Sintering of boron carbide and boron carbide-silicon carbide two-phase materials and their properties

Boron carbide is a high-technological ceramic material (it is used for lightweight armor, neutron absorbers, wear pieces, etc.). Hot pressing (22000C, 40 MPa, Ar atmosphere) and recently high isostatic pressing, are the best know ways for industrial preparation of boron carbide items. Pressureless sintering using metallic, inorganic, B+C, additives is not successful, since, despite having a high density, impurities remain present. Pressureless sintering of boron carbide (or silicon carbide composite) using free carbon addition, produced by in-situ pyrolysis of a Novolaque-type phenol-formaldehyde resin (≅ 9 wt%), is now possible in industry. A promising new method is the use of organic precursors, e.g. polycarbosilane with a small amount of phenolic resin, giving CSi and C by in-situ pyrolysis; the resulting boron carbide ceramics have high density (> 92%) and contain no free carbon and a small amount of SiC (≅ 5 wt%). The mechanical properties of sintered B4C are close to those of hot-pressed; the SiC dispersion in a B4C matrix has no toughening effect. (orig.)