Effects of breakaway oxidation on local stresses in thermal barrier coatings

It is known that fast-growing non-alumina scales in thermal barrier coating (TBC) systems tend to form at features where aluminium depletion is enhanced due to a high surface-area to volume ratio, a phenomenon known as breakaway oxidation. In this work, the influence of breakaway oxidation around protuberances in an MCrAlY bond coat in a TBC system during isothermal oxidation and after cooling is quantified numerically. A finite element approach is used which incorporates elastic, plastic and creep deformation of the bond coat and thermally grown oxide (TGO) and, importantly, the volumetric strains associated with oxide formation during the isothermal exposure. It is shown how volumetric strains which develop during breakaway oxidation can result in the formation of significant (>0.5 GPa) out-of-plane tensile stresses within the yttria-stabilized zirconia top coat at the oxidation temperature. These stresses are located along the flanks of the bond coat protuberances in locations where sub-critical cracks have previously been reported. The magnitude of the stresses increases with bond coat surface roughness, after the initiation of breakaway oxidation and after cooling. Results are also presented for the normal tractions across both TGO interfaces, and the influence of breakaway oxidation on these is discussed.