Tikalon Header

Scratching Diamond

December 6, 2010

It was a common movie cliché, testing whether the diamond was real by scratching a mirror. It was also an example of the Mohs scale of mineral hardness. This scratch test was more believable in the black and white movie era, before cubic zirconia became commercially available. Cubic zirconia has a Mohs hardness of 8.5, and diamond has a Mohs hardness of 10. These will both scratch glass, which has a Mohs hardness of only 6 or 7. There are quite a few minerals that will scratch glass,[1] including quartz, which even looks somewhat like diamond. Only diamond will scratch diamond, as long as you're willing to ignore borazon, a cubic form of boron nitride, and possibly beta carbon nitride. The Mohs Hardness Scale appears below.

Mohs Hardness Sclerometer Hardness Mineral Composition
1 1 Talc Mg3Si4O10(OH)2
2 3 Gypsum CaSO4(H2O)2
3 9 Calcite CaCO3
4 21 Fluorite CaF2
5 48 Apatite Ca5(PO4)3(OH,Cl,F)
6 72 Orthoclase Feldspar KAlSi3O8
7 100 Quartz SiO2
8 200 Topaz Al2SiO4(OH,F)2
9 400 Corundum Al2O3
10 1600 Diamond C

The idea of scratch testing materials goes all the way back to Theophrastus, whom I mentioned in a previous article (Pyroelectric Energy Harvesting, October 15, 2010). In 314 BC, he wrote about minerals that iron could, or could not, scratch in his book, "On Stones."[2] Scratch tests and other tests of gems were noted by Pliny the Elder in his Naturalis Historia (c. 77). Pliny writes that diamond dust is used to cut and polish other gems and that diamond will scratch all other minerals.[3]

Figure caption

Portion of "On Stones" by Theophrastus concerning the fact that iron can shatter minerals that it can't scratch.[2]

You can think of many reasons why a mineral of a higher hardness could scratch one of a lower hardness, the most obvious of these being that the chemical bonds that hold one together are stronger than the other. Also, you can think of reasons why imperfect crystals of the same kind could scratch each other, since more perfect regions of one could occasionally attack less perfect regions of the other. But how can a perfectly crystalline diamond scratch another?

The reason is anisotropy. The wear rate for diamond depends on which crystal face is exposed. Some lattice planes are easier to polish than others. Although this anisotropy was not understood by scientists who study tribology, craftsmen have made use of it since at least Pliny's time. The diamond polishing process employed for the last few hundred years involves pressing diamonds against a rotating iron wheel with fine diamond particles embedded in its surface. These wheels are rotated at about 30 meters per second to allow the craftsman holding the stone to get audible feedback as to when the diamond he's holding is at just the right angle.[4]

A research team at the Fraunhofer Institute for Mechanics of Materials in Freiburg, Germany, has just published a paper in Nature Materials in which they use molecular dynamics modeling to explain how diamonds can be machined.[5] They used quantum mechanics to investigate bond breaking at the surface of diamond in calculations that involved 10,000 carbon atoms. What they found was a change in carbon bond character from sp3 to sp2 that resulted in an amorphous, glass-like, layer of carbon at the surface. The growth rate of this amorphous layer depended on surface orientation and sliding direction, in agreement with experimentally determined wear rates. Carbon is removed from the diamond surface by mechanical scraping; or by oxidation of this layer by ambient oxygen to form carbon dioxide (see figure).[5]

Scratching diamondMaterial removal mechanism during diamond polishing
A sharp-edged diamond particle "peels off" a dust particle from the glass-like phase at the surface of the diamond as oxygen from the air reacts with the carbon at the surface to form carbon dioxide.

(Fraunhofer Institute Illustration)

Diamonds have inspired many things, and also some controversy. One music CD on my bookshelf is "Diamond Music" by Karl Jenkins. Diamond is starting to become a useful electronic material. Polished substrates are important to fabrication of electronic devices, so anything that advances that art is immediately useful.

References:

  1. Steven Miller, "Scratching Diamonds," Argonne National Laboratory, April 29, 2004.
  2. Earle Radcliffe Caley and John F.C. Richards, "Theophrastus on Stones: Introduction, Greek Text, English Translation, and Commentary," Ohio State University (Columbus, Ohio, 1956).
  3. John Bostock and H.T. Riley, "The Natural History. Pliny the Elder," Taylor and Francis (London, 1855), Book 37, chapter 76. Original Latin, here; translation, here.
    Obsianae fragmenta veras gemmas non scariphant, in ficticiis scariphatio omnis candicat. iam tanta differentia est, ut aliae ferro scalpi non possint, aliae non nisi retuso, omnes autem adamante. plurimum vero in iis terbrarum proficit fervor.

    Dust of Obsian stone will not leave a mark upon the surface of a genuine stone: but where the gem is artificial, every mark that is made will leave a white scratch upon it. In addition to this, there is such a vast diversity in their degrees of hardness, that some stones do not admit of being engraved with iron, and others can only be cut with a graver blunted at the edge. In all cases, however, precious stones may be cut and polished by the aid of adamas; an operation which may be considerably expedited by heating the graver.
  4. How to soften a diamond, Fraunhofer Institute for Mechanics of Materials Press Release, November 28, 2010.
  5. Lars Pastewka, Stefan Moser, Peter Gumbsch and Michael Moseler, "Anisotropic mechanical amorphization drives wear in diamond," Nature Materials, nmat2902, Published Online 28 November 28, 2010.