Enhanced Grain Growth and Orientation in Hot-pressed Alumina

Abstract

Hot-pressing of alumina was caried out at 1700°-1900°C and 200-250kg/cm² for 5-60 min in graphite molds heated by high-frequency induction. Three types of structures were microscopically observed in hot-pressed specimens, depending on hot-pressing conditions. Temperature increase under constant high pressures from low to high temperatures tended to give the layered-typed bodies consisting of the structures varied near outer surfaces and in central parts. Application of pressure subsequent to temperature increase gave comparatively homogeneous structures in hot-pressed alumina, in which noticeable orientation was observed under microscope and by X-ray diffraction method. In the oriented-typed specimens, the c-axes of alumina crystallites tended to be parallel to the hot-pressing direction. The orientation degree was determined by X-ray diffraction method in comparison with a standard specimen with an at-random structure. In the cases where temperature was increased by about 50°C during hot-pressing under the conditions which should give the above-described homogeneous oriented-typed structures, enhanced grain growth tended to be caused in alumina specimens which contained 0.6% Na₂O, while the alumina with 0.08% Na₂O showed no enhanced grain growth. The lengths of grain growth were often observed to be about 1mm. The bending strengths of grown-typed specimens were about one forth of those of oriented-typed ones.