Tiny ceramic crystals pack a big punch by protecting carbon composites from their enemy—oxidation
 

Even if carbon weren’t the stuff of life, you’d have to judge it an incredibly versatile substance. Take three examples. In the form of graphite, it serves as an industrial lubricant. In the form of diamond, it crowns things from drill bits to rings. And in the form of carbon composites, it brakes aircraft to a safe landing.

Making carbon composite brakes and other transportation systems last longer and perform better is a goal of SIUC’s Center for Advanced Friction Studies (see below). A new composite material developed by associate scientist Khalid Lafdi uses tiny ceramic particles to protect carbon from oxidizing at high temperatures. 

An expert in carbon science, Lafdi was recently named a scientific consultant to NATO in the field of carbon-carbon composites, one of the University’s leading research areas. Tough and lightweight, these composites are made of carbon fibers embedded in a carbon matrix. They can handle the braking of a jet because they transfer and store heat efficiently, moving it quickly from the brake surface to the interior. This high thermal conductivity allows carbon to hold up to temperatures that would melt steel.

But carbon has an Achilles’ heel: get it hot enough, and it will burn. For example, some degree of burning takes place with every jet landing, reducing the lifespan of the brakes.

At high temperatures (greater than 500°C), the atoms at the edges of a layer of carbon composite, or at the edges of a clump of carbon molecules, are highly reactive—prone to combine with hydrogen, oxygen, or any number of other atoms. If an oxygen atom latches on, it starts a pattern of burning (rapid oxidation) that zips right across the layer or the clump. Once the first carbon atom oxidizes, the next atom in becomes exposed to oxygen, and so forth, in a chain reaction. 

"When carbon oxidizes, the whole layer goes. It burns straight through," says Lafdi. The result is a carbon part that’s full of holes, susceptible to peeling and mechanical failure.

Ceramic materials such as silicon carbide, a very hard crystalline substance, are used to make oxidation-resistant coatings for some carbon composites. Ceramics can withstand high heat (special ceramic tiles protect the space shuttle on re-entry, for example). But the ceramic coating and the underlying carbon composite expand and contract at different rates, leading to crack formation in the brittle ceramic. That necessitates yet another coating, this time of glass, to seal the cracks.

Other techniques, such as chemically depositing silicon carbide inside the carbon composite, also have drawbacks, and the ceramic is still susceptible to fracturing at high temperatures. As Lafdi explains, cracking can occur between the crystals in any large-grained ceramic material.

He thought there had to be a better way. His answer? Nanocrystals. 

"If the ceramic is small enough, it will never crack," he says. He developed a method of growing minute ceramic crystals within and around carbon, creating a hybrid composite. "We want it tightly bonded to carbon at the nanoscale," he says.

The crystals in Lafdi’s new material are 10-15 nanometers long, which is exceedingly small. Try to imagine an average-width human hair split lengthwise into some 20,000 strands. One strand would be about 1 nanometer wide. 

Although the devil is in the details, the process of making the new material is relatively simple. Lafdi uses heat and a solvent to disperse silicon carbide in a carbon precursor, such as coal tar residue or petroleum residue. The carbon is in the form of macromolecules—large structural units. The silicon carbide is in polymer form—also very large molecules, but in long, spaghetti-like chains. 

At about 350°C, the mixture reaches a liquid crystal state, somewhere in between liquid and solid. Tiny spheres form as silicon carbide polymers cross-link with carbon macromolecules. Silicon carbide polymers also surround the spheres.

As the mixture gets hotter, carbon and silicon atoms continue to connect. The small spheres coalesce into larger ones. Impurities boil off. And at about 400°C, you end up with a solid material: microscopic spheres of mostly carbon, in a matrix of silicon carbide nanocrystals. 

The silicon is bound to carbon atoms on the spheres’ surfaces—atoms that would otherwise be vulnerable to oxidation. Thus the crystals protect the carbon from burning. Yet they’re too small to interfere with the carbon’s heat-absorbing properties. 

High-resolution imaging instruments are needed to characterize such materials at the nanoscale level. Lafdi uses a transmission electron microscope to discern the molecular arrangement of the ceramic crystals.

 "Knowing the size of the crystals and how they’re connected tells you the electrical, thermal, and mechanical properties of the material," he says. "The whole idea is to create nanocrystals and include them in a material without changing the desirable properties of the material, but just making it better."

Lafdi spent two years developing and refining the process to produce a good-quality, cost-effective material, and SIUC has filed a patent application on it. 

To make brake discs or other parts for aerospace applications, where high heat resistance is critical, carbon fibers will be coated with the carbon/ceramic mix, then infiltrated into a carbon/ceramic matrix.  "The ratio of carbon to ceramic depends on the application," Lafdi says. Some parts, like braking systems, must absorb a lot of heat and will need a higher ratio of carbon to ceramic. Other parts, like pistons or heat exchangers, require greater heat protection; they will need a higher ceramic content.

Three companies affiliated with the Center for Advanced Friction Studies—BF Goodrich Aerospace, CytecFiberite Inc., and Honeywell (recently acquired by General Electric)—have licensed the technology and will fund work at SIUC on scale-up.

Lafdi has already made small prototype discs, 4 to 6 inches in diameter (aircraft brakes are 16 inches or more in diameter), and has tested them by burning them in pure oxygen. The ceramic crystals substantially decrease oxidation, he says. 

That’s not the only benefit.

In high humidity, a carbon composite loses much of its friction property. Instead, it becomes greasy--more like a lubricant. Adding ceramic prevents this effect. In fact, dynamometer testing shows that the ceramic-enhanced carbon composite has about a four-fold higher coefficient of friction than a standard carbon composite. "It has a lot more braking capability," says Lafdi.

Or, as he puts it, "Buy one [advantage], get one free."

More on CAFS...

The Center for Advanced Friction Studies (CAFS) pulls together the expertise of faculty, staff, and students to conduct research designed to help the automotive and aircraft friction industries. 

Established in 1996 as a National Science Foundation Cooperative Research Center, CAFS is the only university-based applied friction research program in the United States. It receives funding from NSF, the state of Illinois, and 15 industrial affiliates, including every major aerospace brake manufacturer in the world.

CAFS focuses its research in five areas: carbon-carbon brake materials, on-highway and heavy-duty brake materials, vibration and noise effects in braking, thermal effects in braking, and wet friction systems.

Besides involving graduate and undergraduate students in research, faculty and staff who work with CAFS develop courses and seminars in friction science. The chief aim is to train the next generation of researchers in this area and to prepare them for jobs in industry.

For details, see the center’s web site or contact its director, Peter Filip, at (618) 453-1166.

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