How Transparent Aluminum Armor Works

Before we learn about the transparent armor of tomorrow, we're going to learn about the transparent armor of today's most basic component: glass. Understanding how glass forms will help us to later understand how we can see through an aluminum alloy.

Natural heat-producing processes like volcanoes and lightning strikes are responsible for creating various forms of glass, like obsidian, which is just super-heated sand or rock that has rapidly cooled. Humans once used this type of natural glass for decoration, money and the tips of spears or arrows, but before long, ingenious humans discovered their own process for making glass.

Carved into stone tablets about 5,000 years ago in Mesopotamia, the formula still holds true today: You take common sand (silica), soda and lime, mix it up, heat it, shape it into the desired form and cool it back down [source: Martin].

While the glass-making process is more involved than either Pliny or we describe it, this is the basic formula. So we know how it's made, but we don't know why you can see through it. The answer is entirely relevant to how aluminum armor -- which we'll talk about soon -- can be made transparent. When the ingredients of glass are heated, they melt and become liquid.

This change of states is important. Solids are rigid and opaque for a reason -- the molecules that form a solid are aligned with one another, creating a strongly bound lattice of sorts. When a solid turns into a liquid, the lattice loosens as the molecules randomly align with each other. This less orderly molecular structure becomes even more pronounced as the material becomes a gas. This flimsy arrangement of molecules allows light to pass through gases and liquids.

The process of heating and cooling the glass ingredients transforms them into a molecular stew and solidifies them in that same liquidlike state with all of the molecules unaligned with one another, enabling light to pass through the hardened glass. For more on this interesting subject, read What makes glass transparent?

Now we know what glass is, how to make it and why it's transparent. So why doesn't glass stop .50-caliber bullets or a handful of stones? Find out next.

Since its arrival thousands of years ago, glass has been used to make windows, mirrors, telescopes, food-storage containers -- all sorts of handy things. What glass doesn't do, though, is stop bullets from passing through it. Not on its own, at least.

As there's a basic formula for making glass, there's also a recipe for making bullet-resistant glass: press together two thick layers of glass and sandwich between them a layer of a clear plastic called polycarbonate. (For a more in-depth discussion, read How does "bulletproof'" glass work?) This process is called lamination, just like when a piece of paper is sealed between two heated pieces of plastic to make a durable driver's license.

When struck by a bullet, the layers of glass and polycarbonate absorb the energy of the bullet. The outer layer (the layer exposed to gunfire) will shatter and create a cracked spider's web illustrating the outwardly expanding energy as it is absorbed through the pane. The middle layer of polycarbonate, however, typically stands up to the assault and stops the bullet from penetrating the inner glass. The thickness of bullet-resistant glass varies, but generally ranges from less than an inch (2.5 cm) to several inches thick.

This bullet-resistant glass often serves as a transparent armor on embassy buildings, privately owned homes and limousines, Humvees and any other structure (such as a bank or a gas station) in which the occupant is at risk of taking gunfire.

Bullet-resistant glass is commonly -- and mistakenly -- called "bulletproof glass," which doesn't exist, as testified to by its near-helplessness to a .50-caliber round. Don't think that bullet-resistant glass cracks so easily though. It can fend off a round from an AK-47. Essentially, it's hard to say what thickness of bullet-resistant glass will stop what kind of projectile, because so many factors like distance, velocity, type of weapon, type of ammunition and even wind conditions play a part in what will happen when an object strikes an enforced surface.

With that kind of uncertainty, you could see why people reinforce bullet-resistant glass with another pane of the stuff, effectively making a 2-inch (5-cm) window now 4 inches (10 cm) thick. While this adds protection to an armored vehicle, it also leaves it more vulnerable to attack. For one thing, the vehicle will weigh more, thereby slowing it down and making it an easier target. The extra weight will also result in higher fuel use, which reduces the vehicle's range before it needs refueling.

Armor-piercing .50-caliber bullets are designed to pass through bullet-resistant glass. These bullets often have a copper shell casing, which encloses a penetrating shell made of a harder substance, such as depleted uranium or tungsten carbide (which is about as hard as diamond). The outer shell disintegrates on contact with armor, but the penetrating shell inside the copper bracket does just as its name implies -- it continues traveling through the armor and wreaking havoc on the once-protected space behind the armor.­

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­­Now we know how bullet-resistant glass works and how an armor-piercing .50-caliber bullet can pass through it. In the next section, we'll look at a much newer, lighter and stronger transparent armor that may one day find its way onto the battlefield: transparent aluminum armor.

Bullet-resistant glass has some clear drawbacks, namely, that it doesn't bar all bullets and that piling on the protective but heavy material merely slows the vehicle in question. So what can save the lives of soldiers and civilians under fire?

Researchers think the answer may be transparent aluminum armor, a finely polished ceramic alloy that's both lighter and stronger than traditional bullet-resistant glass.

Known commercially as ALON, transparent aluminum armor is made of aluminum oxynitride, a combination of aluminum, oxygen and nitrogen. Before it can end up as a hard transparent armor plate, it begins as a powder. This powder is then molded, subjected to high heat and baked, just as any other ceramic is baked. Once baked, the powder liquefies and then quickly cools into a solid, which leaves the molecules loosely arranged, as if still in liquid form. The resulting rigid crystalline structure of the molecules provides a level of strength and scratch resistance that's comparable to rugged sapphire. Additional polishing strengthens the aluminum alloy and also makes it extremely clear.

Now, just as bullet-resistant glass is made of three layers (two panes of glass and a middle pane of polycarbonate), so too is transparent aluminum armor. The three layers, consist of the following:

Not only can the aluminum armor deflect rounds from small-caliber weapons and still be more clearly transparent than bullet-resistant glass that's been shot, it also passes a much more important test -- it resists .50-caliber armor-piercing bullets and anti-aircraft weapons that typically use .30-caliber rounds. This is an impressive feat, especially since it's half the weight and thickness of traditional transparent armor.

Such a material could save lives and be incorporated in a wide range of vehicles -- everything from lightly armored trucks to low-flying planes, such as the C-130 Hercules or the A-10 Thunderbolt II that are vulnerable to ground fire.

Sounds great. So how come it's not being applied to armored vehicles and aircraft yet? Find out in the next section.

Over time, a regular pane of bullet-resistant glass will be worn away by windblown desert sand, not to mention incoming rounds from an assault rifle or shrapnel from roadside bombs. Transparent aluminum armor, on the other hand, is hardier. It stands up to .50-caliber rounds, is less affected by sand and fends off scratches more easily than traditional transparent armor.

If that's the case, why don't we replace the bulkier, less protective glass in use? Cost, for one thing.

­With a sense of optimism restrained by economic reality, the U.S. Air Force announced in 2005 the results of a series of tests conducted on the materia­l the previous year. While one affiliated researcher described transparent aluminum armor as being "light-years ahead of glass," he acknowledged that it was expensive and difficult to make in large sizes.

Heating and handling larger sheets of transparent aluminum require a sizable investment in infrastructure. For now, there's enough capacity to create 20-by-30-inch (51-by-76-cm) panes [source: CNRS]. That's large enough for most vehicular door windows but too small for front windshields. However, the cost of using these panes to replace existing glass is prohibitive. The bullet-resistant glass currently in use costs around $3 per square inch to produce, a steal compared to the $10 to $15 price tag for the same size piece of transparent aluminum [source: Air Force]. The additional costs of outfitting a vehicle with transparent aluminum armor would, in theory at least, pay for itself over time. Bullet-resistant glass would have to be replaced several times over, while the more expensive material has a much longer life span.

Developed by Raytheon and now commercially marketed by Surmet, transparent aluminum armor, though more expensive than bullet-resistant glass, is cheaper than sapphire, with which it shares common qualities, such as similar degrees of hardness and clarity. Sapphire is used in many different applications, such as semiconductors and bar code sensors, due to its ruggedness. As more industries switch to transparent aluminum for these needs, the price of production could drop to a point that makes it more feasible to build facilities capable of creating larger pieces.

While research continues on the development of this potentially life-saving material, armor-piercing .50-caliber rounds will continue making a mockery of standard bullet-resistant glass both on and off the battlefield.

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