Stress state of TiN/TiAlN PVD multilayers

A multilayer system consisting of TiN and TiAlN layers is deposited by means of a PVD process onto stainless and tool steel substrates. The study is aimed at determining the microstructure and the macrostresses in these layers with X-ray diffraction. The multilayer system is composed of a relative thick TiAlN layer (∼150 nm) and a set of smaller alternating TiN/TiAlN layers of approximately 15–20 nm each with a total thickness of 150 nm. This basic building block of the structure is repeated throughout the coating and is sandwiched between two thicker layers: a TiN layer (400 nm) to achieve good adhesion with the substrate, and a top layer of TiAlN (400 nm). The total thickness of the coating is approximately 4·4 μm. From X-ray diffraction it is concluded that the layers are only slightly textured and there is a weak (311) texture. The strain measurements show a difference in strain for the layers on stainless and tool steels, which is owing to a difference in the linear expansion coefficient for the two substrates. It is possible to determine the unstrained lattice spacing of the TiN and the TiAlN sublayers and to calculate Poisson's ratio for both materials. Furthermore, the residual stresses in the different sublayers could be derived and it was found that they were much higher in the TiN than in the TiAlN. This may be explained by the thermal origin of the residual stress in the TiAlN sublayers, whereas in the TiN sublayers the atomic peening process during deposition introduces an additional residual stress.