Process and In-Service Residual Stresses in Thermal Barrier Systems

Thermal barrier coatings are used in several industries to improve thermal efficiency, for example, of gas turbine engines. The performance and life of thermal barrier coated components depend on many factors. One important factor is the residual stresses in the coating and substrate. Residual stresses can be influenced by the parameters of the application process. Parameters affecting residual stresses include the condition of the substrate, the type of spray application process, and the prespray heat treatment of the substrate. Residual stresses can also change significantly during the life of a thermal barrier coated material. The goal of this work is to quantitatively evaluate the changes in residual stresses of the thermal barrier coating and the substrate during the stages of processing and during simulated in-service testing. Through-thickness residual stresses distributions of the coating and the substrate material were determined using a destructive laboratory method, called the "Modified Layer Removal Method." Thin thermal barrier coatings (less than 0.5 mm) were evaluated in this work. Residual stresses in thermal barrier coated specimens were evaluated at three stages of the processing history: (1) after grit blasting of the Hastelloy substrate, (2) after application of the bond coat, and (3) after spraying the top coat. The effect on residual stresses of substrate temperature during spraying is examined. Changes in the residual stresses for thin thermal barrier coatings are shown at selected stages during the processing history of the coated materials. Differences between residual stresses at the selected stages are identified and discussed. Changes to residual stress distribution due to in-service conditions are examined. The effect of bond coat oxidation is examined by long-term, high-temperature exposure. Also, residual stresses are evaluated for thick thermal barrier coatings after thermal shock testing.