A New Deposition Mechanism of Ceramic Coatings by Fine Solid Particle Spray with the Assistance of Mesoplasma Flow

We recently proposed a new process called a Hybrid Aerosol Deposition (HAD) for deposition of ceramic coatings. The HAD process utilizes a mesoplasma flow, which is a transition plasma from thermal plasma to low-pressure plasma, to enhance particle deposition for aerosol deposition. We actually confirmed an increase in the deposition efficiency by the mesoplasma assistance, yet the detail mechanism of the deposition has not been clarified. Therefore, in this study, we have investigated the microstructure of ceramic coatings obtained through spraying of alpha-alumina fine powder in the HAD process. Transmission electron microscopy and electron energy-loss spectroscopy mapping reveal that alumina coatings consisted of alpha-alumina grains with amorphous alumina on their surfaces. The amorphous layers connected each other and formed a three-dimensional bridge structure. These results indicate that the surface of fine particles was activated by the mesoplasma flow, while the core of the particles was kept solid without melting during flight. The surface activation of particles contributed to increase their sticking probability, which resulted in the increase in the deposition efficiency and the enhancement of the capability of three-dimensional coverage. This deposition mechanism is new and different from either melt deposition or solid consolidation.