Effects of gradient structure and modulation period of Ta/TaN/Ta(C,N)/Ta-DLC multilayer coatings prepared by HiPIMS

Hard coatings, such as nitride, carbide and diamond-like carbon (DLC), have been applied to improve the tribological resistance by forming multilayer or gradient structures. In this study, Ta/TaN/Ta(C,N)/Ta-DLC coatings in gradient content and multilayer structure with different modulation periods were prepared by high-power pulsed magnetron sputtering (HiPIMS). The effect of the modulation period on the microstructure, mechanical and tribological properties was evaluated. The average grain size of the coatings first increased from 15.2 to 17.1 nm and then decreased to 9.9 nm with decreasing modulation period from 1220 to 153 nm. Ta-DLC layer is homogeneously composed of TaC crystals and amorphous carbon. The design of the gradient structure makes the multilayer interface unclear, and the interlayer interdiffusion is obvious. The reduction of the modulation period effectively improved the adhesion strength of the multilayer coating on the substrate. The multilayer coating with a modulation period of 305 nm obtained the highest hardness of 24.5 ± 0.8 GPa and Young's modulus of 263.2 ± 16.6 GPa. Compared with single-layer TaN, Ta/TaN/Ta(C,N)/Ta-DLC multilayer gradient coating had dramatically improved tribological properties, exhibiting the lowest friction coefficient of 0.15 and the lowest wear rate of 2.47 × 10−12 m3N−1 m−1 when the modulation period is 305 nm. By combining a Ta-doped DLC layer with a low friction coefficient and a TaN layer with high hardness, the multilayer gradient coating achieved excellent mechanical and tribological properties.