Delamination of multilayer thermal barrier coatings

Multilayer thermal barrier coatings (TBCs) on superalloy substrates are comprised of an intermetallic bond coat, a thermally grown oxide (TGO) layer, and a porous zirconia top coat that provides thermal protection. The TGO attains a thickness of 1±10 lm prior to failure, while the bond coat and zirconia layer are each about 50±100 lm thick. The preferred method for manufacturing TBCs comprises electron beam deposition. This method produces a thin ``fully dense'' zirconia layer 1 lm or 2 lm thick between the TGO and the thick ``top coat''. Edge-delamination and bucklingdelamination are the expected failure mechanisms. Each is addressed. Both occur at the interface between the bond coat and the TGO. Since low in-plane elastic moduli of the porous zirconia layer promote the latter, but suppress the former, there exists a range of moduli wherein both types of failure can be avoided. Two distinct sizes govern buckling-delaminations. Small scale delaminations arise when the TBC top coat has a very low modulus. They have a characteristic size that scales with the thickness of the TGO plus the fully dense zirconia layer: typically tens of microns. In this domain, the dense TGO/ZrO2 bi-layer buckles by pushing into the thick, more compliant zirconia top layer. The larger scale delaminations occur when the top coat is sti€. They involve not only the bi-layer, but also the zirconia top layer; buckling away from the substrate as a tri-layer. In this case, the total thickness of the TBC determines the extent of the delamination, typically several 100 lm. Ó 1999 Elsevier Science Ltd. All rights reserved.