Investigating Hot corrosion, CMAS, and Thermal Shock Behaviour of Double-layer YSZ/La2Ce2O7 + YSZ Thermal Barrier Coatings

Abstract

In this work, new double-layer YSZ/La2Ce2O7 (LC) + YSZ coatings were developed using air plasma spraying (APS). The surface of the prepared coatings was relatively smooth and consisted of melted and partially melted areas. Their resistance to hot corrosion, CaO-MgO-Al2O3-SiO2 (CMAS), and thermal shock were examined. YSZ was added to the upper layer to enhance the lanthanum cerate (La2Ce2O7, LC) properties. During the hot corrosion tests, the corrosion salt reacted with the upper layer, and the CeO2 phase and new corrosion products were identified. The main phase was LaVO4, and the secondary phases were CeVO4 and YVO4. SEM confirmed the formation of new, cuboidal-shaped corrosion products. The infiltration of CMAS led to the formation of additional new products: Ca4MgxAl4Si(6-x-γ)O14 and Ca2.8(LaxCe1-x).6(SiO4)O6-4x. SEM revealed CMAS infiltration through the upper layer in the form of islands. Following the thermal shock resistance tests, the upper layer gradually peeled off, and the coating survived 67 cycles. Possible failure mechanisms were identified, and failure was attributed to the spallation of the upper layer from the surface layer by layer. After all tests, the top layer showed partial spalling and delamination. This was mainly caused by the reaction of corrosive salt or CMAS with the top layer, which changed its composition, leading to the formation and propagation of cracks and, ultimately, the separation of part of the upper layer. Peeling of the upper layer through mainly horizontal cracks was observed after hot corrosion, CMAS and thermal shocks. The NiCrAlY bond coat and YSZ interlayer remained undamaged.