Secret of Diamond Polishing Understood

It turns outthat diamond polishing is stranger than we imagined.  An amorphous layer is produced that can thenbe removed.  This suggests a new strategyfor preparing fine surfaces on nano engineered material.

It is alittle bit like using pressure to induce failure and electrostatic stickinessto remove debris from the finished surface. It lends itself to fine work.

This impliesthat we will see this used in technology rather quickly on even quite ordinarymaterials.  It is a new useful method.

Secret of diamond polishing revealed

Dec 2, 2010

It is the hardest everyday material on Earth, sowhy does diamond glisten when rubbed against another diamond? Now, the ancientbut mysterious process of diamond grinding may have been explained byphysicists in Germany,who have created a model for explaining the frictional interactions at themolecular level.

For centuries precious-stone merchants havepolished diamonds by grinding them with cast-iron wheels embedded with coarsediamond fragments. It is not clear why this procedure is so effective atcleaning diamonds, but experience suggests that it works far better when thediamond is fixed at certain angles to the wheel than others.

This directional dependence of diamond grindinghas now been investigated by Lars Pastewka at the Fraunhofer Institute forMechanics of Materials who set out to investigate the phenomenon. Working withcolleagues at several other institutes across Germany, he has developed a quantummechanical model to study the atomic interactions in "diamond-like"carbon films, which are often used in industry to reduce friction in machinery.

Diamond in the rough

But when the researchers applied their model todiamond itself, they were surprised to find that it accurately predicted theexperimental wear rates for this material – even though the exact wearmechanism has so far remained poorly understood. "At this point we becamevery excited about this work and analysed our simulations in much more detailto uncover the details of the process," Pastewka

Pastewka's team set about simulating diamondgrinding using 70 computer processors running for a year, and discovered thatduring the grinding the diamond surfaces were being transformed into soft,amorphous layers. These thin films can then be easily removed by eitherchipping them away, or through carbon molecules bonding with oxygen in theatmosphere, leaving behind clean diamond surfaces.

This creation of the amorphous film occurs becauseof existing imperfections at the diamond surface, including the build-up ofdirt over time. As a diamond atom slides over the surface it repeatedly pullsat the diamond crystal's atoms, and sometimes removes an atom from the crystalsurface, which becomes part of the amorphous layer.

Like a stack of paper clips

"Imagine you have a stack of paper clipsneatly arranged on your desk," explains Pastewka. "Now you take amagnet and move that over these clips at a certain height. You cannot keep theheight ideally constant, so if the height is right you will pull some paperclips to your magnet and others will remain on the desk."

Changfeng Chen, a materials scientist at the University of Nevadain the USis impressed by the research and its potential to boost industrial processes."This research is of particular significance in nanotechnology where theorientations of nanoscale crystallites can be well defined andcontrolled," he says. "The predicted orientation-dependentanisotropic amorphization wear mechanism may open doors to a new level ofmaterial processing, ranging from better designer jewellery to superior high-techdevice components."

To develop the work, however, Pastewka's teamintends to further investigate diamond's surface chemistry, and is currentlywriting a paper on the oxidation of the amorphous layer.

This research is described in a research paper in Nature Materials.

Aboutthe author

James Dacey isa reporter for

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