The influence of bond coat surface roughness on chemical failure and delamination in TBC systems

Localised loss of aluminium from the bond coat within thermal barrier coating (TBC) systems is a critical factor determining the lifetime of these coatings. In this paper, it will be demonstrated that electroplated MCrAlY bond coats with asperities of high aspect ratios can experience premature chemical failure during isothermal exposures at 1100 degrees C in air. This can result in the rapid conversion of the asperity into Co- and Cr-rich oxides. The enhanced aluminium depletion within these asperities, which leads to chemical failure, is a consequence of their increased surface area coupled with a limited physical access to the reservoir of aluminium in the bulk of the bond coat. This process has been demonstrated using the 2-D finite difference model ODIN which predicts a depletion to less than 1 at% aluminium within a typical asperity after 1 h exposure at 1100 degrees C. The results also show that the aluminium content of the bulk of the bond coat is sufficient to enable the formation of a continuous protective alumina underneath these regions, as observed experimentally. It is suggested that the volume increase associated with the conversion of the bond coat asperities into breakaway oxide results in out-of-plane tensile stress development at the oxidation temperature. These stresses are anticipated to be at a maximum between the oxide asperities and are thought to nucleate small, sub-critical cracks at these locations at temperature. Further delamination will occur during cooling both by the extension of these cracks and by the growth of crack-like defects within the porous breakaway oxides.