Dr. Lasse Makkonen, Principal Scientist at VTT Technical Research Centre of Finland, has presented a first principles thermodynamic model of sliding friction between solid objects. According to his theory, the amount of friction depends on the thermodynamic surface energy per unit area of the materials in question.
This concept, outlined by Gauss and Gibbs, refers to the excess energy of a surface with respect to an atomically complete contact (as in the bulk). Therefore, this treatment requires considering the growth and disappearance of the real contacts, i.e., the contact areas containing atoms that interact across the interface.—Makkonen 2012
Friction is a key phenomenon in applied physics and engineering, whose origin has been studied for centuries. Friction has a substantial effect on many everyday phenomena, such as energy consumption. Until now, it has been understood that mechanical wear-resistance and fluid lubrication affect friction, but the fundamental origin of sliding friction has been unknown.
According to Makkonen’s theory, the amount of friction is related to the material’s surface energy. Friction originates in nanoscale contacts, as the result of new surface formation. The theory explains the generation of frictional force and frictional heating in dry contact. It can be applied in calculating the friction coefficient of various material combinations.
The model also enables the manipulation of friction by selecting certain surface materials or materials used in lubrication layers, on the basis of the surface energy between them.
As outlined in this paper, sliding of solids can be deﬁned by the formation and disappearance of surfaces. By laws of thermodynamics, this inherently involves consuming kinetic energy and dissipating surface energy at the real contacts. Consequently, friction is generated even when no wear or separation occurs in perpendicular to the interface and when no storing of elastic energy and atomic scale mechanisms within the real contacts are accounted for.
The thermodynamic analysis presented in this paper provides an explanation for the origin of the irreversibility of friction. The key issue is that at the diminishing real contacts kinetic energy must be taken from the motion to form surfaces, whereas at the growing contacts the released surface energy cannot be spent in mechanical work because the local thermodynamic system at them is constrained in such a way that it cannot do mechanical work in parallel to the sliding interfaces.
Qualitatively, the predictions of this theory agree with major experimental evidence. The theory also explains why friction laws are different in macro-scale and nanoscale and predicts the very large frictional force observed in nanoscale measurements. The theory outlines the importance of the contact morphology at the nanoscale. In the extreme case of a complete macroscale contact, i.e., very large d, the theory predicts a negligible frictional force, as observed for adsorbed monolayers. Based on the theory, friction can be manipulated if one can find ways to affect the real contact length d.
It needs to be emphasized that the friction theory derived here considers processes at the scale of the real nanoscale contacts only. Thus, it is in no discrepancy with the postulated atomic scale mechanisms within the real contacts or with various proposed friction and wear mechanisms at a larger scale.—Makkonen 2012
Makkonen’s theory on sliding friction was published as an open access paper in the journal AIP Advances of the American Institute of Physics. The research was funded by the Academy of Finland and the Jenny and Antti Wihuri Foundation.
Lasse Makkonen (2012) A thermodynamic model of sliding friction. AIP Advances 2, 012179 doi: 10.1063/1.3699027