Fracture toughness in some hetero-modulus composite carbides: carbon inclusions and voids

Structure and mechanical characteristics of dense ceramic composites synthesised by reactive hot pressing of TiC–B₄C powder mixtures at 1800–1950°C under 30 MPa were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM and EDX). The results show that during hot pressing solid-phase chemical reaction 2TiC + B₄C = 2TiB₂ + 3C has occurred with final products like TiB₂–TiC–C, TiB₂–C or TiB₂–B₄C–C hetero-modulus composite formation with around one micrometer size carbon precipitates. The fracture toughness depends on the amount of graphite precipitation and has a distinct maximum K1C = 10 MPa m^1/2 at nearly 7 vol.-% of carbon precipitate. The fracture toughness behaviour is explained by the developed model of crack propagation. Within the model, it is shown that pores (voids) and low-modulus carbon inclusions blunt the cracks and can increase ceramic toughness in some cases.