Synergistic methods for the production of high-strength and low-cost boron carbide
Wiley, Charles Schenck
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Boron carbide (B₄C) is a non-oxide ceramic in the same class of nonmetallic hard materials as silicon carbide and diamond. The high hardness, high elastic modulus and low density of B₄C make it a nearly ideal material for personnel and vehicular armor. B₄C plates formed via hot-pressing are currently issued to U.S. soldiers and have exhibited excellent performance; however, hot-pressed articles contain inherent processing defects and are limited to simple geometries such as low-curvature plates. Recent advances in the pressureless sintering of B₄C have produced theoretically-dense and complex-shape articles that also exhibit superior ballistic performance. However, the cost of this material is currently high due to the powder shape, size, and size distribution that are required, which limits the economic feasibility of producing such a product. Additionally, the low fracture toughness of pure boron carbide may have resulted in historically lower transition velocities (the projectile velocity range at which armor begins to fail) than competing silicon carbide ceramics in high-velocity long-rod tungsten penetrator tests. Lower fracture toughness also limits multi-hit protection capability. Consequently, these requirements motivated research into methods for improving the densification and fracture toughness of inexpensive boron carbide composites that could result in the development of a superior armor material that would also be cost-competitive with other high-performance ceramics. The primary objective of this research was to study the effect of titanium and carbon additives on the sintering and mechanical properties of inexpensive B₄C powders. The boron carbide powder examined in this study was a submicron (0.6 μm median particle size) boron carbide powder produced by H.C. Starck GmbH via a jet milling process. A carbon source in the form ofphenolic resin, and titanium additives in the form of 32 nm and 0.9 μm TiO₂ powders were selected. Parametric studies of sintering behavior were performed via high-temperature dilatometry in order to measure the in-situ sample contraction and thereby measure the influence of the additives and their amounts on the overall densification rate. Additionally, broad composition and sintering/post-HIPing studies followed by characterization and mechanical testing elucidated the effects of these additives on sample densification, microstructure development, and mechanical properties such as Vickers hardness and microindentation fracture toughness. Based upon this research, a process has been developed for the sintering of boron carbide that yielded end products with high relative densities (i.e., 100%, or theoretical density), microstructures with a fine (∼2-3 μm) grain size, and high Vickers microindentation hardness values. In addition to possessing these improved physical properties, the costs of producing this material were substantially lower (by a factor of 5 or more) than recently patented work on the pressureless sintering and post-HIPing of phase-pure boron carbide powder. This recently patented work developed out of our laboratory utilized an optimized powder distribution and yielded samples with high relative densities and high hardness values. The current work employed the use of titanium and carbon additives in specific ratios to activate the sintering of boron carbide powder possessing an approximately mono-modal particle size distribution. Upon heating to high temperatures, these additives produced fine-scale TiO ₂ and graphite inclusions that served to hinder grain growth and substantially improve overall sintered and post-HIPed densities when added in sufficient concentrations. The fine boron carbide grain size manifested as a result of these second phase inclusions caused a substantial increase in hardness; the highest hardness specimen yielded a hardness value (2884.5 kg/mm²) approaching that of phase-pure and theoretically-dense boron carbide (2939 kg/mm²). Additionally, the same high-hardness composition exhibited a noticeably higher fracture toughness (3.04 MPa•m¹/²) compared to phase-pure boron carbide (2.42 MPa• m¹/²), representing a 25.6% improvement. A potential consequence of this study would be the development of a superior armor material that is sufficiently affordable, allowing it to be incorporated into the general soldier’s armor chassis.