Formation of Abradable Alumina Ceramic Coatings through Deposition of Semi-Molten Ceramic Particles by Flame Spray

Abstract

High temperature gas turbines require ceramic abradable coatings for sealing to ensure high efficiency. In this study, a novel method is proposed to deposit porous ceramic coatings through deposition of ceramic spray powder particles in the semi-molten state. The commercially available alumina (Al2O3) powders were spheroidized and screened to a particle size range from 40 to 50 µm for spray deposition. Flame spraying was employed for coating deposition. During deposition, the substrate surface was kept at 500°C. The effect of melting degree of spray particles on coating microstructure was investigated by changing the flame power and spray distance. The porosity of flame-sprayed Al2O3 coatings was estimated by image analysis on coating cross-sectional microstructures. The results showed that porous Al2O3 coatings were successfully prepared with a porosity range up to 59% by flame spray. Moreover, spray parameters such as acetylene flow rate and spray distance have significant influences on the particle melting state, thus the microstructure and the porosity of the coating. With the decrease of acetylene flow rate and spray distance, the porosity of coatings increased due to the decrease of the melting degree of the sprayed particles. At a spray distance of 20 mm, when the acetylene flow rate was reduced from 400 to 200 L/h, the porosity increased from 37% to 59%. The results clearly demonstrated the feasibility to prepare porous abradable coatings of high porosity through surface-melted spray particles.