Influence of the nanostructuration of PVD hard TiN-based films on the durability of coated steel

The design of thin hard coatings at a nanometric scale is very promising to improve the surface characteristics of coated parts. Unfortunately, most often only one specific property is really enhanced. Besides, the origin of such a “nanostructure effect” remains still not clearly elucidated, and a wider industrial development requires a better understanding of the relations linking structure and functionality.This paper results from an invited review presented within the framework of the EMRS 2007 spring meeting. Its objective is to present how it is possible to control the films structure to achieve optimized performance in terms of tribological, mechanical, and physico-chemical behaviours. Two types of films architecture are studied, resulting either from a stratification of nanolayers, or from a nanodistribution of a crystallised phase into an amorphous matrix. To illustrate both structures, arc-evaporated nanomultilayered TiN/CrN and TiSiN nanocomposite coatings are more particularly developed, and compared to TiN, CrN and SiN_x references.High wear resistance of TiN/CrN is explained by an original propagation mode of the cracks due to a fluctuating residual stress field, evidenced by TEM and synchrotron measurements. Corrosion behaviour depends mainly on the nature of the outer layer. An external CrN layer, presenting a p-type semiconductive character, affords an enhanced protection. A beneficial role of the nanostratification, susceptible to strongly improve the density of films, was also evidenced. The high oxidation resistance of TiSiN is attributable to the network of refractory SiN_x, which acts as a diffusion barrier for oxygen and insulates the highly reactive TiN nanograins from the aggressive atmosphere.