Chemistry, Types of Diamond and Performance

Carbon forms several allotropes: hexagonal graphite, diamond in the cubic zinc blende structure, non-graphitic carbon, and the cage-like fullerene (C60), discovered in 1985 [JAEG10]. Diamond is the hardest material in nature and very resistant against compaction. This is due to the dense packing of the carbon atoms, their regular, symmetrical order, and the energy rich covalent atom bonds [SEN02, TOLA68].

The crystal unit cell of diamond consists of eight carbon atoms (Fig. 2.13). Four of these (8-1/8 + 6-1/2) define a cubic area-centered lattice. The left four atoms are positioned on the centers of the eighth dice and build a second cubic area-centered lattice that is displaced in the direction of the body diagonal with a quarter of its length [HUTT72]. The lattice constant of diamond is a = 0.355 nm. Every carbon atom in diamond is surrounded tetrahedrally by four atoms, in which the angle between two neighboring atoms amounts consistently to 109.5° and the distance is 1.5445 A [HOLL95, HUTT72, MORT87]. These angles originate from the geo­metrical crystal structure due to the sp3-hybridization: when one of the 2s-electrons is lifted into the empty p-orbit from the basic state (1s22s22p2). From three p-orbits and one s-orbit four new similar orbits are generated. The four hybrid orbits push each other away, so that the orbits are directed into the edges of a regular tetra­hedron [HOLL95, MORT87].

Several diamond materials are used in abrasive tools today [MARI04]:

• Natural diamond grits,

• Synthetic diamond grits,

• Synthetic mono-crystalline diamond logs (MCD),

• Synthetic poly-crystalline diamonds in Co-matrix as logs or plates (PCD) and

• Chemically disposed diamonds without binder as logs or plates (CVD).