Effect of Si-addition on structure and thermal stability of Ti-Al-N coatings

Abstract

The superior mechanical properties and thermal stability of Ti-Al-Si-N coatings have attracted extensive research interests in academia as well as industry. In this study, Ti 0.52 Al 0.48 N, Ti 0.53 Al 0.38 Si 0.09 N, Ti 0.43 Al 0.48 Si 0.09 N, and Ti 0.48 Al 0.38 Si 0.14 N coatings are developed by arc-evaporation. This compositional variation allows to study the impact of Al and Si on structure, thermal stability, and oxidation behavior. Incorporation of Si into Ti-Al-N leads to a nanocomposite structure with an amorphous-like SiN x boundary-phase – encapsulating small crystalline Ti-Al-N grains – and promotes wurtzite-type AlN formation. This causes a significant change in the mechanical and thermal properties of the originally single-phase face-centered cubic structured Ti 0.52 Al 0.48 N. The Si-containing coatings experience an initial increase in hardness from 29.1 ± 1.0 GPa for Ti 0.52 Al 0.48 N to 33.1 ± 1.2 GPa for Ti 0.53 Al 0.38 Si 0.09 N, and then a decrease to 26.4 ± 0.8 GPa for Ti 0.43 Al 0.48 Si 0.09 N and 28.1 ± 0.8 GPa for Ti 0.48 Al 0.38 Si 0.14 N. Alloying with Si improves the thermal stability of Ti-Al-N by retarding the decomposition towards its thermodynamically stable constituents TiN and AlN. Moreover, the oxidation resistance of Ti-Al-N can be largely improved by the Si-addition due to the retarded anatase-to-rutile TiO 2 transformation as well as the formation of a protective oxide scale at the nitride-to-oxide interface. Cross-sectional scanning electron microscopy studies reveal that the oxide scales of the Si-containing coatings exhibit a lamellar structure comprising Al-rich, TiSi-rich, and/or Ti-rich oxide layers. Higher Si and, especially, higher Al content is favorable to the formation of an Al-rich layer, which can act as a diffusion barrier for oxygen. Especially, all Si-containing coatings present an abnormal oxidation behavior, where the consumed nitride layer thickness is not continuously increasing with oxidation temperature. • Both thermal stability and oxidation resistance are enhanced after Si-alloying. • The effect of Si on the oxidation behavior of Ti-Al-Si-N is multifold. • The Si/Ti ratio of 0.21 retards the anatase-to-rutile transformation most. • Si-alloying leads to an abnornal oxidation behavior of Ti-Al-N coatings.