Deformation and fracture of TiN and TiAlN coatings on a steel substrate during nanoindentation

The deformation mechanisms and mechanical properties of both TiN and TiAlN coatings on a V820 nitridable steel substrate have been investigated. Deformation was induced by nanoindentation, and the microstructures of the indented regions were studied using various techniques, including focused ion beam (FIB) and transmission electron microscopy (TEM). Two coatings deposited using a cathodic arc process were investigated: a thicker (1.1 μm) TiAlN–TiN dual-layer coating and a slightly thinner (0.7 μm) TiN coating. Hardness was measured using nanoindentation using a 5-μm-radius spherical indenter. A model developed by Korsunsky et al. was used to obtain the hardness of the coatings, accounting for both the thickness of the coating and the influence of the substrate. Hardnesses of 30 and 24 GPa were obtained for the TiAlN and TiN coatings, respectively. Both coatings exhibited broadly similar mechanisms of deformation. Columnar cracking and shear steps at the coating–substrate interface indicated that coatings deform predominantly by shearing along the columnar grain boundaries; however, significant lateral edge cracking was also observed, especially in the case of the TiAlN coating. The interface between the TiAlN and TiN in the dual-layer coating did not appear to affect the deformation behaviour.