The respective roles of thermally grown oxide roughness and NiAl coating anisotropy in oxide spallation

The spallation of a thermal grown oxide developed on a typical NiAl coating was observed during a compressive mechanical test using a digital image correlation (DIC) technique. The measured surface strain field highlighted the correlation between the strain localisation and the spalled area. This study proposes an in depth analysis of the impact of coating microstructure and surface morphology on strain localisation and associated oxide spallation. An original finite element analysis (FEA) was performed to determine the respective roles of the thermally grown oxide roughness and of the anisotropy of individual NiAl grains on the observed strain scatter. Elastic and plastic behaviour have been studied for both isotropic and anistropic cases. The surface roughness has been shown to determine the strain spread after cooling. The plastic anisotropy of NiAl grains was seen to be the first order parameter, governing spallation, as soon as mechanical loading was applied to the coating.