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. 2025 Sep 5;10(36):41089–41103. doi: 10.1021/acsomega.5c03694

Synergistic Mechanism of Two-Dimensional Layered G/MoS2/h-BN Ternary Composite Additives on the Tribological Properties of Lubricating Grease

Weide Jing †, Junyang Wang ‡,§, Xinlong Yang †, Gewen Yi ‡,§,*, Shanhong Wan ‡,§,*
PMCID: PMC12444512  PMID: 40978356

Abstract

Two-dimensional nanomaterials as grease additives have demonstrated significant research progress in the surface modification of mechanical friction pairs. However, single-component systems often struggle to meet composite lubrication demands under extreme conditions due to the antagonistic effect between antiwear and friction-reduction properties, limiting their engineering application. In this study, five greases were prepared using a multivariate compounding method. Among them, the ternary additive grease (GG2M2B), formulated with a mass ratio of 1:2:2 (1 wt % graphite, 2 wt % MoS2, 2 wt % h-BN), exhibited superior stability and temperature resistance, showing a 6.5% increase in maximum operating temperature. The GG2M2B additive proved effective across wide load ranges (50–100 N) and at temperatures up to 200 °C. Analysis revealed that the additive reduced friction by leveraging its friction-induced physical properties. Specifically, it formed a stable tribo-film on the friction surfaces through a combination of friction-induced physical/chemical adsorption and tribochemical reactions. This resulted in a 49.8–52.7% reduction in the coefficient of friction and a 75–81.3% reduction in wear volume compared to the base grease. Further investigation indicated that the composite additive (G/MoS2/h-BN) responded synergistically to high temperature, shear stress, and frictional heat. It underwent tribochemical reactions with iron (Fe) and oxygen (O) from the subsurface material, generating a series of compounds. These reactions significantly enhanced the friction performance and antiwear effectiveness. This study provides a novel component design strategy and theoretical foundation for developing adaptive composite grease additives.

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1. Introduction

Wear, as one of the dominant factors leading to the degradation of engineering system efficiency and component failure, fundamentally stems from the material loss mechanism at contact interfaces under relative motion. In mechanical systems, sliding wear and friction effects are coupled, resulting in significant energy loss. To address this challenge, lubricating media (such as lubricating oils and greases) are widely used for friction and wear control in rolling/sliding pairs. Among them, grease, as the core functional material of rolling bearings, directly affects the tribological behavior and vibration characteristics of bearings. With the advancement of precision machining technology, the bottleneck of bearing reliability has shifted from geometric accuracy to material performance. Developing greases with excellent wear resistance, temperature resistance, and long service life has become key to enhancing bearing performance. While significant progress has been made, challenges remain. Further innovation is needed to balance ultimate high temperature and high pressure performance (especially long-term oxidative stability in air above 300 °C) with biodegradability, cost-effectiveness and material compatibility. Their weak metal surface adsorption capability leads to insufficient formation of boundary lubrication films, , making it difficult to effectively suppress direct surface contact under extreme conditions. From a compositional perspective, the grease system mainly consists of base oil (75–80%), thickener (20–25%), and functional additives (<1%). Current research focuses on two directions: first, the development of environmentally friendly base oil and thickener systems; second, the exploration of the optimization mechanisms of green additives on the tribological performance of greases. These explorations provide new insights for breaking through the performance limitations of traditional greases.

The lubricating function of grease primarily relies on the characteristic of the base oil retained by the thickener to form a protective oil film on the friction interface. Under ideal hydrodynamic lubrication conditions, this base oil film can effectively achieve lubrication, ensuring the normal operation of mechanical contact surfaces. However, the mixed lubrication and boundary lubrication states prevalent in actual operating conditions impose more stringent requirements on lubrication performance. With the development of modern mechanical equipment toward wide temperature ranges and high loads, the thermomechanical properties of grease have become a critical factor in determining equipment service life and operational reliability. The practical performance of grease under fluctuating temperature conditions is primarily governed by its shear resistance, rheological behavior, and failure mechanisms (such as material softening and oxidative degradation). These critical parameters are fundamentally determined by the structural synergy between the base oil and thickener, as well as the chemical synergistic effects among additives. Notably, although the mass fraction of functional additives is typically maintained below 1%, their reinforcing effect on the lubrication system often proves decisive under boundary and mixed lubrication regimes. In particular, extreme pressure antiwear agents form strongly adsorbed lubricating films on metal contact surfaces, effectively isolating the friction pair surfaces and significantly reducing wear under boundary lubrication conditions.

To enhance the antifriction and antiwear properties of lubricating grease, researchers have systematically explored optimization strategies for single-component additives. , Xu et al. found a significant correlation between the concentration of nano-graphite and the tribological properties of vegetable oil. Hwang et al. revealed the influence mechanism of nanographite preparation processes on its tribological behavior in mineral oil. Among them, exfoliated graphite nanosheets exhibited excellent friction-reducing and antiwear performance, primarily due to their interlayer sliding effect. Hexagonal boron nitride (h-BN), as a novel lubricating additive, demonstrates great potential in extreme-condition lubrication due to its unique two-dimensional layered structure, excellent thermal stability (>800 °C), environmental friendliness, and self-healing capabilities. Studies have shown that adding 0.6 wt % h-BN to lithium-based grease significantly improves its tribological performance. The mechanism of action is closely related to particle size: 60 nm h-BN forms a protective film by filling rough surfaces, while 500 nm h-BN produces a polishing effect by contacting rough peaks. Additionally, h-BN performs remarkably in vibration reduction, with a 10.0 wt % addition significantly enhancing the vibration-damping performance of complex calcium sulfonate-based grease. Synergistic effects with nano-Al2O3 can further suppress full-frequency band vibrations in bearings. In polyurea grease systems, the synergistic effect of 1.0 wt % h-BN and CaCO3 can significantly improve its tribological performance. Molybdenum disulfide (MoS2), as a classic solid lubricant, possesses characteristics such as high load-carrying capacity, strong surface adsorption, and low friction coefficient. Research indicates that a 3 wt % addition can significantly enhance the extreme pressure and antiwear properties of grease without affecting its oxidation stability and corrosiveness. Its performance is closely related to working conditions: larger particle size MoS2 performs better under low-speed vibrations, while smaller particle size MoS2 excels in high-speed sliding. Notably, MoS2 maintains excellent lubrication performance in extreme environments (−196 to 800 °C), with low vacuum volatility and environmental friendliness. In addition, including micro- and nanoscale oxides (CuO, TiO2), the compound additive copper sulfide, as well as graphene and carbon nanomaterials, among others, have also been investigated. ,

Although a single friction modifier and antiwear agent can enhance the tribological properties of lubricants, the combined use of multiple friction modifiers and antiwear agents in practical applications often yields superior results. For instance, Fu et al. study found that the synergistic effect of organic borate esters and phosphorus-based additives in mineral oil resulted in better tribological performance and improved wear scar morphology compared to single-component modifications. Similarly, Qu et al. study demonstrated that the combination of phosphoniumalkylphosphate ionic liquids and zinc dialkyldithiophosphate in oil exhibited enhanced friction reduction and antiwear properties. Additionally, Tong’s research highlighted the synergistic effect of WS2 microparticles and hollow MoS2 nanoparticles in carbon fiber fabric/phenolic composites, which reduced stress concentration, inhibited carbon fiber fracture and pull-out, and thereby improved the material’s load-bearing capacity and tribological performance. Alghani et al. also discovered that the synergistic improvement effect of TiO2/graphene nanoparticles was superior to that of individual TiO2 nanoparticles or graphene alone. Based on these findings, this study utilized Great Wall FPNR grease as the base oil and incorporated three additives-graphite, MoS2, and h-BN-mixed in varying concentration ratios. The physicochemical properties and lubrication performance of the prepared grease were thoroughly investigated. The tribological performance of the grease with different additive ratios was analyzed through fretting friction experiments. Furthermore, the synergistic effects of the additive combinations in the grease were examined, and the friction reduction and antiwear mechanisms were explained by analyzing the morphological changes, chemical composition, and structure of the worn surfaces.

2. Materials and Methods

2.1. Materials

In this study, Great Wall FPNR grease was used as the base grease, and the additives for preparing composite grease include graphite (G), molybdenum disulfide (MoS2, M), and hexagonal boron nitride (h-BN, B). The additives were added to the base grease in specific proportions and mechanically stirred for 30 min at room temperature using a Heidolph Hei Torque Precision 400 mixer to ensure uniform dispersion of the additive particles. Subsequently, a three-roll mill (80E, Exakt) was used for three homogenization cycles to further enhance the uniformity of the grease. The prepared composite greases include the following five formulations: FPNR grease (referred to as G); 1% graphite + 1% molybdenum disulfide (referred to as GGM); 1% graphite + 1% molybdenum disulfide + 1% hexagonal boron nitride (referred to as GGMB); 1% graphite + 1% molybdenum disulfide + 2% hexagonal boron nitride (referred to as GGM2B); 1% graphite + 2% molybdenum disulfide (referred to as GG2M); and 1% graphite + 2% molybdenum disulfide + 2% hexagonal boron nitride (referred to as GG2M2B).

2.2. Friction Tests

The tribological performance of grease samples in terms of antifriction and antiwear properties was evaluated using a fretting friction tester (Optimol SRV-IV, Optimol Instruments Prüftechnik GmbH, Germany). The test employed a friction pair consisting of a steel test ball sliding against a fixed steel test disk, with the test grease applied between them. Both the test ball (diameter: 10 mm, average roughness: 25 nm, Rockwell hardness: 62–65 HRC) and the test disk (diameter: 24 mm, thickness: 7.85 mm, average roughness: 70 nm, Rockwell hardness: 62–65 HRC) were fabricated from 52100 standard bearing steel. The test grease was applied to the stationary disk specimen. A precise amount of grease, 5 mg, was dispensed using a calibrated syringe or micropipette. The grease was then uniformly spread over the entire wear track area on the disk surface using a clean, nonabrasive applicator. The SRV tests were conducted under the following parameters: normal loads of 50 and 100 N, oscillation amplitude of 2 mm, frequency of 20 Hz, test temperature of 200 °C, and duration of 60 min. To ensure data reliability, each test group was repeated three times. Friction coefficient was recorded continuously by the SRV-IV instrument throughout the test duration. The wear volume on the disk was quantified using a noncontact optical profilometer.

2.3. Characterization

The dropping point and cone penetration of the grease were characterized using a DW-168 wide-temperature-range dropping point tester (Dalian Wansheng Petroleum Analytical Instrument Factory, China) and an SYP 4100-I grease cone penetrometer (Shanghai Jingxi Instrument Manufacturing Co., Ltd., China), respectively. This study investigates the influence of additives on the rheological behavior of grease through dynamic rheological analysis. Experiments were conducted using an MCR302 rheometer (Anton Paar, Graz, Austria) under a constant temperature of 25 °C, employing small amplitude oscillatory shear (SAOS) testing. By scanning the angular frequency range from 0.1 to 100 rad/s at a fixed shear stress of 0.1 Pa, the viscoelastic response characteristics of the material were systematically characterized, including storage modulus (G′), loss modulus (G″), and loss factor (tan δ). To ensure data consistency, all test samples were collected from standardized sampling points of the same production batch, effectively controlling experimental systematic errors. In accordance with the GB/T 27761-2011 standard, the high-temperature resistance of the grease was evaluated using an STA449 F3 simultaneous thermal analyzer (Netzsch-Gerätebau GmbH, Selb, Germany) equipped with an S-type (Pt/PtRh) TG-DSC sample holder under a nitrogen atmosphere from 25 to 800 °C, with a heating rate of 10 °C/min. The functional groups of the additives and grease were analyzed using a Fourier transform infrared spectrometer (FT-IR; Thermo Fisher Scientific, Germany). The wear volume of the scars on the test disk was measured using a noncontact 3D surface mapping profiler (KLA-Tencor). Additionally, the surface morphology of the additives and friction specimens was characterized using a scanning electron microscope (SEM; JSM-5600LV, Japan), and chemical analysis/mapping was performed using energy-dispersive spectroscopy (EDS). The surface composition of the additives and friction specimens was analyzed using an X-ray diffractometer (XRD; D8-Discover 25, Bruker, Germany) and a Jobin Yvon HR800 confocal Raman system, with XRD measurements conducted over a range of 10°–90° and Raman spectra collected over a range of 100–4000 cm–1. The chemical composition of the worn surfaces after friction was analyzed using a monochromatic Al Kα X-ray photoelectron spectrometer (XPS; ULVAC-PHI 5000 Versaprobe III, Japan).

3. Results

3.1. Chemical Structure and Morphology Characterization of Additives

The molecular structure of the additives was analyzed by Fourier transform infrared spectroscopy (FTIR), as shown in Figure a. The characteristic absorption peak of graphite at 1614 cm–1 corresponds to the CC skeletal vibration of sp2-hybridized carbon atoms. In the FTIR spectrum of MoS2, the peaks at 1427 cm–1 and 1112 cm–1 are attributed to the stretching vibration of S–Mo bonds and in-plane bending vibrations, respectively. For h-BN, the absorption peaks at 1307 cm–1 and 763 cm–1 originate from the stretching vibration of B–N bonds and the bending mode of B–N–B bonds, respectively. To further elucidate the structural features of the additives, Raman spectroscopy was employed (Figure b). Graphite exhibits a prominent G peak at 1580 cm–1, reflecting the in-plane vibration of sp2-hybridized carbon atoms. The G′ peak (2D peak) at 2700 cm–1 arises from a double-resonance effect in the two-phonon scattering process, with its peak shape highly correlated to the number of graphene layers and stacking orderliness. In the Raman spectrum of MoS2, the E12g mode at 382 cm–1 corresponds to in-plane vibrations of Mo–S bonds, while the A1g mode at 407 cm–1 characterizes the out-of-plane interlayer vibrations of MoS2. The peak separation of 25 cm–1 between these modes indicates a few-layer structure. The E1g mode of h-BN at 1366 cm–1 is ascribed to in-plane atomic vibrations within its hexagonal lattice. The crystal structure of the additives was further confirmed by X-ray diffraction (XRD) analysis (Figure c). Graphite displays sharp diffraction peaks at 26.5° (002), 42.3° (100), and 77.5° (110), with the (002) interplanar spacing (0.335 nm) confirming a highly ordered layered stacking structure. For MoS2, diffraction peaks at 14.4° (002), 32.7° (100), 49.8° (105), and 58.3° (110) are observed, and the (002) interplanar spacing (0.615 nm) verifies its typical layered sulfide structure. The XRD pattern of h-BN exhibits characteristic peaks at 26.7° (002), 41.6° (100), 50.1° (102), and 76.0° (110), with the (002) interplanar spacing (0.333 nm) approaching the theoretical value, demonstrating excellent crystallinity. Scanning electron microscopy (SEM) morphology analysis (Figure d–f) reveals hierarchical composite structures in all three additives. Graphite consists of micrometer-scale layered stacks with curled and wrinkled edges. MoS2 exhibits a pine-like fractal structure, where secondary dendritic branches provide abundant active edge sites. h-BN displays interwoven nanosheets with submicron pores. This hierarchical “layer-sheet” architecture significantly enhances the material’s specific surface area, offering an ideal platform for molecular adsorption and interfacial reactions.

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Composition and SEM morphology of the grease additives. (a) FTIR spectrum, (b) Raman spectrum, (c) XRD pattern, (d) SEM morphology of graphite, (e) SEM morphology of MoS2, (f) SEM morphology of h-BN.

3.2. Compositional Structure and Physical Properties of Lubricating Grease

Figure illustrates the preparation process flow of lubricating grease (Figure a) and its optical morphological evolution. The grease was prepared at room temperature (25 ± 2 °C). First, a specified proportion of additive powders (graphite, MoS2, h-BN, etc.) was incrementally incorporated into the base grease (G). Subsequently, primary dispersion was achieved using a high-speed shear mixer. Finally, a three-roll mill was employed to further reduce particle agglomeration, ensuring uniform dispersion of additives within the grease matrix. Through optical morphology observation (Figure b–g), the base grease G (Figure b) exhibited a homogeneous green colloid with no visible particle aggregation, indicating a dense structure of the pristine grease matrix. Modified greases (Figure c–g) appeared black due to the light-absorbing properties of carbon-based additives (graphite/MoS2). The h-BN nanosheets were uniformly dispersed in h-BN-modified systems (Figure d,e,g), and their high refractive index accentuated the visibility of white particulate morphology.

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Schematic diagram of preparation process and optical morphology of grease, (a) preparation process, (b) G, (c) GGM, (d) GGMB, (e) GGM 2 B, (f) GG 2 M, (g) GG2M2B.

The FTIR spectrum of the lubricating grease is shown in Figure a. The characteristic absorption peaks at 2924 cm–1 (asymmetric stretching vibration, –CH2–) and 2851 cm–1 (symmetric stretching vibration, –CH3) are attributed to the alkyl chains of the base oil. Peaks at 1466.5 cm–1 (scissoring bending vibration) and 1375 cm–1 (symmetric bending vibration) further confirm the long-chain alkane structure of the grease matrix. The absorption peak at 1072.5 cm–1 corresponds to the stretching vibration of the SO bond (sulfate ester group), suggesting the presence of extreme pressure additives or oxidation byproducts in the grease. The results of grease penetration and dropping point analysis are shown in Figure b. The penetration value of the base grease G decreased from 359.25 (0.1 mm) to 330.75 (0.1 mm) for GG2M2B after adding composite additives, indicating a 7.9% improvement in grease consistency. The dropping point ranged from 276.67 °C (G) to 282 °C (GG2M2B), with no statistically significant deviation, confirming that the composite additives do not disrupt the three-dimensional soap fiber network. Thermogravimetric analysis (TGA) is shown in Figure c. GG2M2B exhibited the highest initial decomposition temperature (282.2 °C), representing a 6.5% increase compared to the base grease G (265.6 °C). The thermal weight loss curve of the grease generally shows a two-stage decomposition. The first is the base oil volatilization and thickener degradation within the range of 280–400 °C (mass loss of ∼65%). The second is the oxidation of the residual carbonization layer within the range of 400–550 °C (mass loss of ∼30%). Composite additives significantly delayed stage 1 decomposition, attributed to the physical barrier effect of h-BN/MoS2 and the thermal conductivity enhancement by graphite. Differential scanning calorimetry (DSC) is shown in Figure d. All samples showed a glass transition (T g) temperature near 260 °C, corresponding to the softening of the soap fiber network. For the base grease G, an endothermic peak at 350 °C indicated phase transition to liquid, consistent with the dropping point. A subsequent exothermic peak at 390 °C and oxidative decomposition above 450 °C led to lubrication failure. While GG2M2B exhibited a higher melting temperature (415 °C), indicating that composite additives improved the thermal stability of the grease. This is mainly due to the high thermal conductivity of hexagonal boron nitride accelerated heat dissipation, while its layered structure strengthened interfacial bonding with soap fibers. MoS2 nanosheets filled gaps in the soap fiber network, suppressing structural collapse at high temperatures. Interlayer slip behavior of graphite alleviated thermal stress concentration and delayed crack propagation.

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Compositional structure and physical properties of lubricating grease, (a) FTIR spectra, (b) dropping point and penetration, (c) TGA, (d) DSC.

The rheological properties of lubricating grease are intrinsically linked to its microscopic network structure. The storage modulus (G′) and loss modulus (G″), as critical characterization parameters, reflect the system’s ability to store elastic deformation energy and dissipate mechanical energy, respectively. When G′ dominates (G′ > G″), the material exhibits solid-like behavior, with enhanced confinement of the base oil by the network structure. Conversely, when G″ predominates, the system displays fluid-like characteristics. Experimental data (Figure a,b) demonstrate that the introduction of additives significantly enhances both modulus values, fundamentally revealing the reinforcement of the thickener’s threedimensional network structure. Graphite, characterized by its nonpolar nature, possesses a layered architecture formed by sp2-hybridized carbon atoms. The absence of polarity stems from its perfectly planar hexagonal carbon rings, with interlayer cohesion relying exclusively on weak van der Waals interactions. This chemical inertness hinders substantial interactions with nonpolar hydrocarbon-based oils or metal soap thickeners, allowing graphite to primarily modify the system’s rheological properties through physical filling mechanisms. The exposed layer of sulfur atoms in the S–Mo–S sandwich structure of molybdenum disulfide (MoS2) is highly chemically active. Its special electronic structure (lone pair electrons of sulfur atoms) can trigger three synergistic effects, including the formation of chemisorption films with metal surfaces to enhance extreme pressure performance; the enhancement of network node connection strength through dipole–dipole interactions between sulfur atoms and polar groups (e.g., hydroxyls, carboxylates) of thickeners; and the formation of spatially situated resistance-stabilized colloidal dispersion systems in base oils to prevent particle agglomeration. The ionic character of B–N bonds (∼42% ionic contribution) imparts moderate polarity. This dual nature enables interfacial interactions with polar components while maintaining structural integrity. The mechanism of action mainly includes reinforcement of network interweaving through electrostatic attraction between Bδ+–Nδ− dipoles and charged thickener groups; topological confinement of base oil molecular motion by 2D nanosheets; improvement thermodynamic stability via high thermal conductivity. Polar groups on additive surfaces form hydrogen bonds, coordination bonds, or other secondary interactions with specific functional groups on thickener fibers, increasing network cross-linking density. The stable dispersion of nanosheets in base oils creates a three-dimensional physical barrier that restricts relaxation of thickener fibers. This structural reinforcement elevates the system’s yield stress, enhances thixotropic recovery, and ultimately leads to increased viscoelasticity and reduced fluidity.

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Storage modulus G′, loss modulus G″ (a) and loss coefficient tan δ of greases (b).

3.3. Tribological Properties of Grease

The friction-reduction and antiwear properties of the base grease (G) and the composite additive-modified grease were systematically evaluated under high-temperature conditions at 200 °C using SRV oscillating friction tests, as shown in Figure . The base grease G exhibited significant fluctuations during the friction process (Figure a,b), indicating a nonsteady-state process of continuous rupture and reformation of the lubricating film. After the addition of composite additives, the fluctuations in the friction coefficient curve were significantly improved, with the GG2M2B grease demonstrating the most stable and smooth friction process, even under a high load of 100 N, confirming its excellent high-temperature lubricating film stability. The average friction coefficients of the base grease G at 50 N and 100 N were 0.245 and 0.245, respectively (Figure c), with average wear volumes of 0.091 mm3 and 0.152 mm3, respectively (Figure d). In contrast, the GG2M2B grease exhibited average friction coefficients of 0.123 and 0.116 at 50 N and 100 N, respectively (Figure c), and average wear volumes of 0.017 mm3 and 0.038 mm3, respectively (Figure d). Comparative analysis revealed that the average friction coefficient of the GG2M2B grease at 50 N was reduced by 49.8% compared to that of the base grease G, while the wear volume was reduced by 81.3%. At 100 N, the average friction coefficient of the GG2M2B grease was reduced by 52.7%, and the wear volume was reduced by 75% compared to the base grease G.

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Friction coefficient curves with time for different greases at 50 N (a) and 100 N (b), (c) average friction coefficient, (d) wear volume.

When G-type grease is exposed to a combined thermal field of 200 °C and frictional heating, the base oil components undergo intense volatilization, triggering phase transformation and ultimately leading to a solidified, oil-depleted hardened state, which significantly accelerates the lubrication failure process. Thermodynamic analysis indicates that increasing temperature gradients promote metallurgical bonding tendencies at friction interfaces, resulting in elevated friction coefficients. In contrast, GG2M2B-type grease, through the synergistic effects of two-dimensional layered materials h-BN/MoS2 (as shown in Figure c,d), achieves a thermal decomposition temperature exceeding 280 °C. Consequently, this composite lubrication system rapidly forms a dense boundary film during the initial friction stage, maintaining a stable friction coefficient of 0.1 ± 0.03, demonstrating excellent lubrication performance.

The optical morphology and surface roughness analysis of the wear scars for greases G and GG2M2B under different loads are shown in Figure . Comparative analysis reveals that the wear scar area of the base grease G is significantly larger, with distinct ploughing grooves observed on the worn surface (Figure a,b), indicating severe rupture of the lubricating film during the friction process, which exacerbates abrasive and adhesive wear. In contrast, the worn surface of GG2M2B is smooth and flat (Figure c,d), with only minor surface plastic deformation observed, further confirming the excellent antiwear and friction-reduction properties of the composite additive-modified grease.

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Three-dimensional wear morphology of grease G and GG2M2B under different loads, (a) G-50N, (b) G-100N, (c) GG2M2B-50N, (d) GG2M2B-100N, and (e) surface data of the wear surfaces of G and GG2M2B under different loads (R a: roughness, R z: maximum contour height).

Additionally, the surface roughness (R a) of grease G at 50 N and 100 N is 9.23 and 14.33 μm, respectively, while the maximum profile height (R z) is 30.31 and 47.17 μm, respectively (Figure e). For GG2M2B, the surface roughness at 50 N and 100 N is 1.8 and 6.09 μm, respectively, and the maximum profile height is 7.88 and 23.75 μm, respectively (Figure e). Comparative analysis shows that the surface roughness of GG2M2B at 50 N is reduced by 80.5% compared to that of grease G, while the maximum profile height is decreased by 74%. At 100 N, the surface roughness of GG2M2B is reduced by 57.5%, and the maximum profile height is decreased by 49.7% compared to grease G. The reduction in surface roughness (R a) to 1.8 μm (at 50 N) demonstrates that the composite additives effectively fill the surface asperities of the friction surface. The 74% reduction in R z proves that the nanoadditives smooth the contact surface through a “micropolishing” effect.

3.4. The Characterization of Wear Scar after Tribological Test

Figures and present the SEM morphology and EDS elemental analysis results of the wear surfaces for greases G and GG2M2B under different loads, respectively. Through comparative analysis, the wear mechanisms of the two greases under high-temperature friction conditions and the antiwear mechanisms of the composite additives are revealed. Figure a,b show that the wear surface of grease G exhibits numerous ploughing grooves and spalling pits. As the load increases from 50 N to 100 N, the wear area expands significantly, with deeper grooves and pits forming on the worn surface, indicating severe failure of the lubricating film under high temperatures. EDS elemental mapping reveals that the worn surface is primarily composed of Fe, C, and O elements. The evaporation of the base oil at high temperatures causes the grease to dry out, leading to the breakdown of the lubricating film’s continuity. The presence of O elements confirms that high-temperature oxidative wear occurs on the wear surface under the combined effects of elevated temperature and frictional heat. In contrast, Figure a,b demonstrate that the wear surface of GG2M2B grease is smooth and flat, with only minor surface plastic deformation observed. The wear area is narrow and shallow, with no deep grooves or large spalling pits. The worn surface is mainly composed of Fe, C, O, N, Mo, and S elements, as revealed by EDS analysis. The results indicate that the composite additives form a dynamic protective film on the steel ball surface through ionic adsorption. The layered structures of MoS2 and h-BN release nanosheets during the friction process, filling the surface asperities. As a result, the additives eliminate the inhomogeneity of the metal surface morphology and significantly improve the tribological behavior of the grease.

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SEM morphology and EDS of the wear surface of grease G under different loads, (a) G-50N, (b) G-100N.

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SEM morphology and EDS of the wear surface of grease GG2M2B under different loads, (a) GG2M2B-50N, (b) GG2M2B-100N.

To further analyze the material composition of the wear surfaces and the tribochemical reaction mechanisms when using greases G and GG2M2B as lubricants, systematic XRD and Raman spectroscopy characterizations were performed on the wear scar regions based on the morphological features of the wear tracks (Figure ). The results indicate that the two greases exhibit significantly different surface chemical behaviors during the friction process. The XRD analysis of the wear surface detected only Fe diffraction peaks (Figure b), which may be related to the amorphous structure of the grease decomposition products under high temperatures. The Raman results are shown in Figure b. When grease G was used as the lubricant, no distinct characteristic peaks were observed on the wear surface, indicating that the decomposition products of the grease failed to form an effective protective film. In contrast, when GG2M2B was used as the lubricant, distinct characteristic peaks were observed on the wear surface. The characteristic peaks of organic carbon were observed at 1350 cm–1 (D) and 1620 cm–1 (D′), confirming the formation of a protective layer composed of organic carbides generated from grease decomposition and graphite additives on the wear scar surface. The characteristic peaks at 380 cm–1 (E12g) and 410 cm–1 (A1g) correspond to the in-plane and out-of-plane atomic vibrations of MoS2, respectively, verifying the release and adsorption of MoS2 nanosheets during the friction process. The characteristic peak at 1370 cm–1 (E2g) reflects the in-plane atomic vibration of h-BN, indicating that h-BN participated in the tribochemical reactions and contributed to the formation of a protective film.

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Wear surface XRD (a) and Raman analysis (b) of grease G and GG2M2B at different loads.

X-ray photoelectron spectroscopy (XPS) can accurately resolve the elemental composition and chemical state of wear surfaces and provide molecular-level evidence for the compositional evolution of friction interfaces. In this study, the wear surface of GG2M2B grease under different loads was analyzed by XPS (Figure ), revealing the friction chemical reaction mechanism under the synergistic effect of additive package. Seven elements, Fe, C, O, N, B, Mo, and S, were detected on the wear surfaces of GG2M2B grease under 50 N and 100 N loads (Figure a,b). The presence of nonmatrix elements C, N, B, Mo, and S indicates that the additives are enriched at the friction interface. In order to demonstrate the mechanism of the composite additives, five elements (C, N, B, Mo and S) were further selected for fine spectral analysis. C 1s peak at 284.5 eV was attributed to the C–C bond (Figure a,g). N 1s peak at 398.5 eV was attributed to the B–N bond (Figure b,h). The N 1s peak around 406 eV may be related to Fe4N (iron nitride) (Figure g). S 2p peaks at 163 and 168.5 eV were attributed to the –S–S– bond and the SO4 2– bond (Figure c,i). The B 1s peak at 190 eV is attributed to the B–N bond (Figure d,j).

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High-resolution XPS spectra of the wear surface of GG2M2B at different loads, (a) 50 N, (b) 100 N.

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XPS fine spectrum analysis of the worn surface of GG2M2B at 50 N (a–e) and 100 N (f–j), (a,g) C 1s, (b,h) N 1s, (c,i) S 2p, (d,j) B 1s, (e,k) Mo 3d, and (f,l) Mo 3d.

The appearance of the B 1s peak around 184 eV is usually associated with the oxidation state of boron (B2O3) (Figure i). This is because in some cases the low-valent oxides of boron or oxygen adsorbed on the surface may lead to a shift of the binding energy in the low-energy direction. The Mo 3d peaks at 228 and 232 eV are attributed to the Mo–S-bond and MoO3 (Figure e,k). The lamellar structures of graphite (C–C), h-BN (B–N), and MoS2 (Mo–S) have low interlayer bonding energies, which are prone to interlayer slippage under shear. C–C, B–N, and Mo–S have a hexagonal crystal structure, and the low interlayer bonding in the crystals can provide good lubrication. Meanwhile, the S atom of MoS2 is easy to form chemisorption with Fe surface through lone pair of electrons. h-BN is easy to form coordination bonds with metal oxides at the Bδ+ end of h-BN. It is easy to form adsorption layer during friction. The friction heat triggers the partial oxidation of MoS2 to generate MoO3, which forms a passivation film together with SO4 2–, and the composite oxide film can inhibit the adhesive wear. Graphitic carbon is prone to rearrangement to fullerene-like structure under high pressure, providing an ultralow shear interface. In turn, a reaction layer is formed. h-BN nanosheets fill the surface pits and reduce the contact stress. In addition, the high bonding energy of B–N bonds maintains the structural integrity of the lubrication film at 200 °C. The results of the above analyses show that under boundary lubrication conditions, the additive molecules adsorbed on the metal surface react with elements such as the metal surface during friction to form a stable lubrication film on the friction surface.

4. Discussion

Figure schematically illustrates the GG2M2B grease lubrication mechanism. Panel (a) depicts the steel–steel contact configuration. Figure b–e summarize the frictional lubrication process of GG2M2B at 200 °C and elucidate its dynamic wear repair mechanism during smooth friction. At elevated temperatures (200 °C), significant evaporation of the grease base oil occurs due to both ambient heat and frictional heating, as evidenced in Figures and . This evaporation triggers lubrication failure and leads to severe wear within the contact zone. To mitigate this issue, a composite additive comprising graphene (G), molybdenum disulfide (MoS2), and hexagonal boron nitride (h-BN) was incorporated into the grease formulation. The G/MoS2/h-BN composite interacts with the soap fibers of the grease, forming an intertwined three-dimensional network structure. This interaction results in a looser soap fiber network with a coarser morphology. Crucially, the composite additive impedes the expansion of soap fiber junctions under shear stress. Consequently, significant stretching and directional alignment of the fibers occur only at larger deformations. , Simultaneously, shear forces within the friction pair induce tribochemical reactions, generating electrons and reactive iron ions (Fe+). This process creates a net positive charge on the surface of the steel ball (Figure a). The composite additive, bearing negatively charged oxygen-containing functional groups (e.g., CO, B–O), is electrostatically attracted to this positively charged interface. This electrostatic interaction facilitates the formation of a robust physically adsorbed layer at the friction interface. The inherent properties of the G/MoS2/h-BN compositenamely its high modulus, high strength, and low interlayer shear strengthcontribute significantly to friction reduction. These properties enable the composite to bear high loads while facilitating easy interlayer sliding under shear. Furthermore, during friction, the layered structure of G/MoS2/h-BN allows dissociation along the planes. The resulting nanolamellae effectively fill surface craters and spalling regions. Critically, these lamellae cover the asperities of the contacting surfaces, preventing direct metal-to-metal contact and alleviating localized contact stress concentrations. The effectiveness of this mechanism is concentration-dependent. At low concentrations of the G/MoS2/h-BN composite, the amount of material is insufficient to adequately fill the defects within the wear track, resulting in limited friction reduction (Figure ). In contrast, the GG2M2B formulation, containing a high concentration of the composite additive, exhibits a superior lubrication mechanism. A portion of the G/MoS2/h-BN adheres tenaciously to the wear surface via physical adsorption, establishing a stable lubricating film. The remaining composite particles circulate freely within the grease bulk. This mobile fraction acts as a reservoir, dynamically replenishing the tribofilm in wear-damaged areas. This continuous supply is essential for maintaining the integrity and functionality of the lubricating film under the demanding high-temperature, high-shear conditions, thereby enabling sustained low friction and wear protection.

12.

12

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Schematic of macroscopic lubrication mechanism using GG2M2B as lubricant, (a) schematic diagram of steel–steel friction mates contact, (b–d) synergistic lubrication with composite additives, (e) dynamic wear repair mechanism during smooth friction.

For the friction chemistry mechanism of G/MoS2/h-BN additive package in grease, the lubricant film formation law was revealed by surface morphology analysis and element distribution characterization. As shown in Figure , the friction surface of GG2M2B grease system is smooth under the load condition of 50 N, which is mainly due to the presence of elements such as C, S, Mo, B and N in (G/MoS2/h-BN), which form lubricant film on the wear surface through the combination of physical adsorption and friction chemical reaction. The formation of the lubricant film follows a three-stage mechanism of “physical adsorptionchemical activation–friction reaction”. First, the nanoadditives form a physical adsorption film on the metal substrate by surface charge, and the XPS analysis shows that at this time, the Mo 3d peak is located at 228 eV (corresponding to the characteristic peak of MoS2), and the B 1s peak is located at 190 eV (the characteristic peak of h-BN) (Figure ). Second, with the elevation of contact stress, the frictional heat increases the surface temperature and induces the lattice activation of h-BN. In addition, under the mechanical-thermal synergy, MoS2 undergoes lattice slip while C elements build graphitised transition layers through sp2 hybridization. , At the same time, elements C, S, Mo, B, and N undergo friction chemical reactions with the friction subsurfaces of Fe and O to generate a series of compounds mixed films, including FeS, MoO3, Fe4N, and B2O3. These compounds exist on the wear surface in the form of tribological film or transfer film, which can significantly improve the friction-reduction performance and antiwear effect. Provide stable lubrication performance under extreme conditions (e.g., high temperature, high load).

The boundary film of lubricating grease exhibits multiphase composite characteristics, primarily consisting of thickener fibers, base oil molecular chains, additive nanoparticles (graphite, MoS2, h-BN), and tribochemical reaction products (FeS, B2O3, etc.). Through rheological–tribological correlation analysis (Figures and ), the performance transition mechanism during the evolution of the boundary film has been revealed. The tribological behavior of boundary films in lubrication systems exhibits distinct stage-dependent characteristics. The frictional response is governed by the rheological behavior of the base lubricant medium during the dynamic formation stage of the interfacial film. While upon reaching a steady-state through shear-induced structural reorganization at the interface, the frictional energy dissipation mechanisms become primarily determined by the intrinsic properties of surface-engineered tribofilms. The storage modulus G′ is positively correlated with the strength of the boundary film of the lubricating grease. A high G′ helps enhance the strength and durability of the boundary film, making the boundary film adsorbed on the metal surface less prone to damage. This effectively prevents direct metal-to-metal contact between friction pairs. The storage modulus G′ of GG2M2B grease is higher than that of G grease (Figure ). Thus, the coefficient of friction of GG2M2B grease is the lowest and most stable (Figure ).

5. Conclusion

In this work, the multilevel synergistic mechanism of G/MoS2/h-BN composite additives in FPNR greases is revealed through multiscale characterization, and the main innovative conclusions are as follows:

  • (1)

    The addition of G/MoS2/h-BN composite additive increases the initial decomposition temperature of the grease to 282 °C, which is a 6.5% improvement compared to the initial decomposition temperature of the original grease (265.6 °C). The penetration decreases from 359.25 (0.1 mm) of the original grease (G) to 330.75 (0.1 mm) of the grease with the additive (GG2M2B), resulting in an increase in material consistency of about 7.9%. There is no significant change in the dropping point. Moreover, the addition of the G/MoS2/h-BN composite additive significantly enhances the storage modulus (G′).

  • (2)

    In the wide load range of 200 °C and 50–100 N, the G/MoS2/h-BN additive composite formed a stable friction film on the friction surface through friction-induced physical/chemical adsorption and friction chemical reaction, which resulted in a smooth homogeneity of the GG2M2B system and a reduction of the coefficient of friction by 49.8–52.7% and a reduction of the wear volume by 75–81.3% compared with that of the base grease.

  • (3)

    C, S, Mo, B and N elements present in the composite additive (G/MoS2/h-BN) form a lubricant film on the wear surface through the combination of physical adsorption and friction chemical reaction. The lubrication film includes physically adsorbed film (MoS2, h-BN) and chemically reactive film (sp2 hybridization to build a graphitised transition layer, and mixed reactive film consisting of FeS, MoO3, Fe4N and B2O3).

  • (4)

    The energy storage modulus G′ is positively correlated with the strength of the grease boundary film. The higher G′ is, the strength and durability of the boundary film is enhanced, and the boundary film adsorbed on the metal surface is less likely to be destroyed.

Acknowledgments

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB 0470302), National Natural Science Foundation of China (Grant Nos. 52072380 and 52405229), Major Science and Technology Project of Gansu Province (No. 23ZDGA011), the Basic Research Innovation Group Project of Gansu Province (No: 24JRRA785), the Open Project of State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (No. LSL-2311) and Central Government Guided Local Science and Technology Development Fund (No. 211084337120).

The data that support the findings of this study are available from [THIRD PARTY NAME] but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of [THIRD PARTY NAME].

Weide Jing: Writingreview and editing, Writingoriginal draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Junyang Wang: Writingreview and editing, Visualization, Validation, Software, Data curation. Xinlong Yang: Data curation, Methodology. Gewen Yi: Supervision, Funding acquisition. Shanhong Wan: Supervision, Funding acquisition.

The authors declare no competing financial interest.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The data that support the findings of this study are available from [THIRD PARTY NAME] but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of [THIRD PARTY NAME].

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