Implications of Zircon Formation on Thermal Barrier Coatings Exposed to Siliceous Deposits

This study examined the chemical interaction of ZrO2-(7-8)mole%YO1.5 (YSZ) exposed to a range of calcium-magnesium-alumino-silicate (CMAS) compositions and experimental conditions. Three tests were conducted on electron-beam physical vapor deposited YSZ thermal barrier coatings. This included an airfoil specimen after engine testing, a specimen subjected to burner rig cyclic testing, and a third sample that underwent conventional furnace cycle tests; the latter two were performed at 1150°C. In most scenarios, the chemical reaction between YSZ and CMAS produced zircon as a reaction product, which predominantly formed at the CMAS/YSZ interface. There was no evidence of a tetragonal-to-monoclinic phase transformation in regions where zircon was present, suggesting that the remaining YSZ had not been de-stabilized as frequently observed when zircon is not present. The formation of zircon at the YSZ interface offers an opportunity for mitigating further thermochemical degradation of YSZ thermal barrier coatings by CMAS attack at temperatures up to 1200°C. This research was sponsored by Honeywell Aerospace under the Honeywell-UCSB Alliance for Thermal Barrier Coatings.