In situ synthesis and densification of submicrometer-grained B_4C-TiB_2 composites by pulsed electric current sintering

摘要

B_4C composites with 15 and 30 vol% TiB_2 were pulsed electric current sintered from B_4C-TiO_2-carbon black mixtures in vacuum at 2000 °C. Full densification could be realised when applying an optimized loading cycle in which the maximum load is applied after completion of the B_4C-TiB_2 powder synthesis, allowing degassing of volatile species. The influence of the sintering temperature on the phase constitution and microstructure during synthesis and densification was assessed from interrupted sintering cycles. The in situ conversion of TiO_2 to TiB_2 was a complex process in which TiO_2 is initially converted to TiB_2 with B_2O_3 as intermediate product at 1400-1700 °C. At 1900-2000 °C, B_2O_3 reacted with C forming B_4C and CO. The B_4C and TiB_2 grain size in the fully densified 30 vol% TiB_2 composite was 0.97 and 0.63 μm, combining a Vickers hardness of 39.3 GPa, an excellent flexural strength of 865 MPa, and modest fracture toughness of 3.0MPa m~(1/2).