Development of diamond-like carbon (DLC) coatings with alternate soft and hard multilayer architecture for enhancing wear performance at high contact stress

Diamond-like carbon (DLC) coatings have attracted overwhelming interest due to a wide range of exceptional mechanical and tribological properties. Unfortunately, they have limited tribological applications where high contact stresses are required due to high residual stresses. We proposed that multilayer DLC coatings may have potential for tribological applications at high stresses. In this study, DLC coatings consisting of alternate soft-layer and hard-layer were deposited onto M2 steel substrates using unbalanced magnetron sputtering technique. The tribological performance at high contact stresses (2.5 GPa, 3.2 GPa, and 4 GPa) was evaluated by a ball-on-disc tribometer. The residual stress, the hardness and reduced modulus are increased with percentage of hard-layer thickness. Multilayer coatings with the soft-top layer improve wear performance compared with single-layer DLC coatings. In contrast, multilayer coatings without the soft-top layer show poor wear performance under high stresses. The soft-top layer reduces the wear volume during run-in period by forming a transfer layer so that decreases the total wear. The 50% hard multilayer coating with the soft-top layer provides the best wear performance at an extreme stress of 4 GPa among all coatings in this study because it combines the low magnitude of the residual stress with the acceptable hardness value. Such carbon/carbon multilayer structure offers an alternative to deposit low stress and high wear resistance DLC coatings without introducing other elements.