Diamond is the hardest material known and is commonly used as a superabrasive for removing excess materials. Typical superabrasive products include diamond saws, grinding wheels, polishing pads and the like. Diamond superabrasives have been commercially available in a number of mesh sizes. For example, diamond saws typically incorporate diamond particles having a U.S. mesh size of 18 (about 1 mm) to 60 (about 0.25 mm). Grinding applications often employ diamond particles having a size of 60 to 400 (about 37 microns) U.S. mesh, while polishing applications typically require diamond fines down to about 0.1 micron. Until recently, diamond particles smaller than about 100 nm were not commercially available.
Diamond micron powders are commonly made by pulverizing waste diamond grains that are not otherwise suitable for ultrafine polishing where nanometer scale smoothness is desired. Further, typical pulverized diamond particles include sharp corners and irregular shapes which are not suitable for polishing of expensive workpieces such as silicon wafers, integrated circuitry, and the like. Most expensive workpieces are currently polished using conventional abrasives such as silicon carbide and aluminum oxide.
In addition, it is typically not commercially feasible to pulverize diamond waste to particle sizes below 100 nm. Moreover, very fine diamond particles are too small to be sized by sieving as they tend to plug the screen holes. Hence, sedimentation has been the primary technique to sort diamond fines by size. Sedimentation techniques can take over a week when particle sizes are smaller than 100 nm, however, can be accelerated somewhat using centrifugal force.
Most diamond superabrasives are synthesized from graphite under ultrahigh pressure (e.g. 5.5 GPa) using molten metal (e.g., Fe, Ni, Co or their alloys) as catalyst. Under conditions of high temperature and high pressure, graphite is dissolved or dispersed in the molten catalyst and precipitates out as diamond. The growth rate of a diamond crystal is partially dependent on the pressure and temperature, and can be as fast as about 1 mm per hour. Typically, the materials in a diamond growth reaction cell are not sufficiently uniform to allow precise control of diamond growth. Moreover, the pressure and temperatures have large gradients which vary during diamond synthesis. Consequently, it would not be practical to grow diamond having a size smaller than a few microns.
An alternative method to grow micron sized diamond is by compressing graphite with a shock wave. Graphite powder can be mixed with a cooling metal (e.g. Cu) powder and the mixture is sealed in a steel tube. The tube is then surrounded by an explosive that is capable of generating a shock wave through the tube when ignited. In this case, a high pressure is maintained for merely about one microsecond. The diamond grains thus formed are typically several microns in size. However, these grains are polycrystalline in nature.
In recent years, nanoparticles of diamond have become commercially available. Such nanodiamond particles are commonly formed by explosion. However, instead of graphite being compressed with a shock wave, the dynamite (e.g. TNT and RDX mixture) itself is converted to nanodiamond during less than a microsecond when both the pressure and temperature are high, i.e. over 20 GPa and 3000° C. Nanodiamonds so formed are typically smaller than 10 nm (e.g. 5 nm) and tend to have a very narrow size distribution, i.e. from about 4 nm to about 10 nm. Moreover, the surface of these nanodiamonds contains diamond or diamond-like carbon, such as bucky balls (C60), layered shells, carbon nanotubes, and amorphous carbon. Thus, these nanodiamonds are extremely hard without sharp corners.
Nanodiamond has been used as abrasives for the ultra-fine polishing of hard materials (e.g. gems), hardening wear resistant coatings (e.g. Cr coatings), strengthening soft materials (e.g. rubber), and as a mechanical lubricant (e.g. engine oil additive). However, the properties and applications of nanodiamond particles continue to be explored.
As a result, compositions and methods of using nanodiamond which improve desirable properties of various compositions continue to be sought.