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, Ti0.52Al0.48N, Ti0.53Al0.38Si0.09N, Ti0.43Al0.48Si0.09N, and Ti0.48Al0.38Si0.14N 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 SiNx 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 Ti0.52Al0.48N. The Si-containing coatings experience an initial increase in hardness from 29.1 ± 1.0 GPa for Ti0.52Al0.48N to 33.1 ± 1.2 GPa for Ti0.53Al0.38Si0.09N, and then a decrease to 26.4 ± 0.8 GPa for Ti0.43Al0.48Si0.09N and 28.1 ± 0.8 GPa for Ti0.48Al0.38Si0.14N. 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 TiO2 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.