Deposition of YSZ Coatings in a Chamber at Pressures below 100 Pa Using Low-Power Plasma Spraying with an Internal Injection Powder Feeding

Abstract

By decreasing the chamber pressure, the effective mean free path length in a plasma jet increases, resulting in a distinct decrease in the heat transfer from the plasma jet to the powder feedstock. Thus, instantaneous, complete melting or partial evaporation of the powders in the plasma jet becomes difficult. To create yttria partially stabilized zirconia coatings in a relatively low-power plasma jet and a low-pressure environment of 100 Pa, a new plasma torch with a specialized nozzle with an internal injection powder system was designed. Optical emission spectroscopy indicated that the feedstock powder could be partially vaporized in a 700-A plasma arc in either 40Ar–10H₂ or 20Ar–20He–10H₂ standard liters per minute (SLPM) plasma gas. Denser coatings were deposited at arc current of 700 A by changing the plasma gas mixture from argon–hydrogen to argon–helium–hydrogen. A compact coating was obtained as the amount of vaporized powder increased, but the microstructure of the coating lost its columnar morphology due to the limited amount of vapor phase. In addition, the porosity and microhardness of the coatings were measured.