The Control of Intergranular Oxidation Brittleness in Silicon Carbides by Grain Boundary Engineering

摘要

On the basis of recent findings of the effect of grain boundary character on oxidation and the possibility of controlling oxidation-induced intergranular brittleness in a nickel alloy, the present work was performed to confirm the applicability of grain boundary engineering to the control of intergranular oxidation-induced brittleness in silicon carbides with dopants such as Mg, Al and P. Silicon carbide samples were produced by hot-pressing at 2373K and 40MPa for 300s from high purity β-SiC powder with a sintering aid of 0.5 mass% B. The grain boundary character distributions (GBCDs) in different types of silicon carbides were determined by using the FEG-SEM / OIM before an oxidation test. The weight changes associated with oxidation were measured as functions of oxidation time, temperature and oxygen partial pressure. Fracture behaviour of oxidized specimens whose GBCD had been analyzed was examined by 3-point bending tests at room temperature. The OIM analyses of the GBCD revealed that the frequency of random boundaries, which were preferentially oxidized, and the frequency of low ∑, particularly ∑3 boundaries being the most resistant to oxidation could be changed by selecting the dopant element. It was found that the oxidation-induced embrittlement could be effectively suppressed by decreasing the grain size and increasing the frequency of oxidation-resistant special boundaries. Thus we confirmed that oxidation induced brittleness can be controlled by grain boundary engineering in silicon carbides.