How can a metal object be made antibacterial, self-lubricating or corrosion resistant? By combining metal injection moulding and the space holder technique, it is possible to manufacture porous metal allowing the introduction of chemical agents.
- Text Marianne Mustonen
- Photos Matti Kultamaa, Eeli Gröhn and Mostphotos
Four years ago, researchers at the Department of Chemistry got a proposal to study powder metals. There wasn’t much interest at first – after all, powder metals had already long been developed in different parts of the world. However, then came the idea to study functional metal surfaces.
Metal injection moulding, MIM, combines the long-established plastic injection moulding technique with powder metallurgy. MIM, together with the space holder technique, allows for controlled manufacture of pores in a metal object during moulding, where diverse chemical agents can then be introduced.
“This type of space holder research addressing the use of pores to store functional chemistry is quite unique,” says Professor Mika Suvanto.
“We’ve conducted two projects in this field in collaboration with industrial partners and Karelia University of Applied Sciences. In four years, this resulted in ten scientific articles, seven Master’s theses and two doctoral dissertations.”
Matti Kultamaa is one of the people defending their dissertation on the topic late last year. He studied porous metals already for his Master’s thesis, so continuing with the topic seemed like a natural choice.
“By using metal injection moulding and the space holder technique, it is possible to reserve space for desired chemical agents in a controlled manner. In practice, tiny pores, less than 1 millimetre in size, are manufactured in a metal object and then filled with the desired agent,” he says.
“Using this technique, strong metal objects, such as bearings, can be moulded from powder metal.”
The best thing about this technique is that there is no need for after-treatment, such as heat treatment, after injection moulding.
“In other words, this is a convenient, fast, efficient and affordable mass technique that does not produce waste.”
Chemically enhanced metal prevents the spread of healthcare-associated infections
To reap the benefits of the properties created by metal injection moulding in a metal object, the powder space holder technique, PSH, is needed. It enables the manufacture of steel components that are controlled in terms of their by porosity and pore size.
“Space holders are powder-like particles, such as polymers or inorganic substances or, for example, salt, which I used in my research. It is vital that the space holder does not break down during moulding,” Kultamaa says.
Metal must always contain a binding agent that makes it more fluid and keeps the object intact until the end. A binding agent is a mixture of several components and may contain, for example, a water-soluble polymer such as polyethylene glycol, PEG.
The pores of a metal object can be filled with a variety of agents. In his doctoral research, Kultamaa explored the potential of antibacterial agents, corrosion resistance and lubricants.
In the study, metals with well-known antibacterial tendencies, such as silver, copper and zinc, were introduced into the pores of stainless steel. These metals effectively prevented the growth of the Gram-positive bacteria Staphylococcus aureus – a common cause of healthcare-associated infections – on the surface of steel structures.
“However, silver and copper enhanced metals aren’t a feasible everyday solution for hospitals since grease and dirt from hands will block the pores of metal surfaces. And surface porosity is high precisely because that’s what makes them antibacterial,” Kultamaa explains.
Metal objects’ friction and wear properties can also be enhanced in exactly the same way. It is possible to introduce lubricants into pores: Kultamaa used paraffin in his doctoral research. Pores filled with a lubricant reduce abrasive contact, and when the lubricant is pre-stored in the object, there is no need to add it later on. This method could be introduced even quickly.
A self-lubricating material could be used extensively in the industrial and automotive sectors.
In a study addressing corrosion resistance, the pores of a metal object were filled with zinc, which has long been used in the production of galvanised steel, for example.
“Zinc can be used as a sacrificial anode because it corrodes faster. The object didn’t even have to be coated entirely; small particles were enough,” Kultamaa says.
“We demonstrated in our corrosion experiment that the zinc-containing object got considerably less rusty than the control objects. In ten days, the other objects got entirely covered by rust, although we used stainless steel with the lowest corrosion resistance properties. In other words, zinc would be a very affordable corrosion resistance method.”
In the future, the method can be expanded to steel structures used in very demanding Arctic conditions, for example.
“A single process can be used to mould an object and enhance it with the desired properties. Service intervals and life cycles will get longer, and less energy is needed. Here, sustainability thinking is ingrained,” Suvanto says.
Research and development of metal injection moulding began in 2018 in a project funded by Business Finland. Researchers at the University of Eastern Finland conducted scientific, laboratory-level research, and Karelia University of Applied Sciences was responsible for large-scale injection moulding machines and moulds. Continuing in 2020, the project developed metal injection moulding components for demanding conditions, and they were also tested in practice. Research has also been carried in the field of hard metals under the lead of Professor Jarkko J. Saarinen.
The Department of Chemistry at the University of Eastern Finland has collaborated with the Austria-based Montanuniversität Leoben and the tribology research centre V-Research in material development, research mobility and scientific publishing.
“Now, we are planning new research into how the same space holder technique could be used in the manufacture of porous plastics and composite materials. These could be used in composites or coating techniques to prevent the accumulation of snow and ice on surfaces,” Suvanto says.
Porous materials are light, but the level of porosity must be controlled to ensure their mechanical durability. These materials could be used, e.g., in energy production and in various outdoor plastic-composite scaffolds or supports, which may also include sensors.
Other possible uses include drones, building services engineering, weather observation devices and defence technology – basically any device that gets exposed to the Arctic elements of snow and ice.