A new generation of diamond bits known as polycrystalline diamond compact (PDC) bits were introduced in the 1980’s (Figure 5). These bits have the same advantages and disadvantages as natural diamond bits but use small discs of synthetic diamond to provide the scraping cutting surface. The small discs may be manufactured in any size and shape and are not sensitive to failure along cleavage planes as with natural diamond. PDC bits have been run very successfully in many areas around the world. They have been particularly successful (long bit runs and high ROP) when run in combination with turbodrills and oil based mud.
PCD (Polycrystalline Diamond) is formed in a two stage high temperature, high pressure process. The first stage in the process is to manufacture the artificial diamond crystals by exposing graphite, in the presence of a Cobalt, nickel and iron or manganese catalyst/solution, to a pressure in excess of 600,000 psi. At these conditions diamond crystals rapidly form. However, during the process of converting the graphite to diamond there is volume shrinkage, which causes the catalyst/solvent to flow between the forming crystals, preventing intercrystalline bonding and therefore only a diamond crystal powder is produced from this part of the process.
In the second stage of the process, the PCD blank or ‘cutter’ is formed by a liquid-phase sintering operation. The diamond powder formed in the first stage of the process is thoroughly mixed with catalyst/binder and exposed to temperatures in excess of 14000 C and pressures of 750,000 psi. The principal mechanism for sintering is to dissolve the diamond crystals at their edges, corners and points of high pressure caused by point or edge contacts. This is followed by epitaxial growth of diamond on faces and at sites of low contact angle between the crystals. This regrowth process forms true diamond-to-diamond bonds excluding the liquid binder from the bond zone. The binder forms a more or less continuous network of pores, co-existing with a continuous network of diamond. Typical diamond concentrations in the PCD is 90-97 vol.%.
If one requires a composite compact in which PCD is bonded chemically to a tungsten carbide substrate (Figure 18), some or all of the binder for the PCD may be obtained from the adjacent tungsten carbide substrate by melting and extruding the cobalt binder from the tungsten carbide. The cutters can be manufactured as disc shaped cutters or as stud cutters, as shown in Figure 19.
Institute of Petroleum Engineering, Heriot-Watt University