When the droplet hits the substrate

In thermal spray-coating, high-temperature molten droplets of material are projected at high speed to impact with the target substrate. Professor Margaret Hyland from the Chemical and Materials Engineering Department at the University of Auckland explains her interest in the splat pattern produced after impact. The surface chemistry of the substrate has a role in the type of pattern produced.

Transcript

MARGARET HYLAND
I’m trying to understand some of the really basic processes that are occurring when this molten droplet impacts onto the substrate

The first thing that happens, as you can imagine, is that that droplet starts to slow down and spread into a pancake shape, and we call that pancake shape a splat. And so we build up a coating by a whole series of layers of these splats.

There’s a surprising amount that happens in the microseconds that it takes for a droplet to hit a substrate, spread and then solidify. What we’re interested in, really, is understanding why some of those droplets splash and why others don’t. If you make up a coating that has all the splats which have been splashed, then its properties are not as good as one which is made up from non-splashed or disc splats. So it doesn’t adhere to the substrate as well, its electrical properties aren’t as good. In terms of mechanical properties, it might wear more quickly, it might corrode more quickly, so we want to make sure that we’re depositing droplets that don’t splash. And there’s some really fundamental kind of physics questions and surface chemistry questions all folded into that.

So you can imagine all of these splats having to stick to the substrate, so adhesion to the substrate. It turns out that, when you have splats that are disc shaped so they haven’t splashed, then they adhere or they stick much more strongly to the substrate, and they also stick to each other much more strongly. So they cohere much more strongly, and the better the cohesion of these splats, then the better its mechanical properties would be, and usually the better its electrical properties might be as well, the better its corrosion resistance will be.

Acknowledgements:
Professor Margaret Hyland, Department of Chemical and Materials Engineering, University of Auckland
Dr Anh Tran
Holster Engineering Ltd, Tokoroa