The world’s most wear-resistant alloy consists of platinum and gold

A new alloy developed at Sandia National Laboratories could represent a huge savings for the electronics industry

Although to the general public metals are most durable materials, engineers know that this is not the case. Without specialised coatings or lubricants, wear, deformation and corrosion inevitably follows when metal moves directly against metal.

Chasndross (left) and Argibay with their computer simulation and testing equipment for their high durability alloy. Image: Randy Montoya

Engineers at Sandia National Laboratories in Albuquerque, New Mexico have been studying this problem, and a new approach to the mechanics of frictional wear has led them to develop an alloy of platinum and gold which, they believe, is the world’s most wear-resistant metal as it is more durable than high-strength steel and in the same class as diamond and sapphire. They describe their work in a paper in Advanced Materials.

In general, wear resistance is believed to be related to hardness. However, the Sandia team, led by Nic Argibay and Michael Chandross, proposed a new theory stating that wear is related to how metals react to heat. Using computer simulations to calculate how individual atoms were affecting large-scale properties of the material, in particular how they affected the stability of the nanocrystalline structure of the alloy, they chose a mixture of 90 per cent platinum with 10 per cent gold. Although expensive, these metals have the advantage of being “noble” – that is, far less reactive than other metals and therefore available in very high purity with no need to worry about oxide formation.

“We’re getting down to fundamental atomic mechanisms and microstructure and tying all these things together to understand why you get good performance or why you get bad performance, and then engineering an alloy that gives you good performance,” Chandross said.

Stability of nanocrystals was crucial to the alloy’s properties. “Many traditional alloys were developed to increase the strength of a material by reducing grain size,” said John Curry, first author on the paper. “Even still, in the presence of extreme stresses and temperatures many alloys will coarsen or soften, especially under fatigue. We saw that with our platinum-gold alloy the mechanical and thermal stability is excellent, and we did not see much change to the microstructure over immensely long periods of cyclic stress during sliding.”

The alloy was made using high-tech methods, depositing films atom by atom using a magnetron. The structure of these films consisted of columns with grain sizes about 40nm. The alloy appears like pure platinum, silvery white in colour and little heavier than gold, and is no harder than other platinum gold alloys but is much better at resisting heat and much more wear resistant.

The material had another surprise in store. While measuring its properties in sliding tests, the team noticed that a black film was forming on the surface of the alloy. This turned out to be diamond -like carbon, a highly efficient solid lubricant that is used in high-performance internal combustion engines. Normally, this requires special conditions to manufacture, but the Sandia alloy forms are spontaneously.

“We believe the stability and inherent resistance to wear allows carbon-containing molecules from the environment to stick and degrade during sliding to ultimately form diamond-like carbon,” Curry said. “Industry has other methods of doing this, but they typically involve vacuum chambers with high temperature plasmas of carbon species. It can get very expensive.” This phenomenon is described in a separate paper in the journal Carbon.

The alloy could be highly significant for the electronics industry, where sliding metal contacts are common components in many devices. Because these contacts tend to be very small, they wear out quickly and either need to have expensive protective coatings or be replaced regularly. Using the super durable alloy, even though it is made of expensive materials, could save hundreds of millions of dollars every year materials alone, according to Argibay. Applications can be found in many other industries, including aerospace systems and wind turbines.

Moreover, the diamond-like carbon discovery could lead to simpler and cheaper ways of making this lubricant material, the team added.