X-ray diffraction analysis of the residual stress state in PVD TiN/CrN multilayer coatings deposited on tool steel

Thin multilayer films, especially nitrides of transition metal, exhibit hardness enhancements that are significantly higher than those predicted by the rule of mixtures. In this study, TiN/CrN multilayers, consisting of alternating nanometer-scale TiN and CrN layers, were deposited by arc evaporation technique on M2 steel. Different period thicknesses Λ have been prepared: 10, 20, 40 and 60 nm. A recent study has shown that tribological properties enhancement of hard PVD nanostructured nitride coatings deposited on tool steel is optimum for a special period value of 40 nm with thicknesses of 24 and 16 nm for CrN and TiN layers, respectively. In order to get a better understanding of the mechanical behaviour of the coatings as a function of the modulation wavelength Λ, residual stresses in each nitride systems have been characterized using X-ray diffraction and the sin2ψ method at a synchrotron radiation facility. It is shown that the stresses are compressive and their magnitudes vary according to the type of nitride: CrN is less stressed than TiN. This might explain why the propagation of cracks into the whole multilayer coating is governed by the interfaces.