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7/30/2019 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
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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),
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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.
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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+
ENRSO NR EN60 mld steel
30” 30’ 90” 30”
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.
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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.
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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).
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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].
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(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).
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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.
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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.
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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.
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