Rubber abrasion and wear

Rubber abrasion and wear

Wear, I58 (1992) 213-228 Rubber 213 abrasion and wear A. H. Muhr and A. D. Roberts MRPRA, Brickendonbuy, Hertjordshire SG13 BNL (UK) Abstract Ab...

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Wear, I58 (1992) 213-228

Rubber

213

abrasion and wear

A. H. Muhr and A. D. Roberts MRPRA, Brickendonbuy,

Hertjordshire SG13 BNL (UK)

Abstract Abrasion processes in a highly elastic rubber surface are distinctly different from those of other materials of higher modulus. This review paper describes the main features of rubber abrasion. It is a topic that has received considerable attention over the years in view of its relationship to the wear of road tyres. Beginning with definitions, this paper describes the influence of rubber material properties, how abrasion is initiated, the development of surface abrasion on sharp and blunt tracks and the influence of smear and lubricants. The overall conclusion is that whilst for some simple contact geometries a reasonable prediction of abrasion rate can be made, in practice the wear of rubber articles remains difficult to predict.

1. Introduction

It is with considerable pleasure that this article is offered. John Lancaster may not have published original work on the wear of rubber, but he takes a lively interest in the topic, always seeking to mention the most up-to-date work in his reviews. Kyuichiro Tanaka, published [l] an elegant contact area study of polymer friction in 1961 well before the authors of this paper knew anything about these matters. He also had Yoshitaka Uchiyama as a research student, and Yoshi now leads a school devoted to rubber friction and wear. Wear prediction is of great importance because of the cost of testing actual rubber articles. Laboratory studies of rubber abrasion aim at least to assist in the selection of materials. Although the studies show how for certain arrangements the rate of abrasion can be predicted, the wear of rubber articles in practice is beset with complications. For example, the wear of a road tyre brings into play its gross deformation properties just as much as the actual abrasion resistance of the rubber tread. Road topography, dust, water and other contaminants complicate matters further. Over the years our laboratory has endeavoured to understand the fundamental aspects. Abrasion in itself is hard enough to understand because it involves strength properties and ill-defined loading conditions. This survey begins with a discussion of how parameters such as friction, hardness and resilience may influence wear by affecting the amount of sliding which occurs in the contact zone. Attention is then turned to the effect of material properties on abrasion resistance, with accounts of initiation, severe and mild abrasion. Due consideration is given to the base polymer, tensile strength, crack growth, compound hardness and resilience. Others factors are the effect of lubricants, temperature and smear. The survey concludes with prospects for improving wear prediction.

Elsevier Sequoia

214 2. Definitions

When Schallamach carried out his early work, he defined abrasion as that produced by laboratory machines on rubber test pieces and wear as something that happens to tyres or other rubber products. Abrasion and wear were, and are, difficult to correlate. Thus, for rubber, “abrasion” covers all mechanisms, whereas the word “abrasion” for other materials refers in particular to scoring by hard, sharp particles. When welllubricated rubber or very hard unlubricated rubber is rubbed on abrasive paper, score lines may be produced in the rubber surface, but usually the topography of “abraded” rubber is quite different (Fig. 1). Further terms are severe or mild wear/abrasion. The former simply refers to a higher rate of attrition than the latter and may not necessarily imply a different mechanism. Whether or not the mechanism is different, there are often reversals in ranking of the abrasion resistance of compounds on going from mild to severe abrasion. In the absence of transient effects such as clogging of the abrasive or evolution of an abrasion pattern it is found that the quantity of rubber abraded is proportional to the distance of sliding between rubber and counterface. However, wear of tyres and abrasion on certain laboratory abrasion machines (e.g. the Akron abrader) brings into play gross properties of tyre or test piece which affect the rate of wear (by determining the amount of sliding) just as much as does the abrasion resistance of the compound [2]. For a proper study of abrasion, therefore, it is necessary to design an abrasion apparatus for which the distance of sliding and normal stress (p) are known and the frictional force and weight loss can be measured. Such experiments yield a dimensionless measure of abrasion S(p) (or linear rate of “abrasion”) given by

(4

0)

Fig. 1. Patterns formed during abrasion of unfilled NR on P60 silicon carbide paper (rubber sliding to the left). The test pieces are 25 mm square. (a) Dry (classical abrasion pattern): ~~1.34, 6=2.4x1O-s. (b) Wet (score lines): ~=1.23, 6=0.9~10-~

215

depth of abrasion QJ)=

distance

of sliding

(1)

which may correlate with properties of the rubber and the conditions of abrasion. Since the ultimate goal is to predict the wear rate, knowledge is required not only of the abradibility (susceptibility of the rubber to abrasion) but also of the effect of the rubber properties on the amount of sliding in the contact area. This issue will be addressed first, using tyres as an example, before dealing with abrasion. The “slip” of a rolling elastic wheel is defined to be s= sin 0 for axial slip (crab walk) at a slip angle 0 and V-V ,y= ?I for circumferential slip at travelling speed v and circumferential velocity K This should not be confused with “sliding”, i.e. “relative horizontal motion” between rubber and the ground with which it is in contact, which occurs only towards the rear of the tyre-road contact zone. 3. Influence of material properties on degree of contact sliding 3.1. Friction Sliding of rubber with high frictional forces does not necessarily entail abrasion (as it does for metal-metal contacts). Rather, abrasion of rubber results from mechanical failure due to excessively high local frictional stresses which are most likely to occur on rough tracks. Theories of abrasion thus require details of the local stresses, which together with the strength properties of the rubber may enable the rate of abrasion to be predicted. It seems likely that contributions to friction arising from energy losses associated with bulk deformation, and arising from viscous dissipation in any lubricant film, will not cause large local strains and hence will not contribute to abrasion. Thus the overall average level of friction is of no direct significance for abrasion. However, in some practical situations the level of friction influences the amount of sliding that takes place. Examples of this would be plain unbonded rubber pads [3] (often used in compression as expansion pads in bridge decks or as rail pads) and road tyres. Should sliding occur, prediction of the rate of wear would require a detailed analysis yielding the amount of sliding and the normal load as a function of position. Such an analysis has been given for tyres [4, 51. 3.2. Hardness Despite a dramatic improvement in tyre tread service life by the use of abrasion grades of carbon black, there are circumstances in which the abrasion resistance of vulcanizates is not enhances by their use. For example, on sharp abrasive paper the weight loss of NR may be increased by the incorporation of I%4F black [6]. It has also been observed [7] that a plug of unfilled NR let into the tread of a tyre wears as well as the surrounding filled rubber. This may be explained, in part, by noting that the amount of sliding between rubber and road is not uniquely related to the trajectory of the wheel but is also inversely proportional to the stiffness and directly proportional to the hysteresis of the tyre tread and tyre [S]. The plug is supported by the surrounding black tread compound, preventing its movement relative to the road.

216

In view of the importance of carbon blacks on tread wear, it is surprising that relatively little understanding of the phenomenon has been set out in print. Although for synthetic rubbers such as BR and SBR it may seem unnecessary to look further than the dramatic enhancement of strength properties imparted by the use of particular grades of black, for NR such enhancement is modest and additional mechanisms for the effect of blacks on tread wear should be sought. In any case there is a consensus that high surface area, high surface activity and high structure promote tread wear resistance. Even so, the evidence that carbon black does not necessarily enhance the abrasion resistance of rubber under conditions of equal sliding suggests that the effect of black on tread wear may in part be simply associated with stiffening, and hence reduced sliding, without weakening the compound as a high cross-link density would do. 3.3. Resilience Elastic wheels can roll with a small amount of slip, without sliding over the complete contact length. As it moves into contact, the surface of a wheel adheres to a track and is held on the leading portion of the contact zone. Adhesion is thus maintained at first, but the elastic restoring force of the deformed wheel tries to pull the surface back into its unstrained state. The magnitude of the restoring force depends not only on stiffness but also on resilience. With increasing deformation through the length of the contact the restoring force eventually exceeds the local friction force and the wheel surface slips at the rear of the contact zone with consequent abrasion. In a classic experiment [8] this sequence of events was shown for the case of a small solid rubber wheel in cornering contact with a transparent track (Fig. 2). The adhered region stores elastic energy and the sliding region witnesses its partial release into frictional work. The resulting wear has been calculated by Schallamach and Turner [4]. At small slip they showed that the volume wear rate A is governed by the rubber’s abradibility y, the cornering or circumferential force F, the overall wheel resilience R and its relevant stiffness C according to C

(3)

where abradibility [6] is defined as the volume of rubber lost per unit frictional work input. The unexpected role predicted for resilience was checked experimentally [4] and its importance to the wear of wheels confirmed. Some surprising reversals in ranking of rubber compounds are found when assessed in sliding abrasion (clamped pad) and in crab walk wear (rolling wheel). For example, the sliding abrasion of butyl rubber is typically two to three times greater than that of natural rubber, but its wear in crab walk is only about half that of natural rubber. Epoxidized natural rubber (50 mol%) in crab walk against an Akron abrasion wheel can show negligible wear. This highly hysteretic rubber does not wear because at the rear of the contact zone it is so slow to recover that no slip occurs. Whilst the effect of hysteresis on the wear of slipping wheels appears to be well established, care must be taken when considering the application of laboratory results of model solid wheels to real road tyres. The resilience in eqn. (3) refers to the whole wheel. In the case of road tyres this is dominated by the pneumatic chamber and the carcass and depends only partially on the tread. Analysis to separate the effects of chamber and carcass from the tread has been presented [5]. Without such an analysis, laboratory abrasion tests have no hope of predicting tyre wear. One remarks that a machine such as the Akron abrader has no control on contact sliding, although the

(4

(4 Fig. 2. Model experiments illustrating sliding of sfipping wheels: (a) axial sliding (crab walk); (b) free rolling; (c) slip due to braking torque. The travelling direction is towards the left. amount can be changed by altering the slip angle. Thus the sliding and hence the rubber weight loss will depend upon resilience. The Cabot abrader [9] is another example of a machine that does not control sliding. On the other hand, the blade abrader IlO] and the DIN abrader control sliding, and weight loss depends less on resilience.

3.4. Tem~ra~re In the wear of tyres the temperature of the rubber at the interface needs to be considered in relation to abradibility, which is a function of temperature (see Fig. 5 in Section 6). The theory [4] for the wear of slipping wheels (above, eqn. (3)) neglects any effect of temperature on abradibility through the length of the contact zone. It has been suggested [ll] that the theory might be modified. This was done by estimating the temperature in the contact zone to enable the abradibili~ to be inserted as a function of temperature. Some results [ll] obtained qualitatively explained reversals in wear ranking observed in practice.

4. Initiation

cf abrasion

In the absence of any serious chemical decomposition the abrasion process initially results in the removal of small rubber particles just a few microns in size, leaving pits

218

Fig. 3. Soft polyisoprene hemisphere (R = 18.5 mm) sliding against glass at a speed of 1 mm s-’ under an applied load of 0.2 N. High friction (~=2) is accompanied by the transfer of particles of approximate diameter 8 pm to the glass plate. The hemisphere is sliding from right to left and the contact diameter perpendicular to the sliding direction is 2.8 mm.

behind in the surface. With continued rubbing, larger pieces of rubber of the order of 0.1 mm are removed. Although most weight loss is attributable to the larger pieces, it is thought that the detachment of the smaller particles initiates the abrasion process. The small particles have a characteristic size of l-5 pm, but whether this relates to a structural unit in the rubber compound or is determined by localized stresses is not known. Other suggestions are that mechanical rupture to produce the particles relates to flaws in the rubber, including dirt, or voids that cavitate leading to internal subsurface failure [12]. A rolling experiment suggested that particle detachment might be linked to interfacial adhesion [13]. Microtearing seen for optically smooth rubber hemispheres slid against a glass plate may relate to the initiation process. Photomicrographs of the contact zone were taken in reflected white light through the glass plate [14]. With increasing sliding speed, characteristic Schallamach waves were seen to merge into semistatic gross ridges owing to surface buckling. All contact was concentrated on the crests of the ridges. In the wake of the ridges, distinct wear debris could be seen transferred to the glass surface. Globs of debris were typically 2-15 pm in size regardless of rubber type (hardness 40-60 IRHD). The transfer process was more obvious in the case of a soft (23 IRHD) polyisoprene hemisphere. Here ridges appear distorted by rubber stuck to the glass, with ligaments being pulled out of the rubber surface (Fig. 3) owing to high traction forces. The observed contact ridges are possibly the precursor of abrasion patterns. 5. Abrasion patterns Unidirectional abrasion of rubber often results in surface patterns characterized by ridges one after another at right angles to the sliding direction (Fig. l(a)). A cross-

219

Fig. 4. Cross-sections through abrasion pattern on (a) unfilled NR and (b) a worn tyre tread

PI. section reveals that the ridge shape is asymmetric, the steep side of the ridges facing the direction of attack (Fig. 4). From time to time the crests detach to produce large pieces of debris. The ridge height and the spacing increase with increasing severity of abrasion. The formation of such an abrasion pattern is characteristic of a low modulus elastomer sliding with high friction against a counterface. On sharp tracks the abrasion pattern is replaced by score lines parallel to the direction of motion if the modulus of the elastomer is sufficiently high [15] or the friction is reduced by the presence of a lubricant (Fig. l(b)). If the direction of sliding is continuously changed, the pattern of ridges does not form and the abrasion rate is lower. A finer-scale roughness results (microns rather than millimetres). This is termed “intrinsic” abrasion. Work on abrasion patterns has a very long history. In the MRPRA laboratories Schallamach [7] made pioneering efforts during the early 1950s. In the same period researchers at Dunlop referred to them as buffing ridges and Russian workers noted roll formation (tacky debris from the ridges forming rolls). The patterns themselves are a tool for diagnosis (their orientation giving the direction of abrasion) and have an effect upon abrasion rate. Ridge development during the abrasion of rubber has been investigated. For example, Bhowmick [16] carried out scanning electron microscopy (SEM) studies and found the first step was the occurrence of small particles produced by microtearing. Their size depended upon the frictional stress. Further particles appear which eventually coalesce to form fine ridges that progressively thickened. Uchiyama [17] made direct optical observations of the formation of abrasion patterns on a hemispherical rubber surface in contact with a rotating glass disc. Initially a set of “folds” (buckles) were made in the rubber surface perpendicular to the sliding direction. They moved unstably back and forth (in the contact zone) but subsided if sliding was stopped. Fold crests were bent over and this gave rise to undercutting, so that sets of parallel ridges were found on the rubber surface. The movement (across the rubber surface) of these ridge patterns was proportional to the wear depth and was on a downward slope of 15” into the rubber. The angle was independent of the rubber sample and applied load.

6. Abrasion

on sharp

tracks

On sharp tracks such as abrasive paper Grosch and Schallamach [6] found that s(p) was proportional to the normal stress. If abrasion on sharp tracks is a result of

220

tensile

failure,

then dimensional

arguments

yield

---

(4)

u

EB"B

where U is the energy density at break measured at the appropriate high frequency, Kl is a dimensionless constant and cr, and aB are the strain and stress at break respectively. Equation (4) was supported by results (Fig. 5) showing the same temperature dependence for abradibility (equal to 6/w) and l/U(for both gums and filled compounds). The results also confirm that abrasion does not only depend on frictional energy dissipation. It should be noted that Grosch and Schallamach considered eqn. (4) to be applicable only to “intrinsic” abrasion, i.e. in the absence of an abrasion pattern. Others have proposed similar correlations to eqn. (4). These are

(3 where H is the hardness

where E is Young’s

[18] (primarily

modulus

intended

for wear of plastics),

[19] and

SC K4w -

(7)

PUB

where p is the fractional rebound resilience [20]. It should be noted that eqn. (6) is not complete, in the sense that K3 has dimensions. Uchiyama [19] suggests that insertion of lB into eqn. (6) spoils the correlation, but Schallamach [21] attributed the observed minimum of 6 as the temperature is lowered to the maximum of +,. Grosch and Schallamach [6] reported the surprising result that gum compounds have lower values of 6 than the corresponding filled compounds (SO pphr (parts per hundred of rubber) HAF); and this effect was not accounted for by a change in U, log l/u:log A& I

I

I

I,

-60 Temperature’C

1

I

-20

20

60

100

Fig. 5. Abradibility (broken lines) and reciprocal of the energy density at break at a strain rate of about 10000% SC’ (solid lines) as a function of the temperature of vulcanizates filled with 50 pphr HAF. Arrows on the temperature axis and ref. 6 indicate the zero of the abradibility cun7es.

221 Abrasion

401 35

resistance

# 40

45

index

50

"a~~ess~:tiD~ 65

70

75

, 80

Fig. 6. Variation in abrasion resistance index with natural rubber compound hardness achieved by altering the black level (standard compound S2, BS902 pt A9, 1988). but different values of K, were required in eqn. (4) for filled and unfilled rubbers. However, Uchiyama [19] found that increasing the loading of filler in NR from 40 to 100 pphr HAF (in four stages) led to a nearly monotonic fall in 6. Recently MRPRA [ZZ] has taken a fresh look at the influence of compound hardness as determined by black loading on the DIN abrader (~60 aluminium oxide paper). The results are given in Fig. 6. It is apparent that the DIN abrasion index of NR is retatively insensitive to the black loading. This may reflect the fact that, for NR, reinforcing blacks do not much affect the tensile strength (in some cases it may even be decreased) and they generally decrease the extension at break. In the DIN test the test piece is rotated during abrasion, but often an abrasion pattern is discernible at the end of the test, indicating that the rotating rate is not great enough to fully suppress pattern formation. The photograph of Fig. l(b) shows how a lubricant (soapy water) decreases the rate of abrasion on P60 silicon carbide paper. Thus, although there is evidence in the figure that lubrication increases the cutting damage to the rubber, it appears that such damage does not make the major contribution to abrasion. As pointed out by Schallamach [23] sliding of rubber with high frictional forces does not necessarily entail abrasion (as it does for metal-metal contacts). Rather, abrasion of rubber results from mechanical failure due to excessively high local frictional stresses which are most likely to occur on rough tracks. Theories of abrasion thus require details of the local stresses, which together with the strength properties of the rubber may enable the rate of abrasion to be predicted. 7. Abrasion

on blunt

tracks

In contrast to abrasion on sharp tracks, the rate of abrasion on blunt tracks such as metal gauze, concrete or well-worn grinding wheels is very sensitive to the presence of antioxidants and to the surrounding atmosphere (oxygen or nitrogen). These factors influence abrasion in a parallel manner to their influence on fatigue. This observation points to a fatigue or crack growth mechanism from which [Zl]

where p0 and IZ are empirical constants, with n > 1. Relationships having the form of eqn. (8) have been observed for abrasion on road surfaces [24], scraping by blades (the Pica test 1251 and the MRPRA blade abrader [lo]) and abrasion on metal gauze

POI.

222

Attempts have made to derive theoretical relations of the form of eqn. (8) by Reznikovski [26] and Kragelski and Nepomnyashchi [27] using the empirical equations for fatigue behaviour. The expressions give fair agreement for the value of n with the few experiments run to test them. Equation (8) may also be rationalized by noting with Schallamach [21] that 6

number

of asperities

where dc/dn

per unit length

a-

dc dn

is the cyclic crack growth rate, and conjecturing

that

Tow where T is the tearing energy operating empirical equation of fracture mechanics dc - = RT” dn

on the rubber

during

abrasion.

Thus the

(9)

leads to eqn. (8). However, the values of IZ required for eqn. (8) are generally smaller than those obtained from crack growth experiments. Schallamach [21] suggested that this discrepancy arises because the tracks are intermediate between the ideals of “sharp” and “blunt”. None of these theories appears capable of predicting the value of the constant p0 in eqn. (8). 7.1. Abrasion

by a blade

Southern and Thomas [lo] studied a particularly simple abrasion process scraping of the rubber surface by a razor blade (Fig. 7). In this process an abrasion pattern is developed and a theory relating the rate of abrasion to the growth of cracks at the base of the “tongues” of the pattern was formulated. In this theory the rate of abrasion is related to the crack growth characteristics of the rubber, the angle of crack growth and the frictional force on the blade. The theory is successful for noncrystallizing rubbers, but for NR the rate of abrasion was higher than anticipated from its excellent resistance to crack growth (Fig. 8). The good crack growth characteristic of NR is a consequence of its ability to strain crystallize, so it is as though strain crystallization is ineffective under abrasion conditions. With this proviso the theory in effect explains bothpo and n in eqn. (8), although as yet the crack growth angle,which plays a significant role in determining the rate of abrasion, must be measured since there is no theoretical method of deriving it from material properties. It may appear surprising that scraping by a razor blade seems to relate more closely to abrasion on blunt tracks than on sharp tracks. Further support for this

Fig. 7. A model for the deformation of the abrasion pattern by the blade. Cracks grow at an angle 0 to the rubber surface from the re-entrant points at the root of the “tongues”.

223 dcfdn (am

CYCk-’ f

lo-

0.1.

0.1

Tearing enirgy, T(kN.m+)

Fig. 8. Comparison of dry abrasion results (points) and crack growth results (fun lines) for isomerized NR (0), BR (X), SBR (V), NBR (Cl) and NR (A). classification comes from the fact that antioxidants are effective (at least at lower normal loads) at reducing the abrasion rate of NR by a blade 1281 and also that lubricants dramatically decrease the rate of blade abrasion [29]. This latter observation indicates that frictional stress rather than cutting is primarily responsible for the abrasion. 7.2. Eflect of friction and lubricants Evstratov et al. [1.5] found that abrasion on a ribbed metal surface increases abruptly, by an order of magnitude or so, when the friction coefficient p exceeds about 1.4 (see Fig. 9). Abrasion patterns were observed for p above the critical value, but not for lower values. It did not matter whether JL was an unlub~~ted value for the compound or was determined by the presence of a lubricant. The renowned abrasion resistance of cis-BR compounds may relate to this observation; such compounds have low dry friction and form only very fine abrasion patterns. In spite of their Iow strength, their abrasion resistance can be excellent. Experiments on the blade abrader have led to similar conclusions [29]. When a lubricant is applied, a much finer pattern develops and the rate of abrasion is much lower (Fig. 10). Interestingly, the horizontal force on the blade does not decrease so dramatically, indicating a shortcoming in the theory devised earlier [lo] for unlubricated blade abrasion. The horizontal force as measured by a blunt slider (e.g. the side of a cylinder) is, however, substantiatly lowered by the presence of the lubricant [30]. Blade abrasion experiments [31] with unfilled rubbers incorporating an internal lubricant (e.g. silicone fluid) have also corroborated the conclusions of ref. 10, the pattern spacing getting smaller and the abrasion decreasing. The reverse phenomena occurred when a tackifier was used instead of the lubrication additive. Unfortunately, the silicone fluid appears not to have as substantial and effect when incorporated in tyre tread compounds. 7.3. Smearing and the effect of antioxidants It is well known that for some conditions the surface of rubber becomes tacky during abrasion experiments, drum testing of tyres and sometimes even for tyres on the road. It has been suggested that either exudation of low molecular weight additives or degradation of the polymer to a material of low molecular weight could be responsible. Degradation might result from either thermal or mechanical stress, At high sliding speeds, such as skidding of a vehicle on locked wheels, frictional heating certainly

224 Energy

index of abrasion (mm /J)

0.06’ 1

OESSR

: 2 ER

0.05 : : 0.04

-

3 v5 .

: :

0.03

-

3 Iffl 4 SSMOESSR 5

NR

6

SSR+resh

7

CR

6

SBR

9

NSR

10 SSR-MVP 1,

0.02

-

CSER

12 SR-MVP 13 CER 14

O.Ol-

cis-SR

15 WatNR 16

Wet SSR

Cosffkiint of friction, p After Evstratov et al 1967

Fig. 9. The relationship between abrasion and the coefficient of friction of rubbers tested on a ribbed metal surface at p = 0.63 kgf cm-*: 1, Europrene 1712 (oil-extended styrene-butadiene); 2, SKB; 3, butyl rubber; 4, combination of Europrene 1500 and Europrene 1712 (1:l); 5, NR; 6, SKS-30ARK with resins (15 parts of urea formaldehyde and 12 parts of epoxyamine 89 added to latex); 7, Nairit; 8, Europrene 1500; 9, SKN-26; 10, SKS-25-MVP-5; 11, SKS-30-l; 12, SKMVP15; 13, SKD-1; 14, SKD; 15, NR with water as a lubricant; 16, Europrene 1500 with water as a lubricant [15]. Fig. 10. Effect of lubricants on abrasion of SBR gum by a vertical blade (direction of abrasion is from top to bottom, the test pieces are 12.5 mm wide and 63.5 mm in diameter, the blade is aligned normal to the direction of abrasion and is loaded to 463 N m-l). From left to right: no lubricant, wear rate 22.9 pm cycle -‘, F/h = 748 N m-‘; soapy water lubricant, wear rate 0.4 pm cycle-‘, F/h=543 N m-r; silicone fluid 0.01 Pa s lubricant, wear rate 0.04 pm cycle-‘, F/h=465 N m-’ [29].

causes degradation. However, the phenomenon of “smearing”, discussed in this subsection, is associated with conditions of mild abrasion, e.g. on smooth surfaces, and can occur even for low sliding speeds. Schallamach [32] investigated the factors influencing smearing on the Akron laboratory abrader. He found that smearing could be prevented for NR tyre tread compounds by carrying out abrasion in nitrogen or obviated by feeding a dust (magnesia proved most effective) into the nip between test piece and abrasive wheel. He concluded that oxidative degradation (to which he attributed smearing) affects the rate of abrasion in two distinct ways. If smearing occurs, the rate of abrasion is reduced (presumably because the “smear” acts as a lubricant). When the abrasion of a rubber is low in air, owing to smearing, its abrasion in nitrogen can become greater than in air. However, in air the less grossly degraded rubber is mechanically weakened, so that if smearing

225

is obviated by the use of a suitable dust, the rate of abrasion is greater in air than in nitrogen. Schallamach also showed that the susceptibility of the compound to oxidative degradation can be influenced by the choice of antioxidant and other fo~ulation details. P&ford [ZS] studied antioxidant effects during abrasion of NR tyre tread compounds by a razor blade. He reported that all compounds exhibited smearing at sufficiently low friction loads, but antioxidants reduce the critical frictional force below which smearing occurs. He found that antioxidants reduce the rate of wear for conditions in which smearing occurs but have no effect at higher severities. He considered this to be evidence of two mechanisms of wear, namely degradation at low frictional force and fracture at high frictional force. However, antioxidants also protect against fatigue crack growth, but only at low tearing energies [33]. Thus it may not be necessary to invoke an entirely different mechanism of abrasion when smearing occurs. Instead, smearing can be seen as a complication superimposed on the general fracture mechanism of abrasion. Because of the twofold effect of smearing on abrasion, it is not even clear whether smearing is advantageous or disadvantageous for tyres on the road. Owing chiefly to the efforts of Gent and P&ford [34], the most plausible mechanism of smearing appears to be oxidative consummation of scissions produced by mechanical stress, in much the same way as occurs during cold mastication of NR. They provided rather convincing evidence of mechanochemical degradation of certain rubbers during abrasion by a razor blade. The degradation of filled NR and SBR to a sticky material during blade abrasion occurred only in the presence of oxygen or thiophenol, but not in a nitrogen atmosphere (just as for cold mastication). BR produced only dry debris during abrasion, consistent with the expectation that any free radicals of BR produced by main chain rupture woutd react with the polymer itself, leading to an increase in cross-linking rather than degradation. Similar experimental observations to those of Gent and Pulford were previously obtained by Rudakov and Kuvshinski [35] for abrasion of NR and BR by a smooth indentor in air and in helium. They also gave a calculation suggesting that the rise in temperature of the rubber surface (based on the full surface area of the test piece reaching an equilibrium temperature) was quite inadequate to cause thermal degradation. However, this calculation ducks the possibility of local hotspots: the smaller the region of real contact, the higher is the calculated temperature rise, but we can only conjecture as to the size of the real contacts [36]. Thus, while there is evidence that heat alone may not be responsible for degradation during abrasion at low sliding speeds, the possibility that frictional heating plays some role cannot yet be excluded. Notwithstanding the foregoing, we have obtained [37] some evidence of the possible importance of ambient temperature on smear production. A study was made of the smearing of a peroxide-cured NR wheel (Akron abrasion type) when run on a smooth glass plate with interfacial slip. Both smear and rubber particles were deposited on the glass. In the temperature range from -30 to 60 “C, the higher the ambient temperature, the greater was the production of smear, and the lower the temperature, the greater was the amount of particulate debris left on the plate with less smear. Indeed, below - 20 “C there appeared to be no smear, only copious particle production. In a second experiment the wheel was run against smooth-surfaced ice at -28 “C. It is of interest to see what happens on ice because its acts as an interface temperature limiter (0 “C maximum). In the event, despite noticeable interface traction, only particulate rubber debris was obtained but no smear. From this it would appear that interface temperatures greater than 0 “C are required for smearing.

226

8. Conclusions

Abrasion of rubber can be even more sensitive to detailed conditions than friction. For example, a lubricant may cause a small decrease in frictional force but a dramatic decrease in abrasion. It appears that the main cause of abrasion is tearing or fatigue under the action of high local stresses caused by friction. In the case of fatigue, such as occurs under mild abrasion conditions, the presence of oxygen decreases abrasion resistance in a manner reminiscent of the influence of oxygen on the cyclic growth of cracks in the rubber. Antioxidants can be used to, at least, partially restore the abrasion and crack growth resistance. Smearing may also occur and this has been ascribed to mechano-oxidative degradation of susceptible rubbers such as NR, SBR or EPDM. It appears to be a complication rather than a basic mechanism of abrasion. Cutting may play a role for sharp abradants if the rubber is hard or lubricated, but usually this is a secondary mechanism of abrasion. Wear is proportional not only to abradibility but also to the amount of sliding suffered by a rubber article, and this is often an unknown quantity. The ability to predict from laboratory measurements the wear rate of tyre tread compounds would result in an immense saving to the tyre industry. It is clear that laboratory abrasion experiments must be supplemented by a knowledge (theoretical or empirical) of the effect of the test tread compound on the amount of sliding in the tread contact zone. While some progress has been made on both abradibility and quantification of sliding, the goal of predicting the wear of rubber from laboratory measurements has yet to be achieved. Acknowledgment

The authors are grateful to Dr. A. Schallamach in the preparation of this paper.

for discussions

that have assisted

References

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Appendk NR SBR

Glossary natural

rubber

(cis-polyisoprene)

styrene-butadiene rubber high abrasion furnace black

228

pphr EPDM NBR SKB SKS SKN SKMVP SKD

butadiene rubber international rubber hardness degrees parts per hundred of rubber ethylene propylene dimer rubber acrylonitrile-butadiene rubber sodium catalysed butadiene rubber sodium catalysed styrenebutadiene rubber acrylonitrile-butadiene rubber (2-methyl-5 vinyl pyridene)-butadiene rubber stereoregular c&1,4 butadiene rubber

Note:

The last five rubbers

BR IRHD

refer to synthetic

products

from the former

USSR.