Influence of Substrate Temperature on the Crystalline Structure Formation in Plasma-Sprayed Al2O3 Splats

It has been recently observed that the substrate surface temperature during thermal spray coating deposition has a profound effect on the morphology of the deposited splat, intersplat bonding, and consequently on the microstructure and properties of the deposit. In this study, the substrate temperature effect on the crystalline structure was studied to clarify the crystallographic texture evolution during splat formation. Al₂O₃ splats were deposited by the atmospheric plasma spraying (APS) process on the polished alumina substrate preheated to different temperatures of 300°C, 400°C, 600°C and 900°C. The cross-sections of alumina splats were prepared by focused ion beam assisted scanning electron microscopy (FIB-SEM). The crystallographic texture of splat was examined using high resolution transmission electron microscopy (HRTEM). The results showed that the alumina splats deposited at substrate temperatures higher than 300°C were well bonded with the alumian substrate. Moreover, the crystalline structure changed significantly with substrate temperature. The splats deposited at substrate temperatures of 300°C and 400°C exhibited amorphous structure, while the splats deposited at 600°C exhibited γ-Al₂O₃ crystalline. When the substrate temperature was increased to 900°C, the splat was crystallized to α-Al₂O₃ crystalline structure. The results showed that crystallographic texture of alumian splats changes from amorphous phase through qusistable γ-Al₂O₃ crystalline to α-Al₂O₃ crystalline with the change of the substrate temperature from 300°C to 900°C. The change of the crystalline structure with substrate temperature was correlated to splat cooling rate. The present results clearly indicate that the crystallographic texture formation depends on the temperature of substrate on which droplet impacts.