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From trees to high-performance ceramics

July 22, 2002

When a racing driver brakes, the discs and linings become red-hot. These parts are commonly made of carbon-fiber-reinforced carbon and are black at moderate temperatures. Car manufacturers and their suppliers would dearly like to extend the use of these special brake pads and other hard-wearing parts developed for racing vehicles to perfectly normal family cars - if only the cost was not prohibitive. This may now be possible using a new fiber-composite material that could almost be referred to as a “renewable ceramic”. Its carbon matrix is made of carbonized flax, hemp or wood fibers, which are then permeated with liquid silicon to form a high-strength, wear- and temperature-resistant silicon carbide.

“A great deal of know-how is needed even to produce the sheets of carbon fiber,” reveals Dr. Stefan Siegel, team leader at the Fraunhofer Institute for Ceramic Technologies and Sintered Materials IKTS. “Our starting material are boards of resin-coated plant fibers, which are produced for us by our colleagues at the Fraunhofer Institute for Wood Research WKI. These sheets then undergo a process called carbonization, in which the natural-fiber composite is heated in a nitrogen flooded furnace to temperatures of over 1000 °C.” This process has to be carefully controlled to ensure that the shrinkage of the material - i.e. its reduction in volume - proceeds homogeneously and without distortion. All that is left in the end is carbon; practically all other compounds present in the original material have decomposed and evaporated.” The black sheets resulting from the carbonization of the raw natural fiber can be used, for example, as structural elements or as an insulating lining for furnaces. Alternatively, this intermediate product can be shaped into components using standard industrial machining techniques such as sawing, drilling or milling.

But let us return to the ceramics. At temperatures above 1410 °C, silicon liquefies, and is soaked up like a sponge by the sheets of carbon (at present up to one square meter in size) or the pre-shaped components. The silicon undergoes a fast-acting reaction with the carbon, forming a fiber-composite ceramic material. This stage of the process, unlike carbonization, does not alter the shape or size of the parts. The post-processing of hard ceramic materials like silicon carbide is relatively difficult. The new “wood ceramic”, by contrast, can be shaped before it is hardened, thus presenting manufacturers who use machine tools to produce parts subject to high mechanical and thermal stress with a simple-to-use, low-cost alternative.

Fraunhofer-Gesellschaft