08/21/2003
Patriotic Ceramics
We must have paid our dues. Our 39 hours without power a few weeks ago apparently excused us from the Big Blackout of 2003 last week. We didn’t even have a power dip sufficient to put any of our clocks in the flashing 8s mode. With high humidity and temperatures in the 90s, I must join in commending New Yorkers for the manner in which they handled the crisis. To have come through that experience with so few deaths and no significant increase in criminal activity is downright remarkable.
Uncomfortable as it must have been for those jammed together waiting for ferries, trapped in elevators or having to walk up 20 or 30 flights of stairs, think of our troops in Iraq. With 120 degree temperatures and the bulky uniforms and equipment they carry, it must make our blackout seem like a lark. The batteries they carry that power their radio and other communications equipment also aren’t too happy in such heat and tend to die earlier than at normal temperatures. Hopefully, my colleagues and I at Rutgers have contributed a bit to the battery expertise in the military with our consulting and courses on batteries presented at various Army and Navy facilities.
But let’s concentrate here on the problems of protecting and lightening the load for our soldiers. The comic strip character Hagar the Horrible has effective protection with his shield and metal helmet. Those who follow the strip must marvel at how he survives the plethora of arrows that have penetrated through his primary defense system. Knights of old certainly had effective, if cumbersome protection with their full suits of armor. Tank crews were protected by heavy metal armor but the modern battlefield puts a premium on speed and flexibility. Heavy armor doesn’t promote either of these. An article in the August 11 issue of Chemical and Engineering news (C&EN) titled “Science Transforms the Battlefield” by Mitch Jacoby discusses the role that ceramics are playing in this regard.
When most of us think of ceramics, it’s probably about our coffee mugs or our Corning cookware that can stand up to baking and microwaving. For many years, ceramics have been taking the place of metallic items or components as research and development of tougher ceramics proceeds. One big advantage of ceramics is that they check in typically at about half the weight of their steel counterparts. For comparison, water has a density of 1 gram per cubic centimeter; the density of various types of steel are in the 7 to 8 range, while the density of most ceramics are around 4.
Ceramics have also been getting stronger. A key characteristic is something called “flexural strength”, the ability to stand up to pressure without rupturing. The ceramic in your coffee cup will probably stand up to 5 to 10 thousand pounds per square inch (psi). (Don’t try this on your cup, please – it’ll end up in smithereens!) A sintered and specially reacted form of silicon nitride, a compound of silicon and nitrogen, can handle 140 thousand psi. Replacing the steel armor of a tank with lighter ceramic shielding can make the tank faster, lower the fuel costs to run it and make it more maneuverable. Even more critical is the use of lighter ceramic armor in armored helicopters, in which the lighter armor can make it possible for the helicopter to take off with a load.
Ceramics also play a role in protecting our troops against incoming enemy missiles. Ceramics based on silicon nitride are used in the nose cones of Patriot missiles. The ceramic nose cones have to stand up to the intense heat generated at supersonic speeds. They also have to be transparent to microwave signals emitted and received in order to guide the Patriots to their moving targets.
When it comes to more intimate defense, the old question of body armor arises. Near the end of the Vietnam War, a joint effort by Los Alamos, Lawrence Livermore and Coors came up with body armor made from aluminum oxide. Thinking you might be wondering why the manufacturer of Brian Trumbore’s favorite beer was involved with ceramic body armor, I visited the Web sites coorstek.com and coorsjobs.com. Adolph Coors bought The Golden Brewery in 1880 and in 1910 decided to invest in Herold China and Pottery Company. This proved to be a crucial decision. In the 1920s, with Prohibition in effect, it was the sale of ceramics and, of all things, malted milk that kept Coors solvent!
Back to aluminum oxide, it’s an extremely important material. Aluminum is a very reactive element and, without the tough, thin film of aluminum oxide that forms on the surface of aluminum, items such as aluminum foil and pots and pans wouldn’t exist. Although aluminum oxide has a density of 4, the body armor developed for ceramic breast and back plates still weighs some 30 pounds. This puts a severe limitation on a soldier’s mobility and the C&EN article suggests it is only good for sentry duty.
Scientists have come up with a lighter alternative in the form of the compound boron carbide. This compound of boron and carbon has a density of only 2.5 and today’s soldier is likely to have a jacket with the lighter boron carbide body armor. This also will be better contoured to the wearer than the older rigid aluminum oxide-based armor. Panels of boron carbide are also used to fortify seats, walls and floors of attack helicopters and other military aircraft.
Actually, a suit of body armor is not just a fitted chunk of ceramic. The ceramic will shatter the incoming projectile into bits and pieces but it won’t do a good job of absorbing the energy of the impact. You need a backing of some sort. The backing typically is a composite of some sort that employs fibers that resist the impact. These fibers capture the shrapnel from the encounter of the bullet with the ceramic. DuPont’s Kevlar is one such material. The Kevlar is effective thanks to in part its tendency to line up in orderly arrays of tough fibers bundles.
The ideal body armor would stop all incoming projectiles before reaching the body of the warrior. Boron carbide does a good job of standing up to low energy projectiles such as bullets from a handgun. However, it’s not that good when it comes to higher- energy incoming objects. This puzzled researchers because the properties of boron carbide indicated that it should perform much better. Workers at Johns Hopkins and the Army Research Laboratory’s Aberdeen Proving Ground found the answer, published in the March 7 issue of Science.
Mingwei Chen, lead author of the Science article, and his colleagues looked at fragments of boron carbide recovered from an Army test facility with a high-resolution electron microscope. The boron carbide in the body armor is a crystalline material. What they found was that the impact of the high-energy projectile hitting the crystalline boron carbide, resulted in the presence of some very narrow bands of a glassy form of boron carbide. These bands were only a couple of nanometers wide. They determined that the structure in these tiny bands was disordered, in contrast to the ordered arrangement of atoms in the original crystalline boron carbide. This transformation from crystalline to glassy material has been seen in other materials but apparently not in any material as hard as boron carbide.
The glassy boron carbide is weaker than the crystalline form. As a result, the material in the body armor fractures along the narrow bands of glassy material. The challenge now is for the researchers to come up with some new approaches, perhaps changing the chemistry or additives or grain structure of the boron nitride to up the resistance of the material to higher energy impacts without forming glassy boron carbide.
Wouldn’t it be great if all this work on protecting our soldiers proved unnecessary because everyone decided to live in peace? The way things are going in the world today, I fear it’s more likely that all of us will need to wear body armor in the future. In the meantime, we can savor moments like that memorable closing shot of Shaun Micheel in the PGA.
Allen F. Bortrum
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