Experimental and numerical analysis of a novel reverse-polarity plasma torch with transferred arc hot-wall nozzle for atmospheric plasma spraying of YSZ coatings

A novel reverse-polarity plasma torch (RPT) with transferred arc (TA) hot-wall nozzle was proposed to enhance the energy of the plasma jet and improve the heating and acceleration performance of the refractory ceramic particles during atmospheric plasma spraying, and was applied to prepare high-performance YSZ coatings. First of all, the two RPTs either with TA hot-wall nozzle or with non-transferred arc (NTA) hot-wall nozzle were designed carefully, and their electro-thermal characteristics were tested through experiments to analyze and compare their performance. Results showed that the RPT with TA hot-wall nozzle (TA-RPT) presented higher arc voltage, thermal efficiency and plasma jet mean enthalpy than those of the RPT with NTA hot-wall nozzle (NTA-RPT) and the conventional normal-polarity plasma torches. Subsequently, numerical simulation models of the TA-RPT and NTA-RPT were established to study the influence of the TA hot-wall nozzle on the current density, temperature and velocity distributions of the plasma jet. Results showed that the use of the TA hot-wall nozzle can significantly enhance the energy of the plasma jet, while maintain the temperature and velocity of the plasma jet at downstream region of the RPT exit at a higher level. Finally, YSZ coatings were prepared by the TA-RPT and NTA-RPT respectively at the same spraying parameters. Results showed that the TA-RPT sprayed coatings exhibited denser and more uniform microstructures, fewer defects such as large-scale pores and inter-laminar cracks, and higher deposition rate than those of the NTA-RPT sprayed coatings. Moreover, the porosity, sur-face roughness, microhardness, crack extension forces and stability of the mechanical properties of the TA-RPT sprayed coatings were superior to those of the NTA-RPT sprayed coatings.