Preparation of fine-grained monoclinic zirconia ceramics by colloidal processing

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Preparation of fine-grained monoclinic zirconia ceramics by colloidal processing T. Uchikoshi, Y. Sakka, K. Ozawa, and K. Hiraga National Research Institute for Metals, 1-2-1, Sengen, Tsukuba, Ibaraki 305, Japan (Received 10 September 1997; accepted 6 January 1998)

Fine-grained monoclinic zirconia ceramic was made from well-dispersed zirconia sol prepared by the hydrolysis of zirconium chloride oxide octahydrate. Dechlorinated and concentrated zirconia sol was consolidated by pressure filtration. The relative green density of the compact was improved by the following cold isostatic pressing treatment at 400 MPa. The compact was densified by pressureless sintering to . 98% of theoretical density in air at 1100 ±C, which is lower than that of monoclinic to tetragonal transformation of pure zirconia. The average grain size of the sintered monoclinic zirconia ceramics was 92 nm.

I. INTRODUCTION

Zirconia polycrystal is one of the most promising ceramics as structural ceramics, functional ceramics, and so on. Pure zirconia transforms from monoclinic to tetragonal and cubic at high temperatures.1 1170 ±C

monoclinic

900–1000 ±C 2680 ±C

cubic

$

tetragonal

2370 ±C

$

liquid phase

The transformation between monoclinic smd and tetragonal std is accompanied by a large volume change, ca. 4.6%, and therefore crack forms during cooling. This fatal problem makes it very difficult to sinter monoclinic zirconia powder to a dense body, since the temperatures necessary for sintering of zirconia have always been well above the m-t transformation temperature, i.e., above 1170 ±C. To overcome this inferiority, dopants, such as Y2 O3 , CaO, and MgO, have been commonly dissolved to stabilize the high temperature phases. Until now, some studies have been reported concerning the processing of dense monoclinic zirconia polycrystal.2–8 Most of the previous studies were performed in the cubic or tetragonal stability range using hot pressing and complicated sintering schedules.2–5,7 For example, Sense2 found that it was possible to produce monoclinic zirconia with a density of 5.77 gycm3 by hot-pressing below 1000 ±C and then heating above at least 2250 ±C in vacuum. On the other hand, Yoshimura and S¯omiya6 prepared monoclinic zirconia with a density of 5.85 gycm3 by hydrothermal reaction sintering between zirconium metal and supercritical water at 1000 ±C for 3 h under 98 MPa. Their unique attempt is noteworthy. To make dense monoclinic zirconia ceramics at lower temperatures than m-t transformation temperature, selection of raw material and processing method is very important. Required factors of the raw powder are high 840

http://journals.cambridge.org

J. Mater. Res., Vol. 13, No. 4, Apr 1998

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purity, fine particle, and no flocculation. Dense particle packing is also very important to reduce the densification temperature. Skandan8 first succeeded to sinter monoclinic zirconia to full density in air at temperatures as low as 1125 ±C using zirconia nanoparticles

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