Field Assisted Sintering of Nanoporous Boron Carbide with Hierarchical Microstructure

摘要

Due to its unique combination of properties including high hardness, low density, high strength, thermal stability and high neutron absorption, boron carbide is a potential candidate for various aerospace, nuclear and other applications involving extreme environment. However, the current applications of boron carbide are largely limited due to its intrinsic brittleness as a result of strong covalent bonding. While the most common toughening strategy for boron carbide is crack deflection and micro-crack toughening by introduction of secondary phases such as titanium diboride, a novel toughening strategy by creating nanocrystalline boron carbide and introducing nanoporosity has been demonstrated to increase boron carbide's ability to deform by grain boundary sliding accommodated by nanopore compression in a quasi-ductile manner, potentially leading to enhancement in fracture toughness. In this study, scalable manufacturing of boron carbide and its composites with hierarchical microstructure features, such as micro-grains, secondary reinforcement as well as nano-grains and nanoporosity, is attempted using field assisted sintering technology (FAST) to yield repeatable and tunable microstructures. Compared to traditional sintering methods such as hot-pressing, FAST exhibits multiple benefits including shorter sintering time, lower sintering temperature and a more uniform heating process which can yield finer grains and better control over the microstructure of sintered samples. Using FAST, multiple samples with different grain size distribution and material compositions were successfully sintered to high density. Subsequent mechanical testing and microstructure inspection were carried out, providing information regarding the effects of different hierarchical microstructures on properties including elastic modulus, hardness and fracture toughness of FAST sintered boron carbide.