Improved wear resistance at high contact stresses of hydrogen-free diamond-like carbon coatings by carbon/carbon multilayer architecture

Diamond-like carbon (DLC) coatings are widely used in tribological applications because it can provide high hardness and low friction coefficient. However, hydrogen-free DLC coatings tend to fracture and delaminate where high contact stresses (>1 GPa) are required. We proposed that the carbon/carbon (C/C) multilayer may have the potential for tribological applications under high contact stresses. In this study, C/C multilayer coatings consisting of alternate soft-layer and hard-layer were deposited using unbalanced magnetron sputtering technique. The effect of multilayer architecture (the ratio of hard-layer to soft-layer and bilayer thickness) on tribological behavior was investigated. The tribological performance at different high contact stresses (1.82 GPa, 2.30 GPa, 2.89 GPa and 3.65 GPa) was evaluated by a ball-on-disc tribometer. The C/C multilayer coatings can withstand the maximum contact stress of ~3.65 GPa with low wear rate. In contrast, the single layer hard DLC failed when the stress is higher than 2.30 GPa. The 50% hard multilayer with 61 nm bilayer thickness shows the best overall wear resistance, especially at a stress of 3.65 GPa. The wear rate of our C/C multilayer (~10^-8 mm³/Nm) is 10 times lower than that of DLC coatings (~10^-7 mm³/Nm) from the current literature. The excellent wear resistance of the multilayer DLC can be explained by the combination of good toughness and the lubrication effect of the transfer layer. Such C/C multilayer structure offers an alternative to deposit DLC coatings for high stress applications without introducing other elements.