Thickness regulation of the mechanical properties and failure control of Ti/ TiN and Ti(N)/TiN bilayer stacks

Magnetron sputtering and hot-wire plasma -enhanced nitriding were combined to prepare bilayers of Ti/TiN or Ti (N)/TiN with different thickness ratio deposited on Ti6Al4V (TC4) alloy substrates to form stacks of four bilayers. The structural and mechanical properties and adhesion of the films to the substrate were evaluated. The results showed that the surface morphology of the stacks became smoother as the Ti, Ti(N)/TiN thickness ratios were decreased from 1:2 to 1:8 and 1:17 by reducing the proportion the Ti or Ti(N) layer thickness while increasing the proportion of the TiN layer thickness in the four-bilayer stacks. The primary preferred orientation of Ti (002) and TiN (111) were observed in the multilayer films, while Ti (002) shifted to a lower angle in Ti(N)/TiN, which proved the solid solution of N atoms into the Ti lattices following nitridation of the Ti bottom layers. The hardness, elastic modulus, and wear resistance of the four bilayer stacks also increased with the nitridation of the bottom Ti layer. The mechanical properties of the Ti(N)/TiN bilayer stacks were better than those of the Ti/TiN films while the difference between the samples reduced as the relative proportion of the Ti or Ti(N) layers was decreased. The adhesion between the multilayer stacks and the TC4 substrates, evaluated by both scratch and Rockwell tests, was improved as the thickness ratios of the Ti or Ti(N) bottom layers decreased to 1:8 and 1:17. The adhesion between the films and the substrates were improved by reducing the proportion of soft Ti or Ti(N) and increasing the elastic modulus of the soft layer through nitriding to reduce the modulus difference between the bilayer stacks and TC4 substrate, as well as the Ti or Ti(N) layer and the TiN layer.