Body Armor

Soldiers deployed in Iraq and Afghanistan face danger every day, but thanks to high-tech ceramics developed since the late 1960s, they have protection against numerous ballistic threats. In best-case scenarios, these ceramics can actually shatter a bullet upon impact, leaving nothing more than possibly a bruise on the warrior.

"As threats change, armor has to change," says James W. McCauley, a materials scientist at the Army Research Laboratory who has studied and helped develop many of the armor ceramics in use today.

Unlike steel, which has long been used as body armor for the military, ceramics have the advantage of being lightweight. They also have a very high degree of hardness—in fact, ceramics are some of the hardest materials known—as well as other desirable properties for ballistic protection.

Most of the ceramic powders used in the U.S.'s body armor are made in Europe or China, says Richard Haber, director of the Ceramic & Composite Materials Center at Rutgers University. However, there is still one manufacturer, Washington Mills Ceramics, in the U.S. Firms that make armor ceramics for the Department of Defense must produce lightweight plates that can withstand more than one ballistic impact, and they must also minimize cost and weight.

How to reduce weight is a primary issue, McCauley says, because it has a direct impact on the mobility of the soldier as well as on the stress placed on the warrior's body. Cost is a factor in the military's standards, given that every solider fighting in today's wars is outfitted with this protection.

The three main types of ceramics used to make body armor are boron carbide, silicon carbide, and aluminum oxide. A fourth type of ceramic is aluminum oxynitride—known as ALON—which can be used to make transparent armor for applications such as goggles and windshields.

The powders used to form these ceramics were once primarily used in abrasives because of their hardness, which is also a useful property for armor, Haber explains. Increasing the hardness of these ceramics, he says, is achieved by grinding them into finer powders. But the more the powder particles are reduced in size, the greater the amount of impurities that are introduced from the grinding equipment. This requires additional cleaning processes. Nanoscale ceramic powders are not currently economically viable, Haber says, and other nanomaterials such as carbon nanotubes cannot yet be made affordably in sufficient quantities to be practical.

Producing ceramic powders requires high temperatures. In the common Acheson process, silicon dioxide and graphite or coal-distilled coke starting materials are converted into silicon carbide electrochemically at very high temperatures. The process uses a huge amount of electricity and also produces a huge amount of carbon dioxide, Haber says.

Making boron carbide requires temperatures of 3,000 °C and also produces CO₂, but the ceramic is made in a melt process instead of by electrochemical means. "The melt cools, crystallizes, and is crushed," Haber says. Aluminum oxide and ALON powders are processed via similar high-temperature methods.

With the powders in hand, three manufacturing processes are routes to the final ceramic armor plates, Haber explains. All three processes involve high temperature and are "analogous to what you do in art class" to make ceramics, says Ronald Hoffman, a research physicist at the University of Dayton Research Institute.

Hot-press manufacturing involves shaping the materials in a mold and heating them up to 2,200 °C under pressure. With direct sinter, the materials are shaped and subjected to high heat, but without pressure. And in reaction bonding, the materials are formed to shape, chemically reacted, and heated to about 1,400 °C. These different processes enhance certain performance characteristics of the ceramics and result in cost differences.

Silicon carbide is a little bit softer than boron carbide, which is "the second-hardest material after diamond. Boron carbide literally shatters the bullet," says Marc A. King, president of Ceradyne Armor Systems, a supplier of U.S. military ceramic body armor.

Ceradyne uses the hot-press process at its two manufacturing facilities, located in California and Kentucky, King says. The addition of a composite material on the back side of the ceramic plate—usually a type of high-molecular-weight polyethylene—acts as a catcher's mitt for the bullet fragments, he says. Ceradyne provides both stand-alone ceramic armor and ceramic armor plates that are fitted into vests made of Kevlar, a para-aramid synthetic fiber that is also bullet resistant.

At ALON manufacturer Surmet, "we synthesize our own aluminum oxide powder, form it into a shape we want, then put the material through a series of heat treatments," says Lee Goldman, the firm's vice president of R&D. The material is then cut, ground, and polished to give it transparency. The armor's ability to protect against bullets is based on the overall laminate design, he says, and can withstand armor-piercing rounds and improvised-explosive-device blasts. "We've tested a number of threats," Goldman says.

"A lot of chemical and physical improvements have been made in ceramic manufacturing," Hoffman says. "Improvements in powder chemistry and purity, along with particle-size control coupled with efficient densification control, have led to superior ceramic articles."

Although there is no such thing as "bulletproof," today's armor ceramics provide an unprecedented level of protection and mobility for troops on the ground.

Chemical & Engineering News

ISSN 0009-2347

Copyright © 2011 American Chemical Society