Process Suitability of Plasma Spray-physical Vapor Deposition with Fine Spherical Agglomerated Powders

Plasma spray-physical vapor deposition(PS-PVD) is a novel technology providing a multiphase composite controlled deposition, which is highly flexible in terms of coating structure and performance adjustment owing to the multiphase deposition including solid, liquid, and gas. This process combines the advantages of Air Plasma Spraying(APS) and Electron Beam-Physical Vapor Deposition(EB-PVD). Coatings prepared by this process have the following characteristics, including high efficiency, reasonable cost, long thermal cycle life, and low thermal conductivity. The PS-PVD technology has been of interest owing to its characteristics. However, the progress in the preparation and development of suitable materials and compatibility between materials and process are still not satisfactory. The material used for the PS-PVD process is fine agglomeration granulation particles, which are scarce in the market. The primary focus of this review is the design and preparation of a material suitable for the PS-PVD process. To improve the process suitability of the powder, 8YSZ powders with a "dual-phase", loose spherical agglomerated structure, and fine particles with sizes of 1-20 mu m are developed by a chemical co-precipitation of the raw material. The powder consists of typical fine irregular dense particles, nanoscale and dense agglomerated particles, with a high rate of opening porosity and certain self-fluidity as well as good crushing strength as a key characteristic. The deposition behaviors of the powder and microstructural characteristics of coatings with different spray distances, thermal spray powers, and positions away from the jet center are systematically studied to improve the compatibility of powders and process. When the electric current is between 1.70 and 2.6 kA the spray distance is between 0.4 and 1.2 m, under various other process parameters, we can prepare columnar and quasi-columnar coatings composed of a single metastable tetragonal phase (t') by vapor deposition. A high-vapor-content deposition and excellent compatibility are realized by effectively controlling the powder content and microstructure during the spraying process, at a low thermal spray power, large spray distance, and different positions away from the jet center. By improving the spray powders, the consumption of plasma energy used for vaporing of powders decreases, the vapor deposition improves, and the suitable process parameters for PS-PVD are broadened, thus realizing a high-vapor-content deposition in a high-energy and high-speed plasma. The deposition of the material during the PS-PVD process is constructively analyzed. The deposition process is as follows. First, the powders are stably conveyed into the nozzle, and then the adherents in the powder evaporate and the fine agglomerated granulation particles collapse quickly. The collapsed materials then partially sinter, evaporate, and melt at the high-temperature zone of the anode. During the interaction between collapsed materials and plasma plume, the phenomenon continues in the chamber including a continuous evaporation, melting, atomization of a liquid drop, transmission of a gaseous phase, condensation of a gaseous phase, and ejection of sintered particles and melted liquid droplets from under the jet flow out of the plasma plume due to self-gravity. Finally, the coatings are deposited by a source of a vapor phase and reflected disturbing flow from the outskirts of the plasma plume. The ratio of the gaseous phase in the plasma jet and deposition efficiency are improved to some extent by controlling the powder microstructure, content, and existence of dense particles. The contributions of this study are the breakthrough in the design and controlled preparation of materials suitable for the PS-PVD process, spray powder preparation, property regulation, and compatibility between materials and deposition process.