Effect of Microstructure on Thermal Shock Cracking of Functionally Graded Thermal Barrier Coatings Studied by Burner Heating Test

Abstract

The thermal shock fracture mechanism of metal/ceramic functionally graded thermal barrier coatings was studied by bumer heating tesL Discussions were made on the basis of fracture mechanics with special reference to the effect of microstructure on crack extension behavior. Two types of FGM coatings, having the same graded structure with different microstructures, were fabricated by slurry dipping and HIP sintering process: PSZ/IN 100 FGMs having finely mixed microstructure and PSZ/Inco 718 FGMs having rather coarse microstructure. The fracture toughness of each composition was determine by conventional vickers indentation method on uniform nonFGM specimens. It has been shown that the fracture toughness depends strongly on the microstructure following from the mixing conditions and the particle size of the raw material powders. In PSZ/IN 100 FGMs, the fracture toughness increased with increased in the metal phase content, while in PSZ/Inco718 FGMs it was fairly lower than that of PSZ/IN 100 FGMs, owing to roughly dispersed metal phase in the PSZ matrix. The results of bumer heating test revealed that the crack formation was always observed on the ceramic surface during cooling. By comparison between the fracture toughness and mode I stress intensity factor, the initiated vertical cracks in PSZ/ Inco718 FGMs were considered to extend into the interface of FGM/substrate without deflection. This crack extension behavior was confirmed by observing the cross-section of the tested sapmles. Although vertical cracks in PSZ/IN 100 FGMs tend to be arrested in the FGM coating, with the extension of the cracks into the graded layer, they deflected toward the direction parallel to the surface. The depth of the parallel cracks beneath the surface may correspond to a location of mode II stress intensity being equal to zero.