Grain size of MgO and polymorphic phases of ZrO 2 in zirconia-toughened MgO
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Zirconia-toughened MgO was manufactured and examined by scanning and transmission electron microscopes. It was found that ZrO 2 particles that are present on the MgO grain boundary limit the grain growth of MgO. The cooling rate has an effect on the ZrO 2 phase in zirconia-toughened MgO fired in a cubic ZrO 2 -MgO field, but it does not have an effect on the ZrO 2 phase in specimens fired in a tetragonal ZrO 2 -MgO field. Tetragonal ZrO 2 was retained at room temperature in zirconia-toughened MgO.
ZrO 2 has various polymorphic forms depending on temperature and additive concentration. The transformation of tetragonal ZrO 2 into monoclinic ZrO 2 has been used to enhance the toughness of matrix materials. 1 ^ Zirconia-toughened MgO is one of these materials. It has been shown that MgO can be toughened significantly by dispersing ZrO2.5"8 The fracture stress of MgO can be increased by mixing with ZrO 2 . The KIC was also enhanced by the presence of ZrO 2 . Although these changes in mechanical properties were related to the polymorphic forms of ZrO 2 , no clear evidence for the crystallographic phases of ZrO 2 in zirconia-toughened MgO has been reported. Especially, the effect of cooling rate on the ZrO 2 phases present in the composite was not known. In this communication, we will report the crystalline phases of ZrO 2 in zirconia-toughened MgO prepared by different cooling rates and the existence of tetragonal ZrO 2 in the composite. In the course of the study of zirconia-toughened MgO, it was recognized that the grain size of the MgO matrix was small. In this communication, we will also report on experimental evidence for the limited grain growth of MgO in this composite and will discuss how this can take place. MgO powder (Rare Metallic Co., Tokyo, 99.99%) and ZrO 2 powder (Tosoh Co., Tokyo, TZ-0) were taken at appropriate ratios and ball-milled for 24 h with the presence of isopropanol. After drying, the powder mixture was pressed into disks of 10 mm in diameter and 4 mm in thickness. The disks were sintered at 1450 °C and cooled down slowly in the furnace. The specimens were polished and thermally etched at 1300 °C for 1 h. The etched surface was examined by scanning electron microscope. Grain sizes were determined on the surface by the linear intercept method. The average intercept lengths were multiplied by 1.5 to get the average grain sizes reported in this communication. For the study of identification of ZrO 2 phases, the disks were sintered at 1350 °C or 1600 °C and were cooled down slowly in the furnace. Some specimens were cut into thin plates J. Mater. Res., Vol. 8, No. 11, Nov 1993
( 5 X 5 X 1 mm) and annealed again at 1350 °C or 1600 °C and quenched into liquid nitrogen. A standard technique was used to prepare thin foils from these specimens for the observation by transmission electron microscope (TEM, JEOL, JEM-2000EX). The quenching medium provides very low temperature, but it may not be the best one because the heat of vaporization of liquid nitrogen is small. However, the depth (150 mm) of liqui
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