Improved Ti-Al-N coatings through Ta alloying and multilayer architecture

Abstract

Alloying a fourth substantial element to Ti-Al-N coatings is a promising approach to tailor their properties. Within this work, the effect of Ta-addition and multilayer architecture on structure, mechanical and thermal properties of arc-evaporated Ti-Al-N was investigated. The addition of Ta to Ti-Al-N coatings with chemical compositions right at the border for the face centered cubic metastable solubility limitation, can promote the formation of undesired hexagonal phases. Thereby, also the hardness further decreases from ~ 27.2 GPa for Ti0.42Al0.58N (with a small fraction of hexagonal phases) to 26.2 GPa for Ti0.40Al0.54Ta0.06N and 24.9 GPa for Ti0.34Al0.54Ta0.12N, due to their increased fraction of hexagonal phases. Nevertheless, the Ta-alloyed coatings exhibit higher hardness after vacuum annealing above 1000 °C. When fully stabilizing these Ta-containing nitrides in their face centered cubic structure through a multilayer arrangement with Ti0.52Al0.48N layers, the hardness can significantly be increased to ~ 32.8 GPa for Ti0.40Al0.54Ta0.06N/Ti0.52Al0.48N and ~ 34.3 GPa for Ti0.34Al0.54Ta0.12N/Ti0.52Al0.48N. These multilayers are also superior during the whole temperature range of vacuum annealing up to 1200 °C. Furthermore, only the Ta-containing coatings (monolithically or multilayered) can survive the 20 h exposure to ambient air at 900 °C.