The effect of normal loading on the static friction coefficient between smooth metallic surfaces is ... One of the classical laws of friction, attributed to Amontons,.
I. Etsion M. Amit Department of Mechanical Engineering, Technion, Haifa 32000, Israel
The Effect of Small Normal Loads on the Static Friction Coefficient for Very Smooth Surfaces The effect of normal loading on the static friction coefficient between smooth metallic surfaces is experimentally investigated. Normal loads in the range of 10~3N to 0.3N were applied to small diameter samples made of three different aluminum alloys and contacting a nickel coated surface. The tests were done under controlled humidity and clean air conditions. A dramatic increase in the static friction coefficient was observed as the normal load was reduced to its lower level. This behavior is attributed to the role played by adhesion forces which are more pronounced at small normal loads and smooth surfaces and is in agreement with recent theoretical analyses.
Introduction One of the classical laws of friction, attributed to Amontons, states that the friction force, Q, is proportional to the normal load, F (see Fig. 1). This enables to define a coefficient of friction n in the form _Q ' F
V=
(1)
where it is understood that n is a constant, dependent on the materials of the friction pair but independent on the normal load F. A few experimental studies showed, however, that, in breach of Amontons law, the coefficient of friction is affected by the normal load. Paslay and Plunkett (1953) measured static friction coefficients for shrink fitted steel cylinders as a function of the interface pressure and the mode of surface preparation. It was found that the friction coefficient is lower for rougher surfaces and that the friction coefficient for a given surface roughness decreases as the contact pressure increases. Nolle and Richardson (1974) suggested that the reduction in friction coefficient at high contact pressures is attributed to plastic deformation of the asperities. This eventually results in lower friction coefficient when the material fails in compression. Archard (1961) argued that a decrease in the friction coefficient /x with increasing load F is more pronounced in elastic contacts rather than in plastic ones. According to Archard, for purely elastic deformation of the contacting asperities the friction coefficient should vary as load to the - 1/3 power, whereas when the deformation of the contacting asperities become more plastic the friction coefficient becomes less dependent on the load. The experimental results presented by Archard (1961) show a decrease of the friction coefficient
ix with increasing normal load F for F> 100N. In the low load range 10N
