In a sentence: Boron carbide is a dark gray ceramic which possesses exceptionally good mechanical properties — particularly in its low density and high hardness — and it has seen regular use in the highest-end armor plates over the past couple decades.
Boron carbide’s relevant mechanical properties are below:
Density: 2.52 gm/cc
Hardness (Vickers): 2400-3100 HV1
Fracture toughness: 2.5 – 4 MPa*m1/2
Compressive strength: ~3000 MPa
Nominally “B4C,” boron carbide exists in a wide range of possible compositions, from the carbon-rich extreme of B4.3C, to, on the other end of the spectrum, the boron-rich extreme of B14C. When boron carbide was first discovered in 1899 by Henri Moissan, he assigned it the formula B6C. Only in 1934, by Ridgway, was its wide range of compositions elucidated — and “B4C” became the preferred nomenclature, if for no other reason than because commercial boron carbide is generally far closer to the carbon-rich extreme than to the boron-rich extreme. Indeed, B4.3C is by a wide margin the most common type, and most commercial grades of boron carbide contain additional free carbon as an impurity.
The structure of carbon-rich boron carbide consists of a rhombohedral unit cell composed of eight B11C icosahedrons located at the corners of the unit cell and a three atom chain along the main diagonal. As the boron content of the solid solution is increased, the three-atom chain, initially of the configuration C-B-C, C-C-B, or C-C-C, is substituted with C-B-B or B-C-B chains. As the composition varies to less than 13.3 mol% C — in other words, as you move from the carbon-rich extreme to more boron-rich versions of boron carbide — B11C icosahedrons are replaced with B12 icosahedrons.
Like silicon carbide, boron carbide is wholly synthetic, and is generally produced from the natural oxide borax (B2O3) or from boric acid (H3BO3). There are several synthesis routes, but a popular and representative industrial method is much like the Acheson Process for the production of silicon carbide: Take boron oxide, add petroleum coke, add sufficient heat, and you’ll get boron carbide in a few hours or a couple of days. Because all synthesis routes are extremely dirty in terms of carbon monoxide output (that oxygen has got to bind to something else and go somewhere) boron carbide powders are generally produced in Asia.
Again like silicon carbide, boron carbide has historically been difficult to sinter into dense ceramic parts, so its primary industrial use was as an abrasive. Difficult, but not impossible: It has long been possible to produce dense boron carbide ceramic parts from powder via hot-pressing, but this is an inefficient, expensive, low-throughput technique — which, overall, has severely limited the use of boron carbide in armor and industry.
The 1980s saw the introduction of reaction-bonded boron carbide, which is discussed elsewhere. Through the years, various efforts have been made to increase the sinterablity of boron carbide, primarily via the use of additives such as boron, carbon, alumina, titanium boride, silicon carbide, aluminum, and others additives. These efforts came to fruition in the late 2010s, and high-quality pressureless-sintered grades of boron carbide, at a reasonable price, are now available at last. Our mutual friends at Alchemy Materials Sciences have a particularly fine grade, which we use in our Level IV plates.
Largely on account of its synthetic nature, boron carbide’s purity is usually reasonably high. Free carbon is the most common impurity — and it’s present in such quantities that it can fairly be said that most commercially-available “boron carbide” is a solid solution between boron carbide and carbon. Iron and aluminum impurities are also fairly common, but rarely rise to more than 0.5% by weight. Magnesium can be an impurity — high-quality boron carbide raw materials are sometimes prepared via B2O3 reduction in the presence of magnesium and a carbon source — but this is also very uncommon, and can be considered beneficial.
Aside from the reaction-bonded grades, there are a few variants of boron carbide that are worth mentioning. The pressureless-sintered grades are, necessarily, mixed-up with a number of proprietary polymeric and ceramic sintering aids. Then there are doped hot-pressed grades that are intentionally mixed with small amounts of silicon, aluminum, or magnesium. These light elements can replace carbon in the three-atom chain alongside the icosahedrons, and may therefore stabilize the crystalline structure. There are also grades that are boron enriched, to as far as B14C, for the same exact reason. As to why this might be necessary, suffice it to say that boron carbide exhibits unusual performance characteristics in armor.
Against all common small-arms threats, boron carbide is the best-performing ceramic available. Period. But against threats with tungsten carbide cores, it typically performs anomalously poorly. To such an extent that, in a recent report from noted armor author and researcher Ian Crouch, it was noted that “10mm-thick sintered boron carbide tiles with an UHMWPE backing, in combination with a standard Soft Armour Insert, exhibited the following V₅₀ values against three different 7.62mm AP rounds: 949 m/s against the APM2; 1,002 m/s against the B32; but only 598 m/s against the [tungsten carbide cored] FFV round.”
This was poorly-understood until 2003, when it was discovered that boron carbide’s molecular structure loses integrity when very high shock pressures are applied. It transforms from an ordered crystalline state into a rather chaotic and disordered amorphous state. This has come to be termed “shock induced amorphization.”
A mechanistic understanding of this phenomenon still eludes us. There are competing theories. One of the most compelling will take us back to a discussion of boron carbide’s molecular structure: Murray (2011) stated that boron carbide’s B-C-C chains are weaker than the B-C-B chains, leading to a reduced hugoniot elastic limit, and that the B-C-C chains are more likely to collapse under impact. C-C-C chains are similarly weak. In other words: The carbon-rich side chains that typify carbon-rich boron carbide are inherently unstable.
Subsequently, and perhaps unsurprisingly, it had later been reported that boron carbide can be stabilized via silicon doping, as this inhibits the development of the B11C(CCC) and B12(CCC) polytypes. In-depth research into amorphization suppression and the phenomenon of shock-induced amorphization are continuing. Present indications are that boron-rich boron carbide is less likely than common boron carbide to undergo amorphization upon impact. Further, doped boron carbide — and pressureless-sintered grades mixed with light metallic additives — may also be superior to common boron carbide.
(By “common” boron carbide, I mean the high-carbon, commercially-available, off-the-shelf grades.)
But in any case, notwithstanding how curious and interesting this all is, it’s important to put it into perspective:
– Against steel-core AP and ball rounds, boron carbide exhibits vastly superior performance when compared to silicon carbide and aluminum oxide.
– Tungsten carbide cored AP rounds are vanishingly rare outside of military operations; even then, only NATO, Russia, and China are known to issue them in any substantial quantity.
– The vast majority of Level IV and military-issue plates — whether they’re made of aluminum oxide or silicon carbide — also won’t stop tungsten carbide cored AP rounds. They just weren’t built for it.
So for most use-cases, and certainly for police/domestic use, the best ceramic material available is boron carbide — particularly in doped and pressureless sintered form. This is why we use it exclusively in our ceramic plates.