text: Failure of thermal barrier coating usually manifests as delamination, which arises from the temperature gradient during thermal cycling between ambient temperature and working conditions coupled with the difference in thermal expansion coefficient of substrate and coating. It is rare for the coating to fail completely – some pieces remain intact, and significant scatter is observed in the time to failure if testing is repeated under identical conditions. Various degradation mechanisms affect thermal barrier coating, and some or all of these must operate before failure finally occurs:
  * Oxidation at the interface of thermal barrier coating and underlying bond coat; 
  * Depletion of aluminum in bond coat due to oxidation and diffusion with substrate;
  * Thermal stresses from mismatch in thermal expansion coefficient and growth stress due to the formation of thermally grown oxide layer;
  * Imperfections near thermally grown oxide layer;
  * Various other complicating factors during engine operation.
  Additionally, TBC life is sensitive to the combination of materials (substrate, bond coat, ceramic) and processes (EB-PVD, plasma spraying) used.