Other armor ceramics and an outlook for ceramic armor
Density: 4.5 gm/cc
Hardness (Vickers): 2400-2800 HV1
Fracture toughness: 6.2 MPa*m^1/2
Compressive strength: 5700 MPa
Titanium diboride is an extremely hard, relatively dense compound of boron and titanium. Its mechanical properties are excellent; it boasts a great combination of high strength, high hardness, good fracture toughness, good stiffness, excellent thermal properties, and good chemical stability. Even taking all of that into account, its performance as a ballistic material is surprisingly outstanding. In vehicular armor systems, TiB2 out-performs silicon carbide and boron carbide on a weight basis. This means that a 10mm thickness of TiB2 is almost equivalent to a 20mm thickness of boron carbide!
Titanium diboride’s anomalously good performance characteristics have yet to be explained, and are not well understood. It’s plausible that they have to do with its thermal and molecular stability, combined with its excellent fracture toughness (more than double boron carbide’s), combined with microstructural effects and possibly even some degree of inherent ductility.
Unfortunately, TiB2 is in the same position boron carbide was in 50 years ago: It’s impossible to sinter it without the application of pressure; there are no reaction-bonded grades available anywhere; the powder raw materials are expensive; only hot-pressing and SPS are capable of producing dense ceramic parts. All of this means that TiB2 ceramic production is strictly low-volume and extremely expensive. At this point, it’s not even a “niche” material — it’s practically never used.
There is one exception: There are SiC-TiB2 composite ceramics which perform excellently and are economical.
Density: 2.39 gm/cc
Hardness (Vickers): 2040-2550 HV1
Fracture toughness: Not reported
Compressive strength: ~2000 MPa
Silicon-boron compounds were discovered at roughly the same time as boron carbide, in the same series of experiments, by the same person: Henri Moissan. At the time — that is, in 1900 — he assigned the silicon borides he had found the compositions SiB3 and SiB6. Unlike “B6C” these have largely held-up, though there’s some debate as to whether the original SiB3 was actually SiB4. (SiB3 is elusive and mysterious to this very day, but does seem to exist.)
The silicon borides — be they SiB3, SiB4, or SiB6 –all seem to exhibit similar mechanical properties: They’re strong, hard, and highly resistant to thermal shock. They’re slightly lighter than boron carbide, but at the same time are not quite as hard, they have a marginally reduced compressive strength, and they don’t perform quite as well as abrasives.
The silicon borides have properties that would make them excellent ceramic armor materials — particularly as SiB4 has a substantially lower density than boron carbide — but they are much more difficult to synthesize than boron carbide, and outside of a few exceedingly narrow industrial uses that take advantage of their thermal properties, they are very infrequently encountered. Because the SiB compounds have effectively no large-scale industrial uses, and powder production infrastructure would need to be built from scratch, silicon boride ceramic armor tiles would be very expensive. (Silicon carbide and boron carbide were initially selected for use in armor because they were, and still are, popular industrial abrasives that are readily available in very large quantities.)
Density: 1.89 gm/cc
Hardness (Vickers): 1300 HV1
Fracture toughness: 2 MPa*m^1/2
Compressive strength: Not reported
The beryllium borides are, in a word, superlative.
Be2B and Be4B are the lightest ceramic materials to ever undergo ballistic testing, at just 1.89 and 1.94 gm/cc, respectively. They are extremely light.
Be2B — the better of the two — also melts at just 1300°C, which means that it can be directly cast from a melt using industrial equipment that isn’t too outlandish or unusual. In this sense it is unlike any other armor ceramic.
In the LLNL Wilkins experiments, described elsewhere, Be4B and Be2B were also the best-performing armor ceramics by a wide margin. Indeed, they were the only ones to out-perform boron carbide by a truly significant margin.
There ends the superlative good. Now for the superlative bad.
Beryllium is among the most toxic non-radioactive elements; it’s right up there with arsenic and thallium. Inhalation of beryllium dust — and this includes the dust of beryllium compounds like BeO, Be2B, etc. — can be deadly toxic, even in very small amounts. It’s not a pleasant or swift death, either.
Because it’s so toxic, beryllium has very few industrial uses. In descending order of importance: (1) beryllium-copper tools containing 2% beryllium or less. (2) Niche, high-end aerospace superalloys. (3) Niche, high-end audio equipment that takes advantage of its extremely high sonic velocity and good acoustic properties. (4) Use in X-Ray machines and in various applications that take advantage of its transparency to X-ray wavelengths. (5) Beryllium oxide heatsinks.
Wherever beryllium is used, there are efforts underway to try and replace it with something else. BeO heatsinks are already largely obsolete.
Because it has so few industrial uses, beryllium is produced only in very small amounts. Total global production of beryllium metal and beryllium oxide ceramics is on the order of ~220 tons per year. Recall that silicon carbide is produced in quantities around 1M tons per year, and alumina is produced in quantities over 100M tons per year!
So, ultimately, because beryllium is superlatively scarce and superlatively toxic, it is exorbitantly expensive. A beryllium boride armor plate would cost well over $2000 per unit, just for the ceramic component. In part on account of the raw material cost; in part on account of the precautions and safety measures that need to be taken during production. Such a plate may also pose a hazard to its wearer. And this is why nobody uses beryllium.
Other borides and boron compounds
There are a wide variety of borides and boron compounds that might be appropriate for use in armor, or that have indeed already been investigated for use in armor. As a general rule, these are unattractive development prospects because they’re expensive, scarce, difficult to densify, or all of the above at once.
Boron suboxide (B6O), for instance, is a boron-based ceramic with extremely good mechanical properties. It is structurally similar to boron carbide in that both are comprised primarily of boron icosahedra (12-atom boron clusters) — but it doesn’t feature those three-atom carbon/boron chains between clusters, instead holding them together with shorter one-atom oxygen bridges, so it’s presumably more stable.
Slightly harder than boron carbide, no more dense, and evincing higher molecular stability, boron suboxide should surely be a very interesting ceramic armor candidate material. But there are two problems:
First, there’s no economical way to synthesize it. All known methods involve the reduction of the common boron oxide B2O3 with elemental boron. High-purity elemental boron is extraordinarily expensive; it’s roughly an order of magnitude more expensive than boron carbide powder.
Second, there’s no good way to densify that B6O powder and turn it into large ceramic tiles for armor plates. Even hot-pressing doesn’t work well or reliably for B6O.
Many other borides share one or both problems, to at least some extent. If there’s no good way to make the powder, or no good way to densify the powder once it’s made, then what you’re looking at is not a very practical material for further development.
Ceramic Armor Outlook:
The traditional armor ceramics — Al2O3, SiC, and B4C — have an effectively insurmountable economic advantage, as they’re produced in vast quantities for metallurgical and industrial applications, and their processing into dense ceramic parts is well understood. (In the case of boron carbide, if it isn’t exactly “well understood” just yet, it is getting easier and better understood all the time.) For the foreseeable future, it’s highly likely that those three ceramic compounds retain their dominance in the armor market — though modified and improved versions of those traditional ceramics might yet catch on.
For vehicular armor applications, and in certain special threat plates, a dual-phase SiC-TiB2 composite ceramic shows much promise. For high-end body armor, the improvement of doped and pressureless sintered grades of boron carbide — particularly if they are not so prone to the amorphization problem — is likely the most promising research and development direction.