Corrosion and thermal cycling behavior of plasma sprayed thermal barrier coatings on die steel

Casting strongly depends on its mold (usually die steel) employing in a cyclic thermal condition at temperature around the melting point of casting metal. To protect die steel from cyclic thermal corrosion at 1000 °C, three different kinds of thermal barrier coatings (TBCs), Al₂O₃(AO), La₂O₃-TiO₂-Al₂O₃ (LTA) and La₂Zr₂O₇ (LZO), were deposited on CoNiCrAlY coated steel substrate using a low-cost and time-saving plasma spraying. The corrosion potential and residual stress distribution of coatings were characterized by electrochemical polarization measurement and finite element simulation (FEM). LZO TBCs exhibited the highest corrosion electric potential among three cases. The cycling life was 10, 20 and 50 cycles for AO, LTA and LZO, respectively. FEM results indicated that there appeared the highest residual stress for AO, relatively higher for LTA and the lowest for LZO. Shear stress for AO and LTA cases while axial stress for LZO were the driving force to initiate cracks in coatings. The failure of TBCs originated from their corners and propagated along the direction parallel to the intrinsic layered structure of plasma sprayed coating. The realistic failure modes and the cross-sectional SEM validated this failure mechanism. LZO might be a more promising TBCs compared with AO and LTA for casting application.