Formation mechanism of texture in α-alumina coatings produced by aerosol deposition

To investigate the texture formation mechanism in aerosol deposition (AD), coatings were deposited under various conditions using pure α-Al2O3 powder particles. The deposition rate increased with increasing gas flow rate when the gas flow rate was in the range from 6 to 30 L/min. Furthermore, the deposition rate increased with increasing substrate heating temperature. No large difference in the microstructure of the α-Al2O3 coating was observed. The formed α-Al2O3 coatings was fully dense and crystalline. X-ray diffraction analysis of the coating surfaces revealed specific textures. A texture with (0001) planes tilted approximately 15° from the coating plane was detected in the as-deposited specimen at a nozzle angle of 90°. Whether the nozzle angle was 90 and 60°, almost the same texture was formed indicating that the shear force does not contribute to texture formation. Texture formation was due to plastic deformation of the α-Al2O3 powder during collisions with the substrate. This is thought to be due to activation of basal and pyramidal slip systems. When the substrate was heated during AD, the texture changed to a (0001) fiber texture where the (0001) planes were parallel to the coating surface. The change in texture during substrate heating can be explained by a change in the critical resolved shear stress for the slip systems. The authors have previously reported that (0001) fiber texture has formed when the specimen was heat treated after the coating was deposited at room temperature. However, the mechanism of (0001) fiber texture formation by heat treatment is different form this research. During heat treatment, the texture is formed by the preferential growth of grains having (0001) plane at the surface of the coating.