High velocity materials improvement: here's how better rust protection can be applied to body panels and electrical machines can be created on planar surfaces—both with the same process

Automotive Design & Production, March, 2004 by Gary S. Vasilash

Robert C. McCune, technical leader, Research and Advanced Engineering, Ford Motor Company (Dearborn, MI), talks about snowballs when providing an analogy for a deposition process known as "cold spray." No, he's not talking about the material deposition process, one that has the promise of transforming products from gas tanks to sensors, in the context of "cold" and "snowballs." Rather, it's like this: You have a pile of snowballs. And there's a brick wall. You throw the snow-balls at the wall. The snow begins to stick. Ball after ball, and you begin to build a snow coating on the substrate. "That's what it looks like." On a microscopic scale, that is.

An adjective that might apply as well as "cold" is "ambient." That is, while the process is categorized among the thermal spray processes--which are ordinarily thought of in the context of plasma spray, flame spray, or electric arc spray--a fundamental difference is that in the cold spray approach, the material that's being applied is not melted, not liquefied. Whereas traditional thermal processes may bring the temperature of the coating material to from 1,000 to 1,500[degrees]C, that's not the case with the cold spray process. According to Rick Blose, manager, Engineering Development Dept., Ktech Corp. (Albuquerque, NM)--the firm that has the exclusive U.S. license to make and sell cold spray equipment, provided by the patent holder, Dr. Anatolii N. Papyrin--there is some heating of the particles of material in cold spray. But this is a function of the gas heater that's used in the system to expand the gas for the sake of efficiency. When the powder and the gas meet in a prechamber, there can be some convection heating of the powder. "Relative to other thermal processes," Blose notes, "it is cooler by an order of magnitude or more."

One of the benefits that McCune cites with regard to the fact that the material isn't melted is that there tends to be a higher purity of the coating. He observes that in traditional thermal spray processes, because of the high heat involved, "there is no way to avoid some air entrapment that gives you oxidation of the material. That becomes part of the coating. Try to apply copper by a wire arc process: you're going to have a considerable amount of oxides formed." If that copper is to be used for circuit metallization, then the oxidation is not beneficial.

FROM RUSSIA WITH TECHNOLOGY. The process was initially developed at the Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Science in Novosibirsk in the mid-1980s. Apparently, there was testing done in a wind tunnel, which led to the concept of applying particles to a surface at a high rate of speed. Work in the U.S. began in 1994, when Dr. Papyrin arrived from Russia and the National Center for Manufacturing Sciences sponsored a technology demonstration program.

In operation, particles--which are generally on the order of 1 to 50 microns in size--are accelerated to 500 to 1,500 meters per second, propelled by gas (helium, nitrogen, air, or a mixture) through a nozzle. Think of a jet engine with a focused exhaust propelling particles of materials such as aluminum, copper, zinc, silver, or gold onto a substrate that's located about an inch way from the nozzle.

According to McCune, the thing to think about with regard to the materials that can be applied is their degree of deformability: "The particles you use must be in some way deformable. Right away that rules out materials like ceramics." (Extensive work on the process has been done at the Sandia National Laboratories; one of the terms that is idiomatically used even by the scientists to describe the process is "splat," which is certainly a characteristic of deformability.) McCune adds, however, that while materials such as nickel superalloys aren't particularly deformable, they can be applied: "It's not impossible to do, but it's much more difficult."

HOW DOES IT STICK? A natural question that arises is just how the kinetically applied material sticks to the substrate. "We've done work here on an electron microscope to see how these particles stick together," McCune says. What they've found is that in a number of cases, the coating is bound to the substrate at the molecular level. He points out, "Conventional thermal spray relies almost entirely on mechanical sticking to the surface." That is, often times the surface to be coated is in some way roughed (e.g., grit blasting) and the liquid material adheres to the rough surface. Generally, McCune says, cold spray provides "extraordinary bond strength." As an example, he cites applying aluminum to an iron cylinder bore. With conventional thermal spray, there is a bond strength on the order of 3,000 to 5,000 psi. With cold spray, the numbers are on the order of 9,000 to 10,000 psi.

Blose notes that another aspect of cold spray versus conventional thermal spray is the fact that the residual stresses in the cold-applied coating are low and compressive while those in the liquid-applied coating are higher and tensile, which means that that coating is more volatile and can potentially spall off.