The Nitty-Gritty of Diamond Polishing

Researchers reveal atomic secrets of an ancient art.

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By Rachel Ehrenberg, Science News

You can soften your sweetie with diamonds, but to soften a diamond—the hardest material on Earth—takes some special chemistry.

New research finds that a liquidlike layer of carbon at the interface between a diamond-polishing wheel and a diamond creates the magic that turns a grubby stone into a girl’s best friend.

New polishing tricks may emerge from the research, perhaps allowing scientists to exploit diamonds for use in semiconductors or optics. And the computational modeling approach used in the study, published online November 28 in Nature Materials, could aid in understanding wear in materials such as metals or ceramics.

Gemstones are typically worked with materials that are harder than themselves, but this isn’t possible with diamond—it is one of the hardest natural materials, and anything harder is even more rare. So polishers use diamond on diamond. The technique, which has remained unchanged for some 500 years, involves coating a cast iron disk with olive oil and a layer of diamond grit. The stone to be polished is pressed against this “scaife,” and it’s up to the skill of the polisher to find the grain of the diamond. Polish in the wrong direction and you can damage the gemstone.

It was thought that polishing just fractured off little bits of diamond. But  scientists recently noted little shapeless particles on the surface of the polishing wheel that couldn’t have resulted from fracturing. Now modeling the molecular bonding of 10,000-odd diamonds reveals that those little particles are part of a liquidlike layer of carbon that forms between the scaife and the gemstone. This layer joins forces with the sharp diamond grit and oxygen in the air to polish the diamond.

When the gemstone and wheel first make contact, carbon atoms in the grit and the gemstone form very strong bonds, seeding a strange amorphous layer of carbon, says Michael Moseler of the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, Germany. This liquidlike layer between the two surfaces is very reactive, says Moseler, who led the new work. As the sharp edges of the grit plow by, they scrape off some of the liquidy layer and expose long carbon chains to the air. Oxygen then can swoop in, snatching off carbons to form carbon dioxide and leaving behind a smoother surface than scraping alone would achieve.

The research also explains why polishing in certain directions is difficult and potentially damaging. The latticelike arrangement of carbon atoms in a diamond is such that in some directions the carbon atoms are so tightly bound that they resist becoming part of the amorphous layer.

“The exciting thing about this paper is that molecular dynamics could prove to be the most efficient way of designing new diamond-processing technologies,” says physicist Jonathan Hird of UCLA. The new study describes diamond polishing with unprecedented detail, he says.

Further research is necessary to validate the modeling, which may be tough.  “Studying diamond polishing is experimentally challenging and expensive,” Hird says. “Diamond does not give up its secrets without a fight.”

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