Dr Steven Matthews’s plasma spray gun research is at present focused in two diverse areas. Both involve working in collaboration with other universities and both involve use of thermal plasma spray facilities at Holster Engineering, a commercial company based in Tokoroa.
Using magnesium in body implants
Collaboration with the Department of Chemical and Materials Engineering at the University of Auckland involves plasma spraying hydroxyapatite coatings onto medical implants made from magnesium.
Currently, medical implants are primarily made of materials such as titanium alloys. In applications where the implant supports parts of the body while they repair and regrow, additional surgery is required to remove the implant after the body has healed.
The idea behind using magnesium for implants is that it can be dissolved away slowly in the body by natural processes without the need for additional surgery to remove it after the body has healed. However, corrosion of magnesium in the body releases hydrogen gas. If too much gas is produced, it is difficult for the body to deal with. The concept behind the use of a hydroxyapatite coating on the magnesium is to slow down but not stop the rate of magnesium corrosion so that the body can process the corrosion products. The hydroxyapatite coating has a composition similar to bone and will also dissolve in the body while at the same time helping to support bone growth.
Analysing the spraying process
Since hydroxyapatite is a very high melting-point ceramic, plasma spraying is one of the only ways in which this material can be processed into a coating form suitable for implants. In this application, the coating structure must be tailored to have a controlled amount of interconnected porosity to allow body fluids to penetrate in to generate corrosion of the magnesium and to allow the corrosion products to slowly seep out.
In order to generate this coating structure, numerous plasma spray parameters were analysed, including the gun power, the spray distance, the angle of the gun to the substrate, the powder size distribution and the number of spray passes.
Using plasma spraying to apply titanium coatings
Working in association with the University of Waikato Materials and Process Engineering Group, the second area of research relates to titanium powder metallurgy and the plasma spraying of titanium coatings. It is a national project being championed by the Titanium Industry Development Association (TiDA).
Titanium is a low-density corrosion-resistant metal with a huge number of applications. For example, it is used in the aerospace industry, building industry, sports goods industry and as implants in a number of surgical procedures.
Being able to apply a titanium coating by plasma spraying finely divided titanium powder onto a substrate extends the number of applications for this useful metal. However, there is a problem that needs to be resolved. At the high temperature of the plasma, the molten titanium droplets tend to oxidise as they travel in air from the plasma gun to the substrate. When they splat onto the substrate, the presence of titanium oxide can interfere with the adhesion and cohesion of the splats. The coating formed not only acquires a very thin protective oxide layer on the outside (good) but also oxide clusters within the coating (bad).
Optimising the spraying process
To overcome this oxide problem, a lot of work has been done with optimising the parameters of the spray gun, for example, different plasma gases, different powers of the plasma and spray distances were looked at. In addition, a shrouding system has been trialled that protects the titanium particles from the point where they are injected into the plasma until after they have hit the substrate surface. To date, this research has produced encouraging results.
Atmospheric plasma spraying generates very high noise levels along with UV radiation and intense visible light, so suitable protective eyewear, clothing and earmuffs need to be worn by operators at all times.