钛合金表面微弧诱导 MoS2 纳米复合涂层组织结构与摩擦磨损性能

Introduction Ti-6Al-4V (TC4) titanium alloy is widely used in industrial and biomedical fields due to its excellent mechanical properties and biocompatibility. However, its inherently poor wear resistance significantly limits its further application. Plasma electrolytic oxidation (PEO) as a green and efficient method for in-situ fabrication of ceramic coatings with strong adhesion to the substrate, offering an excellent solution for surface protection of titanium alloys. However, conventional PEO coatings exhibit a porous outer layer composed mainly of high friction TiO2, resulting in insufficient wear and friction reduction performance. To overcome this limitation, incorporating MoS2 (a solid lubricant with a layered structure) can be introduced to the PEO coating to form a composite coating, which has been demonstrated as an effective approach to enhance its tribological properties. Although previous studies have confirmed the potential value of TiO2/MoS2 composite coatings in antifriction, most studies rely on high concentrations of MoS2 additives or prolonged treatment times, which often lead to particle agglomeration and high energy consumption. Even at lower concentrations, the friction coefficient remains high, and systematic studies on the influence of key process parameters, such as applied voltage are still lacking. Therefore, this study aims to systematically investigate the effects of different PEO voltages on the microstructure, chemical composition, and tribological properties of TiO2/MoS2 composite coatings under low MoS2 concentration and short processing time, so as to provide theoretical and practical guidance for the design and fabrication of high-performance wear resistant and antifriction coatings. Methods In this study, Ti-6Al-4V alloy was employed as the substrate. TiO2/MoS2 composite coatings were fabricated in a single-step process via PEO with incorporation of MoS2 nanoparticles. The applied voltage was varied at 400, 500 V, and 600 V. The influence of voltage on the coating surface morphology was characterized using scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). Localized chemical analysis of different regions on the coating surface was performed by energy dispersive spectroscopy (EDS) attached to the SEM. Phase composition was further determined by X-ray diffraction (XRD). Finally, the tribological properties of the coatings were evaluated using a ball-on-disk friction and wear tester. Results and Discussion The test results demonstrate that as the PEO applied voltage increases, the pore size, coating thickness, surface roughness, and deposited MoS2 content of the coatings all increase. Tribological tests on samples prepared at different voltages demonstrated that the coating produced at 500 V exhibited the lowest friction coefficient of approximately 0.2, representing a 69.2% reduction compared to the substrate, indicating excellent antifriction performance. In contrast, coatings prepared at 400 V and 600 V exbibited significantly higher friction coefficients of 0.75 and 0.82, respectively, and suffered from severe adhesive and abrasive wear. The primary reasons for this behavior are as follows: At the lower voltage (400 V), the coating thickness is thin and the MoS2 content is insufficient to provide effective lubrication. During the running-in stage, the coating lacks adequate capacity to accommodate wear debris, leading to inadequate debris removal. This results in pronounced abrasive wear, rapid penetration of the coating, and direct interaction between the substrate and the counterface, thereby increasing the friction coefficient. At the higher voltage of 600 V, although the coating thickness and MoS2 content increase substantially, the surface roughness rises significantly ((4.7 ± 0.3) μm). This leads to the generation of large, coarse debris during the initial running-in stage. Before the coating can effectively accommodate these debris particles, they cause rapid spallation of the coating, generating even more debris and accelerating wear. Under these conditions, the MoS2 particles embedded in the outer layer fail to provide any meaningful lubrication, and the coating is quickly worn through, resulting in a sharp increase in the friction coefficient. At the applied voltage of 500 V, the coating exhibits moderate thickness and surface roughness, maintaining adequate coating thickness and MoS2 content without excessive surface roughness. The friction and wear mechanism of the TiO2/MoS2 composite coating under this voltage can be elucidated as follows: The inherent porous outer layer of the PEO coating undergoes initial smoothing of surface asperities during friction, generating wear debris containing embedded MoS2 particles that fill surface depressions and inherent pores. With continued sliding, the MoS2 particles within the debris gradually spread across the contact interface. Owing to their unique two-dimensional layered structure, these particles undergo interlayer sliding under shear stress, forming a continuous surface film with excellent lubricating properties. Furthermore, the porous structure of the coating not only accommodates wear debris but also functions as a reservoir for MoS2 particles, enabling continuous replenishment of lubricant to areas where the surface lubricating film becomes locally depleted, thereby achieving remarkable self-lubricating performance. Conclusions At the optimized voltage of 500 V, the TiO2/MoS2 composite coating exhibits a moderate thickness ((28.0 ± 0.6) μm) and surface roughness ((3.0 ± 0.2) μm), along with a relatively high MoS2 content. This combination results in the lowest friction coefficient of 0.2, representing a 69.2% reduction compared to the uncoated Ti-6Al-4V substrate; During the friction process, MoS2 particles embedded in the TiO2/MoS2 composite coating are progressively exposed under shear stress and undergo interlayer sliding. This leads to the formation of a lubricating film on the wear track, which provides effective self-lubrication and significantly enhances the tribological performance of the Ti-6Al-4V alloy.