A novel reverse-polarity plasma torch with extended hot-wall nozzle for atmospheric plasma spraying of dense yttria-stabilized zirconia coatings

Abstract

Due to the high heating temperature and low cost, atmospheric plasma spraying (APS) has been widely used in the preparation of ceramic coatings, but its coatings are normally formed by the stacked splats from fully or partially melted feedstock particles, which inevitably leads to some pores, unbonded interfaces or cracks. Thus, in this paper, a novel atmospheric plasma spraying technique using a reverse-polarity plasma torch (RPT) with extended hot-wall nozzle was proposed to fully melt and accelerate feedstock particles for the preparation of dense YSZ ceramic coatings. Firstly, the RPT spray system was designed in detail, and its main electro-thermal characteristics were experimentally studied. The results showed that the RPT exhibited higher arc voltage, thermal efficiency and lower arc voltage fluctuation than that of traditional rod-cathode/normal-polarity plasma torches (NPT) at the similar output power. Secondly, a numerical simulate was carried out to reveal the effect of the extended hot-wall nozzle on the temperature and velocity distributions of the plasma jet. The results showed that the extended hot-wall nozzle remarkably extended the high-temperature and high-velocity region of the plasma jet from the RPT exit to the downstream. Finally, the dense and uniform YSZ coatings with a porosity of about 5.3% were successfully prepared by the RPT with extended hot-wall nozzle. The results showed that the YSZ coating exhibited denser microstructure and fewer defects than that of the coating prepared by traditional APS process. Meanwhile, the hardness and Young's modulus of the YSZ coating were 11.9 ± 1.4 GPa and 197.9 ± 29.4 GPa, respectively, which were relatively higher than that of the coating prepared by traditional APS process, and were very close to that of the coating prepared by very low-pressure plasma spraying (VLPPS) process.