Area of contact between moving surfaces – Bowden and Tabor

Introduction

When two plane surfaces are brought together, the area of intimate contact is much smaller than the apparent area. Even if the surfaces are carefully polished and made as flat as possible, they will still have hills and valleys at the microscopic level. The upper surface will rest on these irregularities, creating large gaps between other areas that are significantly larger than the dimensions of a molecule. The exact size of these irregularities and the degree of flatness of the surfaces are not well understood. This further changes with surfaces in contact under stationary and moving conditions. It is important to study how the area of contact changes within these moving surfaces in contact by understanding their electrical properties. Bowden and Tabors experiments have given greater insights into this field.

Experimental modifications

Previous methodology: Two crossed cylinders were arranged with their axes at right angles. Electrical conductance (A) was measured using a current-potential method. A known current (i) was passed through the contact, and the potential difference (v) was measured using a high-resistance potentiometer or galvanometer. The conductance was calculated as A=i/v. The load on the contact surfaces was varied by: Using water or mercury to adjust the weight (load range: 20 g to 6000 g). Applying larger loads (up to 1000 kg) with a lever machine and a spring balance. The contact surfaces were prepared through fine grinding, polishing, chemical etching, or scraping. Although surface preparation had minimal impact on results, scraping improved reproducibility.

The measurements were extended to moving surfaces, with a method like the previous one, but with the potentiometer replaced by a rapid-response instrument like an Einthoven galvanometer or a cathode-ray oscillograph. Friction was measured using a high-frequency apparatus like that described in the previous paper. The lower surface, in the form of a flat plate, was driven at a constant rate by a water piston. The upper surface, which rested on it, was typically curved. Both the conductance and friction measurements were recorded simultaneously using the same moving-film camera, ensuring a synchronized record of both quantities.

Important findings

1. These experiments clearly demonstrate that the contact area between moving surfaces does not remain constant. While the average value of the conductance is similar to that observed with stationary surfaces, significant fluctuations can occur during sliding. These fluctuations are closely correlated with frictional stick-slip behavior, and the exact nature of these changes depends on the type of metals involved.
2. In the case of a curved slider made of high-melting metal sliding on a low-melting metal, the area of intimate contact increases during the stick phase. However, when slip occurs, there is a sudden decrease in this area. These results strongly support the earlier suggestion that friction under these conditions is primarily caused by small irregularities on the high-melting metal ploughing through the surface of the softer metal. The characteristic “cut scratch” is observed with this behavior.
3. In the case of a low-melting metal sliding on a high-melting-point metal, the behavior is the opposite of that observed with the high-melting metal on low-melting metal. The conductance decreases during the stick phase. However, once slip occurs, the conductance rises to a maximum value. This supports the suggestion that the local high pressure combined with the temperature rise from frictional heat causes the lower-melting metal to weld or solder onto the surface of the higher-melting metal. As a result, the area of contact is maximized. During the stick phase, the increasing pull causes these soldered junctions to become thinner (decreasing conductance) until they suddenly break, leading to slip. This process then repeats itself.
4. When both surfaces are made of the same metal, conductance measurements show that the area of contact is somewhat greater and remains much more constant during sliding.
5. During sliding, only small variations in this conductance occur. The friction is high, exhibiting slow fluctuations, and the characteristic torn track is formed. This behavior is generally observed with similar metals, as long as they are homogenous.
6. These conductance measurements support the suggestion that in the case of similar metals, mutual welding of the two surfaces takes place. The local high pressures and temperatures cause both surfaces to flow, contributing equally to the formation of welded junctions. These mutual junctions are likely easier to form, resulting in a greater real area of contact and increased friction. When movement occurs, both surfaces are damaged, and the metal in the resulting track becomes severely distorted and torn.

Conclusions

Measurements with moving surfaces reveal that the area of contact fluctuates rapidly during sliding. These fluctuations are closely correlated with changes in friction and temperature, indicating intermittent clutching and breaking away of the surfaces, with metallic junctions being formed and broken. The nature of these junctions depends on the physical properties of the metals involved. Even when the metals are lubricated with mineral oils or other lubricants, metallic contact can still occur through the lubricant film. Fluctuations in the area of contact are observed during sliding, and the behavior may be similar to that of unlubricated surfaces.

Reference

[1] Bowden, F.P. and Tabor, D., 1939. The area of contact between stationary and moving surfaces. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 169(938), pp.391-413.