Densification and mechanical properties of mullite/TiO2-coated B4C composites

Mullite/TiO2-coated B4C composites with up to 40 wt.% B4C were fabricated by coating B4C powders with Ti-chelate compounds via a sol-gel method prior to the hot-pressing of mullite/B4C mixtures at 1600 °C for 1 h. The effects of TiO2-coated B4C on the densification and the mechanical properties of mullite/B4C composites were investigated. TiO2 reacts with B4C at high temperatures to produce B2O3 and TiB2, both seems to be favorable for the densification of mullite/B4C composites. The formation of TiB2, besides, may lead to the generation of thermal residual stresses, which is deemed to be beneficial to the fracture toughness but detrimental to the flexural strength. The mechanical properties of mullite/B4C composites such as hardness, flexural strength and fracture toughness, are enhanced remarkably with the increasing addition of B4C. However, with a B4C addition more than 20 wt.%, the flexural strength tends to decrease gradually, probably due to the increasing residual stresses originated from the thermal mismatch of TiB2 with mullite and B4C matrix. B4C contents above 30 wt.% easily cause the aggregation of B4C particles in mullite matrix, which will lower the degree of improvement in fracture toughness.