Synthesis and spark plasma sintering of nano-structured ceramic materials for armour application

The use of extremely high hardness and light weight ceramic-based materials against ballistic threat is gaining popularity over the recent years. However, ceramics materials are often susceptible to tensile cracking and fragmentation due to its inherent brittleness which affects its performance as a protective material. On the other hand, high fracture toughness is highly desirable to minimise the fragmentation on impact but hardness and toughness tend to be mutually exclusive in most materials. Researches on complex structured bio-composites have shown that their outstanding mechanical properties could even exceed that of its constituent materials. Therefore, this leads to new field of research into biomimetic materials to produce a new class of complex hierarchical structure superhard composite with advanced toughening mechanisms. In the current research, various form of superhard nanostructured crystals of boron carbide, B4C, (nano-particles, nano-wires, nano-ribbons and nano-flakes) and boron suboxide, B6O, (star-shaped nano-plate) have been synthesized as potential basic building block for assembling into such complex structured material. Spark Plasma Sintering (SPS) is a novel powder consolidation process that utilises pulse current to achieve rapid densification at a relatively lower temperature than traditional sintering techniques. These reasons make SPS suitable for the consolidation of nano-structured materials that could minimise grain growth during sintering process and also retain the initial structure of the material. SPS was used in the consolidation of synthesized B6O nano-plates. Result from the SPS of B6O show high hardness of 34.8 GPa and fracture toughness of 4.0 MPa.m1/2. These results were some of the highest reported values compared to earlier literature and were only possible through the fast consolidation of SPS sintering technique. Using a novel reactive-SPS technique, the unique core-shell structure of boron carbide and boron nitride (B4C-BN) structured composite material was produced. High localised hardness of 56.0 GPa was registered using Berkovich indentation technique. The result also shows that an optimum amount of BN of up to 3% is desirable to achieve an even redistribution of stress to the load-bearing material structure. The toughness of the SPS-sintered B4C-BN shows a parabolic increasing trend when the sintering pressure increases from 30 MPa to 90 MPa. The increase in toughness was attributed to the control of porosity in the composite which was possible through the introduction of nitrogen and oxygen. The research findings show a clear relation in material toughness enhancement with SPS consolidation. This reactive-SPS technique could also be used to optimise other B4C-based composite materials.DOCTOR OF PHILOSOPHY (MSE