Delamination of multilayer thermal barrier coatings

Abstract

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 μm prior to failure, while the bond coat and zirconia layer are each about 50–100 μm thick. The preferred method for manufacturing TBCs comprises electron beam deposition. This method produces a thin “fully dense” zirconia layer 1 μm or 2 μm 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/ZrO 2 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 μm.