Depending on the geometry of the surface and the distance between the surfaces, different types of lubrication can occur. Figure 3.8 shows the Stribeck curve used in tribology. This figure shows the transition of the lubricating regimes of the slid-ing surfaces. The vertical axis represents the coefficient of friction and the hor-izontal axis the bearing characteristic number, which is ηV /P, where η is shear viscosity, V sliding speed, and P average surface pressure. As the bearing charac-teristic number increases, the lubrication type moves from boundary lubrication to hydrodynamic lubrication via the mixed regime.
Figure 3.8: Schematic of the Stribeck curve showing lubrication regimes.69 η is the fluid viscosity, V is the relative speed of the surfaces, and P is the pressure applied on the interface.
Boundary lubrication
Boundary lubrication occurs when a liquid is under conditions where the solid surfaces are so close together that contact between two is almost possible.70 The friction and wear in boundary condition are determined predominantly by interac-tion between the solids and between the solids and the liquid. The bulk properties of the liquid are less relevant in the friction and wear behaviour, and properties of solid plays a bigger role. Typically, lubricant films of less than 100 nm are formed. This film layer lies between surfaces in the contact area to control the adhesion between solid interfaces and to reduce the resistance, and so to decrease the coefficient of friction. Synovial joints, teeth-saliva during chewing, and the start up and shut down period of engines are examples of boundary lubrication.
Figure 3.9: An example diagram of hydrodynamic lubrication.
In the mixed lubrication regime, the fluid film bears a portion of friction and the coefficient of friction decreases further. In boundary lubrication and mixed lu-brication, a film adsorption layer must be formed on the sliding surfaces to reduce the friction.
Hydrodynamic Lubrication
Hydrodynamic lubrication, also called full film lubrication, occurs when two sur-faces sliding relative to each other are fully separated by a film of fluid, as in a system like Figure 3.9. In this case, the fluid film bears the entire load and the contact area disappears. Friction force is due only to the viscous resistance of the fluid. This means the coefficient of friction reduced to its minimum. However, the viscosity of fluid and sliding speed now increase the bearing characteristic number and also the coefficient of friction. In hydrodynamic lubrication, a thick layer of
film must exist and bear some load even under high pressure and low speed. As a result, the lubricant characteristics of the fluid becomes more important.
Elastohydrodynamic Lubrication
When two surfaces roll, slide or spin relative to each other, the lubricant will be dragged along due to the shear stress exerted on it. In the contact zone, the hydro-dynamic pressure developed in the lubricant causes a further increase in viscosity that is sufficient to separate the surfaces. Because of this high viscosity and the short time required to carry the lubricant through the contact area, the lubricant cannot escape and the surfaces will remain separated. The film thickness will not be affected by pressure because, under the usual pressure to which this type of lubricants is exposed, the lubricant film is harder than the metal surfaces. There-fore, increasing the pressure or load will result in deformation of the metal surface and increase of the contact area, as shown in Figure 3.10. This lubrication mech-anism is called elastohydrohynamic lubrication (EHL). EHL is dominant where the external load on a unit is larger than the stiffness of the material of the moving parts and the contacting surface do not fit well; in other words, the surfaces are non-conformal.71 Such a load is typically applied and removed in hundreds of microseconds.
Figure 3.10: An example diagram of elastohydrodynamic lubrication (EHL).
EHL can be divided into two types: hard or soft. Hard EHL is observed with materials of high elastic modulus such as metals. Up to 3 - 4 GPa of pressure is exerted and a minimum film thickness of 0.1 - 1 µm occurs, but elastic de-formation can be several orders of magnitude higher than the film thickness. The film thickness depends on the applied load, surface moving speed, surface geome-try, elasticity of the surface material, and pressure-viscosity coefficient. Viscosity of the lubricant can be ten orders of magnitude higher than viscosity at ambient pressure. Examples of hard EHL are gears, rolling element bearing, continuously variable speed drives, etc. With materials of low elastic modulus, such as rubber or plastic, soft EHL occurs. Elastic deformation occurs under even light load of up to 1 MPa. The viscosity of the lubricant is not affected by this low pressure, and pressure-viscosity coefficient is not as significant. The minimum film thickness is about 1 µm. Examples of soft EHL are seals, tyres, gaskets, etc.
The friction in an EHL contact is largely determined by high pressure be-haviour. Therefore it is important to know properties of the lubricant at high pressure.