Effects of porosity on thermal loadings of functionally graded Y₂O₃-ZrO₂/NiCoCrAlY coatings

Plasma-sprayed thermal barrier coatings often have problems of spallation and cracking in service owing to their poor bond strength and high residual stresses. Functionally gradient coatings with a gradual composition variation from heat resistant ceramics to fracture-resistant metals are proposed to eliminate these problems. In this study, functionally gradient coatings obtained from five layers were prepared on Ni substrates from Y₂O₃ stabilized ZrO₂ (YSZ) and NiCoCrAlY powders using an atmospheric plasma spray system. The residual stresses of the as-sprayed coatings with different graded layers and different porosities, as well as the changes of residual stresses during thermal cycling were simulated by finite element method (FEM). It was found that residual stresses decreased with an increase in porosity values of layers.