Cold spraying has been developed as a high-quality metallic coating process. On the other hand, cold spraying of ceramic material is still difficult due to a lack of its ductility which is required for this process. There are several reports to fabricate ceramic coatings through this process. However, the deposition mechanism of cold-sprayed ceramic material has not yet been clarified. This study aims to clarify the importance of the microstructure of feedstock powder material on the formation of thick ceramic coating through cold spray process. Yttria powders with different specific surface area were prepared and coldsprayed onto aluminum alloy substrate. Furthermore, the microstructure of feedstock powder materials was carefully observed. The results revealed that the higher specific surface area enhanced the deposition of cold-sprayed ceramic particles and enabled to fabricate thick coatings. Fine primary particles agglomerated with high specific surface area and porous structure can be the most important factor to realize the fabrication of thick ceramic coatings by cold spray.
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 oxyfuelflame 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 alphaalumina 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.