Sandia expand the science of splat

Researchers at Sandia National Laboratories are using experimental and computer-modelling capabilities to improve the world’s fundamental scientific understanding of an emerging manufacturing technique called Cold Spray.

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. The 10- to 50-micron particles hit the substrate so hard they stick.

The research was conducted with a consortium of eight US companies that includes vehicle makers and aircraft engine manufacturers.

The consortium members plan to use new Cold Spray processes 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-bonnet vehicle electronics.

Other possible uses of the technique include fabricating layer by layer low-defect small piece parts, joining chemically dissimilar materials with bonds that gradually transition from one material composition to another, and as a low-temperature alternative to welding.

‘Cold Spray has some significant advantages as a coating and fabrication tool, and it provides capabilities not previously possible,’ said Mark Smith of Sandia. ‘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.’

Conventional thermal spray processes are said to 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 remain at or near room temperature until impact, slamming into the substrate so fast (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.

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

A variety of metals have been deposited at Sandia, including copper and aluminium, as well as several types of steel and nickel-based alloys. The Sandia team also report that a few metal-ceramic composites have been successfully cold sprayed.