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 mu m prior to failure, while the bond coat and zirconia layer are each about 50-100 mu m thick. The preferred method for manufacturing TBCs comprises electron beam deposition. This method produces a thin "fully dense" zirconia layer 1 mu m or 2 mu m thick between the TGO and the thick "top coat". Edge-delamination and buckling-delamination 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 stiff. 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 mu m. (C) 1999 Elsevier Science Ltd. All rights reserved.