EFFECT OF MICROSTRUCTURE ON THERMAL-SHOCK CRACKING OF FUNCTIONALLY GRADED THERMAL BARRIER COATINGS STUDIED BY BURNER HEATING TEST

The thermal shock fracture mechanism of metal/ceramic functionally gra ded thermal barrier coatings was studied by burner heating test. Discu ssions were made on the basis of fracture mechanics with special refer ence 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/IN100 FGMs having finely mixed microstructure and PSZ/Inc o718 FGMs having rather coarse microstructure. The fracture toughness of each composition was determined by conventional vickers indentation method on uniform non-FGM specimens. It has been shown that the fract ure toughness depends strongly on the microstructure following from th e mixing conditions and the particle size of the raw material powders. In PSZ/IN100 FGMs, the fracture toughness increased with increase in the metal phase content, while in PSZ/Inco718 FGMs it was fairly lower than that of PSZ/IN100 FGMs, owing to roughly dispersed metal phase i n the PSZ matrix. The results of burner heating test revealed that the crack formation was always observed on the ceramic surface during coo ling. By comparison between the fracture toughness and mode I stress i ntensity factor, the initiated vertical cracks in PSZ/Inco718 FGMs wer e considered to extend into the interface of FGM/substrate without def lection. This crack extension behavior was confirmed by observing the cross-section of the tested sapmles. Although vertical cracks in PSZ/I N100 FGMs tend to be arrested in the FGM coating, with the extension o f the cracks into the graded layer, they deflected toward the directio n parallel to the surface. The depth of the parallel cracks beneath th e surface may correspond to a location of mode II stress intensity bei ng equal to zero.