The deformation and fracture of thick thermal barrier coatings.

摘要

Plasma-sprayed thick thermal barrier coatings (TTBCs) are being developed for thermal protection of diesel engine components in high temperature service. Comparing to thin thermal barrier coatings used in gas turbine industry, increased thickness causes some TTBCs failure to occur within the bulk of the coating materials and away from the interface. This necessitated the study of mechanical properties of the coating materials independent of the substrate. In order to enhance the performance and to predict the life of TTBCs, we have to understand the materials response under multiaxial stress states, the deformation mechanisms, failure criteria, and the constitutive relations. In this study, the deformation behavior, the deformation mechanisms, and the failure criteria were investigated. The results shows that under combined axial and shear loading, thin walled tubular specimens of ceramic coatings failed in one of two modes, a tensile failure perpendicular to the maximum principal stress when $s1\ge sTf$ or a shear failure through the thickness when $s3\le sCf$. Two apparatuses for in situ SEM torsion and compression testing were developed for deformation mechanisms investigation. The deformation mechanisms were identified as tensile microcracking, crack closing, and crack sliding. A model has been developed for the constitution relation of functionally graded TTBCs. It is shown that with a few simple experiments, this model can be used to predict the cyclic deformation behavior of the functionally graded TTBCs.