Very low-pressure plasma-sprayed dense yttria-stabilized zirconia coatings using an axial bi-cathode plasma torch

In order to increase the plasma power and enhance the heating of sprayed powder, a bi-cathode plasma torch with axial powder injection was developed for the very low-pressure plasma spraying (VLPPS). By using this torch, uniform and dense yttria-stabilized zirconia (YSZ) coatings were prepared at different spraying distances. The plasma characteristics, microstructures and mechanical properties of YSZ coatings were investigated in detail. Results show that the new plasma torch had the ability to vaporize part of the YSZ powder. Meanwhile, the YSZ powder underwent a distinct phase change, at a maximum of 50% monoclinic zirconia phase transformed to the tetragonal-prime zirconia phase. The sprayed coatings had hybrid microstructures, including unmelted particles, molten droplets, and vapor clusters. With increasing spraying distance, the melting state of YSZ coating got better, while the microstructure became looser, leading to the porosity increased from 2.7% to 7.4%. The best mechanical properties were found in the densest coating, whose microhardness and Young's modulus was 11.7 +/- 0.6 GPa, 167.8 +/- 4.0 GPa, respectively, indicating that the mechanical performance of the YSZ coating was more dependent on the density, rather than the melting state. The dense VLPPS ceramic coatings may be applied for corrosion and heat resistance against oxidation, as well as the solid oxide fuel cells.