Observations and analyses of failure mechanisms in thermal barrier systems with two phase bond coats based on NiCoCrAlY

The failure mechanisms operative in a thermal barrier system with a NiCoCrAlY bond coat are ascertained by examining test specimens subjected to thermal cycling in a burner rig. The findings are augmented by observations made on an actual turbine blade. The morphology and microstructure of the thermally grown oxide (TGO) have been characterized: emphasizing heterogeneities, especially “pegs”, and the TGO thickness at failure, htgo. In each case, a dominant delamination has been identified, that extends primarily along the interface between the TGO and the bond coat: a finding that contrasts with the multiple sub-critical cracks found when other bond coats are used. The principal stresses, σ1, have been analyzed on a cycle-by-cycle basis, incorporating plastic deformation of the bond coat and TGO, as well as TGO growth. Energy release rates, G, for putative cracks at the interface have also been ascertained. These analyses reveal that σ1 and G are too small to nucleate interface cracks, except at occasional large edge-imperfections, such as vertical separations in the thermal barrier coating (TBC). Once formed, these cracks become unstable and delaminate the interface. Calculations of the delamination energy release rate, upon comparison with the interface toughness, a critical TGO thickness, (htgo)c≈3 μm, comparable to that found experimentally.