Engine lubricants chemical wear

Anti-wear and load-carrying additives work by reacting with ferrous metal surfaces. The metal surfaces have to be sufficiently reactive themselves for the additive to work, which is the case with traditional steels such as M50 and M50 NiL but problems are encountered with more corrosion-resistant steels. These steels are designed to be chemically less reactive to inhibit corrosion but this affects the ability of the anti-wear additive to react with the metal surface. It is desirable to use these corrosion-resistant steels in engine design and the quest for an additive system that works with corrosion-resistant steels, without adversely affecting other areas of performance, is currently the subject of much research in the aero-engine lubrication community. 

Brian J. Briscoe has been a Lecturer in Interface Science, Department of Chemical Engineering and Chemical Technology, Imperial College, London, since 1978. Prior to his present position, he was Oppenheimer Fellow in Surface Science and Assistant Director of Research at the Cavendish Laboratory, Cambridge University. His research Interests Include the surface and mechanical properties of organic polymers with particular emphasis on adhesion, friction, lubrication and wear. 

Modern lubricants are formulated from a range of base fluids and chemical additives. The base fluid has several functions but it is primarily the lubricant which provides the fluid layer to separate moving surfaces. It also removes heat and wear particles whilst minimizing friction. Many properties of the lubricant are enhanced or created by the addition of special chemical additives to the base fluid, as described in later chapters. For example, stability to oxidation and degradation in an engine oil is improved by the addition of antioxidants whilst extreme pressure, EP, anti-wear properties needed in gear lubrication are created by the addition of special additives. The base fluid acts as the carrier for these additives and therefore must be able to maintain them in solution under all normal working conditions. 

Dependent upon load, speed, viscosity, temperature and the nature of the lubricated surfaces in relative motion, differing degrees of separation of these surfaces occur. When significant oil film penetration occurs, frictional heating begins to cause wear, and then both friction and wear can be controlled by chemical/physical surface reactions. Within an engine a range of the above conditions operate. The influence of these conditions on friction can be seen from the Stribeck curve. Fig. 3.6. 

The other surface-active chemical components include antiwear additives, friction modifiers, and antifoam additives [15]. The functions of these other lubricant additives, as their names imply, are quite different from these two surfactants. For example, antiwear additives, such as zinc dial-kyldithiophosphates, are used to prevent wear on engine parts friction modifiers, such as glycerol monooleate, are used to reduce boundary friction in engine operation to improve fuel economy antifoam additives such as polymethylsiloxane are used to inhibit foam formation. Excessive foam formation in lubricants can cause inadequate oil supply to certain parts of the engine and a high rate of oil loss. 

The applications for hard particle composites He, for example, in internal combustion engines, textile machinery and gas turbine compressors, where combined properties can include resistance to wear, fretting, scuffing and heat and chemical attack. The softer particles display an advantageously low coefficient of friction and effective self-lubricating properties. 

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