DEGRADATION OF PLASMA-SPRAYED ALUMINA AND ZIRCONIA COATINGS ON STAINLESS-STEEL DURING THERMAL CYCLING AND HOT CORROSION

Thermal fatigue cycling and hot corrosion attack of plasma-sprayed ZrO2 and Al2O3 coatings were studied. Transmission electron microscopy (TEM) was also employed to observe two types of extraction replicas from the coating surface and a cross-section of the bond coat on the substrate. The thermal fatigue lifetime of Al2O3 coatings was much lower than that of ZrO2 coatings under the same conditions. During hot corrosion of ZrO2 coatings, molten salt can penetrate into the pores and cracks of the coating, oxidizing the bond coat and substrate and even sulphidizing the substrate. With prolonged attack, it can also react with Y2O3 from the stabilized ZrO2, resulting in structural instability. Fe2O3 and FeS were identified by TEM as the predominant phases in the substrate-bond coat interface region after the hot corrosion test. Occasionally, FeSO3 was present in this interface region. Although both ZrO2 and Al2O3 coatings are ceramic coatings, they have different hot corrosion resistances. The Al2O3 coating can resist hot corrosion attack for much longer than the ZrO2 coating. This is attributed to alkaline dissolution of Al2O3 on the molten Na2SO4. It causes a layer of a dense substance to crystallize along a certain direction, and the coating thus can be further protected from oxidation by air. This layer of a dense substance was hexagonal NaAlO2.