Microstructure, adhesion and tribological properties of CrN/CrTiAlSiN/WCrTiAlN multilayer coatings deposited on nitrocarburized AISI 4140 steel

In this study, the CrN/CrTiAlSiN/WCrAlTiN multilayer coatings have been deposited on polished AISI 4140 steel substrates (named only-coating specimens) and plasma nitrocarburized specimens (named duplex specimens) by cathodic multi-arc evaporation. The microstructure and composition of the specimens were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) with Energy dispersive X-ray analysis (EDX), and transmission electron microscope (TEM) techniques. The micro-hardness and nano-hardness were measured by Vickers hardness tester and nanoindentation, respectively. The adhesion and tribological behavior were evaluated using the Rockwell-C indentation and micro-tribometer, respectively. The results showed that plasma nitrocarburizing gives rise to a modified layer due to diffusion of nitrogen/carbon, leading to surface hardness, H, of 13.0 GPa, compared to H = 6.8 GPa for the untreated steel. Notably, it shows that the super-hardness (11 = 47.9 GPa) multilayer coating consists of a bottom CrN layer, a middle CrTiAlSiN layer, and an outmost superlattice WCrTiAlN layer. The nitrocarburizing pretreatment considerably improves adhesion strength of the duplex specimen compared with the only-coating one. The improved mechanical and tribological properties of the duplex specimen are mainly related to the gradient in the hardness profile of the diffusion nitrocarburized layer.