the mechanisms of erosion of unfilled elastomers

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Wear, 138 (1990) 33 - 46

THE MECHANISMS OF EROSION OF UNFILLED ELASTOMERS

BY SOLID PARTICLE IMPACT*

J. C. ARNOLD and I. M. HUTCHINGS

Department of M at eri als Sci ence and M et all urgy, Uni versit y of Cambridge, Pembroke

Street, Cambridge (U.K.)

The mechanisms of material removal were studied during the erosion

of two unfilled elastomers (natural rubber and epoxidized natural rubber).

The effects of impact velocity and of lubrication by silicone oil were in-

vestigated. The development of surface features due to single impacts and

during the early stages of erosion was followed by scanning electron mi-

croscopy. The basic material removal mechanism at impact angles of both

30” and 90” involves the formation and growth of fine fatigue cracks under

the tensile surface stresses caused by impact. No damage was observed

after single impacts; it was found that many successive impacts are necessary

for material removal. It was found that the erosion rate has a very strongdependence on impact velocity above about 70 m s-l.

1. Introduction

Elastomers show very good resistance to erosive wear under certain

conditions and are used in applications such as ore-handling plant and

pipe linings [ 11. Despite this, understanding of the mechanisms of erosion

and of the properties desirable for erosion resistance is still limited.

There have been several studies attempting to correlate erosion resis-

tance with various mechanical properties of elastomers [ 2 - 41. These have

had some success, although it appears that the dependence of erosion rate

upon mechanical properties is complicated, with no single property domi-

nating. A better understanding of the mechanism of material removal is

needed before any optimization of physical properties for erosion resistance

can be achieved.

The variation in erosion rate with angle of incidence is similar to that

observed in ductile metals, with a high erosion rate at glancing angles of

impact and a much lower erosion rate at normal incidence [3, 51. Themechanism of material removal in the case of metals, namely a cutting

*Paper presented at the International Conference on Wear of Materials, Denver, CO,U.S.A., April 8 - 14, 1989.

0043-1648/90/$3.50 0 Elsevier Sequoia/Printed in The Netherlands



34

and ploughing process, is, however, not generally thought to be responsible

for erosion in elastomers.

Elastomers eroded at glancing incidence show transverse features that

suggest the formation of tears and cracks perpendicular to the erosion direc-tion, possibly by a fatigue mechanism [2, 5, 61. The surface features pro-

duced during erosion bear a similarity to those produced during the abrasion

of elastomers on rough surfaces, to the extent that some workers have

drawn no distinction between the two processes [7, 81. The abrasion of

elastomers is thought to occur by a cyclic crack growth mechanism driven

by tensile stresses in the surface parallel to the sliding direction [9]. The

similarity of the surface features seen during both erosion and abrasion

suggests that a similar process may be occurring in erosion, The fact that

both processes are found to be accelerated by environmental degradation

[lo, 111 also points to some similarity in mechanism. There is, however,

a problem in that elastomers with good abrasion resistance tend to have

poor erosion resistance and vice versa. It has generally been found that

unfilled elastomers with a low modulus and high rebound resilience provide

the best erosion resistance [2, 3, 4, 81, whereas filled elastomers with a

high modulus tend to provide the best abrasion resistance [ 121.

The mode of deformation during wear has been found to be of impor-

tance by Muhr et al . [ 131, who found that lubrication during abrasion

caused a slight reduction in the frictional force, but a much larger reduction

in the wear rate due to an alteration in the mode of deformation. Thenature of the deformation caused by an impacting particle will be different

from that involved in abrasion and will be of great importance in deter-

mining the erosion rate. This could explain the discrepancy noted above

between resistance to the two types of wear.

Bartenev and Penkin [5] and Hutchings et al . [2] related the erosive

wear of elastomers to the amount of kinetic energy absorbed on impact.

Marei and Izvozchikov [ 41 suggested the necessity for a “stress build-up” by

successive impacts, by incomplete relaxation of the elastomer surface be-

tween impacts. It is clear that the impact stresses will play a large part in

determining the erosive wear of elastomers, although the nature of thedeformation induced in a roughened surface by an irregular particle is

likely to be complicated.

In this work, an attempt has been made to determine the material

removal mechanism during the erosion of elastomers by silica particles

under glancing and normal impact. The effects of lubrication were inves-

tigated, and the development of surface features was followed by scanning

electron microscopy (SEM) of the eroded specimens.

2. Experimental methods

2.1. Mater ia ls

The elastomers used in the present study were prepared at the Malay-

sian Rubber Producers Research Association (Brickendonbury, Hertford),



35

in the form of unfilled vulcanized sheets of 5 mm thickness. The sample

thickness was found to be large enough not to have an influence upon the

erosion results, and it did not reduce si~~ic~tly during the test. Two

rubbers with very different dynamic properties were prepared: natural

rubber (NR) with a high rebound resilience, and epoxidized natural rubber

(ENR50) with a much lower rebound resilience. It has been found that

these two elastomers, used in a previous study, show extremes of erosion

behaviour for unfilled gum vulcanizates [21.

2.2. Erosion testing

The erosion tests were performed in a gas blast erosion apparatus,

as described previously [Z]. Test specimens (20 mm X 40 mm) were fixed

to steel backing plates and eroded with silica sand of 130 pm mean par-ticle size. The area of the sample actually eroded was about 10 mm* at

90” and about 20 mm* at 30”. The silica particles used for the erosion

testing are shown in Fig. 1. The velocity of impact in all experiments was

measured by the double-disc method [ 141.

The wear was measured by weighing the specimens to +50 pg, after

erosion by a fixed mass of sand. All the samples exhibited an incubation

period before the onset of steady state erosion and the erosion rate, defined

as mass lost per unit mass of impacting silica, was calculated from a plot

of cumulative mass loss once the incubation period had been passed.

Before weighing, silica particles were removed from the specimen surfaceby an air blast.

To investigate the effects of lubrication, a lubricator was attached

to the air supply of the erosion apparatus. This supplied silicone oil of

60 cSt viscosity (polydimethylsiloxane, Dow Corning) to the air stream

in the form of a fine mist at the rate of 0.1 cm3 min-’ (3.5 cm3 kg-’ of

Si02). Silicone oil was used as it does not react with the samples or swell

(become absorbed into) the rubber surface.

Single impact studies were performed using a gas gun apparatus de-

scribed previously [ 151. Small numbers of 130 ,um silica particles were fired

at 100 m s-l at the elastomer surfaces at impact angles (defined as the angle

between the incoming particle and the sample surface) of 30” and 90”.

Fig. 1. SEM micrograph of the silica particles used for erosion.



36

2.3. Scanning electron microscopy

SEM was carried out with a Cambridge Instruments S2. The samples

were sputter coated with gold before examination. Owing to damage and

embrittlement of the elastomer surface by the electron beam [ 161, it wasunfortunately not possible to perform sequential examination of the same

area during the erosion test. Separate samples were examined after being

eroded with various amounts of silica from 0.1 g to an amount sufficient

to give steady state conditions.

To determine the extent of subsurface damage, sections through the

eroded areas were produced by cutting through the sample with a surgical

scalpel. This method was not completely satisfactory as cutting marks

were visible on the sections. A better method was devised for the samples

eroded at an impact angle of 30”. Two samples were pressed together to

form a sandwich and then cut at right angles to the interface. The cut

surface was then eroded. On peeling the two halves apart after erosion,

a smooth sectional view of the eroded area was obtained. It was found

that fretting between the two surfaces introduced some extraneous fea-

tures, but this was eliminated by a thin smear of silicone oil at the interface.

This method was unfortunately not successful at an impact angle of 90”

since fragments of the silica particles were forced down the interface and

obscured the subsurface features.

3. Results

3.1. Erosion tests

Most of the SEM study was conducted on samples of the two elasto-

mers that had been eroded at impact angles of 30” and 90” at an impact

velocity of 100 m s-l. Plots of cumulative mass loss against mass of silica

are shown in Fig. 2. An incubation period is observed before steady state

erosion for both samples, the extent of which is greater at an impact angle

of 90” than at 30”. This incubation period is accompanied by an increase

in mass, due to small particles of silica embedding in the eroded surfaceand adhering to the remainder of the sample. The erosion rate is higher

at 30” than at 90” and is higher for the less resilient ENR50 than for NR.

3.2. Velocity variations

The variation in erosion rate with impact velocity for NR at impact

angles of 30” and 90” is shown in Fig. 3. Fitting these results to a power

law gives exponents of 2.9 for 30” and 5.1 for 90” impact angles.

3.3. Lubrication

The effect of lubricant is shown in Fig. 4. The erosion rate was mea-

sured with and without lubricant for ENRSO at 30” and 90”, for NR at

30” and for mild steel at 30” all at impact velocities of 70 m s-l-‘. The erosion

rates of the elastomers are substantially lower in the presence of lubricant



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nwthout lubrtcantI1 with lubrlcoq+

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Fig. 4. The variation in erosion rate produced by the incorporation of a silicone oillubricant into the air stream. (For ENR50 at 90” with lubricant, the erosion rate was

lower than the detectable limit of 10b6.)

at both 30” and 90” whereas, for mild steel, lubrication increases the erosion

rate slightly. The erosion rate for ENR50 at 90” with lubricant was lower

than the limits of detection in these experiments (about 10m6 owing to

mass fluctuations), as was the erosion rate for NR at 90” impact.

4. Discussion

4.1. The effects of velocity

As can be seen from Fig. 3, the rise in erosion rate with velocity is

very rapid above about 70 m s-l, particularly for the samples eroded at

an impact angle of 90”. The exponents given by plotting the results as a

power law are much larger than the value of 2 predicted by simple kinetic

energy considerations, and there are obviously other factors playing a part.

Figure 5 shows the surface features produced by erosion at an impact

angle of 30” at the two extremes of velocity (30 and 140 m s-l). In all the

micrographs of surfaces eroded at 30”, the erosion direction is from the top.

(a) (b)

Fig. 5. SEM micrographs of the surface of natural rubber specimens eroded at an impactangle of 30” (erosion direction from the top): (a) impact velocity of 30 m s-l; (b) impact

velocity of 140 m s-l.



39

It can be seen that the features produced at the low velocity are very-

well-defined transverse ridges, as described previously [ 21. The similarity

between these features and the patterns produced by abrasion [9] should

be noted. At the higher impact velocity, however, the ridges although still

present are much more broken up and less well aligned. It seems to be a

general observation that a higher erosion rate leads to less-well-defined

ridges. Hutchings et al. [2] found that NR, with a low erosion rate, pro-

duced more-well-defined ridges than ENR50, with a higher erosion rate,

an observation that is confirmed by the present work.

Figure 6 shows the surface features produced by erosion at an impact

angle of 90”. At 140 m s-i, there is a rapid development of large and very

deep pits, many with silica particles embedded at the bottom. These pits

are absent at lower velocities (below 120 m s-l) and probably contributeto the very rapid rise in erosion rate with velocity at normal incidence.

The surface around the pits in the sample eroded at 140 m s-l is very rough

and almost granular in form. The surface features produced at a lower ve-

locity (90 m s-l) are about the same size, but the surface is much smoother

and can be seen to be merely a network of cracks.

(a) (b)

(cl (d)

Fig. 6. SEM micrographs of the surface of natural rubber specimens eroded at an im-pact angle of 90’: (a), (b) impact velocity of 90 m s-l; (c), (d) impact velocity of 140m s-l.



40

4.2. The development of surface featur es i n nat ural rubber at an impact

angle of 30”

No observable damage was caused by single impacts. The impact sites

were visible when examined by SEM because silica debris in the form ofsmall particles (less than 10 pm) was evident over the impact site. These

particles originate from the surface of the impacting particle and adhere

to the elastomer surface on impact. From the size of the areas covered

with this debris, it seems that the size of the impact site is roughly the

same as the particle size, showing that the elastomer surface deforms appre-

ciably on impact.

The development of the surface features during the incubation period

is shown in Fig. 7. It should be noted in interpreting these micrographs

that, at an impact angle of 30”, 1 g of silica striking the surface leads to

about 200 impacts over each area the size of an impacting particle.

After erosion by 0.1 g of silica (about 20 successive impacts), only

isolated damaged areas are evident. These are in the form of raised ridges,

running roughly perpendicular to the erosion direction, with tears under-

neath (Fig. 7(a)). The facts that single impacts produce no observable

damage and that, after 20 impacts over each area, there are only isolated

areas of damage suggest that many successive impacts are required before

any surface damage becomes apparent.

(a)

(b) (d)

Fig. 7. SEM micrographs of the surface of NR samples at various stages during erosionat an impact angle of 30” and an impact velocity of 100 m s- 1 (erosion direction fromthe top): (a) after 0.1 g; (b) after 0.5 g; (c) after 5 g; (d) after 200 g (steady state).



41

Fig. 8. A higher magnification of one of30” and 100 m s-l.

the ridges seen on the surface of NR eroded at

Fig. 9. SEM micrograph of a section through the eroded area of a sample of NR eroded

at 30” and 100 m s-l (erosion direction from the right).

After erosion by 0.5 g (Fig. 7(b)), the number of ridges is greater

and, after 5 g (Fig. 7(c)), virtually the whole surface is covered with ridges.

The only visible difference between the surface after 5 g of erodent and

that under steady state conditions (i.e. after 200 g (Fig. 7(d))) is that the

steady state surface is slightly more broken up, although the transverse

ridges are still present. Figure 8 is a higher magnification micrograph of

one of these ridges, which seems to be made up of many small rubber

particles. It is likely that the detachment of small parts of these ridges

is the main method of material removal. An unsuccessful attempt at debris

collection showed that there are no wear particles larger than about 20

Pm.

Figure 9 is an SEM micrograph of a transverse section through theeroded area. There is very little subsurface damage and it can be seen that

the ridges are generally sawtoothed in shape, with the steeper face towards

the direction of erosion. These features show a remarkable similarity to

those seen on abraded elastomer surfaces [9].



(a)

(b)

(d)

Fig. 10. SEM micrographs of the surface of NR samples at various stages during erosion

at an impact angle of 90” and an impact velocity of 100 m s-l: (a) after 0.1 g; (b) after

0.5 g; (c) after 5 g; (d) after 200 g (steady state).



43

4.3. The development of surf ace feat ures i n nat ural rubber at an impact

angl e of 90”

As with erosion at 30” impact angle, no damage was observed after

single particle impacts, and even after many impacts (about 40 due to0.1 g of erodent), the damage that is present is in isolated regions. Figure

10(a) shows the surface after erosion by 0.1 g of silica. Many small silica

particles adhere to the surface. There are also occasional fine-scale cracks

or tears on the eroded surface.

After 0.5 g of erodent (Fig. 10(b)), a network of furrows containing

fine cracks has developed. After erosion by 5 g (Fig. 10(c)), the whole

surface has been broken up into a fine-scale, apparently granular structure

by the repeated intersection of the surface cracks seen in Figs. 10(a) and

lO( b).

The steady state surface (Fig. 10(d)) shows a much rougher granular

structure on a scale of about 30 pm. The roughening is almost certainly

caused by the removal of surface material due to the intersection of cracks.

As with the ridges seen at 30”, the features on the surface eroded at 90”

are made up of smaller particles, the detachment of which is probably the

major mechanism of material removal. There is an appreciable amount of

silica embedded in the surface, so that the surface consists of a composite

layer made up of rubber and fine silica particles. It has not been possible

to estimate the relative amounts of each in the surface layer.

An SEM micrograph of a section through the eroded surface is shownin Fig. 11. The granular features seen in Fig. 10(d) are again seen and extend

to a depth of about 30 pm. In contrast with the sample eroded at 30”,

there is a considerable amount of subsurface cracking. The fine lighter-

coloured cracks are associated with the gold-coating process performed

before examination by SEM. The larger cracks extending from the surface

features to a depth of about 30 pm are, however, certainly caused by ero-

sion, since examination of an uneroded section showed no such cracks. The

cracks are not straight but seem to deviate slightly over distances of the

order of 1 pm. It is interesting to calculate the depth removed, on average,

by each particle. On the assumption that the material removed by each

Fig. 11. SEM micrographs of a section

at 90” and 100 m s-l.



44

impact is evenly spread over the impact area, it corresponds to a depth

per impact of slightly less than 1 pm, about the same as the scale of the

irregularities in the subsurface cracks.

4.4. Su r face featu r es i n epoxidi zed nat ural rubber

Micrographs of the steady state surface features of ENR50 eroded at

impact angles of 30” and 90” are shown in Figs. 12(a) and 12(b) respec-

tively. There are no significant differences between the surface features

and their development in ENR50 and NR. .At 30”, the ENR50 has a surface

which is more broken up than that of NR, concomitant with its higher

erosion rate. At 90”, there is very little differences in surface appearance

between the two elastomers.

(a) (b)

Fig. 12. SEM micrographs of the steady state surfaces of samples of ENR50 eroded at an~~~o~t velocity of 100 m s-l: (a) impact angle of 30” (from the top); (b) impact angle

4.5. The mechani sm of mat eri al removal and the effect s of lubr i cat i on

The difference produced in the erosion rates of elastomers by lubrica-

tion (Fig. 4) reveals a great deal about the erosion mechanism. For a cutting

or ploughing process, lubrication would be expected to have little effect

or to increase the erosion rate due to reduced friction on the cutting edge

of a particle. This effect is probably responsible for the increase in erosion

rate with lubrication seen with the mild steel specimens. The dramatic

reduction in erosion rate with lubrication seen with the elastomer samples

at both 30” and 90” shows that it is the surface tensile stresses caused by

the impact that are important. At an impact angle of 30” a reduction in

friction between the elastomer surface and the impacting particle would

cause the surface tractions behind the impacting particle to be lower, leading

to a reduction in the erosion rate.

At normal incidence, as Poisson’s ratio for rubber is approximately

0.5, the surface tensile stresses due to an impacting particle will be pre-

dominantly frictional in nature. A reduction in friction due to lubrication

would cause the surface tensile stresses to be lowered, leading to the ob-

served reduction in erosion rate.



45

From the SEM studies of the development of surface features, it

appears that, both at 30” and at 90”, fine cracks propagate into the surface,

which then intersect to cause material removal. The incubation period

seen with all the samples is due to the gradual growth of these cracks. Only

after a dense network of these cracks has been formed will any material

be removed. The slight deviations in the subsurface cracks, noted on a scale

of about 1 pm, together with the observation that about 1 pm depth of

material is removed on average per impact, suggest that the cracks prop-

agate by about this distance under the stress cycle produced by each

impact.

At an impact angle of 30”, transverse surface ridges are formed ahead

of the subsurface cracks. The similarity between the ridges formed in erosion

and in abrasion suggests that they are formed by similar mechanisms. Thefact that good abrasion resistance in an elastomer normally implies poor

erosion resistance and vice versa does not preclude the similarity between

the two processes. A soft resilient rubber may have poor abrasion resistance,

but because of its low modulus the stresses caused by impact will be less

than for a harder elastomer and may therefore lead to slower cyclic crack

growth and thus to a lower erosion rate.

5. Conclusions

The basic mechanism of material removal in elastomers eroded by

silica particles is by the incremental growth, under cyclic impact loading,

of fine cracks which then intersect, leading to the removal of small par-

ticles. The cracks propagate by distances of the order of a micrometre per

stress cycle. The gradual development of a fine-scale network of these

cracks on a smooth uneroded surface gives rise to the incubation period

seen with most elastomers.

The similarity between the ridges formed during erosion at an impact

angle of 30” and those formed during sliding abrasion suggests that theyoriginate in a similar manner.

Lubrication of the elastomer surface during erosion produces a dra-

matic reduction in erosion rate at impact angles of both 30” and 90”. This

effect is caused by a reduction in the surface tensile stresses associated

with impact.

Acknowledgments

J. C. Arnold thanks the Science and Engineering Research Council

and the Malaysian Rubber Producers Research Association for support

via a CASE studentship. The authors thank Dr. Y. Oka for help with the

single-impact experiments.



46

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and rubber bends in pneumatic conveying system pipelines, in J. E. Field and N. S.Corney (eds.), Proc. 6th Int . Conf. on Erosion by Li qui d and Sol i d Impact, Cavendish

Laborat ory, Cambri dge, Cambri dgeshire, Cavendish Laboratory, Cambridge, Cam-

bridgeshire, 1983, Paper 60.

2 I. M. Hutchings, D. W. T. Deuchar and A. H. Muhr, J. M ateria ls Sci., 22 (1987)

4071 - 4076.

3 R. E. Morris and J. Oser, Rubber Age, 92 (1963) 96 - 103.

4 A. I. Marei and P. V. Izvozchikov, Determination of the wear of rubbers in a stream

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9 E. Southern and A. G. Thomas, Pl ast. Rubber: M at er. Appl ., 3 (1978) 133 - 138.10 A. Schallamach, J. Appl. Poiym. Sci., 12 (1968) 281 - 293.

11 J. C. Arnold and I. M. Hutchings, J. M ater. Sci., 24 (1989) 833 - 839.

12 Y. Uchiyama, In t . Pol ym . Sci. Technol ., 11 (1984) 93 - 102.13 A. H. Muhr, T. J. Pond and A. G. Thomas, J. Chim. Phys., 84 (1987) 331 - 334.14 A. W. Ruff and L. K. Ives, Wear, 35 (1975) 195 - 199.

15 I. M. Hutchings and R. E. Winter, J. Phy s. E, 8 (1975) 84 - 86.16 J. C. Arnold and I. M. Hutchings, Wear, 128 (1988) 339 - 342.

the mechanisms of erosion of unfilled elastomers

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