With a background in surface chemistry, Professor Margaret Hyland, from the Chemical and Materials Engineering Department at the University of Auckland, has focused her plasma spray-coating research on the role the substrate surface plays. Does the chemical make-up of the surface hinder or support good splat formation as the high temperature droplet collides with the substrate surface at high speed?
There’s actually been, you know, a fair amount of research on kinematic or mechanical processes that take place when you have a molten droplet that’s travelling quickly towards something and causing it to splash. And it turns out, when you apply all of those theories to these droplets, it doesn’t explain why some splash and others don’t. And when you see a puzzle like that, you think ‘oh, how interesting’, and we wanted to look at it a bit more.
My background is in surface chemistry, so I immediately thought, well, the surface of the substrate must be playing a role here. And it can do a variety of things, so if you have a rough substrate, that’s going to cause you more splashing. But we thought there’s more to it than just the roughness, which is what we’ve been looking at.
So we have got rid of the roughness effect, and it turns out, when you have a polished substrate, there’s really some quite subtle things that happen just because of the chemistry of the outer layers of the substrate that can cause splashing to occur. Gases actually get released as the droplet is hitting the substrate, and as those gases are being released, you can imagine that they’re collecting underneath this droplet, which is spreading and causing it to just almost explode in some cases.
The temperature is important, so there’s an effect called the transition temperature. At room temperature, you’ll have mostly splashed splats, but if you heat up your substrate, up to some critical temperature – say 300 or 350°C – then you get mostly disc splats. And what we think is happening there is that it’s water in fact on the surface that’s strongly bound – so chemisorbed onto the surface. If you heat it up above 300° certainly you’re going to get rid of most of that gas, and that’s the gas that’s getting in the way when the droplet is trying to spread. So temperature of the substrate makes a really big difference.
Professor Margaret Hyland, Department of Chemical and Materials Engineering, University of Auckland
Dr Anh Tran
Holster Engineering Ltd, Tokoroa