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Taming the Untamable: The Art and Science of Diamond Polishing

By: , Posted on: August 7, 2015

diamonds
https://www.flickr.com/photos/serendipitydiamonds/15658738986

Diamonds can be cut despite being the hardest known material. The process that is used to polish diamonds destined for jewelry has remained unaltered for centuries. Single crystal diamond is pressed against a rotating disc with embedded diamond grit; diamond cuts diamond. Microscopic and atomistic wear mechanisms associated with this process have been studied for over a century. It is now known that polishing the cubic and dodecahedral planes of diamond induces a mechanochemical sp3-sp2 order-disorder transition resulting in an intermediate instable amorphous adlayer while wear on the octahedral plane progresses through microfracture. This wear anisotropy still impedes planarization of polycrystalline chemical vapor deposition diamond coatings.

The word “diamond” derives from the Greek term “adamas,” which means untamable and refers to its main physical feature: its extreme hardness. Although the exact place and date of origin of diamond polishing is unknown (Lenzen, 1966), the fact that the first guild of diamond cutters and polishers (diamantaire) was formed in 1375 in Nuremberg, Germany (Tolansky, 1961) suggests that mankind learned to tame the untamable more than 600 years ago.

Tribological Anisotropy and Associated Phenomena

The scientific story of diamond polishing begins with the work of Tolkowsky (1920) who encapsulated the observations of centuries of lapidaries in the language of crystallography. During the traditional diamond polishing process, a very marked dependence on orientation of the diamond under polish with respect to the rotation of the polishing wheel, or scaife in the parlance of the industry, is observed. The scaife, shown in Figure 1, is a circular disc of cast iron about 0.3 m in diameter which rotates at roughly 3000 rpm (tangential linear velocity of 30 m s−1). In this respect it is not dissimilar from other lapping operations other than the scaife is loaded with diamond grit; viz. the same material that it is to polish. The grit typically has a size <40 μm, the size distribution being chosen to reflect the operation, and must be worked into the polishing wheel prior to its use (see later), and periodically through the life of the scaife plate. The diamond to be polished is usually held in a dop (a small cup or collet) that in turn is held in a hand tool called a tang. Using this apparatus, the diamond is gently placed onto the scaife, and it is at this point that the anisotropy in wear will be noticed. Practical aspects of diamond lapidary techniques can be found by the standard practical text by Watermeyer (1982).

diamantaire
Figure 1. A diamantaire at a typical diamond polishing bench showing the scaife plate, the dop, and the tang. Image: Wikimedia commons.

The polishing directions 〈d〉 on any diamond surface {s} in which a diamond can be worn with relative ease are known as “soft” directions of polish. If the diamond is rotated azimuthally from these directions, the wear rate will decrease to a minimum. These are known as “hard” directions. For the basal planes of diamond, this anisotropy can be stated as follows: for the cube plane ({s  }={100}), the directions of polish 〈100〉 are known to be “soft,” while 〈110〉 are “hard”; that is to say there is fourfold symmetry. The dodecahedral plane ({s  }={110}) has two polishing directions, 〈100〉, on which polishing proceeds fastest, relative to all other crystallographic planes, while 〈110〉are hard. The octahedral plane ({s}={111}), also the predominant cleavage plane in diamond ( Field, 2012), is almost uniformly difficult to polish, but it may be polished in〈112〉 directions, of which there are three. These relationships were first mapped out by Tolkowsky (1920), whose data is replotted in Figure 2, and have since been reproduced numerous times using a variety of techniques ( Bergheimer, 1938Denning, 1953Grillo, Field, & van Bouwelen, 2000Grodzinski & Stern, 1949Grodzinski, 1949 and Grodzinski, 1953Hukao, 1955Knight & White, 1989Kraus & Slawson, 1939;Slawson & Kohn, 1950Whittaker & Slawson, 1946Wilks and Wilks, 1954Wilks and Wilks, 1959 and Wilks and Wilks, 1972Wilks, 1952Winchell, 1946). Grillo, Field & van Bouwelen have correlated this wear anisotropy to a frictional anisotropy; “soft” directions of polish are associated with high friction and “hard” directions with low friction.

polishing
Figure 2. Wear anisotropy exhibited by the three basal planes of diamond as a function of azimuthal angle relative to the direction of rotation of the scaife. The plots are reproduced from the results of Tolkowsky (1920) and show minima at zero. It is likely that wear in these directions does occur but was below the sensitivity of his apparatus.

 

A consequence of the high friction in soft directions is the frictional heating of the diamond that will occur when the correct polishing direction is found. However, the diamond should never be allowed to become candescent, as this will damage the lap of the scaife and increase the probability of thermal fracture of the diamond. Another consequence of the anisotropy in friction is the vibration and audible noise emitted from the diamond under polish which can be used as a guide to orientation.

A well-polished diamond will come off the scaife with a facet having the appearance of a mirror; wear and polishing have occurred simultaneously and this is considered favorable. This is, however, not the only method of producing wear on diamond. Examples of abrasive wear can be observed on natural alluvial diamonds that may have rounded facets. Bruting, a precursor to forming diamond into many gemstone cuts, is also an abrasive wear process which involves the diamond being turned on a lathe using another diamond, appropriately orientated, as the “tool” to produce a cylindrical form.

Learn more about diamond polishing in the article, Taming the Untamable-The Art and Science of Diamond Polishing. This article examines the studies of the scaife preparation technique, the chemical identity of the wear debris and the diamond surface and the scaife surface as well as the new era of polishing technology and insights into the scientific mechanisms which produce wear on diamond.

Read more about properties of diamonds and their applications in the below articles:

Surface Electronic Properties of Diamond by Christoph E. Nebel

Polycrystalline CVD Diamond for Industrial Applications by Eckhard Wörner and Christoph Wild

Diamond Nanoparticles: Surface Modifications and Applications by Anke Krueger

Diamond for Particle and Photon Detection in Extreme Conditions by Eleni Berdermann

Single Color Centers in Diamond: Materials, Devices, and Applications by Igor Aharonovich and Thomas Babinec

Electrochemical Application of Diamond Electrodes by Yasuaki Einaga

Mat Sci Mat Eng Reference Modules CoverThis excerpt is taken from the article Taming the Untamable-The Art and Science of Diamond Polishing by Michael Moseler, Lars Pastewka and Jonathan Hird from the Major Reference Work Comprehensive Hard Materials which will be part of the new Reference Module in Materials Science and Materials Engineering, learn more here.

 

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