Friction and wear behavior of molybdenum-disulfide doped hydrogen-free diamond-like carbon films sliding against Al2O3 balls at elevated temperature

Abstract

Diamond-like carbon (DLC) films quickly experience lubrication failure due to oxidation and mechanical degradation when used above 300 °C, which limits their application. Molybdenum disulfide (MoS2) doping is considered as a feasible method to improve high-temperature properties of DLC films. However, the effect of MoS2 doping on the high-temperature friction behavior of MoS2-DLC composite films and its mechanism remain unclear. Here, MoS2-DLC composite films with different contents of MoS2 were synthesized by a superimposed deposition system consisting of high-power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS). The MoS2 content in the films was controlled by varying the number of MoS2 pellets in the graphite-MoS2 (C–MoS2) mosaic target. The effect of MoS2 doping content on the structure and mechanical properties of MoS2-DLC films was analyzed. In addition, the friction and wear behavior and mechanism of MoS2-DLC composite films sliding against Al2O3 balls at 25–450 °C were studied. The results indicated that MoS2 was successfully incorporated into DLC matrix, and the content of Mo in the films ranged from 0 to 6.15 at. %. The as-deposited MoS2-DLC composite films exhibited a dense and featureless structure. The doping of MoS2 improved the adhesion strength and released the internal stress of MoS2-DLC composite films, but reduced the hardness and elastic modulus of the films. In particular, MoS2-DLC composite films with 3.78–6.15 at. % Mo maintained excellent anti-friction and anti-wear performances in the temperature range of 25–450 °C. Abrasive wear and fatigue wear occurred in the friction test of the films at different temperatures. However, the friction mechanism of the films varied with the temperature of friction test. At 25 °C, MoS2-DLC composite films showed excellent lubrication and anti-wear abilities due to the carbon suspension bond was passivated by H2O molecules. At 250 °C, MoS2 phase precipitated on the surface of the films, resulting in low friction. At 350 °C, MoS2-xOx solid solution was generated on the surface of the wear track to maintain lower friction and wear. Interestingly, MoS2-DLC composite films with high Mo content still achieved relatively low coefficient of friction and wear rate up to 450 °C due to the continuous availability of MoS2-xOx generated on the surface of wear track.