Synergistic effects of Ta and Si alloying on the long-term oxidation resistance of Ti₁₋ₓAlₓN coatings

Abstract

Ti-Al-N coatings are emerging as robust stand-alone protective solutions in turbine applications. To maximize their potential in high-temperature environments, improving their long-term oxidation stability is key. In this work, we investigate the combined effect of Ta and Si alloying on the oxidation of thick (8 to 15 μm) cathodic arc evaporated Ti1-x-y-zAlxTaySizN coatings at 850 ◦C up to 500 h. Combined Ta and Si alloying yields extremely low oxidation kinetics following a logarithmic rate law, demonstrating exceptional long-term stability. After 500 h, the oxide scale thickness decreases from ~1.1 μm for the low-Al Ti0.49Al0.36Ta0.13Si0.02N coating to <600 nm for the high-Al Ti0.40Al0.45Ta0.13Si0.02N variant. Correspondingly, the logarithmic rate constants lie between 0.379 ± 0.019 μm × h􀀀 1 (Ti0.49Al0.36Ta0.13Si0.02N) and 0.243 ± 0.020 μm × h􀀀 1 (Ti0.40Al0.45Ta0.13Si0.02N), indicating a combined impact of Ta and Si in relation to the Al content on the oxidation kinetics. Detailed cross-sectional TEM analyses reveal a multilayered oxide scale consisting of two inner Ti-Ta-Si-containing sublayers and an outer Al2O3 layer. The findings demonstrate that Ti1-x-y-zAlxTaySizN coatings significantly outperform conventional Ti-Al-N in the long-term oxidation regime, where the combined effects of Ta and Si emerge as the decisive factor for retarded oxygen inward diffusion.