A Study of the Microstructure of Yttria-Stabilized Zirconia Deposited by Inductively Coupled Plasma Spraying

Abstract

Zirconia stabilized with 20 wt.% yttria was deposited to a thick free-standing type, similar to7 mm, by (inductively coupled plasma spraying) (ICPS), The spheroidization of particles and the microstructure of deposits were analyzed. Spheroidization fraction dependence on spray parameters such as particle size, H-2 gas mixing quantity, and probe position was studied. Effects of parameters on the spheroidization of particles were analyzed by ANOVA (analysis of variance) (ANOVA). ANOVA results showed that the spheroidization fraction largely depend on H-2 gas mixing quantity and particle size, and there are also some dependence on probe position and H-2 gas mixing quantity. After melting, particles kept their chemical composition homogeneously from the center to their surface without segregation or evaporation. The degree of deformation of the diameter of the splat over the diameter of the spheroidized particle was approximately 320% and splat thickness in the deposit varies between 2 mum and 3 mum depending on the deposition condition. The yttrium concentration gradient of the interlayer boundary appeared linear in the range of 0.5 to 1 mum. X-ray diffraction analysis and a transmission electron microscope (TEM) micrograph showed that low yttrium content particles resulted in tetragonal phase in deposit. The major characteristics of the microstructure of the thick free-standing deposit and solidification mode were studied. Microstructure of the bottom part of the deposit represented equiaxed or cellular structure. Equiaxed small grains prevailed when the droplets were quenched rapidly on substrate. The middle part of the deposit showed large columnar grains, of about 100 mum thick and 300 mum long. This may be due to high substrate or deposit temperatures and results in recrystallization and grain growth. The effects of the parameters, such as H-2 gas mixing quantity, particle size, spraying distance, and probe position, on the microstructure of the deposits were evaluated. The H-2 gas mixing quantity of Ar/H-2 = 120/20 L/min compared to Ar/H-2 = 120/10 L/min resulted in larger grain size and thicker cellular in chill. Grain shapes were affected by the heat removal rate from the deposit to its surrounding. Deposition with larger particle size showed heterogeneous grain size, insufficient particle melting, and incomplete recrystallization, The effect of probe position was less than the others.