Evaluation of fracture strength of porous alumina sintered by hot isostatic pressing (HIPing)

Porous alumina ceramics were sintered by conventional sintering (CSing) and hot isostatic pressing (HIPing) without encapsulating at temperatures between 1200 degrees C and 1400 degrees C under pressures between 0.1 MPa and 200 MPa of Ar gas. Bending strength and Weibull modulus of sintered porous aluminas were evaluated by three-point bending tests, The HIPed samples have well-grown necks and roundish pore shape compared to the CSed ones due to enhanced surface diffusivity, which results in lower specific surface area of HIPed samples than that of CSed ones with the same porosity and pore size, As increasing gas pressure, the bending strength increased for porous materials with same porosity. The bending strength of samples HIPed under 200 MPa was about 30% higher than that of CSed ones having the same porosity of about 40%, The in crease in bending strength by HIPing was significant as decrease in sintering temperature. The enhancement of bending strength by HIPing is caused by well-grown necks that reduce stress concentration around necks. On Weibull plots, kinks were observed for HIPed samples. As increasing gas pressure, Weibull modulus for weak samples increased, while that of strong samples was constant, Enhanced surface diffusivity by high gas pressure causes deviation of defect distribution from single Weibull distribution. As a result, a kink appears on Weibull plots. The increase in Weibull modulus for weak samples is caused by removal of insufficiently grown necks by enhanced surface diffusivity.