Plasma spray research

STEVEN MATTHEWS
So my research into plasma spraying is focused on two diverse areas at the moment. The first one is working on hydroxyapatite coatings, and this is in collaboration with a team at Auckland University. And what we’re looking at there is to develop a hydroxyapatite coating for magnesium medical implants. And the idea about using magnesium instead of traditional materials like titanium is that magnesium can be absorbed by the body.

So what we were looking at doing was to develop a hydroxyapatite coating on the surface of the magnesium with the idea of allowing corrosion of the magnesium but at a controlled rate. So we were looking at hydroxyapatite because that’s a biosorbable ceramic material. It’s got similar composition to the human bone, and it helps promote bone growth in certain areas. But it’s a ceramic material and it’s got a very, very high melting point, and so plasma spraying is one of the only ways in which we can process this material into a coating form that’s suitable for the implants.

We also had to focus on the microstructure of the coatings because we had to develop tailored porosities so that fluids from the body could penetrate through the coating, generate the corrosion of the magnesium and also allow corrosion products to get out slowly. So we were looking at altering the plasma spraying parameters – the angle of the gun, spraying distances, sizes of powder, this kind of thing – so that we could get a good build-up of coating but also have continuous porosity through the coatings. And the idea is that the magnesium implant would be there long enough so that the bone could fuse together and grow and strengthen, and then after that, the magnesium would gradually corrode away.

The second application relates to titanium powder metallurgy and plasma spraying of titanium coatings, and in a bigger context, this is a national project being championed by the Titanium Industry Development Association. So what we’re looking at is applying titanium as a coating for various applications. Now titanium is very, very light, and it’s also very, very corrosion resistant, so it’s got a huge number of applications. But the thing that makes it very corrosion resistant is that it tends to oxidise very quickly and form a very protective oxide layer. And that’s a real problem for us when we try to plasma spray it because we’re melting the titanium particles and we’re spraying them in air, and they tend to oxidise as they’re going from the gun to the substrate. When they hit the substrate, we suddenly get a coating that’s got lots of oxides.

So one of the things we’ve been working on in collaboration with Waikato University has been to develop a shrouding system so that we can protect these titanium particles from the point where they’re injected into the plasma until after they’ve hit the surface and solidified so that the coating we get on the surface can be nice and pure and very dense.

Acknowledgements:
Dr Steven Matthews, School of Engineering and Advanced Technology, Massey University, Auckland
Holster Engineering Ltd, Tokoroa
Madila Awalini
Plasma coating micrographs (3 sets of 4) with permission from: Heat treatment of plasma-sprayed Al2O3 and Al2O3–WO3 coatings between 500 and 1000°C, S. Matthews, F. Taliana, B. James. DOI 10.1016/J.SURFCOAT.2012.09.030. Surface and Coating Technology, Vol 212, Nov 2012, Elsevier.
Bone cross-section micrograph (annotated), with permission from: Biodegradable Orthopedic Magnesium-Calcium (MgCa) Alloys, Processing, and Corrosion Performance.Meisam Salahshoor and Yuebin Guo. Materials 2012,5, 135-155; doi:10.3390/ma5010135. MDPI – Open Access Publishing.
TiDA, Titanium Industry Development Association