Anisotropy effects on microstructure and properties in decomposed arc evaporated Ti1-xAlxN coatings during metal cutting

Abstract

Anisotropy effects on the spinodal decomposition in cathodic arc evaporated cubic “phase c-Ti1−xAlxN coatings have been studied with respect to composition, microstructure and hardness properties before and after a continuous turning operation. Coatings are simultaneously being exposed to both a high temperature and high pressure during the metal cutting process. As evident from the current results, a high Al content coating, x=0.66, when exposed to such extreme conditions decomposes into cubic c-AlN and c-TiN-rich domains. In this case, the evolving microstructure comprises interconnected spatially periodic, elongated and coherent cubic c-AlN and c-TiN-rich regions aligned along elastic compliant <100> crystal direction. A significantly different microstructure with randomly oriented domains is observed for a coating with an elemental composition closer to the isotropic limit, x=0.28, exposed under the same conditions. From a coating hardness perspective, the nanoindentation results display a minor age hardening effect for the c-Ti1−xAlxN coating grown at x=0.28 while the coating grown with x=0.66 exhibits a significant age-hardening effect of about 18%. We conclude that both microstructure and age hardening behavior during spinodal decomposition of c-Ti1−xAlxN correlate to the relative amount of metal Ti/Al ratio and consequently to the elastic anisotropy of the as-grown coating material. These results provide new insights to the understanding of improved wear resistance of c-Ti1−xAlxN with Al content during metal cutting.