Degradation of plasma-sprayed alumina and zirconia coatings on stainless steel during thermal cycling and hot corrosion

Abstract

Abstract Thermal fatigue cycling and hot corrosion attack of plasma-sprayed ZrO 2 and Al 2 O 3 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 Al 2 O 3 coatings was much lower than that of ZrO 2 coatings under the same conditions. During hot corrosion of ZrO 2 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 Y 2 O 3 from the stabilized ZrO 2 , resulting in structural instability. Fe 2 O 3 and FeS were identified by TEM as the predominant phases in the substrate-bond coat interface region after the hot corrosion test. Occasionally, FeSO 3 was present in this interface region. Although both ZrO 2 and Al 2 O 3 coatings are ceramic coatings, they have different hot corrosion resistances. The Al 2 O 3 coating can resist hot corrosion attack for much longer than the ZrO 2 coating. This is attributed to alkaline dissolution of Al 2 O 3 on the molten Na 2 SO 4 . It causes a layer of a dense substrate 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 NaAlO 2 .