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/cm3 | 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 MPa1/2 | 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 carry cuttings away. Drill motors use the energy in this fluid to provide rotation to the drill bit.
Figure 5 is a cross-section of a typical drilling motor. The fluid pumped through the motor progresses down a moving cavity created by the unique geometry between the rotor and stator, inducing the rotor's rotation. The result of this action is a net thrust downward on the rotor that must be carried by a bearing assembly at the base of the motor (also shown in Figure 5). Drilling turbines similarly produce a downward thrust that must be captured by bearings.
Figure 5. Drill string, outlining the rotor, stator and thrust bearing set.
Figure 6. The typical construction of a PDC thrust bearing pair
These bearings have benefited from the use of diamond. Figure 6 shows a typical set of diamond thrust bearings used in a drilling motor. The diamond pads rub against one another as one bearing ring is rotated on the other. The thrust loads and speed carried by these bearings can be substantial. For example, a 6 1/2 in diameter drilling motor might turn at 300 rpm and create bearing loads of 178 KN, all of which might be carried by one pair of thrust bearings.
Diamond strength, thermal conductivity, abrasion resistance and erosion resistance are all necessary for these bearings to operate in a dynamic load environment while being lubricated and cooled with an abrasive-laden drilling fluid. A low coefficient of friction is also important. Depending on the exact conditions, the authors have measured coefficients of friction between 0.03 and 0.08. Although these values are low compared to almost any other bearing material pair, there is still significant heat generated because of the high loading and fast speeds. Mixed-mode film lubrication is present at best, and frequent contact between the bearing pads yields associated heat generation and wear. The ability of these pads to make physical contact and still function well is an important attribute in the success of PDC in drill motors and turbines.
Because there is physical contact between the bearing pads, there is also finite wear and an associated bearing life when using diamond bearings in drilling motors. In motor and turbine applications, bearings have lasted between 500 and 3,000 hours depending on the combination of load, speed, cooling and vibration severity. Longer bearing life is possible under more ideal operating circumstances.
Potential Applications in the Pump Sector
PCD may provide a benefit in thrust bearings, radial bearings and other wear parts in pump applications. It may be particularly useful in pumps where bearings and wear components are exposed to abrasive-laden process fluids. As described above, PCD bearings have proven to be successful in oil and gas drilling tools where rock particles in the drilling mud have a tendency to quickly wear out conventional rolling-element or plain bearings. PCD bearings may also provide a benefit in pumps where large loads are causing existing bearings to fail prematurely.
Laboratory tests conducted by US Synthetic have shown that PCD bearings are capable of sustaining high loads. Figure 7 shows eight data points representing laboratory failures induced in PCD thrust bearings. In these tests, the speed for each individual test remained constant while the axial load was incrementally increased. The test terminated when the PCD thrust bearing failed. These failures were thermal in nature and resulted when the frictional heat generation in the bearing caused the diamond temperature to exceed approximately 800 deg C, at which temperature PCD begins to break down. When plotted together, these eight points form a curve that defines the operating envelope for this bearing set. Loads and speeds below the curve will result in safe operation while loads and speeds above the curve will result in bearing failure.
Figure 7. Laboratory PCD thrust bearing failures.
Summary
Polycrystalline diamond possesses mechanical and physical properties that make it suitable for a number of applications. Successful applications in the oil and gas sector may provide clues regarding where it may be successful in the general pump sector. Bearing applications seem particularly promising and should be explored.
Pumps & Systems, November 2009