CVD diamond forms as a fine crystalline columnar structure. There is a certain amount of preferred crystallographic orientation exhibited; more so than, for example, PCD, but far less than in single crystal diamond. Wear characteristics are therefore much less sensitive to orientation in a tool. Again, the CVD diamond is not used as an abrasive but is proving very promising when fabricated in the form of needle-shaped rods for use in dressing tools and rolls. Fabrication with CVD is slightly more difficult as it contains no metal solvents to aid EDM wire cutting and diamond wetting also appears more difficult and must be compensated for by the use of an appropriate coating.
5.6.12 Development of Large Synthetic Diamond Crystals
In the last 10 years, increasing effort has been placed on growing large synthetic diamond crystals at high temperatures and pressures. The big limitation has always been that press time and hence
cost goes up exponentially with diamond size. The largest saw grade diamonds are typically 30 to 40#. The production of larger stones in high volume, suitable for tool and form-roll dressing applications, is not yet cost-competitive with natural diamond. However, there has recently proved to be an exception to this, namely, the introduction, first by Sumitomo, of needle diamond rods produced by the slicing up of large synthetic diamonds. The rods are typically less than 1 mm in cross section by 2 to 5 mm long (similar in dimensions to the CVD diamond rods discussed above) but orientated along the principal crystallographic planes to allow optimized wear and fracture characteristics when orientated in a dressing tool. Several companies now supply a similar product.