Elsevier

Tribology International

Volume 34, Issue 9, September 2001, Pages 585-591
Tribology International

The significance and use of the friction coefficient

https://doi.org/10.1016/S0301-679X(01)00050-0Get rights and content

Abstract

The quantity known as the friction coefficient (or ‘coefficient of friction’) has long been used in science and engineering. It is easy to define, but not easy to understand on a fundamental level. Conceptually defined as the ratio of two forces acting, respectively, perpendicular and parallel to an interface between two bodies under relative motion or impending relative motion, this dimensionless quantity turns out to be convenient for depicting the relative ease with which materials slide over one another under particular circumstances. Despite the fact that both static and kinetic friction coefficients can be measured with little difficulty under laboratory conditions, the time- and condition-dependent characteristics of friction coefficients associated with both clean and lubricated surfaces have proven exceedingly difficult to predict a priori from first principles.

The shaky nature of friction's fundamental underpinnings, has not prevented investigators from compiling lists of friction coefficients and publishing them for general use. Problems often arise, however, when engineers attempt to use tabulated friction coefficients to solve specific problems in mechanical design or failure analysis. The systems-dependence of frictional behavior is sometimes ignored, leading to misapplication of published data. This is particularly true for applications in nano-technology and others that differ from typical laboratory size scales. This paper will review the measurement and use of static and kinetic friction coefficients, discuss their usefulness, and describe the sources of frictional resistances in terms of shear localization.

Section snippets

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,Ef=Eout+EstFor 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.

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