Characterization of a cyclic displacement instability for a thermally grown oxide in a thermal barrier system

The mechanism responsible for the performance of a commercial thermal barri er system upon thermal cycling has been investigated. It comprises an elect ron beam physical vapor deposited (EB-PVD) yttria-stabilized zirconia therm al barrier coating (TBC) on a (Ni.Pt)Al bond coat. At periodic interfacial sites, the thermally grown oxide (TGO) that forms between the TBC and the b ond coat at high temperature displaces into the bond coat with each thermal cycle. These displacements induce strains in the superposed TBC that cause it to crack. The cracks extend laterally as the TGO displaces, until those from neighboring sites coalesce. Once this happens, the system Fails by la rge scale buckling. The displacements are accommodated by viscoplastic flow of the bond coat and "vectored" by a lateral component of the growth strai n in the TGO. They depend upon the initial morphology of the metal/oxide in terface. The observed responses are compared with the predictions of a ratc heting model. (C) 2001 Acta Materialia Inc. Published by Elsevier Science L td. All rights reserved.