Composite Wear Reductions

After years of applying wear-resistant coatings to metals, the engineers at General Magnaplate Inc. have now set their sights on enhancing the surface properties of a very different kind of substrate - carbon-fiber composites.

"We've focused on thin-film coatings for metals for 50 years, so composites represent a change in direction for us," says Corey Wesnitzer, the company's sales and marketing vice president. "But there's a growing demand to improve some of the properties that composites lack."

Chief among those properties is wear resistance. "The improvement would depend on the composite, the coating, and the application conditions, but we've seen exponential improvements in wear," he says. Wesnitzer adds that Magnaplate is also working on coatings that boost impact performance and release properties.

Magnaplate engineers believe they can improve these properties through custom-formulated synergistic coatings similar to those used on metals. These coatings typically consist of a polymer matrix embedded with combinations of polymer, metal and ceramic particles. "The exact combination depends on what properties you want to impart," says Larry Campbell, Magnaplate's vice president of engineering. Magnaplate usually applies these films - using various plasma deposition techniques - at thicknesses less than 5 mils.

Applying the coatings to carbon-fiber-reinforced plastics opens up a Pandora's box of technical difficulties. Campbell notes that composites can be sensitive to the mechanical and chemical surface preparation methods used for metals. They also exhibit different adhesion characteristics.

"But the really big issue is thermal," he says. Magna-plate's coatings are applied under temperatures that can range from 350 to 700F. Campbell points out that many epoxies can tolerate temperatures well into the middle of that range - but for decreasing periods of time as temperatures rise. "Our challenge has been to develop coatings whose polymers cross link quickly so that the composite part is not exposed to elevated temperatures for any length of time," he says.

Campbell stresses that Magnaplate has not developed an off-the-shelf coating solution for carbon-fiber-reinforced plastics - nor does it plan to do so. "We always work on a case-by-case basis," he says. But the company has made concrete strides in solving the application difficulties.

In fact, the composites coating work can be seen as an outgrowth of the company's Custom Moldmaking Process Technology, or CMPT. It creates molds for composites lay-up by building up a pure metal surface directly over a composite master. At 0.38 to 0.50 inch thick, this tooling surface is much thicker than a coating. Yet Campbell says Magnaplate's work on the thick tooling surfaces has helped it solve some of the thermal issues relate to thinner composite coatings. "CMPT taught us how to create distortion free surfaces by controlling the differential expansion and contraction of metals and composites," he says, "and that ability translates to thinner coatings for composites."

Wesnitzer reports that Magnaplate is already at work on some confidential composite applications unrelated to tooling. A couple are for the military, including improving composite's resistance to ballistics by adding an energy- absorbing constituent to a relatively thick coating. The company has also coated composite rollers used in web processes in order to improve their release properties. And when asked if the company is ready to talk to other engineers wanting to improve the surface properties of composite parts, Wesnitzer says "absolutely."