Superhard Diamond Alloy May Make Superior Cutting Tools

Bond movie titles can be deceiving-- diamonds are not forever when it comes to demanding tasks like cutting steel. Now, researchers have combined diamond with cubic boron nitride, a hard human-made material, to make an alloy that outperforms commercial cutting tools. If the material can be made on a practical scale, it could be useful in the construction, automotive, or aerospace industries.

Top: Samples of diamond-cubic boron nitride alloy materials. Bottom: Polished rake faces of cutters... [+] made from the alloy. (Credit: D. W. He/SCU)

The work, from a team led by Duanwei He of Sichuan University's Institute of Atomic and Molecular Physics, in Chengdu, China, appears in the journal Applied Physics Letters.

Let's say you're on a construction project that requires you to machine steel and then cut granite. Tools that cut, grind, and polish have two essential requirements: they have to be harder than the material they're cutting, and they mustn't degrade as a result of the heat that the cutting process generates. Diamond is as hard as they come, but it isn’t chemically impervious. Its surface can oxidize at the temperatures needed to work iron-containing alloys such as steel. Cubic boron nitride has superior stability, so it is the go-to abrasive for many kinds of steel, among other materials. Yet it’s not as hard as diamond, so it can’t always best diamond’s performance.

It seems logical to somehow mix diamond with cubic boron nitride to get the best of both worlds-- a universal cutting material. That’s a tall order. One big challenge, it turns out, has been obtaining enough material to test for stability and cutting performance.

He and colleagues have solved the size problem. Their new alloys reach 3 millimeters in diameter. That's about one-eighth of an inch, large enough to process into cutting tools. To make the alloys, the researchers subjected 1:1 mixtures of powdered diamond and cubic boron nitride to a range of simultaneous high pressure (about 100,000 to 200,000 times Earth's air pressure at sea level) and high temperature conditions (from 1300 to 2600 degrees Kelvin, about a third as hot as the sun's surface).

The best of He’s alloys outperformed commercial tools containing either polycrystalline diamond or polycrystalline cubic boron nitride in high-speed cutting tests on hardened steel and granite. The alloys also resulted in longer tool life compared to the other two materials. Importantly, the alloys have the expected blend of properties-- nearly as hard as diamond and almost as inert as cubic boron nitride.

Two questions will be the focus of future work. First, what chemical insights can be gleaned from this alloy to design even better materials? The team's measurements suggest that bonds may have formed between boron and carbon as well as carbon and nitrogen, most likely at the interfaces between grains of the two materials. The quantity and nature of the bonds isn't clear.

Second, will this high-pressure, high-temperature synthesis scale up consistently and cheaply? It takes energy to heat and press this alloy into existence. The team is working to synthesize centimeter, rather than millimeter, sized alloys. Time will tell whether those will compete at the bank as well as they do at the machine shop.