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Fretting

Fluid flow or mechanical rubbing can cause removal of or damage to a protective oxide, increasing the proximity of the bare metal and the attacking medium. This can result in increased attack rates because a stable oxide layer is frequently a corrosion rate limiter. An example is erosion corrosion caused by high flow rates of domestic hot water in copper pipes, especially around fittings, which can generate turbulence. Another example involves the press fit of gears, wheels, pulleys, etc., onto shafts that experience elastic torsion, or bending. Small relative motions occur at the contacting surfaces which mechanically break up protective oxide layers. This type of corrosion is known as fretting corrosion.
(Standard Handbook of Machine Design (3rd Edition), Edited by: Shigley, Joseph E.; Mischke, Charles R.; Brown, Thomas H. Jr., p44-28)


Fluid friction

Fluid friction is internal friction in the lubricating film between the friction body surfaces, the surfaces being completely separated by the lubricating film. A difference is frequently made between fluid friction in conformal contact surfaces (hydrodynamics) and in nonconformal contact surfaces (elastohydrodynamics). While rigid surfaces and lubricant viscosity only dependent on temperature are generally assumed in the first case, this cannot be assumed in the second case, in which not only the deformations of the surfaces but also the lubricant viscosity’s dependence on pressure, temperature, and shear rate must be taken into account.
(Springer Handbook of Mechanical Engineering, Grote, Antonsson (Eds.),part B, p302)


Boundary friction

In solid friction the friction acts between material zones that exhibit solid properties and are in direct contact. If the friction occurs between solid boundary layers with modified properties compared with the bulk material, e.g., between reaction layers, then this is boundary-layer friction. If the boundary layers on the contact surfaces each consist of a molecular film coming from a lubricant, then this is called boundary friction. In boundary friction, the lubricant’s hydrodynamic effect can be disregarded because the velocity is very low and/or only a very small quantity of lubricant, insufficient to fill the lubrication gap, is present.
(Springer Handbook of Mechanical Engineering, Grote, Antonsson (Eds.),part B, p302)


Mixed friction

Whenever lubricated surfaces slide together at low sliding speeds or with a high applied normal load, the lubricant may not separate the two solid surfaces completely. However, the lubricant can still significantly reduce the friction coefficient by reducing the shear strength of adhesive junctions between the two surfaces. In this so-called boundary lubrication regime, the effectiveness of the lubricant can be improved if the lubricant molecules adhere well to the solid surfaces. This is best accomplished by introducing a lubricant or additive that forms a surface film through adsorption, chemisorption, or chemical reaction with the surface. The ensuing reduced shear strength of the surface film can lower the friction coefficient by as much as an order of magnitude from the dry friction value.
When a good supply of a viscous lubricant is available, the separation between the surfaces will increase as the sliding speed increases or the normal load decreases. As the separation increases, the amount of solid/solid contact between the surfaces will decrease, as will the friction coefficient and wear rate. In this Òmixed frictionÓ regime, friction is determined by the amount of plowing deformation on the softer surface by the harder surface asperities and by adhesion within the solid/solid contacts. When the surfaces become completely separated by a self-acting or externally pressurized lubricant film, the lubricating regime is hydrodynamic, wear is reduced to nearly zero, and friction reaches a low value governed by viscous shear of the lubricant. Friction coefficients in such cases can be 0.001 or lower, depending on the surface velocities and the lubricant viscosity. This is the case for most journal or thrust bearings (see subsection on fuid film bearings).
(Mechanical Engineering Handbook, Ed. Frank Kreith, p3-133)