Multicontact Solid Friction: A Macroscopic Probe of Pinning and Dissipation on the Mesoscopic Scale
- PDF / 849,950 Bytes
- 6 Pages / 576 x 777.6 pts Page_size
- 64 Downloads / 159 Views
In this framework, the AmontonsMRS BULLETIN/JUNE 1998
Coulomb (AC) law F = ^,FN amounts to stating that Ar is proportional to the normal load FN where \x is the coefficient of friction. When considering soft metals, Bowden and Tabor noticed that / < < 1 entails that the nominal local pressure p on the real contacts—of the order of F N / (4>A0)—generally overcomes the yield strength Y so that the contacting asperities flow plastically until p = H ~ 3Y, the "hardness" of the (softer) material. So,
• the fraction of real contact = A,/ Ao « 1 and • the number of microcontacts N is large enough for a statistical approach to be valid. As the normal load is increased, the average distance between the two surfaces decreases. Two processes then compete: the growth of previously formed microcontacts and the creation of new ones. Taking advantage of the fact that surfaces of engineering interest exhibit shorttailed distributions of asperity heights, Greenwood has shown that (1) the mean area of a microcontact (a2) is independent of the load FN—it is a "topographic" parameter, typically of the order of 1100 mm 2 , involving the width of the height distribution and the mean radius of curvature of asperity tips—(2) N aFN; and (3) these statements hold true regardless of the nature of the local deformation (from pure Hertzian to fully plastic). Greenwood's analysis therefore leads to several important conclusions as follows: • It explains the robustness of the AC law for multicontact interfaces. • It shows that this law is essentially a geometrical property of rough interfaces under load. • From this it appears that the information about the physical processes at work in multicontact friction should be looked for in the detailed study of the friction coefficient(s) = F/FN.
(2)
Such studies have been pioneered by the works of Tabor and Rabinowicz, and more recently of earth scientists4 such as Dieterich, Rice, and Ruina. In this article, Ar = FN/H. we identify and analyze various experimental and theoretical paths toward This analysis was later reconsidered a better understanding of the physical and extended by Greenwood1 in the light origin of the dissipation mechanisms reof a statistical description of random sursponsible for dry friction between exface profiles, to the case of contact defortended solids and of the scale(s) on mation that is not fully plastic. This which they take place. description is concerned with the case of what we will call multicontact interfaces The "tribological object" we restrict for which ourselves to—the multicontact interface—is composed of a sparse ( =2.5 X 1CT4 and N * 2.5 X 102"3. This condition also holds for Dieterich's work on granite at heavier loads (=103-104 N). This contrasts with some works on hard metals5 reporting highly noisy friction forces that we attribute to the fact that, under the light loads used (0.2-1 N), the estimated number of contacts is =3. In this case, the friction coefficient is no longer an average quantity but rather a probe of the randomness of surface profiles.
Data Loading...
