Preparation And Mechanical Properties Of Boron Carbide Composites

The unique combination of extremely high hardness and low density makes boron carbide an attractive structural ceramic material. So far, the application range of boron carbide has been greatly limited by its brittleness, low strength, poor sinterability, and the need of very high sintering temperatures.TiB2/B4C composite was fabricated with disperse TiB2 phase, which was introduced by in situ pressureless sintering of TiO2, glucose additives. Al8B4C7/B4C composite was fabricated by pressureless sintering technique. By using XRD analysis, scanning electron microscope (SEM), as well as mechanical testing, the microstructure and mechanical properties of the two kinds of composites were examined.It was shown that at a temperature of 2150℃in continuous pressureless sintering, the relative density of the specimens containing 25 vol.% TiB2 was 96%. At a temperature of 2100℃in isothermal pressureless sintering for 60min, the relative density of the specimens containing 25 vol.% TiB2 was 98%. The optimum sintering cycle included heating with a rate of 10℃/min to the optimum sintering temperature, followed by a 60 minute isothermal hold. The presence of fine TiB2 particles provides the material with high driving force for sintering and allows a significant reduction of sintering temperature.The apparaent activation barrier for sintering was found to be reduced in the presence of from 717kJ/mol for pure boron carbide to 266kJ/mol for boron carbide with 25 vol.% TiB2.It was found that the presence of the TiB2 phase has a major positive effect on the mechanical properties of boron carbide composite. Maximum values of bend strength (502MPa), fracture toughness (4.6MPam1/2) and Vickers hardness (33GPa) were observed in the specimens containing 15 vol.% TiB2. The strength and fracture toughness were found to be controlled by thermal mismatch stress generated by the TiB2 particles as well as by the microcracking of the particle-matrix interface.Maximum values of relative density (98.5%), fracture toughness (6.2MPam1/2) and Vickers hardness (39GPa) were observed in the Al8B4C7/B4C specimens containing 5 wt% Al. The fracture toughness was found to be controlled by stress relaxation and flaw passivation generated by the Al-B4C interface.