Thin Film Lubrication, Lubricants and Additives

Abstract

Friction will be generated when two solid bodies are pressed over or slide against each other, and it acts opposite to the direction of relative motion. Lubricants are frequently used to reduce friction which otherwise may result in high machine wear and energy losses. Depending upon the phenomenon, lubrication can be classified into four different regimes: boundary, mixed, elastohydrodynamic and hydrodynamic. In boundary regime, the frictional response is mainly governed by the properties of the surfaces and it generally involves adsorption of lubricant molecules onto the mating surfaces. Therefore, in this regime, properties other than bulk properties of the lubricants play a significant role in determining the frictional response. Mixed or thin film lubrication (TFL) is a bridge that mark the transition from boundary to Elasto-Hydrodynamic (EHL) [or hydrodynamic (HL)] regimes. In TFL the load is partly supported by direct contact of the surface asperities and partly by the fluid. EHL regime is a type of HL regime which is characterized by the formation of sufficiently thick fluid film which fully separates the surfaces from direct contact thus reducing friction. Elastic deflections of the surfaces in contact in EHL regime influence the shape and thickness of the lubricant film significantly. HL differs from EHL due to negligible elastic deformation of the surfaces at the contact interface. In EHL/HL, load is fully supported by the lubricant where the bulk property of the lubricant and entrainment velocity of the tribo pairs determines the film thickness and friction. Transition between different lubrication regimes is well described by Stribeck curve. In this chapter, the mechanism of transition between different regimes and factors influencing the frictional response, different types of lubricants and additives types and their key features will be covered.KeywordsFrictionLubricationThin filmLubrication regimes