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The Adhesion Theory of Friction

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1957 Proc. Phys. Soc. B 70 98

(http://iopscience.iop.org/0370-1301/70/1/314)

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98

The Adhesion Theory of Friction

BY R. T. SPURR AND T. P. NEWCOMB Ferodo Limted, Chapel-en-le-Frlth, Stockport, Cheshire

MS. received 31st July 1956

Abstract. It is shown that the static friction between a specimen of bituminous material and a cast iron disc is due to adhesion between specimen and disc, and that the friction is reduced when the normal load is removed because of elastic interaction between the surfaces.

9 1. INTRODUCTION R U E contact between two surfaces occurs only at small areas dispersed over the nominal area of contact. I t is generally considered that adhesion T occurs between the surfaces at these contact areas and that the frictional

resistance to sliding is due to the force required to overcome this adhesion. This theory has been criticized (e.g. Bikerman 1941) on the grounds that if the adhesion is large enough to be the cause of frictional resistance it should be experimentally measurable when separating the surfaces normally, but in general no such adhesion has been observed. This criticism has been ingeniously met by Bowden and Tabor (1939) who suggested that when the normal load is removed elastic recovery of the surfaces breaks the junctions formed at the localized contact areas and so adhesion is not detectable. Bowden and Rowe (1956) have shown that adhesion can occur between metals if they have been heated above their annealing temperatures in order to reduce the elastic stresses at the junctions.

The bituminous material ' Mexphalte ', (from the Shell Petroleum Co. Ltd.) adheres to a cast iron surface and in the course of an investigation into the relation- ship between the adhesion and friction of ' Mexphalte ', some observations have been made which support the adhesion theory and show the importance of elastic recovery.

9 2. EXPERIMENTAL DETAILS Nominally flat specimens of ' Mexphalte ' (about 1 in. square) were used.

The specimens were fixed in a hinged mount at the end of a torque arm which could rotate freely about the same axis as a polished cast iron disc (35cm in diameter) driven by a 3 h.p. electric motor. The disc could be heated to various temperatures by a length of electrical heating strip. Before making any measure- ment the specimen was ' run in ' under load against the rotating disc until it had a smooth polished appearance. The disc was then clamped stationary, the specimen was loaded against it for a set period of time and the friction measured by pulling at the torque arm with a spring gauge until the specimen slipped. Similarly, to measure the adhesion a vertical force was applied through a spring gauge, the counterweighted hinged mount permitting the specimen to swing up from the plate when the adhesion was overcome. Loads of from 150 to 2000 grammes were used. The measured values of the friction and the adhesion were dependent upon the time the surfaces had been in contact. The static

The Adhesion Theory of Friction 99

1500

coefficient of friction measured after the surfaces had been loaded together for 30 seconds was called pori. Other measurements were made in which the normal load was removed after 30 seconds and then 30 seconds later the tangential force necessary to shear through the junction formed between specimen and disc measured. From this force

poii = (tangential force)/(normal load) was obtained.

am = (normal load)/(adhesion) was measured in the same way as poff except that a normal instead of a tangential force was applied. As there was some scatter in the results, a number of sets of measurements, each set consisting of adhesion and friction measurements using the one specimen, was made over the range of loads, several measurements being made at each load.

§ 3. RESULTS AND DISCUSSION

The coefficient of adhesion

Measurements in each set were made in random order.

It was found that over the experimental range of loads pori was proportional to L-O 6o ; poff to L-O 70 and ooff to L-Oi4, where L is the normal load. It appears that within the experimental error of the measurements &off = with K = 1-35. In figure 1 the frictional force (normal load removed) is plotted against adhesion for the whole range of loads. In the ooff measurements the tensile strength of the junctions was measured whereas in the poff measurements the shear strength was measured ; apparently the tensile strength of a junction is a definite fraction of its shear strength.

T o determine whether the difference could be due to the areas of contact increasing in size when the tangential load was applied to measure pOn (McFarlane and Tabor 1950) the normal load was applied for 30 seconds, a tangential load just short of that necessary to bring about slip then applied to the specimen for 5 seconds, the tangential and normal loads removed and 30 seconds later the adhesion measured. If the

pon was greater than poff at room temperature.

.

+ J + ++++

B P L 500

0 RMeron ($1

Figure 1. Frictional force (after removal of normal load) plotted against adhesion. G-2

100 R. T. Spurr and T. P. Newcomb

tangential load had caused an increase in contact area, the adhesion after the tangen. tial load had been applied should be correspondingly greater than the value obtained when no such load had been used. There was a small Increase but this was found to be due to the extra five seconds of normal loading.

pon could also be greater than poff because of elastic recovery reducing the true contact area when the normal load was removed to measure poff . TO obtain an idea of the magnitude of the visco-elastic recovery in ' Mexphalte ', in. hemi- spheres were loaded against a glass plate and the area of contact measured with a microscope. The load was applied for 120 seconds and then removed ; a typical area-time curve is shown in figure 2, and it can be seen that the recovery is considerable. Further poff measurements were therefore made, but the interval between the removal of the normal load and the measurement of poif was varied between 1 and 120 seconds in the expectation that poff should approach pon as the interval was decreased. However, any variation in poi! with variation in the time between removing the load and measuring the friction was smaller than the experimental error in the measurements. Presumably, the recovery in nominal flats is almost entirely elastic and consequently very rapid.

Figure 2. Creep and recovery curve of ' Mexphalte '. The normal load was removed after 120 seconds.

More indirect methods, however, showed that the difference between pori and po f f was due to recovery phenomena. If a ' Mexphalte ' specimen was left in contact with the stationary disc, the friction increased with the length of time the surfaces were in contact because of creep, whereas the elastic forces remained practically unchanged. Consequently, if pori and poff are measured for different pre-load times, poff should approach pori with increasing times. Values of pon and poff are shown in figure 3 for a 550g load and pre-load times varying from 2 to 1000 seconds, and it can be seen that poff approaches pori with increasing time. Similarly, at higher temperatures, when the area of contact and therefore the adhesion and pon increase to very high values, poff and pori converge, e.g. at 550 g load pof f = 0.85 pon at ~ O " C , but p o f t = 0.95 pori at 3Ooc, and at 3 5 " ~ pon and p o f i had the same values within the limits of experimental error over the entire load range. Conversely, the elastic forces can be made important compared with the adhesive forces by allowing the surfaces to become contaminated. Measurements of pon and pm were made at room temperature at various times with 550 g load without any attempt to clean the surfaces and are shown in figure 4. It can be seen that if Won is less than about 1.75 the elastic recovery is sufficient to overcome the adhesion, i.e. poff=O.

The Adhesion Theory of Friction

3 -

2 -

PdT

I -

101

/ /

/ /

/’ /

/ /

/

5

3 CL

2

0

Figure 3 . Values of pori (upper curve) and poff (lower curve) plotted against log (pre-load time).

P m

Figure 4. Values of pLoa plotted agalnst pori.

0 4. CONCLUSION It appears that, with ‘ Mexphalte’, static friction is due to adhesion between

the specimen and the opposing surface, and that elastic recovery of the surfaces on removal of the load tends to reduce the friction, and can, if the adhesion is low enough, reduce the friction to zero. I t is suggested that such behaviour is not limited to materials like ‘ Mexphalte ’ but is quite general.

ACKNOWLEDGMENTS T h e authors wish to thank the staff of the Technical Division of Ferodo

Limited for assistance and criticism and the Directors of Ferodo Limited for permission to publish this paper.

REFERENCES BIKERMAN, J J., 1941, Plzzl. Man., 32, 69. BOWDEN, F. P , and ROWE, G. W , 1956, Proc Roy. Soc. A, 233,429. BOWDEN, F. P , and TABOR, D., 1939, Pvor Roy Soc. A, 169, 391. MCFARLANE, J. S., and TABOR, D ,1950, Proc. Roy. Soc . A, 202,244