Accelerated Durability Testing of Coatings for Gas Turbines

Oxidation resistance and thermal barrier coatings for components on the hot section of gas turbine engines are desired to have lifetimes on the order of tens of thousands of hours. This presents a problem in evaluating new coatings and modifications to existing coatings tests, which completely replicate the operating conditions, could take years to complete. Therefore, a reliable accelerated testing protocol is required. In this paper efforts directed toward developing a mechanism-based protocol for evaluating the life times of oxidation resistant coatings under thermal cyclic and hot corrosion conditions and thermal barrier coatings under thermal cyclic conditions is described. The cyclic lifetimes of oxidation resistant and thermal barrier coatings are determined by spalling behavior. Spallation is a function of oxide thickness and stress level, which control the elastic energy available to drive spallation, and the structures and morphologies of the various layers and interfaces in a given system, which control the fracture toughness at possible planes of weakness. Efforts to evaluate these quantities in relatively short duration tests are described. Specific techniques include acoustic emission studies, indentation techniques, and detailed metallographic observations. The extrapolation of results from high temperature tests, where failure can be achieved in relatively short times, to lower temperatures, which are characteristic of service conditions is also described. An approach to control these variables in a manner to produce accelerated failures under conditions, which allow estimation of lifetimes under typical operating conditions, are described and preliminary results are presented.