Wear, 43 (1977) 141- 150 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
OF THE SLIDING
WEAR OF RUBBER*
&I. BARQUINS and R. COURTEL
1 Place Ark-tide Briand, 92190 Meudon
(Received December 31, 1976)
Summary The mechanism of the sliding wear of rubber was investigated using a hard sphere on a plane of filled rubber. Viscoelastic folds produced in front of the hard asperities of the rider enter the true area of contact and are abraded. It is suggested that the experimental method that was adopted could be used as a quick test to assess the sliding wear resistance and the wear behaviour of rubbers.
Introduction The wear mechanism of rubber is an important industrial problem for tyres and seals, especially in the landing of heavy aircraft. As it is difficult to determine simple laws for the wear of industrially used materials, work has been carried out on a more specific rubber than that used in practice but one which has similar mechanical properties.
Using an apparatus described elsewhere [ 1] the non-lubricated sliding of a hard sphere on a plane of filled rubber was found to produce viscoelastic folds (or ridges) in front of the rider; these did not disappear or propagate when the rider was lifted [Z] . The folds might enter the area of contact when the rider moved forward (Fig. 1) and they might be abraded by the rider. This process did not occur when surface relaxation was effected by the formation and propagation of Schallamach waves (detachment waves  ) instead of visco-elastoplastic folds. The domain of mechanical properties (E, Y, the viscosity, the relaxation time, the hardness produced by the concentration of the filler) which favour the formation of folds requires special study. *Paper presented at the 2nd Israel Tribology Conference, Haifa, October 20 - 22, 1976.
(c) Fig. 1. The progression of the rider over the folds of the rubber formed in front of it. The motion of the rider is from the bottom to the top of the photographs and that of the rubber is in the reverse direction.
To elucidate the correlation between folds and sliding wear, e.g. the fretting of antivibration rubber plates, the apparatus was adapted to give an alternating motion (frequency 4 Hz, amplitude 50 pm in the tangential
direction to the contact) to a sample of natural rubber that was vulcanized with dicumyl peroxide and filled with 30 wt.% SiOz (E = 9 X lo7 dyn cme2, shore hardness A = 65, Si02 particle size = 25 nm). The rider was a glass hemisphere 4 mm in diameter under a load of 0.2 N. Experiments were conducted under conditions of preliminary displacement, i.e. where the displacement of the rider was too small to produce continuous sliding everywhere in the area of contact. According to the theory of Mindlin  the central part of the area of contact is a no-slip region; slip takes place in an annular zone, the size of which depends on the applied tangential force T, on the elastic constants E and v of the rubber and on the “true” friction coefficientf[4-61. When T varies sinusoidally between -T and +T (T < T,,, which provokes the global sliding), rapid wear of the annular slip zone is observed. The formation of rolls resulting from abrasion of the crests of the folds may be observed by ultra-rapid cinematography. These rolls are ejected from the area of contact, while degraded rubber sticks to the surface of the rider.
Fig. 2. The pattern of the contact
area after fretting
for 40 000 cycles (magnification
The rubber surface after the experiment is shown in Fig. 2. From the centre of the contact, four concentric zones may be distinguished: (1) the central no-slip adhesion zone; (2) a “fold” region; (3) the circular hollow wear ring; (4) heaps of degraded or abraded rubber piled up on the outer side of the wear print. The formation of the folds in zone 2 is a fatigue process. The alternating tangential stress is maximal on the internal edge of the wear ring. Figure 3 shows a part of zones 2 and 3 at higher magnification. The saw tooth wave pattern in zone 3 is characteristic of a unidirectional process of abrasion, as observed by Schallamach  and confirmed by another series of experiments not described here. It may therefore be deduced that the
Fig, 3. Magnification
of zone 2 (the “fold”
abrading process in the front and the back zones takes place only during the forward motion. During displacement sliding occurs in the front part of the area of contact, but detachment occurs in the rear part. In contrast, there is an alternating motion in the two side zones. Figure 4 shows the successive phases of the degradation of the rubber, from one stroke to 40 000 cycles. zone of alternative
Fig, 4. Successive
zone of onedirection
stages in the degradation
of the rubber.
The abrasion mechanism described is not specific to an SiQa filling. It appears to be the same with a carbon black filled industrial rubber (Figs. 5 and 6) which was tested during 1300 single passages. The heaps of degraded rubber were toluene soluble, i.e. they were true polymers, although they resulted from a depolyme~za~on of the rubber. Complementary to the fundamental study, the influence of time and amplitude on the fretting wear of rubber was demonstrated and was correlated with the friction forces. A special device necessary to perform the experiments was constructed on the basis of past experience [8,93 (Fig. 7). A
Fig. 5. Comparison between samples worn by 1300 single passages, fillings: (a) 30% carbon black; (b) 20% SiO2.
having two kinds of
(a) Fig. 6. The abrasion pattern (a) before and (b) after washing degraded rubber was dissolved (Filler 30% carbon black).
beat frequency oscillator supplied a small vibration exciter and a piezoelectric gauge fixed on the exciter measured the applied force. The rider was a stainless steel sphere. The rubber samples were stuck on two ball slideways and were pressed together with a fixed load N. Another acceleration gauge fixed on the rider measured the acceleration. Both signals were amplified and analysed with the help of a cathodic analyser and the phase angle was also measured.
Fig. 7. A schematic diagram of the experimental
Fig. 8. The application of Fresnel’s rule to the composition
of the forces.
Since the ball diameter was 5 mm and the load was 0.3 N, the frequency was taken as u = 160 Hz to avoid the natural frequencies of the apparatus [S] and to accelerate wear. As a first approximation one may consider that, for a viscoelastic material, the force Fa measured by the piezoelectric gauge is the sum of three terms: the elastic term F,, the viscous term Fv and the inertial force Fi; these are respectively in phase with the displacement X, the speed and the acceleration. Using the Fresnel diagram (Fig. 8) and taking cpas the phase angle between FR and x, we can write IFEI = IFal cos rp + IFi1 IFvI = IF,1 sin rp In the experiments cp < 90”, i.e. vwork< vresonance which implies that FE = F,. The evolution of the wear pattern as a function of the amplitude (10 - 79 pm) at a fixed number of cycles (120 000) is shown in Fig. 9. A reduction in the size of the central zone and a widening of the wear ring could be observed. Figure 10 shows the variation in the correlation of the different mechanical parameters during the experiment. In another series of experiments the amplitude of the displacement was fixed (50 pm) and the number of cycles varied between 10 000 (30 s) and 1 200 000 (60 min). Essentially the same phenomena were observed as previously (Figs. 11 and 12). To interpret these results, it must be considered that they do not correspond exactly to the same total energy dissipated. For comparative purposes, experiments were made on three different samples at the same frequency (160 Hz), at the same power of the vibrator and for the same time (60 min) (Fig. 13). The three samples were: (a) natural vulcanized rubber, which showed a small wear area; (b) natural vulcanized rubber filled with 30% carbon black, which showed a larger wear area; and (c) natural vulcanized rubber filled with 30% silirsa. which showed the largest wear area. The
Fig. 9. Evolution of the wear pattern as a function of the amplitude (IO - 79 i.lm) at a fixed number of cycles (120 000); the amplitudes (in micrometres) were (a) 10, (b) 17, (c) 26, (d) 29, (e) 35, (f) 40, (g) 44, (h) 46, (i) 55, (j) 64, (k) 72 and (1) 79.
Fig. 10. The correlative variations of the mechanical parameters for the experiment shown in Fig. 9.
quantitative wear could not be measured, as the variation small. The total energy dissipated was evaluated as t
of weight was too
Fig. 11. Evolution of the wear pattern (20X) as a function of the number of cycles (10 000 - 1 200 000) at a fixed amplitude (50 pm). The times of abrasion (in minutes) were (a) %, (b) 1, (e) 2, (d) 5, (e) 10, (f) 20, (g) 30 and (h) 60. The displacement was in the vertical direction.
Fig. 12. The correlative shown in Fig. 11.
of the mechanical
for the experiment
PCtj being the value of the dissipated power at the time t, calculated on one cycle, i.e. P(tt = nxF,/T. The values of W for the three cases described above were found to be (a) 1.6 mW h, (b) 11 mW h and (c) 10 mW h. There was a considerable difference between the behaviour of the unfilled rubber and that of the two filled rubbers. To interpret this difference,
Fig. 13. A comparison (b) natural vulcanized filled with 30% silica.
between three samples of rubber: (a) natural vulcanized rubber; rubber filled with 30% carbon black; (c) natural vulcanized rubber
it was considered that it would be necessary also to take into account the thermal properties of the different samples but at present little information is available on this aspect.
To compare results obtained with different materials (mainly with different fillers), it is essential to maintain all the mechanical and physical parameters constant. From a mechanical point of view it appears to be necessary to compare samples of similar hardnesses (e.g. rubber with 30% carbon black and rubber with 20% pyrogenic SiOs). From thermal considerations, the apparatus should be adapted to dissipate the same amount of energy during the same time in all samples. The wear mechanism of viscous rubber filled with either Si02 or carbon black is the same. Viscoelastic folds produced in front of the hard asperities of the rider enter the true area of contact and are abraded. The experimental method presented gives some information on the progressive evolution of the process and the method could be used as a quick test to compare the sliding wear resistance and the wear behaviour of different filled rubbers.
Acknowledgment We thank the Direction Des Recherches et Moyens d’Essais (French Army Department) for the grant which supported this work.
References 1 2 3 4 5 6 7 8 9
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