Reaction-bonded boron carbide for lightweight armor:
The Interrelationship between processing, microstructure, and mechanical properties
By Shmuel Hayun
With adequate understanding of processing parameters and resulting material properties, reaction bonding offers a relatively inexpensive alternative fabrication method for lightweight ceramic armor.
Since the dawn of history, weapons and armor have been in a life-and-death struggle. During the last three decades of the 20th century, a variety of ceramics, including aluminum nitride (AIN), aluminum oxide(Al2O3), boron carbide(B4C), silicon carbide(SiC), titanium diboride(TiB2), tungsten carbide(WC), and zirconium oxide(ZrO2), were investigated as armor materials.
Light ceramics particularly are attractive for personal well as land and airborne vehicle protection. The most commonly used ceramics are Al2O3, SiC, and B4C. Al2O3 is the most economical alternative, but its final protection solutions are heavier, because Al2O3 has the highest density and lowest ballistic efficiency of the three light ceramics. B4C is the hardest ceramic, but it undergoes an amorphization process at high impact pressures (such as with WC-cored bullets), which weakens the armor. Although SiC has no amorphization issues, its higher density (3.2 g/cm³) compared with B4C (2.52 g/cm³) limits its use.
We must consider some other points when choosing an adequate armor material. For instance, low porosity in the ceramic tile generally results in better ballistic performance. Moreover, smaller grain size increase ballistic performance. In addition, ease of fabrication and cost are of paramount importance in considering a particular material for armor applications. Full density of B4C or SiC is a prerequisite for achieving acceptable ballistic resistance, but can be attained only by hot-pressing fine powder (<2µm) in the presence of sintering additives at relatively high temperatures (>2,473). Further, production method strongly affects properties of the ceramic: hot-pressing tiles often results in a harder ceramic, which is optimal against a single hit, whereas reaction-bonding tiles provide better multibit performance. However, there is no clear correlation between quasi-static and/or dynamic mechanical properties and the ballistic behavior of ceramics. Nonetheless, some parameters, an elastic hardness, fracture toughness, and elastic modulus, are expected to have an influence.
Elevated hardness values are, by common consensus, crucially important for good ballistic resistance, because a material with sufficiently high hardness deforms or fragments a projectile upon impact. Moreover, ceramic fragments may continually abrade the projectile during the rest of the penetration process. It is, however, unclear if harder is always better, because one of the main failure modes of thin ceramic tiles is related to fracture from tensile stresses, which higher hardness does not improve.
Competition between high performance of carbide ceramics and the high cost of conventional fabrication methods led to the development of relatively inexpensive alternative fabrication methods capable of providing adequate mechanical properties. One approach is based on the reaction-bonding technique. According to this approach, ceramic powder (SiC, B4C, or B4C-SiC mixture) is mixed with free carbon, compacted, and subsequently infiltrated with molten metal. Molten metal reacts with free carbon and with carbon and with carbon that originates in B4C to form a ceramic composite. The resulting composite has high cohesive strength and elevated hardness values and is an effective ballistic impact-resistant material. Several variants of reaction-bonding processes, as well as the properties of final composites, are described in scientific journals and in patents.
One crucial drawback associated with reaction-bonded composites, however, is the fraction of the residual metal/alloy that significantly reduces the composite’s mechanical properties. This fraction strongly depends on initial porosity of the preforms and on the fraction of additional free carbon. Several approaches can reduce initial preform porosity, including partial sintering, use of multimodal powder mixtures, addition of elements that react with the alloy/metal to form stable phases, and addition of elements (e.g., titanium or iron) or compounds (e.g., TiC) that react with B4C and release additional free carbon.
Thus, knowledge of the effect of processing parameters on the microstructure of infiltrated composites, their static and dynamic mechanical properties, and microstructure-property relationships in necessary to understand and develop more efficient armor.