Measurements of Microhardness in Aluminum Oxide Crystals

Abstract

The variations of microhardness in aluminum oxide crystals were examined; (1) the effects of loading conditions in a single crystal of ruby and (2) characteristics of crystal grains in a polycrystal of corundum with 0.8mol per cent MgO. The ruby was produced by the Verneuil method and the corundum was prepared by a molten method in argon atmosphere at 2100°C.A thin section was used for the measurement of microhardness in the corundum, since subgrain boundaries and precipitates in grains could not be observed under a microscope attached in a microhardness tester, but detected under a polarized microscope.The results are summerized as follows:(1) The diagonal of indentation figures d (μ) for the (0001) surface of ruby varied according to a formula, P=1.82d1.85, depending on the load applied P(g).(2) A temperature higher than 250°C was preferable for the measurement of microhardness in the ruby, since cracks or fractures around the indentation marks disappeared at such temperature.(3) The microhardness numbers (300g) on (0001) and (1010) surface of the ruby at room temperature were 2065±95kg/mm2 and 2230±140kg/mm2, respectively. As the temperature increased, from room temperature to 1000°C these numbers decreased almost lineary.(4) The microhardness of grain boundaries in the corundum tended to become lower than that of grain centers.(5) On the plane nearly parallel to c-axis in the corundum, the microhardness of subgrain boundaries tended to become lower than that of subgrain centers. On the plane nearly perpendicular to c-axis, the microhardness of subgrain boundaries tended to become slightly higher than that of subgrain centers.The effects of loading conditions on the variation of microhardness were almost neglegible as far as cracks or fractures were not formed. It will be deduced from the above observations that subgrain boundaries and precipitates in the grains caused a marked variation of microhardness in the corundum.