Optimizing the Wide-Temperature Tribological Properties of HfO₂/WS₂ Coatings by Tuning Deposition Pressure

Abstract

To enhance the wear resistance and load-bearing capacity of WS₂ coatings, this paper employs unbalanced magnetron sputtering technology to fabricate HfO₂/WS₂ composite coatings by regulating the deposition pressure (0.6–1.4 Pa), leveraging the superior properties of HfO₂. The microstructure, mechanical properties, and tribological behavior across a wide temperature range (room temperature to 450 °C) are systematically investigated. The results demonstrate that deposition pressure significantly modulates the coating structure and properties. At a deposition pressure of 0.6 Pa, a pronounced secondary bombardment effect leads to coarse surface particles, a thickness of only 1.525 μm, and a high hardness of 9.332 GPa, but inferior tribological performance with an average friction coefficient of 0.703. When the deposition pressure is increased to 1.4 Pa, the secondary bombardment effect weakens, resulting in an increased coating thickness of 2.125 μm, a decreased hardness of 3.88 GPa, and a significantly improved friction coefficient of 0.072. At an optimal deposition pressure of 1.0 Pa, the sputtered atoms possess moderate energy and optimal surface mobility, promoting the formation of a dense structure. The coating demonstrates a synergistic balance between mechanical load-bearing capability (hardness: 6.38 GPa) and a highly crystalline WS₂ structure, yielding superior frictional behavior characterized by a mean coefficient of friction (COF) of merely 0.062. High-temperature tribological evaluations indicate that the COF displays a non-monotonic trend, declining at first before ascending as the temperature elevates. A minimum value of 0.015 is reached at 300 °C, corresponding to a wear rate of 1.127 × 10⁻⁸ mm³·N⁻¹·m⁻¹. At 450 °C, partial oxidation of WS₂ to WO₃ causes the friction coefficient to rise to 0.045, accompanied by fluctuations. Microstructural analysis confirms that HfO₂ doping effectively suppresses the oxidation of WS₂ at elevated temperatures and promotes the preferred growth orientation of the WS₂(002) plane, thereby synergistically optimizing the wide-temperature-range lubrication performance of the coating. This study provides a novel technical approach for the design of lubricating coatings intended for high-temperature and harsh operating conditions, such as those encountered in aero-engine bearings.

1. Introduction

As a critical component of aerospace transmission systems, rolling bearings perform essential functions, including load support, friction reduction, and motion transmission, making their performance and reliability a subject of continuous concern. With the ongoing advancement of aviation technology, the operating conditions for bearings in aerospace transmission systems have become increasingly severe, often requiring them to function under extreme circumstances such as high temperatures (≥300 °C), high rotational speeds, and alternating loads. Particularly during flight, abrupt changes in aircraft attitude can easily lead to insufficient lubrication supply and localized high temperatures, further exacerbating the harshness of the bearing operating environment. These extreme conditions impose significant challenges on the high-temperature resistance, wear resistance, corrosion resistance, and low-friction characteristics of bearings. High temperature (above 300 °C) poses a systematic and potentially fatal threat to aerospace bearings, primarily manifested through lubrication system failure: lubricating oils or greases undergo oxidation and carbonization under extreme heat, resulting in a loss of lubricating capability. This is accompanied by material performance degradation; for example, bearing steel experiences a decrease in hardness due to the tempering effect, along with a marked deterioration in fatigue strength and toughness [1]. In response, recent research has concentrated on surface engineering of aero-engine bearings via magnetron sputtering processes to engineer protective layers capable of providing lubrication at elevated temperatures. The primary objective of this strategy is to improve both mechanical and tribological performance, marking it as a pivotal area of study within the discipline [2].

WS₂ is a layered solid lubricant whose crystal structure consists of stacked “S-W-S” units. Within each layer, atoms are bonded by strong chemical bonds, while adjacent layers are held together by relatively weak van der Waals forces [3]. Compared to the commonly used lubricant MoS₂, WS₂ exhibits a higher interlayer bonding energy and stronger intralayer covalent interactions. Consequently, WS₂ demonstrates superior thermal stability and more excellent high-temperature tribological performance than MoS₂ [4]. However, WS₂ coatings are plagued by intrinsic limitations, including a porous and loosely packed surface morphology, limited mechanical hardness, and vulnerability to hydration and oxidative degradation under atmospheric conditions. These limitations restrict their practical application.

To address this issue, researchers have found that doping WS₂ coatings with certain metals [5,6,7,8,9], non-metallic elements [10,11], and compounds [12] can enhance their mechanical properties and high-temperature tribological performance. Cai Haichao et al. prepared La-Ti/WS₂ composite films using unbalanced radio-frequency magnetron sputtering. They investigated the influence of target power on the film structure and tribological performance at 500 °C. The results indicated that under a 1N load, the La-Ti/WS₂ composite film deposited at a target power of 20 W exhibited excellent tribological performance, characterized by the highest H/E ratio and the lowest average friction coefficient of 0.012. This is primarily attributed to the formation of rare earth oxides at the high-temperature friction interface, which helps maintain effective lubrication of WS₂ even when it suffers thermal degradation [13]. Liu Jinlong et al. fabricated WS₂/TiB₂ composite films via unbalanced magnetron sputtering and studied the effect of the TiB₂ sputtering current (i.e., TiB₂ content) on the tribological performance across a wide temperature range (25–500 °C). The results showed that the film possessed a low friction coefficient (0.034) at 100 °C. As the temperature increased to 300 °C, the friction coefficient rose to 0.051. At 500 °C, the film failed rapidly. This behavior is mainly attributed to the increased film density due to TiB₂ doping and the formation of a lubricious TiO₂ (001) crystalline phase under the combined action of high ambient temperature and frictional heating [14]. Lu Zhaoxia et al. prepared Ta-doped MoS₂/WS₂ multilayer films by magnetron sputtering. Their results demonstrated that the Ta-doped MoS₂/WS₂ multilayer films maintained a stable friction coefficient between 0.05 and 0.08 in high-temperature air at 370 °C, which is 30% lower than that of the undoped films. The Ta doping enhanced the interlayer bonding strength, while the formed Ta₂O₅ acted as a high-temperature lubricious phase, providing sustained lubrication and delaying oxidation-induced failure [15]. V. An et al. investigated the tribological properties of nanolayered WS₂ doped with zinc oxide nanoparticles. According to the tribological test results, the ZnO nanoparticles did not significantly affect the friction coefficient of nanolayered WS₂ in air at 25 °C. However, they positively influenced the wear resistance of nanolayered WS₂ at 400 °C, primarily due to the formation of hexagonal ZnO. The small size and hexagonal structure of the ZnO nanoparticles (n-ZnO) play an important role during lubrication by filling microcracks on the friction surface [16]. Li Xuemu et al. [17] investigated the effect of WS₂ addition on the microstructure and mechanical properties of ZrO₂ coatings. The results indicated that the incorporation of WS₂ reduced the microhardness of the ZrO₂ coating by 39%, and decreased the average friction coefficient from 0.43 to 0.2. Existing studies have demonstrated that doping with single elements or compounds can improve, to a certain extent, the mechanical properties and high-temperature friction and wear performance of WS₂ coatings. However, their tribological properties still fall short of meeting increasingly stringent service requirements. As aerospace equipment continues to evolve toward higher performance and enhanced reliability, the performance standards required for WS₂-based coatings have become more demanding. In view of this, to address the complex and harsh service environments encountered by next-generation aerospace equipment, it is imperative to explore and develop novel WS₂ composite coating systems to satisfy the critical demand for high-performance solid lubricant coatings capable of operating at elevated temperatures.

Hafnium oxide (HfO₂) is a wide-bandgap metal oxide characterized by an extremely high melting point, excellent thermal stability, high hardness, and superior wear resistance in terms of its physical properties. Chemically, it exhibits strong chemical stability, good corrosion resistance, and outstanding oxidation resistance [18], demonstrating great potential for improving the high-temperature tribological performance of WS₂ coatings. Building on this foundation, our preliminary research involved fabricating HfO₂/WS₂ composite coatings by adjusting the sputtering power of the HfO₂ target [19]. The results indicated that HfO₂ can enhance coating densification, thereby increasing its hardness. Furthermore, an increase in HfO₂ content promotes an amorphous transformation in the WS₂ matrix. Meanwhile, finely dispersed HfO₂ particles serve to shield the sliding contact zone from the steel counterface, thereby yielding a minimal coefficient of friction (COF) at room temperature by suppressing adhesive wear. Based on the aforementioned findings, this paper focuses on exploring and elucidating the role and mechanisms of HfO₂ doping in improving the high-temperature tribological performance of WS₂ coatings.

Building on our previous work, this study further fabricated HfO₂/WS₂ composite coatings under various deposition pressures to investigate their influence on friction and wear performance. Following the optimization of the process parameters, tribological tests across a wide temperature range were conducted to elucidate the underlying mechanism by which HfO₂ doping enhances the high-temperature friction and wear performance of the WS₂ composite coating. This work aims to provide both a theoretical framework and a technical foundation for the design of HfO₂/WS₂ composite coatings and their application as lubricants in high-temperature environments.

2. Materials and Methods

Prior to deposition, G13Cr4Mo4Ni4V bearing steel discs (30 mm diameter) and monocrystalline silicon wafers (7 mm × 7 mm × 1 mm) were prepared as substrates for tribological testing and cross-sectional analysis, respectively. The surface conditioning procedure involved sequential grinding with SiC abrasive papers (800 to 2000 grit), followed by mechanical polishing using diamond pastes (3.5 W, 1.5 W, and 0.5 W). Subsequently, the substrates were degreased via ultrasonic baths in acetone and anhydrous ethanol for 10 min, dried under nitrogen flow, and loaded into the chamber. The composite coatings were prepared in a JGP045CA magnetron sputtering unit manufactured by Shenyang Scientific Instruments Co., Ltd. (Shenyang, China). Sputtering sources comprised 99.99% pure WS₂ and HfO₂ targets (50 mm diameter, 3 mm thickness), driven by DC and RF power supplies, respectively. Upon stabilizing the base pressure at 5 × 10⁻⁴ Pa, the substrates underwent Ar plasma etching for 15 min. A Cr adhesive interlayer (~210 nm) was subsequently deposited for 15 min under the following conditions: 0.6 Pa working pressure, 40 sccm Ar flow, and 60 W power. Throughout the coating growth, both sputtering targets were operated simultaneously, with the substrate stage spinning at 20 rpm [19]. HfO₂/WS₂ composite coatings were fabricated under various deposition pressures. Preliminary experiments revealed that when the deposition pressure was set below 0.6 Pa, it was difficult to ignite or sustain a stable plasma on the HfO₂ target, preventing coating formation. Consequently, the deposition pressure was maintained above 0.6 Pa. Further investigation indicated that when the deposition pressure exceeded 1.4 Pa, collisions among the sputtered particles increased and their mean free path shortened, resulting in a significant reduction in the kinetic energy of particles arriving at the substrate surface and a consequent deterioration in coating properties. Based on these findings, the deposition pressure was ultimately selected within the range of 0.6 Pa to1.4 Pa. The detailed process parameters are listed in

Table 1. Fabrication parameters of HfO₂/WS₂ composite coatings.

Background pressure/Pa	5 × 10−4
Argon flow rate/sccm	40
Deposition pressure/Pa	0.6–1.4
HfO2 target power/W	80
WS2 target power/W	150
Temperature/°C	300
Deposited time/min	95

.

A field-emission scanning electron microscope (FE-SEM; Zeiss Sigma 360, Carl Zeiss AG, Oberkochen, Germany) was utilized to examine the microstructure, surface morphology, and worn surface features of the HfO₂/WS₂ composite coatings. This system included an energy-dispersive X-ray spectroscopy (EDS; Xplore, TESCAN, Brno, Czech Republic) unit to analyze the elemental composition. The crystalline structure of coatings deposited at different pressures was examined by X-ray diffraction (XRD; Bruker D8 Advance, Bruker AXS, Karlsruhe, Germany) using Cu Kα radiation (λ = 0.15405 nm) at 40 kV and 30 mA, with diffraction patterns recorded from 10° to 80° (2θ) at a sweep speed of 1°/min. To minimize substrate interference and evaluate temperature-induced phase evolution, selected samples after high-temperature testing were further analyzed by grazing-incidence X-ray diffraction (GIXRD; Rigaku Smartlab 3 kW, Rigaku, Akishima, Japan) at a fixed incident angle of 1°, scanning from 5° to 90° at 9°/min. X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) was employed to investigate the chemical bonding states of the coating constituents. Mechanical properties, including hardness and elastic modulus, were evaluated using an iNano nanoindenter (Nanomechanics Inc., Oak Ridge, TN, USA) utilizing a three-sided pyramidal Berkovich indenter. Nanoindentation measurements were conducted at nine different locations on each coated steel substrate under a peak load of 50 mN, with the indentation depth carefully controlled to within 10% of the total film thickness to minimize substrate effects. A holding time of 10 s was applied at the maximum load to allow for stabilization prior to unloading. The reported values represent the average of these measurements to ensure statistical reliability [19].

Dry sliding tests were conducted on an HT-1000 ball-on-disk rotational tribometer to characterize the tribological performance in an ambient atmosphere with a controlled relative humidity of 30%. The tests were conducted in a circular rotation mode. During the friction and wear tests, GCr15 steel balls and silicon nitride (Si₃N₄) ceramic balls, both with a diameter of 5.5 mm, were selected as the counterpart materials. The use of Si₃N₄ ceramic balls under high-temperature testing conditions was primarily based on the following considerations: at room temperature, the friction coefficients of the steel and ceramic balls are relatively comparable, rendering both suitable as counterpart materials. However, under elevated temperatures or during prolonged testing, steel balls are susceptible to significant wear due to oxidation and thermal softening, which may interfere with the accurate evaluation of the intrinsic friction and wear behavior of the coating. In contrast, Si₃N₄ ceramic balls exhibit excellent high-temperature stability, making them more suitable for characterizing tribological performance under such conditions. A constant load of 10 N, a rotational speed of 330 rpm, and a test duration of 15 min were applied, with a test diameter of 8 mm. Each tribological test was repeated at least three times to ensure reproducibility. Following the wear tests, the wear track profiles of the HfO₂/WS₂ composite coatings were examined with a high-accuracy surface profilometer. The mean cross-sectional worn region (S) was quantified through integration of the scanned profile data collected at various points along the wear track. The wear volume (V) was subsequently derived by the product of the mean wear area (S) and the circular path length (L). Finally, the specific wear rate was determined using Equation (1).

$$W={\displaystyle \frac{V}{F\cdot L}}$$

(1)

where W denotes the specific wear rate (mm³·N⁻¹·m⁻¹), V represents the volumetric wear loss (mm³), F corresponds to the normal contact load (N), and L signifies the total sliding distance (m). To reduce measurement uncertainties, the final wear rate value was obtained by averaging three separate tribological tests.

3. Results

3.1. Composition and Morphology Analysis of HfO₂/WS₂ Composite Coatings

Figure 1 presents SEM images illustrating the surface morphology of the HfO₂/WS₂ composite coatings. All coatings, irrespective of deposition pressure, exhibit a “dome-shaped cellular” morphology. At a deposition pressure of 0.6 Pa, the surface is characterized by relatively large cellular structures, resulting in high surface roughness and poor compactness. As the deposition pressure increases beyond 0.6 Pa, the size of these cellular structures decreases, accompanied by a noticeable improvement in coating densification. When the deposition pressure is raised to 1.0 Pa, the coating surface displays the finest and most densely packed cellular structures, achieving the highest surface smoothness and optimal compactness under the tested conditions. However, a further increase in deposition pressure leads to a significant enlargement in the spacing between the cellular structures and agglomerated grains. These observations indicate that the surface compactness of the coating first increases and then decreases with rising deposition pressure. At the specific pressure of 1.0 Pa, the grain size of the HfO₂/WS₂ composite coating is effectively refined, the pore dimensions are minimized, and the coating consequently demonstrates excellent overall densification.

Figure 2 illustrates the cross-sectional morphologies of the HfO₂/WS₂ composite coatings prepared under varying pressures, revealing that the deposition pressure significantly influences both the thickness and microstructure of the coatings. At a deposition pressure of 0.6 Pa, the coating thickness is only 1.525 μm. As the deposition pressure increases, the coating thickness exhibits an increasing trend, reaching 2.125 μm at 1.4 Pa. Regarding the cross-sectional microstructure, under the conditions of 0.6 Pa and 0.8 Pa, the coatings display a relatively fine nanocrystalline or amorphous structure, with no obvious columnar features. The analysis suggests that the deposition pressure significantly influences the coating growth process by modulating the kinetic energy of sputtered species upon arrival at the substrate. Under relatively low deposition pressure (0.6 Pa), the density of gas molecules within the vacuum chamber is low, resulting in fewer collisions between sputtered particles and gas molecules during their transport to the substrate and consequently minimal energy loss. Thus, the particles arrive at the substrate with high kinetic energy. This high-energy particle bombardment effect—commonly referred to as the “atomic peening effect” in physical vapor deposition—induces resputtering of the already deposited surface [20,21,22], leading to a relatively small coating thickness (1.525 μm). As the deposition pressure increases to 1.0 Pa, the collision frequency among particles rises, and their energy is moderately reduced. This favors enhanced surface adatom mobility and promotes the formation of a dense structure, resulting in improved coating compactness and increased thickness. Upon further increasing the pressure to 1.4 Pa, the gas molecule density becomes higher, and sputtered particles undergo frequent collisions, leading to thermalization. Consequently, their kinetic energy upon reaching the substrate is significantly diminished, and surface adatom mobility is reduced. Under these conditions, coating growth tends to favor the formation of a relatively porous columnar structure, with the thickness further increasing to 2.125 μm. This trend of increasing thickness with rising deposition pressure aligns with the classical description in the Thornton structure zone model concerning the influence of sputtered particle energy on thin film growth [23].

Table 2. Element composition of the HfO₂/WS₂ composite coatings.

Deposition Pressure/Pa	Elemental Concentration/(at.%)
	S	Hf	W	O
0.6	30.82	11.29	39.39	18.50
0.8	32.33	10.95	38.10	18.62
1.0	38.16	11.24	32.91	17.69
1.2	36.50	11.31	33.29	18.90
1.4	34.64	10.82	33.14	21.40

details the chemical constituents of the coatings, which were characterized via energy-dispersive X-ray spectroscopy (EDS). With increasing deposition pressure, the sulfur (S) content in the coatings rises from 30.82 at.% to 34.64 at.%, while tungsten (W) exhibits a decreasing trend. The hafnium (Hf) content remains largely unchanged. This compositional variation can be attributed to the pressure-dependent deposition dynamics. Under lower pressure, the mean free path is longer, resulting in fewer collisions between W atoms and Ar atoms. Consequently, most W atoms retain sufficient kinetic energy to reach the substrate, leading to high deposition efficiency with minimal energy loss. In contrast, under higher pressure, the shortened mean free path increases the collision frequency for both W and S atoms with Ar atoms. These collisions cause significant kinetic energy dissipation and may alter the trajectory of the atoms. Notably, due to the substantial mass disparity between W and Ar, momentum conservation dictates that W atoms are more prone to trajectory deflection during collisions, contributing to their reduced content.

3.2. Multispectral Characterization of HfO₂/WS₂ Composite Coatings

Figure 3 displays the X-ray diffractograms of the HfO₂/WS₂ composite coatings deposited at different pressures. As shown in Figure 3, all coatings exhibit characteristic diffraction peaks of WS₂ with varying intensities, primarily including the (002) peak at approximately 2θ ≈ 14.3° and the (102) peak at approximately 2θ ≈ 35.9°, indicating a polycrystalline structure with both basal and edge plane orientations. It is worth noting that diffraction peaks corresponding to the Cr interlayer—specifically the (110), (200), and (211) planes (PDF#06-0694)—remain detectable in all composite coating systems due to X-ray penetration through the relatively thin coatings. With increasing deposition pressure, the intensity of the WS₂ (002) diffraction peak first increases and then decreases. Since the (002) plane represents the primary lubricious phase of WS₂, its highest intensity at 1.0 Pa correlates well with the optimal tribological performance observed for the coating deposited under this condition. Notably, no distinct diffraction peaks corresponding to crystalline HfO₂ are observed in the patterns. This absence can be attributed to the following possible reasons: (1) the HfO₂ content is below the detection limit of XRD or overlaps with other stronger peaks; (2) HfO₂ exists in an amorphous or nanocrystalline form within the WS₂ matrix. Regarding the effect of deposition pressure on the intensity of the WS₂ (002) peak, we attribute this phenomenon primarily to the evolution of preferred orientation during film growth. At lower deposition pressures (0.6–0.8 Pa), high-energy particle bombardment disrupts the layered growth order of WS₂, thereby inhibiting the formation of the (002) plane. As the pressure increases to 1.0 Pa, the kinetic energy of the sputtered species becomes moderate, leading to enhanced adatom mobility and promoting the preferred growth of WS₂ along the (002) basal plane. However, when the pressure further increases to 1.2–1.4 Pa, the excessively low kinetic energy of the arriving species results in diminished adatom mobility, leading to disordered crystal growth and a weakening of the (002) texture [24].

To elucidate the effect of working pressure on the bonding configurations within the coatings, XPS measurements were conducted to characterize the surface chemistry of the composite coating deposited at 0.6 Pa (the corresponding spectrum for the 1.0 Pa condition is provided in Figure S1). Figure 4a,b present the fitted XPS spectra of the Hf and W elements, respectively, for the HfO₂/WS₂ composite coating deposited at 0.6 Pa. As depicted in Figure 4a, the binding energy peaks positioned at 17.5 eV and 19.1 eV are assigned to the Hf 4f5/2 and Hf 4f7/2 photoelectron peaks of Hf4+ in HfO₂, exhibiting a spin–orbit splitting of 1.6 eV, verifying the formation of Hf–O bonds [25,26]. Figure 4b reveals that the peaks at 32.4 eV and 34.3 eV are attributed to the W 4f7/2 and W 4f5/2 states of W4+ in WS₂ [27], while the signals observed at 36.1 eV and 38.1 eV can be ascribed to W6+ in WO₃, representing the W 4f7/2 and W 4f5/2 orbitals [28]. Compared with Figure S1b, the characteristic WS₂ peaks in Figure 4b exhibit reduced intensity and broader full width at half maximum (FWHM). This change may originate from the increased collision frequency between particles and gas molecules when the deposition pressure is raised to 1.0 Pa, leading to a reduction in their kinetic energy and consequently a more moderate energy upon reaching the substrate. The attenuation of the secondary bombardment effect reduces the associated damage to the growing surface, thereby creating more favorable conditions for the WS₂ crystals to arrange and grow in an ordered manner according to their intrinsic layered structure.

To investigate the specific form in which HfO₂ exists within the WS₂ coating, the microstructure of the coating was analyzed using transmission electron microscopy (TEM). The results are presented in Figure 5. Lattice fringes with a spacing of approximately 0.615 nm, matching the standard interplanar distance of the WS₂ (002) plane, were observed in all micrographs. Furthermore, lattice fringes with a spacing of approximately 0.264 nm, corresponding to the HfO₂ (002) plane [26], were detected in Figure 5a–c. The TEM images also reveal that at a deposition pressure of 0.6 Pa, a larger quantity of HfO₂ grains precipitated and were distributed more widely within the coating. In contrast, the coating deposited at 1.0 Pa exhibited a more balanced distribution between the HfO₂ and WS₂ grains. These TEM results indicate that the HfO₂ dopant is dispersed within the WS₂ matrix in the form of nanocrystals. Due to their fine grain size and consequently weak diffraction signal, no distinct characteristic diffraction peaks for HfO₂ appeared in the XRD patterns.

3.3. Analysis of Mechanical Properties of HfO₂/WS₂ Composite Coatings

The mechanical properties of the HfO₂/WS₂ composite coatings were characterized using a nanoindenter, and the results are presented in Figure 6. The results indicate that both the hardness and elastic modulus of the coatings exhibit a decreasing trend with increasing deposition pressure. At a deposition pressure of 0.6 Pa, the coating achieved its maximum hardness (9.332 GPa). When the pressure was increased to 0.8 Pa, the hardness slightly decreased to 9.14 GPa. A further increase in deposition pressure led to a significant drop in hardness, reaching only 3.88 GPa at 1.4 Pa. The variation in hardness is also closely related to the particle energy modulated by the deposition pressure. Under the low-pressure condition of 0.6 Pa, the sputtered particles possess high kinetic energy, and the pronounced atomic peening effect induces grain refinement and introduces lattice distortion [23], thereby achieving a high hardness of 9.332 GPa. The cross-sectional morphologies in Figure 2a,b reveal a dense structure and fine grains under this condition, which aligns with the refinement mechanism induced by high-energy bombardment. As the deposition pressure increases above 1.0 Pa, the kinetic energy of the particles decreases significantly, the atomic peening effect weakens, and the inhibition of grain growth is correspondingly reduced. This leads to larger grain sizes and consequently lower hardness. The trend observed for the elastic modulus is consistent with that of hardness, further corroborating the intrinsic relationship whereby deposition pressure modulates particle kinetic energy, thereby influencing the microstructure and mechanical properties of the coatings.

3.4. Analysis of the Tribological Properties of HfO₂/WS₂ Composite Coatings

3.4.1. Coatings’ Tribological Properties Under Normal Temperature Conditions

The results presented in Figure 7 demonstrate that the mean coefficient of friction (COF) for the composite coatings remains below 0.1 when the deposition pressure is higher than 0.8 Pa. Specifically, at pressures of 0.6 Pa and 0.8 Pa, the mean coefficients of friction (COF) are measured as 0.703 and 0.518, respectively. When the deposition pressure is increased to 1.0 Pa, the average friction coefficient decreases significantly to 0.062, with a minimum value reaching 0.048. Concurrently, the friction coefficient curve under this condition exhibits the smallest fluctuation. Further increasing the deposition pressure results in an average friction coefficient of 0.069 at 1.2 Pa, with evident instability observed in the COF response. At 1.4 Pa, the average friction coefficient is 0.072, with similarly significant curve fluctuations. These findings indicate that an appropriate deposition pressure significantly enhances the tribological performance of the coatings. In conjunction with the XRD analysis from Figure 3, it is observed that the intensity of the WS₂ (002) diffraction peak first increases and then decreases with increasing deposition pressure. Under the low deposition pressure conditions of 0.6 Pa and 0.8 Pa, the diffraction peak intensity of the lubricious WS₂ (002) crystallographic plane is relatively weak, indicating that the coating lacks a preferred orientation conducive to low shear stress. Meanwhile, as observed in Figure 1, the coating surfaces under these two conditions exhibit a higher density of pores and structural defects, resulting in poor compactness. Combined with the hardness data presented in Figure 6, although the coatings deposited at 0.6 Pa and 0.8 Pa possess relatively high hardness, excessively high hardness is often accompanied by increased brittleness, making the coatings susceptible to brittle fracture and premature spallation. Consequently, under these two deposition pressures, localized failure occurs shortly after the onset of friction testing, leading to a persistently high friction coefficient with significant fluctuations and overall inferior lubricating performance.

To elucidate the underlying mechanisms governing the frictional response of the HfO₂/WS₂ composite coatings, the wear morphologies were systematically analyzed. Figure 8 presents the SEM images of the wear morphologies under different deposition pressures. At deposition pressures of 0.6 Pa and 0.8 Pa, the wear track exhibits almost complete coating removal, accompanied by flaking at the edges. Upon raising the working pressure to 1.0 Pa, the removal is significantly alleviated, with only localized delamination observed in the central region of the wear track. Upon further increasing the deposition pressure to 1.2 Pa and 1.4 Pa, localized delamination was observed on the coating surface. The delamination occurring at higher pressures can be attributed to multiple factors. As shown in Figure 2d,e, coatings deposited at 1.2 Pa and 1.4 Pa exhibit a columnar structure characterized by larger grain sizes and lower interface density. Combined with their low hardness presented in Figure 6, this indicates reduced resistance to plastic deformation, rendering the coating susceptible to localized fracture under contact stress. In contrast, the extensive delamination observed at 0.6 Pa and 0.8 Pa originates from a different mechanism. Under low-pressure conditions, excessive ion bombardment induces high compressive stress and extreme hardening, imparting brittle characteristics to the coating. Consequently, under sliding contact, the coating undergoes brittle fracture rather than progressive wear, leading to large-area spallation. At 1.0 Pa, the coating displays a finer and more uniform surface morphology, a stronger (002) diffraction peak, and moderate hardness (indicative of good toughness). As a result, only mild abrasive wear is observed under this condition. Based on Equation (1), the wear rate of the coating deposited at 1.0 Pa is calculated to be 1.571 × 10⁻⁸ mm³·N⁻¹·m⁻¹. This low wear rate corresponds to the densified surface structure shown in Figure 1c, indicating superior tribological performance under this condition.

3.4.2. Thermal Environment-Induced Coating Tribological Properties

Figure 9 and Figure 10 present the evolution of the friction coefficient and the wear rate data of the HfO₂/WS₂ composite coatings, fabricated at a sputtering power of 80 W and a deposition pressure of 1.0 Pa, under different temperatures. The results indicate that as the test temperature increases, the mean coefficient of friction (COF) for the coating exhibits an initial decrease followed by an increase. At 150 °C, the average friction coefficient is 0.022, with a corresponding wear rate of 1.297 × 10⁻⁸ mm³·N⁻¹·m⁻¹. When the temperature rises to 300 °C, the friction coefficient reaches its minimum, averaging 0.015 (with a lowest recorded value of 0.011), while the wear rate also achieves its minimum of 1.127 × 10⁻⁸ mm³·N⁻¹·m⁻¹. Upon further increasing the temperature to 450 °C, the average friction coefficient rises to 0.042 and the wear rate increases to 5.34 × 10⁻⁸ mm³·N⁻¹·m⁻¹. At this temperature, the friction coefficient begins to exhibit a noticeable rise and fluctuation after approximately 5 min of testing. The underlying reasons are as follows: At a temperature of 450 °C, WS₂ undergoes significant oxidation to form WO₃. During the initial 5 min, the original contaminants and a thin oxide layer on both the coating and the counterpart ball surface are gradually worn away, exposing fresh WS₂. Subsequently, the sustained mechanical action of friction continuously removes the surface layer, while the accumulated frictional heat accelerates oxidation reactions within the contact region. The resulting WO₃ possesses a significantly higher shear strength than WS₂ and lacks its superior layered lubricating properties, thereby leading to the observed increase in the friction coefficient. These findings demonstrate that the friction coefficients of the HfO₂/WS₂ composite coatings at elevated temperatures are consistently lower than those at room temperature, confirming its favorable lubrication performance in high-temperature environments. Furthermore, according to the investigation of WS₂ coatings by Cai et al. [29], the average friction coefficient of pure WS₂ coatings at 500 °C was reported to be 0.147, coupled with pronounced oscillations in the COF trace. In the present study, the HfO₂/WS₂ composite coatings achieve a minimal COF of 0.015 at 300 °C, while maintaining a value superior to pure WS₂ even at 450 °C. These findings confirm that incorporating HfO₂ significantly improves the elevated-temperature frictional behavior of WS₂-based coatings.

Figure 11 presents the wear track morphology of the HfO₂/WS₂ composite coating tested at various temperatures. Observations reveal that the wear volume remains relatively low at temperatures below 300 °C, aligning with the data presented in Figure 10. Under room temperature and 150 °C conditions, only minor debris accumulation is detected along the boundaries of the wear track, with the primary wear mechanism being adhesive wear. This phenomenon originates from the shear-induced sliding of the coating under sliding contact, resulting in wear debris pile-up on either side of the wear track. At 300 °C, the wear track is less distinct, exhibiting only mild characteristics of adhesive wear. However, when the temperature increases to 450 °C, evident brittle spalling appears within the wear track. At this temperature, the wear mechanism is dominated by the combined action of abrasive wear and spalling.

Figure 12 presents the surface morphology of the HfO₂/WS₂ composite coatings after exposure to different temperatures. It can be observed that all coatings maintain good surface flatness and exhibit a “dome-shaped cellular” morphology. As the temperature increases, the size of these cellular structures on the coating surface gradually enlarges. This phenomenon is primarily attributed to the combined effects of thermally activated surface diffusion and grain growth. With rising temperature, the atomic diffusion capacity within the coating is enhanced, promoting the coalescence and coarsening of nanocrystalline grains. The higher temperature provides a stronger driving force and greater atomic mobility, allowing the cellular structures to grow more fully. This process fundamentally represents the evolution of the coating towards a more thermodynamically stable state under thermal activation.

To investigate the material transfer behavior at the interface during the friction process, the wear scars on the ceramic balls surfaces after tests at various temperatures were characterized via field-emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Figure 13). The results indicate that a continuous transfer film formed on the ceramic balls surface under all tested temperature conditions. This transfer film exhibited good plasticity and strong adhesion, effectively protecting and lubricating the contact interface during sliding. This observation is consistent with our previous findings regarding the formation of a lubricious transfer film from the HfO₂/WS₂ composite coating and its role in improving tribological behavior [19]. Specifically, our prior work systematically demonstrated that doping HfO₂ into the WS₂ matrix can significantly enhance the stability of the transfer film. The related mechanisms, such as the interfacial passivation effect, provide a crucial reference for understanding the frictional behavior observed at different temperatures in this experiment.

To investigate the influence of coating composition on its tribological performance at different temperatures, XRD patterns of the coatings after tests at various temperatures were analyzed. The results are shown in Figure 14: at 150 °C and 300 °C, the coatings exhibit diffraction peaks corresponding only to the WS₂ (002) and (102) planes. However, at 450 °C, in addition to the characteristic peaks of WS₂, a distinct peak attributed to the WO₃ (200) plane appears near 2θ ≈ 24.4°. This indicates significant oxidation of the coating at this temperature. This phenomenon originates from the gradual reaction of sulfur atoms within the layered WS₂ structure with oxygen when heated in air, leading to its transformation into tungsten oxide. The emergence of the WO₃ (200) peak suggests partial conversion of WS₂ into WO₃. The hard WO₃ particles can act as abrasive phases during sliding, generating a plowing action on the coating surface and consequently increasing the friction coefficient. This mechanism accounts for the significant increase in the friction coefficient observed at 450 °C in Figure 9.

As shown in Figures S2–S4, at 150 °C, no characteristic diffraction peaks of WO₃ were observed for HfO₂ sputtering powers ranging from 40 W to 120 W. At 300 °C, only weak WO₃ characteristic peaks were detected for coatings deposited at 40 W and 60 W. These results indicate that the doping of HfO₂ effectively suppresses the oxidation of WS₂ at elevated temperatures. Furthermore, all three sets of figures reveal that, across different temperatures, the diffraction intensity of the WS₂ (002) plane initially increases and then decreases with increasing HfO₂ doping, reaching its maximum at approximately 80 W. This suggests that an appropriate amount of HfO₂ promotes the preferred growth orientation of the WS₂ (002) plane. From Figure S3 (450 °C), it can also be seen that as the HfO₂ sputtering power increases, the intensity of the WO3 (200) diffraction peak gradually weakens, reaching its minimum at 120 W. This further confirms that HfO₂ inhibits the oxidation of WS₂—i.e., the higher the HfO₂ doping, the less pronounced the oxidation of the WS₂ coating. Notably, after heating to 450 °C, no distinct diffraction peaks were detected in the XRD pattern of the coating prepared at 40 W. This suggests that coatings deposited at low power possess a more metastable nature. Their initial nanocrystalline/amorphous composite structure may undergo complete amorphization or form a disordered solid solution at high temperature, resulting in the loss of long-range ordered structure. This result highlights the crucial role of sputtering power in regulating the thermal–structural stability of the coatings.

4. Conclusions

In this study, HfO₂/WS₂ composite coatings were fabricated via magnetron sputtering. The influence of deposition pressure on the structure and properties of the coatings was systematically investigated, and the tribological behavior of the optimally prepared coating was examined across a wide temperature range. The main conclusions are as follows:

(1)

Deposition pressure significantly modulates the mechanical and tribological properties of the coatings. Coatings deposited at a lower pressure (0.6 Pa) exhibit higher hardness but inferior tribological performance. As the deposition pressure increases to above 1.0 Pa, the coating hardness moderately decreases, while the tribological performance is markedly improved. The primary wear mechanism for these coatings is spalling.

(2)

Temperature significantly affects the structural evolution and frictional performance of the coatings. As the test temperature rises, the size of the cellular structures on the coating surface gradually increases. Appropriate HfO₂ doping promotes the preferred orientation of the WS₂ (002) crystallographic plane and suppresses the oxidation of WS₂ in high-temperature environments.

(3)

The HfO₂/WS₂ composite coating exhibits excellent tribological performance across a wide temperature range. The friction coefficient shows an initial decrease followed by an increase with rising temperature. From room temperature to 300 °C, the friction coefficient continuously decreases, reaching a minimum value of 0.015 at 300 °C, corresponding to a wear rate of 1.127 × 10⁻⁸ mm³·N⁻¹·m⁻¹. At 450 °C, due to the oxidation of WS₂ and formation of WO₃, the friction coefficient increases and exhibits fluctuations. The dominant wear mechanisms at elevated temperatures are adhesive wear and abrasive wear.

(4)

As a hard phase, HfO₂ synergistically optimizes the tribological performance of WS₂ coatings from ambient to elevated temperatures, thereby achieving wide-temperature-range lubrication capability for the HfO₂/WS₂ coating system. This study provides a new material design concept and a technical pathway to meet the critical demand for long-lasting, stable lubricating materials in high-temperature and harsh operating conditions, such as those encountered in aero-engine bearings.