Impact of V, Hf and Si on oxidation processes in Ti–Al–N: Insights from ab initio molecular dynamics

Abstract

Increasing requirements on hard coatings in high-performance machining processes like high speed and dry cutting demands further developments even of the already well established TiAlN coatings. The aim of this work is to present a comparative study on oxidation behavior of Ti0·5Al0·5N coatings alloyed with X (X = V, Hf, Si) elements. Ab initio molecular dynamics results showed that a pronounced layered oxide appears due to the incorporation of these alloying elements thus affecting the thermal stability and antioxygenic properties. The additions of Hf and Si facilitate the formation of layered oxides with Ti-rich and Al-rich oxides, respectively, which improves the antioxidant performance at high temperatures. Contrarily, the Al-rich layer is absent during oxidation of V-containing coating. The Ti-oxides on the surface of TiAlVN and TiAlHfN coatings have different structures due to the discrepancy of O–Ti–O angle. The present findings clearly show that the V-addition results in more rutile-like TiO2 whereas the Hf-addition leads to more anatase-like2 with O–Ti–O angle of 156.3°. With further oxidation, the structures of coatings vary significantly with the alloying elements, which indicates that the alloying of Hf and Si retards the inward diffusion of O atoms, especially, the alloying of Si leads to an earlier formation of a dense sublayer rich in Al2O3, and thus promotes the oxidation resistance. The different diffusion barrier energies of Ti and Al in TiN bulk contribute to the formation of layered oxides on the surface of coating and the different oxidation behaviors. This study provides an atomistic insight to the initial as well as further stages of the oxidation process of quaternary TiAlXN, which together with the electronic structure analysis and diffusion of metal atoms gives a new basis for understanding of its oxidation behaviors.