Comparative study of the failure mechanism of atmospheric and suspension plasma sprayed thermal barrier coatings

Among numerous works on suspension plasma sprayed thermal barrier coatings, only a few focused on the relationship between the thermal shock resistance of SPS coatings and their mechanical properties. In this study, several SPS yttria-stabilized zirconia coatings were deposited with various spraying parameters including plasma gas composition, stand-off distance, and suspension formulation, then they were subjected to thermal cyclic test at 1150 °C. The results showed that despite segmented/columnar microstructure most of them failed after fewer cycles than the benchmark APS coating with conventional lamellar microstructure. Changes in the mechanical properties of the coatings such as Young's modulus and toughness after exposure to elevated temperature were evaluated. Most of the SPS coatings showed unusual change of mechanical properties, i.e., significant decrease of either Young's modulus or toughness or both. It was shown that such peculiar evolution of mechanical properties cannot be explained by the tetragonal to monoclinic transformation of YSZ, but it is rather related to the formation of very fine pores smaller 1 μm after heat treatment. The evolution of mechanical properties was linked with the results obtained in the thermal cyclic test, showing that thermal cycling resistance of SPS coatings was predominantly controlled by the mechanical properties after heat exposure, for which such phenomena as sintering-induced embrittlement may give crucial influence.