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Diamond Bearing Tribology Fundamentals

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.

Diamond Bearing Tribology Fundamentals

Diamond as a Bearing Material

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.

Physical and Mechanical Properties of Bearing Materials


Properties Polycrystalline Diamond (PCD) Tungsten Carbide Steel (4140) Silicon Nitride Silicon Carbide
 
Coefficient of Friction 0.05-0.08** 0.2-0.25† 0.42‡ -- --
Thermal Conductivity (W/m*K) 543 70 42.6 30 85
Fracture Toughness (MPa√m) 13-15 10-25 50 4 3.5-4
Hardness (GPa, Knoop) 49.8 1.8 0.2 1.8 2.4
Compressive Strength (GPa) 6.9-7.6 2.68 -- -- 2.5
Young’s Modulus (GPa) 841 669-696 205 296 434
Tensile Strength (MPa) 1,300-1,600 334 415 520 500

*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

Coefficient of Friction

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Thermal Conductivity

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Hardness

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Fracture Toughness

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