Phase Separation and Hydrothermal Degradation of 3 mol% Y2O3-ZrO2 Ceramics

Tokyo Research Laboratory, Tosoh Corporation, 2743–1, Hayakawa, Ayase-shi, Kanagawa 252–1123
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Key-words : Zirconia, Phase separation, Hydrothermal degradation, Rietveld analysis
1. Introduction Numerous studies of Y2O3-stabilized tetragonal ZrO2 polycrystal Y-TZP have been made in the past last several decades. Especially, 3 mol! Y2O3 doped systems 3Y-TZP have been studied extensively because of its high toughness and high strength.1 The tetragonal structure in sintered body is strongly related to such mechanical property because the enhanced toughness and strength are attributed to the stress-induced phase transition of tetragonal phase.2 Tetragonal to monoclinic phase transition is accelerated under hydrothermal condition. This phenomenon is called hydrothermal degradation of tetragonal zirconia. Although many attempts have been taken to explain hydrothermal degradation, the mechanism is still controversial. So far, roughly two major theories have been proposed to explain the degradation mechanism. The first theory is drawing out of yttrium from Y-TZP by means of formation of other compound, such as Y OH 3,3 or direct leaching.4 The second mechanism is termination of Zr–O–Zr bond near the surface due to attack of H2O molecule. Sato and Shimada.5 reported that the termination of Zr–O–Zr bonds leads to reduce strain near surface. The reduction of lattice strain reduces the difference of Gibbs free energy between tetragonal and monoclinic phase. Yoshimura et al.6 suggested that terminations of Zr–O–Zr bonds allow penetrating OH ion into tetragonal lattice, consequently, lattice strain reduce an energy barrier of nucleation for tetragonal to monoclinic phase transition. Although tetragonal to monoclinic phase transition socalled martensitic phase transition plays an important rule for not only the high toughness mechanism but also the hydrothermal degradation mechanism, few studies on the crystal structure of sintered body have been made so far. Recently, a heterogeneity caused by yttrium segregation and cubic formation were reported in 3Y-TZP ceramics with highresolution TEM.7 ,8 Matsui et al.8 reported that enrichment
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of yttrium exclusively occurs near the grain boundary and solute drag effect of yttrium play an important rule for the grain-growth mechanism. In this study, the structural changes of the sintered body were analyzed with the Rietveld analysis as a function of sintering temperature. Aging kinetics of tetragonal to monoclinic phase transition was also analyzed with the equation for nucleation-growth mechanism. The correlation between phase separated state of the sintered body and hydrothermal degradation phenomena was discussed.
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Phase separation and hydrothermal degradation in 3 mol! Y2O3-stabilized tetragonal ZrO2 polycrystal 3YTZP were studied by X-ray diffractometry. The Rietveld analysis showed that crystal structure of sintered body remarkably changes at high temperature. The sintered body is phase separated into high and low yttrium region with increasing sintering temperature. Both regions are assigned to the tetragonal phase. The high yttrium region can be analyzed using tetragonal phase model with c a೾1. The amount of monoclinic phase induced by hydrothermal degradation increased with increasing degradation time. Aging kinetics of tetragonal to monoclinic phase transition was analyzed with Johnson–Mehl–Avrami equation. Fraction of transformable phase evaluated with the equation is in good agreement with the fraction of low yttrium region determined with the Rietveld analysis. The quantitative analyses clearly show that the low yttrium region selectively transforms into monoclinic phase under the hydrothermal condition, while the high yttrium region does not transform. Transformable fraction during the hydrothermal condition in 3Y-TZP ceramics is governed by the amount of low yttrium region. The formation of low yttrium region due to phase separation is responsible for hydrothermal =Received February 27, 2005; Accepted May 19, 2005? degradation in sintered body.
Journal of the Ceramic Society of Japan
113 [ 8 ] 530–533 (2005)
Paper
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Isao YAMASHITA and Koji TSUKUMA