An Improvement of Low Power Plasma Torch for Very Low Pressure Plasma Sprayed Thin and Dense Ceramic Coatings

As a promising thermal spray technology, the very low pressure plasma spray (VLPPS) process has been significantly used to deposit thin, dense and homogenous ceramic coating materials for special application needs in recent years. In this study, a home-made transferred arc nozzle was mounted on a low power F100 plasma torch for enhancing the plasma jet energy under very low pressure conditions. Thin and dense alumina (Al₂O₃) and yttria-stabilized zirconia (YSZ) ceramic coatings with an average thickness of 30-40 µm were successfully elaborated by VLPPS process below 1 mbar. Optical emission spectroscopy (OES) was used to analyze the properties of the plasma jet with or without powder injection. The microstructures of the coatings were observed by means of scanning electron microscopy (SEM). It was found that the YSZ coatings displayed a bimodal microstructure that was composed of splats formed by melted particles and a little amount of vapor condensation from evaporated particles. However, vapor condensation could not be observed in the Al₂O₃ coatings formed by lamellar splats only. The Vickers microhardnesses of both coatings were also evaluated.