Self-Lubricating Behavior of Graphite Reinforced Metal Matrix Composites

Abstract

Lubricants are extensively used between the contacting surfaces to reduce friction and wear. These lubricants are usually toxic and not readily biodegradable and thus these lubricants can cause considerable damage to the environment. The use of external lubricants can be eliminated by designing selflubricating composite materials. These composite materials have the ability to achieve low friction and wear at the contact surfaces without any external supply of lubrication during sliding. The metal matrix composites reinforced with various self-lubricating particles such as graphite, molybdenum disulfide are being used as self-lubricating materials for various engineering applications. In this paper, the tribological behavior of metal matrix composites reinforced with graphite particles has been reviewed. More specifically, aluminum-graphite, magnesium-graphite, copper-graphite, silver-graphite and nickel-graphite composites have been studied. The influence of various variables on the friction coefficient and wear rate is discussed. It was found that the amount of graphite released on the wear surface forms a tribo film on the contact surfaces. This reduces the overall friction coefficient and wear rate. The formation and retention of this tribo layer 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 and depend on the nature of the sliding surface, the test condition, environment and the graphite content in the composite. The presence of graphite tribo layer also Keywords Friction Á Wear Á Self-lubrication Á Metal matrix composites IntroductionA lubricant is a material used to facilitate relative motion of solid bodies by minimizing friction and wear between interacting surfaces. A lubricant functions by introducing a layer of material with lower shear strength than the surfaces themselves between the sliding surfaces [1]. In some lubricated systems, the lubricant may not completely prevent asperity contact, although it will reduce it and may also reduce the strength of the junctions formed. In other cases, the lubricant completely separates the surfaces and no asperity junctions are formed at all [2]. In general, there are three categories of lubricants-liquid, solid and gaseous.Nearly all lubricants used in the automotive and manufacturing sectors are oil or grease-based. These oils include engine oils, transmission fluids, hydraulic fluids and gear oils. These products are not typically environmentally friendly or biodegradable and can introduce significant quantities of pollutants into the waste stream [3]. Thus, disposal of this very large volume of material obviously represents an environmental burden. Hence, it is very important to develop either green lubricants or composite materials which have self-lubrication properties.Composite materials are a special class of materials and are made from two or more constituent components with significantly different physical or chemical p...

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