Effect of B Element Doping on High-Temperature Tribological Properties of WS₂-Based Composite Coatings

Abstract

WS₂ coating, as a solid lubricating material, plays a significant role in the lubrication of rotating components in spacecraft. During the launch process, however, spacecraft are exposed to high-temperature and humid atmospheric environments, which can lead to oxidative failure in the coating, thereby limiting its engineering applications. By doping with B elements, B/WS₂ was successfully prepared as a composite coating. The results demonstrate that the fabricated coating exhibits excellent high-temperature tribological performance in atmospheric environments. The mechanism through which B doping improves the high-temperature friction and wear properties of the WS₂ composite coating was revealed through high-temperature friction and wear tests. With the incorporation of B elements, the average friction coefficient of the coating was 0.071, and the wear rate was 7.63 × 10⁻⁷ mm³·N⁻¹·m⁻¹, with the wear mechanisms identified as abrasive wear and spalling. Due to high-temperature oxidation, thermal decomposition effects, and the formation of WB₄ during sputtering, the wear resistance and anti-plastic deformation capability of the coating were further improved. Compared to room-temperature test conditions, the B/WS₂ composite coating at different high temperatures exhibited superior friction coefficients and wear rates. Notably, at 150 °C, the average friction coefficient was as low as 0.015, and the wear forms were abrasive wear and adhesive wear.

1. Introduction

With continuous improvements in the thrust-to-weight ratios of aeroengines, the operating temperature continues to rise, posing increasingly severe high-temperature challenges in bearing systems. The main bearing of a specific large passenger aircraft engine reaches a working temperature of nearly 300 °C during cruise conditions, far exceeding the service limits of traditional bearings [1]. High temperatures can lead to a series of issues, such as reduced material strength, the degradation of lubricant performance, and the thermal expansion of components, significantly affecting the service life and reliability of bearings. It is therefore necessary to develop a new generation of lubricating coatings for bearings that demonstrate self-lubricating properties, high-temperature resistance, and low friction [2]. Compared with grease lubrication and oil lubrication, solid lubricating coatings can avoid dependency on working environments and overcome replenishment difficulties.

As a solid lubricating material, WS₂ coating is widely applied in the aerospace field [3,4] due to its excellent tribological properties, which arise from the interlayer shear slippage between S-W atoms under weak shear forces coupled with the formation of WO₃ (a high-temperature lubricant) through oxidation [5]. The porous microstructure [6] and reactive dangling bonds at crystal edges [7], however, make WS₂ coatings prone to oxidation through interactions with atmospheric oxygen in humid environments, resulting in limited wear resistance, which hinders their application in high-load and long-service-life conditions.

In recent years, researchers worldwide have significantly enhanced the performance of WS₂ coatings through the doping of metallic elements (e.g., Cr [8], Ti [9], Ag [10], Ni [11], Ta [12], and Al [13]) or non-metallic elements (e.g., C [14] and N [15]), as well as various compounds. The results indicate that, on the one hand, the doped elements improve the microhardness and elastic modulus of the coating, enhancing its wear resistance and load-bearing capacity. On the other hand, they alter the microstructure and crystallographic orientation of the coating, leading to a denser surface structure for the WS₂ coating and thus improving its tribological properties. When doped with reactive rare-earth metal elements (e.g., La [16] and Ce [17]), these elements act as oxygen scavengers, simultaneously enhancing the coating’s tribological and oxidation resistance properties. Ma [18] investigated the fabrication and high-temperature tribological performance of Ag-doped WS₂-based composite coatings. By precisely controlling the Ag content during the deposition of WS₂-Ag composite coatings, they systematically examined the influence of the Ag concentration on the friction and wear characteristics across a broad temperature range (RT~800 °C). Furthermore, their study elucidated the underlying wear mechanisms governing the WS₂-Ag composite coatings under elevated temperatures. Xu et al. [19,20,21] incorporated Al, Cu, and Ni elements into WS₂ coatings via radio-frequency co-sputtering. Their systematic investigation revealed distinct roles for different dopants: Cu doping promoted coating amorphization and enhanced structural densification; Ni doping effectively suppressed the columnar grain growth of WS₂, with this inhibitory effect becoming more pronounced at higher Ni concentrations; and Al doping significantly improved environmental resistance by protecting the coating against water vapor and oxygen penetration, thereby enhancing oxidation resistance in humid environments. Dai et al. [22]. fabricated WS₂-doped diamond-like carbon coatings using mid-frequency magnetron sputtering with ion source-assisted deposition technology. Their results demonstrated that the incorporation of WS₂ into DLC coatings led to the formation of WC1-x and WS₂ nanocrystalline clusters, which were uniformly dispersed within the carbon-based network. This unique microstructure endowed the W-S-C composite coatings with both the high hardness and excellent wear resistance that are characteristic of DLC coatings, as well as the superior self-lubricating properties inherent in WS₂.

The rapid development of aviation equipment has led to higher demands in the performance of WS₂ coatings, requiring enhanced friction and wear resistance, as well as high-temperature durability. However, the research on incorporating high-temperature-resistant materials into WS₂ solid lubricant coatings remains relatively limited. Few studies have systematically investigated the doping of thermally stable additives in WS₂-based solid lubricant coatings. As a non-metallic element, boron (B) exhibits distinctive physical properties such as a low density, metalloid characteristics, a high hardness, and excellent thermal resistance [23,24]. The incorporation of B into WS₂ coatings to form B/WS₂ composite coatings through structural modulation enables the strategic combination of soft-phase WS₂ components with hard B-phase constituents. This synergistic design effectively harnesses the intrinsic advantages of both materials, maintaining superior lubricating properties while simultaneously enhancing the mechanical strength and tribological performance.

2. Materials and Methods

2.1. Fabrication of B/WS₂ Composite Coatings

This study employed a JGP045CA magnetron sputtering deposition system (Shenyang Scientific Instrument Co., Ltd., Shenyang, China) to fabricate B/WS₂ composite coatings. High-purity WS₂ and B targets (99.99% purity) with dimensions of 50 mm in diameter and 3 mm in thickness were selected for the deposition process. The WS₂ target was sputtered using radio-frequency power, while the B target was deposited via a direct current power supply. During the sputtering process, 9Cr18 bearing steel substrates with a diameter of 30 mm were selected for the evaluation of the coating’s tribological properties, while single-crystal silicon wafers (8 mm × 8 mm × 1 mm) were used to characterize the mechanical properties and cross-sectional morphology of the coatings. The pre-deposition treatment process was conducted as follows: the substrate surface was sequentially ground using silicon carbide abrasive papers with varying grit sizes (800, 1000, 1500, and 2000), followed by precision mechanical polishing using a diamond polishing compound (Henan Xinhong Grinding Materials Co., Ltd., Xuchang, China) (1.5 μm and 0.5 μm particle sizes). Subsequently, the substrates were ultrasonically cleaned in anhydrous ethanol and acetone for 10 min each, dried with nitrogen gas, and then transferred to a vacuum chamber. When the base pressure reached 5 × 10⁻⁴ Pa, argon gas was introduced for plasma sputtering. Prior to deposition, the substrates were subjected to 10 min of plasma cleaning, followed by the deposition of a 15 nm thick Cr interlayer. During the deposition process, dual magnetron sputtering targets were operated simultaneously while the substrate holder rotated at a constant speed of 20 revolutions per minute to ensure uniform film thickness and composition.

B/WS₂ composite coatings were deposited under varying sputtering power conditions. The as-deposited B/WS₂ composite coatings were subjected to high-temperature tribological tests at 150 °C, 300 °C, and 450 °C to evaluate their thermal stability and wear resistance. For comparative analysis, three types of coatings were selected, pure WS₂ coatings, B/WS₂ composite coatings, and B/WS₂ composite coatings after high-temperature tribological testing, to systematically investigate the influence of different elevated-temperature conditions on their tribological performance. The detailed processing parameters are summarized in

Table 1. B/WS₂ composite coating preparation and experimental parameters.

Vacuum Degree/Pa	5 × 10−4
Sputtering power of target B/W	70
Argon flow rate/Sccm	50
Sputtering temperature/°C	300
WS2 target power/W	200
Deposition time/min	60
Friction and wear test temperature/°C	25–450

.

2.2. Microstructure and Property Characterization of B/WS₂ Composite Coatings

The surface morphology, cross-sectional microstructure, and wear track characteristics of the B/WS₂ composite coatings were systematically investigated using scanning electron microscopy(SEM; Zeiss Sigma 300, Carl Zeiss AG, Oberkochen, Germany). The phase composition and elemental distribution of the B/WS₂ composite coatings were characterized using energy-dispersive X-ray spectroscopy. The crystal structure of the B/WS₂ composite coatings was analyzed using X-ray diffraction, and the test conditions were as follows: an X-ray wavelength (λ) of 0.15405 nm; an operating voltage and current of 40 kV and 30 mA, respectively; a scanning range of 10~80°; and a scanning rate of 1°/min. The chemical valence states of the B/WS₂ composite coating were analyzed using X-ray photoelectron spectroscopy. The interplanar spacing of the B/WS₂ composite coating was characterized via high-resolution transmission electron microscopy.

The hardness and elastic modulus of the B/WS₂ composite coating were analyzed using a nanoindentation tester manufactured by Nano Mechanics, San Jose, CA, USA. A Berkovich indenter was employed to perform single-point hardness tests on a monocrystalline silicon wafer. To minimize experimental errors, measurements were conducted at nine different locations, and the average value was calculated as the final result. The tests were performed under a load of 50 mN, with the maximum indentation depth set to be less than one-tenth of the coating thickness to avoid substrate effects.

The high-temperature tribological properties of the coating under dry friction conditions were evaluated using an HT-1000 rotary friction wear tester (HT-1000, Lanzhou Zhongke Kaihua Technology Development Co., Ltd., Lanzhou, China). Tests were conducted in four different environments, an ambient atmosphere, 150 °C, 300 °C, and 450 °C, employing a circular motion mode. A GCr15 steel ball (GCr15 steel ball, Shenzhen Dingte Steel Ball Co., Ltd., Liaocheng, China) with a diameter of 6 mm served as the counterpart. The experimental parameters included a normal load of 10 N, a rotational speed of 330 rpm, and a test duration of 15 min. To ensure reliability, each friction wear test was repeated at least three times.

The wear profile of the B/WS₂ composite coating after tribological testing was characterized using a precision profilometer. The average wear cross-sectional area (S) was determined by integrating the profile curves at multiple locations along the wear track. Subsequently, the wear volume (V) was calculated by multiplying S by the circumferential length of the wear track. The wear rate was then computed using the standard wear rate formula [17].

$$W={\displaystyle \frac{V}{FL}}$$

Here, W represents the wear rate, with the unit mm³/(N·m); V denotes the worn volume of the wear scar, with the unit mm³; F stands for the applied load force, with the unit N (Newtons); and L indicates the total friction distance, with the unit m (meters).

The wear rate, determined by averaging the results of three repeated friction tests to reduce experimental deviations, was adopted as the evaluation metric for the coating’s wear performance in this study.

3. Results

3.1. Morphology Analysis of B/WS₂ Composite Coating

Figure 1 shows the surface morphology of the B/WS₂ composite coating. As can be seen in Figure 1, the pure WS₂ coating exhibits large granular clusters with clearly defined grain boundaries, a relatively large grain size, numerous surface defects, and poor surface compactness. After the introduction of B, the surface morphology undergoes a significant transformation, shifting from coarse granular clusters to a “cellular-like” protrusion structure. The coating exhibits finer surface particles, improved flatness, and reduced surface roughness. Analysis suggests that the enhanced coating densification induced via B incorporation can be attributed to multiple effects, including significant room-temperature strengthening, surface hardening [25], and the formation of WB4 compounds. The surface morphology analysis of the B/WS₂ composite coating after heating revealed that the cellular-like protrusion structure became more pronounced at 150 °C, accompanied by improved coating densification. With an increasing temperature, enhanced grain stacking was observed, leading to the further optimization of surface flatness. When the temperature reached 450 °C, a lamellar stacking phenomenon emerged in the coating. This can be attributed to high-temperature oxidation effects, which progressively enhanced the coating densification. Remarkably, the B/WS₂ composite coating maintained its structural integrity even at 450 °C without undergoing morphological transformation compared to its room-temperature state.

3.2. Phase Analysis of B/WS₂ Composite Coatings

Figure 2 presents the XRD diffraction patterns of the B/WS₂ composite coatings. As shown in Figure 2, the pure WS₂ coating exhibits characteristic diffraction peaks corresponding to the (002) and (101) crystal planes of WS₂, with a predominant (002) basal plane orientation. The (002) basal plane orientation is primarily responsible for the coating’s lubricating properties, effectively reducing the sliding resistance and enhancing performance. Upon the incorporation of boron (B), the B/WS₂ composite coating exhibited a noticeable reduction in peak intensity and broadening of the (002) diffraction peak, indicative of grain refinement and enhanced grain boundary strengthening. Concurrently, the WS₂ (101) diffraction peak disappeared, while new diffraction peaks corresponding to the (101) and (112) crystallographic planes of WB₄ emerged, confirming the formation of a WB₄ compound with a preferred basal orientation. The results indicate that doping with B element significantly refines the grain size, resulting in grain boundary strengthening and solid solution strengthening effects. Additionally, the formation of WB₄ compounds as a hard phase enhances both the densification and lubricating properties of the WS₂-based solid lubricating coating. The B/WS₂ composite coating was subjected to XRD analysis after high-temperature heating. At 150 °C, the peak width of the WS₂ (002) diffraction plane showed no significant change, while the preferred orientation of the WB₄ (101) and WB₄ (112) diffraction planes intensified slightly, though the overall variation remained minor. When the temperature increased to 300 °C, the WS₂ (002) peak broadened with a reduced intensity, and the preferred orientation of the WB₄ (101) and WB₄ (112) planes weakened. This trend became more pronounced at 450 °C. The analysis suggests that the heating temperature significantly alters the peak intensity and width of the WS₂ and WB₄ diffraction planes due to high-temperature oxidation and thermal decomposition, thereby affecting the lubricating properties and surface densification of the coating.

3.3. Analysis of Mechanical Properties of B/WS₂ Composite Coatings

Figure 3 presents the microhardness and elastic modulus of the B/WS₂ composite coatings. As evident from the figure, the pure WS₂ coating exhibited a relatively low microhardness of only 0.83 GPa. In contrast, the B-doped B/WS₂ composite coating demonstrated a significantly enhanced microhardness of 3.604 GPa, representing a 4.34-fold increase. Similarly, while the elastic modulus of pure WS₂ measured 32.9 GPa, the B/WS₂ composite coating achieved a substantially higher value of 54.7 GPa. Remarkably, after exposure to 450 °C, the B/WS₂ coating showed further improvement in mechanical properties, with its microhardness increasing from 3.604 GPa to 4.17 GPa and its elastic modulus rising from 54.7 GPa to 55.84 GPa.

Figure 4 presents the H/E and H³/E² ratios of the B/WS₂ composite coatings. Our analysis reveals that the pure WS₂ coating exhibited an H/E ratio of 0.0131, while B doping significantly enhanced this value to 0.065 for the B/WS₂ composite coating, corresponding to a 4.99-fold improvement in wear resistance. After high-temperature exposure at 450 °C, the H/E ratio further increased to 0.076, representing a 5.84-fold enhancement compared to the undoped coating. More remarkably, the H³/E² ratio, which reflects resistance to plastic deformation, showed even more dramatic improvements. The pure WS₂ coating displayed an H³/E² value of 5.842 × 10⁻⁴, whereas the B-doped composite coating achieved a value of 0.015: a substantial 25.6-fold increase. This enhancement became even more pronounced after thermal treatment at 450 °C, with the H³/E² ratio reaching 0.0241 (41.26 times higher than the undoped coating). These results clearly demonstrate that boron doping can dramatically improve the mechanical properties of WS₂ coatings. Notably, the B/WS₂ composite coating maintains excellent mechanical performance even under elevated temperatures up to 450 °C, suggesting its potential for high-temperature tribological applications.

Analysis suggests that, due to the smaller atomic radius of B, its doping enhances the bond energy between adjacent atoms in the coating, contributing to grain boundary strengthening and solid solution strengthening. Additionally, the magnetron sputtering process generates WB₄ compounds, the high hardness of which improves dislocation defects between unit cells, thereby enhancing the coating’s performance. At elevated temperatures, the oxidation of the coating affects the hardness and elastic modulus of the fibers. The primary composition of the B/WS₂ composite coating, however, remains largely unchanged with an increasing temperature. The doping and higher content of B lead to improved mechanical properties of the coating. Furthermore, the thermal decomposition effect of the B/WS₂ composite coating is pronounced, providing favorable conditions for high-temperature applications. These factors collectively contribute to the enhancement of the microhardness, elastic modulus, wear resistance, and resistance to plastic deformation of the B/WS₂ composite coating.

3.4. Analysis of Tribological Properties of B/WS₂ Composite Coatings

In the high-temperature friction and wear test, the coating was subjected to three cyclic wear tests, with a track radius of 2 mm, 2.5 mm, and 3 mm. The wear time was 15 min, and the speed was 330 r/min. The GCr15 steel ball was replaced after each test to ensure the accuracy of the test data. During the test, the experimental equipment was in a closed space, and a dehumidifier and refrigerator were used to ensure that the equipment was in an environment with a temperature of 25 °C and less than 30% humidity.

Figure 5 displays the friction coefficient curves of the B/WS₂ composite coatings. As shown in Figure 5, the pure WS₂ coating exhibits significant fluctuations in its friction curve, with intensified oscillations occurring around 10 min, yielding an average friction coefficient of 0.11. With the incorporation of B element, the fluctuation of the coating’s friction curve markedly decreases, achieving an average friction coefficient of 0.071, which is substantially lower than that of the pure WS₂ coating. This result demonstrates that B doping can effectively enhance the tribological performance of the coating. The high-temperature tribological tests of the B/WS₂ composite coating deposited at 70 W sputtering power reveal that its average friction coefficient initially decreases and then increases with a rising temperature. The coating exhibits the most favorable friction performance at 150 °C, achieving an ultralow average friction coefficient of 0.015. At 300 °C, the friction coefficient slightly increases to 0.024. Further temperature elevation to 450 °C leads to a more pronounced rise in the average friction coefficient, reaching 0.052. According to a study by Yan et al. [26] on Ag-doped WS₂, when the Ag content is 6.24 at.%, the coating achieves optimal tribological performance, with an average friction coefficient of 0.023. Rengifo et al. [27] conducted research on Al-doped WS₂, finding that, when the composition is Al-2 vol% WS₂, the coating’s friction coefficient reaches a minimum of 0.55. Pei et al. [28] investigated Cu-doped WS₂, noting that, when the Cu target sputtering power is 15 W, the coating’s friction coefficient reaches a minimum of 0.10. Additionally, according to the research on WS₂ coatings conducted by Cai et al. [29], a pure WS₂ coating exhibits an average friction coefficient of 0.147 at 500 °C, with relatively significant fluctuations in its friction coefficient curve. In our study, the B/WS₂ composite coating shows a friction coefficient of 0.015 at 150 °C, and its friction coefficient at 450 °C is lower than that of the pure WS₂ coating. It can therefore be concluded that B doping effectively enhances the high-temperature tribological properties of WS₂ coatings.

Table 2. Wear rate of B/WS₂ composite coatings.

Sample Number	Coating Name	Target B Power	Test Temperature	Wear rate/mm3·N−1·m−1	Wear Rate SD Rate	Average Friction Coefficient Curve
1	WS2	0 W	25 °C	3.63 × 10−6	1.56266 × 107	0.11
2	B/WS2	70 W	25 °C	7.63 × 10−7	3.81655 × 108	0.071
3	B/WS2	70 W	150 °C	1.89 × 10−7	9.46452 × 109	0.015
4	B/WS2	70 W	300 °C	2.26 × 10−7	1.13475 × 108	0.024
5	B/WS2	70 W	450 °C	8.96 × 10−7	4.4836 × 108	0.053

presents the wear rates of the B/WS₂ composite coatings. As shown, the pure WS₂ coating exhibits a relatively high wear rate in the order of 10⁻⁶ mm³·N⁻¹·m⁻¹ (3.63 × 10⁻⁶ mm³·N⁻¹·m⁻¹). However, with the incorporation of B, the coating demonstrates enhanced microhardness and an enhanced elastic modulus, leading to improved wear resistance and a significantly reduced wear rate (7.63 × 10⁻⁷ mm³·N⁻¹·m⁻¹). High-temperature tribological tests conducted on the B/WS₂ coating deposited at 70 W sputtering power reveal that the wear rate initially decreases and then increases with a rising temperature. The lowest wear rate (1.89 × 10⁻⁷ mm³·N⁻¹·m⁻¹) is achieved at 150 °C, corresponding to the minimum average friction coefficient. At 300 °C, the wear rate slightly increases to 2.26 × 10⁻⁷ mm³·N⁻¹·m⁻¹. Further elevating the temperature to 450 °C results in a more substantial increase in the wear rate (8.96 × 10⁻⁷ mm³·N⁻¹·m⁻¹), which, although higher than that of the B/WS₂ coating at room temperature, remains lower than that of pure WS₂. These findings confirm that B doping significantly enhances the wear performance of the coating.

Figure 6 presents the wear scar morphology of the B/WS₂ composite coatings. As observed in Figure 6a, the pure WS₂ coating exhibits distinct grooves and wide wear tracks, indicating poor wear resistance with the characteristics of typical adhesive wear. This behavior originates from the coating’s low hardness and inferior wear resistance, making its surface susceptible to shear-induced sliding under cyclic loading, with wear debris accumulating along the groove edges. After doping with B element, the width of the wear track decreases slightly, and minor scratches appear at the edges of the wear track, while brittle spalling occurs inside the track [30]. The wear mechanisms of the coating involve both abrasive wear and spalling (Figure 6b). This is because external forces generate microcracks in the wear track during friction, and when these cracks propagate to a critical length, the coating delaminates from the substrate. Compared to the pure WS₂ coating, the B/WS₂ composite coating formed through B doping exhibits a more compact microstructure, improved wear resistance, and enhanced resistance to plastic deformation. Consequently, the wear mechanism of the B/WS₂ composite coating shifts from adhesive wear to spalling and abrasive wear.

When the friction and wear test conditions shift from room temperature to elevated temperatures, the wear mechanism of the B/WS₂ composite coating undergoes corresponding changes. At 150 °C, only slight wear traces are observed on the wear track of the B/WS₂ composite coating, with abrasive wear being the dominant mechanism (Figure 6c). A similar trend is observed at 300 °C. At 450 °C, however, debris accumulation appears along both sides of the wear track, and the coating exhibits a combination of abrasive wear and partial adhesive wear (Figure 6e). Analysis suggests that the B/WS₂ composite coating possesses internal residual stress. Under high-temperature testing conditions, this stress is released, enhancing the coating’s resistance to cyclic external loading and preventing premature failure. Due to its dense microstructure and superior wear resistance, the dominant wear mechanism at elevated temperatures is still abrasive wear [31]. At 450 °C, however, the high-temperature environment facilitates rapid reactions between the coating and oxygen/other gaseous impurities, generating additional abrasive particles. Consequently, the combined abrasive action of the steel counterpart and these wear particles intensifies. As the wear debris accumulates beyond a critical threshold, partial debris forms piled-up ridges along the wear track edges, while the remainder transforms into a transfer film, contributing to lubrication. This transition alters the wear mechanism to adhesive wear [32].

Figure 7 shows the wear scar morphology and EDS spectra of steel balls with different coatings. It can be observed that transfer films are formed on the pure WS₂ coatings, B-doped B/WS₂ composite coatings, and B/WS₂ composite coatings under various test temperatures. The generation of transfer films contributes to the excellent friction and wear performance of the coatings. The pure WS₂ coating produces a relatively large amount of transfer film, accompanied by considerable wear debris. This is attributed to the low hardness and loose porous structure of the pure WS₂ coating, which leads to severe wear under cyclic loading, resulting in increased transfer film formation in the wear scar. In contrast, the B-doped WS₂ coating exhibits higher hardness and resistance to plastic deformation. On the one hand, the superior mechanical properties of the coating enable it to withstand the alternating stresses caused by cyclic loading, minimizing damage to the coating structure and generating less wear debris. Some of the debris form transfer films, while the rest participate in the wear process as fine abrasive particles [33]. On the other hand, the transfer films present participate in the friction and wear process, resulting in lower transfer film formation for the B-doped B/WS₂ composite coating during sliding. Under high-temperature testing conditions, the oxidation and thermal decomposition effects of the B/WS₂ composite coating further enhance its hardness, wear resistance, and resistance to plastic deformation, thereby improving its tribological performance. Consequently, smaller wear scars and fewer transfer films are observed, which can be attributed to the superior wear resistance of the coating. Excessively high temperatures, however, degrade the original mechanical properties of the B/WS₂ composite coating, leading to larger wear scar areas and increased transfer film formation.

EDS analysis of the oxygen distribution in the wear scars of the steel balls revealed the presence of oxygen across all coatings. In the pure WS₂ coating, oxygen accumulation was observed at the edges of the wear scar, whereas the B-doped WS₂ composite coating exhibited no significant oxygen enrichment, indicating that boron doping effectively enhanced the oxidation resistance of the WS₂ coating. Under high-temperature testing at 150 °C, the B/WS₂ composite coating still showed no oxygen aggregation. As the test temperature increased to 300 °C and 450 °C, however, distinct oxygen enrichment became apparent. This phenomenon can be attributed to frictional heating and plastic deformation during wear, which generate localized heat and surface activation, promoting oxidation. At elevated temperatures, tungsten (W) oxidizes to form WO₃, while the combined effects of frictional heat and environmental heating further accelerate the oxidative degradation of the coating.

To gain deeper insights into the chemical composition of the B/WS₂ composite coating, XPS analysis was conducted to examine the wear morphology and scar composition at both room and elevated temperatures. Figure 5, Figure 6, Figure 7 and Figure 8 present the XPS fitting results of the coatings. As shown in Figure 8, W, S, and B elements were detected on the surface of the B-doped WS₂ coating at room temperature. The deconvolution of the W4f peak for the pure WS₂ coating revealed minor metallic W (W4f7/2 31.6 eV, W4f5/2 33.7 eV), a moderate amount of WB₄ (W4f7/2 36.1 eV, W4f5/2 38.2 eV), and a dominant WS₂ phase (W4f7/2 35.6 eV, W4f5/2 36.1 eV) [34,35]. Notably, XPS analysis confirmed the presence of W-B bonds, with the binding energies consistent with the standard values for tungsten tetraboride (WB₄). The high hardness and superior wear resistance of WB₄ significantly enhanced the mechanical properties of the coating, reducing the average friction coefficient to 0.069 and substantially decreasing the wear rate (7.63 × 10⁻⁷ mm³·N⁻¹·m⁻¹). Combined with the wear scar morphology in Figure 7, the transfer film formed during room-temperature friction primarily consisted of W and S elements, along with trace amounts of O. XPS analysis of the B/WS₂ composite coating at 450 °C revealed the disappearance of metallic W (W4f7/2 31.6 eV, W4f5/2 33.7 eV), with the coating mainly composed of WS₂, WB₄, and WO₃. The formation of WO₃ occurred via two pathways: (1) the partial oxidation of metallic W during high-temperature friction reacting with atmospheric oxygen and impurities to form minor WO₃ (accounting for the depletion of metallic W) and (2) the extensive oxidation of WS₂ in the coating under elevated temperatures, generating abundant WO₃, the distribution of which matched the oxygen enrichment pattern observed in Figure 7. The high-temperature transfer film was predominantly composed of WO₃.

To further investigate the friction and wear mechanisms of the B/WS₂ composite coating, wear debris samples were prepared on microgrid copper meshes for transmission electron microscopy (TEM) analysis (Figure 9). The TEM results demonstrate that the wear debris of the B/WS₂ composite coating at room temperature primarily consists of WS₂, the excellent lubricating properties of which contribute to the coating’s favorable friction coefficient (0.11). Notably, the WB₄ lattice can be clearly observed in the wear debris. Combined with XRD and XPS analyses, this finding confirms the formation of WB₄ during the friction process. The presence of WB₄ significantly enhances the load-bearing capacity of the coating through both solid solution strengthening and its inherent high hardness characteristics (with the coating’s microhardness increasing from 0.83 GPa to 3.604 GPa). After exposure to 450 °C, the wear debris exhibits spherical particle accumulation, with a morphology resembling WO₃. The measured interplanar spacing of approximately 0.39 nm matches the standard value for WO₃, which is further corroborated via the EDS analysis of the wear scar (Figure 7) and the XPS spectra of the steel ball’s wear scar (Figure 8), confirming the formation of WO₃ compounds during high-temperature friction. Our analysis suggests that WO₃ functions as a high-temperature solid lubricant with a moderate lubricating capability. At room temperature, the coating contains abundant inherent WS₂, the superior lubricity of which surpasses that of WO₃, resulting in favorable friction coefficients. Additionally, B doping enhances the load-bearing capacity of the coating, while improved hardness and wear resistance further optimize its tribological performance. Under elevated temperatures, the combined effects of thermal conditions and grinding heat promote the reaction between W and O to form WO₃. The high-temperature lubricating properties of WO₃, coupled with the rolling effect of its spherical particles, collectively reduce the friction coefficient, endowing the coating with excellent friction and wear resistance, even at 450 °C.

4. Conclusions

A comparative analysis of pure WS₂ coatings, B/WS₂ composite coatings, and B/WS₂ composite coatings under high-temperature friction and wear test conditions yielded the following conclusions: The pure WS₂ coating exhibited poor wear resistance, with its wear mechanism dominated by typical adhesive wear. With boron doping, the average friction coefficient decreased from 0.11 to 0.071, while the wear rate reduced from 3.63 × 10⁻⁶ mm³·N⁻¹·m⁻¹ to 7.63 × 10⁻⁷ mm³·N⁻¹·m⁻¹. The wear mechanism of the composite coating manifested as a combination of abrasive wear and spalling. The B/WS₂ composite coatings tested at elevated temperatures (150 °C and 300 °C) demonstrated superior friction coefficients and wear rates compared to those tested under room-temperature conditions, exhibiting wear characteristics of both abrasive and adhesive wear. Notably, even at 450 °C, the B/WS₂ composite coating maintained excellent tribological performance at a sputtering power of 70 W, with its wear mechanism consisting of abrasive wear accompanied by adhesive wear.