EXPERIMENTAL CHARACTERIZATION OF ELASTIC STIFFNESS AND DELAMINATION TOUGHNESS IN THERMAL BARRIER COATING SYSTEMS

Reliable high-temperature propulsion materials are crucial to improved engine efficiency, reduced fuel costs and decreased life cycle costs. Multilayered thermal barrier coatings (TBC) are an effective way of extending the temperature capabilities of gas turbine engines, but accurate assessment and life prediction of these disparate material systems requires detailed knowledge of the constitutive behavior of each layer and the adhesion between layers. The focus of this study is to measure the properties relevant to thermal barrier coatings and elucidate how variations in the processing affect the mechanical response. Working closely with industrial suppliers at General Electric (GE) and Pratt & Whitney (P&W), novel Compression Edge Delamination (CED) tests were used to measure the Mode II interfacial toughness of two TBCs. GE’s standard diffusion aluminide bond coat/yttria stabilized zirconia (YSZ) topcoat TBC system was characterized at various stages of gradient thermal cycle life and compared to previous results for furnace cycled specimens with the same TBC. Both sets of samples exhibited comparable interfacial toughness and a decrease in interfacial strength with thermal cycling. The drop in interfacial strength was correlated with TGO and bond coat rumpling and found to be greater in the thermal gradient test, in part because these specimens experienced higher bond coat temperatures. CED testing was also performed on P&W’s low-pressure plasma sprayed NiCoCrAlY bond coat/YSZ topcoat TBC system at various stages of furnace cycle life. The delamination of this TBC resulted in the spallation of the topcoat in one large piece, facilitated by the absence of rumpling. The interfacial toughness of these specimens decreased with thermal cycling and was attributed to thickening of the thermally grown oxide (TGO), which in the absence of rumpling leads to increased stored strain energy that promotes delamination. A parallel investigation with P&W investigated the influence of off normal processing conditions on the intrinsic microstructure and mechanical properties of YSZ topcoats manufactured via electron-beam physical vapor deposition. Microstructural variations were subtle, but 3-point bend experiments on free-standing topcoats showed a strong tension/compression asymmetry and variations in modulus as a function of processing conditions