The rhombohedral phase produced in partially-stabilized zirconia

Abstract

It has recently been reported that the rhombohedral phase (r-phase) is produced on the abraded surface of partially-stabilized zirconia (PSZ) [1]. The r-phase found in yttria-stabilized PSZ (Y-PSZ) has a different lattice parameter and composition with the ordered phase Zr3Y4012, which is an equilibrium phase in the ZrO2-Y203 system [2, 3]. The r-phase is considered to be a metastable phase which is induced by mechanical polishing, although the stability of the phase has not been clarified yet. This paper aims to report that the r-phase is formed not only on abraded surfaces but also in bulk Y-PSZ of a particular composition. Five ZrO2--Y203 alloys with Y203 content 2.0, 3.1, 4.0, 5.0 and 6.2mo1% were prepared from zirconia and yttria powders with 99.9% purity produced by Rare Metallic Co. Ltd. The oxide powders were mixed in a ball mill followed by pelleting in a steel die. The pellets were sintered at 1400 ° C for 1 h, and then arc-melted. The average weight of arc-melted samples was about 1.5 g. As reported previously [4], the arc-melted samples were black due to oxygen deficiency, but annealing at 1400 ° C for a few hours restored the usual colour. The oxygen deficiency was estimated to be about 1 at% from the increase in weight of a pure zirconia sample by the annealing. EPMA analysis has shown that the samples have a uniform concentration of Y203 on the cross-sectional plane within an accuracy of less than 0.5mo1%. The structures of the alloys were examined by X-ray diffraction, optical and electron microscopy. X-ray diffraction studies have revealed that the arc-melted alloys with 5.0 and 6.2mo1% Y2Oz were composed of cubic and tetragonal phases. Fig. 1 shows an electron micrograph of ZrO25.0molY203 alloy in which the two-phase structure is seen. It was found that the diffraction pattern from the matrix in the two-phase structure was indexed by an fc c lattice with a fluorite structure. However, the forbidden reflections in the fc c lattice often appeared from the lath-like phase in Fig. 1 [5]. From X-ray and electron diffraction data, the lath-like phase was recognized to be a tetragonal phase (t-phase) embedded in the cubic matrix (c-phase). On the other hand, the structure of ZrO2-2 .0mol%Y203 alloy was composed of monoclinic and tetragonal phases. It was found that a phase with a characteristic structure was developed in ZRO2-3.1 and 4.0 mol%Y203 alloys together with cand t-phases. Fig. 2 is an electron micrograph of the phase with a "herring-bone" appearance, which has a different microstructure to the c-phase or t-phase. The structure in Fig. 2 resembles the rhombohedral /3-phase in ZrO2-Sc203 alloys [6, 7], which is believed to be an equilibrium phase [7, 8]. An examination of the diffraction pattern in Fig. 2 has revealed that the pattern is slightly distorted from that of the f c c lattice with a zone axis close to [100] . The angle between 0 2 0 and 0 0 2 reflections is about 91 °, and the reciprocal vector 0 22 is slightly larger than 02 2. Such a distortion is expected from the rhombohedral distortion in the (111) direction of the fluorite lattice. The