Wear, 61(1980) 273 - 282 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
FRICTION AND BEAR IN RUBBERS AND TYRES*
DESMOND F. MOORE Department
College, Dublin (Eire)
(Received July 26,1979)
Summary A unified approach to the entire subject of friction and wear in rubbers and tyres is presented. Friction is regarded as a fundamental energy dissipative mechanism as a result of which surface degradation and wear occur. At the same time the rolling tyre is considered as a combination of rubber elements undergoing prescribed motions in the contact patch, and this elementary macroscopic modelling of tyre frictional behaviour provides a basic understanding of rubber friction and tyre mechanics simultaneously. An analogy between friction and wear mechanism is developed. Thus the adhesion and hysteresis contributions to friction on rough surfaces produce abrasive and fatigue wear respectively, whereas waves of detachment on smooth surfaces appear to produce wear by rolled fragments. The paper concludes with a description of an elaborate experimental rig designed to test elementary friction blocks of rubber or a complete tyre in various braking modes.
1. Friction and wear mechanisms This paper offers a unified approach to the entire subject of friction and wear in rubbers and tyres, and presents a clear understanding of the physical phenomena involved. Thus friction is regarded as a fundamental energy dissipative mechanism as a result of which surface de~adation and wear occur. At the same time the rolling tyre is considered as a combination of rubber elements undergoing prescribed motions in the contact patch, and such elementary macroscopic modelling of tyre frictional behaviour [l] provides a fundamental understanding of rubber friction and tyre mechanics simultaneously. Figure 1 is a schematic diagram of friction and wear mechanisms in rubber-like materials. There are two principal components of rubber friction, i.e. adhesion and hysteresis as shown [ 1 - 31. The adhesion contribution is a *Paper presented at the International Rubber Conference, Kiev, U.S.S.R., October 10 - 14,197s.
Fig. 1. Schematic
diagram of the friction and wear mechanisms
in rubber-like materials.
surface effect having its origin as a molecular kinetic stick-slip action between rubber and the underlying surface, whereas the hysteresis factor is a bulk phenomenon within the body of the rubber slider. It is seen that, depending on the surface conditions, adhesion and hysteresis give rise to several distinct wear effects. Thus on a perfectly smooth texture adhesion may give rise to wear by roll formation in cases where the tear strength of the particular rubber is low. A more common experience occurs on harsh textures where the adhesion mechanism gives rise to abrasive or cutting wear. Should the nature of the surface texture of the substrate be such that its asperities are smooth and rounded rather than harsh, the hysteresis mechanism of friction gives rise to fatigue wear. It has been shown [l] that the fatigue mechanism is relatively mild in intensity but continuous, whereas the abrasive wear phenomenon is severe and usually of short duration. For example a rolling unbraked tyre exhibits the fatigue mechanism because of the repeated and cyclic loading and unloading of discrete asperities in the road surface if the latter are smooth and rounded. Under wet conditions and with severe braking the same tyre will experience locked wheel sliding, and on this particular surface hysteresis friction will then induce a more severe form of the fatigue wear mechanism. In contrast, locked wheel sliding of a tyre on a dry harsh texture causes severe and perhaps irreversible abrasive wear resulting in local overheating and the production of rubber wear fragments. Fatigue wear is also present in the latter case but it is of such minor proportions as to be negligible. We see then that the various mechanisms of friction and wear occur in different
combinations when a tyre is subjected to braking, driving, free rolling and cornering manoeuvres. It is impo~~t to stress the fact that, irrespective of the various components of each, friction and wear are broadly classified as cause and effect in circumstances where an energy dissipative mechanism is seen to occur at a surface or boundary. Furthermore, the division of rubber friction into adhesion and hysteresis components is simply a convenient method of identifying features and properties of the frictional mechanism as a whole, whereas in reality no such distinction necessarily exists. As an example consider a progressive reduction in scale of the macrotexture of a solid substrate subjected to sliding contact by a rubber-like material Ultimately the scale is sufficiently small to cause microhysteresis within a very thin layer of rubber at the sliding interface and it becomes virtually impossible to distinguish this mechanism from macroadhesion. In the normal case we use adhesion and hysteresis merely as convenient physical mechanisms for visualizing a total frictional effect. Consider next the relation between friction and wear as depicted in Fig. 2. Here the energy index of abrasion (defined as the ratio of worn rubber layer thickness to the work of friction) for various tread rubbers is plotted as a wear factor against the corresponding measured coefficient of friction. It is seen that the fatigue mechanism for frictional coefficients of less than unity produces comparatively little wear, whereas severe abrasive wear (on harsh rough surfaces) or roll formation (on smooth substrates) correspond to frictional coefficients greater than 1.25.
Ccsefficimt of Friction,
Fig. 2. Broad correlation of friction and wear.
2. Viscoelasticity In contrast with metal friction both the adhesive and hysteretic mechanisms of friction for rubber-like materials are viscoelastic, This is
exemplified by characteristic friction uersus frequency (or sliding speed) plots at a specified temperature as illustrated in Fig. 3. Each curve of the total coefficient of friction exhibits two viscoelastic peaks. The first of these is due to adhesion and occurs at a sliding speed of about 1 cm s-l which is a typical value of slip speed in the contact patch of a rolling tyre. The second or hysteresis peak occurs at about 150 - 200 km h- ’ at room temperature and corresponds to sliding velocities in the contact patch for high speed panic locked wheel braking. Each total curve with its twin peaks moves to higher frequencies or sliding speeds as the temperature is raised. From a tyre design viewpoint both peaks should be used at the extreme limits of driving behaviour, i.e. slow speed rolling and high speed sliding, to maximize tyre/ road friction. By using the well-known Williams-Landel-Ferry transform it is possible to convert the friction uersus frequency plots at a given temperature as shown in Fig. 3 into equally useful friction uersus temperature plots at a specified frequency. This technique has been used to extend the range of validity of experimental data and in certain cases to obtain considerably more data with given measuring equipment than would otherwise be possible. It has also been shown that the characteristic peaks of rubber friction correspond to characteristic troughs of resultant wear or abrasion [l] which is to be expected if both mechanisms, i.e. friction and wear, are viscoelastic.
Fig. 3. Viscoelastic
The adhesion peak of rubber friction has been predicted by a number of theories based upon the fundamental molecular kinetic stick-slip action which is believed to underlie viscoelastic properties [ 21. In contrast the hysteresis peak can be simply shown with mechanical models using springs and dashpots in various arrays of complexity [l] . What is not at all certain as yet is whether it is valid to make any distinction at all between adhesion and hysteresis as complementary mechanisms, and if so whether to ascribe both to a common origin. The present state of the art appears to be that viscoelastic behaviour for the adhesional mechanism can be traced back to molecular kinetic and therefore microscopic origin, whereas viscoelastic behaviour for hysteresis is usually ascribed to mechanical modelling of rubber or macroscopic origin.
3. Tyre tread motions The path of a tyre tread element entering the contact patch of a rolling tyre is complex [l] with a squirming motion of microscopic proportions occurring in the forward part of the contact area and macroslip occupying the rearmost part. Such a squirming motion produces microslip which gives rise to adhesional friction, whereas the region of macroslip shows a progressively increasing velocity of slip of rubber elements relative to the road surface according as the rearmost point of contact is approached during rolling action. Figure 4 shows the non-linear increase in slip velocity towards the rear of the contact patch for braking, driving, left cornering and right comering conditions [l] . It should be noted that the direction of this increasing slip velocity is forward, backward and lateral for these respective driving modes.
Fig. 4. Increasing velocity of slip in the contact patch for a braking, driving or cornering tyre.
Rather than being concerned with details of tyre and carcass construction which ultimately determine slip motion in the contact area, the elementary macroscopic modelling technique assumes that individual tread elements follow the motions shown in Fig. 4 which have been determined by experiment. Vibrational effects are of course superimposed on this gross macromotion but these are not considered to have a measurable influence on tyre friction. Figure 5 shows the traction ellipse corresponding to all modes of driving behaviour and therefore complementary to Fig. 4. The boundary of this ellipse defines the limits of traction in the broadest sense, and the internal accident avoidance envelope is a statistical interpretation of actual driving behaviour. It is interesting to note that with the exception of braking all modes of driving fall well within the tractive capability of the vehicle.
278 S = Srakmg
L = Left
c = Cornering
R = Ri*t
D = Driving ‘RC
Fig. 5. Traction ellipse and accident avoidance envelope.
4. Elastohydrodynamics The slip velocity distribution shown in Fig. 4 in the case of wet surfaces generates elastohydrodynamic pressures on the slopes of individual asperities of the road texture, thereby attempting to separate tread and surface as shown in Fig. 6. It has been shown [ 1, 21 that the positive pressure increments far exceed the negative contributions so that a net uplifting force occurs. The severity of this force increases towards the rear of contact where slip velocities are greatest. This theory of viscous hydroplaning was developed by Moore in 1966, and it explains the important role of microtexture in combating a dangerously slippery condition. The iterative sequence in calculating the extent of the elastohydrodynamic separating effect is shown in Fig. 7. In simple terms the generation of hydrodynamic pressures along the leading slopes of surface asperities due to the slipping Profile
Fig. 6. Elastohydrodynamic surface.
pressure generation on the individual asperities of a wet road
279 Inflation Plsuure
I hwiw Loading
LOad swpwt 4 W-W Ratio
Hitial Film Thickness
Fig. 7. Iterative sequence in the elastohydrodynamic
action of lubricated tread elements opposes the elastic pressures created by initial contact, and under certain conditions the former exceed the latter and produce a continuously slippery film at asperity peaks where dry contact formerly prevailed. The condition of viscous entrainment can be effectively counteracted by introducing a microroughness at road asperity tips (usually by adding silica sand during road construction) such that the amplitude of this roughness exceeds the film thickness which would otherwise have existed as a consequence of elastohydrodynamic fluid entrainment. The mathematical details of the entire mechanism can be found elsewhere [l] and the consensus of informed opinion today is that the range of amplitude for such microroughness is 10 - 70 pm. This is shown in Fig. 8 which also illustrates the compromise which must be reached between wet friction and tyre abrasion. The design texture band from 10 to 70 pm appears to offer
Fig. 8. Design texture band for microroughness.
reasonably high coefficients of wet friction with a tolerable level of abrasion as shown. The role of road surface texture is of course fundamental in determining the nature and extent of tyre friction. Thus a surface with distinct asperities or macrotexture gives both adhesion and hysteresis in high speed sliding under dry conditions but only hysteresis under wet conditions. This is because of viscous or dynamic hydroplaning or both which have the effect of producing a film of water at asperity peaks. The use of microtexture at the peaks of the macrotexture elements restores the adhesional mechanism under wet conditions by counteracting fluid entrainment and permitting solid peak contact. Microtexture without macrotexture is effective only for very thin films, even though the hysteresis contribution to friction is absent, but there is no provision for bulk water to escape for normal heavy rainfall. 5. Recent work One of the more interesting phenomena observed in recent years is the occurrence of “waves of detachment” in the contact area of sliding rubber . These waves are folds in the rubber surface produced by buckling. Whereas adhesion due to molecular kinetic stick-slip behaviour undoubtedly occurs between the waves, the buckling itself can be attributed to compressive tangential stresses which exceed the pressure forces tending to maintain uniform contact. The motive force driving the waves of detachment across the surface of contact appears to be a tangential stress gradient. These waves have been observed to propagate at right angles to the imposed sliding velocity  and in the same direction as that in which the rubber surface moves relative to the other frictional member. They occur both between a hard slider and a rubber track and between a rubber slider and a hard track. It is interesting to note that the propagation of waves of detachment is accompanied by hysteresis losses due to the microscopic undulation. Thus from a fundamental friction viewpoint the loss of potential adhesion between folds is compensated for, at least to some extent, by the creation of a microhysteresis component of friction. This viewpoint supports the unified theory of rubber friction by equating adhesion and microhysteresis as discussed earlier. If we now extend the concept of undulating waves of detachment as an observable adhesional mechanism of friction between smooth surfaces to the probable resultant wear effect we conclude that wear by roll formation is an extremely likely candidate for a number of reasons [ 2, 51. Thus the most likely conditions of contact, i.e. a soft rubber with low tear strength sliding on a smooth hard base, for both waves of detachment and wear by roll formation are identical, and additionally the necessary geometrical configuration required to initiate rolled wear fragments is surely the region of periodic surface detachment due to buckling. The analogy between friction and wear mechanisms as shown in Fig. 1 is now complete. The adhesion and hysteresis contributions to friction on rough surfaces produce abrasive and fatigue wear respectively, whereas waves of detachment on smooth surfaces would appear to produce wear by rolled
fragments. It is unlikely that either waves of detachment or rolled wear fragments will be observed for hard rubbers with high tear strength. A second interesting phenomenon observed in recent years is the “stiffening” effect in rubbers according as the scale of roughness of the hard frictional base is reduced for a given speed of sliding . Thus the effective hardness of rubber-like materials is altered depending upon the frequency of indentation by asperities of the hard base surface during sliding. The effect of this property on the frictional mechanism is two-fold: (1) the microhysteresis contribution to friction becomes a truly adhesional effect since the discrete periodic indentation caused by the very small surface asperities disappears; (2) the onset of viscous hydroplaning by the viscoelastohydrodynamic entrainment of lubricant over the peaks of macroasperities is most effectively opposed. 6. Experimental facility In conclusion a brief description and illustration is given of a unique braking test rig which now forms part of the Tribology Laboratory at University College Dublin. This rig has the dual capability of using either a test rubber block for measuring friction and wear or of testing the effectiveness of tyre tread pattern and braking mode on traction. As the illustration in Fig. 9 shows the rig is at present adapted to test the effectiveness of a new and truly adaptive anti-skid braking system for all forms of road vehicle. A test axle with variable wheel inertia, speed tachometer, test wheel, slip-ring assembly, torsional strain gauges, drum brake, accelerometer sensing unit and dummy support wheel is loaded by gravity against a simulated moving roadway which consists of an endless stainless steel belt with variable texture passing over end pulleys and driven by a variable-speed motor control unit. The system is unique in that the pulsing sequence normally associated with anti-skid systems is designed so that both the shape and intensity of each braking pulse exactly matches the slipperiness of the road surface. In this manner optimum braking is continually and automatically achieved. The Tribology Laboratory contains a number of additional unique items of equipment which have been developed and constructed within the University for measuring rubber/tyre properties. These include in particular a complex fatigue machine for cyclically applying three-dimensional stressing to an elastomeric work sample, a new design of profile-measuring equipment and a semi-automatic hydraulic texture meter for recording surface roughness. Standard friction and wear measurement machines are also available, as are viscometers and a range of calibration surfaces. The following unique research goals were achieved during the period 1971 - 1978: (1) measurement of the lowest coefficient of friction ever recorded (0.0003); (2) measurement of infinitesimal differential wear by a hydraulic leakage flow method;
Fig. 9. Overall view of the anti-skid braking rig and road friction simulator.
(3) development of a theory of elastohydrodynamic separation for rubber-like materials slipping and sliding on lubricated textured substrate; (4) measurement of texture by hydraulic leakage flow. The Laboratory has the overall capability of measuring and recording the dynamic properties of rubber-like materials under all conditions of stressing including vibration, fatigue and impact. References 1 D. F. Moore, The Friction of Pneumatic Tyres, Elsevier, Amsterdam, 1975. 2 D. F. Moore, The Friction and Lubrication of Elastomers, Pergamon Press, Oxford, 1972. 3 D. F. Moore, Principles and Applications of Tribology, Pergamon Press, Oxford, 1975. 4 A. Schallamach, Elementary effects in the contact area of sliding rubber, Symp. on the Physics of Tire Traction, Plenum Press, New York, 1974, pp. 167 - 177. 5 D. I. James, Abrasion of Rubber, McLaren, London, 1967. 6 D. F. Moore, Scale effects in elastohydrodynamic lubrication for rubberlike materials, Proc. 4th Leeds-Lyon Tribology Conf., Lyon, September 1977, Mechanical Engineering Pubhcations, London, pp. 315 - 320.