685

Friction and W
13. Friction and Wear

13.2.3 Interaction with Other Degradation

Almost all mechanical systems, artificial or natural,

Part D 13
Mechanisms ................................ 692
involve the relative motion of solid components.
13.2.4 Experimental Planning
Wherever two surfaces slide or roll against each
and Presentation of Results ........... 692
other, there will be frictional resistance, and
wear will occur. The response of materials to this 13.3 Tribological Test Methods ...................... 693
kind of interaction, often termed tribological, 13.3.1 Sliding Motion ............................. 693
depends not only on the precise nature of the
13.3.2 Rolling Motion ............................. 693
13.3.3 Abrasion ..................................... 694
materials, but also on the detailed conditions of
13.3.4 Erosion by Solid Particles............... 695
the contact between them and of the motion.
13.3.5 Scratch Testing............................. 695
Friction and wear are system responses, rather
than material properties. The measurement of 13.4 Friction Measurement ........................... 696
tribological behavior therefore poses particular 13.4.1 Friction Force Measurement........... 696
challenges, and a keen awareness of the factors 13.4.2 Strain Gauge Load Cells
and Instrumentation .................... 696
that influence friction and wear is essential.
13.4.3 Piezoelectric Load Sensors ............. 697
This chapter provides definitions of the key
13.4.4 Other Force Transducers ................ 697
concepts in Sect. 13.1, and provides a rationale
13.4.5 Sampling and Digitization Errors .... 698
for the design and selection of test methods 13.4.6 Calibration .................................. 698
in Sect. 13.2. Various standard and other tribolog- 13.4.7 Presentation of Results ................. 700
ical test methods are comprehensively reviewed
in Sect. 13.3, which is followed by descriptions of 13.5 Quantitative Assessment of Wear ........... 701
13.5.1 Direct and Indirect Quantities ........ 701
methods used for the quantitative assessment
13.5.2 Mass Loss .................................... 701
of both friction (Sect. 13.4) and wear (Sect. 13.5).
13.5.3 Dimensional Change ..................... 701
Methods used for characterizing worn surfaces and 13.5.4 Volume Loss ................................ 702
wear debris are addressed in Sect. 13.6. 13.5.5 Other Methods ............................. 704
13.5.6 Errors and Reproducibility
13.1 Definitions and Units ............................ 685 in Wear Testing ............................ 704
13.1.1 Definitions .................................. 685 13.6 Characterization of Surfaces and Debris .. 706
13.1.2 Types of Wear .............................. 686 13.6.1 Sample Preparation ...................... 706
13.1.3 Units for Wear.............................. 687 13.6.2 Microscopy, Profilometry
13.2 Selection of Friction and Wear Tests ....... 689 and Microanalysis ........................ 707
13.2.1 Approach to Tribological Testing..... 689 13.6.3 Wear Debris Analysis..................... 709
13.2.2 Test Parameters ........................... 690 References .................................................. 709

13.1 Definitions and Units

Tribology is the science and technology of interacting mance, such as corrosion or biodegradation, both friction
surfaces in relative motion, which includes the study of and wear result from exposure of the material to a par-
friction, wear and lubrication. Friction and wear rep- ticular set of conditions. They do not therefore represent
resent two key aspects of the tribological behavior of intrinsic material properties, but must be measured and
materials. Like other measurements of material perfor- defined under well-characterized test conditions.
686 Part D Measurement Methods for Materials Performance

13.1.1 Definitions ues under the specific conditions of measurement. The

coefficient of friction usually lies in the range from 0 to
Wear can be defined as damage to a solid surface, gener- 1, although there is no fundamental reason why it need
ally involving progressive loss of material, due to relative do so. Negative values of μ may be thought of as being
motion between that surface and a contacting substance rather artificial, since they result from adhesive forces
or substances [13.1]. Materials in contact are subjected between the bodies involved which result in a tangential
to relative motion in many different applications. In force while the bodies are subjected to a tensile normal
Part D 13.1

some cases this motion is intentional: for example in force, and “friction” is perhaps not a helpful term to de-
rotating plain bearings, pistons sliding in cylinders, auto- scribe this phenomenon; but values of μ greater than
motive brake disks interacting with brake pads, or in the 1 are physically quite reasonable, and can be encoun-
processing of material by machining, forging or extru- tered, for example, in the interaction between a car tire
sion. It may also be unintentional, as in the small cyclic and a dry road surface, or in the sliding of certain ductile
displacements known as fretting which can cause wear metals in the absence of oxygen.
in certain structural joints under oscillating loading. If A large number of specialist terms used in tri-
hard particles are present, for example as contamination bology have been formally defined, most recently by
in a lubricant, or intentionally as in abrasive machining, ASTM [13.1]. An earlier compilation including many
then these particles will have a profound influence on other terms, as well as their translations into French, Ger-
the resulting wear process. man, Italian, Spanish, Japanese, Arabic and Portuguese,
The friction force is defined as the force acting tan- was published by an international group [13.2]. A par-
gentially to the interface resisting motion, when, under ticularly useful term is “triboelement”, defined as one of
the action of an external force, one body moves or tends the two or more solid bodies involved in a tribological
to move relative to another. The friction force F may contact.
be associated with sliding motion or with pure rolling
motion of the bodies; in some practical engineering ap- 13.1.2 Types of Wear
plications such as the contact between ball and track
in a ball-bearing or between a pair of mating gear It is important, especially when describing wear, to dis-
teeth, there may be more complex motion involving both tinguish clearly between the nature of the relative motion
sliding and rolling. responsible for the wear and the physical mechanisms
The coefficient of friction μ is a dimensionless num- by which the material is removed or displaced in wear.
ber, defined as the ratio F/N between the friction force F Wear tests are designed to produce specific types of
and the normal force N acting to press the two bodies relative motion, which can often lead to different mech-
together. The kinetic coefficient of friction μk is the anisms of wear in different materials, or at different
coefficient of friction under conditions of macroscopic loads or speeds. Methods that can be used to distinguish
relative motion between the two bodies, while the static between wear mechanisms are discussed in Sect. 13.6.
coefficient of friction μs is the coefficient of friction Figure 13.1 illustrates some common types of rel-
corresponding to the maximum friction force that must ative motion that can result in wear. These represent
be overcome to initiate macroscopic motion between idealized examples and can be further subdivided. For
the two bodies. The coefficient of friction is a conve- example, sliding and rolling motions may be either con-
nient method for reporting friction force, since in many tinuous or interrupted, and may occur along the same
cases F is approximately linearly proportional to N over track on a rotating counterbody or on a continuously
quite large ranges of N. The equation fresh track. They may involve constant relative veloc-
ity (continuous sliding), or varying velocity (such as
F = μN (13.1)
reciprocating motion with perhaps sinusoidal or linear
is sometimes called “Amontons Law”. velocity variation).
The value of μ can be expected to depend sig- When analyzing the motion in a tribological con-
nificantly on the precise composition, topography and tact, the nature of the contact must also be examined. It
history of the surfaces in contact, the environment to is helpful to consider how the contact region moves with
which they are exposed, and the precise details of the respect to the surfaces of the bodies in contact. For ex-
loading conditions. Although tables of coefficients of ample, in the simple example of a small block sliding on
friction have been published, they should not be regarded a larger counterbody shown in Fig. 13.1, the contact re-
as anything more than general indications of relative val- gion is fixed relative to the block, which is in continuous
 Friction and Wear 13.1 Definitions and Units 687

contact, whereas it moves along the larger triboelement,

so areas of this experience contact only for a certain pe- Sliding abrasion
riod. In the case of simple rolling shown in Fig. 13.1, (“two-body”)
the contact region moves relative to the surfaces of both
triboelements, and areas on both of these surfaces are Rolling abrasion
(“three body”)
therefore in intermittent contact.
In many practical situations there is a lubricant film
Scratching

Part D 13.1
present, but in some others, as well as in many laboratory
studies, the sliding or rolling occurs unlubricated in air.
Relative motion normal to the surface of a triboele- Fig. 13.2 Examples of abrasive wear due to the presence of
ment may result in impact. Repeated impact by a large hard particles in a sliding contact, and scratch damage
counterbody on an essentially single-contact region can
lead to wear, as can impact by a large number of small dragged over the counterbody in a process described as
particles or even liquid droplets, in which case the im- sliding abrasion. This type of abrasion is often termed
pact sites are usually distributed over an area of the “two-body” abrasion, since the particles effectively form
surface. Wear can also result from the collapse close to part of one of the two triboelements. If the particles are
a solid surface of vapor or gas bubbles produced by local free, and roll between the sliding bodies, then their in-
pressure fluctuations in a cavitating liquid. teraction with them is different and can be described
The presence of small particles in the contact re- as rolling abrasion; this is also known as “three-body”
gion, usually harder and thus less deformable than one abrasion, since the particles form a distinct “third body”
of the moving bodies, leads to a significantly different which moves with respect to the two sliding bodies. The
wear process to that for plain sliding. The most common terms “two-body” and “three-body” can lead to confu-
types of wear associated with hard particles in a sliding sion, since the motion of free particles in a sliding contact
contact are illustrated in Fig. 13.2. If the particles are at- will depend on the relative hardnesses of the surfaces and
tached to one of the sliding bodies, either forming part on the applied pressure, and can vary between sliding
of it (for example, as hard discrete phases in a softer and rolling even in different regions of the contact. The
matrix, or as particles fixed to the surface, as in abra- conditions of abrasive wear are sometimes described as
sive paper) or embedded in the surface, then they will be either “low-stress”, in which the abrasive particles them-
selves are relatively undamaged in the wear process, or
“high-stress”, where the particles experience extensive
Sliding fracture.
If surface damage results from the action of a single
r 1ù 1 = r 2ù 2 hard asperity, or a small number of relatively large as-
Rolling ù1 ù2
r1 r2 perities, then it is often termed “scratching”, and scratch
testing can be used to characterize the response of a ma-
Sliding + ù1 ù2 terial to this type of deformation (see Sect. 13.3.5).
rolling r1 r2
Scratching is also shown schematically in Fig. 13.2.
Impact (large body)
13.1.3 Units for Wear

Particle impact Since wear is often associated with the removal of ma-
terial from a surface, it is usually quantified in terms
of the volume of material removed, the mass removed,
Liquid impact or the change in some linear dimension after wear. All
three can be used as primary measurements. For ex-
ample, the volume of material worn from the end of
Cavitation a spherically-ended pin to form a flat area on the pin
may be calculated from a measurement of the diameter
Fig. 13.1 Examples of tribological contacts involving rela- of the flat area; volume changes in more complex geome-
tive motion of triboelements, as well as erosion by particle tries may be derived computationally from the difference
impact, liquid impact or cavitation in a liquid between profile traces recorded before and after wear.
688 Part D Measurement Methods for Materials Performance

Measurement of mass loss is in principle comparatively as the volume or mass loss per unit mass of impinging
straightforward, although great care must be taken in the particles.
experimental technique to reduce errors. Linear changes Many quantitative models for sliding wear have been
in dimensions (for example, the reduction in length of developed, but one of the simplest, due to Archard, is
a pin specimen, or the change in clearance in a bearing) useful for comparing wear rates and material behavior
are also in principle straightforward to measure, but are under different conditions. The Archard model leads to
prone to error and so careful experimental procedures the equation:
Part D 13.1

are needed. Measurements of mass change can be af-

Q = KW/H , (13.2)
fected by material transfer and oxidation; dimensional
changes can also be influenced by thermal expansion. where Q is the volume of material removed from the sur-
These and other aspects of experimental techniques are face by wear per unit sliding distance, W is the normal
discussed in Sect. 13.5. load applied between the surfaces, and H is the indenta-
In some cases material may be lost from both tri- tion hardness of the softer surface. Many sliding systems
boelements, or significant transfer of material may occur do show a dependence of wear on sliding distance which
between the triboelements, and particular care is then is close to linear, and under some conditions also show
needed in both measuring and describing the magni- wear rates which are roughly proportional to normal
tude of wear. If wear is associated with the movement load. The constant K , usually termed the Archard wear
of material within the surface of a single triboelement, coefficient, is dimensionless and always less than unity.
it will lead to changes in the topography of the sur- The value of K provides a means of comparing the sever-
face which can be detected by profilometry, as discussed ities of different wear processes. Equation (13.2) can be
in Sect. 13.6.2. applied to abrasive wear as well as to sliding wear; an
The amount of wear that occurs when two surfaces analogous equation can also be developed for erosion
slide over each other tends to increase with sliding dis- by solid particle impact, from which a value of K can
tance, and hence with time. The wear rate can then be be derived.
defined as the rate of material removal or dimensional Figure 13.3 shows, very approximately, the range
change per unit time, or per unit sliding distance. Be- of values of K seen in various types of wear. Under
cause of the possibility of confusion, the meaning of the unlubricated sliding conditions (so-called dry sliding),
term “wear rate” must always be defined, and its units K can be as high as 10−2 , although it can also be
stated. In the case of erosion the wear rate is usually as low as 10−6 . Often two distinct regimes of sliding
quoted as the mass or volume loss per unit time; for ero- wear are distinguished, termed “severe” and “mild”. Not
sion by solid particle impact, it may also be expressed only do these correspond to quite different wear rates
(with K often above and below 10−4 respectively), but
they also involve significantly different mechanisms of
material loss. In metals, “severe” sliding wear is asso-
Erosion
Wear by hard ciated with relatively large particles of metallic debris,
particles while in “mild” wear the debris is finer and formed of
Abrasion
oxide particles. In the case of ceramics, the “severe”
Severe wear regime is associated with brittle fracture, whereas
Unlubricated “mild” wear results from the removal of reacted (often
sliding Mild hydrated) surface material.
Boundary When hard particles are present and the wear pro-
Lubricated
cess involves abrasion (by sliding or rolling particles)
EHL
sliding or erosion (by the impact of particles), then the highest
HL values of K occur; the relatively high efficiency of the
removal of material by abrasive or erosive wear explains
10–14 10–12 10–10 10–8 10–6 10–4 10–2 1 why these processes can also be usefully employed in
K
manufacturing.
Fig. 13.3 Typical ranges of wear coefficient K exhibited under The values of K that occur for unlubricated sliding,
various tribological conditions: hydrodynamically lubricated (HL); or for wear by hard particles, are generally intolera-
elastohydrodynamically lubricated (EHL); lubricated by a boundary bly high for practical engineering applications, and in
film; unlubricated sliding; abrasive wear; and erosive wear most tribological designs lubrication is used to reduce
 Friction and Wear 13.2 Selection of Friction and Wear Tests 689

the wear rate; the effect of lubrication in reducing wear volume lost (mm3 ) per unit sliding distance (m) per
is far more potent than its effect on friction, and the in- unit normal load on the contact (N). These units are
crease in life which results from the reduction of wear commonly used when quoting experimentally measured
is generally much more important than the increase in rates of wear. It is useful to note that for a material with
efficiency from the lower frictional losses. As Fig. 13.3 a Vickers hardness H of 1 GPa (≈ 100 HV), then the nu-
shows, even the least effective lubrication can reduce merical value of K (dimensionless) is the same as that
the wear rate by several orders of magnitude, and as the of k (in units of mm3 /Nm).

Part D 13.2
thickness of the lubricant film is increased in the pro- Although k (or K ) is effectively constant over quite
gression from boundary, to elastohydrodynamic (EHL) large ranges of load or sliding speed in many cases of
and then to hydrodynamic lubrication (HL), the value sliding wear, in some instances sharp transitions can
of K falls rapidly. In the hydrodynamically lubricated occur, and k may change by a factor of 100 or even
components of a modern automotive engine, values of 1000 for a relatively small change in the conditions. This
K as low as 10−19 are achieved. behavior is associated with a change in the predominant
The quantity Q/W(= K/H ), given the symbol k, mechanism of material removal; for this reason it is
and sometimes termed “specific wear rate” (UK) or always dangerous to extrapolate wear data to predict the
“wear factor” (USA), is also useful. The units in which k likely rate of wear in a system from data obtained under
is expressed are usually mm3 /Nm, representing the different conditions.

13.2 Selection of Friction and Wear Tests

Friction and wear testing is often carried out to ensure 13.2.1 Approach to Tribological Testing
that good tribological performance is achieved from ma-
terials. This information is required at different times in There are many different approaches to friction and wear
the life cycles of products. Designers require data that testing, illustrated schematically in Fig. 13.4, that differ
will enable them to design products that will not wear in the scale and complexity of the element that is being
out or have unacceptably low efficiency, or even fail tested. Ultimately field testing of the final product will
to function, due to frictional losses; process engineers always be required for final proof of effectiveness, but it
need equipment to make the products that will function can often be prohibitively expensive to evaluate several
cost-effectively without premature failure from wear; different alternative materials or design variations by
and those marketing the products need to know that they this method. It also suffers from a lack of control of
will have good tribological performance. Other tests are the parameters that define the wear processes that are
performed for more fundamental reasons to investigate occurring, so that an understanding of the dependence
wear mechanisms or material performance, perhaps as of wear mechanisms on important factors such as load
part of a program of material development. and speed cannot be developed.
Friction and wear are important in most industrial As Fig. 13.4 demonstrates, it is possible to break
sectors, but there is now also increasing attention paid to down the complete product into its functional elements.
the application of tribological testing to societal needs. The final stage of this process of abstraction is to use
Traditionally, the evaluation of friction and wear has laboratory tests that simulate a particular tribological
been of major concern in areas such as mining and element of the complete product. Moving away from
drilling, transport power plant, power generation and field trials towards laboratory testing, in general, re-
machine tools. Now there is increasing attention to ar- duces the costs of testing and increases the ability to
eas such as the tribological performance of biomedical control the important factors that affect the magnitude
implants and the degradation of decorative finishes. In of friction and wear. This enables programs of testing
recent years there has also been much attention given to to be designed that allow for the cost-effective devel-
the design and development of microelectromechanical opment of materials and tribological systems through
systems (MEMS) where friction and wear are strongly better understanding of the tribological processes.
influenced by the scale of these small components, There are two main situations for which laboratory
hampering the development of new devices. tests are required. The first is to provide information on
690 Part D Measurement Methods for Materials Performance

Fig. 13.4
Increasing Increasing
Category Type of tests Symbol cost complexity Schematic illus-
tration of types
Machinery of tribological
I
field tests test [13.3]

Machinery
Part D 13.2

II
bench tests

Systems
III
bench tests

Components
IV
bench tests

V Model tests

VI Laboratory tests Increasing Increasing

control flexibility

the tribological performance of materials when they are As it is often difficult to achieve an exact simula-
being developed or selected for a general field of applica- tion in the laboratory, it is always important to check
tion, but when the specific conditions that will be applied that the same mechanisms of wear occur in the labora-
are not well-defined. In this case there is little informa- tory test as in the real application. When this is achieved,
tion to guide the specification of the test parameters to the laboratory test is likely to give results that will reli-
be used in any test program, and indeed the specific test ably predict performance in the application. Figure 13.5
that should be used is not always clear. However, it is shows an example of such a comparison, from tests per-
important to consider carefully how the materials will formed to simulate the abrasion of cemented carbide
be used in order to eliminate inappropriate tests that will extrusion tools used to form ceramic roof tiles. The fea-
only lead to misleading results. Thus, for materials that tures of the worn surfaces of the cermets were very
are being developed for applications where abrasion is similar in both laboratory tests and the samples recov-
extremely likely to occur, sliding wear tests cannot be ered from the practical application, and subsequent field
expected to give relevant data. Nevertheless, so long as trials showed an excellent match between the results of
the likely mode of wear can be predicted, useful infor- the laboratory tests and the lifetimes of the components
mation can be obtained on the likely performance of the in service.
materials in practice.
The second situation is where there is a requirement 13.2.2 Test Parameters
to simulate a specific application. Here the conditions
that are operating in the tribological contact should be Many parameters control and define the conditions in
better defined, so that the design of a program of lab- a tribological contact, and hence influence the resulting
oratory wear tests reduces to choosing the appropriate wear and friction. Important examples are listed in Ta-
test and test parameters to achieve the correct simulation ble 13.1. Some of these parameters are only applicable
of the practical application. Although in some cases the to specific types of wear; for example the shapes of abra-
best approach is to design and manufacture a specific rig sive or erodent particles are relevant only to the processes
that is intended to reproduce the conditions in the ap- of abrasion or erosion.
plication precisely, in most cases a well-established test A full description of the effects of these test pa-
can be used, perhaps with some adjustment of the test rameters is well beyond the scope of this chapter. The
parameters, to provide a reasonable match with the ap- reader should refer to other sources for a fuller descrip-
plication. The selection of these parameters is the key to tion [13.4, 5], but some aspects are discussed briefly
generating a valid test. here.
 Friction and Wear 13.2 Selection of Friction and Wear Tests 691

Table 13.1 List of some important parameters that influence tribological processes
Parameter Units or type of information
Normal load N
Sliding speed m/s
Materials of triboelements All aspects of composition and microstructure, at surface as well as sub-
surface
Test environment (including any lubricant present) Composition, chemical and physical properties of gas and/or liquid sur-

Part D 13.2
rounding tribological contact
Temperature ◦C

Type of relative motion between surfaces Trajectory: sliding, rolling, impact etc.
Contact geometry (form) Shapes of surfaces
Contact geometry (local) Roughnesses of surfaces
Abrasive/ erosive particles Material, shape (and distribution), size (and distribution)
Contact dynamics Stiffness, damping, inertial mass

In laboratory testing it is normal to positively control humidity), the bulk temperature of the samples, and the
only a few test parameters such as the applied normal mechanical dynamics of the test system. These parame-
load, relative speed and contact geometry. Other param- ters are normally held constant from one test to another,
eters that can have a major effect on the resulting wear either by the application of a specific control method-
and friction are the presence of lubrication, the envi- ology, or simply by using the same test system for the
ronment and atmosphere of the test (including the air whole sequence of tests in the program.
The contact geometry is a crucial parameter in the
design of a wear test where two bulk surfaces are in mov-
a) ing contact, as in sliding. The ideal situation, in which
the nominal contact area, and hence nominal contact
pressure, would not vary during the test, would be to
achieve self-aligning conformal contact between the test
samples. This can be achieved in some cases by care-
ful design of the samples and their holders, but often it
cannot be achieved, and then high initial contact stresses
occur at the edges of the misaligned contacts. To avoid
this uncontrolled nonconformal geometry, it is common
to use a controlled nonconformal geometry with one tri-
boelement in the form of a ball or spherically-ended
pin loaded against a flat sample. A ball diameter of
about 10 mm is typically used. The disadvantage of this
b) approach is that although the initial contact geometry
is well controlled, the initial contact stresses are high.
The ball or pin then wears rapidly until a larger con-
tact area is produced, resulting in a substantially lower
contact pressure. This change may lead to a change in
wear mechanism from the initial high-stress regime to
a subsequent low-stress, larger contact area regime. An
approach that reduces this effect is to use one triboele-

Fig. 13.5a,b Example of comparison of worn surfaces from

a laboratory simulation and a practical application. The
SEM images show WC/Co tools used to manufacture con-
crete tiles: (a) worn surface from production tool; (b) worn
surface of sample after laboratory wear testing
692 Part D Measurement Methods for Materials Performance

ment with a large contact radius so that although the material lost. There is positive synergy in most cases,
initial contact geometry is still well defined, the initial so that the mass loss is greater than the sum of the con-
contact pressure is lower, and changes in mechanism are tributions expected from pure corrosion or pure wear
less likely. alone. Exceptionally, in negative synergy, corrosion can
The temperatures of the contacting triboelements form a mechanically stronger layer on the surface which
can affect the tribological behavior through changes in acts to protect the surface and thus reduce the total wear
their mechanical and chemical properties. As well as rate. Tribochemical reactions of materials may also oc-
Part D 13.2

these bulk effects, the areas of the samples in contact cur; ceramics, for example, can react with water vapor
become hotter through frictional heating; the power dis- to form hydroxides that reduce friction and protect the
sipated in the contact is given by μNv where μ is the wearing surface.
friction coefficient, N is the normal load, and v is the When metals are heated, either through a bulk
relative sliding velocity. The local temperatures at the increase in temperature or by frictional heating, consid-
contact areas can therefore become much higher than the erable oxidation can occur. At first this may reduce wear,
bulk temperatures [13.6]. This factor needs to be taken as seen in the mild wear of steels, but as the temperature
into account when designing wear tests or interpreting increases further, the production of weak oxide may in-
test results. crease to such an extent that the wear rate increases once
As outlined in Sect. 13.1.3, lubrication of the con- more [13.4].
tacts will also affect the wear, often in terms of In many tribological systems, the surfaces of the
mechanism as well as amount. Under certain condi- contacting materials are subjected to alternating stresses.
tions of sliding speed, load and lubricant viscosity, These stresses may be high enough to initiate and grow
the two surfaces become separated by a continuous, fatigue cracks in the material that may contribute to the
hydrodynamically-generated lubricant film, and the material loss caused by other wear processes. Rolling
wear rate can fall to a very low level, as indicated contact fatigue is an important degradation mechanism
in Fig. 13.3. Hydrodynamic lubrication (a thick lubricant in certain applications of materials, but lies outside the
film) is favored by low normal pressure, high lubricant scope of this chapter.
viscosity, and high sliding speed. For conditions where
the lubricant film thickness becomes comparable with 13.2.4 Experimental Planning
the roughness of the triboelements, elastic deformation and Presentation of Results
of these bodies becomes important and the pressure in
the lubricant is high enough to cause a significant local When planning a program of tribological tests, the
increase in viscosity; this is the regime of elastohydro- choice of test conditions must be determined, either
dynamic lubrication (EHL). For even thinner films (at by the relevant engineering application, or in order to
low speeds or high contact pressure), the system enters achieve the appropriate mechanism of wear in more
the regime of boundary lubrication, where appreciable fundamental studies. In the development of materials,
interaction occurs between asperities on the two sur- a range of conditions can be chosen to represent those
faces. Here the chemical composition of the lubricant likely to occur during actual use of the material.
has a major effect on the nature and properties of the There are many test factors that need to be con-
chemically formed surface films and on the wear that trolled during a test. These can be grouped into those
occurs. concerned with the mechanical test conditions (such as
contact load or pressure, speed, motion type and test
13.2.3 Interaction with Other Degradation environment), and sample parameters (such as material
Mechanisms composition, microstructure and the initial surface fin-
ish of the samples). A full program of testing under all
Other degradation mechanisms such as chemical reac- combinations of these factors would be time-consuming
tion and fatigue can also make a major contribution to and costly, and may not be required. Often a single fac-
the overall loss of material from moving contacting sur- tor can be identified as “key” to the material response,
faces. Interactions often occur with the test environment and in this case a good approach is to set all the other
through chemical reactions of the test materials. In wear factors at constant values and vary the chosen factor
corrosion, the triboelements are exposed to an aqueous in a controlled way in a series of tests. This approach
medium, and corrosion of the materials acts in conjunc- is termed a parametric study. More complex statistical,
tion with mechanical processes to alter the amount of experimental design procedures can be used, but care
 Friction and Wear 13.3 Tribological Test Methods 693

needs to be taken in the design of experiments to ac- tions in friction and wear behavior as sudden changes
count for the variability that is normally observed in the in wear rate or friction coefficient. The mapping proce-
results of wear tests. dure is a very efficient way of determining the overall
Mapping techniques can be used where two (or behavior of a material because it provides useful infor-
more) factors are changed in a controlled way (nor- mation about the position of transitions in wear behavior
mally more coarsely than in parametric studies), with in a systematic and controlled way. This comes at the
the friction and wear results plotted either as individ- expense of a reduction in the detailed knowledge of the

Part D 13.3
ual points or as contours. Regions on the map are then variation of friction and wear with any one factor, but
delineated on a mechanistic basis, with the region bound- once the regime of interest is better defined through the
aries either defined by identifying the wear mechanism use of mapping studies, then a more detailed parametric
by microstructural techniques, or by identifying transi- study can be conducted.

13.3 Tribological Test Methods

This section gives some information on the tribological
tests that are in common usage, and which have in many
cases been standardized, particularly through the Amer-
ican Society for Testing and Materials (ASTM). Due
to constraints on space, only a brief description can be
given. Further information can be obtained from other
sources [13.5,7–12]. The main test parameters, the mea-
surements that can be made, and the standards associated Pin on disc Pin on ring
with each type of test are listed.

13.3.1 Sliding Motion

Wear tests for materials in sliding contact are character-

ized by continuous contact between the two samples in Pin on flat Crossed cylinder Thrust washer
tribological contact. There are two distinctly different
types of test. Fig. 13.6 Examples of test geometries used for tribological
In many tests, the extent of relative movement is tests involving sliding motion
sufficiently large (typically greater than 300 μm) so that
all contact points on at least one of the bodies are out of the test system can be similar in magnitude to the re-
of contact for some of the test period. This is in contrast quired sample motion, leading to difficulties controlling
to the case of fretting, where at least part of the contact and measuring sample displacement.
area on both samples is always in contact. In fretting Often balls are used instead of pins, and in some
tests, the debris is trapped at the interface between the cases blocks (with large contact areas) are also used. The
two samples, leading to different behavior from that in choice of a conformal or nonconformal contact geometry
tests involving larger extents of movement, in which the and the frictional generation of heat are factors that need
debris is more readily lost from the contact region. to be carefully considered.
Common tribological test geometries involving slid-
ing motion are shown in Fig. 13.6. Pin-on-disc and 13.3.2 Rolling Motion
reciprocating geometries are used most often, but pin-
on-disc testing with continuous rotary motion is often Tribological contacts often involve rolling motion. Suit-
not appropriate since many applications involve recipro- able test geometries are shown in Fig. 13.7. If the
cating motion. This difference is important, as it affects application is concerned with bearing races, then this
the retention of debris at the wear interface and the stress is one situation where laboratory tests can be carried out
history on the surfaces. Fretting tests are always car- relatively easily on the components themselves. This is
ried out with a reciprocating geometry; particular care clearly appropriate for roller bearing applications, but
is needed in apparatus design as the elastic deformation the complexity of the test geometry means that it is of-
694 Part D Measurement Methods for Materials Performance

13.3.3 Abrasion
F
Abrasion testing can be carried out with a range of tests,
F as shown in Fig. 13.8. The simplest type of testing is
carried out by loading a sample pin against abrasive-
coated paper supported on a solid backing. As well as
using a rotating abrasive-covered disc, tests can also be
Part D 13.3

carried out with abrasive paper held on to a drum, or

with a belt of abrasive paper. An important issue with
these tests is that, as abrasion of the test sample takes
Four ball test Balls in bearing race Roller on roller place, the abrasive paper degrades by clogging with wear
debris and due to damage to the contacting regions of
Fig. 13.7 Examples of test geometries used for tribological the abrasive particles, so that the wear of the test sample
tests involving rolling motion will typically decrease as the test progresses; in extreme
cases the wear mechanism may change as sliding then
ten difficult to develop a fundamental understanding of occurs predominantly against debris rather than against
the wear and friction processes that occur. the abrasive particles. The use of a spiral track on the
The four-ball test has been popularly used for many abrasive paper can achieve a steady process of wear by
years to test, in particular, the maximum load and speed ensuring abrasion against fresh particles.
for failure at the contact points in this test geometry, from In another common type of abrasion test, a sam-
mechanisms such as scuffing and seizure. However, re- ple is pressed against a rotating wheel in the presence
sults from this test, which can involve very high contact of a continuous supply of free abrasive particles. An
pressures and frictional power dissipation, are not read- important parameter in these tests is the stiffness of
ily extended to practical applications, and the method the wheel or other counterbody. Tests can also be car-
also suffers from a lack of control of the detailed motion ried out dry or in fluids of different types, allowing
of the balls during the test. tests to be made in the presence of corrosive media.
In two-roller testing, on the other hand, there is in- Some tests with rotating wheels involve the recir-
dependent control of the motion of the rollers, and it is culation of abrasive during a single test (ASTM B
possible to adjust the amount of relative sliding between 611 [13.8]), while in others a continuous stream of
them as well as the rolling speed. The cylindrical edge abrasive passes through the wear interface and is not
of the rollers sometimes has a flat cross-section, but is reused (such as the dry sand rubber wheel abrasion test,
also commonly crowned. ASTM G 65 [13.8]).

F
F

Pin on rotating Plate against rotating Block on a moving plate

abrasive disc wheel with feed of abrasive in a bath of slurry
F

Drive shaft Abrasive

slurry

Pin on rotating abrasive Wear of pin in rotating Micro-scale abrasion test

disc with spiral track pot of slurry with rotating ball on plate
in presence of abrasive Fig. 13.8 Examples of test geometries
involving abrasion
 Friction and Wear 13.3 Tribological Test Methods 695

A test that has received some prominence recently a)

is the microscale abrasion test, in which a ball is rotated
against a flat sample in the presence of abrasive. This test
is particularly suited to determining the abrasion rate of
a coating.

13.3.4 Erosion by Solid Particles Gas in

Part D 13.3
b)
There are two main groups of particulate erosion tests, in
which the particles either strike the sample in a gas (usu-
ally air), or are transported in a liquid, which is usually
termed slurry erosion. Examples are shown in Fig. 13.9.
In the centrifugal accelerator, the erodent particles
are accelerated along radial tubes in a rotor so that
a stream of erodent particles emerges at some angle
to the rotor periphery (which depends on the detailed
design) to strike one or more samples located in a ring c) Ejector
around the rotor. This test has the advantage that sev- Test chamber
Pressure
eral different samples can be tested simultaneously, but Specimen gauge
there is usually some uncertainty in the angle with
which the particles strike the samples, and only a frac- Sand bed
tion of the particles fed into the apparatus actually
strike the samples. Some designs of centrifugal ac-
celerator impart significant rotation to the particles,
Flow
which may influence the resulting erosion rate. In the Funnel gauge
gas-blast test a compressed gas stream (usually air) ac-
celerates the erodent particles along a nozzle towards
the sample, which is held at a controlled angle to the
Test
stream. In this case only one sample can be tested at solution
a time. Pump
In slurry erosion testing a pump can be used to gen-
erate liquid flow through a nozzle. Erodent particles are Fig. 13.9a–c Examples of tests involving erosion by solid
incorporated into the jet, being either picked up through particles: (a) gas-blast test; (b) centrifugal accelerator;
a venturi system or suspended in the erodent slurry. The (c) fluid jet rig
resulting jet of slurry is directed against the test sample
held at a controlled angle. cracked, or the coating delaminates from the sub-
In all erosion tests the particle impact speed and the strate.
angle of impingement of the erodent stream against the When the test was developed, the adhesion of
sample are the most important variables. typical engineering coatings was quite poor and the
scratch test provided reasonable results, representative
13.3.5 Scratch Testing of the adhesion of the coating. More recent coatings
are much more adherent, and although the scratch test
Scratch testing was originally developed to evaluate remains an extremely valuable test for coatings, the
the adhesion of coatings. In the scratch test an in- results are more difficult to interpret, giving informa-
denter of well-defined geometry is pressed onto and tion on the complex response of the coating–substrate
moves relative to a sample under a fixed or increas- composite system to the movement of the inden-
ing normal load. The tangential force resisting the ter.
motion (usually described as the friction force) and In the context of wear testing, scratch testing can be
acoustic emission from the sample are often both used as a model abrasion test; the response of a material
measured continually. The acoustic emission signal to the scratching of the indenter is treated as an analog
typically increases sharply when a brittle coating is of its response to a hard asperity or abrasive particle.
696 Part D Measurement Methods for Materials Performance

Parameters such as the frictional force generated and can be made both in the same position or intersecting
the scratch width can be measured. Multiple scratches one another to extend the model range.

13.4 Friction Measurement

Part D 13.4

As discussed in Sect. 13.1.1, the measurement of fric- is determined from the combination of these individ-
tion involves the measurement of the friction force. ual measurements of strain. Each elastic element is
For a rotating component it may be useful to define designed to measure the force acting in a particular
the friction torque, the measurement of which also in- direction, and not to be affected by other components
volves a force measurement combined with a length such as side loads. The material used for the elastic el-
measurement. ements is usually tool steel, stainless steel, aluminium
or beryllium-copper. The material should exhibit a lin-
13.4.1 Friction Force Measurement ear relationship between the stress (proportional to the
force applied) and the strain (output), with low hysteresis
A friction force measurement system is made up of one and creep in the working range. A further requirement
or more force transducers mounted between one of the is a high repeatability between force cycles, with no
triboelements and the base frame of the tribometer, and fatigue effects.
their associated instrumentation. A practical force trans- The most common materials used for strain gauges
ducer usually consists of a chain of several transducers. are copper-nickel, nickel-chromium, nickel-chromium-
For example, the force may act upon and bend a metal molybdenum and platinum-tungsten alloys. The foil
beam, and the bending then alters the electrical resis- strain gauge is the most widely-used type because it
tance of a strain gauge bonded to the surface of the has significant advantages over the other types and
beam by an amount proportional to the force. For many is employed in the majority of precision load cells.
types of force measurement system, the term “load cell” A foil strain gauge consists of a metal foil pattern sup-
is commonly used in place of “force transducer”. ported on an electrically insulating backing of epoxy,
Most force transducers employ some type of elas- polyimide and glass-reinforced epoxy phenolic resin.
tic load-bearing element or combination of elements. It is constructed by bonding a sheet of thin rolled
Application of force to the elastic element causes it metal foil, 2–5 μm thick, on a backing sheet 10–30 μm
to deflect, and this deflection is then sensed by a sec- thick. The measuring grid pattern including the ter-
ondary transducer which converts it to an output. The minal tabs is produced by photoetching, in a process
output may be in the form of an electrical signal (as similar to that used in the production of printed circuit
from a strain gauge or LVDT, a linear variable differ- boards.
ential transformer), or a mechanical indication (as in Semiconductor strain gauges are manufactured from
proving rings and spring balances). However, the most n-type or p-type silicon. The output from a semiconduc-
common method is for longitudinal and lateral strains to tor gauge is about 40 to 50 times greater than that from
be sensed, and when this is done by electrical resistance a metal foil gauge. While the output is not linear with
strain gauges the transducer is known as a “strain gauge strain, they do exhibit essentially no creep or hystere-
load cell”. sis and have an extremely long fatigue life. Because of
their high temperature sensitivity, careful matching of
13.4.2 Strain Gauge Load Cells the gauges and a high level of temperature compensation
and Instrumentation is required.
Thin film strain gauges are produced by sputtering
A strain gauge load cell is based on an elastic ele- or evaporation of thin films of metals onto an elastic
ment to which a number of electrical resistance gauges element. Wire strain gauges are used mainly for high-
are bonded. The geometric shape and elastic modulus temperature transducers. The wire is typically 20 to
of the element determines the magnitude and distri- 30 μm in diameter and is bonded to the elastic element
bution of the strain field produced by the force to with ceramic material.
be measured. Each strain gauge responds to the local The rated capacities of strain gauge load cells range
strain at its location, and the measurement of force from 0.1 N to 50 MN, at a typical total uncertainty of
 Friction and Wear 13.4 Friction Measurement 697

Fig. 13.10
Full-bridge strain gauge circuit
Example of
Strain gauge Strain gauge strain gauges
(stressed) (stressed)
connected into
a full-bridge
Cantilever beam circuit
V

Part D 13.4
1 or 2 strain gauges
(on top and/or bottom)

F
Fixation Strain gauge Strain gauge
(stressed) (stressed)

0.02% to 1% of full scale. The range of capacity depends a stiff sensor and the advantage of a high-frequency
on the type of gauges, with thin film gauges having the response in the measuring system without introducing
highest sensitivity (typically 0.1 N to 100 N), followed geometric changes to the force measuring path. To en-
by semiconductor gauges (1 N to 10 kN), and foil gauges able measurement of both tension and compression,
(5 N to 50 MN). piezoelectric force sensors are usually pretensioned by
The response of a load cell can be maximized a bolt.
using one or more strain gauges aligned to respond A typical piezoelectric transducer deflects by only
to a longitudinal strain and another set aligned ei- 0.001 mm under a force of 10 kN. The high frequency
ther to a longitudinal strain of the opposite sign or to response, typically up to 100 kHz due to the high stiff-
the transverse strain. When connected electrically in ness, makes piezoelectric crystal sensors very suitable
a Wheatstone bridge configuration, this has the addi- for dynamic measurements. The typical range of rated
tional advantage of minimizing temperature effects that capacities for piezoelectric crystal force transducers is
act equally on all gauges. The resistance change is de- from a few N to 100 MN, with a typical total uncertainty
tected by measuring the differential voltage across the of 0.3% to 1% of full scale. While piezoelectric trans-
bridge (see for example Fig. 13.10). The load cell forms ducers are ideally suited to dynamic measurements, they
part of the measurement chain and requires an a.c or cannot perform truly static measurements because there
d.c. excitation voltage to be supplied, and amplification is a small leakage of charge inherent in the charge am-
and conditioning of the output signal, before it can be plifier, which causes a drift of the output voltage even
used. The whole chain must be incorporated into the with a constant applied force.
calibration procedure. Piezoelectric sensors are suitable for measurements
in laboratories as well as in industrial settings because
13.4.3 Piezoelectric Load Sensors of their small dimensions, the very wide measuring
range, rugged packaging, and their insensitivity to over-
Another commonly used type of force measurement load (typically by > 100% of full scale). They can
transducer is based on the piezoelectric phenomenon operate over a wide temperature range (up to 350 ◦ C),
exhibited by certain crystalline materials, in which an and can be packed to form multicomponent transduc-
electric field is generated within a crystal which is ers (dynamometers) to measure forces in two or three
proportional to the applied stress. To make use of the orthogonal directions.
device, a charge amplifier is required to provide an
output voltage signal that is proportional to the ap- 13.4.4 Other Force Transducers
plied force and large enough to measure. Piezoelectric
crystal sensors are different from most other sensing Interesting but less commonly-used examples of force
techniques in that they are active sensing elements. No measurement transducers are the “vibrating-wire trans-
power supply is needed for the sensor (although it will ducer” and the “gyroscopic load cell”. The first uses
be for the amplifier) and the mechanical deformation a taut ferromagnetic wire which is excited to resonate in
needed to generate the signal is very small, which gives transverse vibration. The resonant frequency is a mea-
698 Part D Measurement Methods for Materials Performance

Friction force signal (arb. units)

13.4.5 Sampling and Digitization Errors
Friction force signal
TiO2 400 °C
In the case of a strain gauge load cell, the output sig-
1 m/s 10 N nal from the whole force measurement chain depends
on the mechanical properties of the elastic element to
which the strain gauges are bonded and on the elec-
tronic circuitry used to convert the resistance changes in
Part D 13.4

the strain gauges (caused by the bending force applied

to the elastic element) into a recordable electrical signal
proportional to the force. Similar considerations apply
to other types of load cell. The design of the elastic ele-
ment defines the resonance frequency of the load cell. If
0 50 100 150 200 250 300 350
Time (ms)
the dynamic properties of the force to be measured ex-
cite this natural frequency, then the output signal will no
Fig. 13.11 Friction force signal from a load cell recorded with high longer be proportional to the applied force.
time resolution, showing vibration at its resonant frequency super- Figure 13.11 shows the output signal from a strain
imposed on the more slowly varying friction force gauge load cell, recorded with a high sampling rate; the
friction force was generated between a pair of titania
sure of the wire’s tension and hence the applied force at specimens sliding at 1 m/s at a temperature of 400 ◦ C.
that instant. The advantage is its direct frequency output, The dominant component of the alternating signal oc-
which can be handled by digital circuitry, eliminating the curs at the natural resonant frequency of the load cell,
need for analog-to-digital conversion. due to vibration induced by the friction between the
Gyroscopic load cells exploit the force-sensitive triboelements. Derivation of a friction force from such
property of a gyroscope mounted in a gimbal or frame a signal will clearly lead to error. Such effects will still
system. The force to be measured is applied to the lower occur but not be obvious if the sampling rate is too low or
swivel and a couple is produced on the inner frame, caus- if the electronic circuitry used has a long time constant,
ing the gimbals to precess. The time taken for the outer integrating over the rapid signal variations. Recording
gimbal to complete one revolution is then a measure of the output from a load cell with an analog chart recorder
the applied force. The gyroscopic load cell is essentially usually filters out such fast signal changes and integrates
a rapidly responding digital transducer and is inherently them.
free of hysteresis and drift. Another example of a dynamic friction signal is pre-
sented in Fig. 13.12. The friction signal, measured with
a piezoelectric sensor, for one sliding cycle of a silicon
Friction force (N)
15
nitride couple exhibits different extents of variation for
the two half-cycles, indicating that the sliding couple
10 under these test conditions tends to stick/slip motion.
The difference in behavior is caused by the difference
5 in the stiffness of the construction elements in the two
0 directions. For such a system it is not possible to de-
fine a quantitative friction level because the measured
-5 signals are dominated by the vibrations of the elastic
sensor element and are thus not proportional to the fric-
-10 tion forces. Results of this type only allow the tendency
-15 to stick/slip motion to be deduced.
–5 –4 –3 –2 –1 0 1 2 3 4 5
Material Position (mm) Test-No.: LB1504_A
Si3N4/Si2N4 r.h.: 50 %
13.4.6 Calibration

Fig. 13.12 Friction force signal for one sliding cycle of a silicon Even with good transducers and a good overall sys-
nitride couple in reciprocating sliding, showing asymmetric behavior tem design, measurements of forces cannot be relied
due to differences in the stiffness of the system in the two sliding upon without calibration. Instruments capable of per-
directions [13.13] (r.h. = relative humidity) forming force calibrations are known as “force standard
 Friction and Wear 13.4 Friction Measurement 699

Table 13.2 Types of force standard machine

Type Principle Uncertainty attainable Category
Deadweight A known mass is suspended in the Earth’s gravita- < 0.001% Primary or secondary
machines tional field and generates a force on the support
Hydraulic ampli- A small deadweight machine applies a force to < 0.02% Secondary
fication machines a piston-cylinder assembly and the pressure thus gen-
erated is applied to a larger piston-cylinder assembly

Part D 13.4
Lever amplifica- A small deadweight machine with a set of levers < 0.02% Secondary
tion machines which amplify the force
Strain-gauged hy- The force applied to an instrument is reacted against < 0.05% Secondary
draulic machines strain-gauged columns in the machine’s framework
Reference force A force transfer standard is placed in series with the < 0.05% Secondary
transducer instrument to be calibrated
machines

machines”. Primary standards in force measurement are ement (Fig. 13.13). The bending of the cantilever is
machines whose uncertainty can be verified directly sensed by a segmented photodiode which measures the
through physical principles to the fundamental base units intensity of laser light reflected from the back of the
of mass, length and time. Secondary standards can be cantilever. The piezoelectric driving voltage is adjusted
compared with primary standards through the use of during the scan over the surface to hold the resulting dif-
a force transfer standard, which is a calibrated force ference in photocurrent between segments (1 + 2) and
transducer, frequently a strain gauge load cell. Stan- segments (3 + 4) constant. A constant photocurrent im-
dards document ISO 376 describes the calibration and plies constant beam deflection, which is equivalent to
classification of transfer standards. a constant force. The piezoelectric voltage needed to
Table 13.2 lists types of force standard machines that hold the photocurrent constant is thus a measure of the
have been developed for the calibration of static forces topographical height at the instantaneous position of the
acting along a single well-defined axis. The principles tip. The height resolution of an AFM is in the range of
used for the calibration of multicomponent force sensors atomic dimensions.
remain the same, but cross-talk between the different If the direction of scanning is perpendicular to the
axes must also be considered. For the measurement of axis of the cantilever, the torsion of the cantilever pro-
dynamic forces it is assumed that the statically-derived vides a measure of the friction force between the tip and
force transducer sensitivities are applicable, but atten- the sample. The torsion is measured by the difference
tion must also be paid to the natural frequencies of the between the photocurrents from segments (1 + 3) and
load cell. segments (2 + 4). The calibration of the relevant spring
The calibration of a force transducer can be per- constants of the cantilever (in bending and torsion) is
formed with the transducer in its permanently installed not a trivial procedure [13.16–18].
position by using a transfer standard, or prior to its instal-
lation and by removal as required for further calibration.
More information on force measurement and cali- Segmented
bration can be found elsewhere [13.14, 15]. photodiode
r
se
For very small forces, as measured with an atomic 1 2 Piezo La
force microscope (AFM), special calibration procedures
are needed. An AFM can be used to measure fric-
tion forces in the nN range. The calibration methods 3 4
used for this force-measuring machine also use known
masses under gravity, but the lowest attainable uncer- Cantilever
tainty is much greater than that described above for
macroscopic forces. The AFM scans a probe tip over AFM tip
the surface under examination. It operates like a sty- Sample surface
lus profilometer with constant contact force, which is
maintained by steering the bending of the cantilever Fig. 13.13 Principle of friction measurement with an AFM
beam carrying the probe tip using a piezoelectric el- (atomic force microscope)
700 Part D Measurement Methods for Materials Performance

Fig. 13.14 Example of online measurement of linear wear

Normal force (N) / Friction force (N) /
Total displacement (ìm) (total displacement of ball relative to disc) and friction force
15 for continuous sliding of an alumina ceramic ball on an
alumina disc 
Normal force (N)
10 Table 13.4 Tribological tests involving rolling motion
Total displacement (ìm)
Test Parameters
5
Part D 13.4

Friction force (N)

Materials composition and structure
0 Load
Disc: Al2O3 Relative slip of rolling elements
Ball: Al2O3
–5 Velocity: 0.1 m/s Rolling speed
FN: 10 N
r.H.: 50 %
Temperature
–10 Surface condition
0 2000 4000 6000 8000 10 000 Environment
Time (s)
Lubrication
Measurements
Table 13.3 Tribological tests involving sliding motion Direct change of dimension
Test Parameters Volume (usually via weight loss and density)
Materials composition and microstructure Profilometry
Load Torque
Speed Continuous measurement of wear during test
Temperature Examination of worn surface
Surface condition Examination of wear debris
Environment
Lubrication this technique, complementary images of the friction
Contact time and interval between loading of contact areas on force can be obtained by simultaneously recording
two samples spatially-resolved electrical contact resistance and fric-
Measurements tion measurements. The technique provides information
Direct change of dimension about the history of friction and wear processes [13.19].
Volume (usually via weight loss and density) When measuring the electrical contact resistance, care
Profilometry must be taken that the electrical sensing current re-
Friction mains low, because otherwise the wear process may be
Continuous measurement of wear during test influenced.
Examination of worn surface
Examination of wear debris 13.4.7 Presentation of Results
Standards
ASTM G 99 In most cases the specification of a single coefficient of
ASTM G 133 friction is not adequate. This can be seen from the exam-
ISO 20808 ples in Fig. 13.12 and Fig. 13.14, depicting the evolution
of friction during individual experimental runs. At the
very least, all relevant test and system parameters should
A technique known as “triboscopy” monitors the be specified. The list in Table 13.1 provides a general
evolution of time-dependent local phenomena with guide, and is supplemented by the more specific lists of
a spatial resolution limited by contact size. With parameters in Table 13.3 and Table 13.4.
 Friction and Wear 13.5 Quantitative Assessment of Wear 701

13.5 Quantitative Assessment of Wear

13.5.1 Direct and Indirect Quantities the act of interrupting the test in order to weigh the spe-
cimen may alter the progress and even the predominant
The amount of wear can be specified in terms of direct mechanism of wear.
or indirect quantities. The sensitivity of this method of wear quantification
Indirect quantities are often used in technical as- is relatively low. The method is applicable only for wear

Part D 13.5
sessments of the lives of machinery and in practical rates high enough that a significant mass loss (typically
engineering. Examples include: of the order of at least 1 mg) is reached after a reason-
able time for a test sample not heavier than about 0.5 kg.
• wear-limited service life (used for cutting tools for
For materials which experience significant mass changes
example: h, d, number of parts)
from other causes, such as the absorption or loss of wa-
• wear-limited throughput (used for the flow of abra-
ter by certain polymers, or the oxidation of metals at
sive materials or objects through pipelines for
high temperatures, special care must be taken to ensure
instance: number of parts, m3 , kg)
that the mass changes measured are genuinely associ-
Direct wear quantities specify the change in mass, geo- ated with tribological processes and are not due to other
metrical dimensions or volume of the wearing body. phenomena. Suitable control specimens, not subjected
Examples include: to wear but otherwise exposed to the same conditions as
the triboelements, may be helpful for eliminating such
• wear amount:
effects and also for correcting for any long-term drift in
– mass loss (kg)
the calibration of the balance.
– linear dimensional change (m)
The accuracy of weighing may be limited by the ac-
– volume loss (m3 )
curacy or sensitivity of the balance (especially in the
• wear resistance = 1/(wear amount) (m−1 , m−3 ,
case of heavy triboelements), by changes in humidity
kg−1 )
between the two weighings, or by particle or debris at-
• wear rate = (wear amount)/(sliding distance or time)
tachment or detachment. Material transfer and oxidation
(m/m, m3 /m, kg/m, m/s, m3 /s, kg/s)
during the wear process often complicate the interpre-
• wear coefficient, or specific (or normalized) wear
tation of the measured mass values and can sometimes
rate (also sometimes called wear factor) = (wear
lead to erroneous interpretation of the wear behavior.
rate)/(normal force) (m3 N−1 m−1 ).
Wear is sometimes expressed in units of volume, and
The primary measurement from which these quantities in order to convert mass loss to volume loss, the density
are derived is usually mass loss, dimensional change or of the worn material must be known. For a homogeneous
volume loss, although other methods can also be used bulk material this will usually pose no problems, but if
(see Sect. 13.5.5). wear is occurring from a coating or treated surface layer
then the relevant density may not be accurately known.
13.5.2 Mass Loss
13.5.3 Dimensional Change
The loss of substance from the surface of a triboelement
can be determined by weighing it before and after wear. Changes in linear dimensions due to wear are frequently
Continuous sensing of the wear process in terms of mass measured on-line (continuously) during friction tests.
loss is usually not possible. Mass loss measurement at This has advantages over making measurements by in-
defined intervals, to obtain information about the devel- terrupting the test, because one can then get information
opment of wear (and hence to investigate running-in, about the continuous evolution of wear during the test
stability of wear rate, and so on), require the test to be and, by detecting transitions in the measured linear wear
halted and the triboelement(s) removed for weighing. In rate, about changes in the dominant wear mechanism.
this process the danger exists that the microcontact ge- In most cases a change in the distance between the
ometry will be changed on reassembly. Such a change mounting fixtures of the two triboelements is measured.
in microgeometry can influence the wear rate. Debris This implies that only the sum of the linear wear con-
present in the contact region is also likely to be dis- tributions from both elements can be obtained. Material
turbed, and often removed. The most important period transfer from one triboelement to the other may lead to
in the development of the wear rate may be missed, and misinterpretation. For example, in the frequently used
702 Part D Measurement Methods for Materials Performance

test arrangement with a ball or pin sliding continuously For on-line measurements of the changes in linear
on a disc surface, transfer of material from the disc to the dimensions or displacements, inductive or capacitative
ball will change the distance between them. Depending sensors are frequently used. Inductive sensors can attain
on the amount of wear of the disc or the ratio of wear a resolution down to 1 μm, and capacitative sensors can
to transfer, the measured distance can be reduced, elim- reach a resolution in the nanometer range. Both types
inated or increased. This last case may occur if much of sensors and their associated electronic circuits ex-
of the material worn from the larger wear scar on the hibit some temperature drift and a limited bandwidth
Part D 13.5

disc is transferred to the smaller wear area on the ball (frequency range over which the defined specification
or pin. Figure 13.14 shows an example of such behavior of repeatability, resolution and accuracy is achieved).
for an alumina ball sliding on an alumina disc. For up to However, in most cases, the dimensional changes due to
about 3500 s of sliding, the displacement of the sample temperature variations in the test samples, their fixtures,
increases steadily, with the system apparently showing and the mechanical construction of the apparatus in-
“negative” wear. The reason for this is that wear debris duce greater measurement errors than those of the sensor
is accumulating in the contact region. The agglomerated system itself. Capacitative displacement sensors require
particles of worn material then suddenly detach, leading a relatively clean environment: dirt, dust, water, oil or
to a negative total displacement, which is then followed other dielectric media in the measuring gap will influ-
by further accumulation of debris. The friction force also ence the measurement signal. Inductive sensor systems
changes, which is associated with a change in the nature are generally less expensive and also less sensitive to
of the surface interaction. environmental influences.
In some cases wear of one or both triboelements
leads to a significant change in the local geometry 13.5.4 Volume Loss
of the contact. An example of this behavior is shown
in Fig. 13.15. As wear occurs on the steel ball the contact The volume loss from a specimen in a tribological test
area progressively increases, and the wear rate detected can be derived from the mass loss if the density of the
in terms of a change in the distance between the two material is known. As indicated in Sect. 13.5.2, there
triboelements falls rapidly with time, although the as- may however be uncertainty in the density, especially in
sociated volume wear rate varies much less. In this the case of a coated or surface-treated sample. In princi-
experiment there was also a substantial change in friction ple, the volume loss can also be derived from measured
force. dimensional changes. In many cases this method will be
more accurate than weighing a sample before and after
50 testing.
Linear wear W1 (µm)

For tests such as the ball-on-disc, pin-on-disc or

block-on-ring geometries, this is particularly true for
40 W1 1
the two limiting cases when the wear occurs on only one
of the triboelements. For example, for reciprocating slid-
30 0.8
Coefficient of friction f

ing tests with the ball-on-plate configuration, relatively

simple equations for the wear volume can be deduced.
20 0.6 If wear occurs only on the ball, then the wear volume
of the ball Wv,b is given by
10 0.4  
Wv,b ≈ πda2 dp2 /64R . (13.3)
f
0 0.2 The total linear wear is given by
Wl ≈ da2 /(8R) (13.4)
0
0 0.5 1.0 and therefore
Time t (h)
Wv,b = Wv,tot ≈ π RWl2 , (13.5)
Fig. 13.15 Example of online measurement of linear wear
(total displacement of ball relative to disc Wl ) and friction where the symbols are defined below.
force ( f ) for reciprocating steel ball on a diamond-coated If wear occurs only on the plate (so the ball wear
disc (courtesy of D. Klaffke) volume is zero), then the wear volume for the plate Wv,d
 Friction and Wear 13.5 Quantitative Assessment of Wear 703

dp diameter of the wear scar on the ball, parallel to

the sliding direction,
da diameter of the wear scar on the ball, perpendic-
ular to the sliding direction,
Wq planimetric wear (plate),
Wl total linear wear,
Δx stroke of motion,

Part D 13.5
R radius of the ball.
Combining (13.4), (13.5), and (13.8a) leads to:
3/2
Wv,d = Wv,tot ≈ (4/3)Δx(2R)1/2 Wl . (13.9)

This equation allows the evolution of the volumetric

wear to be related to the evolution of the continuously
measured linear wear.
Usually, however, wear occurs on both bodies. In
Wq
this case a “rule of mixtures” can be used
3/2
Wv,tot = απ RWl2 + β(4/3)Δx(2R)1/2 Wl ,
(13.10)

where α and β describe the contribution of ball and plate

10 ìm to the wear, respectively.
500 ìm However, the values of α and β are not known be-
forehand, and can change during the test. Therefore, the
Fig. 13.16 Example of determination of wear volume for determination of the wear volume offline from the di-
a flat specimen after a sliding test with ball-on-flat geom- mensions of the wear scars is usually more accurate
etry, by measurement of the cross-section of the wear scar than online determination from the measurement of the
on the flat. The upper image shows the end of a linear change in linear dimensions during the test.
wear scar produced by reciprocating sliding, and the lower Measurements of volumetric wear from topographic
graph shows a profilometer trace across it at a representative changes determined with a stylus profilometer, laser pro-
position filometer or white light interferometer in most cases
deliver more accurate results than online measure-
is given by: ment of dimensional changes. With these methods, the
  three-dimensional profiles before and after the test are
Wv,d ≈ πda2 dp2 /64R + ΔxWq . (13.6) determined and the wear volume derived by subtract-
The planimetric wear is ing one from the other. A precondition for this method
is the ability to align the two profiles correctly. With
Wq ≈ da3 /12R (13.7) wear scars which are extended in one direction (such
as scars formed in linearly reciprocating or rotating tri-
and therefore boelements), line scans can be performed across the
  wear scars and the wear volume calculated from the
Wv,d = Wv,tot ≈ Δxda3 /12R + (πda2 dp2 /64R)
resulting two-dimensional cross-section profile and the
(13.8)
length of the wear scar (Fig. 13.16). This method may,
≈ Δxda3 /12R (13.8a) however, lead to large errors if the depth of the wear scar
is not much greater than the roughness of the surface.
since the second term in (13.8) is small compared with
If the cross-sectional area Wq of a wear scar is
the first one in the case of reciprocating sliding, where
measured by profilometry after a ball-on-flat test with
Δx is large compared with da and
reciprocating sliding, the wear volume of the ball can be
Wv,d wear volume of the plate, calculated from:
Wv,b wear volume of the ball,    
Wv,tot = Wv,d + Wv,b total wear volume, Wv,b = πda2 dp2 /64 · (1/R) − (1/R∗ ) , (13.11)
704 Part D Measurement Methods for Materials Performance

be calculated from the diameter of the wear scar and the

radius of the ball, the change in which during the test can
usually be assumed to be negligible. The diameter of the
wear scar may be measured with a stylus profilometer
or an optical microscope.

13.5.5 Other Methods

Part D 13.5

Very small amounts of wear can be determined in real

time, for example during the operation of internal com-
bustion engines, by radionuclide (or radioactive tracer)
techniques [13.20, 21]. Before starting the wear test,
a very thin layer at the surface of one or both triboele-
Fig. 13.17 Example of wear scar produced by a ball cra- ments (such as the piston ring and cylinder liner in an
tering test on a coated sample (optical micrograph); the internal combustion engine) is activated by irradiation
boundary between the titanium nitride coating and the steel with heavy ions from a particle accelerator. Since only
substrate is clearly seen. Typical wear scar diameter is a thin layer is activated, only rudimentary radiation pre-
1–2 mm cautions are required. The wear debris from the activated
triboelements during the operation of the engine are ra-
where R∗ is the radius of the wear area of the ball. This dioactive and are transported within the oil circuit to
value can be calculated from the profile of the wear scar a filter. A γ -ray detector close to the filter measures
on the flat the radiation activity, as shown in Fig. 13.18. The in-
crease in this activity with test duration is proportional
R∗ = da3 /(12Wq ) . (13.12) to the amount of wear. If the two triboelements (such as
the piston ring and cylinder liner) contain different al-
The wear volume of the flat is given by
loying elements, the activation will lead to radioactive
 
Wv,d ≈ πda2 dp2 /64 (1/R∗ ) + ΔxWq . (13.13) isotopes of these elements which emit γ -rays with dif-
ferent characteristic energies. In this case, the wear of
In the case of well-defined wear scars of known geo- each triboelement, and not only the sum of both, can be
metry, for example those produced in ball-cratering separately measured online.
abrasion tests (Fig. 13.17), the volume of wear can also Measurement of the electrical contact resistance dur-
ing a friction test between metallic triboelements, with
one contact partner insulated, can provide information
Irradiated about the formation and removal of reaction layers, such
rings as oxides. Special arrangements involving insulation and
embedded conductive wires or sensors in a test sample
can be used to detect a wear limit [13.22–24].

Gamma ray 13.5.6 Errors and Reproducibility

detector
in Wear Testing
Irradiated
rod As mentioned above in Sect. 13.4, the filtering effect of
bearings
Lead the electronic circuits used for signal recording, as well
shielding as the sampling rate in digital signal recording, may
Sump introduce errors into measurements of dynamic fric-
tion force signals. Vibrations excited at the resonant
External Heat frequency of the sensor system may be superimposed
oil pump exchanger
on the friction force signal, leading to variations in the
Fig. 13.18 Principle of wear determination in an internal combus- measured friction force which would not be present in
tion engine by the radionuclide measurement method (courtesy of the absence of the sensor system. These vibrations can
Southwest Research Institute, USA) also influence the wear rate of the system. In the case of
 Friction and Wear 13.5 Quantitative Assessment of Wear 705

a very unsteady friction signal, friction force measure-

Volumetric wear Wv (10–3mm3)
ments must be interpreted with care; it is problematic to 100
deduce a unique coefficient of friction from such tests. Fn
Use of a bandwidth that is too low in the electronic
recording system can mask the dynamic properties of 80
rel. humidity 7 %
friction forces resulting, for example, from stick/slip 100Cr6
behavior. 60 100Cr6
Äx = 160 ìm

Part D 13.5
The processes involved in sample preparation, espe-
cially cleaning of the test specimens, can have a major 40 rel. humidity 100 %
influence on friction and on the wear of triboelements.
Surface machining and polishing often involve the use
20 rel. humidity 50 %
of cutting fluids or lubricants in contact with the sample,
which must be removed by a thorough cleaning proce-
dure. Ultrasonic cleaning with a sequence of suitable 0
0 100 200 300 400 500
solvents is often necessary to obtain acceptably clean Number of cycles n (103)
surfaces.
In friction measurements with very low external Fig. 13.19 Example showing development of wear volume for
loads, “intrinsic” adhesion forces such as van der Waals a steel/steel couple with the number of sliding cycles (reciprocating),
or capillary forces must be taken into account in the for different relative humidities of the surrounding air
interpretation of measured friction force.
Material transfer during a tribological test can also 100Cr6 ball
lead to erroneous interpretation of the results, for friction against
as well as for wear. It is therefore important to analyze SiC Si3N4 SiC Si3N4
30
(10-6 mm3/Nm)
Wear coefficient

the worn areas to examine the processes which have

occurred. The example shown in Fig. 13.14 involved 22.6
transfer, agglomeration and compaction of wear par- 20
ticles, which affected not only the wear rate but also the 13.2
friction.
The wear rate measured in a tribological test will 10
usually be highly specific to the mechanical design of 1.8 1.4
the apparatus used and its applicability to other tribo- 0
logical systems may be doubtful. Measurements derived 5 % r.h. 50% r.h.
from a test may be misleading if they are obtained af- “Winter” conditions “Summer” conditions
f 0.41 0.54 0.23 0.53
ter an inappropriate test duration. An example is shown
in Fig. 13.19 for a steel/steel sliding couple. If the tests kSiC/kSi3N4 12.5 0.11
had been stopped after 100 000 cycles, the wear rates Fig. 13.20 Example showing the strong effect of atmospheric hu-
deduced from the tests would have been the same for midity on tribological behavior for two ceramic materials. Specific
the dry (7% r.h.) and the humid (≈ 100% r.h.) condi- wear rates and mean coefficients of friction for reciprocating slid-
tions. Only longer term tests reveal that in this system ing of a 100Cr6-steel ball on SiC and Si3 N4 discs in air with relative
the wear rate in fact depends strongly on the humidity humidities of 5% and 50%
of the surrounding atmosphere. Atmospheric humidity
is a parameter which is often not controlled in tribo- times that of the steel/silicon nitride couple, whereas
logical tests, even though it is known to affect the with the higher “summer” humidity the specific wear
tribological behavior of some systems, especially ce- rate of the steel/silicon carbide couple was about one
ramics, very strongly. Figure 13.20 shows an example tenth of that of the steel/silicon nitride couple. The dif-
of the influence of humidity on tribological behavior for ferences in friction coefficient, in contrast, were quite
two ceramics, with very different results for tests under small.
“winter” (low humidity) and “summer”(high humidity) In a reciprocating sliding test the length of the stroke
conditions. and the period of each cycle can influence the wear
The specific wear rate of the steel/silicon carbide rates if reactions with the environment, for example with
couple under “winter” conditions was more than twelve oxygen or water vapor, occur. Similar effects may also
706 Part D Measurement Methods for Materials Performance

occur in continuous sliding on a rotating disc or ring. which may be from a relatively low-wear to a high-wear
For these types of test, the time for which the wear scar regime. Predicting system wear behavior based on such
surface is exposed to the atmosphere, out of contact with an accelerated test can lead to errors of one or even
the other triboelement, depends on the detailed geometry two orders of magnitude. Much more reliable informa-
of the triboelements and the kinematics of their relative tion can be provided by “wear mapping”, as described
motion. in Sect. 13.2.4.
The interpretation of the specific wear rate k derived One reason for variability in friction and wear results
Part D 13.6

from a tribological test also needs care. This quantity is inhomogeneity of the test samples. Material properties
is defined as the wear volume divided by the sliding can vary between positions on a test sample, and a test
distance and the normal force (Sect. 13.1.3). Specific which involves a large area of the triboelement will gen-
wear rates at higher normal forces can therefore be lower erally be more reproducible than one which samples
than those at low normal forces, despite the total wear only a small area. Fretting tests, for example, typically
being the same or higher. This may happen if a system show lower reproducibility than tests involving continu-
can sustain loads up to a certain limit without any change ous sliding, since a fretting test with a ball on a flat, with
in damage or wear rate. a very small amplitude of motion, may be located close
Accelerated tribological tests, achieved by increas- to a flaw in the material, or on an essentially flaw-free re-
ing the load (or pressure) or velocity, for example in gion. In a continuous sliding test, however, the response
order to estimate component life-time, run the danger of the material will tend to be averaged over a much
of encountering a transition in the wear mechanism, larger region.

13.6 Characterization of Surfaces and Debris

Worn surfaces, and the debris resulting from wear, may performed within a vacuum system so that imaging and
be examined for several reasons: surface analysis can be carried out during or immediately
after the experiment.
• to study the evolution of wear during an experi-
However, most tribological tests (and most tribo-
ment, or during the life of a component in a practical
logical applications of materials) occur in air, and the
application,
presence of atmospheric oxygen and sometimes humid-
• to compare features produced in a laboratory test
ity often plays an important role in the wear process.
with those observed in a practical application,
Further exposure of the specimen to dry air will prob-
• to identify mechanisms of wear,
ably have little effect on the surface film, although any
• (by studying debris) to identify the source of debris
significant corrosion that might occur in steel samples
in a real-life application.
should be avoided by keeping them dry (for example in
a desiccator).
13.6.1 Sample Preparation Test samples are often contaminated with lubricant.
Wear debris may be present, as may corrosion prod-
Samples for surface examination may vary greatly in ucts and, in the case of samples retrieved from field
cleanliness. At one extreme are those which have been trials or components exposed to wear in service, even
prepared and tested under very clean laboratory condi- extraneous dirt introduced while dismantling the ma-
tions, perhaps even in ultrahigh vacuum, and at the other chine and extracting the sample to be studied. These
extreme, those which have been salvaged from field trials features pose particular challenges, since on the one
on heavy machinery operating under dirty conditions. hand they may obscure or confuse the examination of
After tribological testing under vacuum conditions, the worn surface, but on the other hand, as in the case
or in a controlled atmosphere, the specimen may be of wear debris, they may be valuable in providing ex-
moved directly through a transfer port into a vacuum tra information about the wear process. Although each
chamber for instrumental examination (for example by case may present individual problems, it is possible to
SEM or the various surface analytical techniques de- provide some general guidelines. The sample should
scribed in Sect. 13.6.2 below) without being exposed to be removed as soon as possible after the test appara-
air. In some experiments, the tribological tests may be tus or machine has stopped, to avoid possible additional
 Friction and Wear 13.6 Characterization of Surfaces and Debris 707

corrosion, which might for example be enhanced by the processes involved in wear, and the subsurface material,
condensation of water in a system which cools below the very close to the worn surface, must be studied. It may
dew point after running at a higher temperature. Organic also be valuable to measure mechanical properties by
lubricants can be removed with organic solvents, and micro- or nanoindentation in this region. Cross-sections
visual and low-magnification optical inspection should of samples can be prepared for microscopy by conven-
reveal the presence of any gross solid contaminants. tional metallographic or ceramographic techniques, but
Corrosion products may not be readily distinguishable since the regions of interest in the tribological context

Part D 13.6
from wear debris, and indeed in many cases the wear will be at or very close to the surface, special attention
debris will result from chemical reaction of the sub- must be paid to retaining features in this region, and
strate material with the environment; but any debris to avoiding the introduction of damage during prepara-
which is localized in and around the worn region is tion. The surface can be protected by applying metallic
more likely to result from the wear process than ma- coatings (such as electroplated nickel) to metal spec-
terial which is more widely distributed on the sample imens, or by using a hard embedding resin, preferably
and thus probably a product of general corrosion. When containing a hard filler (such as carbon or glass fibers, or
preparing the surface, a balance must be struck regard- ceramic particles), before the sample is cut perpendicu-
ing the amount of debris to remove to allow the material larly to the worn surface and then ground and polished.
surface to be studied. Solvent cleaning in a gentle ul- Similar grinding and polishing rates should be aimed
trasonic bath, for example with ethanol, isopropanol or for in the protective material and in the worn sam-
propanone, followed by hot-air drying, should produce ple itself, to achieve a perfectly plane cross-sectional
a surface which is clean enough for microscopic exam- surface for examination. Ceramics are particularly dif-
ination, but such a process may remove debris, which ficult to protect in this way, but edge protection may
can provide valuable information about the wear mech- be achieved by clamping two samples of the same
anism. It is sometimes possible to retrieve such debris ceramic together, with the worn surface at the inter-
by careful filtration of the solvent, and to examine the face, before embedding and sectioning the composite
debris on the filter, or to further classify it, for example sample.
by ferrography. Taper sectioning may also be used to study near-
Replication involves the preservation of the sample’s surface microstructure and reduce the effect of polishing
surface topography by casting or molding a replicating artefacts close to the surface. This technique involves the
medium against the surface, and then removing it. Care- examination of a section cut and polished at a shallow
ful experimental technique and the use of an appropriate angle to the surface. For example, on a section cut at an
replicating medium can lead to excellent results, and angle of 5.7◦ to the surface, a distance of 10 μm normal
the reproduction of surface features on a submicrome- to the edge of the sample corresponds to a depth beneath
ter scale. Replication allows techniques such as SEM the surface of only 1 μm (since tan (5.7◦ ) ≈ 0.1).
and profilometry to be applied to regions of large tri- Focused ion beam (FIB) milling provides a powerful
bological contacts in the field from which conventional technique to make cuts perpendicular to a worn surface,
samples cannot be cut, or to which access is very dif- either for subsequent examination of the near-surface
ficult. It also allows a sequence of records to be made regions by SEM, or as a first step in the production of
from a single specimen at intervals during a wear test, a sample for TEM. Since it is carried out on a fine scale
showing the evolution of surface features, since it is and in regions which can be examined in detail by SEM
a nondestructive technique. The ability to build up a before the milling takes place, it is possible to study
“library” of replicas of the same region of the speci- the subsurface features associated with specific surface
men during a test allows the investigator to subsequently areas of interest.
study in detail the evolution of features which only be-
come known to be of particular interest at the end of the 13.6.2 Microscopy, Profilometry
test sequence. and Microanalysis
Control experiments can be valuable in providing
assurance that the observations on the specimen are not Worn surfaces are often imaged in order to characterize
associated with artefacts introduced by the method of their surface topography and provide information about
specimen preparation. the wear mechanism. In many cases, accurate diagnosis
In some cases, examination of the surface of a sam- of wear mechanism may not be possible from imaging
ple does not provide sufficient information about the alone, since information on the local chemical composi-
708 Part D Measurement Methods for Materials Performance

tion and microstructure of the near-surface material, as (for example, from secondary electron detection) and
well as subsurface defects such as cracks or pores, may compositional (such as the mean atomic number from
be required. Thus a wide range of techniques may be back-scattered electrons) information. Stereo imaging,
employed to study tribological surfaces and subsurface which involves the capture of two images of the same
regions. These vary in the information they provide, the area of the surface tilted relative to each other by a small
accuracy of that information, and the dimensional scale angle, can be used to provide qualitative information
(in both lateral extent and depth into the surface) over on topography, and the images can also be processed by
Part D 13.6

which they provide information. suitable software to yield quantitative topographic maps.
Table 13.5 summarizes commonly used techniques This method supplements the more traditional methods
and their attributes. These include imaging techniques of profilometry, which use a stylus or optical means to
and methods of microstructural and compositional anal- map the surface heights (Sect. 13.6.2). Atomic force mi-
ysis. Resolution and sensitivity figures are approximate croscopy can be used to explore topography at the very
guides to relative performance, and there is often finest level.
a trade-off between resolution and sensitivity. Further in- Low-vacuum and environmental SEMs can be
formation on methods of surface chemical analysis can especially valuable for studying some of the contam-
be found in Sect. 6.1 of this volume, and information on inated and poorly conducting (such as polymer and
surface topography analysis in Sect. 6.2. ceramic) samples encountered in tribological investiga-
Scanning electron microscopy is very widely used tions. Ion channeling contrast can be used in a focused
to examine worn surfaces and, through the use of dif- ion beam (FIB) microscope to reveal phase distribu-
ferent imaging modes, can provide both topographic tions and deformed microstructures, and as mentioned

Table 13.5 Surface examination techniques

Technique Spatial Depth Analytical sensitivity (Z is Applicability (Z is atomic
resolution resolution average atomic number) number of element)
OM 0.2 μm – – –
SEI (SEM) 1.5 nm – – –
NCOP 1 μm 3 nm – Topography
CP 0.5 μm 1 nm – Topography, but may damage
soft samples
XRD 10 mm 10 μm 5% Crystal phase, lattice strain,
particle/grain size
XRF 10 mm 10 μm 1 –10 ppm Z >4
XPS 5 μm 3 monolayers 0.3% Z >2
LIMA 2 μm 1–2 μm 10– 100 ppm All Z
EDS (SEM) 1 μm 1 μm 0.1% Z >4
WDS (SEM) 1 μm 1 μm 100 ppm Z >4
BEI (SEM) 50 nm 50 nm 0.1 Z
SSIMS 1 μm 2 monolayers 0.01% All Z
DSIMS 20 nm 10 monolayers <1 ppm All Z
SAM 5 nm 3 monolayers 0.3% Z >2
EDS (STEM) 10 nm 20 nm 0.1% Thin foil, Z > 4 peak overlap
EELS (STEM) 10 nm 20 nm 1% Thin foil, Z > 3
APFIM 1 nm 1 monolayer 0.1–1% All Z
EBSP 100 nm 50 nm – Crystal phase, orientation
Key: OM = optical microscopy; SEI = secondary electron imaging; BEI = back-scattered electron imaging; XRD, XRF, XPS
= X-ray diffractometry, fluorescence and photoelectron spectroscopy; LIMA = laser-induced mass spectroscopy: EDS = energy-
dispersive X-ray analysis; WDS = wavelength-dispersive spectrometry; SEM = scanning electron microscopy; SSIMS, DSIMS =
static and dynamic secondary ion mass spectrometry; SAM = scanning Auger spectrometry; STEM = scanning transmission electron
microscopy; APFIM = atom probe field ion microscope; EBSP = electron back-scattered diffraction pattern; EELS = electron energy
loss spectrometry; NCOP = noncontact optical profilometer; CP = contact profilometer
 Friction and Wear References 709

in Sect. 13.6.1 the FIB can also be used to cut sections distributed according to size. Filtration can be used to
beneath the specimen surface for microscopy. separate nonmagnetic particles. The particles can then
be examined (for example by optical or scanning elec-
13.6.3 Wear Debris Analysis tron microscopy), chemically analyzed, and their sizes
and shapes characterized. Optical methods (such as
Debris poses special problems in examination; the first laser light scattering) are commonly used to determine
challenge is to obtain a representative sample. In a lubri- particle size distributions. Automated systems exist to

Part D 13
cated system the technique of ferrography can be used evaluate and describe particle shapes in wear debris, and
to separate and grade magnetic debris. In ferrography, link them to wear mechanisms [13.25]. The methods of
a suspension of wear debris flows through a magnetic microscopy and microanalysis outlined in Sect. 13.6.2
field gradient and the particles become separated and can be used for debris as well as for worn surfaces.

References

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Materials (ASTM International, West Conshohocken 13.13 D. Klaffke, M. Hartelt: Stick-Slip Untersuchungen
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13.2 M. B. Peterson, W. O. Winer (Eds.): Glossary of terms stofftech. 31, 790–793 (2000)
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13.10 M. G. Gee, A. Gant, I. M. Hutchings: Rotating wheel 254, 810–817 (2003)
abrasive wear testing NPL Good Practice Guide 55 13.21 D. C. Eberle, C. M. Wall, M. B. Treuhaft: Applica-
(National Physical Laboratory, Teddington 2002) tions of radioactive tracer technology in real-time
13.11 M. G. Gee, I. M. Hutchings: General approach and measurement of wear and corrosion, Wear 259,
procedures for erosive wear testing NPL Good 1462–1471 (2005)
Practice Guide 56 (National Physical Laboratory, 13.22 Standard B794-97: Standard Test Method for Dura-
Teddington 2002) bility Wear Testing of Separable Electrical Connector
13.12 M. G. Gee, A. Gant, I. M. Hutchings, R. Bethke, Systems using Electrical Resistance Measurements,
K. Schiffmann, K. Van Acker, S. Poulat, Y. Gachon, American Society for Testing and Materials (ASTM
J. von Stebut: Ball cratering or micro-abrasion International, West Conshohocken, 2005)
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13.23 I. Buresch, P. Rehbein, D. Klaffke: Possibilities of sliding of SiC/SiC-TiC. In: Proc. 2nd World Tribology
fretting corrosion model testing for contact sur- Congress, ed. by F. Franek, W. J. Bartz, A. Paus-
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A. Pauschitz (The Austrian Tribology Society ÖTG, 13.25 S. Raadnui: Wear particle analysis – utilization of
Vienna 2001) quantitative computer image analysis: a review,
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ages on friction and wear in water lubricated
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