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Science & Technology

July 27, 2009
Volume 87, Number 30

Material Processing

From Sand To Ceramic

Intensive processing converts simple materials into sophisticated ceramics

Sophie L. Rovner

Washington Mills
SILICON FURNACE Washington Mills produces silicon carbide in outdoor furnaces. Each furnace contains a mix of silica sand and petroleum coke. Applying electricity to the furnace heats the mixture, which is covered with a blue tarp to retain heat and gases produced during the reaction (background). The tarp is removed when the reaction is complete (foreground). The furnace is then cooled and broken apart to recover silicon carbide.
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Washington Mills
ARMOR ANTECEDENTS Silicon carbide crude (left rear) is ground into powder before it can be used to make armor ceramic.
Washington Mills
QUALITY CONTROL Silicon carbide powder samples are collected for analysis to ensure consistent quality.

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The complex, highly engineered ceramics used in armor have their origin in some of the simplest ingredients on Earth. But these raw materials must be transformed through brute-force techniques before they can be crafted into armor components.

Armor ceramics are made from ceramic powder produced primarily by companies outside the U.S., says Michael J. Normandia, chief scientist for armor development at Ceradyne, based in Costa Mesa, Calif. The few remaining U.S. manufacturers include Washington Mills, of Hennepin, Ill., and Chicago-based Superior Graphite.

To make silicon carbide, Washington Mills uses the Acheson carbothermic reduction process, in which silica sand and petroleum coke are mixed together and placed in a large mound on the ground. Once fully built, the mound or "furnace" is charged with megawatts of electricity that heat the mixture to about 2,400 °C and convert it to a variety of silicon carbide products. After about 10 days, the furnace is fully "cooked," at which point it is broken apart to separate and recover the silicon carbide materials.

Boron carbide is produced by placing carbon and boric acid or boric oxide in an enormous crucible that contains electrodes the diameter of telephone poles, explains Richard A. Haber, a Rutgers University materials scientist whose research focuses on armor ceramics. Megawatts of electricity create an arc between the electrodes that melts the constituents to form boron carbide and carbon monoxide.

Both of these processes create large lumps of solid "crude" that then have to be crushed and ground down to very small particle sizes in order to create a ceramic powder.

Ceramic powder grains are extremely hard—somewhere around 9.5 on the Mohs scale, where diamond is 10, Haber notes. So grinding the powder grains down from, say, 5.0 to about 0.5 μm in diameter is "really energy intensive," he says. "And you have to be very careful that you're not introducing impurities from whatever you use to reduce its size, because you will wear the grinding material down. In many cases, you have to use the same materials to grind it."

Because the market isn't big enough, ceramic powder is generally not manufactured specifically for armor production, so armor ceramists must make do with what's commercially available, Normandia says. Exceptions include ceramic powders processed in Kempten, Germany, by Ceradyne's ESK division, he adds.

Armor ceramists carefully analyze the powder to identify any impurities it may contain. Impurities are difficult to remove, so researchers are trying to learn how they affect the behavior of the powder, as well as the performance of the finished ceramic, Normandia notes.

Ceramists mix the powder with additives or suspend it in a slurry to improve handling and processing. Then, by means of a method such as extrusion, pressing, or casting, they form the mixture into the desired shape, whether that's a flat tile for a vehicle or a gently curved panel for body armor. This "green body" component is then fired in a furnace at temperatures around 2,000 °C using one of several possible techniques.

The simplest method is "pressureless" sintering, in which the green body is simply popped into the furnace.

Alternatively, pressure can be applied to the green component during firing to enhance densification and lower the firing temperature. Pressure-sintering techniques include hot pressing, which uses a die press to aid densification, and isostatic pressing, in which the green component is squeezed by a gas in a pressurized vessel. Isostatic pressing can be used on far more complex shapes than the hot-pressing technique.

Another method is reaction sintering, which is also known as reaction bonding. For this process, silicon carbide or boron carbide powder is mixed with carbon and formed into the green component, which is then placed in a molten bath of silicon in a furnace. During firing, the silicon works its way into the porous component and reacts with the carbon to form silicon carbide bonds between the grains.

The goal of these firing processes is to produce a high-density ceramic by bonding the constituents together and reducing porosity between the grains.

For armor applications, "we worry a lot about how the ceramic grain boundaries come together," notes Christopher Hoppel, force protection research area manager in the Weapons & Materials Research Directorate of the Army Research Laboratory, in Aberdeen Proving Ground, Md. That's because, like impurities, pores or voids in the finished ceramic act as "damage initiators" that make a piece of ceramic more prone to fail when it's hit by a projectile, he says. "You want a very clean ceramic system to get the least defects you can."

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Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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