On the growth of functionally graded self-lubricating layer during a plasma-assisted thermochemical treatment of M50NiL steel

Diamond-like carbon (DLC) with excellent mechanical properties has aroused great interest. However, the weak adhesion between the DLC film and steel substrate restricts its application. Considering that the addition of interlayers needs cumbersome steps, fabricating a functionally graded self-lubricating layer through one-step remains a challenge. We reported a novel integrated self-lubricating layer, inspired by the gradient structure formed during the traditional thermochemical treatment. That integrated layer from DLC structure gradual transition to nitrided diffusion layer is in-situ fabricated on the M50NiL steel through a plasma-assisted thermochemical treatment. The gradient structure possesses excellent wear resistance due to the self-lubricating of the DLC structure and provides sufficient adhesion strength. A deep insight into the relation between Fe₅C₂ crystal facet and growth of DLC is conducted by first-principle calculation and find low Miller Index of Fe₅C₂ facets are favorable for the growth of DLC. The thermodynamic calculation reveals that gradient structure formation is the collaboration effect of non-equilibrium and equilibrium processes.