Study of diamond/mullite composites by sol/gel and hot press sintering methods

Abstract

A study has been conducted into the synthesis of 10wt% diamond/ mullite composites through two methods: First through the hot press sintering of alumina and silica in stoichiometric composition for 3:2 mullites (mullite formed in situ) at 1400, 1450 and 1500oC. Second through the sol/ gel process. The sol/ gel method only provided the basis for future development with no further discussion of the results while the hot press sintering method yielded composites with residual cristobalite and corundum phases. Achieved densities of the composites were 93.7, 94.6 and 95.8% of the theoretical density with respect to sintering temperatures of 1400, 1450 and 1500oC for compact samples by the first method. Hardness – measured by Vickers indentation – of the composites decreased with increase in temperature with 15.5 ± 0.33GPa achieved at the lowest sintering temperature investigated. The decrease in hardness was attributed to the structural degradation of diamond to non-diamond carbon forms with increase in temperature as observed from Raman spectra of each of the composites. X-ray traces showed an increase in the mullite content with increase in temperature. The fracture toughness of compacts initially hot press sintered from alumina and silica in stoichiometric composition for 3:2 mullite with no diamond added decreased with increase in sintering temperature with 4.75 ± 0.10MPa·m1/2 achieved at the lowest sintering temperature investigated. Further discussion to the structure and physical properties is presented.