Paper III (i) Reduction in friction coefficient in sliding ceramic surfaces by in-situ formation of solid lubricant coatings
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Cited by 3 publications
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Production trends for bearings and bearing materials closely parallel general industrial activity; Ball and roller bearings represent the largest business segment; except for rolling‐element bearings, only a few bearing types are of general commercial significance. U.S. shipments include plains, bearings, powdered metal bearings, jewel bearings, air bearings and for use in light‐load, high speed applications such as air circulators in aircraft, lasers, and dental drills. Environmental concerns have resulted in diminished use of lead in recent years in lead babbitt, porous metal bearings, and related bearing materials. Some other bearing materials, eg, filled plastics such as nylon, acetal resin, PTFE, and phenolics formed and molded into bearings in a wide variety of mechanical structures, and tin, lead, and bronze alloys used for oil‐film bearings in heavy industrial and power‐generating equipment, also find extensive use. Even bearings operating primarily with full oil‐film lubrication may rub the shaft. During this sliding contact, the bearing material must avoid either welding to the shaft or scoring and galling under the localized high surface strains and high temperature at microscopic asperities. Adding more of a good element, eg, lead in a copper alloy, generally improves score resistance, whereas adding more of poor metal zinc will degrade score resistance. Various plastics and other nonmetallics also provide excellent compatibility, low friction, low wear, and good scoring resistance. Their application is usually limited to slow surface speeds, however. When a shift surface forces dirt, machining chips, or grinding debris against a bearing, the bearing material is required to absorb the foreign particles to minimize scoring and wear. The soft babbitts are unsurpassed for both embedability and conformability. An alloy too low in strength is prone to extrude under load, whereas too high strength may be accompanied by brittleness and poor embedding of foreign particles. Corrosion by the organic acids and peroxides formed in lubricating oil during its oxidation in service can be minimized by selecting oils with good oxidation inhibitors and by keeping the operating temperature low. Conducting frictional heat out through a bearing can be a significant requirement, particularly for high speeds. Lubricant‐film bearings primarily employ the white‐metal babbitts and a variety of copper and aluminum alloys. Steel and cast iron structural parts are frequently used as oil‐film bearing materials. Porous bronze and iron, a variety of plastics, carbon–graphite, wood, and rubber are widely used in dry sliding or under conditions of sparse lubrication. These materials have commonly allowed design simplifications, freedom from regular maintenance, and good performance at low speeds and with intermittent lubrication. As the temperature limits for lubricating oils (150–250°C) and solid lubricants (350–400°C) are exceeded, bearing materials must accommodate either dry, low speed sliding or operate with very poor lubricants such as gas, pressurized water, or liquid metals. Continually higher temperatures are being encountered by bearings in gas turbines, diesel engines, automotive engines, superchargers, nuclear plant equipment, and rocket engines. High temperature strength often leads to selection of alloys of nickel, cobalt, and chromium for use from 500–850°C. Ceramics find high temperature use to over 800°C. Advanced ceramics finding interest include alumina, partially stabilized zirconia, silicon nitride, boron nitride, silicon carbide, boron carbide, titanium diboride, titanium carbide, and sialon (Si–Al–O–N). Desirable bearing material properties offered by ceramics are high compressive strength, fatigue resistance, corrosion resistance, low density, and retention of mechanical properties at elevated temperatures. Ball bearings are almost exclusively made with through‐hardened materials such as the industry standard 52100 and stainless 440C. Case‐hardened steels, commonly containing a lower carbon content of about 0.20%, are used for the rollers and races in many roller bearings for automobiles and railroad equipment. Ceramic materials, and especially silicon nitride, Si 3 N 4 , are being applied in a variety of demanding applications.
Production trends for bearings and bearing materials closely parallel general industrial activity; Ball and roller bearings represent the largest business segment; except for rolling‐element bearings, only a few bearing types are of general commercial significance. U.S. shipments include plains, bearings, powdered metal bearings, jewel bearings, air bearings and for use in light‐load, high speed applications such as air circulators in aircraft, lasers, and dental drills. Environmental concerns have resulted in diminished use of lead in recent years in lead babbitt, porous metal bearings, and related bearing materials. Some other bearing materials, eg, filled plastics such as nylon, acetal resin, PTFE, and phenolics formed and molded into bearings in a wide variety of mechanical structures, and tin, lead, and bronze alloys used for oil‐film bearings in heavy industrial and power‐generating equipment, also find extensive use. Even bearings operating primarily with full oil‐film lubrication may rub the shaft. During this sliding contact, the bearing material must avoid either welding to the shaft or scoring and galling under the localized high surface strains and high temperature at microscopic asperities. Adding more of a good element, eg, lead in a copper alloy, generally improves score resistance, whereas adding more of poor metal zinc will degrade score resistance. Various plastics and other nonmetallics also provide excellent compatibility, low friction, low wear, and good scoring resistance. Their application is usually limited to slow surface speeds, however. When a shift surface forces dirt, machining chips, or grinding debris against a bearing, the bearing material is required to absorb the foreign particles to minimize scoring and wear. The soft babbitts are unsurpassed for both embedability and conformability. An alloy too low in strength is prone to extrude under load, whereas too high strength may be accompanied by brittleness and poor embedding of foreign particles. Corrosion by the organic acids and peroxides formed in lubricating oil during its oxidation in service can be minimized by selecting oils with good oxidation inhibitors and by keeping the operating temperature low. Conducting frictional heat out through a bearing can be a significant requirement, particularly for high speeds. Lubricant‐film bearings primarily employ the white‐metal babbitts and a variety of copper and aluminum alloys. Steel and cast iron structural parts are frequently used as oil‐film bearing materials. Porous bronze and iron, a variety of plastics, carbon–graphite, wood, and rubber are widely used in dry sliding or under conditions of sparse lubrication. These materials have commonly allowed design simplifications, freedom from regular maintenance, and good performance at low speeds and with intermittent lubrication. As the temperature limits for lubricating oils (150–250°C) and solid lubricants (350–400°C) are exceeded, bearing materials must accommodate either dry, low speed sliding or operate with very poor lubricants such as gas, pressurized water, or liquid metals. Continually higher temperatures are being encountered by bearings in gas turbines, diesel engines, automotive engines, superchargers, nuclear plant equipment, and rocket engines. High temperature strength often leads to selection of alloys of nickel, cobalt, and chromium for use from 500–850°C. Ceramics find high temperature use to over 800°C. Advanced ceramics finding interest include alumina, partially stabilized zirconia, silicon nitride, boron nitride, silicon carbide, boron carbide, titanium diboride, titanium carbide, and sialon (Si–Al–O–N). Desirable bearing material properties offered by ceramics are high compressive strength, fatigue resistance, corrosion resistance, low density, and retention of mechanical properties at elevated temperatures. Ball bearings are almost exclusively made with through‐hardened materials such as the industry standard 52100 and stainless 440C. Case‐hardened steels, commonly containing a lower carbon content of about 0.20%, are used for the rollers and races in many roller bearings for automobiles and railroad equipment. Ceramic materials, and especially silicon nitride, Si 3 N 4 , are being applied in a variety of demanding applications.
Lubricants are extensively used between contacting surfaces to reduce friction and wear. Typically, liquid lubricants are used to achieve low friction and wear. However, these lubricants are not effective in elevated temperature applications or vacuum environments. For these reasons, solid lubricants are utilized to meet these operational needs, where liquid lubrication is impractical. Solid lubricants are only effective as long as they are present in the tribo-interface. Therefore, it is desirable to provide a constant supply of solid lubricant material to the contacting surface. This is often achieved by incorporating solid lubricants as a second phase in the base material. These composite materials have the ability to achieve low friction and wear at the contact surfaces without any external supply of lubrication during sliding. Metal matrix composites reinforced with lamellar solid lubricant particles such as graphite are being used as self-lubricating materials for various engineering applications. In this chapter, the tribological behavior of metal and ceramic matrix composites reinforced with graphite particles has been reviewed. More specifically, copper-graphite, nickel-graphite, magnesiumgraphite, silver-graphite, aluminum-graphite, silicon nitride-graphite, and alumina-graphite composites are studied. The influence of various environmental and mechanical parameters on the friction coefficient and wear rate is discussed. It was found that the amount of graphite released on the worn surface forms a thin transfer film on the contact surfaces. This transfer film reduces the overall friction coefficient and wear rate. The presence of the graphite-based transfer film increases the seizure resistance and enables the contacting surfaces to run under boundary lubrication without galling. The formation and retention of this transfer film on the sliding surface as well as its composition, area fraction, thickness, and hardness are important factors in controlling the friction and wear behavior of the material. The effectiveness of the transfer film also depends on the nature of the sliding surface, the test condition, environment, and the graphite content in the composite.
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