Many of us non-engineers think of metals as being incredibly tough and durable. And while it is true that these forged materials are very tough by the technical definition – they don’t break or fracture when force is applied – and also very strong – a high level of force has to be applied to them before they deform – both pure metals and even our best alloys, like steel, are lacking in terms of hardness.
To a materials scientist, hardness refers to a substance’s ability to stay together when friction is applied. Metals don’t hold up well in this regard; in engines and other mechanical devices where metal parts grind against other metal parts, the friction will cause the outer layers to wear away over time. This is why protective lubricants like oil are used on gears and pistons and why saws and screws have diamond-tipped blades and bits.
In electronics with moving metal-to-metal connections, engineers will often coat cheaper metals with more naturally resistant precious metal alloys to increase the device’s longevity; but even then, this coating will eventually get scraped away due to repetitive motion.
Yet now, a team of researchers from Sandia National Laboratories has developed a new type of metal that shatters world records for wear resistance. Results from their analyses, published in Advanced Materials, show that their 9 to 1 platinum-gold alloy is 100 times more durable than high-strength steel. It is the first all-metallic material to exhibit wear rates comparable to the hardest known materials, sapphire and diamond-like carbon (DLC).
In laboratory experiments where the metal was slid along a stainless steel friction track over and over, the surface of the alloy showed negligible signs of structural change upon scanning electron microscope imaging after 100,000 sliding passes.
“These wear-resistant materials could potentially provide reliability benefits for a range of devices we have explored,” Chris Nordquist, a Sandia engineer not involved in the study, said in a statement. “The opportunities for integration and improvement would be device-specific, but this material would provide another tool for addressing current reliability limitations of metal microelectronic components.”
The Sandia team notes that the impressive properties of 9 to 1 platinum-gold alloys are already well documented. Their formulation’s exceptional resistance comes from the unique way they are created: very thin layers of platinum-gold are deposited, one at a time, on top of a high-purity platinum-gold base using a vacuum-sealed magnetic chamber. The inspiration to use this fabrication method came from computer models of how individual atoms would interact.
Although the stability under mechanical stress was predicted by their simulations, the platinum-gold alloy also had some unexpected benefits. In an experiment documented in the journal Carbon, the material spontaneously created its own lubricant – a coating of DLC – during a wear test. DLC is one of the best lubricants because it has the hardness of diamond but slickness of graphite.
“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,” first author John Curry explained. “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.”