Diffusion behavior of C, Cr, and Fe in arc evaporated TiN- and CrN-based coatings and their influence on thermal stability and hardness

Hard coatings like TiN, Ti1 − xAlxN, CrN, and Cr1 − xAlxN are widely used to protect tools in numerous machining and forming applications during which they have to sustain high temperatures and tribological loads. Thin layers of C, Cr, and Fe, which are common transfer-elements during machining, were deposited on arc evaporated TiN, Ti0.5Al0.5N, CrN, and Cr0.3Al0.7N coatings to act as infinite diffusion sources. The diffusion processes within these coatings are significantly determined by their microstructural changes upon annealing, especially by the phase decomposition and dissociation processes. Therefore, we have selected these four material systems. Due to the decomposition of supersaturated Ti1 − xAlxN phases towards the stable constituents cubic TiN and hexagonal wurtzite-type AlN, the fraction of grain and phase boundaries and dislocations increases with increasing annealing temperature. This is in contrast to TiN coatings, where no phase decomposition takes place, and the fraction of structural built-in defects generally decreases upon annealing. Consequently, the diffusion of C, Cr, and Fe is more pronounced within Ti0.5Al0.5N than in TiN, especially at elevated temperatures where the supersaturated Ti1 − xAlxN phase decomposes. The dissociation of CrN bonds towards Cr under nitrogen release within CrN based coatings is postponed to higher temperatures by the addition of Al. Therefore, the diffusion of C, Cr, and Fe is less pronounced within Cr0.3Al0.7N than in CrN, especially at temperatures where CrN already dissociates. These results allow for a better understanding of the complex interaction between coated tool and work-piece and the thereby obtained diffusion driven wear.