Advancing fracture behavior of boron carbide with arc melt processing
Creators: name: Christian, Kent
Publisher: name: No Publisher Supplied
Year: 2022

description: Among armor materials, boron carbide is of great interest due to its high hardness and low density. The hardness of boron carbide is surpassed only by diamond and cubic boron nitride, and its density is significantly lower than that of metals, and low even among other hard ceramics. Boron carbide represents a strong contender for ballistic armor materials.However, under high shear stresses, it has been found that boron carbide undergoes a sudden and unexpected brittle fracture. For this reason, many recent studies have undertaken efforts to improve boron carbide’s fracture resistance to better take full advantage of its ballistic capability. Several solutions to the limitations of boron carbide have been proposed in recent years, notably among them atomic doping and compositing with secondary phase particle reinforcement.In this thesis, a novel arc melt processing route for boron carbide is explored. The process is modeled off of commercial production methods for pure boron carbide, and builds on experimental studies regarding addition of dopant elements and secondary phases into the boron carbide system. In the proposed process, boron carbide and dopant/reinforcement powders are mixed and reacted in the liquid state at very high temperatures under an ionized electric arc in a “pre-reaction” process. Melted ingots of boron carbide are then crushed and milled into fine powder, which is sintered to produce dense doped boron carbide composites. Along with this processing technique, a multiscale approach to improve fracture behavior is pursued by exploring the addition of several different types of secondary reinforcement phases into the microstructure of doped boron carbide using these processing methods.Comparisons will be made between the results of this new method and the traditional processing methods, for both doped and composite systems, in terms of their chemistry, microstructure, mechanical properties, and susceptibility to high-stress amorphization.Comparisons will show that arc melting of boron carbide allows for successful silicon doping using either SiB6 or silicon powder, with resultant chemistry and structure virtually indistinguishable between the two. Despite the tendency for high temperature reactions during arc melting, TiB₂ successfully remains unreactive with boron carbide and boron/silicon dopant powders, although several other boride/carbide additives are shown to react. Materials produced through arc melt processing are shown to have exceptionally low oxygen content, which becomes beneficial for sintering after further processing steps.High-speed circulatory milling is shown to be effective at reducing melted boron carbide material into a fine, sinterable powder which maintains the chemistry achieved during melting, effectively making a “pre-reacted” powder. Greater than 99% of the theoretical density can be achieved by hot press sintering with this powder in a very short amount of time, and can be done using B₄C, doped boron carbide, and doped composite boron carbide.Results also show that sintered materials produced from this melt-processed powder have a much lower concentration of secondary oxide phases compared to similar samples produced with commercially available powder. The melt-processed powder also experiences much less grain growth compared to traditional samples. Finally, resistance to amorphization under stress is shown to be greatly increased by the boron/silicon doping, while hardness and toughness are shown to be increased by TiB₂ addition.