Ceramic Coatings Deposited from Fine Particles by Different Spraying Processes

Plasma spraying of fine particles promises uniform microstructure and improved properties for ceramic coatings due to the formation of small splats with reduced residual stress and pore size. Although spraying fine particles is challenging due to the poor rheological properties of particles (e.g., low flowability and agglomeration), the potential improvements to the coatings make it an attractive option. In this study, we discussed the feasibility of fine particle spraying via three different spraying techniques, namely low-power direct current (DC) plasma spraying, axial suspension plasma spraying (ASPS), and plasma-assisted aerosol deposition or hybrid aerosol deposition (HAD). Low-power DC plasma was sufficient to melt and deposit fine particles in an ambient atmosphere without using a liquid carrier. The fabricated coatings had similar features to those of ASPS coatings of small splats (5-8 mu m in diameter without internal cracks). The fabricated coatings by the low-power DC plasma showed almost equivalent properties to that of the conventional plasma-sprayed coating while providing low energy consumption. Fine particles deposited via the HAD process led to the coatings with improved microstructure without pores or cracks. During the HAD process, particle melting was not required for deposition, and using the plasma assisted the surface activation and improved the deposition efficiency of the aerosol deposition process. The fabricated HAD coating revealed improved microstructure with the highest hardness, Young's modulus, and adhesion strength and lack of pores or cracks compared to the coatings fabricated by other plasma spray techniques. In addition, similarities and differences among the techniques for spraying fine ceramic particles were investigated.