides, silicon oxides, graphite and residual organic compounds generated on the worn surface. This tribofilm possesses higher mechanical properties and lubricating ability, contributing to the excellent tribological behaviors of the SP additive. Ulteriorly, Zhang et al. [32] characterized the formed tribofilm by scanning electron microscope and focused ion beam (SEM/FIB) work station and transmission electron microscope (TEM). It was found that an amorphous SiOx film with amorphous SiOx particles inserted had formed on the worn surface, attributed to the frictional chemical reaction between serpentine particles and metal worn surfaces (Figure 5). Yu et al. [33] added 1.5 wt% surface-modified serpentine mineral powders to 500 SN mineral base oil and evaluated the tribological behaviors of the oil sample through a CETR UMT-2 multi-specimen test system (CETR, Billerica, MA, USA). It was found that the friction coefficient and wear rate of the base oil were both significantly reduced by the additive. A nanocrystalline tribofilm, with a thickness of 500–600 nm, formed on the worn surface under the effect of serpentine mineral powders. The tribofilm is composed of Fe3O4, FeSi, SiO2, AlFe, and Fe3C. A phenomenological model of the tribofilm generated by the serpentine additive (Figure 6). The formation of nanometer crystallites with an average size of 20 nm, along with the existence of the complicated compound leads to high surface hardness (about 8.0 GPa within 100 nm) and a low modulus approach to steel level (<240 GPa within 100 nm) for the tribofilm (Figure 7). The excellent mechanical properties, the reinforced phase of embedded particles, and the porous structure of the tribofilm contribute to the reduction of friction and wear.
Bai et al. [34] evaluated the tribological properties of antigorite as a friction and wear-reducing additive to lubricating oil (CD15W-40) using an MM (multifunctional friction and wear testing machine)-10W four-ball friction tester and an MM (multifunctional friction and wear testing machine)-200 friction tester in a ring-on-block contact configuration. The friction coefficient, the average wear scar diameter of steel balls, wear scar depth and wear volume in the system with antigorite powders were reduced by 19.3%, 16.41%, 30.77% and 33.33%, respectively as compared with the base oil. The friction-reducing and anti-wear mechanisms of antigorite powders are attributed to their intrinsic morphological and structural properties, as well as friction-induced formation of a ceramic-like film on the surface of the metals. Zhang et al. [35] studied the tribological performance and self-repairing performance of surface-modified nanoscale serpentine powders as lubricant additives in mineral base oil (5-CST (Centistoke)). The results showed that when adding 1.00 wt% serpentine into 5-CST, the friction coefficient reduced by 14.80%, and the wear scar diameter (WSD) decreased by 11.82% under a load of 294 N. The results of X-ray absorption near edge structure and XANES showed that adsorption and tribochemical reactions occur to form self-repairing lubrication films consisting of iron oxides, silicon oxides, magnesium oxide, and residual organic compounds. Wang et al. [36] prepared lubricant oil with a mixture of serpentine powder and KTL32 (Kunlun tongyong extreme pressure long-life 32) turbine oil (mass ratio of 1:199) (Petrochina Co., Ltd., Beijing, China). An ABLT (angular contact bearing life strengthening testing machine)-1 bearing test machine was used for 1350 h, and an MM-W1 three-pin-on-disk apparatus was used to investigate the lubricity of the prepared oil. The results show that the energy-saving effect was improved after adding serpentine powder to the oil and that both the friction coefficient and mass loss were dramatically decreased. In the disc-pin experiments, a compact and uniform auto-reconditioning surface layer mainly consisting of magnesium (Mg) and silicon (Si) elements with a thickness of 4 μm was formed on the worn surface of pins lubricated with oil containing serpentine. Wang et al. [37] milled natural serpentine by high-energy ball milling to obtain nanoscale serpentine powder (NSPs) and studied its influence in liquid paraffin on the friction and wear behaviors using a four-ball friction tester. The results showed that by adding 0.5 wt% of NSPs, the friction coefficients and wear spot diameters were reduced by 22.8% and 34.2%, respectively. Moreover, the long-term tribological test shows that the wear scar diameter decreased slightly after 3 h, reaching a state of dynamic balance between wear and repair. The outstanding tribological performance should be attributed to the formed bilayer tribofilm, where the outermost first layer is composed of lubricant-encapsulated nanoparticles that minimize friction and protect the surfaces in contact, while the underlying second layer consists of nanoparticles that are densely packed and bonded to the surface of the steel ball, providing a resilient barrier against wear and contributing to the tribofilm’s self-repair capabilities. Yu et al. [38] carried out thermal activation of serpentine ultrafine powders at different temperatures and studied the tribological properties of surface-coated serpentine ultrafine powders after thermal activation as liquid paraffin additives. It was found that the serpentine powders suspended in liquid paraffin presented excellent tribological properties. Thermal activations at 300 °C to 600 °C increased the film-forming ability and tribofilm completeness of the serpentine, maintained the layer structure, and accordingly further improved the tribological properties. Yin et al. [39] investigated the effect of surface-modified serpentine natural mineral powder on the friction and wear of tin bronze against steel under oil lubrication. Under certain conditions, the addition of 0.1 wt% serpentine to the base oil provides a reduction in friction coefficient by 23.7% and a decrease in wear volume by 45.7%. During the friction process, a non-conductive tribofilm consisting of metal oxides, oxide ceramic particles, graphite, and organics was formed on the tin bronze worn surface by serpentine mineral. The nanohardness and elastic modulus of the film demonstrated gradient variation, being low on the surface and high in the sub-surface, which is favorable for improving tribological behaviors. In addition, some researchers investigated the tribological properties of serpentine powder as a lubricating oil additive under high temperatures. In response to critical temperatures exceeding 400 °C, where conventional lubricating oils are rendered ineffective, Qi et al. [40,41] investigated the tribological properties of nanoscale serpentine as a lubricating oil additive at 400 °C. The tribological test results showed that with nanoscale serpentine powder added to lubricating oil, self-repairing protective layers were found on the worn surface of specimens under high temperature. The protective layer mainly consisted of SiO2, silicates, MgO, and anhydrous magnesium salts. Moreover, Qi et al. [42] also studied the friction and wear behaviors of serpentine (SP) mineral powder as an additive in a steel–chromium plating pair at 400 °C. Self-repairing protective layers were generated on the worn surfaces of specimens, whether on the steel matrix or chromium plating, under high temperatures. Isomorphic replacement between Fe/Cr and SP mineral silicate occurred, which was the essence of the tribochemical reaction during the metal wear process of self-repair.
Furthermore, researchers also combined serpentine with other lubricating additives. Zhang et al. [43] investigated the effect of Cu nanoparticles (NPs) on the tribological behaviors of serpentine powders (SPs) as additives in diesel oil. It was found that the optimum mass ratio of Cu NPs to SPs is 7.5:92.5. Compared with SPs alone, the tribological properties of the oil containing the mixture were more excellent. A smoother and more compact tribofilm, mainly composed of iron oxides, silicon oxides, species enriched in Si-O structures, graphite, organic compounds, and Cu0, Cu1+, and Cu2+ species, formed on the worn surface, which is responsible for the further reduced friction and wear. Zhao et al. [44] synthesized serpentine/La(OH)3 composite particles via the sol–gel method and investigated the tribological properties of the composite particles as a lubricating additive for CD15W-40 lubricating oil using the MM (multifunctional friction and wear testing machine)-10W multi-functional friction abrasion tester and MHK (multi huan kuai)-500 ring-block wear testing machine. The lubricating oil containing serpentine/La(OH)3 composite particles exhibited better tribological and self-repairing properties than the lubricating oil containing only a single kind of particle. A tribofilm containing O, Si, and Fe elements formed on the surface of the rubbing pair by the iron exchange reaction between iron atoms and Mg2+ of the serpentine, with lanthanum acting as a catalyst during the reaction. Zhao et al. [45] investigated the tribological performance of serpentine in combination with ZDDP as an additive for MCT (medium chain triglycerides)-10 base oil using a plint high-frequency friction tester at room temperature and 100 °C. It was found that a combination of serpentine and ZDDP helps reduce the friction of the oil blend and exhibits better anti-wear properties than base oil. With the effect of the combination additive, the wear scar width (WSW) of the pin was reduced by 30.4% at room temperature and 68.6% at 100 °C. XANES spectra indicated that a tribofilm containing serpentine and Zn polyphosphate, ZnS (from the decomposition of ZDDP), was generated by the base oil containing serpentine and ZDDP. Qin et al. [46] prepared a serpentine/Ni composite and investigated its tribological performance as an oil-based additive in PAO10 by using ball-disk wear tests. The composite, as a lubricant additive, exhibited friction reduction (15.1%) and superior anti-wear (78.13%) performance compared to MSH and Ni alone. During the friction process, the composite powder can form a smoother tribofilm, mainly containing Fe3O4, FeOOH, SiOx, and NiO on the surface.

2.5. Attapulgite

Attapulgite, with the molecular formula Mg5Si8O20(OH)2(OH2)4⋅4H2O, is a crystalline hydrated magnesium aluminum silicate with a natural nanofiber structure [47]. Attapulgite has a structure similar to that of serpentine. Some researchers indicated that attapulgite can also be used as a lubricating additive to decrease friction and wear. Nan et al. [48] added surface-modified natural attapulgite powders into 150 SN base oil and investigated the tribological properties of the oil containing attapulgite powders using an optimal SRV-IV oscillating friction and wear tester. It was found that a moderate amount of attapulgite can significantly improve the friction-reducing and anti-wear properties of base oil. A complex tribofilm mainly composed of FeO, Fe2O3, FeOOH, and SiOx, formed on the worn surface (Figure 8). It possesses excellent lubricating ability and contributes to the excellent friction-reducing and anti-wear properties of the attapulgite additive. Ulteriorly, Nan et al. [49] prepared ultrafine attapulgite spherical particles (UAP), with an average size of about 330 nm, from natural attapulgite powders through the ball-milling dispersion method. The tribological properties of UAP suspended in mineral lubricating oil were evaluated. UAPs possess better friction-reduction and anti-wear properties compared to natural attapulgite powders. A tribofilm mainly composed of FeO, Fe2O3, FeOOH, and SiO formed on the worn surface lubricated with oil containing UAP. Yu et al. [50] designed an orthogonal experiment to study the effects of load, frequency, duration and concentration on the tribological properties of the natural attapulgite additive using an SRV reciprocating wear tester. Results indicate that the order that affect the tribological performances of the additive is load, concentration, frequency, and duration. A tribolayer consisting of oxides, ceramics, silicates, and graphite was generated on the rubbing surface by the effect of attapulgite (Figure 9). Such layer with good lubricity, high hardness, and a hardness/elastic modulus ratio is conducive to the excellent tribological behaviors of the attapulgite. The applied load achieved the greatest influence on the contact stress, shearing force, and local temperature. The attapulgite content affects the generation rate of the tribolayer and the total amount of tribochemical reaction products. Frequency affects the formation of the tribolayer by influencing the temperature and shearing force on the friction surfaces. Finally, sufficient time is necessary for both the formation and abrasion of the tribolayer. The friction and wear tests in the above studies were conducted on carbon steel friction pairs. In addition, Nan et al. [51] investigated the tribological properties of attapulgite as a lubricant additive on electric-brush plated Ni coating using an SRV-IV friction and wear tester. It was found that the attapulgite nanofibers can improve the friction-reducing and anti-wear properties of 150 SN for electric-brush plated Ni coating. Under the action of attapulgite, a tribofilm mainly composed of Ni, NiO, Al2O3, SiO2, graphite, and organic compounds formed on the worn surface of the Ni coating, contributing to the decrease in friction and wear.
Like other phyllosilicates mineral powder, attapulgite can achieve synergistic friction-reducing and anti-wear effects with other lubricating additives. Wang et al. [52] used surface-modified attapulgite clay as a thickener and synthetic oil (PAO 40) as the base oil to prepare a lubricating grease and investigated the effect of KB3O5, MoS2, and a graphite/MoS2 mixture (mass ratio 3:2) on the tribological properties of the grease using an Optimol SRV reciprocating friction and wear tester. The addition of MoS2 and the graphite/MoS2 mixture to the grease resulted in better friction-reducing ability. KB3O5 showed relatively better anti-wear ability. MoS2 and the graphite/MoS2 mixture were able to increase the load-carrying capacity of the grease. Nan et al. systematically studied the tribological behavior of attapulgite combined with metals [53,54], rare earth oxides [55], and graphene [56,57]. The research results demonstrated that attapulgite can achieve synergistic friction-reducing and anti-wear function with nanoparticles, including Cu, Ni, La2O3, and graphene. Under the effect of attapulgite and other nanoparticles, tribochemical reactions between attapulgite and carbon steel friction pairs occurred, forming a tribofilm mainly composed of iron oxides and silicate oxide. Meanwhile, other nanoparticles can also be deposited and adsorbed onto the worn area. Some metal nanoparticles can be oxidized to generate metallic oxides. La2O3 can serve as a catalyst to accelerate the tribochemical reactions between attapulgite and friction pairs, thus forming a thicker and smoother tribofilm. As for graphene nanoparticles, they can form a physical deposition film on the surface of friction pairs (Figure 10). Wang et al. [58] prepared molybdenum-dotted palygorskite (Amo-PMo) nanoplatelets by an aqueous miscible organic solvent treatment method. Subsequently, the effect of prepared Amo-PMo on the lubricity of ULTRA-S 150N base oil was conducted using an MS-10A four-ball tribometer (Xiamen Tenkey Automation Co., Ltd., Xiamen, China). It was found that the sample of 0.5 wt% Amo-PMo exhibited the best tribological properties with a coefficient of friction of 0.09. Moreover, the resulting wear scar diameter and wear volume of the sliding ball surface were 63% and 49.6% of those lubricated with base oil, respectively. Its excellent lubricating performance and self-repairing ability were mainly attributed to the generated MoS2 adsorbed on the contact surfaces during the tribochemical reaction, effectively preventing the direct collision between asperities on sliding solid surfaces.
Based on a comprehensive analysis of the tribological performance of phyllosilicates mineral powders as lubricant additives, a comparative synthesis of their chemical structures, physical and chemical properties, and their respective merits and demerits is presented. These properties are summarized in Table 1, which outlines the unique properties and potential applications of each phyllosilicate in lubricating formulations:

3. Fundamental Mechanisms of Lubrication

The fundamental mechanisms of lubrication involving phyllosilicates mineral powders are pivotal to understanding their performance as lubricating additives. This section reviews the key mechanisms identified in the literature and their implications for the tribological properties of lubricants.

3.1. Role of Layered Structure

The layered structure of phyllosilicates is integral to their lubricating properties, as it allows for intercalation and exfoliation under shear, reducing frictional forces. Studies have shown that the layer structure and formation of tribofilms composed of various compounds, including FeOx, FeOOH, carbon, silicate, and organic carbides, contribute to the friction-reducing and anti-wear properties [22,23]. The interplay between muscovite and thermal-treated muscovite, as well as composites with La2O3, has been shown to enhance the grease’s anti-wear properties and form even smoother worn surfaces [22,23]. Talc, another phyllosilicate, has been found to improve the wear response of leaded-tin bronze bush samples, reducing wear loss and frictional heating [26]. In the study by Wang et al. [58], molybdenum-containing palygorskite (Amo-PMo) nanosheets, prepared by the method of treatment with water-soluble organic solvents, showed excellent dispersion in oil-based lubrication systems. During friction, these laminated nanosheets were able to form MoS2 on the contact surfaces, significantly reducing the coefficient of friction, proving the friction-reducing effect of the laminated structure in actual lubrication processes. Sleptsova et al. [59] studied the tribological properties of PTFE/laminated silicate composites. It was found that the introduction of laminated silicates contributed to the formation of a protective layer on friction surfaces, achieved through the formation of a laminated structure exfoliation and rearrangement under shear, providing the material with additional wear resistance. Vasiliev et al. [60] demonstrated the potential of laminated silicates and carbon fibers as fillers in Polytetrafluoroethylene (PTFE) matrices to improve the mechanical properties and wear resistance of polymers by forming a hard friction film on the surface, effectively reducing friction and wear. These studies have shown that the layered structure of silicate minerals is effective in promoting interlayer sliding in shear, reducing friction, and improving wear resistance by forming a protective layer—properties that make them ideal for use as lubricant additives.

3.2. Chemical Activity and Film Formation

Layered silicate mineral powders exhibit excellent anti-wear properties due to their reactivity with metal surfaces, which leads to the formation of protective friction films that are essential for reducing friction and prolonging the service life of mechanical components. For example, montmorillonite reacts with oxides on metal surfaces at elevated temperatures to produce lubricating phases with low shear strength [29,30]. The formation of these friction films is influenced by the specific layered silicate used, as well as the operating conditions (e.g., temperature and loading) [25,27]. The use of inorganic modified montmorillonite (IOMMT) has been shown to enhance friction reduction and wear resistance by forming protective friction films through physisorption and chemical reactions [29]. Formation of such friction films not only improve the anti-wear properties of the lubricant but also provide continuous lubrication under various operating conditions by forming a stable protective layer on the surface of mechanical components.

3.3. Self-Healing Ability

The self-healing ability of layered silicate mineral powders as lubricant additives is one of their significant advantages, a property that enables them to repair wear-induced surface defects and maintain lubricant film integrity. Vermiculite powders, in particular, have been shown to continuously release lubricating substances during friction, replenishing wear areas and forming a friction film consisting of iron oxides, silicon oxides, graphite, and residual organic compounds [31,32,33]. Experimental studies [39,40] have shown that these mineral powders are capable of repairing friction surfaces and maintaining low coefficients of friction by continuously releasing materials under severe operating conditions, such as high temperature and high pressure. In addition, the self-repairing behavior is not limited to the initial wear phase but continues throughout the operating cycle, a property that has great potential for applications in difficult-to-maintain environments, such as aerospace and deep-sea exploration [43,44].

3.4. Synergistic Effects with Other Additives

The use of layered silicate mineral powders in combination with other additives often produces a synergistic effect that significantly enhances the overall performance of the lubricant. For example, the combined use of talc/carbon sphere composites and MWCNTs/talc nanoparticles has been shown to reduce friction and wear more significantly than the use of these additives alone [27,28]. This synergistic effect is attributed to the complementary effects between the different materials, such as the ball-bearing effect of carbon spheres and the self-healing properties of talc [27]. The use of vermiculite in combination with other lubricant additives, such as copper nanoparticles, lanthanum hydroxide, and zinc sulfide phosphate esters, has also been shown to improve tribological properties and form smoother and denser friction films [43,44,45]. Experimental studies have further shown that using these mineral powders in combination with metal particles, surfactants, or specific polymers results in the formation of a more stable and tougher protective layer on the friction surface through physical or chemical reactions, leading to a significant reduction in the coefficient of friction and wear [45,46]. The combined use of additives such as molybdenum disulfide or organophosphates has been shown to further enhance the wear resistance and adaptability of lubricants under extreme operating conditions [47,48]. The discovery of these synergistic effects provides new avenues for the development of more efficient and multifunctional lubricant additives that can help customize solutions for specific industrial applications and meet higher performance requirements.

3.5. Environmental Adaptability

The performance of layered silicate minerals as lubricant additives is significantly affected by environmental factors such as temperature and loading. Studies at high temperatures have shown that nanosized vermiculite powders are capable of forming self-healing protective layers that are mainly composed of silica, silicates, magnesium oxide, and anhydrous magnesium salts, providing a powerful solution for mechanical systems subjected to high stresses [40,41]. Experimental studies have further confirmed that these mineral powders maintain low coefficients of friction and wear rates even under extreme temperature and humidity conditions, exhibiting excellent anti-wear and anti-corrosion properties [49,50,51,52]. In addition, their excellent performance in chemical media further demonstrates their potential for industrial applications, especially in difficult-to-maintain environments such as aerospace and deep-sea exploration [53,54]. The discovery of these properties provides new directions for the design of high-performance lubricant additives capable of delivering long-lasting lubricating performance in a variety of environmental conditions.

4. Conclusions and Perspectives

Phyllosilicates mineral powder have been widely researched as lubricating additives owing to their unique physicochemical properties and self-repairing effect for friction pairs. It has been demonstrated that phyllosilicates mineral powder can improve the friction-reducing and anti-wear properties of lubricating oils and greases. In addition, the improvement effect of phyllosilicates mineral powder can be further enhanced by combining them with other additives. However, there are still some issues that have not been resolved.
(1)
For phyllosilicates mineral powder lubricating additives, the dispersion stability in lubricants is an important issue in its exertion of performance and practical application. Traditional organic modifiers, such as oleic acid, silane coupling agent, and so on are presently used to modify phyllosilicates mineral powder to improve their dispersion stability in lubricants. However, the improvement effect is not ideal. Consequently, how to prepare long-term equilibrium solid-liquid two-phase lubrication systems still needs further investigation.
(2)
Natural phyllosilicates mineral powder need to undergo several processes, including crushing, refining, purification, and organic modification, before being added to lubricating oils. Hence, the cost of phyllosilicates mineral powder as lubricating additives is high for industrial applications. Efforts are still needed to improve preparation efficiency and reduce the preparation costs of phyllosilicates mineral powder lubricating additives.
(3)
The friction-reducing and anti-wear improvement effect of phyllosilicates mineral powder is presently limited. The friction coefficient of the lubricants containing phyllosilicates mineral powder is mostly higher than 0.1. Further reduction of the friction coefficient, or even achieving superlubricity, is important for the research of phyllosilicates mineral powder as additives.
(4)
It has been found that tribofilms are generated on worn surfaces, but most discussions on tribological mechanisms are based on the characterization and analysis of the surfaces on the tribofilms. Little is known about the internal structure and formation process of the tribofilms. Hence, the friction-reducing and anti-wear mechanisms of phyllosilicates mineral powder are still unclear. Using more advanced detection equipment and simulation methods [61,62,63,64,65,66,67,68] for an in-depth revelation of the tribological mechanisms is needed in the future.

Author Contributions

Writing—original draft preparation, N.J.; writing—review and editing, N.J. and F.N.; funding acquisition, F.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant number 51705511).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM micrographs of wear scar surfaces: (a) base oil and (b) muscovite + 5 wt% La2O3. Reprinted with permission from ref. [23].
Figure 1. SEM micrographs of wear scar surfaces: (a) base oil and (b) muscovite + 5 wt% La2O3. Reprinted with permission from ref. [23].
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Figure 2. (a) Dynamic and (b) static coefficient of friction of 1.5% talc in 5W-30 motor oil at various temperatures. Reprinted with permission from ref. [25].
Figure 2. (a) Dynamic and (b) static coefficient of friction of 1.5% talc in 5W-30 motor oil at various temperatures. Reprinted with permission from ref. [25].
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Figure 3. Morphologies of the worn surfaces lubricated with insulating greases at 200 N, 5 Hz, and RT. (a,a’) OMMT grease, (b,b’) IOMMT grease, (c,c’) MMT grease, (d,d’) SiO2 grease. Reprinted with permission from ref. [29].
Figure 3. Morphologies of the worn surfaces lubricated with insulating greases at 200 N, 5 Hz, and RT. (a,a’) OMMT grease, (b,b’) IOMMT grease, (c,c’) MMT grease, (d,d’) SiO2 grease. Reprinted with permission from ref. [29].
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Figure 4. (a) Effect of SP concentration on the friction coefficient and wear rate of the ball and disc; (b) SEM morphology of the worn surface on the disc lubricated with the base oil and the oil containing 0.5 wt% SP. Reprinted with permission from ref. [31].
Figure 4. (a) Effect of SP concentration on the friction coefficient and wear rate of the ball and disc; (b) SEM morphology of the worn surface on the disc lubricated with the base oil and the oil containing 0.5 wt% SP. Reprinted with permission from ref. [31].
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Figure 5. The observation cross-section image and corresponding TEM image of the protective film for the oil containing serpentine. Reprinted with permission from ref. [32].
Figure 5. The observation cross-section image and corresponding TEM image of the protective film for the oil containing serpentine. Reprinted with permission from ref. [32].
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Figure 6. Phenomenological model of the tribofilm generated by the serpentine additive. Reprinted with permission from ref. [33].
Figure 6. Phenomenological model of the tribofilm generated by the serpentine additive. Reprinted with permission from ref. [33].
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Figure 7. Variation of H/E with the indenter displacement for the worn surface with mineral base oil and selected points on the tribofilm formed under the lubrication of oil containing serpentine. Reprinted with permission from ref. [33].
Figure 7. Variation of H/E with the indenter displacement for the worn surface with mineral base oil and selected points on the tribofilm formed under the lubrication of oil containing serpentine. Reprinted with permission from ref. [33].
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Figure 8. (a) Friction coefficient; (b) SEM morphology of the worn surface on the disk; (c) XPS spectra of the elements on the worn surface lubricated by the base oil and the oil containing 0.5 wt% AP. Reprinted with permission from ref. [48].
Figure 8. (a) Friction coefficient; (b) SEM morphology of the worn surface on the disk; (c) XPS spectra of the elements on the worn surface lubricated by the base oil and the oil containing 0.5 wt% AP. Reprinted with permission from ref. [48].
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Figure 9. Cross-sectional SEM images and element line scanning of the worn surface lubricated with: (a) base oil, (b) oil containing 0.5 wt% attapulgite. Reprinted with permission from ref. [50].
Figure 9. Cross-sectional SEM images and element line scanning of the worn surface lubricated with: (a) base oil, (b) oil containing 0.5 wt% attapulgite. Reprinted with permission from ref. [50].
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Figure 10. Schematic of the lubricating models of the base grease and attapulgite grease with graphene. Reprinted with permission from ref. [56].
Figure 10. Schematic of the lubricating models of the base grease and attapulgite grease with graphene. Reprinted with permission from ref. [56].
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Table 1. Comparative analysis of phyllosilicates as lubricant additives.
Table 1. Comparative analysis of phyllosilicates as lubricant additives.
Phyllosilicate TypeChemical StructurePhysical/Chemical
Properties		ProsConsTribological
Mechanisms
MuscoviteKAl2[AlSl3O10] (OH)2Average coefficient of friction: 0.05–0.15
Average wear diameter: 30–50% reduction		Excellent heat resistance and chemical stability
Forms an effective anti-wear protective layer		Higher preparation costs
Surface modification may be required during application		Layer structure and formation of tribofilm
TalcMg6Si8O20(OH)4Average coefficient of friction: 0.03–0.10
Average wear diameter: 20–40%		Relatively low costRespirable particles can be harmful to health
Performance degradation in extreme conditions		Formation of oxide transfer films
Montmorillonite(Na,Ca)0.33(Al,Mg)2[Si4O10](OH)2·nH2OAverage coefficient of friction: 0.03–0.10
Average wear diameter: 20–40%		Excellent lubricity
Relatively low cost		Requires specific surface treatments to improve performanceLayered structure and formation of protective tribofilm
SerpentineMg6[Si4O10] (OH)8Average coefficient of friction: 0.06–0.18
Average wear diameter: 25–50%		Good heat and corrosion resistance
Forms a self-healing protective layer		Performance is strongly influenced by temperature and environmental conditions
Further research is needed to optimize performance		Formation of a protective tribofilm
AttapulgiteMg5Si8O20(OH)2(OH2)4⋅4H2OAverage coefficient of friction: 0.04–0.13
Average wear diameter: 20–40%		Excellent self-healing ability
Excellent chemical stability		Sensitive to heat
Further surface modification may be required to improve performance		Formation of a protective tribofilm
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MDPI and ACS Style

Jiang, N.; Nan, F. Applications of Phyllosilicates Mineral Powder as Anti-Wear Lubricating Materials in Lubricating Oil and Grease: A Review. Lubricants 2024, 12, 324. https://doi.org/10.3390/lubricants12090324

AMA Style

Jiang N, Nan F. Applications of Phyllosilicates Mineral Powder as Anti-Wear Lubricating Materials in Lubricating Oil and Grease: A Review. Lubricants. 2024; 12(9):324. https://doi.org/10.3390/lubricants12090324

Chicago/Turabian Style

Jiang, Nan, and Feng Nan. 2024. "Applications of Phyllosilicates Mineral Powder as Anti-Wear Lubricating Materials in Lubricating Oil and Grease: A Review" Lubricants 12, no. 9: 324. https://doi.org/10.3390/lubricants12090324

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