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00202967.1978.11870471

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Transactions of the IMF
The International Journal of Surface Engineering and Coatings
ISSN: 0020-2967 (Print) 1745-9192 (Online) Journal homepage: http://www.tandfonline.com/loi/ytim20
Wear Mechanisms of Metals and Polymers
J. K. Lancaster
To cite this article: J. K. Lancaster (1978) Wear Mechanisms of Metals and Polymers,
Transactions of the IMF, 56:1, 145-153, DOI: 10.1080/00202967.1978.11870471
To link to this article: http://dx.doi.org/10.1080/00202967.1978.11870471
Published online: 08 May 2017.
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Date: 26 October 2017, At: 03:38
Wear Mechanisms of Metals and Polymers
J. K. Lancaster
Procurement Executive, Ministry of Defence, Materials Dept,
Royal Aircraft Establishment, Famborough, Hants
MS received 9 January 1978
Downloaded by [Chalmers University of Technology] at 03:38 26 October 2017
SUMMARY--;A revie~ is given of the vario~s wear processes which may occur during the sliding of metals and
polymers e1thc~ agamst themselves or agamst each other. For convenience a formal classification is adopted in
!erms of abrasiOn, adhesion, ~orrosion, and .fatigue, but it i~ shown that,' in practice, these processes are often
mterr~lat~. Methods of reducm~ wear are. discussed, ap~r'?pnate t~ e!lch type of wear process, and include an
exammatlon of the r6le of matenal properties and compos1t10n, lubncat1on, surface roughness and surface coatings.
INTRODUCfiON
THE formal definition of wear agreed by an OECD committee on Terms and Definitions in Tribologyt is - "the
progressive loss of substance from the operating surface of
a body as a result of relative motion at the surface". Behind
this deceptively simple choice of words lie a whole range
of complex phenomena associated with the production of
wear debris, and depending not only on material properties
but also on the externally imposed conditions of sliding. It is
important to emphasize this dual dependence from the very
beginning; wear is not an intrinsic material property in the
same sense as, for example, tensile strength or elastic
modulus, and even minor changes in the conditions of
sliding can sometimes induce very large changes in wear.
A general idea of the overall range of wear rates and
coefficients of friction which can be obtained in different
situations is given in Fig 1. Wear rates cover a range of
more than l3 orders of magnitude, so that it is hardly
surpri~ing to find that many different processes are involved.
WEAR RATES
(-DEPTH/"" SLIDING DISTANCE)
101
-
10 4
101
10 1
CHALK ON ILACKIIOARD
-
TYAES (DAY SKID)
-
PENCil. ON PAPER
-
STEEL ON STEEL (DRY)
The most frequently adopted classification of these processes is based on the causative agents believed to be
responsible 2 , and these are listed on the left-hand side of
Fig 2. The four main groups are also often subdivided into
more descriptive processes, as shown on the right-hand side.
As the figure illustrates, however, many of these descriptive
processes involve a number of causative agents acting
together. For example, fretting - "wear resulting from
surfaces in oscillatory relative motion of small amplitudes"
- can involve contributions from abrasion - "displacement of material by hard particles or protruberances" - ,
from adhesion - "transference of material from one surface to another by a process of solid-phase welding"- and
from corrosion - "wear involving chemical or electrochemical reactions with the environment".
The objective of this review is to provide a general,
qualitative description of the various wear processes listed
in Fig 2. The information presented, however, is necessarily
a subjective choice only, because of the wide range of
materials and conditions of sliding involved. Consideration
is given first of all to metals, and the wear behaviour of
polymers is then treated by comparison. Finally, an attempt
is made to show how the understanding of wear processes
leads to improved, wear-resistant materials, with particular
reference to surface coatings.
METALS
10
1o·•
ABRASION
The simplest wear situation to examine first is that of the
abrasive wear of a soft metal by a harder, rougher surface,
and from which material is removed by shearing or cut.
-SHOE SOLES
1o·1
•o-•
-
STARTER MOTOR BRUSHES
TYAES (NORMAL USE)
Jo-•
-
PLASTIC IEAAINGS
ur•
-
~;~~~:RYs~~~~ICATION)
T
STEEL ON STEEL
(HYDRODYNAMIC LUUICATION)
1o· 4
10""'
10""1
ABRASIVE
COEFFICIENTS OF SLIDING I'RICTION
pAY MATERIALS
10
LUBRICATED METALS
ADHESIVE
-METALS (VACUUM)
SCUFFING
I
0·1
-
METALS (AIR)
-CERAMICS
-PLASTICS
-CARBONS
••••• PTFE
-DRY
-
BOUNDARY LUBRICATION
-
SOLID LUBRICATION
CAVITATION
0·01
0•001
OIL - , ••••
GAS -
0·0001
fofqlQ~Jf~[Nlo}J
FATIGUE
HYDRODYNAMIC
PITTING
PRESSURISED
GAS
-.
Fig 1. Range of friction coefficients and wear rates.
Fig 2. Types of wear.
14S
Transactions of the Institute of Metal Finishing, 1978, Vol 56
ting. Geometrical considerations based on model asperities
of conical shape lead to the relationship 3
Downloaded by [Chalmers University of Technology] at 03:38 26 October 2017
·
V
KLs ( 2
=H
-;tan a
)
where V is the volume of wear, L is the applied load, s
is the distance of sliding, H is the indentation hardness and
a is the base angle of the cone. The constant K expresses
the fact that only a proportion of the groove volume
appears as loose wear debris, typically 10- 30%". Two things
should be noted about this relationship. Firstly, it introduces the concept of a "specific wear rate", volume removed
per unit distance of sliding per unit load, V 1sL, and
secondly, this specific wear rate then depends on a material
property, hardness, and on a geometrical factor defining
the conditions of sliding. The relationship between wear
r~te and har~ness has been examined experimentally many
ttmes, a~d F1g 3 shows a selection of results by Kruschov
and Babtchev5 for the wear of metals on abrasive papers.
In these results wear is expressed as a "wear resistance"
the reciprocal of the wear rate relative to a particula;
standard material, and the hardness is that of the annealed
metals. For commercially pure metals wear resistance is
directly proportional to hardness, but f~r cold worked and
heat treated alloys some discrepancies occur. The lack of
dependence of wear on the degree of cold working suggests
that the particular hardness relevant to wear should be that
of the fully strain-hardened layer generated by the abrasion
process itself, and this has been confirmed experimentally
by Richardson 6 • For heat treated steels, several other fac~ors also enter into the picture. Firstly, heat treatment
mfluences the microstructure of a steel. For carbon steels,
abrasion resistance increases with increasing pearlite content and decreasing cementite particle spacingT, whilst for
alloy steels wear resistance increases with increasing carbide
c~ntent up to an optimum value of around 30%&. Secondly,
Wit~ two-phase alloys, differential abrasion may occur in
wh1ch the softer phase is preferentially removed. Finally,
the degree of abrasion depends on the relative hardness
of the metal and abrasive, and wear resistance starts to
increase significantly when the metal hardness exceeds about
half that of the abrasive 9 •
ADHESION
The elementary process of adhesive wear postulates that
the interatomic forces between two surfaces in localised
contact result in adhesion and a process of solid phase
welding. During relative motion further plastic deformation
results in a process of junction growth10 until shear ultimately occurs. If the shear strength of the junction exceeds
that of the subsurface layer of one of the metals, a fragment is then transferred to the other.
Considerable attention has been paid to examining the
extent to which adhesion is influenced by the properties
of the contacting metals. In ordinary environments, metal
surfaces are invariably covered by adsorbed films of oxygen, water vapour, etc, and adhesion only becomes significant when these films are disrupted by plastic deformation.
Simple adhesion experiments by the so-called "twistcompression" technique11 have demonstrated that whilst
there is a general inverse trend between the coefficient of
adhesion and hardness, the precise relationships depend on
the particular crystal structure of the metals involved.
Close packed hexagonal metals, such as Cd, Zn, Be and Co
generally exhibit relatively low adhesion because their restricted slip properties during plastic deformation limit the
growth of intermetallic junctions.
The strength of adhesion between two metals reflects
the extent to which matching can occur between the crystal
lattices, and a practical criterion for this is the degree of
solid solubility of one metal in another. It was noted many
years ago by Roach et a/1 2 that insoluble metal pairs generally exhibited less tendency to scoring (scuffing) during
sliding than soluble pairs, and this concept has recently
been refined and extended by Rabinowicz1 s. Most of the
commonly used metals in bearing alloys, for example Pb.
Sn, Cd, Ag, are either insoluble or only sparingly soluble
in Fe.
A simple analysis of adhesive wear 14 leads to the relationKLs Th"ts IS
. basically
.
the same as that derived
sh .tp V = ~·
for abrasive wear, although it no longer contains any
geometrical factor relating to the conditions of sliding. As
before, the constant K expresses the fact that only a pro-
RELATIVE
W[AA RAT[
WEAR
RESISTANCE
10
',,
80
..
..
l I" ON Pt
)
•
'
"• ON
r ..
6 At AUO'I' (e S 10•0;
w!
60
•
1. /L.. · .
/
40
t MoON ( t
10
-~
$f(LI.IT(11
l A,. ON No
o41 IP\ ON No
~~
•o-•
""9 ON
o6 .,, 0 ..
WON
STELLITE 20
..............
•""'
Fe
-.....0 COLO • WORKED
- - - - · C• STEEL
20
Co
N•
,.,
Cu
zn/
0
- - - - - - - 9e:•Cq
- - - - - AI• Cu
~
::..__.,....-'
141 10-,.AI•C" ON No
1S IP,
Sn
aoo
.200
Joo .aoo
HARDNESS
500
600
7oo eoo
oo·• L...__ __.__ __,__ ___,
oo•
I
16hlo
·~·L---~----~----~.
10
100
1000
HAROI'IIESS, VPN
k9/mm2
Fig 3
Fig 4
Fig 3. Abrasive wear resistance of metals (Krushchov and Babichev5).
Fig 5
Fig 4. Variation of severe wear rate with load (on 18% W tool steel unless otherwise indicated).
FigS. Variation of severe wear rate with hardness (on 18%W tool steel unless otherwise indicated).
146
..
oC WON SftLLif( 2.:
CARBON STEELS
Vz
L
Cr
...
, •o1•0 c .. H .. •2•1•o•
c ..
c ..
Lancaster: Wear Mechanisms of Metals and Polymers
portion of contacts actually result in the production of a
transferred fragment. Experimental confirmation of the
predicted linear relationships between the .volume of ~ear,
distance of sliding and load has been obtamed many. times,
and Fig 4 shows some examples of the proportionally
between wear rate, V 1s, and load for a number of metal
combinations during dry sliding. It should be emphasized,
however, that such simple relationships are only observed
when changes in one variable, such as the load, do n~t
induce changes in any other, such as temperature. Com~h­
cations of this type appear to preclude a well defined mverse relationship between adhesive wear rate and hardness
for different metals, although as shown in Fig 5, a general
inverse trend does nevertheless exist.
Downloaded by [Chalmers University of Technology] at 03:38 26 October 2017
FATIGUE
It has already been noted that adhesion between two metals
can only result in transfer if the shear strength of a
junction exceeds that in t~e s.ubsurface .lay~r of one or
other of the metals. This situation can anse either because
an adhesive junction becomes very strong, or because the
subsurface layer becomes very weak. In ordinary atmospheric environments, a junction must inevitably contain. th.e
disrupted remains of oxide or adsorbed films, and It IS
therefore difficult to envisage its shear strength ever exceeding that of the bulk material. A more plausible picture is
thus one in which the subsurface layer becomes progress·
ively weaker as a result of repeated contacts until, e~en·
tually, even weak adhesion across a junction ca':l become
sufficient to detach a fragment and transfer 1t to the
opposite surfaceu. This pr~ess introduces .the concept of
localised fatigue as a contnbutory factor m wear, and a
'delamination' theory based upon these lines has recently
been proposed by Suh and coworkers16 • As a result of
repeated contacts and subsurface deformation, dislocations
are generated beneath the surface and begin to pile up
around discontinuities such as impurity particles. These
dislocations eventually coalesce leading to voids, stress
concentrations and crack formation, and the subsurface
layer thus gradually weakens until fragme.nts can be
detached via adhesion or possibly even by simple mechanical interlocking. It 'follows from this analysis that, in
general, wear particles are likely to be pla~e-like in. shape,
and examination of the debris produced m a vanety of
different sliding situations has confirmed that such particles
are extremely common17 •
In some conditions where adhesive interactions are minimal, fatigue can often become the predominant m~chani~m
of wear. These situations arise in components mvolv~ng
lubricated concentrated contacts, such as gears or rollmg
element bearings_, where the localised stresses are hig~ ~nd
relative motion occurs mainly by rolling rather than shdmg.
Extensive investigations have been made into the way in
which fatigue of rolling elements, leading to pitting, is
influenced by metallurgical properties of the mate~ials and
the environmental conditions18 • Some of the factors mvolved
are listed in Fig 6.
SURFACE
FINISH
HARDNESS
-
POSSIBLE
OPTIMUM
{
BALLS 10
°/0
STRESS RELIEF
LOW RETAINED AUSTENITE
HEAT- TREATMENT
-
{
STEEL PROCESSING
-
{
LUBRICANT
-
HARDER THAN RACES
POSITION IN BILLET
VACUUM- REMELTING
(INCLUSIONS)
VISCOSITY
rYPE OF
STEEL
{
ADDITIVES
(CORROSION)
WATER (HYDROGEN EMBRITTLEMENT}
TOOL STEELS
-
{
STAINLESS
STEELS
Fig 6. Factors affecting the fatigue life of rolling bearings.
observed when wear is predominantly adhesive in type, and
they result from the fact that the corrosion products protect
the surfaces against intermetallic contact and the formation
of strong, adhesive junctions. For unlubricated metals
sliding in the ordinary atmosphere, oxide film formation i~
the primary source of protection. It is frequently observed
that many combinations of metals can exhibit two distinct
types of wear, depending on the particular conditions of
sliding imposed1 D. These are illustrated schematically in
Fig 7. Above a critical load or speed, the wear rate tends
to be high, there is appreciable metal-to-metal contact
producing metallic wear debris, and the surfaces are
severely roughened. This regime has been described as
'severe wear', and the wear process is essentially one of
adhesion supplemented by fatigue, as already described.
Below a critical load or speed, however, wear rates become
much lower, little or no intermetallic contact occurs, the
debris is largely metal oxide, and the worn surfaces are
relatively smooth. This regime is therefore categorised as
one of 'mild wear'. Fig 7 shows that there can also be
a further transition from severe back to mild wear at very
heavy loads or high speeds.
The occurrence of these transitions can be qualitatively
explained in terms of a competition between the rate of
exposure of fresh metal surface from intermetallic contact
and adhesive wear, and the rate of oxidation of this surface
between repeated contacts 20 • At light loads the contact
areas are small and widely distributed, and the rate of
production of fresh surface is sufficiently low to enable
oxidation to form a protective film inhibiting further intermetallic contact. At moderate loads the contact areas
increase in both size and number, and the formation of a
protective oxide film is no longer possible. However, at
very heavy loads the temperature rises due to frictioMl
heating, and the associated increase in the rate of oxidation
(exponential with temperature) now suffices to enable a
LOG
WEAR RATE
LOG
WEAR RATE
CORROSION
In the general sense of chemical reaction between metals
and the surrounding environment, corrosion is frequently
involved as a supplementary mechanism to other forms of
wear and its influence can be either detrimental or beneficial: Detrimental effects may occur if the corrosive
environment selectively attacks part of a multiphase structure such as the ferrite in steels, thus removing support
for Ute harder pearlite or carbide constituents .. Corrosion
may also enhance crack propagation during fatigue wear.
Beneficial effects of corrosion on wear are most commonly
LOG LOAD
LOG SPEED
Fig 7. Schematic variation of wear rate with load and speed
for brass on steel at two temperatures, T 2 Tr
>
147
Downloaded by [Chalmers University of Technology] at 03:38 26 October 2017
Transactions of the Institute of Metal Finishing, 1978, Vol 56
protective film to reform. Similar considerations apply to
the effects of speed on wear in Fig 7(b). At low speeds
sufficient time is available between repeated contacts to
establish a protective oxide, and at very high speeds the
increased rate of oxidation resulting from frictional heating
more than compensates for the reduced time between contacts. An increase in ambient temperature, by increasing
the rate of oxidation at all loads and speeds, extends the
mild wear regime at both ends of the spectrum.
The particular conditions of sliding at which transitions
occur between mild and severe wear depend on the composition of the metals involved. For example, Fig 8(a)
shows the different transition loads for several Cu-Zn alloys,
and it may be noted that the addition of 2% Pb to 60/40
Cu/Zn limits the mild wear regime to extremely light loads
only. The main reason for this is the preferential segregation of lead at the grain boundaries which, after deformation of the surface layer, weakens the subsurface and
facilitates severe wear. With carbon steels - Fig 8(b) the effects of composition on the transition are complicated
by phase changes 21 • At light loads the transition for the
0·52%C steel is essentially similar to that of the Cu/Zn
alloys in Fig 8(a), but at heavy loads the localised asperity
WEAR RATE
mmlJm
10
/l.s2"1.c
v
~0.026°/0 C
Ia" I
II
I
-·
10
I
u:>'
~GAINST
18°/o W
(a)
TOO~
STEE~
I
I
I
STEEL
AGAINST
THEMSE~VES
(b)
Fig 8. Variation of transition load for brass and steel with
composition.
contact temperatures become sufficiently high to initiate a
phase change and produce a hardened surface layer. The
reduced deformation associated with this hardened layer
then facilitates the formation of a protective oxide film
and, in turn, a further transition back to mild wear. With
steels of higher carbon content, phase transformations occur
more readily, and the two transitions at low and high
speeds merge together and eliminate the severe wear regime
entirely.
The precise mechanisms of wear occurring within the
mild wear regime are still somewhat uncertain. Simplified
theories have been developed in which it is assumed that
the wear rate is controlled either by the rate of formation
of oxide 22 ,23, 24 or by its rate of removal2 5 • A major difficulty, however, is that the protective surface films developed
during sliding in the mild wear regime appear to bear little
resemblance in either structure or composition to the oxides
produced during static oxidation experiments 15 • The balance
of evidence, at present, suggests that a number of different
wear processes may all be operating simultaneously within
the mild wear regime. These include:- intermetallic contact and adhesive wear on a greatly reduced scale ; direct
removal of oxide films by mechanical interactions; blistering and flaking of oxide films as a result of the build-up of
compressive stresses; and abrasion by loose oxide. A simiJar confused situation occurs during fretting where it has
148
been concluded26 that intermetallic contact and adhesive
wear is supplemented by oxidation of the fresh metal surface exposed and abrasion by this oxide after removal from
the surface.
POLYMERS
There are two main differences between polymers and
metals relevant to their wear behaviour. The first is that
polymers, in general, have moduli of elasticity which are
typically only about 1/100-1/lOth those of metals. For
any given surface topography, therefore, the localised
deformation of asperity contacts involving polymers is
much more likely to be elastic than for metals. Secondly,
whilst adhesion undoubtedly exists between polymers and
themselves, or metals, 21 •2 s it is not analogous to the solid
phase welding exhibited by metals, and bonding occurs
primarily by van der Waals' forces 29 • There is no junction
growth during sliding, and no strain hardening in the conventional sense, so that polymers are not susceptible to
scuffing or seizure.
ABRASION
An essential requirement for abrasive wear, as defined
earlier, is that plastic deformation occurs beneath an indent·
ing asperity or abrasive grain. One criterion to define the
onset of plasticity has been given by Halliday 3 o and can
be written in the form tan a = C ~ (1 • v2) where a is the
base angle of the asperity, H and E are the hardness and
modulus of elasticity respectively, v=Poisson's ratio and
C = 0·8 for tht.! onset of plasticity and 2·0 for full plasticity.
Inserting typical values for H and E. it can be shown that
whereas plastic deformation and abrasive/cutting type wear
occurs with metals for a ~ 1•, the corresponding values
for polymers are of the order of 5- to•. The latter angles
are only likely to be encountered on very rough surfaces,
of the order of 1·25 p.m Ra or greater, or on abrasive
papers 31 •
Ratner et a/3 2 have derived a simple theory of the abrasive wear of polymers which relates the volume of wear, V.
per unit sliding distance, s. to the polymer properties ;
VIs= p.L/ HSe where p. is the coefficient of friction, L is
the load, H is the hardness, S is the ultimate breaking
strength, and e is the elongation to break. The most important parameters appear to be S and e, and Fig 9 demonstrates that there is indeed a significant inverse correlation
between the wear rates of 18 polymers against rough steel
and their values of Se. The product Se is also related to
impact strength, and Ratnec3 3 has observed a correlation
between the wear rates of polymers on abrasive paper and
their notched impact strengths. Although theoretical considerations also predict an inverse relationship between
wear rate arid hardness, this appears to be much less well
defined than the corresponding relationship for metals. The
main reason is that the concept of hardness for a polymer
has a different physical significance from that for a metal.
During indentation of a polymer by a spherical or pyramidal indenter, a significant proportion of the load is
supported elastically so that neither the depth of penetration nor the dimensions of the permanent impression correspond to full plasticity over the whole volume of the
impression. Despite this complication, however, indentation
hardness measurements can nevertheless be used to provide
a very rough comparison of the abrasive wear rates of polymers and metals in the same conditions of sliding34, Fig 10
shows such a comparison for wear against abrasive papers,
and it may be noted that all the rigid polymers exhibit
higher wear rates than the majority of common metals.
This conclusion does not, of course, extend to include
Lancaster: Wear Mechanisms of Metals and Polymers
WEAR RATE
WEAA RA'Tl
..,mJ/NIII
I NYLON,
I
POlYACI:TAL.
J POLY (METHYL METHACRYLATE
t
4 POLYETMYLENE, S POLYPROPYLENE, t
mmltNm
eo·•
7
J,
POLYSTYRENE,
PTF[
WEAR RATE
/"",.
, ..
,/.
I/)
••
10
mm 3 /Nm
18
10. 3
TOOL
STEEL
•o·•
"
10.. 5
,
•.'·
'
Downloaded by [Chalmers University of Technology] at 03:38 26 October 2017
t.
PMIU
fOLTfTKlLfiiE(LD)
POLT$TYI\EII(
AC(TAL COPOL'"ll\
Ml'lOtf 0 6
f.Tfl
••
fOL1'U.T£R
•
II
f. fOL'I'PIIOPYUN[
tron
•'·
fMI'IA•ACRYLONITI\IL(
. ......
,,
,,
lt1'LON II
••
''· "'
" rvoc
oo'l------',o-----,.;.o''------:'
oo'
HARDNESS, VPN
J
POLYETHYLENE ILO)
PTft.
POLYPROPYLENE
S PVC
7 FEP
8 PMMA
10
II
12
PTFCE
POLVCARBONATE
NYLON 6
•
ACETAL
9
13
NYLON 6 6
ft)L~ULfHON[
PUC(
ll fOLTCAII.IOIIAT(
PPO
·o·•
01
R0 {"m)
(a)
10 1
oo•
'ovC~o~m)
(b)
Fig 10
Fig 11
Fig 9
Fig 9. Correlation between wear rates and 1I Se during single traversals of polymers over rough mild steel (1·2 pm R).
Fig 10. Variation of wear rate with hardness for abrasion on coarse carborundum paper (100).
Fig 11. Variation of wear rate with counterface roughness during single traversals over mild steel.
elastomers for which the deformation during both wear
and indentation hardness measurements is almost wholly
elastic. The moduli of elasticity of elastomers are so low
that abrasive/cutting-type wear can only occur against
extremely sharp indenters, such as needles etc3 ~. Against
more conventionally rough surfaces, whilst the term
'abrasive wear' is still used, the wear mechanism for elastomers differs from that for rigid polymers. A bulge is formed
ahead of each indenting asperity and a tensile stress develops at the rear ; localised failure then occurs via tearing
in a direction at right angles to the direction of sliding.
Several attempts have been made to correlate wear with
the mechanical properties of elastomers under these con·
ditions, and Schallamach36 has observed a variation in
wear rate with speed for an unfilled styrene-butadiene
rubber which is exactly paralleled by the variation in tensile
strength with speed.
FATIGUE
When polymers slide against relative smooth surfaces in
conditions of localised elastic deformation, it has been
postulated that fatigue begins to play a major role in
wear37.s 8 • A simple theory of fatigue wear, assuming sliding
of a polymer of modulus E over a rigid surface containing
hemispherical asperities of radius r under an applied load
L leads to the relationship.
V
, - 2(t -l)/3 L(t + 2)/3 E2(t -l)/3
-
s
ADHESION
0::
t
IT
0
where t is the exponent in the fatigue relationship,
( n =-ITITo
)t,
n
The most significant prediction from the above analysis
is that the wear rates of polymers are likely to be extremely
sensitive to very small changes in the topography of the
counterface against which they slide. Confirmation of this
is provided by the results in Fig ll (a), which shows the
variations in wear rate of several polymers during single
traversals over steel surfaces of different roughnesses. It
can be seen that the dependence of wear rate on roughness
is much smaller for the two soft metals where plastic
deformation occurs and wear is of the abrasive/cuttingtype. From a computer analysis of profiles of the topography on the steel, the average radius of curvature of the
asperities can be derived, and Fig 11 (b) shows the wear
rates plotted against these values. The slopes of the lines
enable the values of t in the fatigue relationship to be determined, and these are in very reasonable agreement with
those obtained from conventional fatigue tests.38
Despite the fact that the above results provide strong
circumstantial evidence in favour of a fatigue wear process
in polymers, direct microscopic evidence for crack form.
ation and propagation is very difficult to find. A major
problem in observing such cracks results from the marked
changes produced on polymer surfaces by sliding, such as
flow (creep), oxidation, or thermal degradation, and also
from the fact that appreciable elastic recovery takes place
after removal of the load.
being the number of
applied stress and
IT
0
c~cles
to failure,
IT
the
the ultimate failure stress. Again, it
should be noted that there are two components to the above
expression; mechanical properties, as defined by t, IT 0 and
E. and the conditions of sliding, as defined by the topo·
graphy r. Values of t, derived from conventional fatigue
data range from 1·5- 3·5 for elastomers and 3 • 10 for the
more rigid thermoplastics and thermosets.
As with metals, adhesion is difficult to isolate as a unique
mechanism of wear for polymers. However, it must undoubtedly play a role in other processes such as fatigue.
by modifying the magnitude and distribution of stresses
around a localised contact. In most practical applications
of polymers in tribology, for example dry bearings, the
polymer slides repeatedly over the same wear track on a
relatively smooth metal surface. It is in these conditions
that adhesive effects are likely to predominate, via transfer.
and it is therefore pertinent to examine the wear phenomena characteristic of this situation.
The influence of load, speed and temperature on the rate
of wear of several polymers sliding repeatedly against steel
149
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Transactions of the Institute of Metal Finishing, 1978, Vol 56
is shown in Fig 12. It can be seen that in many instances
there are critical values of load, speed and temperature
above which the wear rate begins to increase rapidly. These
transitions are not analogous to those between the mild and
severe wear regimes for metals, but result from localised
softening or melting of the polymer. The localised temperatures can be calculated from theoretical considerations39,
and the arrows in Fig 12(c) show that the calculated speeds
corresponding to the melting point temperatures agree very
closely with those at which the wear rate begins to increase
rapidly. Because of the relatively low thermal conductivities
oi polymers, typically 100 times lower than that of steel,
the thermal conductivity of the counterface is the most
important parameter affecting the dissipation of frictional
heat. The wear rates of polymers sliding against themselves
at low speeds are not greatly different from those for polymers sliding against metals, but the critical speeds at which
melting occurs are greatly reduced, for example 0·05 m/s
for acetal on itself compared to 10 m/s for acetal on steel
at one particular load. The use of polymer f polymer sliding
combinations should therefore always be restricted to very
low speeds and/or light loads.
POLYTETRAFLUOROETHYLENE (PTFE)
PTFE is one of the most widely used polymers in sliding
applications for three main reasons. Firstly, it is very inert
chemically and has a relatively high softening point,
~ 325•c; secondly, its coefficient of friction, as a thin
film on a hard substrate, is lower than that of any other
WEAR RATE
(mm 3 /m)
I
2
PO~YETHY~ENE
NY~ON 6
3 PMt.AA
4 PTFE
5 PO~YACETA~
6 NY~ON 6.6
7 PO~YPROPYLENE
10
LOAD (N)
polymer, ~ 0·05; and thirdly, when suitably filled or reinforced, the wear rates of PTFE composites can be very
low. It is now considered that the low friction of PTFE
arises primarily from its smooth molecular profile, which
enables the polymer chains to slip readily over each other
during shear4°. As a consequence of this, PTFE usually
transfers to metals during sliding in the form of thin, uniform sheets with the chains oriented in the direction of
sliding, and it is the presence of these transferred films
which is ultimately responsible for both the low friction
and wear properties. With unfilled PTFE it is relatively
easy to draw out thin sheets or filaments during shear and
the wear rate tends· to be high : by introducing fillers, however, this drawing-out process is impeded and the wear
rates are then greatly reduced. The particular choice of a
suitable filler, or reinforcing fibre, depends on which combination of composite properties is most important for the
intended application, for example friction, wear rate, dimensional stability, stiffness, thermal and electrical conductivity,
cost, etc. Some examples of the values of the specific wear
rates of various PTFE composites during unlubricated sliding against stainless steel are shown by the full lines in
Fig 13. It may be noted that the wear rates of all these
composites are at least 100 times lower than those of the
unfilled polymer.
There is still some uncertainty about the precise mechanisms involved in the formation of transfer films of
PTFE on a metal counterface. Mechanical interlocking
within the surface depressions appears to play some part,
but there is also evidence for chemical effects. The addition
of bronze and lead fillers to PTFE greatly improves transfer film formation, and reduces wear, and it has been
suggested that the bronze catalyses a chemical reaction
between degradation products of the PTFE and the leadH.
The importance of the transfer film in reducing the wear
rate of PTFE composites becomes very evident when sliding
in the presence of a contaminating fluid, such as water. In
dry conditions, the transfer film confers a relatively smooth
topography to wear track on the counterface, facilitating
low wear. In the presence of water, however, this film is
unable to form, the rough topography of the counterface
is maintained, and the wear rate remains high. The magnitude of this increased wear rate in water for a number of
composites is illustrated by the hatched lines in Fig 13.
One reason for the wide differences between the waterlubricated wear rates of different PTFE composites is that
in the absence of transfer the abrasive action of the fillers
on the counterface topography becomes more significant4 2 •
This conclusion can be exploited43 by deliberately adding
very small amounts of finely divided abrasives to PTFE
composites in order to maintain a smooth counterface and
low wear in water-lubricated conditions.
WEAR' RAT£
to-7~--~------~----~------~-----J
40
80
120
160
200
TEMPERATURE (°Cl
•o·•
FILLER
NONE
POLY (P-HYOROXY BENZOIC AC!O)
ro·•
1=-::-=--=-=-="'-=-:-::-=--=-:-::-:-=_-:_;-,_=-=_-::_:-::_::-:_==--=-=-=-_ _ _
_ _ _ _
_
STAINL.ESS STEEl.. ~-""'-=--=--=-'""-""-~
GRAPHITE
1111111J/Nra
•a·•
_
_ _ _ - - - - ._.
_______ -~
1=-::-,-.,-==--=-'-=-=-=-=--="- _ -____ - - - _ ~
MOLYBDENUM ~-,...,-=--=--""'=-""'-
25a/o t.AICA
1::-::_,-:_::-:_,.,_=-:::!
___________ ._
25°/o I CARBON FIBRE+ 11°/o Pb !::-::_:-::_:-::_:-::_:-!.______ - - ..
25°/o ASBESTOS 1=-=-:-::_:-::_:-::_:-!_
_ _ ----------------1~
CERA~IC
25°/o GLASS FIBRE
POLY!~
IDE
BRONZE•GRAPHITE
SPEED (m/s)
Fig 12. Variation of wear rate with load, temperature and
speed.
8R0NlE-Pb:~Oa
Fig 13. Wear rates of PTFE composites on stainless steel.
Full lines, dry; hatched lines, in water.
Lancaster: Wear Mechanisms of Metals and Polymers
CoRROSION
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In so far as polymers are concerned, the term corrosion
implies oxidative degradation at high temperatures. The
influence of this phenomenon on polymer wear is largely
restricted to thermosetting resin composites sliding in severe
conditions. such as those characteristic of brake operation.
In such situations, steady state surface conditions are seldom reached and it is only possible to relate wear to the
parameters controlling sliding by empirical equations44 of
the type--wea.r ex: PaVbte-EJRT-where P is the pressure.
V is the speed, t is the time of sliding, E is the activation
energy for oxidative, or thermal degradation, and T is the
temperature.
REDUCfiON OF WEAR
The importance of the mechanical properties of a metal,
and in particular its hardness, in determining resistance to
abrasive wear has already been mentioned. In general, the
surface hardness can be increased by cold working, by
heat treatment, by hard facings or by diffusion treatments.
For low stress abrasion, the most wear resistant ferrous
alloys are those with a martensitic matrix containing a
uniform distribution of secondary carbides of, for example,
chromium, molybdenum, vanadium or tungsten45 • Small
carbide particles are usually superior to larger ones since
the latter may, after detachment, cause secondary damage.
Many abrasive situations, however, involve high stresses
and impacts, such as in rock crushing or ball milling. In
those conditions, toughness becomes of equal or greater
significance than hardness, and the most resistant ferrous
alloys are those with an austenitic matrix which can rapidly
work-harden during operation. The literature on wear
resistant coatings, produced by electroplating, plasma
spraying, arc deposition and diffusion treatments is volum- ·
inous, but unfortunately, there are few general guidelines
from which to select the most appropriate solution for a
particular application. A convenient summary of the various
products and processes available is given in the Tribology
Handbook46 , but for details of performance it is usually
necessary to consult the producer. A general review of the
most widely used diffusion treatments has been given by
Gregory4 7 •
At the other extreme of the materials spectrum, coatings
of soft elastomers, and in particular, polyurethane, are of
considerable value. in conditions of low stress, three-body
abrasion or during erosion by free-flowing abrasive particles. The primary requirements are a combination of low
elastic modulus, toughness, and resistance to tearing. In
erosive conditions, there appears to be a reasonable correlation between the wear resistance of an elastomer and
its ultimate resilience; the latter can be expressed as
t(tensile strength) 2 / elastic modulus and relates to the
amount of energy which can be absorbed before deformation or cracking occurs4 5 • Polyurethane coatings also provide excellent resistance against erosion by rain on the
forward facing surfaces of high speed aircraft•8 •
For wear processes in which adhesion is present as a
significant factor, the essential requirement in reducing
wear is to minimise the extent of metal-metal contact. The
most obvious way of doing this is, of course, to introduce
a fluid lubricant to separate the surfaces either by a complete fluid film-hydrodynamic lubrication-or by a monomolecular film of some adsorbed surface-active speciesboundary lubrication. The protection afforded by boundary
lubricant molecules depends on their type and structure
(for example fatty acids or esters), on their mechanism of
adsorption on the metal substrate (physical or chemical)
and on the surface topography of the substrate. Whilst
very rough surfaces are clearly undesirable because high
localised stresses increase the possibility of penetration of
the ad~orbed films, some types of very smooth surfaces
can also be undesirable, and lead to premature film failure
and scuffing' 9 • The optimum surface topography for most
effective boundary lubrication still remains to be precisely
defined.
For sliding situations in which fluid lubrication is either
inadmissible, for example to prevent contamination, or
ineffective, for example at low or high temperatures, numerous techniques are available for providing lubrication by
solids. The simplest is to use a rubbed film of a lamellar
solid, such as graphite or molybdenum disulphide, on a
metal substrate, the particles being attached to the metal
partly by mechanical interlocking within the surface depressions and partly by chemical interaction. Whilst such films
are capable of preventing metal-to-metal contact under extremely high stresses, film thicknesses are relatively low,
< 1 p.m. and their endurance during repeated sliding thus
also tends to be low. Thicker films, up to about 25 J.l.ffi•
leading to longer lives, can be obtained by bonding the
particles to themselves, and the substrate, with synthetic
resin binders. With both rubbed films, and those incorporating binders, life depends on the topography of the substrate metal, and there is an optimum surface roughness of
about 0·5 ,urn Ra 5o,
As an alternative to lamellar solid lubricant films, soft
metal coatings are also sometimes used to reduce friction
and wear, for example copper or silver to prevent fretting,
or lead for the lubrication of rolling bearings in space applications51. According to the delamination theory of wear16,
providing that a soft metal film is sufficiently thin for the
dislocations generated during deformation of the substrate
to escape through it, delamination and wear of the substrate
will be greatly reduced. Thick, soft-metal films prevent
deformation and wear of the substrate entirely but will,
however, suffer delamination wear themselves. It follows
that there should be an optimum film thickness for minimum wear of the film/substrate combination, and experimental observations by Suh and coworkers 52 , in Fig 14,
demonstrate that this thickness is in the range 0·1. 1 ,urn.
For relatively hard films on softer substrates, such as those
used to improve resistance to abrasive wear, the situation
is rather different. The most important criterion here is
the total prevention of substrate deformation, and there is
consequently a minimum film thickness, which depends on
the relative hardness of the coating and substrate. For
chromium on copper, for example, a minimum thickness of
around 200 J.l.ffi has been suggested53.
Failing the possibility of any form of conventional lubrication, by liquids or solids, it then becomes necessary to
rely on the reaction products between the metal and the
environment to provide protection against metal-to-metal
contact. This type of "lubrication" is particularly important
at elevated temperatures and can be encouraged by the
inclusion of alloying elements in a metal which are able to
segregate at the surface and oxidise preferentially, for example the addition of silicon to nickel-base alloys 54 , The
most effective alloys for high temperature wear resistance
are those which develop an oxide 'glaze' on the surface,
which not only prevents intermetallic contact but is also
capable of resisting flaking and blistering. Detailed examinations of such glazes on nickel and cobalt alloys have
shown them to be of complex structure5 s.
The results in Fig 10 have demonstrated that the abrasive
wear rates of the more rigid engineering plastics are not
particularly low in comparison to those of metals, and,
unfortunately, there is little which can be done to alleviate
this situation. The incorporation of fillers or reinforcing
fibres may well increase the breaking strength, S, of the
polymer, but the elongation to break, e, is invariably
1Sl
Transactions of the Institute of Metal Finishing, 1978, Vol 56
WEAR RATE
g(cm xi0-
100
1
'
''
'
\ NICKEL
10
''
'' '
''
'
\
I
X/
Downloaded by [Chalmers University of Technology] at 03:38 26 October 2017
)(
0.1
0
0.1
10
FILM THICKNESS I'm
Fig 14. Variation of wear rate with coating thickness (Suh52).
reduced ; the parameter Se of a filled polymer may thus be
lower than that of the unfilled material, and the resistance
to abrasive wear in consequence reduced 56 • For wear processes in which adhesion plays a major role, lubrication
by fluids is one way of reducing the wear of polymers, but
the effects of fluid lubricants are, in general, less marked
than those with metals, and can sometimes even be deleterious57. Acetals, nylons, reinforced thermosetting resins
and elastomers usually show reduced wear in the presence
of fluids. However, those materials which rely on transfer
film formation to provide low wear in dry conditions, such
as PTFE composites and polymers containing solid lubricant fillers, generally suffer an increase in wear in the
presence of fluids. The fluid impedes, or wholly prevents,
transfer film formation, as already described for water. In
dry conditions, solid lubricant additions of graphite, MoS 2
or PTFE are widely used as additives to reduce the coefficient of friction, and these, in turn, often reduce wear
rates also.
Probably the most significant factor influencing the wear
rate of polymers in conditions of repeated sliding and
adhesive/fatigue wear is the topography of the mating
metal counterface. There are two aspects to consider ; the
initial topography and that generated by the sliding process
itself. The initial topography influences the wear rate during
running in, as implied by the results in Fig 11, but it is
also often suggested that there is an optimum initial roughness for minimum wear even in the subsequent, steady
state conditions of sliding. Evidence for this, however, is
conflicting. Lewis 58 reports a minimum wear rate for PTFE
against low carbon steel at a roughness of about 0·2 J.tm Ra,
but no minimum for a polyimide. Buckley59 shows a pronounced minimum in the wear of polyethylene on stainless
steel at around 0·4 pm Ra, but Dowson et al60 find only
a very shallow minimum for a similar combination at
0·1 pm Ra. No minima in wear have yet been observed
with reinforced polymers 6 1, but with such materials, however, it would be unreasonable to expect one. As already
mentioned, most fillers and reinforcements are slightly
abrasive towards metals and modify the initial topography
of a counterface during repeated sliding. It is the magnitude
and type of this self-generated topography which then
determines the steady state rate of wear. Although the
relative abrasiveness of different fillers can readily be
!52
assessed by simple test methods 62 , there is, unfortunately,
no quality control on this aspect for the fillers used commercially. It is thus not uncommon to find that the wear
properties of one particular polymer composite from one
manufacturer differ significantly from those of an apparently similar material from another.
Of all the various possibilities for reducing wear mentioned above, the use of thin coatings continues to be the
most attractive. There are several reasons for this: one is
that coatings can often be applied to a component without
significant changes in either materials or design ; another
is that cheap substrate materials can be chosen at the design
stage ; and a third is that the possibility exists of depositing
materials which are either uneconomic, or impractical, to
use in bulk form. The past few years have seen a surge of
interest in relatively sophisticated coating techniques, such
as RF sputtering of PTFE and MoS 2 63 , chemical vapour
deposition (CVD) of nitrides and carbidesf;4, electrochemical
codeposition of carbides in metal matrices 65, ion implantation69 and ion plating67 . Applications of these techniques
to the production of improved wear resistant coatings are
likely to expand enormously once the- composition and
structure of the most promising materials have been adequately defined.
·
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Copyright© Controller HMSO London 1977
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Lancaster: Wear Mechanisms of Metals and Polymers
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153
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