Diamond Bearings 101

Polycrystalline diamond is known for its high thermal conductivity, low coefficient of friction, high toughness and other preferred physical and mechanical properties. Having a bearing material with high thermal conductivity reduces localized temperature extremes that lead to bearing degradation. During starting and stopping, a high thermal conductivity will reduce the likelihood of causing localized welding of the surfaces, which in turn leads to scoring and galling of the bearing surface. In sliding bearings, low coefficients of friction are desired in order to decrease heat generation and reduce power loses. A bearing material exhibiting a large fracture toughness will decrease the likelihood of race damage during extreme operation conditions. Because of its extreme hardness, polycrystalline diamond is very resistant to wear from abrasive particles in lubricants or process fluids.

*ASI 4140 Steel, annealed at 815°C (1500°F) furnace cooled 11°C (20°F)/hour to 665°C (1230°F), air cooled, 25 mm (1 in.) roun(1100°F) temper)
** PCD on PCD in H2O, dynamic, dynamic
†Tungsten Carbide on Tungsten Carbide, static
‡Steel (Hard) on Steel (Hard), dynamic
YAt 100˚C
Sources: Bertagnolli, US Synthetic; Roberts et al., De Beers; Cooley, US Synthetic; Jiang Qian, US Synthetic; Glowka, SNL; Sexton, US Synthetic; Lin, UC Berkeley, MatWeb.com, Cerco

Diamond bearings utilize Polycrystalline diamond (PCD) in their primary working surfaces. In general, bearings allow relative movement or rotation of two machine elements with respect to one another while minimizing heat generation and maintaining alignment between the respective machine elements. There are several classes of bearings. Three of the most common would be rolling element, sliding and fluid film bearings.

Polycrystalline diamond could, in principle, be used in all three bearing types. Currently, however, they are found in only sliding and fluid film bearings. Diamond bearings consist of precision machined polycrystalline diamond surfaces that run relative one to another. In operation diamond bearings may act as dry sliding bearings with little or no lubricant, semi-lubricated sliding bearings or fully lubricated fluid film bearings.

Because of the properties of polycrystalline diamond, diamond bearings are capable of operating under harsh conditions which include operating with process fluids and severe loading. They are extremely resistant to abrasion and provide bearing lives that are 2 to 8 times longer than tungsten carbide or other hard metal bearings when operating in abrasive fluids. Successful applications include the near-bit bearing set in drilling mud motors and turbines. In these applications, bearing life has been extended, overall tool length has been shortened and bearing reliability improved. Diamond bearings have also found application in power generation and rotary steerable tools where bearing life has been at least tripled when compared to conventional tungsten carbide bearings. The outstanding toughness, thermal conductivity and hardness are the main properties of polycrystalline diamond that make it such an outstanding bearing material.

Tribology comes from the Greek word ‘tribos’ which means ‘rubbing’. It is a relatively new science that considers the friction, wear and lubrication at moving contacts. Diamond bearing tribology looks at the physics between two diamond surfaces moving relative to one another, most of the time in the presence of a lubricant.