Thermo-electrical properties of the alumina coatings deposited by different thermal spraying technologies

Abstract Thermal and electrical properties of alumina (Al 2 O 3 ) coatings produced via Atmospheric Plasma Spraying (APS) and Detonation Gun Spraying (DGS) were investigated and compared in this study. A combination of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) analysis, image analysis and X-ray Diffraction analysis (XRD) were used to better understand microstructural characteristics of as-sprayed alumina coatings. Microstructure formation of coating layers and its main features such as splats, pores, and boundaries were experimentally observed for both deposited coatings. The measurement of splats average thickness indicated that splat sizes, as building blocks of coatings were approximately similar for both coatings. The XRD showed that feedstock was pure α-Al 2 O 3 , but both deposited alumina coatings contained the mixture of α-Al 2 O 3 and γ-Al 2 O 3 phases. However, the amount of α-Al 2 O 3 phase was two times higher in DGS deposited coating. The DGS deposited coatings were also had a higher hardness compared to APS deposited one. The electrical properties of alumina coatings were measured using the pseudo two-contact impedance spectroscopy method that allowed separation of the electrical resistivity of splats and their boundaries within the coating microstructure. The obtained results showed significant difference between these two features in terms of electrical resistivity, namely splats had 5 orders of magnitude smaller resistivity compared to the splat boundaries. In addition, DGS deposited coating was characterized with higher electrical resistivity of splats and splat boundaries compared with APS coating. Finally, the experimentally measured thermal conductivities of alumina coatings showed higher thermal conductivity in DGS deposited coatings compared to the APS ones. These values were validated using analytical models and Finite Element Analysis (FEA).