Reaction, Microstructures and Mechanical Properties of Model Ceramic Ware in the K2O-Al2O3-SiO2 System

Abstract

This paper reports on the reaction, microstructures and mechanical properties of model ceramic ware produced using nonplastic raw materials of feldspar, alumina and silica. The feldspar of median size 2.60 μm and γ-alumina of 34 nm or α-alumina of 0.2 or 0.6 μm, were mixed with tetraethyl orthosilicate at the composition of feldspar/alumina/silica = 38.3/16.6/45.1 (mass%). After hydrolysis of tetraethyl orthosilicate with HCl solution, the dried powders were compacted and sintered at 1200-1300°C for 1-10 h. Fine γ-alumina particles reacted with feldspar and silica to produce mullite whiskers of 1 μm length, cristobalite and a liquid phase rich in silica. Cristobalite formed by the crystallization of silica component in the liquid phase during the cooling process. In the ceramic ware using submicrometer-sized α-alumina particles of median sizes 0.2 and 0.6 μm, little α-alumina particles dissolved in the liquid phase, indicating the formation of metastable phases of α-alumina-liquid. Crystallization of silica component from the liquid during the cooling process was disturbed by the presence of α-alumina particles. These samples contained spherical air bubbles of 10-80 μm in diameter in the glassy matrix. In the sample with 34 nm-γ-alumina (30 mass%) and 0.6 μm-α-alumina (70 mass%), both stable and metastable phases of mullite-α-alumina-liquid were produced. The ceramic ware of metastable phases (α-alumina-glassy matrix) including air bubbles showed a lower Young's modulus and a larger strain at fracture than the rigid ceramic ware of stable phases (mullite-cristobalite-glassy matrix) with interlocking mullite whiskers. However, the flexural strength was comparable between samples of the stable and metastable phases. Vickers hardness was higher for the sample with mullite whiskers than for the sample with α-alumina particles because of an increased resistance by the interlocking of mullite whiskers to the applied Vickers load.