Don’t wear it out

Materials World magazine
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24 Sep 2018

A new take on an old material is believed to have created the world’s most wear-resistant alloy, as Ellis Davies reports.

A platinum-gold alloy, developed by researchers at Sandia National Laboratories, USA, is reportedly the most wear-resistant in the world. It exhibits resistance of up to 100 times more than any other known metal, even under aggressive conditions, and is just one example of a range of materials that could come out of the study, which focuses on creating a stable nanocrystaline metal by adjusting its reaction to heat.

‘What we did was make a connection between the microstructure of the metal and its frictional performance,’ Michael Chandross, a Sandia researcher involved in the study told Materials World. The nanocrystalinity of a metal can give it advantageous strength, but there has not been much work into the mechanical stability of such metals, researchers say. ‘It’s been known for a long time that nanocrystalline metals have superior properties – higher hardness for example. We, and others, have shown that they have more resistance to friction. But the grains – each being a distinct crystal – of a nanocrystal are not stable. If you leave that piece of metal sitting on the counter those grains are going to grow and lose those properties,’ says Chandross.

Metal alloys are known to exhibit sensitivity to heat, and can cause stress. To alter this trait, researchers modified the grain boundaries of the alloy using solute segregation – enriching the atoms of a region. When the segregation is strong enough, researchers say, there will be a substantial reduction or elimination of heat force for grain growth, increasing wear resistance.

‘This is the first demonstration of the phenomenal mechanical properties of these alloys,’ says Nic Argibay, materials scientist at Sandia. ‘Platinum and gold don’t have oxides, so we could rule out any question of them playing a role in inducing stability. This is an extraordinary demonstration of higher performance with something that nominally doesn’t oxidise.’

Researchers used sputter deposition of a platinum-gold film at the grain boundaries to synthesise the alloy, which prevents grain growth and stabilises the structure, but they were keen to point out that this is not the only technique that will work. ‘The trick was knowing what to do with it and what structure you want to achieve. We like to point out that platinum and gold alloy is commercially available, but the trick is processing it into a form that is very stable,’ said Argibay.

Old alloy, new methods

Platinum and gold are not new materials, and the combination isn’t a breakthrough either – but the methods and theory behind the alloy are. Researchers focused on the relationship between the alloy and heat, not the conventional path of looking at friction based on hardness. They came to this theory after observing that many materials developed to have reduced grain size to improve strength still softened under extreme pressure or temperature. The platinum-gold alloy does not display the same fatigue, with the team observing 100,000 sliding passes with negligible surface wear in testing, and no sign of grain growth after one week of annealing at temperatures as high as 500°C.

Part of the reason researchers were able to produce the alloy is down to computer modelling. ‘We developed models that could study the interactions between platinum and gold and carry out large-scale atomistic simulations of pure platinum structures and platinum-gold structures to observe how they performed under thermal and mechanical stresses, to validate what we were seeing in our experiments,’ says Chandross. ‘I think the modelling has provided a lot of insight, especially during development,’ added Argibay. ‘When the material wears or fails, we can see how and why it is doing so. We’ve learned that the grain structure is extremely fatigue-resistant, which has implications for other applications with other metals.’

The wider market

The primary application for the platinum-gold alloy will be in electrical contacts as the metals provide good conductivity as well as high wear resistance. But, any application that requires sliding metal contact could benefit from the alloy – tools, bearings, gears, musical instruments and watches. The synthesis and theory can also be applied to cheaper metals. ‘We really don’t think this behaviour is limited to the platinum-gold alloy, it’s just one of dozens in this category of alloys that ought to have superior performance. For some of these applications, where conductivity isn’t needed, another alloy could be used to make it cheaper,’ explained Chandross.

Moving forward, the team is trying to find industry partners and ways to achieve transfer to consumers and industry. ‘This is very important,’ stressed Argibay. ‘Even though we picked conventional processing methods, consistently producing a good alloy is generally time consuming, there are practical considerations such as confirming that we have the right processing parameters.’ The team is also looking into non-precious alloys for bulk applications and continuing to develop further theories covering a broader set of materials.