In this article, we will describe what PCD is by discussing how it is made, its unique material structure and the resulting physical and mechanical properties that make this type of synthetic diamond an interesting material. We will then discuss applications in the oil and gas sector where diamond has been applied successfully and has contributed to lowering costs and improving reliability. Finally, we will discuss where in the pumping and systems sector we feel the use of diamond might make sense. Of course, there are a number of applications we have not considered. As stated previously, we intend to encourage the creativity of engineers working in the pumps sector to develop new applications that can take advantage of the unique benefits of synthetic diamond.
Background
Polycrystalline diamond is sintered by placing diamond grit under high pressure and temperature, 1,500 deg C and 6 GPa respectively, in the presence of a metal catalyst. This is accomplished in a high-temperature, high-pressure press (see Figure 1 ). At these conditions, which are similar to those where diamond grit is synthesized, the diamond grains will grow together. In the end, a microstructure is formed of cemented diamond grains with interstitial pore space filled with the remnants of the metal catalyst. Typically, the catalyst may comprise 10 percent or less of the total volume. Shown in Figure 2 is a typical micrograph of PCD material.
Figure 1 . A high-temperature, high-pressure cubic press suitable for sintering polycrystalline diamond. The press shown stands approximately 3 m high.
Figure 2 . Scanning Electron Microscope (SEM) micrograph of PCD at 500X magnification. The dark portions are diamond grains that are surrounded by the cobalt catalyst.
Because of the random orientation of its diamond grains, PCD is tougher than natural diamond, synthetic diamond coatings and Carbon Vapor Deposition (CVD) diamond. It is most often sintered on a tungsten carbide substrate that provides the catalytic metal and elastic support for the diamond layer, although it can be sintered as a standalone slug. Shown in Figure 3 are examples of polycrystalline diamond compacts (PDCs). The black layer seen on the top is the diamond layer (PCD), which ranges from 1 to 3 mm thick depending on the application. This additional thickness provides properties and abilities that coatings cannot match.
Figure 3. PDCs typically used as cutting elements in PDC drag bits and roller cone drill bits.
In addition to high strength and fracture toughness, PCD is heat conductive, hard and abrasion resistant. Table 1 provides a partial list of the physical and mechanical properties of diamond that make it effective for many difficult applications. Nevertheless, because synthetic diamond is sometimes thought of as an exotic and expensive material, it is often not routinely considered for applications where it could be suited.
Table 1 . Typical thermal and mechanical properties of PCD
|
Property |
Value |
Source: |
|
Density |
3.90 g/cm 3 |
Bertagnolli, US Synthetic |
|
Compressive Strength |
6.9-7.6 GPa |
Roberts, Debeers |
|
Tensile Strength |
1.3-1.6 GPa |
Cooley, US Synthetic |
|
Young's Modulus |
841 GPa |
Roberts, Debeers |
|
Fracture Toughness |
13-15 MPa 1/2 |
Jiang Qian, US Synthetic |
|
Hardness |
49.8 GPa (Knoop) |
Roberts, Industrial Diamond Review |
|
Coefficient of linear expansion |
1.3-3.9*10-6/°C |
Glowka, Sandia National Laboratory |
|
Coefficient of friction PCD on PCD in H2O |
0.05-0.08 |
Sexton, US Synthetic |
|
Thermal Conductivity |
543 W/m°C |
Lin, UC Berkeley |
Applications in Oil and Gas: Drill Bits, Drill Motors and Turbines
Diamond, of course, is the hardest material known to man with a Mohs hardness of 10 and a Knoop hardness of 49.8 GPa. It is precisely this hardness that permits diamond to penetrate the hard minerals that constitute rock and allow diamond to operate effectively as cutting elements in drill bits. Diamond offers fracture toughness and benefits in abrasion resistance required to withstand the intermittent loading associated with cutting rock. High thermal conductivity keeps the heat from building up too rapidly, thus maintaining the cutting edge of the PDC.
Examples of a PDC drag bit and a roller cone bit can be seen in Figure 4. PDC is used in both bit types, but it has found the most extensive use in PDC drag bits. In 1974, PDC drag bits were introduced. By 1991, they were used in perhaps 5 percent of the drilling done for oil and gas and only in the softest rock formations. Through the years, with improvements in toughness and wear resistance of PCD material, market share has expanded. Now, approximately 60 percent of the footage is drilled with PDC drag bits, including some hard and difficult rock formations.
Figure 4 . Examples of PDC drag bit and roller cone drill bit
Drill Motors and Turbines
During drilling, drilling fluid is pumped through the drill string to cool the bit, stabilize the well bore and
