Hot Corrosion Behavior of Plasma Sprayed Thermal Barrier Coatings and Acoustic Emission Response to Corrosion Monitoring

Four kinds of stabilized zirconia coatings, 8Y2O3·ZrO2, 7.6MgO·ZrO2, 10CaO·ZrO2 and 15CeO2·ZrO2 undercoated with NiCrAlY were sprayed on Type 304 stainless steel substrate by plasma spraying. The hot corrosion resistance of these materials were evaluated by investigating the coatings after heating in furnace for 3 hours at 1273K with corrosive ashes, and by monitoring the Acoustic Emission (AE, hereafter) of coatings with corrosive ashes in the 1273K_??_RT thermal cycle test. Corrosive components such as V, Na and S penetrated through the porous sprayed coatings, and caused the coatings to peel off due to corrosion of the undercoat. Stabilizing elements, MgO and CaO readily reacted with V and S compounds, but Y2O3 and CeO2 were relatively resistant to hot corrosion. In the thermal cycle test, the AE signal counts were detected immediately at the beginning of the test for coatings with corrosive ashes, and the coatings were spalling or broken down. On the other hand, the sprayed coatings without corrosive ashes required a number of cycle repetitions before AE signal could be measured. AE analysis allows not only the beginning of coating damages but also corrosion progressing to be detected in satisfactory response. These results also show that the AE monitoring method is applicable to evaluate the hot corrosion resistance for the thermal barrier coatings.