Impact of deposition temperature on crystalline structure of plasma-sprayed Al₂O₃ splats revealed by FIB-HRTEM technique

The effect of substrate surface temperature on the crystalline structure of plasma-sprayed Al₂O₃ splats was studied to clarify the phase selection process during the splat quenching for the development of crystalline structure. Al₂O₃ splats were deposited by atmospheric plasma spraying on the polished alumina substrate preheated to different temperatures from 573 K to 1173 K. The TEM samples of cross-sectional alumina splats were prepared by focused ion beam assisted with scanning electron microscopy (FIB-SEM). The crystalline structure of splats was examined using high resolution transmission electron microscopy (HR-TEM). Results showed that the alumina splats deposited at the substrate surface temperature higher than 573 K were well bonded to the alumina substrate. Moreover, the crystalline structure changed significantly with substrate temperature. The crystalline structure of alumina splats changed from amorphous phase through meta-stable γ-Al₂O₃ crystallites to α-Al₂O₃ crystallites with the increase of the substrate temperature from 573 K to 1173 K. The present results clearly indicate that the crystalline structure formation depends on the surface temperature of the substrate on which droplet impacts. Thus, it is possible to control the splat crystalline structure by controlling substrate surface temperature prior molten droplet impact.