ASSERT Grant: Evolution of Stress and Damage in Coatings for Thermal Protection Systems
Final rept. 1 Jun 96-31 May 99
CALIFORNIA UNIV SANTA BARBARA DEPT OF MATERIALS
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The goals of this AASERT were to establish quantitative relationships between the stress in alumina scales formed beneath thermal barrier coatings during high-temperature exposure, the oxidation conditions and accompanying microstructural changes. The technique utilized was that of photostimulated Cr3 luminescence piezospectroscopy developed under the parent grant. Piezospectroscopy measurements of the residual stress evolution during prolonged oxidation exposure have revealed that, contrary to expectation in the community, the stress initially changes rather rapidly and then attains a constant value that remains essentially unchanged until failure occurs. Thus, failure is not associated with the residual stress gradually increasing until exceeding some critical stress occurs. Instead, as both programs proceeded it became clear that failure usually occurs by a progressive failure mode, one in which localized regions of damage nucleate, then grow until finally linking up to form a critical-sized defect that can cause spalling. From this new insight, a detailed investigation was made into the microstructural origins of possible nucleating defects, in particular those associated with the early stages of deposition of zirconia TBC coatings. The microstructure and crystallographic texture of thermal barrier coatings deposited on a variety of substrates was investigated and related to the thermal conductivity of the coatings.
- Coatings, Colorants and Finishes