Microstructure and mechanical property of dense yttria-stabilized zirconia coating fabricated by an axial bi-cathode plasma torch under very low pressure

Very low-pressure plasma spray is a promising technique for preparing dense metal/ceramic coatings due to the high-speed plasma jet. However, at the same time, the low-pressure environment weakens the ability of the plasma jet to heat sprayed materials. In order to improve this shortcoming, a novel bi-cathode plasma torch was firstly designed in this study, which could enhance the plasma power from 40 kW to 81 kW and realize the axial powder feeding. By using this torch under 6 mbar, a dense and uniform yttria-stabilized zirconia coating with a porosity of 2.7% was fabricated, which had a hybrid microstructure that deposited with the form of unmelted particles, melted droplets, and vapor clusters. Meanwhile, 40% of the zirconia phase changed from the monoclinic type to the tetragonal type, indicating that the yttria-stabilized zirconia powder was melted well. The microhardness and elastic modulus of the coating was high compared with those prepared with other plasma spraying processes, which was 11.7 +/- 0.6 GPa and 167.8 +/- 4 GPa, respectively. The high density and mechanical property of the YSZ coating demonstrated that using the bi-cathode plasma torch at low pressure has the potential for the preparation of SOFC electrolytes.