Fabricating toughened super-hard B4C composites at lower temperature by transient liquid-phase assisted spark plasma sintering with MoSi2 additives

Abstract

Toughened, super-hard B4C triplex-particulate composites were densified by spark plasma sintering with MoSi2 additives (5, 10, and 15 vol.%) at temperatures in the range 1750–1850 °C at which the reference monolithic B4C ceramics are porous. It is proved that MoSi2 is a reactive sintering additive that promotes densification by transient liquid-phase sintering, thus yielding fully-dense B4C-MoB2-SiC composites at relatively lower temperatures. Specifically, the MoSi2 first reacts at moderate temperatures (<1150 °C) with part of B4C to form MoB2, SiC, and Si. This last is a transient component that eventually melts (at ∼1400 °C), contributing to densification by liquid-phase sintering, and then (at 1500–1700 °C) reacts with free C present in the B4C starting powders to form more SiC, after which densification continues by solid-state sintering. It is found that these B4C-MoB2-SiC composites are super-hard (∼30 GPa), tough (∼3–4 MPa m1/2), and fine-grained, a combination that renders them very appealing for structural applications. Finally, research opportunities are discussed for the future microstructural design of a novel family of toughened, ultra-hard/super-hard multi-particulate composites based on B4C plus refractory borides and carbides.