Title

Failure mechanisms and mechanisms-based life predictions for electron beam physical vapor deposition thermal barrier coatings

Date of Completion

January 2005

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

This research is designed to define failure mechanisms and to develop and experimentally validate non-destructive life prediction methodologies for electron beam physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs). It is shown that for the two TBCs of this study ((Ni,Pt)Al and NiCoCrAlY bond coated TBCs) different failure mechanisms are exhibited, and therefore, the selected life prediction methodologies are accordingly different. ^ For the (Ni,Pt)Al bond coated TBC tested at three temperatures, progressive rumpling of the thermally grown oxide (TGO) and bond coat interface is responsible for the failure at a critical rumpling value. Rumpling is a single value function of TGO thickness, suggesting that TGO growth strains are critical to rumpling, and the TGO growth controls rumpling, which in turn controls spallation life. Associated with rumpling, the TGO stress, as measured by the Photoluminescence Piezospectroscopy (PLPS) technique, decreases linearly with thermal cycles. Longer life specimens exhibit shallower slopes. ^ The relationships among rumpling rate, stress relaxation rate and spallation life are defined: as temperature increases, rumpling and stress relaxation rates increase, and spallation life decreases. The rumpling of the TGO provides a physical basis for use of TGO stress measurements as a non-destructive method for TBC damage initiation, progression and life prediction. ^ Temperature-blind remaining life predictions were made successfully using regression and neural network methods based on only TGO stress measurements at three temperatures. The lowest root mean square error for the prediction using neural networks and regression methods was 6.6% and 14.7%, respectively. ^ Bimodal luminescence spectra, obtained using PLPS, are shown to be related to TGO cracking. The degree of cracking increases initially as &thetas;- transforms to α-Al2O3, then decreases as the cracks heal, and then increases again prior to spallation. Area stress maps, based on the bimodal luminescence and average fraction of bimodal spectra with cycles, show damage progression and have the potential for non-destructive prediction of spallation failure. ^ For NiCoCrAlY bond coated TBCs, damage initiates at localized debonds at the TGO/bond coat interface due to an increasing out-of-plane tensile stress. The spallation of the coating is driven by the strain energy stored in the TGO. (Abstract shortened by UMI.)^