Transparent aluminum armor
A palm-sized .50-caliber round fired from a Barrett M107 rifle travels through the air at 823 meters per second, and it can take out a lightly armored vehicle from more than a mile away. It will reach its destination faster than the sound of it being fired.
A weapon like this deals not only a mortal blow to opposing forces, but also a crippling psychological blow. Even the most dedicated opponent will consider taking up a new vocation after witnessing the human carnage caused by a direct hit from a .50-caliber round fired from about 17 football-field lengths away. In fact, the diameter, or caliber, of these bullets is so large that it's generally the biggest shell you'll see before you're dealing with an explosive projectile fired by cannon. These weapons can destroy parked aircraft; penetrate an inch of armor from almost 37 meters. What they can't do is penetrate transparent aluminum armor.
Engineers here are testing a new kind of transparent armor -- stronger and lighter than traditional materials (half as heavy and thick as bullet-resistant glass) -- that could even stop a round from an anti-aircraft gun. The Air Force Research Laboratory's materials and manufacturing directorate is testing aluminum oxynitride – ALON™ -- as a replacement for the traditional multi-layered glass transparencies now used in existing ground and air armored-vehicles.
The test is being done in conjunction with the Army Research Laboratory at Aberdeen Proving Grounds, Md., and University of Dayton Research Institute, Ohio.
We’ll learn what transparent aluminum armor is how it's made and whether we can expect to see it on the battlefield any time soon. First though, we'll take a look at how regular glass is made, and how you can beef it up to stop bullets.
Understanding how glass forms will help us to later understand how we can see through an aluminum alloy. You take common sand (silica), soda and lime, mix it up, heat it, shape it into the desired form and cool it back down.
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 liquid like state with all of the molecules unaligned with one another, enabling light to pass through. 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? 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.
Bullet-resistant glass shows the marks from an insurgent sniper shot fired at a U.S. military post in Iraq's Anbar province.
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.
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.
ALON: 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, known commercially as ALON, a combination of aluminum, oxygen and nitrogen, a finely polished ceramic alloy that's both lighter and stronger than traditional bullet-resistant glass.
ALON DATASHEET
Aluminum Oxynitride Approximate Formula: Al23O27N5
Aluminum Oxynitride Powder Description:
Aluminum Oxynitride is an extremely durable crystalline material with excellent optical transparency in the near ultraviolet wavelength.
Aluminum Oxynitride Powder Typical Chemical Properties Available:
Typical chemistries available (99.0%, 99.9%, 99.99% and 99.999%)
Aluminum Oxynitride Powder Typical Physical Properties Available:
a) Single crystals, sintered pieces, rods, targets, pellets, whiskers, coarse grains for fillers, fine powder and foil.
b) Granules and powder are available from as coarse as 60 mesh (250 microns) and finer down to a 0.5 micron product. The finest commercial grade available in tonnage lots has an average agglomerated Particle size of approximately 1.8 microns.
Aluminum Oxynitride (ALONTM) Powder Typical Applications:
High thermally conductive ceramics, translucent ceramics, Si-Al-O-N compounds, high temperature materials, additives, heat sinks, power and multichip modules, refractories, ceramic armor, break rings, coatings, insulators, heat radiation plates, optoelectronic devices, metal matrix composites, thermally conductive filler, IC packages and substrates
Aluminum Oxynitride or ALON optical ceramic is transparent material, developed and patented by Raytheon, which is very similar to sapphire, being comprised mostly of Al2O3 with a small amount of additional nitrogen. This nitrogen addition has the effect of producing a cubic material whose optical and mechanical properties are isotropic. Importantly, this means that it can be produced by powder processing methods, which are scalable to larger sizes, and at lower prices than can be achieved by the single crystal growth techniques that are used to grow sapphire. Furthermore, its isotropic properties make it much easier to grind and polish than sapphire.
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:
1. An outer layer that's exposed to gunfire and made of baked aluminum oxynitride
2. A middle layer of glass
3. A rear layer of polymer backing
The new armor combines the transparent ALON™ piece as a strike plate, a middle section of glass and a polymer backing. Each layer is visibly thinner than the traditional layers.
ALON™ is virtually scratch resistant, offers substantial impact resistance, and provides better durability and protection against armor piercing threats. In a June 2004demonstration, an ALON™ test pieces held up to both a .30 caliber Russian M-44 sniper rifle and a .50 caliber Browning Sniper Rifle with armor piercing bullets. While the bullets pierced the glass samples, the armor withstood the impact with no penetration.
This ability to add the needed protection with only a small amount of material is very advantageous, said Ron Hoffman, an investigator at University of Dayton Research Institute. 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?
The Future of Transparent Aluminum Armor
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 1st Lt. Joseph La Monica, transparent armor sub-direction lead said,” The substance itself is light years ahead of glass," adding that it offers "higher performance and lighter weight and also acknowledged that it was expensive and difficult to make in large sizes. In extensive testing, ALON™ has performed well against multiple hits of .30 caliber armor piercing rounds -- typical of anti-aircraft fire, Lieutenant La Monica said. Tests focusing on multiple hits from .50 caliber rounds and improvised explosive devices are in the works.
The lieutenant is optimistic about the results because the physical properties and design of the material are intended to stop higher level threats. "The higher the threat, the more savings you're going to get," he said. "With glass, to get the protection against higher threats, you have to keep building layers upon layers. But with ALON™, the material only needs to be increased a few millimeters."
"When looking at higher level threats, you want the protection, not the weight," Mr. Hoffman said. "Achieving protection at lighter weights will allow the armor to be more easily integrated into vehicles." Mr. Hoffman also pointed out the benefit of durability with ALON™. "Eventually, with a conventional glass surface, degradation takes place and results in a loss of transparency," Mr. Hoffman said. "Things such as sand have little or no impact on ALON™, and it probably has a life expectancy many times that of glass. It all comes down to survivability and being able to see what's out there and to make decisions while having the added protection." The Army is looking to use the new armor as windows in ground vehicles, like the Humvee, Lieutenant La Monica said. The Air Force is exploring its use for "in-flight protective transparencies for low, slow-flying aircraft. These include the C-130 Hercules, C-17 Globe master III, A-10 Thunderbolt II and helicopters.
While some see the possibilities of this material as limitless, manufacturability, size and cost are issues the lab is dealing with before the armor can transition to the field, the lieutenant said. "Traditional transparent armor costs a little over $3 per square inch. The ALON™ Transparent Armor cost is $10 to $15 per square inch," Lieutenant La Monica said. "The difficulties arise with heating and polishing processes, which lead to higher
costs. But we are looking at more cost effective alternatives."
Lieutenant La Monica said experimenting with the polishing process has proven beneficial. "We found that by polishing it a certain way, we increased the strength of the material by two-fold," he said.
Currently, size is also limited because equipment needed to heat larger pieces is expensive. To help lower costs, the lieutenant said researchers are looking at design variations that use smaller pieces of the armor tiled together to form larger windows.
Lowering cost by using a commercial grade material is also an option, and the results have been promising. "So far, the difference between the lower-grade material and higher purity in ballistic tests is minimal," he said.
Lieutenant La Monica said once the material can be manufactured in large quantities to meet the military's needs, and the cost brought down, the durability and strength of ALON™ will prove beneficial to the warfighter. "It might cost more in the beginning, but it is going to cost less in the long run because you are going to have to replace it less," he said.
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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