In a sentence: Silicon carbide is a hard, dark gray, chemically-stable ceramic that, in recent years, has become the workhorse ceramic armor material in high-end and military body armor systems.
Silicon carbide was accidentally discovered in 1891 by by Edward Goodrich Acheson, formerly an assistant to Thomas Edison, during experiments on the synthesis of diamonds in electric arc furnaces. After heating a mixture of clay and coke to very high temperatures, Acheson noticed blue crystals on the furnace’s carbon electrode, and had assumed that the substance was a reaction product between the carbon he was using in his diamond experiments and the alumina (or corundum) contained in the clay, which is why SiC was named carborundum. It quickly became apparent that carborundum contains neither corundum, nor aluminum, nor oxygen, but the name stuck.
Several years after this initial discovery, Acheson developed a process for the industrial production of SiC, and this Acheson Process is still in use today. It involves heating silica sand and coke in an electric furnace. The initial application of heat ignites a self-sustaining endothermic reaction, which results in reaction temperatures which often approach 2600°C. Relatively pure SiC is produced via this process — often in very large crystals that need to be milled down to size — but the fact that it liberates tremendous quantities of CO and CO2 gas has restricted its use in Western nations in recent years. Thus the majority of today’s silicon carbide powder production takes place in China.
Besides the old Acheson Process, there are other methods for producing silicon carbide powder — but, generally, every industrially useful method involves mixing SiO2 with a carbon source and applying heat.
Silicon carbide has, historically, been difficult to sinter into bulk ceramic parts. For most of the decades since its discovery, its major industrial use was as an abrasive in powder form. When bulk ceramic parts are made, they generally have the following mechanical properties.
Density: 3.2 gm/cc
Hardness (Vickers): 2200-2800 HV1
Fracture toughness: 3-4 MPa*m^1/2
Compressive strength: 3500-4900 MPa
As for purity: Because of its wholly synthetic nature, silicon carbide, unlike alumina, is generally highly pure. The powders available are invariably 98%+ pure. And just as you can easily figure out a star’s temperature from its color, you can gauge SiC’s purity from its color: Absolutely pure SiC is colorless and transparent. As it picks up small amounts of metal impurities, it turns yellow and then green, and grows opaque. As it picks up more impurities, it turns dark blue, and then grey/black. The first crystals that Acheson saw were blue. The SiC powders utilized in armor ceramic production are invariably black, on account of iron, aluminum, and free carbon impurities. Even if white or green powders are used, the dense ceramic part is typically black, on account of impurities picked up during the sintering process.
Sintering aids used in the production of SiC ceramics include Al2O3, rare earth oxides, elemental carbon, and boron. These are generally added at under 1% by weight.
There is no widespread industrial use of SiC variants, nor are there very many SiC variants to speak of, but a mixture of silicon carbide and boron carbide has recently been introduced as an armor ceramic. For example, Saint Gobain sell a 70% SiC–30% B4C composite ceramic called SB70, alongside a 50:50 blend called SB50.
Along similar lines, our mutual friends at Alchemy Materials Sciences sell a composite ceramic comprised of silicon carbide and titanium diboride, specifically intended for armor purposes.
SiC is a semiconductor, and, for use in electronic devices, SiC is sometimes doped with elements such as boron or phosphorous to modify its electronic, thermal, and physical properties. This has no relevance towards armor. But it does bring us to…
SiC’s chemical and crystalline nature is simple in some respects, maddeningly complex in others. To briefly review: SiC is comprised of two light elements that prefer covalent, sp3 bonding. Each Si atom is tetrahedrally bonded to four carbon atoms, and each carbon atom is similarly bonded to four silicon atoms, so it can rightly be said that SiC is made up of alternating C4Si and Si4C units.
The primary types of SiC are beta-SiC, which is cubic, and alpha-SiC, which is a catchall term for all of the hexagonal or rhombohedral types. There are hundreds of SiC polytypes all called alpha-SiC. Further discussion of them would be highly technical and is outside the scope of this short review; suffice it to say that all of the alpha polytypes appear to perform similarly in ceramic armor applications, and have apparently similar mechanical properties.
The cubic (beta) form transforms to a hexagonal (alpha) form when subjected to temperatures above 1700°C — and, as a rule, sintering temperatures for SiC ceramics are higher than that, so the very vast majority of the SiC ceramics used in armor are alpha-SiC. The beta form may have slightly superior mechanical properties , and polycrystalline beta-SiC can be optically transparent if impurities are kept low and grain size is optimized, but, for all practical intents and purposes, it is never encountered in ceramic armor.
Silicon carbide (as alpha-SiC) has become increasingly popular in body armor over the past 25 years. It had started as a sort of also-ran: Not nearly as cheap as alumina — so that cheaper ceramic was initially preferred in vehicular and low-end body armor applications — nor quite as light as boron carbide — so that lighter ceramic was initially preferred in high-end body armor applications.
But as SiC sintering technologies are advancing quickly, alumina’s cost advantage is growing ever more narrow. The old rule of thumb was that SiC would cost at least 3-5x more than aluminum oxide. Today, it’s much closer to 2.5x alumina’s price, on average, and decreasing. A multi-curve, pressureless-sintered, 10×12” SiC tile, suitable for a Level IV body armor plate, can now be had for well under $100.
Alumina can also exhibit unpredictable performance against tungsten carbide-cored threats, largely on account of alumina’s low intrinsic hardness. SiC doesn’t share this weakness; it can be trusted to always be harder than a WC-Co AP bullet penetrator, and perform reliably.
And, despite great hardness, it turns out that against tungsten carbide-cored projectiles, boron carbide has performance issues of its own. These are discussed in much greater detail in our boron carbide section. Here, suffice it to say that boron carbide’s crystal structure breaks down at very high pressures. The pressures which catalyze this phase transition can be attained upon ballistic impact — particularly at very high velocities, or when boron carbide is impacted by a projectile with a tungsten carbide core. This can lead to premature, seemingly anomalous failure. SiC doesn’t share this weakness, either; its chemical structure is rock solid.
So silicon carbide, though neither the lightest nor the cheapest of the common armor ceramics, is the only one that performs reliably against all small-arms threats. Steel core? Lead core? Tungsten core? None of those are a problem for silicon carbide.
It is also quite light. Boron carbide is only 20% lighter. In practice, the difference in weight between a boron carbide body armor plate and a silicon carbide body armor plate usually amounts to less than a pound.
For improved reliability, at reduced cost, at just slightly increased weight, many of the world’s militaries have, lately, come to prefer SiC as their first choice in ceramic armor.
– It’s unclear whether or not this is the case. By some accounts, the hardness and strength of pure beta-SiC are inferior to those of alpha-SiC. In any case, they’re all quite similar.