Turning the Hardest Known Abrasive Into a Dry Lubricant

Diamond.  The world's hardest commonly available mineral (yes there are harder oddities that have never been synthesized in industrial quantities, as diamond has) and a favorite abrasive for as long as they've been known.  Diamond coated tools are widely used in industry, and I personally think the whole "granite kitchen counter" movement couldn't have happened without widely available diamond tools.

So when I think about diamond, I think about uses as an abrasive; perhaps a coating on a machine tool.  I'd never think of it as lubricant, but according to Machine Design some researchers at Argonne National Laboratory have found just that.  When they mixed molybdenum disulfide, commonly used as a lubricant in high temperature applications, with tiny diamond particles they call nanodiamonds, the diamonds turned into a form of carbon that creates a nearly frictionless lubricant that essentially doesn't wear out. 

The most commonly used solid lubricants on the market today take the form of graphite paste. They are used as lubricants to grease doorknobs and bike chains, among other things.

In 2015, one of the researchers, Anirudha Sumant, made a breakthrough in solid-lubrication technology by demonstrating superlubricity (near-zero friction) at engineering scale for the first time by using graphene combined with nanodiamonds. This approach was revolutionary, and since then his group has continued to further develop the technology.

Graphene, of course, is one of today's "wonder materials" that's being researched for just about everything.   Its structure is essentially the same as graphite's, in that it's two dimensional hexagons of carbon atoms in sheets looking much like atomic-scale chicken wire.  Unlike graphite, it forms large, continuous sheets a single carbon atom in thickness but of macroscopic sizes. 

Most recently, Sumant replaced graphene in the process with molybdenum disulfide to see how other materials would behave. He was expecting the process to resemble the one observed with graphene-nanodiamond lubricant. However, the team was surprised when they couldn’t see nanodiamonds in the material. Instead, they found balls of onion-like carbon.

The molybdenum disulfide was breaking up into molybdenum and sulfur and reacting with the nanodiamonds to convert them into onion-like carbon. Onion-like carbon consists of several layers of spherical graphitic shells that can be used as a dry lubricant. And the process of combining molybdenum disulfide and nanodiamonds automatically creates this form of carbon without any additional chemical application. The lubricant is also self-generating and readjusts itself continuously, so it lasts longer.

These carbon balls sustain high contact pressure and, due to their unique nanostructure, glide easily, creating superlubricity. The team concluded that the sulfur diffusion increased the strain in the nanodiamonds, subsequently breaking them and converting them into onion-like carbon.

The term "onion-like carbon" is one I've never heard; I imagine something like shells of something like graphite arranged similar to the layers of an onion.  They estimate the friction of this combination to be 1/10 of fluropolymers (I assume they mean Teflon and similar), which means moving parts will perhaps last 10 times longer before wearing out.   They also point out that there won’t be any hazardous liquid residue or the need to use and dispose of rags as part of the clean-up process (no word on its toxicity to parrots).  Machine Design speculates it could also be used to make parts that can’t be made today, especially with metal stamping.

While molybdenum disulfide is a bit more expensive than graphene, less is needed in this process.

“The amount is so small — a few drops for kilometers of sliding — that cost is not an issue,” Sumant said.

I suppose it's the counterintuitive result that adding a fine abrasive to an ingredient used in grease creates a better lubricant rather than grinding paste that appeals to me about this story.  Argonne National Laboratory already has three patents on the superlubricity technology, with a patent pending on this breakthrough, and will soon be licensing the technology. 

The formation of the "onion-like carbon"; the left picture shows the nanodiamonds (brown) lying on the molybdenum disfulide sheet, then being wrapped up in the MoS2, and eventually turning into the "OLC" on the right.  Full story in their paper in Nature.