Mystery of diamond`s soft side solved
London: A German research has decoded the atomic mechanism behind diamond grinding, explaining how the hardest known material in the world can be machined.
A team of researchers at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, Germany, said their findings would have broader implications for understanding friction and wear on materials.
Diamonds have been ground by craftsmen for hundreds of years using cast iron wheels studded with fine diamond particles turning at around 30 meters per second at the outer rim.
A highly tuned sense of sound and feeling enable an experienced diamond grinder to hold the rough diamond at just the right angle to achieve a smooth and polished surface.
The fact that diamonds react directionally has been known for a long time.
The physical phenomenon is known as anisotropy. The carbon atoms in the diamond lattice form lattice planes, some of which are easier to polish than others, depending on the angle at which the diamond is held.
For hundreds of years, researchers were looking for a logical way of explaining this empirical phenomenon, and have so far been unsuccessful.
Equally, no one was able to explain why it is possible that the hardest material in the world can be machined.
Dr. Lars Pastewka and Prof Michael Moseler have answered both these questions with the help of a newly-developed calculation method.
“The moment a diamond is ground, it is no longer a diamond,” says Moseler.
Due to the high-speed friction between the rough diamond and the diamond particles in the cast iron wheel, a completely different ‘glass-like carbon phase’ is created on the surface of the precious stone in a mechanochemical process.
The speed at which this material phase appears depends on the crystal orientation of the rough diamond.
Moseler said the new material on the surface of the diamond is then ‘peeled off’ in two ways - the ploughing effect of the sharp-edged diamond particles in the wheel repeatedly scratches off tiny carbon dust particles from the surface.
The second, equally important impingement on the normally impenetrably hard crystal surface is due to oxygen (O) in the air.
The O2 molecules bond with carbon atoms (C) within the instable, long carbon chains that have formed on the surface of the glassy phase to produce the atmospheric gas CO2, carbon dioxide.
“We looked extremely closely at the quantum mechanics of the bonds between the atoms at the surface of the rough diamond breaking. We had to analyze the force field between the atoms in detail,” said Pastewka.
“This provided the basis for investigations into the dynamics of the atoms at the friction surface between a diamond particle on the wheel and the rough diamond itself,” he added.
He and Moseler calculated the paths of around 10,000 diamond atoms and followed them on screen. Their calculations paid off - their model is able to explain all the processes involved in the dusty and long misunderstood method of diamond grinding.
The study is published in the current online issue of Nature Materials.
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