From Smithells Metals Reference Book, Eighth Edition

The friction and wear characteristics of materials are not intrinsic properties but, rather, depend on a large number of variables including the physical, chemical and mechanical properties of the material and surfaces and the environment.

25.1 Friction

25.1.1 Friction of Unlubricated Surfaces


Friction is the resistance to motion when two bodies in contact slide on one another. The frictional force F is the force required to initiate or maintain motion. If W is the normal reaction of one body on the other, the coefficient of friction is defined as = F/W.


If the force to initiate motion of one of the bodies is F s and the force to maintain its motion at a given speed is F k, there is a corresponding coefficient of static friction s, = F s /W and a coefficient of kinetic friction k = F k/W. In some cases these coefficients are approximately equal; in most cases s, > k and there is a tendency for intermittent or 'stick-slip' motion to occur.


The two classic laws of friction, which are valid over a wide range of experimental conditions, state that:

  1. The frictional force F between solid bodies is proportional to the normal force between the surfaces, i.e. is independent of W.

  2. The frictional force F is independent of the apparent...

Products & Services
Friction Materials
Friction materials include brake pads, friction liners, clutch plates, friction bands, brake pads, brake shoes, friction rolls, bonded assemblies and other components used to generate controlled friction for braking or power transfer while minimizing wear.
Electric Brakes
Electric brakes are assemblies consisting of electrical elements for the slowing or stopping of shafts in equipment drives. Electrical power is required to activate the brake.
Industrial Lubricants
Industrial lubricants are oils, fluids, greases and other compounds designed to reduce friction, binding or wear and exclude moisture. Specialized characteristics may enhance thermal conduction across thermal interfaces or reduce electrical resistivity across electrical joints.
Mechanical Brakes
Mechanical brakes convert kinetic energy to other energy forms, primarily heat. They decelerate shafts and linked components via friction between a rotating body and wear-resistant material.
Linear Brakes
Linear brakes are used to slow or stop linear motion in conveyors, web systems, and vehicles.  Technologies include friction, eddy current braking, and hysteresis.

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Marina Ruths, Alan D. Berman, and Jacob N. Israelachvili Summary In this chapter, we describe the static and dynamic normal forces that occur between surfaces in vacuum or liquids and the different...

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