Influence of MgO on microstructure and properties of mullite–Mo composites fabricated by pulse electric current sintering

Mullite–Mo (10 vol.%) composites and monolithic mullite were fabricated using a pulse electric current sintering technique. Both monolith and composites of mullite were sintered up to theoretical density at 1500°C within few minutes. MgO of 0.25 wt.% was added as a sintering aid to both the mullite and composites. Addition of MgO significantly increased the bending strength of the monolithic mullite and mullite/10 vol.% Mo composites to 441 and 634 MPa respectively. The apparent increase in the bending strength of the composites was attributed to the combinational effect of Mo and MgO present in the composites. The fracture toughness of the composites also increased from 2 to 3.9 MPa.m0.5 for the mullite/10 vol.% Mo composites, which was nearly twice that of the mullite. Crack-bridging and frontal process-zone elongation were expected to be the toughening mechanisms operated in these composites. The addition of Mo having high thermal diffusivity slightly increased the thermal diffusivity of the composites, because the 10 vol.% Mo particles were well dispersed and discontinuous in the matrix. Elongated mullite grains were observed for the composites without MgO, whereas the composites with MgO have a controlled microstructure.