Thermal Barrier Coating Life Prediction Model Development

Ceramic thermal barrier coating tests show that the coating fails by ceramic spallation. Analysis of life data indicates that cyclic thermal loading and thermal exposure play synergistic roles in controlling the spallation life of the coating. A life prediction algorithm has been developed, based on a damage accumulation algorithm that includes both cyclic and time-dependent damage. The cyclic damage is related to the calculated cyclic inelastic strain range in the ceramic coating; the time-dependent damage is related to the oxidation kinetics at the bond-ceramic interface. Cyclic inelastic strain range is calculated using a modified form of the Walker viscoplastic material model. Calculation of the oxidation kinetics is based on traditional oxidation algorithms using experimentally determined parameters. A relation between oxide growth and cycle parameters was derived from test data. The life prediction model was evaluated by predicting the lives of a set of thermal cyclic tests whose heating and cooling rates were significantly greater than those used to correlate the life parameters. Correlation between the actual and predicted spallation lives is within a factor of 3. This is judged to be satisfactory, relative to fatigue life prediction scatter in metals.