Accelerated cyclic oxidation testing protocols for thermal barrier coatings and alumina-forming alloys and coatings

The oxide spallation resistance of oxide scales and ceramic thermal barrier coatings is a key design factor for developing high-temperature alloy systems. Determination of the lifetimes of such alloy and coating systems is highly desirable. However, as improved systems are developed, lifetimes become so long that the time required to test a system to failure becomes prohibitive. Therefore, reliable protocols for accelerated testing and lifetime prediction are needed. This paper describes two attempts at developing such protocols. The first involves modification of the NASA COSP model to predict cyclic oxidation behavior of alloys and metallic coatings and the incorporation of acoustic emission data into this model. The second involves use of an indentation technique to induce spalling of thermal barrier coatings (TBCs) after short-term thermal exposures. The first effort involves using the COSP Model, developed at NASA, as the basis for the prediction of oxide spallation. Acoustic emission measurements are used in an attempt to obtain critical parameters in the model from short-time experiments for a varietyof alloys and coatings which rely on alumina scales for oxidation resistance. The model is then used to predict the lives of these alloys and coatings when subjected to cyclic oxidation at 1100 °C. A principal concern with ceramic thermal barrier coatings (TBCs) used in gas turbines is their loss of adhesion during service, leading to coating spallation. In this paper, an overview is given of an indentation test for brittle coatings on ductile substrates which is used to quantify decreases in interfacial toughness of TBC systems due to cyclic high-temperature exposures. The indentation test involves penetration of the TBC and the oxide layer below it, inducing plastic straining in the underlying metal bond coat and superalloy substrate. The indentation strains cause an axisymmetric delamination of the TBC and oxide layers. Measurement of the extent of the delamination, coupled with finite-element modeling, provides a measure of the adherence of the coating. Test results are presented tracking the loss of interfacial toughness for EBPVD TBC systems cyclically exposed at 1100 °C.