Device Development for the Analysis of Non-equiliblium Fracture of Ceramic Composites under Ultra Severe Condition

点击翻译按钮获取中文摘要

The bending strength and the work-of-fracture of polycrystalline Al2O3 and one reinforced with SiC whiskers were measured at elevated temperatures using an equipment developed in this study. Four-point bend test with an inner span of 10mm and an outer span of 30mm was carried out for a beam with a dimension of 3 * 4 * 36mm3 in order to estimate the bending strength. For the estimation of the work-of-fracture, 3-point bending with a span of 30mm was done against the beam with a chevron notch at its center. Remarkable plastic deformation was not observed in each material from room temperature to 1200。C, so that linear fracture mechanics could be applied to analyze the mechanical properties measured.There was almost no change in the work-of-fracture of the polycrystalline Al2O3, which was 20J/m2. On the contrary, the work-of-fracture of the composite increased with increasing temperature from about 50J/m2 at room temperature up to the maximum of 120J/m2 at 1100。C.This was caused by the re … More laxation of residual thermal stresses which was due to thermal expansion mismatch between Al2O3 and SiC, and was leaded the increase in the number as well as the pull-out length of bridging whiskers. The rationality of the argument was supported by the result of model calculation in which both frictional and pull-out bridging of whiskers were considered, and by the observation of pulled-out whiskers on a fractured surface.The bending strength of polycrystalline Al2O3 was constant (about 450MPa) below 1000。C, and gradually decreased with increasing temperature from 1200。C.On the contrary, the bending strength of the composite did not change from room temperature to 900。C, and steeply decreased from about 400MPa to about 300MPa at 1000。C.The bending strengths changed with correspond to the change in crack extension resistance, because it was experimentally confirmed that there was no crack healing and macroscopic plastic deformation in both materials below 1200。C.The significant decrease in the bending strength of the composite at 1000。C was caused by the decrease of crack closure stress owing to the relaxation of compressive residual thermal stress around whiskers which bridged natural flaws at room temperature. Less