Oxynitride c-Al0.7Cr0.3OxN(1-x) arc-PVD hard-coatings, processing, mechanical properties & stability at high temperatures

Abstract

Crystalline fcc Al0.7Cr0.3OxN(1-x) oxynitride-based dense coatings were deposited in a semi-industrial coater using the arc-PVD technique. The microstructural analysis of the as-coated oxynitrides showed the increased formation of defects such as porosity, droplets, and pinholes upon the O-raise during deposition, while the columnar-like features oriented in the (200) direction were maintained. Meanwhile, the c-Al0.7Cr0.3N reference nitride coating presented a similar microstructure but was mostly texturized in the (111) direction, maintaining the fcc crystal structure. These microstructural characteristics correlated well with the mechanical response of both nitrides. In the c-Al0.7Cr0.3N, significant plastic deformation was registered compared to the c-Al0.7Cr0.3OxN(1-x) coating. However, the interface mismatch did not affect the adhesion strength of the oxynitride. Upon treatment at 950 °C, the c-Al0.7Cr0.3N nitride displayed increased average roughness due to the oxidation and formation of plate-like oxides, probably an α-(CrAl)2O3 phase, while tiny oxide whiskers grow on the c-Al0.7Cr0.3OxN(1-x) coating. The presence of other oxide phases upon thermal treatment was identified. Only slight changes in the nano-hardness & elastic modulus are registered on the thermally treated coatings at 950 °C, which is confirmed by the/E (elastic strain to failure) parameter analysis, indicating that the thermal treatment for the c-AlCrN and the c-AlCrON coatings did not affect their cracking behavior. However, in the as-coated state, the c-AlCrON shows less tendency to form cracking than the c-AlCrN. This cracking behavior agrees with what was found in the scratch testing, supporting the idea that O increases the cracking resistance in the c-AlCrON and results in mixed cohesive + adhesive failure modes in this oxynitride.