Issue Date: July 27, 2009
If you're deployed in Iraq or Afghanistan, you're no doubt the epitome of a modern soldier. But if you hear the hiss and thud of incoming bullets and live to tell the tale, you may well owe your life to a material invented many thousands of years ago.
That material is ceramic. As far back as 24,000 B.C., humans were already crafting ceramic figurines from clay and other materials and firing them in kilns, according to the American Ceramic Society. Ceramic was first deployed on the battlefield in World War II, when it served to protect a Sherman tank, says Ronald J. Hoffman, a physicist who specializes in armor design and testing at the University of Dayton Research Institute, in Ohio. Ceramic armor was employed again during the Korean and Vietnam Wars, but it wasn't utilized in significant volume until the first Gulf War, which began in 1991.
Today, ceramic is widely used as a component in soldiers' body armor, says Christopher Hoppel, force protection research area manager in the Weapons & Materials Research Directorate of the U.S. Army Research Laboratory (ARL), in Aberdeen Proving Ground, Md.
Ceramic armor is not yet common on military vehicles, although it can be found on some U.S. combat vehicles including the Stryker and the Armored Security Vehicle. However, it is gaining ground in this application as a replacement for the increasingly heavy metal armor needed to defeat modern threats such as improvised explosive devices (IEDs), says Stan Lyons, manager of the Phoenix-based Composite Technology Center in the Security & Survivability Division of BAE Systems, a major defense contractor and producer of ceramic armor. Ceramic armor also protects some helicopters, fixed-wing aircraft, and sea vessels.
Ceramic armor weighs about 50% less than traditional steel armor, Hoffman says. The lighter weight means that a vehicle protected with ceramic armor is quicker and more maneuverable or can carry more personnel or equipment, including weapons, Hoffman says.
Other positive attributes of ceramic armor include the high compressive strength and high hardness of the ceramic, says Michael J. Normandia, chief scientist for armor development at Ceradyne, a company based in Costa Mesa, Calif., that produces millions of pounds of armor ceramic each year.
At the same time, this hardness can make it difficult to machine the material to precise dimensions, says Michael P. Bakas, a materials scientist at Idaho National Laboratory, an Idaho Falls facility that researches, develops, and tests ceramic armor.
Armor ceramic has other shortcomings. For one, it's far more difficult to make a complex shape from ceramic than from metal, he adds.
Armor ceramic also suffers from low tensile failure strength, which means the material is brittle, Normandia notes. And ceramic armor typically costs 50–200% more than a metallic equivalent on a weight basis, although he says this cost differential is somewhat mitigated by the lighter weights required for ceramic armor.
Because of ceramic's potential benefits, researchers are hard at work trying to understand and improve its properties and reduce its cost. The quest for providing soldiers with better protection—whether that's more protection for a given weight or reduced weight for a given amount of protection—continually drives this research onward, Hoppel says.
When it's used in armor, ceramic is partnered with other materials that complement its strengths and weaknesses. The entire assemblage, which is designed to be able to withstand multiple hits, can have as many as a dozen layers in all and is referred to as a "system."
"When used right in a system, ceramics are basically impenetrable," Normandia says. "But the desire to get the lightest-weight system possible to defeat the threat means you have to lighten the system components, which then invokes failure."
The ceramic layer can take the form of a single monolith or a mosaic-like array of tiles shaped like squares, cylinders, or hexagons, Normandia says. In the mosaic structure, the "grout" between the individual tiles can be metal or a polymer such as rubber or polyurea.
The ceramic itself is typically aluminum oxide, silicon carbide, or boron carbide, Hoppel says. Other ceramics that have been used in armor include titanium diboride, aluminum nitride, silicon nitride, and tungsten carbide.
These ceramics are far harder than those used to make consumer goods and in some cases are nearly as hard as diamond. They are designed to break up an impacting bullet or other projectile. In so doing, some of the ceramic also fractures, Lyons says.
Other layers in the armor minimize this fracture and catch the bullet and ceramic fragments, Lyons says. They also deflect and absorb energy from the impact to minimize injury to a soldier or damage to a vehicle.
These additional armor layers might consist of a fiber-reinforced composite made of an aramid like Kevlar, glass, graphite, or ultra-high-molecular-weight polyethylene; rubber; or a metal such as aluminum, titanium, or steel.
Achieving the right mix of materials requires extensive testing and consideration of multiple criteria. The Army, for example, evaluates ceramic armor based on ballistic performance, weight, durability, availability, and cost, Hoppel says. ARL works with its government and industrial partners to develop, produce, and evaluate ceramic armor and then passes the manufacturing processes and designs to industry.
Evaluation of the performance of ceramics is by no means straightforward. "Although there are standards for testing ceramic armor systems, each particular research institute or company might have their own favorite way of evaluating the ceramic materials used within that system," Normandia says. "There is no accepted standard nonballistic set of experiments to compare ceramics." Furthermore, "the results you get from one particular test to rank the performance of ceramics do not necessarily translate to how they will perform in a given application or in another application," he adds. So researchers are trying to develop a universal test for evaluating a ceramic's performance, and they're also looking for methods to quantify damage.
Making matters even more complicated is the fact that "evaluating ceramics in isolation is not the same as evaluating them in the system," Normandia says. "It's very difficult to choose one ceramic over another without actually putting them in a system."
Once a particular system design has been selected, other evaluation techniques are required for quality control of the ceramic component during manufacturing. Richard A. Haber, a Rutgers University materials scientist whose research focuses on armor ceramics, is working on nondestructive evaluation methods that could be used to make sure that each piece of armor ceramic that is produced is free of internal defects. These techniques include ultrasound and microfocus X-ray radiography.
Some of the defects that arise during manufacturing can be traced to impurities in the powder used to make ceramics, says Haber, who heads Rutgers' Ceramic & Composite Materials Center. Because impurities are difficult to remove from the powder, his group is trying to learn which impurities might be turned to advantage and which are detrimental.
Haber is also studying the additives that are intentionally mixed with ceramic powder before it is pressed into a specific shape and fired in a furnace. The firing process consolidates and "densifies" the material, reducing porosity that could diminish the finished ceramic's performance.
"We're working on how additives are put into batches," Haber explains. "We're worrying about the uniformity of mixing in small levels of additives. If you're mixing together a powder and 1% of an additive, you don't want to have that 1% all in one area and 0% in the other area."
Haber is also looking at new firing technologies that might improve the densification process, including spark plasma sintering and microwave sintering.
Another focus of research concerns the dimensions of grains in ceramic powder and finished ceramics. Grains in ceramic powder are typically less than 1 µm in diameter. During the firing process, the grains grow, typically reaching about 20 µm or less in diameter. Growth is held in check with the help of additives or through careful control of pressure and temperature; otherwise, excessively large grains could reduce the ceramic's hardness and compressive strength, Normandia says.
"It's a common belief that the smaller the grain size in the finished armor ceramic, the better the performance," at least down to the size of a few micrometers, Normandia says. There's a lot of research into submicrometer—nanosize—materials because of this belief, he adds. But when the grain size drops too far, he says, the ceramic's fracture toughness likely declines, and the grain boundary surface area increases dramatically.
That's probably bad, but no one knows for sure. In many ways, ceramic behavior and properties are still baffling. "Higher hardness and compressive strength are good," Normandia says. "Higher fracture toughness? Unsure. Billions of dollars have been spent on developing higher fracture toughness materials, and they have not necessarily translated into better armor ceramics."
One technique that's being used to better understand the ballistic behavior of the material is computational modeling. ARL is applying this approach to gain "insight into how ceramics respond to a projectile impact dynamically and how to improve that performance," Hoppel says. "We're trying to understand better the dynamic properties of ceramic materials." During certain high-pressure impacts, for example, some ceramics undergo a phase transition that gives the material a small amount of plasticity; this property can be exploited to enhance armor performance.
"We're trying to achieve improved damage tolerance, which ties back into the manufacture of the ceramic," Hoppel adds. Most of the damage that results when a ceramic is hit by a projectile originates from defects introduced during the manufacturing process. "So driving down defects in the ceramics generally increases their performance," he says. "Other things we're looking for are higher hardness, higher dynamic compression strength, and lower density."
To address some of these goals, ARL is experimenting with modification of grain boundaries at the nanoscale with additives to enhance plasticity, development of better backing and cover layers to improve damage tolerance and reduce loads on the ceramics, and creation of ceramic composites, Hoppel says.
Some of the composite materials being studied include pairings of titanium diboride with aluminum oxide, or silicon carbide with either boron carbide or aluminum nitride, Normandia says.
Production and processing is another focus of development. Some researchers are experimenting with chemical vapor deposition to produce armor ceramic. This process eliminates the need for additives, resulting in a ceramic without impurities, Normandia says.
Manufacturers are also trying to work out how to make larger pieces of armor ceramics—on the order of 3 sq ft, Hoffman says. At that size, pieces tend to crack or splinter apart during the production process.
One of the hottest areas of research concerns transparent ceramic armor, according to Hoffman. Armored windows currently are made of multilayered glass or polycarbonate panels, Bakas says. Coating or even replacing these materials with transparent ceramic can reduce the weight and boost the hardness of armored windows.
Some ceramic windows consist of polycrystalline magnesium aluminate spinel or aluminum oxynitride (called AlON). Others are made of a single large crystal of sapphire (aluminum oxide), Haber says.
The windows can be produced by growing single crystals from a molten bath known as a melt. Alternatively, the windows can be made via hot pressing. In this process, pressure is applied to the window during firing to improve densification and rid it of pores that would prevent it from being transparent. Pressure can be applied by means of a press and die or by gas.
These methods are costly, complicated, and limited in terms of how large and complex the window shape can be, Bakas says. So "there's a lot of demand for processes and technologies that are cheaper and give you more flexibility in the kind of ceramic shape you can make," he says.
One solution is to move to a process in which the window is fired in a furnace without the application of pressure. To achieve this goal, Bakas is experimenting with sintering aids that are mixed with the ceramic powder before firing. In the heat of the furnace, these aids form a liquid phase between the powder grains that assists densification.
One sintering aid Bakas is testing is aluminum orthophosphate. With this additive, along with some other process modifications, he can now produce nearly transparent AlON armor from aluminum oxide and aluminum nitride without applying pressure during firing. The additive even cleans up after itself: After forming a liquid phase, it gradually decomposes into P2O5 gas, which escapes, and alumina, which is incorporated into the ceramic, so it leaves no residue behind that could cloud the armor.
Although modern-day ceramists and armor manufacturers have made tremendous progress since the days of antiquity, the bar keeps getting higher for ceramic armor—as in every other military endeavor. "You come out with a new piece of armor," says Haber. "Then the bad guys come out with a new weapon. So armor has to constantly be improved to match the next type of weapon."
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