Next Article in Journal
The Low Friction Coefficient and High Wear Resistance UHMWPE: The Effect of Pores on Properties of Artificial Joint Materials
Previous Article in Journal
International Perspectives on Skid Resistance Requirements for Pavement Markings: A Comprehensive Synthesis and Analysis
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Lubricants
Volume 13
Issue 1
10.3390/lubricants13010030
lubricants-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Preparation and Tribological Performance of the Ag/BN Nanocomposite as Additives for Lithium-Based Grease

by
Yijun Chen
1,†,
Chuan Li
1,2,†,
Xiaodong Wang
2,
Li Zhang
1,
Xu Tan
2,
Yubin Peng
3,* and
Xiaoyong Xu
2,*
1
Huangshan Shangyi Rubber and Plastic Products Co., Ltd., Huangshan 245500, China
2
School of Chemistry and Material Engineering, Chaohu University, Hefei 238000, China
3
School of Mechanical and Electrical Engineering, Anhui Jianzhu University, Hefei 230009, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Lubricants 2025, 13(1), 30; https://doi.org/10.3390/lubricants13010030
Submission received: 21 November 2024 / Revised: 8 January 2025 / Accepted: 9 January 2025 / Published: 11 January 2025
Browse Figures
Versions Notes

Abstract

:
This study synthesized a nanocomposite composed of silver nanoparticles and boron nitride (Ag/BN nanocomposite) by depositing Ag nanoparticles on BN surfaces. The chemical composition, structure, micromorphology, and tribological properties of the Ag/BN nanocomposite were investigated using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and a high-speed reciprocating tribometer. Under a 10 N load, 30 min test duration, 50 mm/s sliding speed, and 2 wt% additive concentration, lithium-based grease (LBG) containing Ag/BN nanocomposite exhibited the lowest average friction coefficient of 0.33 and a wear rate of 1.08 × 10−14 m3/(N × m), representing reductions of 35.2% and 41.6%, respectively, compared to pure LBG. A further analysis of the friction mechanism was conducted using 3D laser scanning microscopy, scanning electron microscopy (SEM), and XPS. The results indicate that the Ag/BN nanocomposite effectively reduced friction and wear on the friction pair surfaces through repair mechanisms, the formation of lubricating films, and micro-bearing effects.

1. Introduction

In the automotive industry, economic losses due to friction are substantial. Friction originates from sliding and rotating components in engines, including crankshafts, camshafts, and pistons. Reducing friction between crankshaft journals and bearings, cams and cam followers, pistons and piston rings, and cylinder walls is crucial for decreasing fuel consumption [1]. Consequently, high-quality lubricating greases play an indispensable role in enhancing the friction reduction and wear resistance of mechanical parts in automotive engines, and their quality is vital for protecting the performance of automotive components. As technology advances, addressing the reliability and stability of automotive engines and transmission systems during operation has become imperative. Lithium-based greases (LBG) incorporate a wide range of additives, including antioxidants, extreme pressure and anti-wear agents, rust inhibitors, metal deactivators, viscosity index improvers, demulsifiers, pour-point depressants, and lubricating additives, each serving a unique purpose [2]. Traditional lubricating greases often contain certain amounts of heavy metals and harmful organic substances, which can contaminate the environment when released [3]. Furthermore, traditional lubricants often have strong, irritating odors and high corrosiveness, and their performance may not meet requirements under certain harsh conditions. Therefore, developing novel lubrication solutions has become a key focus for industry development. Researchers have significantly enhanced the performance of base greases by incorporating various types of micro- and nanoparticles [4,5,6].
As lubricant additives, boron nitride (BN) nanomaterials have exhibited excellent lubrication effects and stability, finding applications in various scenarios [7]. BN nanomaterials can enhance the oxidation resistance and temperature tolerance of lubricating greases, improve extreme pressure and anti-wear performance, strengthen corrosion prevention, and lower the pour point, thereby optimizing the overall performance of automotive lubricating greases and providing robust support for the rapid development of the automotive industry [8]. Tao et al. reported on the friction and wear behavior of steel–steel friction pairs under BN-modified LBG. The results indicated that the optimal addition of BN nanoparticles in LBG is 0.60%, ensuring the best friction reduction and anti-wear performance [9]. Wu et al. prepared ultra-thin alkyl-functionalized BN nanosheets using a combination of ball milling and surface modification techniques. This nanomaterial exhibits an ultra-thin structure and good compatibility. As additives, BN nanosheets possess excellent friction reduction and anti-wear properties capable of reducing friction and wear by approximately 45% and 89%. This is attributed to the effective protection of friction pair surfaces via BN nanosheets [10]. Rasul et al. incorporated BN nanosheets into polyethylene materials. Compared to pure polyethylene, the BN-containing polyethylene materials demonstrated significantly reduced wear rates and notably improved thermal conductivity [11]. Liu et al. obtained functionalized BN nanoparticles by ultrasonically exfoliating h-BN in water and further modifying it. Compared to pure PAO10, the addition of 2 mg/mL functionalized BN nanoparticles reduced the friction coefficient and wear rate by 42% and 29%, respectively, indicating that functionalized BN nanoparticles can form a protective friction film on the friction surface and achieve a sliding lubrication effect. This demonstrates that the use of BN nanoparticles exhibits excellent lubrication effects [12].
In recent years, Ag nanoparticles have attracted widespread attention in tribology due to their low shear strength, excellent ductility, small particle size, high surface activity, and outstanding thermal conductivity [13,14,15]. Currently, there are numerous methods to obtain nano-silver, primarily categorized into physical, chemical, and biological approaches, with chemical approaches dominating due to their high efficiency. Common chemical approaches to synthesizing nano-Ag include liquid-phase chemical reduction, microemulsion, and hydrothermal techniques [16,17,18]. For instance, Tan et al. synthesized Ag nanoparticles of various shapes and sizes by controlling the addition method of sodium borohydride (NaBH4) as the reducing agent and obtained nano-Ag particles with diverse morphologies [19]. Meng et al. synthesized graphene nanocomposites modified with nano-silver through chemical reduction and further investigated the tribological properties of these nanocomposites as lubricant additives in engine oil using a four-ball tester. Their research revealed that Ag nanoparticles anchored on graphene can expand the interlayer spacing of graphene nanosheets, thereby inhibiting their re-stacking during the friction process and fully exploiting the friction reduction and anti-wear properties of this nanocomposite material. Additionally, Ag nanoparticles can form a protective film on the friction pair, thus preventing direct interaction between the friction pairs during sliding [20]. In summary, Ag nanoparticles represent a highly promising lubricant additive.
While Ag nanoparticles can enhance the friction reduction and anti-wear of base lubricants, they are expensive, and their load-bearing capacity during friction is sub-optimal. BN nanomaterials with high stability and carrying capacity can make up for these shortcomings and obtain better lubrication properties. Therefore, focusing on the development of Ag/BN nanocomposite can both effectively reduce economic costs and significantly improve the lubricating and load-bearing properties of base lubricants. The Ag/BN nanocomposite combines the advantages of Ag and BN, and their application as grease additives in automotive components subjected to severe friction and wear can effectively mitigate these issues.
In this study, Ag/BN nanocomposites were successfully prepared by depositing Ag nanoparticles on BN nanoparticle via the chemical reduction method. The prepared nanocomposite was used as an additive of LBG, and the lubrication properties of LBG, LBG+BN, LBG+Ag, and LBG+Ag/BN greases were compared. The lubrication behavior under three working conditions of addition amount, load, and velocity was studied on a high-speed reciprocating tribometer. The results demonstrate that Ag/BN nanocomposites exhibit significantly superior tribological performance compared to either Ag or BN nanoparticles alone.

2. Experimental Details

2.1. Synthesis of Ag/BN Nanocomposite

Some chemical reagents were used to synthesize the Ag nanoparticles and Ag/BN nanocomposites, including silver nitrate (AgNO3, analytical-grade), anhydrous ethanol (C2H5OH, analytical-grade), dodecyl mercaptan (C12H26S, analytical-grade), tert-butylamine borane (C4H14BN, analytical-grade), acetonitrile (C2H3N, analytical-grade), and toluene (C7H8, analytical-grade).
The steps for preparing Ag nanoparticles are as follows [21]: First, use a pipette to transfer 2 mL of acetonitrile into a clean, dry 250 mL three-neck flask. Sequentially add 0.17 g AgNO3 and 30 mL toluene solution. Insert a magnetic stirrer bar, and mix in a constant temperature water bath for 30 min while heating to 70 °C. Then, add 0.48 mL of the capping agent, dodecyl mercaptan, to the above solution, and continue mixing for 30 min. Finally, add 0.87 g of tert-butylamine borane as a reducing agent to the mixed solution, and allow the reaction to proceed at 70 °C for 2 h. After the reaction is complete, add anhydrous ethanol, and centrifuge at 4000 rpm for 10 min, repeating this process twice. Lastly, dry the product in a vacuum-drying oven at 60 °C for 3 h to obtain dry Ag nanoparticles.
The preparation of Ag/BN nanocomposite: First, use a pipette to transfer 2 mL of acetonitrile into a clean, dry 250 mL three-neck flask. Sequentially add 0.0745 g of BN, 0.17 g of AgNO3, and 30 mL of toluene solution. Insert a magnetic stirrer bar, and mix in a constant temperature water bath for 30 min while heating to 70 °C. Then, add 0.48 mL of dodecyl mercaptan as a capping agent to the above solution, and continue mixing for 30 min. Finally, add 0.87 g of tert-butylamine borane as a reducing agent to the mixed solution, and allow the reaction to proceed at 70 °C for 2 h. After the reaction is complete, add anhydrous ethanol, and centrifuge at 4000 rpm for 10 min, repeating this process twice. Lastly, dry the product in a vacuum-drying oven at 60 °C for 3 h to obtain dry particles of Ag/BN nanocomposite.
The preparation of LBG+BN, LBG+Ag, and LBG+Ag/BN: Weigh appropriate amounts of BN, Ag, and Ag/BN particles, and add them to LBG to prepare lubricant samples with mass fractions of 0.5%, 1.0%, 2.0%, 3.0%, 4.0%, and 5.0%. Grind the mixtures in an agate mortar for 30 min to ensure thorough and uniform mixing.

2.2. Characterization and Tribological Performance Evaluation

The synthesized Ag nanoparticles and Ag/BN nanocomposite were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy.
An HSR-2M high-speed reciprocating tribometer was employed to evaluate the tribological properties of LBG containing BN nanoparticles, Ag nanoparticles, and Ag/BN nanocomposite. The friction pair is a ball–disk contact model. Both the ball and disk are made of GCr15 steel, and the diameter of the ball is 6 mm. The friction disk was pre-fixed on the platform of the tribometer via a compression ring. The upper surface of the compression ring is 2 mm above the disk, which creates a simple oil box with a height of 2 mm and a diameter of 18 mm. A scraper was used to evenly apply grease (LBG, LBG+BN, LBG+Ag, and LBG+Ag/BN) to the disk surface until the oil box was completely filled. Thus, the amount of grease added in each friction test was subjected to mold filling, which was basically the same, at about 0.5 mL. The friction coefficient was monitored in real time using software integrated with the tribometer. The wear width and wear volume of the wear disk were analyzed and calculated using a 3D laser scanning microscope. In order to better compare the wear resistance, the measured wear volume was converted to the wear rate with the following equation:
W = V F × n × t
where W was the wear rate, m3/(N × m), V was the wear volume, μm3, F was the load during friction, N, n was the sliding speed, mm/s, and t was the friction time, min.
Tribological tests were conducted with additive concentrations of 0.5%, 1%, 2%, 3%, 4%, and 5% under a 5 mm friction stroke, a 30 min friction test duration, a 10 N load, and a 50 mm/s sliding speed. The impact of friction conditions on the tribological properties of the samples was further investigated, with test parameters including loads (10 N, 20 N, and 50 N) and sliding speeds (25 mm/s and 50 mm/s). For the ball–disk contact model, the contact pressure could be calculated using the Hertz contact theory with the following equation:
p m a x = 0.388 F E 2 R 2 3
where pmax was the maximum Hertz contact pressure, GPa, F was the load, N, E was the equivalent elastic modulus, 208 GPa, and R was the equivalent curvature radius, 3 mm. The loads of 10, 20, and 50 N correspond to the maximum Hertz contact pressures of 1.41, 1.78, and 2.41 GPa, respectively.
The microscopic morphology of the worn surface was characterized via scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) data were obtained. The chemical composition of the friction film was analyzed using XPS.

3. Results and Discussion

3.1. Chemical Composition, Structure, and Morphology Analysis

Figure 1 shows the XRD spectra of BN nanoparticles and the Ag/BN nanocomposite. Based on their characteristic peaks, both additives exhibit diffraction peaks at 2θ = 26.6°, 41.5°, 50.1°, and 55.1°, corresponding to the (002), (100), (102), and (004) crystal planes of BN, respectively. This indicates that the Ag/BN nanocomposite retains the crystal structure of BN. Additionally, the Ag/BN nanocomposite exhibits diffraction peaks at 2θ = 38.1° and 64.5°, which correspond to the (111) and (220) planes of Ag, respectively. This confirms that Ag/BN nanocomposite also incorporates the crystal structure of Ag. In addition, the characteristic peaks of BN are more pronounced in the XRD spectrum of BN nanoparticles but appear weaker in the XRD spectrum of the Ag/BN nanocomposite. This change is attributed to the high crystallinity of Ag nanoparticles deposited on BN, thus resulting in strong diffraction peak intensity for Ag, which consequently leads to a significant attenuation of the diffraction peak intensity for BN [22].
Figure 2 presents the Raman spectra of BN nanoparticles and the Ag/BN nanocomposite. The peak observed at a wavenumber of 1366 cm−1 is characteristic of BN. Following the deposition of Ag nanoparticles on the BN surface, the Raman spectrum exhibits peaks at 1341 cm−1 and 1356 cm−1, which are also characteristic of BN [23]. This indicates that the incorporation of Ag during synthesis induces a slight shift in the Raman peaks of BN, possibly due to the dispersion of silver nanoparticles on the surface of BN nanoparticles.
Figure 3 illustrates the XPS spectra of the Ag/BN nanocomposite. Analyzing the fitting curves in Figure 3 reveals the following: Figure 3a shows the Ag3d spectrum, with peaks at 368.2 eV and 374.3 eV corresponding to Ag3d3/2 and Ag3d5/2 of Ag, respectively [24,25]. Figure 3b shows the B1s spectrum, with a central peak at 190.9 eV attributed to the B-N bond, which is consistent with findings in the literature [26]. In Figure 3c, the N1s spectrum of Ag/BN exhibits a center peak at 398.3 eV, corresponding to the B-N bond. The C1s spectrum peak is centered at 284.8 eV, attributed to C-C bonds, primarily originating from organic functional groups on the surface of the Ag/BN nanocomposite. These results confirm the presence of elements such as Ag, B, and N in the nanocomposite material, indicating that Ag is successfully attached to the BN surface.
Figure 4 presents the FETEM-EDS images of the Ag/BN nanocomposite. Figure 4a illustrates the morphology of Ag/BN and reveals Ag nanoparticles deposited on BN nanoparticles. There is a large amount of Ag nanoparticles less than 10 nm distributed on BN, in addition to some agglomerated silver nanospheres with an average diameter of 30 nm. These Ag nanoparticles are generally well dispersed, showing no significant aggregation and demonstrating excellent dispersion. Figure 4b displays the EDS spectrum, which shows the characteristic elements Ag, B, and N, further confirming the successful loading of Ag nanoparticles onto the surface of BN.

3.2. Tribological Analysis

Figure 5 illustrates the impact of varying nanoparticle concentrations on the average friction coefficient and wear rate of LBG under conditions of 10 N load, 30 min test duration, and 50 mm/s sliding speed. Figure 5a reveals that the average friction coefficient of pure LBG is 0.51. When BN nanoparticles, Ag nanoparticles, and the Ag/BN nanocomposite are added to LBG at concentrations of 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, and 5 wt% respectively, the average friction coefficients of the additive-containing greases all decrease to varying degrees. Notably, LBG+Ag/BN exhibits the lowest average friction coefficient across all concentrations. This indicates that the friction reduction performance of LBG+Ag/BN is superior to that of LBG+BN and LBG+Ag, with the performance differences among various nanoparticles closely related to their composition and structure. As the Ag/BN concentration increases, the average friction coefficient initially decreases and then increases. The changing trend of friction coefficient with Ag/BN content is similar to that of doped Ag nanoparticles in coatings. The high doping concentrations in the coating will affect the nanostructure of the coating and reduce the surface hardness and elastic modulus. This makes the lubrication conditions of nano silver worse, resulting in an increase in friction coefficient [27]. However, for grease additives, it is mainly due to the lack of Ag/BN lubricating materials at low concentrations and the agglomeration problem of too-high concentrations.
At 2 wt%, the average friction coefficient of LBG containing Ag/BN reaches its lowest value of 0.33, representing a 35.2% reduction compared to pure LBG. This is attributed to the initial low content of Ag/BN nanocomposite, resulting in a lubricating film that is insufficiently continuous. When an optimal amount of Ag/BN nanocomposite is added, a more complete lubricating film forms on the friction pair surface, thus effectively reducing friction-induced losses. However, once the additive content is too high, the dispersion of nanomaterials decreases and likely leads to agglomeration, which can adversely affect the lubrication performance of LBG.
Figure 5b illustrates the impact of varying nanoparticle concentrations on the wear rate of LBG. The graph reveals that the wear rate of pure LBG is 1.85 × 10−14 m3/(N × m). Within the concentration range studied in this paper, BN nanoparticles slightly improve the anti-wear performance of pure LBG, while adding Ag nanoparticles to LBG enhances the anti-wear performance of the lubricated friction pair surfaces. When Ag/BN nanocomposite replaces Ag nanoparticles as an additive in LBG, the anti-wear performance of LBG is further improved. Compared to pure LBG, at 2 wt%, LBG+Ag/BN exhibits the lowest wear rate of 1.08 × 10−14 m3/(N × m), representing a 41.6% reduction in the wear rate relative to pure LBG. As the concentration of Ag/BN nanocomposite increases, the wear rate initially decreases and then increases, indicating that an optimal amount of Ag/BN nanocomposite can effectively enhance the anti-wear performance of LBG, while excessive amounts may be detrimental. These results demonstrate that the Ag/BN nanocomposite is a more efficient lubricating additive compared to BN nanoparticles and Ag nanoparticles individually, with BN and Ag in Ag/BN nanocomposite exhibiting a significant synergistic effect in friction reduction and wear resistance [28].
Figure 6 shows the effect of different loads on the average friction coefficient and wear rate of LBG and LBG containing nanoparticles under 2 wt% additive concentration, 30 min test duration, and 50 mm/s sliding speed. Figure 6a indicates that the average friction coefficients of pure LBG and LBG+BN decrease as the load increases. The average friction coefficient of LBG+Ag remains almost unchanged with an increasing load, which may be attributed to the low elastic modulus of Ag nanoparticles, allowing them to exhibit good plasticity and ductility under certain loads [29,30,31]. Under different loads, the average friction coefficient of LBG+Ag/BN is consistently lower than that of LBG, LBG+BN, and LBG+Ag.
Figure 6b illustrates that the wear rate of pure LBG, LBG+BN, LBG+Ag, and LBG+Ag/BN all increase with an increasing load. The addition of BN nanoparticles, Ag nanoparticles, and Ag/BN nanocomposite as lubricating additives results in decreased wear rates. Among these, LBG+Ag/BN exhibits the smallest wear rate compared to LBG+BN and LBG+Ag. Under a 10 N load, the wear rate of pure LBG is 1.85 × 10−14 m3/(N × m), while that of LBG+Ag/BN is 1.08 × 10−14 m3/(N × m), representing a 41.6% reduction. At 20 N, the wear rate of pure LBG is 1.79 × 10−14 m3/(N × m), compared to 1.44 × 10−14 m3/(N × m) for LBG+Ag/BN, a 19.6% reduction. At 50 N, pure LBG exhibits an wear rate of 2.40 × 10−14 m3/(N × m), while that of LBG+Ag/BN is 1.52 × 10−14 m3/(N × m), which is a 36.7% reduction. These results demonstrate that LBG+Ag/BN maintains excellent anti-wear performance across all loads. The findings indicate that, under various loads, the lubrication performance of LBG with added Ag/BN nanocomposite significantly surpasses that of LBG with BN nanoparticles or Ag nanoparticles alone. The Ag/BN nanocomposite provides effective protection and repair against pressure and shear forces during the friction process, substantially enhancing the friction reduction and wear resistance of the friction pair.
Figure 7 illustrates the impact of different sliding speeds on the average friction coefficient and wear rate of LBG and LBG containing nanoparticles under 2 wt% additive concentration, a 30 min test duration, and a 10 N load. As depicted in Figure 7a, at various sliding speeds, the average friction coefficients of LBG+BN, LBG+Ag, and LBG+Ag/BN all decrease compared to pure LBG. The friction coefficients of pure LBG at 25 mm/s and 50 mm/s are 0.34 and 0.51, respectively, while those of LBG+Ag/BN at the same speeds are 0.30 and 0.33, respectively. This represents reductions of 11.8% and 35.2% in the average friction coefficients of LBG+Ag/BN compared to pure LBG. Across different speeds, Ag/BN added to LBG demonstrated significant effects of friction reduction.
Figure 7b reveals that, at varying sliding speeds, the wear rates of LBG+BN, LBG+Ag, and LBG+Ag/BN all decrease compared to pure LBG. The wear rates of pure LBG at 25 mm/s and 50 mm/s are 1.07 × 10−14 m3/(N × m) and 1.85 × 10−14 m3/(N × m), respectively. Notably, LBG+Ag/BN exhibits the most significant reduction in the wear rate, with values of 0.55 × 10−14 m3/(N × m) and 1.08 × 10−14 m3/(N × m) at 25 mm/s and 50 mm/s, respectively. These figures represent substantial reductions of 48.6% and 41.6% compared to pure LBG. These results demonstrate that the Ag/BN nanocomposite maintains excellent anti-wear performance across different sliding speeds.
Figure 8 illustrates the 3D morphology of friction surfaces under different lubricants. Figure 8a depicts the 3D morphology of the friction surface lubricated with pure LBG, and numerous deep furrows can be observed, which is indicative of severe wear on the surface of the friction pair. In particular, the wear is deepest in the center of the wear scar, where the contact pressure is greatest. There are a lot of spalling pits leading to a deeper wear depth. Figure 8b presents the 3D morphology of friction surfaces lubricated with LBG+BN. The worn surface exhibit shallower and narrower furrows compared to the LBG surface, except for some deep spalling in the central high-load area. This suggests that adding BN has mitigated wear on the friction pair surface. Figure 8c,d show the 3D morphology of the friction surface lubricated with LBG+Ag and LBG+Ag/BN, respectively. It is evident that the furrows on the worn surface have become even shallower, and the wear depth of the entire wear scar is further reduced. This significant improvement is attributed to the silver component (Ag or Ag/BN) added to the lubricating grease. Comparing the four experiments, LBG+Ag/BN has the smallest wear cross-sectional area, indicating that the best wear resistance of Ag/BN additive.
Figure 9 shows the SEM images of wear scars on steel plates and their corresponding elemental distribution under various lubricants. The figure reveals the presence of C, O, and Fe in the wear scar areas across different lubricants. These elements primarily originate from the steel substrate and the absorption film of LBG. When lubricated with LBG+BN, characteristic elements B and N from BN nanoparticles are detected in the wear scar area. For LBG+Ag, the characteristic element Ag from Ag nanoparticles is observed. In the case of LBG+Ag/BN, characteristic elements Ag, B, and N from the Ag/BN nanocomposite are present in the wear scar area. This indicates that BN, Ag, and Ag/BN all participate in the formation of lubricant film during friction. The findings suggest that Ag nanoparticles disperse at the tribological interface of the friction pair, filling the worn surface and providing a repair function [32,33,34,35,36]. BN nanoparticles can deposit on the friction pair surface during sliding, forming a protective friction film [37]. Ag and BN particles in the Ag/BN nanocomposite exert their respective advantages, contributing to lubricating film formation, surface repair, and micro-bearing effects and thereby achieving synergistic lubrication.
Figure 10 presents the XPS spectra of wear scars on steel plates under various lubricants. In Figure 10a, the peak at 284.9 eV is attributed to C-C bonds. Figure 10b shows peaks at 530.3 eV and 531.3 eV, corresponding to Fe2O3, C-O, and H-O-C, respectively. These originate from metal oxides produced during friction, as well as the adsorption and tribochemical deposition of LBG on the friction pair surface [38]. Analyzing the valence spectrum of Fe2p in Figure 10c reveals characteristic peaks at 710.6 eV (2p3/2) and 724.6 eV (2p1/2) for Fe2O3. In addition to these same elements as steel, several other elements (Li, B, N, and Ag) affected by the composition of grease were detected on the worn surface. This means that LBG and the added composites (Ag, BN and Ag/BN) can participate in the friction process and improve the friction performance. In Figure 10d, all four experiments have a peak at 55.7 eV attributed to Li2CO3 [39], which comes from the adsorption and tribochemical reaction of LBG. The B1s spectrum (Figure 10e) exhibits a peak at 189.5 eV, corresponding to B-N. The N1s spectrum in Figure 10f shows a peak at 399.2 eV belonging to N-B, indicating BN nanoparticle or Ag/BN nanocomposite contained in LBG can be adsorbed on the friction surface to form a lubricating film. Figure 10g presents the XPS spectrum of Ag3d, with peaks at 368.2 eV and 374.1 eV attributed to Ag3d3/2 and Ag3d5/2 of Ag, confirming the presence of Ag nanoparticles or Ag/BN nanocomposite on the friction surface. In summary, when LBG+Ag/BN is used as a lubricant, the friction film generated on the wear scar surface comprises Ag, BN, Fe-containing compounds, and organic compounds, demonstrating the participation of Ag/BN nanocomposite in forming a lubricating film.
Compared to Ag and BN nanoparticles individually, Ag/BN nanocomposite as an additive more effectively enhances the lubrication capability of LBG. Based on the results of the tribological test and wear surface analysis, the mechanism of Ag/BN nanocomposite in LBG is as follows: Although LBG exists between friction interfaces, direct contact between friction pairs remains inevitable. Adding the Ag/BN nanocomposite allows it to penetrate the friction interface and deposit to form a lubricating film during friction, providing a repairing function. Furthermore, Ag nanoparticles produced via exfoliation during friction act as “micro-bearings” at the friction interface. Consequently, the synergistic lubrication effect of Ag and BN nanoparticles in the Ag/BN nanocomposite effectively protects the friction interface.

4. Conclusions

This study prepared an Ag/BN nanocomposite to enhance the tribological properties of LBG. The friction and wear mechanisms of the Ag/BN nanocomposite were investigated through a series of analyses. The key findings are as follows:
  • Ag nanoparticles were successfully synthesized on the BN surface, and the Ag/BN nanocomposite was obtained.
  • The Ag/BN nanocomposite significantly improves the friction reduction and anti-wear performance of LBG across various loads and sliding speeds. Notably, under the conditions of a 10 N load, a 30 min test duration, a 50 mm/s sliding speed, and a 2 wt% additive concentration, LBG containing a Ag/BN nanocomposite exhibits the lowest average friction coefficient and wear rate of 0.33 and 1.08 × 10−14 m3/(N × m), respectively. These values represent reductions of 35.2% and 41.6% compared to pure LBG.
  • Compared to Ag and BN nanoparticles individually, the Ag/BN nanocomposite as an additive enhances the lubrication capability of LBG more effectively. The lubrication mechanisms of worn surfaces were explored using 3D laser microscopy (SEM/EDS and XPS). The improved lubrication performance of LBG with the Ag/BN nanocomposite can be attributed to its repairing effect, the formation of a lubricating film composed of Ag, BN, and Fe-containing compounds, and its micro-bearing effect.

Author Contributions

Conceptualization, C.L.; data curation, C.L., X.W. and Y.P.; formal analysis, X.W., Y.P. and X.X.; funding acquisition, Y.C., C.L. and X.X.; investigation, Y.C. and L.Z.; methodology, Y.C. and X.T.; writing—original draft, Y.C. and C.L.; writing—review and editing, Y.P. and X.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Open Project of Anhui Province Key Laboratory of Critical Friction Pair for Advanced Equipment (LCFP-2408), Key Research & Development (R&D) Plan of Anhui Province under Grant (2022a05020019), the Support Program for Outstanding Young Talents in Anhui Province Colleges and Universities (gxyq2022079), the Excellent Research and Innovation Teams Project of Anhui Province’s Universities (2022AH010092), the Discipline Construction Quality Improvement Project of Chaohu University (kj22fdzy03, XLZ202307), the School-level Scientific Research Project of Chaohu University (XLY-202112), the Scientific Research Planning Project of Anhui Provincial (2022AH051726), the Anhui Province University Science and Engineering Teachers’ Internship Program in Enterprises (2024jsqygz89), the Anhui Province College Students’ Innovation and Entrepreneurship Training Program (S202410380020), the Anhui Province Postdoctoral Research Project (2024A773) and the Horizontal Research Project of Chaohu University (hxkt20240219).

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

Authors Yijun Chen and Li Zhang were employed by the company Huangshan Shangyi Rubber and Plastic Products Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Gangopadhyay, A. A review of automotive engine friction reduction opportunities through technologies related to tribology. Trans. Indian Inst. Met. 2017, 70, 527–535. [Google Scholar] [CrossRef]
  2. Shen, Y.H.; Wang, Y.S.; Lin, J.H.; Zhang, P.; Gao, X.D.; Wang, Z.J. Study on anti-wear and friction-reducing compounding additives in lithium greases. Ind. Lubr. Tribol. 2023, 75, 546–553. [Google Scholar] [CrossRef]
  3. Syahir, A.Z.; Zulkifli, N.W.M.; Masjuki, H.H.; Kalam, M.A.; Alabdulkarem, A.; Gulzar, M.; Khuong, L.S.; Harith, M.H. A review on bio-based lubricants and their applications. J. Clean. Prod. 2017, 168, 997–1016. [Google Scholar] [CrossRef]
  4. Tuero, A.G.; Bartolomé, M.; Gonçalves, D.; Viesca, J.L.; Fernández-González, A.; Seabra, J.H.O.; Battez, A.H. Phosphonium-based ionic liquids as additives in calcium/lithium greases. J. Mol. Liq. 2021, 338, 116697. [Google Scholar] [CrossRef]
  5. Wang, J.B.; Zhang, H.; Hu, W.J.; Li, J.S. Tribological properties and lubrication mechanism of nickel nanoparticles as an additive in lithium grease. Nanomaterials 2022, 12, 2287. [Google Scholar] [CrossRef] [PubMed]
  6. He, Q.; Li, A.L.; Guo, Y.C.; Liu, S.F.; Zhang, Y.; Kong, L.H. Tribological properties of nanometer cerium oxide as additives in lithium grease. J. Rare Earths 2018, 36, 209–214. [Google Scholar] [CrossRef]
  7. Fan, X.Q.; Gan, C.L.; Feng, P.; Ma, X.L.; Yue, Z.F.; Li, H.; Li, W.; Zhu, M.H. Controllable preparation of fluorinated boron nitride nanosheets for excellent tribological behaviors. Chem. Eng. J. 2022, 431, 133482. [Google Scholar] [CrossRef]
  8. Wang, L.; Qi, J.J.; Zhang, S.; Ding, M.C.; Wei, W.; Wang, J.H.; Zhang, Z.H.; Qiao, R.X.; Zhang, Z.B.; Li, Z.H.; et al. Abnormal anti-oxidation behavior of hexagonal boron nitride grown on copper. Nano Res. 2022, 15, 7577–7583. [Google Scholar] [CrossRef]
  9. Wang, T.; Li, Z.J.; Li, J.B.; He, Q. Impact of boron nitride nanoparticles on the wear property of lithium base grease. J. Mater. Eng. Perform. 2020, 29, 4991–5000. [Google Scholar] [CrossRef]
  10. Wu, H.X.; Yin, S.C.; Du, Y.; Wang, L.P.; Yang, Y.; Wang, H.F. Alkyl-functionalized boron nitride nanosheets as lubricant additives. ACS Appl. Nano Mater. 2020, 3, 9108–9116. [Google Scholar] [CrossRef]
  11. Rasul, M.G.; Kiziltas, A.; Hoque, M.S.B.; Banik, A.; Hopkins, P.E.; Tan, K.T.; Arfaei, B.; Shahbazian-Yassar, R. Improvement of the thermal conductivity and tribological properties of polyethylene by incorporating functionalized boron nitride nanosheets. Tribol. Int. 2022, 165, 107277. [Google Scholar] [CrossRef]
  12. Liu, C.L.; Tang, G.B.; Su, F.H.; Xu, X.; Li, Z.J. Functionalised h-BN as an effective lubricant additive in PAO oil for MoN coating sliding against Si3N4 ball. Lubr. Sci. 2020, 33, 33–42. [Google Scholar] [CrossRef]
  13. Xia, Y.Q.; Cao, Y.N.; Feng, X.; Haris, M.P. A comparative study on the electrical and tribological characteristic of magnetron sputtered Ag, Cu and Al films under current-carrying friction. Ind. Lubr. Tribol. 2021, 73, 1219–1225. [Google Scholar] [CrossRef]
  14. Pirhayati, P.; Aval, H.J.; Loureiro, A. Characterization of microstructure, corrosion, and tribological properties of a multilayered friction surfaced Al-Mg-Si-Ag alloy. Arch. Civ. Mech. Eng. 2022, 22, 176. [Google Scholar] [CrossRef]
  15. Dong, L.R.; Li, C.S.; Tang, H. Tribological properties of Ag/BSCCO self-lubricating composites. Int. J. Mod. Phys. B 2010, 24, 2664–2669. [Google Scholar] [CrossRef]
  16. Wang, H.S.; Qiao, X.L.; Chen, J.G.; Ding, S.Y. Preparation of silver nanoparticles by chemical reduction method. Colloid Surf. A 2004, 256, 111–115. [Google Scholar] [CrossRef]
  17. Gao, H.F.; Yang, H. Preparation and characterization of cinnamaldehyde/polyvinyl alcohol/silver nanoparticles ternary composite films. Int. J. Polym. Anal. Charact. 2021, 26, 24–36. [Google Scholar] [CrossRef]
  18. Liu, H.; Wang, S.Z.; Li, Z.C.; Zhuo, R.S.; Zhao, J.N.; Duan, Y.W.; Liu, L.; Yang, J.Q. Experimental study on the preparation of monodisperse nano-silver by hydrothermal synthesis. Mater. Chem. Phys. 2024, 314, 128902. [Google Scholar] [CrossRef]
  19. Tan, D.Q.; Dong, M.Y.; Xia, Y.J.; Wang, X.Y.; Wei, H.G. Chemically reduced silver nanoparticles: Preparation and applications. Emerg. Mater. Res. 2023, 13, 56–64. [Google Scholar] [CrossRef]
  20. Meng, Y.; Su, F.H.; Chen, Y.Z. Supercritical fluid synthesis and tribological applications of silver nanoparticle-decorated graphene in engine oil nanofluid. Sci. Rep. 2016, 6, 31246. [Google Scholar] [CrossRef]
  21. Kumara, C.; Luo, H.M.; Leonard, D.N.; Meyer, H.M.; Qu, J. Organic-Modified Silver Nanoparticles as Lubricant Additives. ACS Appl. Mater. Interfaces 2017, 9, 37227–37237. [Google Scholar] [CrossRef]
  22. Wang, W.; Chang, W.J.; Lv, F.F.; Xie, Z.L.; Yu, C.C. Preparation and tribological properties of fluorinated boron nitride nanosheets water-based additive. Chin. J. Mater. Res. 2024, 38, 410–422. [Google Scholar]
  23. Charoo, M.S.; Wani, M.F. Tribological properties of h-BN nanoparticles as lubricant additive on cylinder liner and piston ring. Lubr. Sci. 2017, 29, 241–254. [Google Scholar] [CrossRef]
  24. Ahmadi, M.; Kaleji, B.K. TCA (Ag doped TiO2-CuO) mesoporous composite nanoparticles: Optical, XPS, and morphological characterization. J. Mater. Sci. Mater. Electron. 2021, 32, 13450–13461. [Google Scholar] [CrossRef]
  25. Corro, G.; Vidal, E.; Cebada, S.; Pal, U.; Bañuelos, F.; Vargas, D.; Guilleminot, E. Electronic state of silver in Ag/SiO2 and Ag/ZnO catalysts and its effect on diesel particulate matter oxidation: An XPS study. Appl. Catal. B Environ. Energy 2017, 216, 1–10. [Google Scholar] [CrossRef]
  26. Kumar, R.; Singh, R.K.; Yadav, S.K.; Savu, R.; Moshkalev, S.A. Mechanical pressure-induced chemical cutting of boron nitride sheets into boron nitride quantum dots and optical properties. J. Alloys Compd. 2016, 683, 38–45. [Google Scholar] [CrossRef]
  27. Pogrebnjak, A.D.; Bagdasaryan, A.A.; Pshyk, A.; Dyadyura, K. Adaptive multicomponent nanocomposite coatings in surface engineering. Physics-Uspekhi 2017, 60, 586–607. [Google Scholar] [CrossRef]
  28. Bondarev, A.V.; Fraile, A.; Polcar, T.; Shtansky, D.V. Mechanisms of friction and wear reduction by h-BN nanosheet and spherical W nanoparticle additives to base oil: Experimental study and molecular dynamics simulation. Tribol. Int. 2020, 151, 106493. [Google Scholar] [CrossRef]
  29. Bian, S.N.; Yu, L.H.; Xu, J.H.; Xu, J.D. Impact of Ag on the microstructure and tribological behaviors of adaptive ZrMoN–Ag composite lubricating films. J. Mater. Res. Technol. 2022, 19, 2346–2355. [Google Scholar] [CrossRef]
  30. Zuo, B.; Yu, L.H.; Xu, J.H. Effect of Ag content on friction and wear properties of TiCN/Ag films in different service environments. Vacuum 2023, 212, 112029. [Google Scholar] [CrossRef]
  31. Ju, H.B.; Guo, J.L.; Yu, L.H.; Xu, J.H.; Luan, J. Enhancement of the mechanical and tribological properties of self-lubricant Mo2N–Ag composite film by adding amorphous SiNx. Ceram. Int. 2024, 50, 8463–8471. [Google Scholar] [CrossRef]
  32. Zheng, X.; Su, L.H.; Deng, G.Y.; Zhang, J.; Zhu, H.T.; Tieu, A.K. Study on lubrication characteristics of C4-alkane and nanoparticle during boundary friction by molecular dynamics simulation. Metals 2021, 11, 1464. [Google Scholar] [CrossRef]
  33. Li, K.S.; Zhang, Y.B.; Tan, W.K.; Wang, J.W.; Xu, Z.H.; Li, Z.J.; He, Q. Investigation of friction and vibration performance of lithium complex grease containing candle soot on aviation electrical machine. Wear 2024, 550–551, 205401. [Google Scholar] [CrossRef]
  34. Luo, T.; Wei, X.W.; Huang, X.; Huang, L.; Yang, F. Tribological properties of Al2O3 nanoparticles as lubricating oil additives. Ceram. Int. 2014, 40, 7143–7149. [Google Scholar] [CrossRef]
  35. Lai, S.Q.; Yue, L.; Li, T.S.; Hu, Z.M. The friction and wear properties of polytetrafluoroethylene filled with ultrafine diamond. Wear 2005, 260, 462–468. [Google Scholar] [CrossRef]
  36. Wang, P.; Deng, G.Y.; Zhang, H.B.; Yin, J.; Xiong, X.; Zhang, X.; Zhu, H.T. Microstructural feature and tribological behaviors of pyrolytic carbon-coated copper foam/carbon composite. J. Mater. Sci. 2019, 54, 13557–13568. [Google Scholar] [CrossRef]
  37. Demas, N.G.; Timofeeva, E.V.; Routbort, J.L.; Fenske, G.R. Tribological effects of BN and MoS2 nanoparticles added to polyalphaolefin oil in piston skirt/cylinder liner tests. Tribol. Lett. 2012, 47, 91–102. [Google Scholar] [CrossRef]
  38. Li, C.; Wang, X.X.; Zhang, Q.Q.; Tan, X.; Liu, Y.F.; Li, H.L.; Liu, H.; Hu, E.Z.; Hu, X.G. Synthesis, characterization, and tribological performances of nano-CeO2/biodiesel carbon soot composites as a novel lubricant additive in polyalphaolefin. J. Ind. Eng. Chem. 2023, 126, 432–443. [Google Scholar] [CrossRef]
  39. Hu, E.Z.; Xu, Y.; Hu, K.H.; Hu, X.G. Tribological properties of 3 types of MoS2 additives in different base greases. Lubr. Sci. 2017, 29, 541–555. [Google Scholar] [CrossRef]
Lubricants 13 00030 g001
Figure 1. XRD spectra of BN and Ag/BN.
Figure 1. XRD spectra of BN and Ag/BN.
Lubricants 13 00030 g001
Lubricants 13 00030 g002
Figure 2. Raman spectra of N and Ag/BN.
Figure 2. Raman spectra of N and Ag/BN.
Lubricants 13 00030 g002
Lubricants 13 00030 g003
Figure 3. XPS spectra of Ag/BN nanocomposite: (a) Ag3d, (b) B1s, (c) N1s, and (d) C1s.
Figure 3. XPS spectra of Ag/BN nanocomposite: (a) Ag3d, (b) B1s, (c) N1s, and (d) C1s.
Lubricants 13 00030 g003
Lubricants 13 00030 g004
Figure 4. FETEM (a) image and corresponding elemental mapping (b–d) images of Ag/BN.
Figure 4. FETEM (a) image and corresponding elemental mapping (b–d) images of Ag/BN.
Lubricants 13 00030 g004
Lubricants 13 00030 g005
Figure 5. Influence of nanoparticle concentrations on the average friction coefficient and wear rate of LBG (10 N, 50 mm/s, and 30 min): (a) average friction coefficient and (b) Wear rate.
Figure 5. Influence of nanoparticle concentrations on the average friction coefficient and wear rate of LBG (10 N, 50 mm/s, and 30 min): (a) average friction coefficient and (b) Wear rate.
Lubricants 13 00030 g005
Lubricants 13 00030 g006
Figure 6. Effect of different loads on the average friction coefficient and wear rate of LBG and LBG containing nanoparticles (2%, 30 min, and 50 mm/s): (a) average friction coefficient and (b) wear rate.
Figure 6. Effect of different loads on the average friction coefficient and wear rate of LBG and LBG containing nanoparticles (2%, 30 min, and 50 mm/s): (a) average friction coefficient and (b) wear rate.
Lubricants 13 00030 g006
Lubricants 13 00030 g007
Figure 7. Influence of varying sliding speeds on the average friction coefficient and wear rate of LBG and LBG containing nanoparticles (10 N, 2%, and 30 min): (a) average friction coefficient and (b) wear rate.
Figure 7. Influence of varying sliding speeds on the average friction coefficient and wear rate of LBG and LBG containing nanoparticles (10 N, 2%, and 30 min): (a) average friction coefficient and (b) wear rate.
Lubricants 13 00030 g007
Lubricants 13 00030 g008
Figure 8. Three-dimensional morphology of steel plate friction surfaces under various lubricants (2 wt%, 10 N, 50 mm/s, and 30 min): (a) LBG, (b) LBG+BN, (c) LBG+Ag, and (d) LBG+Ag/BN.
Figure 8. Three-dimensional morphology of steel plate friction surfaces under various lubricants (2 wt%, 10 N, 50 mm/s, and 30 min): (a) LBG, (b) LBG+BN, (c) LBG+Ag, and (d) LBG+Ag/BN.
Lubricants 13 00030 g008
Lubricants 13 00030 g009
Figure 9. SEM images of wear scars on steel plates and corresponding elemental distribution under different lubricants (2 wt%, 10 N, 50 mm/s, and 30 min): (a) LBG, (b) LBG+BN, (c) LBG+Ag, and (d) LBG+Ag/BN. All scale bars are 5 μm.
Figure 9. SEM images of wear scars on steel plates and corresponding elemental distribution under different lubricants (2 wt%, 10 N, 50 mm/s, and 30 min): (a) LBG, (b) LBG+BN, (c) LBG+Ag, and (d) LBG+Ag/BN. All scale bars are 5 μm.
Lubricants 13 00030 g009
Lubricants 13 00030 g010aLubricants 13 00030 g010b
Figure 10. XPS spectra of wear scars on steel plates under different lubricants (2 wt%, 10 N, 50 mm/s, and 30 min): (a) C1s, (b) O1s, (c) Fe2p, (d) Li1s, (e) B1s, (f) N1s, and (g) Ag3d.
Figure 10. XPS spectra of wear scars on steel plates under different lubricants (2 wt%, 10 N, 50 mm/s, and 30 min): (a) C1s, (b) O1s, (c) Fe2p, (d) Li1s, (e) B1s, (f) N1s, and (g) Ag3d.
Lubricants 13 00030 g010aLubricants 13 00030 g010b
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Chen, Y.; Li, C.; Wang, X.; Zhang, L.; Tan, X.; Peng, Y.; Xu, X. Preparation and Tribological Performance of the Ag/BN Nanocomposite as Additives for Lithium-Based Grease. Lubricants 2025, 13, 30. https://doi.org/10.3390/lubricants13010030

AMA Style

Chen Y, Li C, Wang X, Zhang L, Tan X, Peng Y, Xu X. Preparation and Tribological Performance of the Ag/BN Nanocomposite as Additives for Lithium-Based Grease. Lubricants. 2025; 13(1):30. https://doi.org/10.3390/lubricants13010030

Chicago/Turabian Style

Chen, Yijun, Chuan Li, Xiaodong Wang, Li Zhang, Xu Tan, Yubin Peng, and Xiaoyong Xu. 2025. "Preparation and Tribological Performance of the Ag/BN Nanocomposite as Additives for Lithium-Based Grease" Lubricants 13, no. 1: 30. https://doi.org/10.3390/lubricants13010030

APA Style

Chen, Y., Li, C., Wang, X., Zhang, L., Tan, X., Peng, Y., & Xu, X. (2025). Preparation and Tribological Performance of the Ag/BN Nanocomposite as Additives for Lithium-Based Grease. Lubricants, 13(1), 30. https://doi.org/10.3390/lubricants13010030

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop