In mechanical assemblies, bearings are crucial components that enable rotation and power transmission of axial components. Bearing materials must exhibit high hardness, exceptional wear resistance, and a low friction coefficient. Typically, their matrix is crafted from tempered alloy steel to ensure high strength and toughness. Common bearing materials include chromium bearing steel (Cr15SiMn), carburized bearing steel (20Cr2Mn2Mo), high-temperature bearing steel (Cr15Mo4), and tungsten carbide.
Polycrystalline Diamond in Bearings
Polycrystalline Diamond (PDC) is an ideal bearing material due to its extremely high hardness, very low friction coefficient, and excellent thermal conductivity. With the rapid development of artificial polycrystalline diamond, PDC composite sheets have found widespread use in various fields. PDC is produced by pressing micron diamond powder under high temperature and pressure with a catalyst. The sintering process creates strong carbon-carbon bonds, and the final product is shaped through wire cutting, grinding, and polishing.
Structure of PDC Composite Sheets
PDC composite sheets typically consist of two parts: a diamond layer on the upper surface, 1 to 2 mm thick, and a cemented carbide base. To reduce stress during interface preparation and enhance bonding strength, a transition layer is often added between the diamond and the carbide base. This composite structure combines the hardness of polycrystalline diamond with the strength and toughness of the cemented carbide base, allowing for integration with other parts of the workpiece.
Despite the benefits of PDC composite sheets, challenges remain. Thermal stress during brazing or flame welding can cause delamination and fracture. Additionally, the high temperature resistance and brittleness of cobalt-based polycrystalline diamond present certain defects in bearing applications. Nonetheless, due to its superior physical and mechanical properties such as high thermal conductivity, low friction coefficient, and high toughness, polycrystalline diamond remains a popular material for high-quality bearings.
High thermal conductivity in bearing materials can effectively reduce local extreme temperatures, preventing damage. During start-stop cycles, high thermal conductivity minimizes local bonding on the bearing surface, reducing surface scratches and wear. In sliding bearings, low friction coefficients help lower heat generation and power loss. Polycrystalline diamond materials, with their extreme hardness and large fracture toughness, perform exceptionally well under extreme operating conditions.
Applications of PDC Bearings
PDC bearings, including radial, thrust, cutting blade, and TC radial bearings, are widely used as anti-friction bearings in downhole motors. These bearings find extensive applications in oil and gas drilling, geological exploration, coal mining, and other industries. In downhole drilling tools, hydraulic loads from drilling and mud flow can generate significant thrust loads. PDC bearings are a high-performance alternative to traditional bearings, resistant to erosion by abrasive particles in drilling mud, which typically accelerate ball bearing wear.
PDC radial bearings, designed for downhole drilling tools, can withstand radial or lateral loads. Compared to tungsten carbide alternatives, PDC bearings offer greater wear resistance, extending tool life, enhancing reliability, and increasing lateral load capacity. In applications requiring bearings to withstand both thrust and radial loads, PDC bearings excel due to their superior performance.
Conclusion
Polycrystalline Diamond (PDC) is a promising bearing material owing to its excellent physical and mechanical properties. Although there are challenges in its application, ongoing technological advancements will likely expand and deepen the use of PDC bearings across various fields.
