The significance and use of the friction coefficient☆
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Historical underpinnings and definitions of the friction coefficient
The dimensionless quantity known as the friction coefficient, or coefficient of friction as it is sometimes called, evolved from the work of many philosophers, scientists and engineers; in particular, da Vinci [1], Amontons [2], and Coulomb [3]. These thinkers attempted to rationalize the sliding resistance between solid bodies with a universal law that explained observations of their day. In early work with simple machines and macro-scale tribometers, it was observed that the proportionality
Tabulations of friction coefficients
As young science students, we are given the erroneous impression that all friction problems can be solved either by conducting simple experiments or by looking up values in published tables of friction coefficients [7]. Engineers and scientists confronted with real friction problems in machinery or industrial processes often find this simple approach insufficient to explain observations or to enable them to select from among numerous candidate materials and lubricants. Furthermore, as Table 1
Factors affecting frictional behavior
The forces that resist sliding occur in the regions near and between solid surfaces. The problem of establishing exactly which attributes of the contact conditions and the materials contribute most to the friction force is a major one for developing friction tests and analytical friction models. Models for friction have used geometric arguments (surface roughness and asperity interlocking), mechanical properties-based arguments (shear properties of the solids and of the substances between the
Interfacial shear localization
The non-conservative friction force, be it static or kinetic, arises in response to the work needed to enable relative motion between two bodies. In different tribosystems, the energy associated with this work is distributed differently. Some of the energy goes into heat, some of it into the creation of new surfaces (wear), and some is used in deforming the materials. In well-lubricated systems, the force of friction is largely a result of shearing within the lubricant film or the boundary
Selection of test methods
As described elsewhere [7], six categories can be used to characterize friction testing devices:
- 1.
Gravitation-based devices
- 2.
Direct linear force measurement devices
- 3.
Torque measurement devices
- 4.
Tension-wrap devices
- 5.
Oscillation-decrement devices
- 6.
Indirect indications
Gravitation-based devices have been proposed for at least 500 years, and some of them are shown in the notebook sketches of da Vinci [1]. In some configurations, like flat-on-flat testing or pin-on-disk testing, the friction force can be
Ambiguity in friction testing terminology
There are cases in which a ‘friction coefficient’ is reported, yet there is some ambiguity as to whether that term is appropriate. Two cases will illustrate this point. One case involves the term ‘stick-slip’ as applied to a periodic instability in the relative motion between bodies. Fig. 3 typifies a tangential force trace associated with a sliding system in which there is intermittent motion. The linear, upward sloped portions of the curve, labelled ‘s,’ indicate times when there is no
The relationship between friction and wear
The energy that is transformed as a consequence of frictional contact can be stored in the tribosystem or dissipated in a number of different ways. If Ef is the energy resulting from sliding contact, Eout is the energy leaving the tribosystem, and Est is the energy remaining in the tribosystem,For example, mechanical energy from sliding can be converted to heat, vibrations (like sound), to material deformation, or the creation of new surfaces (by fracture). Likewise, it can be stored
Concluding remarks
The friction coefficient is an established, but somewhat misunderstood, quantity in the field of science and engineering. It is a convenient and useful parameter for engineering, but care should be exercised when ascribing to it a fundamental significance. For hundreds of years, friction coefficients have served many useful purposes, like aiding in the design of machines and buildings, improving devices for enhanced safety (like brakes, floor waxes, tires, and walkways), and improving
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Research sponsored by the US Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technologies, as part of the High Temperature Materials Laboratory User Program, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.