Sandia is studying the science of splat.

Working with a consortium of eight US companies that includes automakers and aircraft engine manufacturers, researchers at Sandia’s Thermal Spray Research Laboratory are using experimental and computer-modeling capabilities to improve the world’s fundamental scientific understanding of an emerging

manufacturing technique called Cold Spray™ (Lab News, Jan. 26, 2001).

Cold Spray involves injecting microscopic powdered 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-wide particles hit the substrate, they splat so hard they stick — like a bug to a windshield.

Building a reputation

Consortium members want to use new Cold Spray processes refined at Sandia to create wear-resistant coatings on car- or aircraft-engine components, for instance, or to deposit layers of reactive metals such as aluminum or copper onto substrates for use as heat-tolerant circuits.

The Sandians ultimately want to employ successfully commercialized Cold Spray processes, which originated, ironically, at a Soviet-era research lab in Siberia, to improve US nuclear weapons components. (See "The promise of Cold Spray processes" below right.)

"Cold Spray has some significant advantages as a coating and fabrication tool," says Mark Smith, Manager of Joining and Coating Dept. 1833. "We think the best way to legitimize Cold Spray for use in the nuclear weapons program is to have its use proven in US industry, and to support the development of a commercial supplier base."

Members of the consortium — Alcoa, DaimlerChrysler, Ford Motor Co., The Jacobs Chuck Manufacturing Co., Ktech Corp., Pratt & Whitney, Praxair, and Siemens/Westinghouse — met at Sandia recently to discuss progress they’ve made toward readying the Cold Spray processes for widespread commercial use.

Cold Spray 101

Cold Spray more appropriately might be called "room-temperature spray."

Conventional "thermal spray" processes require preheating the sprayed materials so the particles are in a semi-molten state when they reach the substrate, allowing them to splash across the surface. But as the "splats" cool, they contract slightly, creating residual (stored) stresses or flaws at the interface that can cause defects later.

Cold-sprayed materials typically remain at or near room temperature until impact, slamming into the substrate so hard (travelling at 500 to 1,500 meters per second) that a tight bond is formed without the undesirable chemistry changes and stresses associated with conventional processes.

Although the science behind this bonding process is not yet well understood, the researchers think the 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.

Understanding splat

Unlike thermal-sprayed materials, cold-sprayed particles experience little to no defect-causing oxidation during flight and exhibit remarkably high densities and conductivities once fabricated, the researchers have found. In addition, deposition rates comparable to traditional thermal spray processes can be achieved with Cold Spray.

"This is the logical conclusion of research thrusts in thermal spray technology during the last two decades toward faster-and-faster and cooler-and-cooler methods," says Richard Neiser (1833).

To advance the state of fundamental understanding and improve the usefulness of Cold Spray, the Sandians are combining modeling expertise in Engineering Sciences Center 9100 with experimentation in Materials and Process Sciences Center 1800.

The team has examined gas dynamics, aerosol physics, and plastic deformation during splat-to-substrate impact. Current efforts focus on avoiding fouling of nozzles with powder residue; experimenting with varying materials, particle sizes, and impact velocities; and characterizing splat patterns and Cold Spray-fabricated bulk materials.

The researchers also want to design better aerodynamic lenses that focus or spread out the spray pattern like a thumb held over the end of a garden hose.

A variety of metals have been deposited, 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.

Revolutionary changes

The Cold Spray consortium supplies $400,000 a year for three years toward Sandia’s R&D efforts, plus in-kind contributions by each member. Sandia also is collaborating with several individual member companies on proprietary Cold Spray R and D.

Cold Spray technology came to the US in 1994, ten 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 Novosibirsk.

One of its discoverers, Prof. Anatolii Papyrin, who holds the US patent for Cold Spray, now works for Ktech in Albuquerque, which hopes to supply fabrication equipment to a broadened Cold Spray market.

Sandia is among just a few R&D institutions in the world successfully turning improved understanding of Cold Spray science into marketable technology, says Mark.

"We think Cold Spray provides capabilities not previously possible," he says. "It’s a new enough technology that we don’t yet know all the possible applications, but it has the potential to make truly revolutionary changes in the way some products are manufactured."