In a sentence: Aluminum oxide, or alumina, is an ubiquitous ceramic material that dominates the vehicular and body armor markets — but it is heavier than alternative ceramic materials like boron carbide and silicon carbide, and it doesn’t perform quite as well.
Aluminum oxide, or alumina, is an ivory-colored ceramic which is by a very wide margin the most commonly-used ceramic in armor applications. This is for three key reasons:
– Alumina powder is produced in the millions of tons per year, throughout the world. In 2019, China produced 73 million metric tons, and Australia produced 20 million tons. (To put this into perspective, silicon carbide is the second most popular armor ceramic. Total global production of SiC stands at roughly one million metric tons per year.) Just between those two countries, that’s almost a trillion pounds of alumina. Most of this vast quantity is used towards the production of aluminum metal and its alloys — for drink cans, airplane fuselages, etc. But there’s so much alumina powder out there, at any given time, that it is a true commodity, and is thus very cheap.
– Second, alumina is a very unusual armor ceramic from a processing perspective. Unlike the others, alumina is very easy to work with. Unlike carbides and borides, alumina can be sintered at low temperatures, in some cases under 1000°C. And again unlike carbides and borides, alumina can be sintered in air, for it is a stable oxide and doesn’t require any protection from the highly reactive oxygen in the atmosphere. Sintering boron carbide and silicon carbide requires expensive tanks of argon or complicated vacuum furnaces, but alumina tile production needs none of that. Between the lower temperatures required and stability such that it can be sintered in air, alumina ceramic production is relatively cheap and easy.
– Finally, alumina is, perhaps surprisingly, the hardest and strongest metal oxide — and also among the very lightest. To be sure, there are other oxides that are almost as abundant and as easy to sinter as alumina — such as iron oxide (Fe2O3), titanium oxide (TiO2), and magnesium oxide (MgO) — but these are, without exception, much softer than alumina, much heavier than alumina, or both at once. None of them are suitable for use in armor.
But alumina, though still relatively heavy, is eminently suitable for use in armor. Its relevant mechanical properties are as follows:
Hardness (Vickers): 1200-1600 HV1
Fracture toughness: 3-5 MPa*m1/2
Compressive strength: >2000MPa
Purity in most armor grades is 85 – 99.9%. Other oxides such as MgO, CaO, and especially SiO2 (as mullite) are common impurities. These impurities are all quite soft and all exhibit low densities: MgO at 3.6 gm/cc, CaO at 3.34 gm/cc, and mullite at roughly 2.9 to 3.4 gm/cc. Low-purity grades of alumina therefore exhibit correspondingly lower density and lower hardness and strength than high-purity grades.
Against AP rounds, high-purity grades of alumina are strongly preferred, as the impure grades tend to exhibit poor performance on account of their impaired mechanical properties. Against ball rounds such as M855, the jury is still out, for it could be that the impurity-derived reduction in density, which is a positive factor, outweighs the reduction in hardness.
Impure alumina ceramics are not a novelty. 85%-pure alumina armor ceramics were initially pioneered by CoorsTek as AD85, and that grade has seen much use in the years since its introduction.
It’s worth noting that the mechanical properties listed above only apply to highly-pure alumina. Impure grades such as AD85 can exhibit hardness values well under 1000 HV, and densities as low as 3.4 gm/cc.
When it’s not a natural impurity, MgO is often intentionally added to alumina powder as a sintering aid, at 0.25 to 1% by weight. In this role, it helps to reduce the temperatures required for sintering, inhibits grain growth, and reduces porosity.
Speaking of intentional doping, there are a number of modified alumina ceramics with altered mechanical properties. The most noteworthy is ZTA — or zirconia toughened alumina — which contains roughly 15-20% ZrO2 in addition to sintering aids. It is tougher than baseline alumina, but at the same time is substantially heavier and more expensive, so it is not used in body armor. It has been tested towards vehicular armor applications, but the results obtained with it were puzzling and inconclusive. (Suffice it to say that in some experiments it performed much better than unmodified alumina; in other experiments, it performed worse.) In any case, ZTA — along with all other modifications of alumina — is not particularly relevant towards ceramic body armor applications at this time.
The chemical nature of alumina is fairly straightforward: Alumina is aluminum in its highest oxidation state. It is thus incapable of further oxidation, and it’s difficult to reduce, so it’s very stable. Although there are many crystalline phases of alumina, they all transform to alpha-Al2O3 (“corundum”) at high temperatures — so, following sintering, corundum is almost always the sole or predominant phase in dense ceramic parts.
Alumina dominates the ceramic armor market. Virtually all armored vehicles that use ceramic armor utilize alumina. It’s also exceedingly popular in body armor, particularly in low-cost plates for police and civilian use. It is much less popular, indeed is nonexistent, in high-end armor plates for frontline military units. Here’s why:
– Generally, as a hard and fast rule, all armor ceramics perform similarly when used at the same thickness. Against a steel-core AP round, an 8mm-thick silicon carbide tile won’t perform much better or worse than an 8mm-thick alumina tile.
– Silicon carbide weighs roughly 20% less than alumina. Boron carbide weighs roughly 35% less.
– Corollary: You can make an armor plate with an alumina tile that weighs 4 pounds, or you can make an armor plate with a silicon carbide tile at 3.2 pounds, or you can make an armor plate with a boron carbide tile at 2.6 pounds. After bonding the ceramic to a backer, etc. — and all else being equal  — total armor plate weights would be roughly 6.75 pounds, 5.2 pounds, and 4.5 pounds, respectively. The silicon carbide and boron carbide options are substantially lighter, but all of those plates would perform similarly if not identically against steel-core AP and ball rounds.
– Silicon carbide costs 2-5x as much as high-purity alumina. Boron carbide costs 5-10x as much.
…So the cost savings are hugely significant, and the lower weight of carbide ceramic armor plates doesn’t always justify their much higher cost. With boron carbide, you could end up paying as much as 20x more  for a plate that’s perhaps as much as 40% lighter. Sometimes this is justified. Sometimes it isn’t.
There is, however, another factor: The hardness of alumina peaks at 1600 HV, and is often somewhere within the 1100 to 1400 HV range. This is frequently softer than the tungsten carbide penetrators used in modern AP rounds, which are typically at 1200 – 1800 HV. For this reason, and because it’s axiomatic that the ceramic strike-face needs to be harder than the bullet penetrator it must defeat, alumina-based ceramic armor plates can exhibit poor performance against tungsten-cored threats. Silicon carbide doesn’t share this weakness; against tungsten-cored threats, silicon carbide is simultaneously lighter and better-performing than alumina. This is another reason militaries around the world prefer to utilize silicon carbide in general-issue body armor. (Boron carbide, though extremely hard, has problems of its own against tungsten-cored threats, discussed at length elsewhere on this site.)
- – To distinguish it from ternary oxides, such as the lithium aluminate AlLiO2. As a general rule, ternary compound space is largely unexplored, and it may be that interesting surprises await.
- – This is rarely the case. Usually, when boron carbide and silicon carbide are used, they are paired with higher-end, lighter, and more costly materials.
- – See