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

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 boundaryphase - 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 facecentered 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 Siaddition 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 Alrich 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.

Automated summary

Ti-Al-Si-N coatings exhibit superior mechanical properties and thermal stability. The addition of Si improves the hardness and thermal stability of the coatings, and enhances their oxidation resistance. Si and Al content have a significant impact on the structure, stability, and oxidation behavior of the coatings.