Mullite-refractory metal (Mo, Nb) composites

Two refractory metals were chosen as a second phase to fabricate mullite-metal composites: (a) molybdenum which is thermodynamically compatible with mullite up to 1650 °C and (b) niobium, a metal which is solid state incompatible with mullite giving rise to compounds such as NbO and Nb5Si3 which are both electrical conductors and more oxidation resistant than the Nb metal. The mullite-refractory metal electrically conductive composites with a content of metal (30–50 vol%) were obtained by a wet-processing route and subsequent hot pressing at 1650 °C. The effect of Mo particle size and volume fraction on the mechanical properties (σf, damage resistance, crack growth resistance i.e., R-curve behaviour, etc.) and wear resistance of the mullite–Mo composites have been assessed. Additionally these composites could be machined by electro discharge machining (EDM) technique. In the case of mullite–Nb system mullite–Al2O3–Nb5Si3–NbO composites showed electrically conducting properties. Its microstructural feature, mechanical properties (σf and R-curve behaviour) and EDM were studied and compared with the results obtained for the mullite–Mo system.