Influence of Cr additions on the structure and oxidation resistance of multilayered TiAlCrN films

Ti1 − xAlxN coatings with NaCl-type structure have been widely used in advanced machining and other high temperature applications due to their excellent mechanical, thermal and tribological properties. Here, we investigated the influence of Cr additions on the structure, mechanical properties and oxidation resistance, including the oxide scale characterization, of multilayered TiAlN/CrAlN coatings deposited by magnetron sputtering. The properties of Cr rich coatings were compared to a TiAlN film deposited as reference. XRD diffraction analysis revealed that all coatings showed an fcc NaCl-type structure. However, for the TiAlN monolayer Ti and Al are forming a solid solution whilst, for Cr rich coatings, a multilayer structure alternating TiAlN and CrAlN layers, also forming solid solutions, was grown as a result of the geometry of the targets distribution inside the deposition chamber in combination with a slow rotation of the sample holder and a sufficiently high deposition rate. TGA measurements showed that Cr additions increased the oxidation performance of the coatings. For Ti0.47Al0.46N, dual oxide layers occur when tested at 800 °C, being the porous inner one of TiO2 and the outer a compact and continuous layer of Al-oxide which protects the coating from the oxidation. At 900 °C the oxidation resistance of this film degraded due to the fast Ti ions diffusion to the surface which impedes the formation of the continuous and protective Al oxide layer. Cr rich coatings showed different oxides scales depending on their chemical composition. For Ti0.30Al0.46Cr0.26N and Ti0.28Al0.34Cr0.42N, a Ti-O rich layer is formed on the top of a protective Al-Cr-O layer when the samples were tested at 900 °C and 1000 °C, being the oxide layers thicker for the higher temperature. Concerning the coating with the highest Cr content (Ti0.28Al0.31Cr0.51N), a compact and continuous Cr(Al)2O3 oxide layer was formed at 900 and 1000 °C with residual TiO2 islands on the top, contributing for a significant enhancement of the oxidation resistance.