The Massachusetts Institute of Technology, Cambridge, announces that its researchers have developed a high-speed camera in which 16 separate charged-coupled-device imaging chips can record images of thermal-spray particles moving at supersonic speeds in just three nanoseconds. The team used this camera, which can shoot up to 300 million frames per second, to observe a spray-painting-like process similar to those that apply a metallic coating to surfaces in many industries.
While such processes are widely used, until now their characteristics have been determined empirically, since the process itself is so fast "you can't see it, you can't tell what's happening, and no one has ever been able to watch the moment when a particle impacts and sticks," says ASM Fellow Christopher Schuh, the Danae and Vasilis Salapatas Professor of Metallurgy and head of the Department of Materials Science and Engineering. As a result, there has been ongoing controversy about whether the metal particles actually melt as they strike the surface. The new technology means that now the researchers "can watch what's happening, can study it, and can do science," he says.
The new images make it clear that under some conditions, the particles of metal being sprayed really do melt the surface — and that, unexpectedly, prevents them from sticking. The researchers found that the particles bounce away in much less time than it takes for the surface to resolidify, so they leave the surface that is still molten.
If engineers find that a coating material is not bonding well, they may be inclined to increase the spray velocity or temperature in order to increase the chances of melting. However, the new results show the opposite: Melting should be avoided.
It turns out the best bonding happens when the impacting particles and impacted surfaces remain in a solid state but "splash" outward in a way that looks like liquid. It was "an eye-opening observation," according to Prof. Schuh. That phenomenon "is found in a variety of these metal-processing methods," he says. Now, it is clear that "to stick metal to metal, we need to make a splash without liquid. A solid splash sticks, and a liquid one doesn't."
The findings could be relevant for processes used to coat engine components in order to reuse worn parts rather than relegating them to the scrap-metal bin. "With an old engine from a large earth-moving machine, it costs a fortune to throw it away, and it costs a fortune to melt and recast it," Prof. Schuh says. "Instead, you can clean it off and use a spray process to renew the surface." But that requires that the sprayed coating remain securely bonded.
The research was carried out by postdocs Mostafa Hassani-Gangaraj and David Veysset, and professors Keith Nelson and Christopher Schuh. It was reported in two papers, in the journals Physical Review Letters and Scripta Materialia.
The work was supported by the U.S. Army through MIT's Institute for Soldier Nanotechnologies, the U.S. Army Research Office, and the U.S. Office of Naval Research.
http://news.mit.edu/2017/how-get-sprayed-metal-coatings-stick-1122
