Solid Lubricants: Overview and Recent Developments
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Applications of Solid Lubricants
A renaissance in research and development of solid lubricants has taken place during the past 10 years. This renaissance has been driven by the needs of advanced technological applications, for whiçh liquid lubricants alone hâve become either ineffective or undesirable. Increasing the ef firiency of jet and automobile engines, which entails engine opération at significantly higher températures, and increasing the longevity of space vehicles hâve been central technology drivers. In the past 10 years, many new solid lubricants hâve been developed to address thèse needs for higher température opération and longer life. Their development, for the most part, has taken advantage of what is known concerning their mechanism of action. Although this mechanistic knowledge is incomplète, particularly at high températures, significant advances in material performance hâve been made. In addition, new methods of applying solid lubricant coatings, namely plasma and ion-beam-assisted vacuum déposition methods, hâve recently corne into their own and produced improved materials. This article will highlight developments in the 1980s which produced new solid lubricants and the advanced technologies which thèse materials will enable. Several reviews of solid lubricants survey the area in more détail than is possible hère.1"3 For overview purposes, this article will briefly describe the classes of materials used as solid lubricants, their mechanism of action, and areas of application.
Solid lubricants are solid materials which reduce friction and wear and prevent damage between contacting surfaces in relative motion. By far the three most widely used solid lubricants are graphite, molybdenum disulfide (MoS2) and polytetrafluoroethylene (PTFE). Thèse and many other inorganic and polymeric solid lubricants are used in a wide variety of applications and are applied in many forms: as thin films, as components of "self-lubricating" composite bodies, and as dispersions in oils. Solid lubricants are used in applications for which liquid lubricants are either ineffective or undesirable. Solid lubricants hâve advantages over liquid lubricants for use in extrême environments: vacuum, in which liquids évapora te; high températures, at which liquids décompose; cryogénie températures, at which liquids solidify; high pressures, at which liquids cannot support the applied load; and ionizing radia^ tion, in which liquids décompose. Solid lubricants hâve prplonged storage stability, no maintenance requirements and do not migrate to contaminate their environment. Thèse advantages define the primary application areas for solid lubriçants. Among thèse are jet and missile components, satellites and other space vehicles, metalworking opérations, robots, x-ray equipment, electrical contacts, nuclear reactors, compressors, optical equipment, mining equipment, automotive components, engine oil additives and consumer durable items. Solid lubricants hâve some disad-
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vantages relative to liquid lubricants in certain applicati
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