Microstructure and properties of porous abradable alumina coatings flame-sprayed by semi-molten particles

High efficiency gas turbine needs high temperature sealing by abradable porous ceramic coatings. In this study, porous Al₂O₃ coatings were deposited by flame spraying through controlling melting of spray powder particle in a semi-molten state. The effect of melting degree of spray particles changed via spray conditions on coating microstructure and porosity was investigated. The melting degree of spray particles was characterized by using 3D confocal laser microscopy. The porosity of the coating was estimated by image analyzing. The results showed that the melting degree of alumina particles can be changed from 80 down to 30% and thus the coating porosity can be increased from 30% up to about 60%. The standard hardness test yielded no effective data for the porous coatings deposited by spray particles of a melting degree less than 60%, and hardness of 32-75 HR15Y for Al₂O₃ coatings deposited by spray particles with a melting degree higher than 60%. The pin-on-disk abrasion test of Inconel 738 nickel- based superalloy spherical pin of 5 mm in diameter at room temperature against porous alumina coating was conducted to evaluate abradability of porous Al₂O₃ coatings. It was found that for the coatings of hardness less than 32HR15Y and porosity over 40% the wear weight loss of the IN738 pin became negligible despite high wear rate of the coating. It is evident that the flame-sprayed porous alumina coatings of high porosity by the present approach are promising abradable coatings applicable to gas turbine operating at high temperature.