Al₂O₃ZrO₂ composite coatings for thermal-barrier applications

Abstract

Graded alternate layers of A1₂O₃ and 8% Y₂O₃ ZrO₂ and their admixtures were plasma sprayed onto bond-coated mild steel. They were evaluated for thermal-shock resistance, thermal-barrier characteristics, hot corrosion resistance (molten NaCl corrodant) and depth of attack, adhesion strength and the presence of phases. Although front-back temperature drops of 423-623 K were observed, some of the coatings showed good adherence even after 100 thermal shock cycles. In the sequence of the graded layers, the oxide which is directly in contact with the bond coat appears to influence the properties especially in coatings of 150 and 300 µm thickness. Molten NaCl readily attacks the films at high hot-face temperatures (1273 K for 1h) and the adhesive strength falls significantly by 50-60%. Diffusion of alkaline elements is also found to depend on the chemical composition of the outer coating directly facing the molten corrodant.