Densification of boron carbide at relatively low temperatures by hot pressing and hot isostatic pressing

The poor sinterability of B4C limits its widespread application because both high temperatures and high pressures are required for a complete densification. Moreover, B4C suffers from a low strength and fracture toughness, possesses, however, a high potential because of its extreme hardness. Reaction hot pressing of B4C-WC-TiC-Si-Co mixtures resulting in B4C-TiB2-W2B5 composites of high density exhibit remarkable mechanical properties. The influence of hot isostatic pressing (HIP) on the microstructure and the mechanical properties is investigated in cooperation with participants of the COST 503 activities and related to the strengthening and toughening mechanisms. Difficulties during densification by HIP arise from the evaporation of adsorbed volatiles as well as from the strong swelling of the powder compact due to the sintering reaction. Several HIP cycle designs were tested in order to prevent the bloating of the capsule and to control internal stresses due to the misfit of the thermal expansion of the entire phases. In comparison to single phase B4C ceramics, bending strength was improved to 1030 MPa, KIc to 5.2 MPa/m, while hardness was comparable with HV1=38 GPa. Wear test were performed and related to the toughening mechanisms. (orig.) With 56 refs., 9 tabs., 64 figs