...

Williams2005-WearandWearParticles.pdf

by user

on
Category: Documents
5

views

Report

Comments

Transcript

Williams2005-WearandWearParticles.pdf
Tribology International 38 (2005) 863–870
www.elsevier.com/locate/triboint
Wear and wear particles—some fundamentals
John A. Williams*
Engineering Department, Cambridge University, Trumpington Street, Cambridge CB2 1PZ, UK
Available online 10 May 2005
Abstract
Each of the various processes by which material can be lost from a surface in service leaves its fingerprint both in the topography of the
worn surface and in the size, shape and number of the particles which make up the wear debris. To use debris examination as a diagnostic aid
in assessing the health of operating plant, which may contain many tribological contacts, requires not only careful and standardised
procedures for debris extraction and observation but also an appreciation of the mechanisms by which wear occurs and the regimes in which
each of the contacts of interest operates when displayed on an appropriate operational map.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Wear particle; Debris examination; Topography; Wear mechanism
1. Introduction
The phenomenon of wear was given a formal definition
in 1968 by the OECD as ‘the progressive loss of material
from the operating surface of a body occurring as a result of
relative motion at its surface’ [1]. This might today be
thought a slightly curious definition as there is no mention of
what it is that is moving relative to the surface in question,
or indeed, of the need for the transmission of some contact
force between the wearing surface and its counter-face—
even if this transmission is through a low-strength,
intermediate film such as that formed by a lubricant.
Notwithstanding this difficulty, common usage of the term
wear implies some mechanical action on the wearing
surface—so that, for example, corrosion per se would not
be classified as a wear mechanism whereas corrosive wear
involving some form of mechanical interaction in a
corrosive environment certainly would be.
Having divided wear mechanisms into two principal
categories, the first adequately described as ‘mechanical
wear’ and a second covering those situations where there is
an additional element of active chemistry (typically
oxidation and/or corrosion) it is temping to subdivide the
former large category into some number of smaller
classes—the extent of this sub-division being to some
* Tel.: C44 1223 765237; fax: C44 1223 332662.
E-mail address: [email protected]
0301-679X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.triboint.2005.03.007
extent a function of the enthusiasm of the author. Fig. 1
illustrates a typical set of mechanical wear processes
grouped under four general headings and arranged, within
each of these, in increasing order of severity. All these
mechanisms have the capability of removing material from
the surface in question though the rates of degradation,
measured as rates of loss of mass per unit time, can vary
over many orders of magnitude. All can, and often do,
operate in the presence of a lubricant, very often a mineral
oil, whose function may be both to limit friction and to
convect away heat so reducing the severity of thermal
stresses or distortions in vital machine elements. Mechanisms of wear are not mutually exclusive and in some
complex pieces of machinery several may be operating
simultaneously at different sites and different internal
contacts. Techniques of wear debris analysis are based on
the hypothesis that the morphology of debris particles
examined in a representative sample of the lubricant
circulating through the machine can indicate which is the
most active operating wear mechanism and, furthermore,
that changes in the concentration of such particles within the
circulating fluid will be indicative of changes in the state
of surfaces of these potentially critical components [2,3].
A reduction in the rate of particle production and detection
in the early stages of device operation is associated with the
running-in process as a benign and acceptable wear regime
is established; whereas a later increase in the rate of particle
concentration may herald a transition to a higher wear
regime as the surfaces wear out—perhaps with catastrophic
results.
864
J.A. Williams / Tribology International 38 (2005) 863–870
Nomenclature
H
K
P
p
k
R
t
indentation hardness
constant of proportionality
applied load
pressure
flow stress
representative length dimension
time
Surveys of industrial wear problems often highlight
abrasion as being of particular concern. Within the general
area of abrasive wear a distinction is often made between
so-called two-body abrasion when material is removed or
displaced from the softer surface by the asperities or
protuberances on the harder surface and three-body abrasion
when the damage is done by some form of free, discrete
abrasive particles rolling and sliding between the opposing
surfaces of the contact: often such particles are contaminants form the outside environment. In practice, the
distinction between the two sub-divisions may be somewhat
blurred, as if the free particles become lodged in one of the
bearing surfaces the situation either temporarily, or even
permanently, becomes one of two-body abrasion. So-called
open three body abrasion occurs when the two surfaces are
sufficiently far apart to be effectively independent of one
another; for example, this is the type of wear to which earth
moving equipment is subject when soil particles abrade the
shovel faces. If the particle velocities are large, say more
than a few m/s, perhaps because they are carried in a gas
stream or entrained in a flow of liquid, the wear process
becomes one of erosion.
2. The modelling and the mapping of mechanical wear
No simple and universal model is applicable to all
situations. In the dry, unlubricated or perhaps marginally
lubricated sliding of a pair of, usually dissimilar, loaded
surfaces, i.e. two-body conditions, the rate of surface
V
w
k
m
t
q
J
sliding speed
wear dimension
thermal diffusivity
coefficient of friction
shear stress
slope or angle
plasticity index
degradation or damage of each depends (at least) on the
factors of Table 1.
When a third body is present at the interface wear may be
inhibited, though not entirely eliminated, for example if the
third body is a lubricant or low shear strength film with a
thickness dimension at least comparable with the mean
surface roughness, or enhanced as in the case of contamination by entrained dirt or even just the retained debris from
previous wear events. Contamination by debris both harder
and softer than the opposing solid surfaces is likely to be
detrimental to the life of the contact though may involve
different detailed mechanisms: the distribution of sizes,
shapes and mechanical properties of the third bodies are all
influential variables.
For the relatively elementary of two loaded surfaces, one
hard the other softer, sliding over one another we might
suppose that the loss of linear dimension, say w, in
the wearing surface will depend on the applied load P, the
imposed sliding speed V, the coefficient of friction m, the
hardness H of the softer surface the time they slide together t
and the size of the contact measured by some representative
length dimension R. Hardness, as a plastic property, is
included rather than the elastic modulus since, for ductile
metals, wear is generally only achieved after significant
plastic flow. It is thus possible to write
w Z ƒðP; V; m; H; t; RÞ;
(1)
Buckingham’s rule can then be used to reduce this equation
to a simpler non-dimensional form,
w
P Vt
Zƒ 2 ; ;m ;
(2)
R
R H R
experimental studies have failed to find any simple
dependence of wear rates on coefficients of friction and
so, if, for the moment, this is excluded and we suppose
Table 1
Factors influencing dry wear rates
Fig. 1. A classification of mechanical wear processes.
Normal load
Relative sliding speed
Geometry (both macroscopic and local or topographic)
Initial temperature
Local environment
The thermal, mechanical and chemical properties of the materials involved
J.A. Williams / Tribology International 38 (2005) 863–870
the simplest linear functional relation between the other
non-dimensional groups, this equation simplifies to
w_ Z
w
PV
pV
ZK 2 ZK
;
t
H
R H
(3)
an expression for the change in dimension of a load–bearing
surface as a result of wear in which K is some constant and p
is the nominal pressure on the contact. We should not be
surprised if this dimensional wear is dependent on the
imposed pV value and indeed this is a well established
design parameter in simple dry sliding situations. Eq. (3) is
known in the tribology community as either the Archard or
the Rabinowicz equation but is also familiar in the semiconductor industry, in connection with the polishing of
silicon wafers, where it is known as the Preston equation
[4–6]. The quantity p/H is non-dimensional and so there
might well be situations in which dimensional wear
increases more rapidly than linearly with load. These are
indeed often observed as transitional regimes between two
regimes—each separately adequately described by the
linear Archard equation—when, over some relatively
small range of surface loading, the observed wear rate is
much more sensitive to load than simple linearity. Over this
transition region wear increases with load raised to a power
more very much greater than unity. These transitions, often
referred to as representing a change from mild to severe
wear, are characteristic of many sliding systems. The form
and the morphology of the resultant debris may be very
different either side of the change reflecting a change in the
physical mechanism of its generation.
Abrasive wear is but one of the various mechanical
processes which can lead to material loss from a surface1. In
addition to erosion, referred to above, the mechanisms of
adhesion and surface fatigue can also lead to surface
distress, damage and, ultimately, the generation of wear
debris with a characteristic morphology. If two atomically
clean metallic surfaces are loaded against one another, then
at points at which opposing asperities make contact, strong
adhesive junctions will be formed. If the surfaces are now
forced to move tangentially, not all these junctions may
shear along the original material interface, but some may be
deformed in such a way that fragments of the softer surface
are plucked out and removed: these particles constitute
adhesive wear debris. Of course, the majority of engineering
surfaces are not atomically clean, nor do they always carry
normal loads sufficiently high to generate enough surface
plastic deformation that will lead to the formation of such
adhesive junctions; consequently, significant adhesive wear
is unlikely in equipment operating within normal design
constraints. However, this does not mean that adhesion
cannot be important—sometimes dramatically so. It can be
a particular problem when both members of the tribological
1
The literature on wear is voluminous. More details of published sources
are available at: http://www.shef.ac.uk/mecheng/tribology/tools/tools.html.
865
contact are made of the same sort of materials or when
sliding speeds are high; severe damage of this type can
result in the tearing of macroscopic chunks of material from
the surface—this is known as galling. The term scuffing is
used specifically to describe the onset of adhesive wear
between lubricated surfaces which has arisen from the
breakdown or failure of the lubricant film for whatever
reasons—although this is generally associated with an
excessive increase in temperature. The phenomena of
fretting failures in bolted or interference joints subject to
vibration or oscillation is another example of a form of
tribological failure in which adhesion can play a part.
Surfaces which are loaded repeatedly, even if they
display some plastic deformation in the initial stages of their
history, can generate patterns of subsurface protective
stresses which are sufficiently strong to enable the applied
loads to be carried entirely elastically in the longer term
after many applications of the load cycle—they are said to
shake-down [7]. Despite the alleviation of stress levels that
this generates, surfaces that have shaken down can still wear
subsequently by processes of surface fatigue or allied
phenomena. Subsurface cracks may be nucleated at microstructural defects or inclusions in the material, giving rise to
characteristic pitting fatigue generating wear or debris
particles which are more or less equiaxed. Where the
friction or traction forces are sufficient to deform the surface
layer, material may be lost, in the form of thin flakes or
platelets. In such delamination wear subsurface plastic shear
is associated with the formation and propagation of cracks
nucleating from pre-existing voids or inclusions present in
the material structure. The thickness of the wear sheet is
controlled by the location of these subsurface cracks, and is
typically of the order of a few microns. In relatively defectfree materials, for example vacuum remelted bearing steels,
local incremental plastic strains can again build up, cycle by
cycle, producing a form of local surface collapse, sometimes
known as ratchetting, occurring on an even finer scale and
producing debris which is again characteristically lamellar
or sheet-like in form, but is now less than a micron in
thickness.
Of the various forms of abrasive mechanical wear,
polishing is the mildest form, and is unusual, in that it leads
to an improvement in surface smoothness; this is in contrast
to other abrasion processes that invariably lead to surface
roughening. Other terms sometimes used in association with
three-body abrasive wear, as opposed to two-body wear, are
high-stress and low-stress abrasion. In high-stress abrasion,
the crushing strength of the abrasive particles is exceeded at
typical concentrated contacts so that they are broken up
during the wear process—the active components in ball
mills and other forms of crushing equipment are subject to
this form of damage. In low-stress abrasion, the particles
remain essentially intact—pipework, hoppers and conveyors carrying solid particulates are typically subject to
low-stress abrasion. The term gouging abrasion is also
866
J.A. Williams / Tribology International 38 (2005) 863–870
sometimes used to describe particularly severe forms of
high-stress abrasion.
3. Mapping mechanical wear processes
The purposes of laboratory testing [8] can range from
very specific ‘trouble-shooting’, i.e. the entirely empirical
solution of operating problems with an existing machine, to
much more general and fundamental studies of the micromechanics and materials science aspects of the physical
processes operating during the phenomenon of wear itself.
Wear rates in successfully operating industrial equipment can vary enormously, from very high values under
particularly aggressive conditions, to very low values in
more benign circumstances—this is reflected in the
numerical value of the coefficient K in Eq. (3) which can
vary from more than 10K2 in situations of very severe wear
to less than 10K9 in much less hostile conditions. If
component or material tests are to be carried for service in
such low wear regimes, then, in order to achieve
dimensionally reliable measurable amounts of wear,
laboratory simulations must be run for several hundred
hours. Industrial scientists and research engineers will often
find themselves placed under pressure to reduce the time
scale of such wear tests, and thus to speed up the production
of data; it is important to appreciate that this procedure is
potentially hazardous, as there is always the danger, for
example when increasing loads or speeds, of moving from a
regime of operation within which one specific form of wear
mechanism is dominant to another controlled by quite
different physical phenomena. Such pitfalls can often be
prevented, and the way indicated in which changes in
service conditions might be expected to influence wear
response, by constructing an appropriate ‘map’ of wear
behaviour for the surfaces concerned. Empirical mechanism
maps are built up by plotting experimental data for wear
rates on suitable axes, identifying at each point the
mechanism by direct experimental observation. This
approach can be complemented by the use of physical
modelling: model based equations describing the wear rate
of each mechanism are combined so that each ‘country’ on
the map represents the combination of plotted variables for
which one physical degradation mechanism is dominant.
An immediate difficulty is the choice of the most
appropriate coordinate axes on which to display the chosen
data. In the case of dry, or marginally lubricated, metallic
sliding contacts, two, to some extent complementary,
approaches have been suggested. The first, proposed by
Lim et al. [9,10], plots the severity of the load expressed as
the applied normal pressure p versus the sliding speed V.
Such a map, which is specific to a particular material
(surface topography, grain size, hardness, etc.) has the merit
that it can also incorporate regimes of different chemical or
surface reaction behaviour which are associated with
temperature effects—although this is at the expense of
detail on the mechanical forms of wear associated with
different states of surface topography and asperity interaction. The general form of such a map, for a metal, is
illustrated in Fig. 2 in which pressure and speed have been
respectively, by
expressed non-dimensionally as p and V,
using the relations:
pffiffiffiffiffiffiffiffi
p Z p=H and V Z V=k ! A=p
H is the hardness of the wearing material and k its
thermal diffusivity (equal to the value of thermal conductivity divided by the product of density and specific heat).
The normalised speed can be thought of as a measure of the
magnitude of the sliding speed compared to the speed of
heat flow through the wear material. Regions of the map
associated with different wear mechanisms are traversed by
contours of equal wear rate. Characteristically, the map is
divided into two regions by a roughly vertical field
boundary. To the left of this divide, wear is controlled by
essentially mechanical processes; here the wear rate
depends on the normal pressure (or load) but is not greatly
dependent on sliding velocity so that contours of wear rate
are more or less horizontal. On the other hand, to the right of
this division, thermal and chemical effects (invariably under
normal atmospheric conditions these involve oxidation)
become the dominant influence and the contours of wear
rate become functions of both load and velocity. For steels,
this fundamental mechanism transition corresponds to dry
sliding speeds of about 1 m sK1; below this, surface heating,
and so oxidation, is relatively insignificant. The map
immediately highlights the dangers of increasing the rate
of data collection in simulation experiments by increasing
the sliding speed. For example, if actual service conditions
and those in the proposed accelerated test are represented by
points in different countries of the map then it is clear that in
moving from one set of circumstances to the other not only
will the rate of wear be different but it will have its origins in
quite different physical and chemical mechanisms; this then
throws into doubt any evidence of material performance
from such an accelerated programme.
In the regime dominated by mechanical wear, there are
regions within which the spacing between contours of wear
rate is equal to that between changes of load of the same
factor, for example as K goes from 10K9 to 10K8 or from
10K5 to 10K4 the load also changes by a factor of 10. In
each of these cases wear rate and load are directly
proportional to each other, i.e. in accord with Eq. (3). But
this is not the case throughout the region; the gap between
the contours for K equal to 10K8 to that for 10K5, covering
three orders of magnitude corresponds to a load change of
only one order. This very rapid increase in wear rate is an
example of the transition from ‘mild’, and hence usually
acceptable levels, to ‘severe’ (and usually unacceptable)
values brought about by an increase in load. Dry or
marginally lubricated contacts in machines between steel
surfaces often operate at nominal pressures of a few
J.A. Williams / Tribology International 38 (2005) 863–870
867
Fig. 2. Load–speed wear mechanism map for medium carbon steels based largely on pin-on-disc data. Load and speed both normalised as described in the text.
Thick lines delineate different wear mechanisms and thin lines are contours of equal wear rates. Chain lines represent constant values of the pV factor. From
Williams JA. Wear modelling: analytical, computational and mapping: a continuum mechanics approach. Wear 1999;225–229:1–17.
megapascals and at sliding speeds of the order of something
less than a metre per second; in other words just at this level
of wear uncertainty. Note that at a given level of load but at
high sliding speeds, surface oxidation can be protective and
provide reductions in loss of material, i.e. dips in the
contours of K, when the softened oxide film acts as a
protective almost lubricant layer over the metallic substrate.
The relevance of the pV factor has been mentioned as an
important design guide to the consequences of dry rubbing
not least as it represents the energy input per unit area. Since
the map of Fig. 2 is drawn on log–log scales, constant pV
values will plot as straight lines with a negative slope and
three such lines are shown in Fig. 2 for specific energies of
0.1, 1 and 10 MW mK2 which are typical of dry running
machine contacts.
An alternative presentation of wear data, suggested by
Childs [11], and subsequently developed [10], which gives
more emphasis to the mechanical—and so strictly abrasive—aspects of surface damage (but less to the thermal,
since velocity is not considered as an independent variable)
plots the regimes of wear on axes representing the shear
strength of the interface between the two materials (usually
normalised by the shear strength of the weaker material k
versus some roughness parameter, such as the angle q
representing the average slope of asperities on its surface.
The general form of such a plot is shown in Fig. 3; note that in
order to include the region of elastic response the roughness
Fig. 3. Wear mechanism map for a soft surface abraded by a harder rougher
counter-face. The ratio t/k represents the relative strength of the interface
and q the mean slope of the rough surface (or the attack angle of a single
asperity). Thicker lines delineate different wear mechanisms and thin lines
are contours of equal wear rates. From Williams JA. Wear modelling:
analytical, computational and mapping: a continuum mechanics approach.
Wear 1999;225–229:1–17.
868
J.A. Williams / Tribology International 38 (2005) 863–870
axis is calibrated logarithmically. This axis could also be
calibrated in terms of the Greenwood and Williamson
plasticity index J [12] whose value indicates the relative
importance of elastic as opposed to plastic contact
conditions: the boundary between the predominantly elastic
and plastic zones corresponds to a value of J close to unity.
When the surfaces are comparatively smooth, so that the
value of q is small, elastic deformations cannot be
neglected—indeed they may be sufficient to accommodate
the applied loads alone—so that wear in repeated traversals,
which is the usual situation in most tribological devices,
depends on some form of fatigue or damage accumulation
mechanism. At steeper values of surface slope, i.e. higher
values of q or J, wear may be due to a combination of elastic
and plastic effects, as is the case in delamination wear. If the
surface is made even rougher, abrasion is initiated; this
always involves severe plastic deformation and can take the
form of a combination of ploughing (in which, although the
surface topography is much modified, only a small
proportion of the displaced material is actually detached
from the surface) and micro-machining (where a much
higher proportion of the plastically deforming material is lost
as wear debris). Once significant volumes are lost by micromachining, then reducing the interfacial she stress has the
effect of increasing the volumetric loss, because the
efficiency of the micro-cutting operation is improved; we
have situation in which reducing friction (perhaps by
providing or im proving lubrication) enhances wear. This
is in contrast to the circumstances over the rest of the map in
which reducing the surface shear stress lowers the wear rate.
4. Polymers and ceramics
The common glassy polymers, such as poly-methylmethacrylate, poly-carbonate and poly-styrene, are not often
used as bearing materials but rather as optical windows and
in this application their resistance to abrasive wear or
scratch resistance is if obvious interest. Work, principally
with PMMA, has demonstrated that damage evolves
through a range of severity as the imposed strain is
increased: visco-elastic smoothing or ironing is followed
by plastic or visco-plastic grooving, then extensive plastic
flow and tearing, pronounced fracture or tearing and finally
micro-cutting or chip formation [13]. As with metallic
materials, each mechanism of material removal leaves its
signature in the morphology of the debris which its
operation generates. These regimes are illustrated in the
wear map of Fig. 4: the abscissa is effectively the attack
angle of an assumed conical asperity or, by extension, the
slope of a rough surface which might be thought of as an
array of such individual asperities.
Ceramic materials have attractive tribological properties
but their Achilles’ heal has been their poor resistance to
sudden and potentially catastrophic failure by fracture.
Improvements in both material design, integrity and
Fig. 4. Scratching mode map for PMMA Nominal contact strain taken as
0.2 tan q. Velocity 0.004 mm/s. From Briscoe BJ, Sinha PK. Wear of
polymers. Proc Inst Mech Eng Part J 2002;401–413.
fabrication procedures have in recent years seen levels of
fracture toughness both increase in magnitude and, most
importantly, reduce in scatter, so that rational tribological
design is feasible [14]. However, although it is well
established that ceramic materials (just as more ductile
metals and polymers) can wear by a number of different
mechanisms it is not obvious what material parametric
groups should be used as map coordinates when additional
physical, chemical and environmental quantities must be
included. One such map based on an investigation of a
number of ceramics sliding against a specimen of the same
material in air at 20 8C is shown in Fig. 5 where the two
parametric groups Sc;m and Sc;t are, respectively, measures
of the susceptibility of the transition to severe wear by
intergranular fracture and thermal tensile stress cracking
Fig. 5. A proposed generic wear map for ceramic materials indicating the
transition form mild to severe wear conditions illustrating the effect of the
coefficient of friction. From Adachi K, Kato K, Chen N. Wear map for
ceramics. Wear 1997;203–204:291–301.
J.A. Williams / Tribology International 38 (2005) 863–870
[15]. An important feature of ceramic tribology is the
influence that chemical reactions with the environment exert
on both friction and wear. This chemical action can take
several forms depending on the particular combination of
materials involved and the imposed service and environmental conditions; it can consist in modifications of surface
composition and topography that decrease wear and friction,
in a purely chemical form of wear by dissolution, or by
chemically induced cracking and fracture which can
increase wear rates. Water vapour in particular has a
pronounced effect on silicon nitride, amongst other
important engineering ceramics, changing not only the
rate of wear but also the mechanism by which it is
generated. In the absence of water vapour such ceramics can
lose material at rates of up to 100 times that observed when
operated with high values of relative humidity.
Fig. 6. Wear map showing wear rates and mechanisms for TiN in dry
sliding against HSS pin material. The TiN wear rates have units of g mK1!
108. Experiments were conducted using the pin-on-disc geometry, at room
temperature in air (RHZ13%). (a–e) SEM micrographs (back-scattered
electron contrast (BSE)) of worn TiN surface morphologies from Regimes
I–IV (see text for explanations): the arrow indicating TiN disc sliding
direction relative to HSS pin applies to all micrographs in a–e; the
continuous longitudinal features perpendicular to the sliding direction are
the polishing marks left over from the original grinding process. From
Wilson S, Alpas AT. Tribolayer formation during sliding wear of TiN
coatings. Wear 2000;245:223–229.
869
When ceramic are applied as thin coatings then further
additional possibilities exist in the effect of surface
degradation of combinations of load and speed which will
be reflected in the form of the observed debris: a map for
the sliding of TiN coatings on a High Speed Steel (M2) disc
against pins of a similar material is shown in Fig. 6 [16].
Wear rates (in units of g mK1!108) are indicated at the
various loads and speeds investigated and the map divided
into four regimes on basis of the observed operating wear
mechanisms.
5. Corrosive wear and erosion–corrosion maps
Corrosive wear might be defined as covering those
situations in which chemical or electro-chemical reaction
with the environment predominate over mechanical interactions. However, this is not really a very satisfactory
definition as the effects of mechanical wear and chemical
wear may be synergistic and result in rates of material loss
and surface degradation much greater than a simple sum of
the two mechanisms observed independently. Any definition
of corrosive wear will encompass the re-oxidation of
exposed metal in a worn surface—and in general this is a
beneficial phenomenon: it will also include the action of
extreme-pressure or anti-wear additives in lubricants which
technically rely on a form of controlled ‘corrosion’ to
generate low shear strength and protective boundary films.
All chemical reactions take place more rapidly at elevated
temperatures—as a rough rule of thumb reaction rates may
double with every 10 8C increase in temperature so that
corrosive wear problems tend to be exacerbated at higher
temperatures. In circumstances in which loss of material is
due to chemical activity alone with no addition mechanical
abrasive or erosive element, the two most important
variables are the electro-chemical potential applied to the
surface and the pH of the surrounding medium. The surface
may be immune, become passivated or actively corroded
Fig. 7. Erosion–corrosion map for steel in aqueous conditions, pH 7. From
Stack MM, Corlett N, Zhou S. A methodology for the construction of the
erosion–corrosion map in aqueous environments. Wear 1997;203–
204:474–488.
870
J.A. Williams / Tribology International 38 (2005) 863–870
and these regimes can be displayed on a Pourbaix diagram
[17]. When, in addition, abrasion or erosion occurs, as is
often the case, the form of the map becomes more complex—
Fig. 7 illustrates a possible form of the interaction between
particle velocity and applied potential for a steel surface in
an aqueous solution of pH 7: the boundaries between
the different areas of the map are established from
independently determined models of rates of mass loss due
to either mechanical interaction or chemical corrosion and
the details for this particular simulation are given in [18,19].
In the field, the effect of erosion and corrosion may be either
simply ‘additive’ or, more usually, ‘synergistic’—this latter
term implying that the effect of corrosion is to change the
mechanical properties of the surface in such a way as to
enhance the wear rate arising from the mechanical
interaction. Wear debris in situations ion which corrosion
is active tends to be finely divided and fully reacted—with
very little unreacted metal present. The existence of such
debris is, however, no guarantee of corrosive wear as in
abrasive wear it is possible for the debris to become fully
transformed after removal form the surface.
6. Conclusions
When material is lost from a loaded surface either
entirely or principally through some form of mechanical
interaction the concentration, size and shape of the debris
particles carry important information about the state of
surfaces from which they were generated and thus, by
implication, the potential life of the contact and of the
equipment of which this forms a part. The full exploitation of
this information and the ability to be able to predict
quantitatively the future performance or life requires an
understanding of the sources and mechanisms of generation
of the extracted and sampled particulate debris. In many
cases, it is instructive to display the running conditions of a
given contact on some form of operational or wear map. This
both enables the implications for wear of changes in design,
material or operating parameters to be assessed and allows
sensible correlations to be made between laboratory-based
experimental investigations and observations in the field.
References
[1] Lansdown AR, Price AL. Materials to resist wear: a guide to their
selection and use. Oxford: Pergamon; 1986.
[2] Roylance BJ, Williams JA, Dwyer-Joyce R. Wear debris and
associated wear phenomena—fundamental research and practice.
Proc Inst Mech Eng 2000;214J:79–105.
[3] Hunt TM. Handbook of wear debris analysis and particle detection in
liquids. London: Elesvier; 1993.
[4] Archard JF. Contact and rubbing of flat surfaces. J Appl Phys 1953;24:
981–8.
[5] Rabinowicz E. Friction and wear of materials. New York: Wiley;
1965.
[6] Preston FW. Theory and design of plate glass polishing machines.
J Soc Glass Technol 1927;11:156–214.
[7] Williams JA, Dyson IN, Kapoor A. Repeated loading, residual
stresses, shakedown and tribology. J Mater Res 1999;14:1548–59.
[8] Williams JA. The laboratory simulation of abrasive wear. Tribotest
1997;3:267–306.
[9] Lim SC, Ashby MFA. Wear mechanism maps. Acta Metall 1987;35:
1–24.
[10] Williams JA. Wear modelling analytical, computational and mapping;
a continuum analysis approach. Wear 1999;225–229:1–17.
[11] Childs THC. The mapping of metallic sliding wear. Proc Inst Mech
Eng 1988;C202:379–95.
[12] Greenwood JA, Williamson JPB. Contact of nominally flat surfaces.
Proc Royal Soc 1966;A295:300–30.
[13] Briscoe BJ, Evans PD, Pelilio E, Sinha SK. Scratching maps for
polymers. Wear 1996;200:137–47.
[14] Jahanmir S. Friction and wear of ceramics. New York: Marcel
Dekker; 1994.
[15] Adachi K, Kato K, Chen N. Wear map for ceramics. Wear 1997;203:
291–301.
[16] Wilson S, Alpas AT. Tribolayer formation during sliding wear of TiN
coatings. Wear 2000;245:223–9.
[17] Pourbaix M. Atlas of electro-chemical equilibria in aqueous solution.
Houston: NACE; 1974.
[18] Stack MM. Mapping tribo-corrosion processes in dry and in aqueous
conditions. Tribol Int 2002;25:681–9.
[19] Stack MM, Corlett N, Zhou S. A methodology for the construction of
the erosion–corrosion map in aqueous environments. Wear 1997;203–
204:474–88.
Fly UP