Besides a direct transformation of graphite into diamond and the most common synthesis technique with molten catalyst, there are more synthesis methods [WEDL77]. These are Shock Wave Synthesis, growth from carbon molt and chemical diamond deposition from gas phase (Chemical vapor deposition, CVD). During CVD synthesis, carbon-containing gas like methane is disintegrated in presence of hydrogen at 2000 °C or in plasm sparks. At normal pressure, the dissoluted products condense on appropriate areas. Graphite seeds are hydrated quickly to methane, but diamond seeds grow faster than they are decreased by hydrogenation [HOLL95]. The resulting diamond layer is polycrystalline.
2.3.4.1 Grinding Tools
After the successful synthesis of diamond, natural diamond in grinding tools was replaced more and more by synthetic diamond [NOTT80]. The first commercially available synthetic diamond in 1955 was friable and polycrystalline, arising presumably from a lack of control in the early days of diamond synthesis [DYER79]. However, this type of grit had advantageous self-sharpening abilities not displayed by natural diamond.
Today, grinding tools of resin bond consist mostly of friable diamond, such as natural diamonds of lesser purity or synthetic diamonds with defined defects. In metallic bonds, cubic diamond grits with high toughness are applied. Naturally, a grinding wheel with blocky diamonds has lower wear rates than a tool with friable grits; in contrast, the grinding forces are higher due to the higher friction between flat grit areas, workpiece, and chips [WIMM95, BAIL99].
Diamond wears because of diffusion and graphitization during grinding of ferrous materials with low carbon content [KOMA76]. The diamond turns to graphite in the surface layers, which is accelerated by oxygen as catalyst [KOMA76]. Then the carbon diffuses from graphite into the ferrous material.
Research is ongoing on engineered wheels with defined grit patterns or CVD diamonds as abrasive layer (see Sect. 9.2 “Innovative and More Sustainable Tools”).