Design of a TiAlON multilayer coating: Oxidation stability and deformation behavior

Abstract During the cutting of high-speed steel, thermal and mechanical loads occur. Thin hard coatings like TiAlN deposited by physical vapour deposition (PVD) are applied on the tool surface to prevent wear and oxidation. In the scope of this article, a multilayer coating concept was developed to improve the cutting performance of the tools. It consists of a Ti bond coat, a TiN/AlN nanolayer as an interlayer and an oxynitride TiAlON toplayer. The manuscript presents results of the investigation on the influence of the oxygen content in the reactive gas flow during an industrial direct current/high power pulsed magnetron sputtering hybrid process on the coating properties and on the oxidation stability. The reaction layer forming after the PVD process under ambient atmosphere on the surface of hard coatings has a thickness of a few nanometers. The chemical composition of the coatings and of the reaction layer were measured using electron probe micro analysis (EPMA) and X-ray photoelectron spectroscopy (XPS), respectively. At a high oxygen content of y(O / (O + N) = 85% the oxynitride TiAlON shows the presence of both, crystal and amorphous phases, as proven by transmission electron microscopy. As investigated by nanoindentation, indentation hardness, indentation modulus and resistance against plastic deformation decrease with increasing oxygen content in the coatings. When the oxygen content increases from y = 49% to y = 85%, the oxidation stability decreases. This might be caused by a decreased aluminum ratio of the reaction layer of x(Al / (Al + Ti) = 73% compared to x = 49%.