Criteria for the delamination of thermal barrier coatings: with application to thermal gradients

A mechanism for the delamination of thermal barrier systems has been described and analyzed. It is based on the observations that, under some circumstances, vertical separations develop in the coating, creating an ‘internal’ edge. The associated gaps, width w, allow the coating to displace in the presence of a strain misfit with the substrate. In turn, the displacements induce an energy release rate, G. Delaminations develop from the base of the vertical separations when G exceeds the mode II toughness of the coating. The analysis reveals that, when the gaps are narrow (small w), they close before the full misfit strain can be accommodated. Once they close, the energy release rate decreases, causing the cracks to be stable. That is, the size of the delaminations is controlled explicitly by the gap width. Beyond a critical w, the cracks become unstable and spall the entire coating. One of the mechanisms for forming the vertical separations and widening them has been proposed and analyzed. The mechanism is directly connected to the constraint on the thickening of the thermally grown oxide (TGO) that forms near ridges on the bond coat surface. Since such growth cannot be accommodated, stresses are induced that form and widen the gap. The gap width increases in proportion with the thickness of the TGO. This phenomenon connects delamination failure to the thermal history.