Boron carbide is a material well known for its high hardness, wear resistance, low density and high chemical stability. This combination of properties has made it a very attractive option for several high performance applications, including wear resistant components, abrasive media, armor plates and corrosion resistant components. The production of highly densified compacts without grain growth, avoiding a consequent degradation of mechanical properties, is one of the main challenges in the  sintering technology of boron carbide.

In this work, we have used high pressure/high temperature processing as an alternative route for the sintering of B4C based composites. The samples were produced from starting mixtures of microsized boron carbide powder (H.C. Starck, grade HS) with different metal powders (Ti, Nb, W, Fe, Co, Ni, Si), used as sintering additives (10 mol%). The compacts were produced at high pressures (7.7 GPa) and high temperatures (up to ~ 2000°C) using a toroidal-type high-pressure chamber. The heating and cooling rates were about 300 °C/min and the samples were kept at the highest temperature during 5 min.
After surface polishing with diamond paste down to 0.25 mm, the Vickers microhardness of the sintered bodies was measured. Their phase composition and microstructure were investigated by X-ray diffraction (XRD), and scanning electron microscopy (SEM) with X-ray energy dispersive spectrometry (EDS).

The XRD results showed, besides the presence of B4C, the formation of new boride phases in all the cases, except for the sample produced using Si as sintering additive. In that case the formation of silicon carbide was observed. For all the samples, no diffraction peak associated to the starting metal phase was observed, which indicates that it was totally consumed during the high pressure reaction sintering process.
The microstructure observed by SEM and the EDS results are consistent with that conclusion. In addition, no significant grain growth was observed for the B4C phase and the metal boride/carbide phases were formed along the B4C grain boundaries.
The values ​​of the Vickers microhardness varied according to the metal used as sintering additive. The lowest (~ 17 GPa) and the highest (~ 24 GPa) mean values were obtained for the samples produced using Co and Ti, respectively.

The results showed that the different systems reached advanced sintering stages. The compacts preserved the high hardness of the ceramic matrix even with the insertion of metal sintering agents, which resulted in the formation of new hard boride/carbide phases.