Additional better-quality homogeneous microstructures can be rendered available by the use of finer ceramic particles in thermal spraying, which in-turn requires more precise and advanced evaluation approaches for assessing their microstructures and properties, such as a nanoindentation method. However, it is important to examine whether the required microstructure and phase information can be accurately obtained using the nanoindentation method. In this study, the hardness and Young's modulus were measured by the nanoindentation method and were statistically evaluated by the Weibull distribution. As a case study, alumina coatings deposited by atmospheric plasma spraying (APS) and high-velocity oxyfuel-flame spraying (HVOF) were examined. When medium-sized alpha-alumina powder was sprayed by APS, the coating consisted of alpha- and gamma-alumina, and the Weibull plot of the hardness showed a bimodal distribution. Conversely, in the case of small-sized powder sprayed by APS, the coating exhibited a gamma-phase and a unimodal distribution. When finer alpha-alumina powder was sprayed using HVOF, it consisted of alpha, gamma, and non-crystalline phases, and the Weibull plot revealed a bimodal distribution. The gamma and non-crystalline phases were considered to appear from molten states and as the alpha phase was believed to originate from the unmolten states of the particles. Therefore, the unimodal distribution was ascribed to the molten state of the particle, while the bimodal distribution to the molten and unmolten states.
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