SURFACE THERMAL-CRACKING OF THERMAL BARRIER COATINGS OWING TO STRESS-RELAXATION - ZIRCONIA VS MULLITE

The initiation of surface cracks in zirconia based multilayer thermal barrier coatings is related to stress relaxation which occurs at the t op surface of the coating at high temperatures. An analytical model is used to show that the compressive stresses decrease over time and bec ome tensile upon cooling of the coating, thus resulting in cracks. Mul lite, which exhibits significantly reduced stress relaxation behavior, is shown to remain in compression, thus preventing the initiation of surface cracks. This behavior is compared for the cases of equal heat flux and equal surface temperature conditions.