(A) study on the characteristics of Yttria stabilized zirconia deposit by plasma spraying

Abstract

A plasma spraying technology has been applied to the coating by using the metals of low melting point and small particles. The present work is to apply this technique to the fabrication of a ceramic pellet with a high density. Powder melting, spheroidization, thick deposition and pellet forming on the various conditions were carried out and their microstructures were characterized according to the solidification theories. Yttria stabilized zirconia (YSZ) was used for the deposit material. Effects of parameters for the melting, such as particle morphology, particle size, plasma gas composition, spraying chamber pressure and plasma plate power, were evaluated. A statistical tool, analysis of variance (ANOVA), was applied to analyze the effects of parameters quantitatively and their correlations on the density of a deposit. It was found that the particle morphology was the major factor limiting the density of a deposit. Dependence of spheroidization on variables, such as particle size,$H_2$ gas fraction in plasma gas, particle flight direction in plasma flame, and probe position in the torch, was studied. The result showed that the spheroidization of a particle largely depended on the $H_2$ gas fraction and particle size. ANOVA showed that the $H_2$ gas fraction had an impact on the spheroidization by 25%. In addition, there was a close correlation between the probe position and $H_2$ gas fraction. A higher $H_2$ gas fraction resulted in larger and thicker cellular grains on the graphite substrate. Long grains with needle-like endings were observed more in a dense deposit than in a porous one. These shapes seemed to be affected by the heat removal rate. The heterogeneous grain size and insufficiently melted particles were frequently observed in the deposit sprayed with a large particle size. The solute (yttrium) concentration of the spheroidized particles was homogeneous from the center to the surface of a particle, which confirmed that there was no selective vapo...