Consortium tests splattering technique

Mark A. DeSorbo

Cold spray technology may have indirect impact on manufacturing in cleanrooms

ALBUQUERQUE, NM—Bug guts splattering across the windshield are often regarded as a rather gross occurrence.

But to a consortium of eight U.S. companies and researchers at Sandia National Laboratories, the velocity and the untimely impact of insect entrails typifies a science that could change the face of manufacturing in controlled environments.

Mark Smith of Sandia National Laboratories uses what amounts to a laser version of a police officer’s radar gun?a laser velocimeter system?to measure the speed of tiny metal particles shooting toward the work surface during cold spray deposition.
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Affectionately referred to as the “science of splat,” the consortium is experimenting with computer-modeling to gain a better understanding of the “cold spray” manufacturing technique, which involves injecting microscopic particles of metal or other solids into a supersonic jet of rapidly expanding gas and shooting them at a target surface. When these 10- to 50-micron particles hit the target, they splat so hard they stick—like a bug to a windshield.

The cold spray technique would not presently be conducive in a cleanroom, but the technology could have an indirect impact on manufacturing applications within controlled environments, says consortium member Rick Blose, manager of the engineering development department for Ktech Corp., an Albuquerque, NM-based semiconductor equipment and systems integration firm.

Cold spray technology, he explains, may find uses in making targets for wafer sputterers, electrical conductors for printed circuit boards and coatings to strengthen and protect glass, ceramics, plastics and metals from static electricity. It may also be used to seal metal-to-metal, metal-to-ceramics or ceramics-to-ceramics.

“Right now, it would be disadvantageous to use in cleanrooms because you would have particles flying around, but it could be used to lay lines on printed circuit boards or repair damaged metal parts in an airplane wing,” Blose says.

And because helium and nitrogen gases are the needed thrust in cold spray technology, companies like cleanroom gas supplier Praxair Corp. (Danbury, CT), also a consortium member, are exploring new avenues of not only selling products, but also recycling expensive propellants.

“We do not yet know all the possible applications, but it has the potential to make truly revolutionary changes in the way some products are manufactured,” says Mark Smith, a researcher at Sandia.

Along with Sandia, Ktech and Praxair, the consortium includes aluminum fabricator Alcoa Co. (Sidney, OH); automakers DaimlerChrysler (Auburn Hills, MI) and Ford Motor Co. (Dearborn, MI); drill component maker The Jacobs Chuck Manufacturing Co. (Clemson, SC); aerospace contractor Pratt & Whitney (East Hartford, CT); and Siemens Westinghouse Power Corp. (Orlando, FL).

The secret of splatter
Cold spray may also be called “room-temperature spray,” according to Sandia Labs.

Smith says conventional thermal spray processes require heating materials so the particles are in a semi-molten state, allowing them to splash across the surface of a substrate. But as the “splats” cool, they contract slightly, creating residual flaws that can cause oxidation defects later.

Cold-sprayed materials typically remain at or near room temperature until impact, slamming into the substrate at a rate of 500 to 1,500 meters per second, forming a tight bond without the undesirable chemistry changes and stresses associated with conventional processes.

Although the science behind this bonding process is not fully understood, the consortium believes high-velocity impact disrupts thin metal-oxide films on the particle and substrate surfaces, pressing their atomic structures into intimate contact with one another under momentarily high interfacial pressures and temperatures.

Cold spray technology first appeared in the U.S. in 1994, 10 years after its Russian inventors first recognized its potential significance while conducting high-velocity wind tunnel tests at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Sciences in Siberia. One of its discoverers, Anatolii Papyrin, who holds the US patent for the technology, now works for Ktech.

Never-before-seen capabilities
Smith says consortium members plan to use cold spray processes refined at Sandia, for instance, to create tough new coatings on car or aircraft engine components made from lighter-weight composites, or to deposit layers of conductive metals onto substrates for use as heat-tolerant under-hood automobile electronics.

Filling a seam or repairing a scratch on an automobile body panel is one example of how cold spray could be used, Smith explains. “Usually, material is heated to repair that type of damage, which creates oxidation. With cold spray, scratches or seams in automotive body panels could be filled without heating it,” he says. “The process does not require a cleanroom either.”

A variety of metals have been deposited at Sandia, including copper and aluminum, as well as several types of steel and nickel-based alloys. Even a few metal-ceramic composites have been successfully cold sprayed.

Blose says combining metals to make targets for wafer sputtering, which is conducted in a cleanroom, was not possible before this technology. Any combination of aluminum, copper, cobalt, chromium, nickel, titanium and zinc, he says, can be done precisely with cold spray technology.

“It would be a less-expensive way to produce target material. This could also replace coating processes that give off volatile organic compounds, which are currently used and difficult as well as too expensive to get rid of,” Blose says. “These types of applications could not be done before this technology.”



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