Charaterization of failure mechanisms of duplex and graded thermal barrier coatings exposed to thermal shocks

Thermal-barrier coatings (TBC's), which have insulating properties and hot-corrosion resistant, have increasing utilization in the hot sections of gas turbine engines. All such coatings are applied by plasma spraying. The major benefit of the TBC is the reduction of heat transferred into air-cooled components that they provide. The beginning of failure in thermal shock tests of (ZrO2 7% Y2O3)/ (Ni-22% Co-17% Cr-1205 Al-0.6% Y) duplex and graded coating systems on Inconel 617 and 738 Lc has been characterized. The used burner rig provided a temperature fluctuation of the substrate between 265 and 765 degree C, with 40 and 80 sec heating and cooling periods, respectively. The appearance of the first bright spot on the coating surface during heating was considered as the beginning of failure. Stresses due to thermal expansion mismatch on cooling are the probable cause of coating failure