Bearing materials are characterized by low coefficient of friction, sufficient fatigue strength, good running-in properties, and good corrosion resistance. Commonly used bearing materials include bearing alloys (Babbitt metal), copper alloys, powder metallurgy, gray cast iron, and wear-resistant cast iron.
Non-lubricated bearing materials mainly fall into three categories: polymers, carbon graphite, and special ceramics.
Polymers
Polymers, also known as organic polymer materials or engineering plastics, commonly include phenolic resins, nylon, and polytetrafluoroethylene (PTFE). Non-lubricated bearings made of plastics (such as PTFE) are resistant to strong acids and weak alkalis, and exhibit good embedding, friction reduction, and wear resistance. PTFE sheets are stamped into lip seals, bearing shells, piston rings, and gaskets for use in belt conveyors, typewriters, sewing machines, record players, water pumps, textile machinery, and agricultural machinery.
Polymers are lightweight, insulating, friction-reducing, wear-resistant, self-lubricating, corrosion-resistant, have simple molding processes, and high production efficiency. Compared to metallic materials, their tribological properties are highly sensitive to ambient temperature and humidity, and their viscoelasticity is significantly affected, resulting in a larger clearance between the bearing bush and journal. Furthermore, their low mechanical strength, low elastic modulus, and poor lubricant absorption limit the operating speed and pressure of the bearing bush.
Carbon-Graphite
Carbon-graphite bearing bushes can be used in harsh environments. The higher the graphite content, the softer the material and the lower the coefficient of friction.
Carbon-graphite generally exhibits good electrical conductivity, heat resistance, wear resistance, self-lubrication, good high-temperature stability, strong chemical corrosion resistance, higher thermal conductivity than polymers, and a low coefficient of linear expansion. Under atmospheric and room temperature conditions, the coefficient of friction and wear rate with chrome-plated surfaces are very low. Its self-lubricating and friction-reducing properties depend on the amount of water vapor adsorbed, but it loses its lubricating properties at very low humidity. Applying a wear-resistant coating can improve the wear resistance of carbon-graphite. Carbon-graphite can also be used as a water-lubricated bearing bush material.
Graphite can be used not only as a solid lubricant and added to materials such as resins, metals, and ceramics to increase their friction-reducing properties, but also directly as a friction pair material. Examples include bearings, high-temperature sliding bearings, seals, piston rings, and scrapers for oil-sensitive applications such as papermaking, woodworking, textiles, and food processing. The symbol for "class" of carbon graphite materials used in mechanical engineering is M, and there are four series: carbon graphite materials, electrographite materials, resin-carbon composite materials, and metallic graphite materials.
Ceramics
Ceramics are non-metallic materials made from inorganic non-metallic natural minerals or artificial compounds through crushing, shaping, and high-temperature sintering. They consist of numerous inorganic non-metallic small crystals and a glassy phase. Traditional ceramics are made from inorganic non-metallic natural minerals such as clay, feldspar, and quartz; special ceramics are made from artificial compounds. Mechanical engineering ceramics are generally special ceramics made from artificial compounds such as alumina, magnesium oxide, zirconium oxide, lead oxide, titanium oxide, silicon carbide, boron carbide, silicon nitride, and boron nitride.
The properties of ceramics are largely determined by their microstructure, including grain size and distribution, the composition and content of the glassy phase, and the nature, content, and distribution of impurities. This microstructure, in turn, is determined by the raw materials, composition, and manufacturing process. Common characteristics of ceramics include high hardness and compressive strength, high temperature resistance, wear resistance, oxidation resistance, good corrosion resistance, brittleness, poor impact resistance, and lack of ductility.
Ceramics are a relatively new material for unlubricated bearing bushes, especially SiC and Si3N4, which exhibit excellent strength, heat resistance, corrosion resistance, and tribological properties.