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Room-Temperature Injection Molded, Pressurelessly Sintered Boron Carbide
Boron carbide (B4C) is a promising material for ballistic armor applications due to its extreme hardness and low density. However, utilizing these unique properties requires sintering B4C components to full density. This dissertation explores methods for sintering B4C to full near fully density without the application of external pressure. These pressureless sintering methods allow for B4C to be produced in a variety of geometries that would be impossible to achieve using current industrial methods. Highly loaded (51 vol. %) aqueous B4C suspensions were developed and injection molded at room temperature followed by pressureless sintering up to 2075 °C. Three different sintering aids (Y2O3, Al, and Al2O3) were used to aid the densification process. B4C parts were sintered to high density (up to 97.7 % relative density) and high hardness values (up to 3200 Vickers). The flexural strength of the samples was limited by grain pullout during polishing of the tensile surface; the strength was correlated to the maximum grain pullout flaw measured at the intersection of the tensile surface and the fracture surface (R2 > 0.98). Five compositions (undoped B4C, 5 wt. % WC, 5 wt. % WC + 10 wt. % Y2O3, 10 wt. % Y2O3, and 10 wt. % ZrB2) were studied for their relative density (up to 96.0 %), hardness (up to 3458 Vickers), and differential efficiency factor (up to 3.7 ± 0.7) during depth of penetration ballistic impact testing. The differential efficiencies of the samples were comparable with a commercially hot-pressed sample but were limited by their relative densities as opposed to their hardnesses, indicating that increasing densities through different sintering parameters could further improve performance relative to commercial armor alternatives. Potentially bonding B4C either in-situ or post-sintering is also explored as a method of overcoming the critical cracking thickness of B4C suspensions. Powder mixtures of B4C +Y2O3 were used during the sintering process, with low concentrations of Y2O3 yielding the most complete bond formation. Powder mixtures of SiC + C + Si with additional Si infiltration were used create bonds between B4C with a strong cohesive interface, but samples cracked due to a mismatch in thermal expansion coefficients between B4C and the bonding layer. This dissertation demonstrates the potential of room-temperature injection molding and pressureless sintering as an alternative means to produce highly dense B4C components with complex geometries not possible in traditional processing methods.