Breakaway oxidation of a low-Al content nanocrystalline coating at 1000 °C

Abstract

Nanocrystalline coating though with low Al reservoir shows high oxidation and spallation resistance, owing to its high density of defects and grain boundaries. It now attracts growing attention. However, with oxidation going on, its stability is always a significant concern since such a high-temperature protective coating should be designed with a long life-expectancy when the nano-sized grains have grown and the reserved Al has been consumed a lot. Yet, this issue has never been clarified. To identify the exact mode of breakaway oxidation, a nanocrystalline coating was prepared with very low Al content that could decrease fast to a critical value in a very short oxidation time. Its oxidation behaviour related with the stability was studied in detail at 1000 °C. Results indicate that an intact external alumina scale still forms at the surface of the nanocrystalline coating initially. However, after 100 h exposure, grains growing and the continuous consuming of Al of the nanocrystalline coating lead to oxidation of Ti and doping of titanium oxide in the alumina scale, which was accompanied with the formation of abundant micropores and cracks. Thereafter, scale spalls off and abundant less-protective oxides, e.g. TiO2, NiCr2O4, NiTiO3, form, and breakaway oxidation occurs.