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An Evaluation of Tests for the Adhesion of Thermal Barrier Coatings

The results of different test procedures to measure adhesion of thermal barrier coatings (TBCs) are compared. Two TBC types have been studied as a function of time at temperatures up to 1100 oC. The tests evaluated were the transverse scratch test, indentation, tensile straining and piezospectoscopy to determine the residual stress of the thermally grown oxide (TGO). Generally, EB-PVD coatings were more responsive to the different test methods with failure generally occurring at the bondcoat/TGO interface, but only in the case of transverse scratching could quantitative values of interfacial strength be obtained. Piezospectrocopy to determine the residual stress of TGO offers promise for the non-destructive evaluation of the spallation resistance. It was also observed that metastable -alumina developed during ageing, suggesting that areas of disbond had occurred. Maps relating residual stress to -alumina formation have been generated showing the heterogeneous nature of bonding at the TGO/TBC interface. TBC systems with Pt aluminide and MCrAlY bond coats were aged for periods up to 4000 h in cyclic tests without initiation of spallation of the TBC. Spectra from these samples have been obtained at various stages during the ageing cycles and changes interpreted. The Piezospectroscopy method has also been applied to turbine blade both in the used and unused condition. For the used blade deposits present interfered with measurements and attempts made to remove these deposits had limited success.