Structure and properties of plasma-sprayed ceramic coatings

Plasma-sprayed ceramic coatings are increasingly being used in a variety of applications. Among these, ZrO2-based thermal barrier coatings (TBC) are often used in applications involving thermal cycling such as in the case of gas turbine engines. Coating failure under these severe conditions is due to factors that include: stresses resulting from the thermal expansion mismatch between the ceramic coating and the metallic substrate; residual stresses; adhesion and interfacial reactions; coating composition; and phase transformations. In this study the effect of these factors on the integrity of ZrO2-based and other oxide coatings are examined. Acoustic emission (AE) was used to study a variety of plasma-sprayed oxide coatings subjected to thermal cycling in the range 250 to 6500C. Coatings of Al2O3, Al2O3-TiO2, TiO2, ZrO2-5 wt% CaO and ZrO2-20 wt% Y2O3, plasma sprayed onto the ends of a steel substrate, with and without a NiCrAl bond coat, were examined to relate acoustic activity to coating integrity. Al2Onumber and TiO2 coatings generated more AE counts than multicomponent Al2Onumber-TiO2 coatings. ZrO2-based coatings yielded the least number of AE counts consistent with their better thermal cycling properties. The presence of the bond coat reduced AE counts in general. Results also indicated the formation of microcrack network during the initial cycles, in some coatings, which stabilized during subsequent cycles