Spray it, Don't Melt it

Too hot. Too dirty. Too hazardous. Too slow. Too expensive. That just about sums up some of the knocks against conventional metal coating and plating technologies. But Ralph Tapphorn, a nuclear physicist by training, has found a way around these problems with a patented solid-state coating technology called Kinetic Metallization (KM).

Available from Inovati, the company Tapphorn co-founded, KM sprays micron and sub-micron metal powders onto a wide variety of metallic substrates-including steel, aluminum, copper, and magnesium. The system works by first fluidizing the powders in a stream of helium or other inert gas. The system then sprays the powder and gas mixture at nearly sonic velocities through a directed nozzle that moves just above the substrate surface. "The powders impact upon the substrate surface with sufficient force to establish an interfacial bond," Tapphorn explains, noting that the powders deform as they hit the substrate to create a flat, continuous coating. Inovati has used KM to form coatings from a wide variety of metal powders-including aluminum, copper, nickel, niobium, titanium, braze alloys, and chromium. He reports that the system can achieve a range of thicknesses, such as a 50-mil wear resistant coating. Much thicker sprays are also possible. In fact, Tapphorn says that KM can serve as a spray forming system to fabricate near-net-shape components.

The chief benefit of the KM system mostly relates to heat-or rather, the lack of heat. KM normally deposits the metal powders at temperatures of 500 to 1000F, or hundreds of degrees below their melting points. "Thermal spray methods require the melting of metal powders," he says, noting that KM's low processing temperatures avoid problems with both for the coating itself and the substrate. With the coating, low temperatures help prevent the formation of brittle metallic oxides that can compromise the coating interface. With the substrate, low temperatures can head off thermal stresses that diminish the fatigue strength of the finished component. And the system isn't just friendly from a thermal standpoint; it also avoids the nasty chemicals used in electroplating operations. Finally, KM requires no surface preparation other than a wipe-down to remove oil or grease. "It doesn't need grit blasting that some deposition methods require," Tapphorn adds.

KM is by no means the only solid-state deposition system, but it does have a few twists that set it apart and may give it an edge in the economics department. The system, for example, features a patented fluidized bed and plasma heating system designed to boost the efficiency of the process-both in terms of the time it takes and the amount of gas and powder it consumes. But the biggest key to the system's economic performance can be found further downstream in its patented nozzle design. This "friction compensated" nozzle maintains the velocity of the gas and powder mixture just below helium's sonic velocity. Other solid-state deposition methods, by contrast, tend to have supersonic gas velocities. Stepping down from supersonic speed, the system uses less gas. "At higher velocities, the density of the pushing gas falls, so you need to use more," Tapphorn points out. At the

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