Boron Carbide is one of the hardest materials known, ranking third behind diamond and cubic boron nitride. It is the hardest material produced in tonnage quantities. Originally discovered in mid 19th century as a by-product in the production of metal borides, boron carbide was only studied in detail since 1930.
Boron carbide powder (see figure 1) is mainly produced by reacting carbon with B2O3 in an electric arc furnace, through carbothermal reduction or by gas phase reactions. For commercial use B4C powders usually need to be milled and purified to remove metallic impurities.
In common with other non-oxide materials boron carbide is difficult to sinter to full density, with hot pressing or sinter HIP being required to achieve greater than 95% of theoretical density. Even using these techniques, in order to achieve sintering at realistic temperatures (e.g. 1900 - 2200°C), small quantities of dopants such as fine carbon, or silicon carbide are usually required.
As an alternative, B4C can be formed as a coating on a suitable substrate by vapour phase reaction techniques e.g. using boron halides or di-borane with methane or another chemical carbon source.
Figure 1. Boron nitride powder and shapes (photo courtesy of CERAM Research Ltd)