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German reserachers have uncovered the atomic mechanism behind diamond grinding, explaining why some planes are easier to polish than others.

In a paper published today in the journal Nature Materials, a team of researchers at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, Germany, say their findings will have broader implications for understanding friction and wear on materials.

Gemstone polishers have long known that diamonds have 'hard' and 'soft' surfaces, the soft surfaces being easier to polish. They knew 'softness' related to the crystal arrangement on different surfaces, and the polishing direction.

Polishing on 'hard' crystal planes is laborious and leads to inferior facets, but until now the reasons were not fully understood.

The various planes are numbered to refer to their crystal lattice orientation. Dr Lars Pastewka, of the Fraunhofer Instiute, explains: "The hard plane (the {111} plane) is rather flat and closely packed. The atoms sit much closer to each other than on the other surfaces. The {110} plane consists of parallel rows, similar to the furrows in a freshly ploughed field; and the {100} surface looks like an array of rooftops."

Initially it was surmised that during the polishing process small chips of diamond crystals were removed as a result of unequal crystalline toughness on different crystal axes.

But with the advent of electron microscopes it could be seen that such a process accounts for wear only on the hard surfaces, which explains why they produce poor facets.

Pastewka says that under electron microscopy it was possible to see that amorphous carbon - carbon lacking a crystal structure - was somehow playing a role, but it wasn't until the current research at the Fraunhofer Institue that the link was understood.
When a diamond isn't diamond

"The moment a diamond is ground, it is no longer a diamond", the head of the research group Professor Michael Moseler explains.

He says the high-speed friction between a rough diamond and the polishing wheel creates a glass-like carbon phase on the surface of the stone. The speed at which this material phase appears depends on the crystal orientation of the rough diamond.

The new material is then removed in two way. The diamond particles on the wheel scratch off tiny carbon dust particles from the surface. This would not be possible in the original diamond state, in which the bond forces are too great, thus the diamond is too hard.

The second mechanism at work is the action of oxygen on the diamond's surface. The oxygen molecules bond with carbon atoms within the unstable long carbon chains that have formed on the surface, to produce carbon dioxide.

"Originally we developed a method that allowed us to investigate the properties of 'diamond-like' carbon films," Pastewka says. "Out of curiosity we applied this method to diamond, and found the amorphisation process."

"We then looked into the scientific literature on diamond polishing and found that our model actually predicts the experimental wear rates and that the exact wear mechanism had so far not been understood.

"At this point we became very excited about this work and analysed our simulations in much more detail to uncover the details of the amorphisation process."
Easily removed

Pastewka says the amorphisation leaves a disordered, loosely bonded layer on the diamond's surface.

"The amorphous phase (on the {100} surface) is much softer than the diamond and can be chipped away," he says. "It is also susceptible to oxidation, which means it partially burns away."

Pastewka says this finding is important because it is one of the first wear-processes to be understood on the atomic scale.

"Wear is an important phenomenon in engineering," he says. "You either want it, because you want to machine let's say a piece of steel, or you don't want it because the bearings in a motor should run for years and should not increase fuel consumption."

"So far most wear models are empirical. That is, it is known how wear proceeds but it is not known exactly why the materials wear. These insights might help us devise ways to control wear and tailor it for specific applications."

"As far as diamonds go, this could speed up the polishing process, and hence produce cheaper (anti-friction) diamond films."

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