Effect of additives on self-healing of plasma sprayed ceramic coatings

MgAl2O4 coatings, used for electrical insulation along the metallic seals in high temperature fuel cell stacks, tend to fail prematurely due to generated stresses during on-off thermal cycles. To enhance stress bearing capability and the durability of these coatings, self-healing was investigated by introducing additives consisting of SiC as primary additive along with a secondary additive material. Secondary additives were one of the following compounds: BaO, CaO, ZnO, Y2O3, Al2O3, La2O3, TiO2, GeO2, Ce0.9Gd0.1O1.95 (GDC) or Ta2O5. Crack healing can be attained due to reaction between SiC, secondary additive and oxygen that was transported to the additive due to a crack. Such reaction would be associated to a phase formation in the additive material linked with mass and volume increase assisting in closure of the advancing crack. Using TGA/DSC reaction temperatures and mass gains of SiC with or without secondary additives were identified. SiC+Y2O3 and SiC+ZnO were opted as promising additive materials. 20 wt% (SiC+Y2O3) containing MgAl2O4 coatings were produced by plasma spraying. In these developed coatings, healing was demonstrated after heat treatment at 1050°C in air for 10 hour. Defect healing in spinel coating with SiC+ZnO is under investigation