Aluminium oxide spallation on NiAl coating induced by compression

Aluminium rich intermetallic coatings are used to protect the substrate of aeronautical turbine blades from detrimental oxidation. Unfortunately, severe thermal and mechanical loading cause spallation of the protective thermally grown oxide (TGO). Complex service loading can be simulated by laboratory isothermal compression tests. We present an experimental and numerical study of spatial heterogeneities of oxide spallation occurring in such compression tests. The combined use of a digital image correlation technique (DIC) to measure the strain field, and scanning electronic microscopy (SEM) to measure the spallation shows that oxide spallation is a function of the local strain. Spatial heterogeneity of spallation is assumed to be related to measured strain gradients. A finite element analysis (FEA) taking into account local elastic anisotropy and oxide ridge morphology clarifies the physics of the onset of spallation events: ridge morphology and local elastic anisotropy are found to be associated respectively with low strain spallation, and grain-to-grain spatial heterogeneity under increasing load.