How to Get Sprayed Metal Coatings to Stick

Surprisingly, melting hurts rather than helps, MIT research reveals.

How to Get Sprayed Metal Coatings to Stick
Micrographs of a metal surface after impact by metal particles. Craters are formed due to melting of the surface from the impact. Courtesy of the researchers

Bonding metals to each other is a common task in both home-based DIY projects and commercial production. Soldering is a commonly used method, where molten metal must be introduced between the pieces. But melting metals can actually inhibit metal bonding rather than promote it.

According to MIT scientists, the findings could have serious implications for the design of certain metal coatings or for 3-D printing, which both require getting materials to stick together and stay that way.

Christopher Schuh, Professor of Metallurgy explained that one of the papers outlines ‘a revolutionary advance in the technology’ for observing extremely high-speed interactions, while the other makes use of that high-speed imaging to reveal that melting induced by impacting particles of metal can impede bonding.

How to Get Sprayed Metal Coatings to Stick
The top row of photos shows a particle that melts the surface on impact and bounces away without sticking. The bottom row shows a similar particle that does not melt and does stick to the surface. Arrows show impact sprays that look like liquid but are actually solid particles.
Courtesy of the researchers

The optical setup, with a fast camera, that utilization 16 isolates charged-coupled gadget (CCD) imaging chips and can record pictures in only 3 nanoseconds, was basically created by Veysset. The camera is fast to the point that it can track singular particles being splashed onto a surface at supersonic speeds, an accomplishment that was already impractical.

The group utilized this camera, which can shoot up to 300 million edges for each second, to watch a shower painting-like process like ones used to apply a metallic covering to surfaces in numerous businesses.

Such techniques are widely used as they are so faster that can’t be seen and can’t tell what’s happening. No one has ever been able to watch the moment when particle impacts and sticks.

Schuh said, “As a result, there has been an ongoing controversy about whether the metal particles actually melt as they strike the surface to be coated. The new technology means that now the researchers can watch what’s happening, can study it, and can do science.”

This new analysis shows that under some conditions, the particles of metal being sprayed at a surface really do melt the surface and that, unexpectedly, prevents them from sticking. The particles bob away in considerably less time than it takes for the surface to resolidify, so they leave the surface that is as yet liquid.

If coating material is not bonding properly, they may be inclined to increase the spray velocity or temperature in order to increase the chances of melting. But the study suggests something different- melting should be avoided.

Schuh said, “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.”

“The phenomenon is found in a variety of these metal-processing methods. Now, it is clear that to stick metal to metal, we need to make a splash without liquid. Solid splash sticks and a liquid one doesn’t.”

Postdoc Mostafa Hassani-Gangaraj said, “With the new ability to observe the process. By precise measurements, we could find the conditions needed to induce that bond.”

Moreover, this new information could also help in the design of some metal-based additive manufacturing systems, known as 3-D printing. It also promises an accurate and mathematical approach to determining the optimal conditions to ensure a solid bond.