Czichos H., Saito T., Smith L.E. (Eds.) Handbook of Metrology and Testing

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758 Part D Materials Performance Testing

Normal force (N) /Friction force (N) /

Total displacement (μm)

Normal force (N)

Friction force (N)

Total displacement (μm)

Ball: Al

Disc: Al

Velocity: 0.1 m/s

10 N

50 %

FN:

r.H.:

0 10 000

Time (s)

–10

–5

8000600040002000

Table 13.3 Tribological tests involving sliding motion

Test Parameters

Materials composition and microstructure

Load

Speed

Temperature

Surface condition

Environment

Lubrication

Contact time and interval between loading of contact areas on

two samples

Measurements

Direct change of dimension

Volume (usually via weight loss and density)

Proﬁlometry

Friction

Continuous measurement of wear during test

Examination of worn surface

Examination of wear debris

Standards

ASTM G99

ASTM G 133

ISO 20808

segments (2 +4). The calibration of the relevant spring

constants of the cantilever (in bending and torsion) is

not a trivial procedure [13.16–18].

A technique known as triboscopy monitors the evo-

lution of time-dependent local phenomena with a spatial

Fig. 13.14 Example of online measurement of linear wear

(total displacement of ball relative to disc) and friction

force for continuous sliding of an alumina ceramic ball on

an alumina disc 

Table 13.4 Tribological tests involving rolling motion

Test Parameters

Materials composition and structure

Load

Relative slip of rolling elements

Rolling speed

Temperature

Surface condition

Environment

Lubrication

Measurements

Direct change of dimension

Volume (usually via weight loss and density)

Proﬁlometry

Torque

Continuous measurement of wear during test

Examination of worn surface

Examination of wear debris

resolution limited by contact size. With this technique,

complementary images of the friction force can be ob-

tained by simultaneously recording spatially-resolved

electrical contact resistance and friction measurements.

The technique provides information about the his-

tory of friction and wear processes [13.19]. When

measuring the electrical contact resistance, care must

be taken that the electrical sensing current remains

low, because otherwise the wear process may be

inﬂuenced.

13.4.7 Presentation of Results

In most cases the speciﬁcation of a single coefﬁcient

of friction is not adequate. This can be seen from the

examples in Figs. 13.12 and 13.14, depicting the evo-

lution of friction during individual experimental runs.

At the very least, all relevant test and system pa-

rameters should be speciﬁed. The list in Table 13.1

provides a general guide, and is supplemented by

the more speciﬁc lists of parameters in Table 13.3

and Table 13.4.

Part D 13.4

Friction and Wear 13.5 Quantitative Assessment of Wear 759

13.5 Quantitative Assessment of Wear

13.5.1 Direct and Indirect Quantities

The amount of wear can be speciﬁed in terms of direct

or indirect quantities.

Indirect quantities are often used in technical as-

sessments of the lives of machinery and in practical

engineering. Examples include

•

wear-limited service life (used for cutting tools for

example: h, d, number of parts)

•

wear-limited throughput (used for the ﬂow of abra-

sive materials or objects through pipelines for

instance: number of parts, m

, kg).

Direct wear quantities specify the change in mass, geo-

metrical dimensions or volume of the wearing body.

Examples include

•

wear amount:

– mass loss (kg)

– linear dimensional change (m)

– volume loss (m

)

•

wear resistance = 1/(wear amount) (m

−1

−3

−1

)

•

wear rate = (wear amount)/(sliding distance or

time) (m/m, m

/m, kg/m, m/s, m

/s, kg/s)

•

wear coefﬁcient, or speciﬁc (or normalized) wear

rate (also sometimes called wear factor) = (wear

rate)/(normal force) (m

−1

The primary measurement from which these quantities

are derived is usually mass loss, dimensional change or

volume loss, although other methods can also be used

(see Sect. 13.5.5).

13.5.2 Mass Loss

The loss of substance from the surface of a triboelement

can be determined by weighing it before and after wear.

Continuous sensing of the wear process in terms of mass

loss is usually not possible. Mass loss measurement at

deﬁned intervals, to obtain information about the devel-

opment of wear (and hence to investigate running-in,

stability of wear rate, and so on), require the test to be

halted and the triboelement(s) removed for weighing. In

this process the danger exists that the microcontact ge-

ometry will be changed on reassembly. Such a change

in microgeometry can inﬂuence the wear rate. Debris

present in the contact region is also likely to be dis-

turbed, and often removed. The most important period

in the development of the wear rate may be missed, and

the act of interrupting the test in order to weigh the spe-

cimen may alter the progress and even the predominant

mechanism of wear.

The sensitivity of this method of wear quantiﬁca-

tion is relatively low. The method is applicable only

for wear rates high enough that a signiﬁcant mass loss

(typically of the order of at least 1 mg) is reached af-

ter a reasonable time for a test sample not heavier than

about 0.5 kg. For materials which experience signiﬁcant

mass changes from other causes, such as the absorption

or loss of water by certain polymers, or the oxidation

of metals at high temperatures, special care must be

taken to ensure that the mass changes measured are gen-

uinely associated with tribological processes and are

not due to other phenomena. Suitable control speci-

mens, not subjected to wear but otherwise exposed to

the same conditions as the triboelements, may be help-

ful for eliminating such effects and also for correcting

for any long-term drift in the calibration of the balance.

The accuracy of weighing may be limited by the ac-

curacy or sensitivity of the balance (especially in the

case of heavy triboelements), by changes in humidity

between the two weighings, or by particle or debris at-

tachment or detachment. Material transfer and oxidation

during the wear process often complicate the interpreta-

tion of the measured mass values and can sometimes

lead to erroneous interpretation of the wear behavior.

Wear is sometimes expressed in units of volume,

and in order to convert mass loss to volume loss, the

density of the worn material must be known. For a ho-

mogeneous bulk material this will usually pose no

problems, but if wear is occurring from a coating or

treated surface layer then the relevant density may not

be accurately known.

13.5.3 Dimensional Change

Changes in linear dimensions due to wear are frequently

measured on-line (continuously) during friction tests.

This has advantages over making measurements by in-

terrupting the test, because one can then get information

about the continuous evolution of wear during the test

and, by detecting transitions in the measured linear wear

rate, about changes in the dominant wear mechanism.

In most cases a change in the distance between the

mounting ﬁxtures of the two triboelements is meas-

ured. This implies that only the sum of the linear wear

contributions from both elements can be obtained. Ma-

terial transfer from one triboelement to the other may

Part D 13.5

760 Part D Materials Performance Testing

lead to misinterpretation. For example, in the frequently

used test arrangement with a ball or pin sliding contin-

uously on a disc surface, transfer of material from the

disc to the ball will change the distance between them.

Depending on the amount of wear of the disc or the ra-

tio of wear to transfer, the measured distance can be

reduced, eliminated or increased. This last case may oc-

cur if much of the material worn from the larger wear

scar on the disc is transferred to the smaller wear area

on the ball or pin. Figure 13.14 shows an example of

such behavior for an alumina ball sliding on an alumina

disc. For up to about 3500 s of sliding, the displace-

ment of the sample increases steadily, with the system

apparently showing negative wear. The reason for this

is that wear debris is accumulating in the contact re-

gion. The agglomerated particles of worn material then

suddenly detach, leading to a negative total displace-

ment, which is then followed by further accumulation

of debris. The friction force also changes, which is

associated with a change in the nature of the surface

interaction.

In some cases wear of one or both triboelements

leads to a signiﬁcant change in the local geometry

of the contact. An example of this behavior is shown

in Fig. 13.15. As wear occurs on the steel ball the

contact area progressively increases, and the wear rate

detected in terms of a change in the distance between

the two triboelements falls rapidly with time, although

the associated volume wear rate varies much less. In

1.0

Time t (h)

Coefficient of friction f

Linear wear W

(µm)

0.8

0.6

0.4

0.2

0 0.5

Fig. 13.15 Example of online measurement of linear wear

(total displacement of ball relative to disc W

) and friction

force ( f ) for reciprocating steel ball on a diamond-coated

disc (courtesy of D. Klaffke)

this experiment there was also a substantial change in

friction force.

For on-line measurements of the changes in linear

dimensions or displacements, inductive or capacitative

sensors are frequently used. Inductive sensors can attain

aresolutiondownto1μm, and capacitative sensors can

reach a resolution in the nanometer range. Both types

of sensors and their associated electronic circuits ex-

hibit some temperature drift and a limited bandwidth

(frequency range over which the deﬁned speciﬁcation

of repeatability, resolution and accuracy is achieved).

However, in most cases, the dimensional changes due

to temperature variations in the test samples, their ﬁx-

tures, and the mechanical construction of the apparatus

induce greater measurement errors than those of the

sensor system itself. Capacitative displacement sensors

require a relatively clean environment: dirt, dust, water,

oil or other dielectric media in the measuring gap will

inﬂuence the measurement signal. Inductive sensor sys-

tems are generally less expensive and also less sensitive

to environmental inﬂuences.

13.5.4 Volume Loss

The volume loss from a specimen in a tribological test

can be derived from the mass loss if the density of the

material is known. As indicated in Sect. 13.5.2, there

may however be uncertainty in the density, especially in

the case of a coated or surface-treated sample. In princi-

ple, the volume loss can also be derived from measured

dimensional changes. In many cases this method will be

more accurate than weighing a sample before and after

testing.

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

block-on-ring geometries, this is particularly true for the

two limiting cases when the wear occurs on only one of

the triboelements. For example, for reciprocating slid-

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

simple equations for the wear volume can be deduced.

If wear occurs only on the ball, then the wear vol-

ume of the ball W

v,b

is given by

v,b

≈



πd

/64R



. (13.3)

The total linear wear is given by

≈d

/(8R) (13.4)

and therefore

v,b

= W

v,tot

≈π RW

, (13.5)

where the symbols are deﬁned below.

Part D 13.5

Friction and Wear 13.5 Quantitative Assessment of Wear 761

10 μm

500 μm

Fig. 13.16 Example of determination of wear volume for

a ﬂat specimen after a sliding test with ball-on-ﬂat geom-

etry, by measurement of the cross-section of the wear scar

on the ﬂat. The upper image shows the end of a linear

wear scar produced by reciprocating sliding, and the lower

graph shows a proﬁlometer trace across it at a representa-

tive position

If wear occurs only on the plate (so the ball wear

volume is zero), then the wear volume for the plate W

v,d

is given by

v,d

≈



πd

/64R



+ΔxW

. (13.6)

The planimetric wear is

≈d

/12R (13.7)

and therefore

v,d

= W

v,tot

≈



Δxd

/12R





πd

/64R



(13.8)

≈Δxd

/12R (13.8a)

since the second term in (13.8) is small compared with

the ﬁrst one in the case of reciprocating sliding, where

Δx is large compared with d

and

v,d

wear volume of the plate,

v,b

wear volume of the ball,

v,tot

= W

v,d

v,b

total wear volume,

diameter of the wear scar on the ball, parallel to

the sliding direction,

diameter of the wear scar on the ball, perpendic-

ular to the sliding direction,

planimetric wear (plate),

total linear wear,

Δx stroke of motion,

R radius of the ball.

Combining (13.4), (13.5), and (13.8a) leads to

v,d

= W

v,tot

≈(4/3)Δx(2R)

1/2

3/2

. (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

this case a rule of mixtures can be used

v,tot

=απ RW

+β(4/3)Δx(2R)

1/2

3/2

(13.10)

where α and β describe the contribution of ball and plate

to the wear, respectively.

However, the values of α and β are not known be-

forehand, and can change during the test. Therefore, the

determination of the wear volume ofﬂine from the di-

mensions of the wear scars is usually more accurate

than online determination from the measurement of the

change in linear dimensions during the test.

Measurements of volumetric wear from topographic

changes determined with a stylus proﬁlometer, laser

proﬁlometer or white light interferometer in most cases

deliver more accurate results than online measure-

ment of dimensional changes. With these methods, the

three-dimensional proﬁles before and after the test are

determined and the wear volume derived by subtract-

ing one from the other. A precondition for this method

is the ability to align the two proﬁles correctly. With

wear scars which are extended in one direction (such

as scars formed in linearly reciprocating or rotating tri-

boelements), line scans can be performed across the

wear scars and the wear volume calculated from the

resulting two-dimensional cross-section proﬁle and the

length of the wear scar (Fig. 13.16). This method may,

however, lead to large errors if the depth of the wear scar

is not much greater than the roughness of the surface.

If the cross-sectional area W

of a wear scar is

measured by proﬁlometry after a ball-on-ﬂat test with

reciprocating sliding, the wear volume of the ball can

be calculated from

v,b



πd

/64





(1/R) −(1/R

∗

)



, (13.11)

Part D 13.5

762 Part D Materials Performance Testing

Fig. 13.17 Example of wear scar produced by a ball cra-

tering test on a coated sample (optical micrograph); the

boundary between the titanium nitride coating and the steel

substrate is clearly seen. Typical wear scar diameter is

1–2mm

where R

∗

is the radius of the wear area of the ball. This

value can be calculated from the proﬁle of the wear scar

on the ﬂat

∗

/(12W

) . (13.12)

The wear volume of the ﬂat is given by

v,d

≈



πd

/64



(1/R

∗

)+ΔxW

. (13.13)

In the case of well-deﬁned wear scars of known geo-

metry, for example those produced in ball-cratering

abrasion tests (Fig. 13.17), the volume of wear can also

Irradiated

rings

Irradiated

rod

bearings

Gamma ray

detector

Lead

shielding

External

oil pump

Heat

exchanger

Sump

Fig. 13.18 Principle of wear determination in an internal combus-

tion engine by the radionuclide measurement method (courtesy of

Southwest Research Institute, USA)

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 diam-

eter of the wear scar may be measured with a stylus

proﬁlometer or an optical microscope.

13.5.5 Other Methods

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-

ments (such as the piston ring and cylinder liner in

an internal combustion engine) is activated by irradia-

tion with heavy ions from a particle accelerator. Since

only a thin layer is activated, only rudimentary radiation

precautions are required. The wear debris from the ac-

tivated triboelements during the operation of the engine

are radioactive and are transported within the oil circuit

to a ﬁlter. A γ -ray detector close to the ﬁlter measures

the radiation activity, as shown in Fig. 13.18. The in-

crease in this activity with test duration is proportional

to the amount of wear. If the two triboelements (such as

the piston ring and cylinder liner) contain different al-

loying elements, the activation will lead to radioactive

isotopes of these elements which emit γ -rays with dif-

ferent characteristic energies. In this case, the wear of

each triboelement, and not only the sum of both, can be

separately measured online.

Measurement of the electrical contact resistance

during a friction test between metallic triboelements,

with one contact partner insulated, can provide informa-

tion about the formation and removal of reaction layers,

such as oxides. Special arrangements involving insula-

tion and embedded conductive wires or sensors in a test

sample can be used to detect a wear limit [13.22–24].

13.5.6 Errors and Reproducibility

in Wear Testing

As mentioned above in Sect. 13.4, the ﬁltering effect

of the electronic circuits used for signal recording, as

well as the sampling rate in digital signal recording,

may introduce errors into measurements of dynamic

friction force signals. Vibrations excited at the resonant

frequency of the sensor system may be superimposed

on the friction force signal, leading to variations in the

measured friction force which would not be present in

the absence of the sensor system. These vibrations can

also inﬂuence the wear rate of the system. In the case of

Part D 13.5

Friction and Wear 13.5 Quantitative Assessment of Wear 763

a very unsteady friction signal, friction force measure-

ments must be interpreted with care; it is problematic to

deduce a unique coefﬁcient of friction from such tests.

Use of a bandwidth that is too low in the electronic

recording system can mask the dynamic properties of

friction forces resulting, for example, from stick/slip

behavior.

The processes involved in sample preparation, espe-

cially cleaning of the test specimens, can have a major

inﬂuence on friction and on the wear of triboelements.

Surface machining and polishing often involve the use

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

solvents is often necessary to obtain acceptably clean

surfaces.

In friction measurements with very low external

loads, intrinsic adhesion forces such as van der Waals

or capillary forces must be taken into account in the

interpretation of measured friction force.

Material transfer during a tribological test can also

lead to erroneous interpretation of the results, for fric-

tion as well as for wear. It is therefore important to

analyze the worn areas to examine the processes which

have occurred. The example shown in Fig. 13.14 in-

volved transfer, agglomeration and compaction of wear

particles, which affected not only the wear rate but also

the friction.

The wear rate measured in a tribological test will

usually be highly speciﬁc to the mechanical design of

the apparatus used and its applicability to other tribo-

logical systems may be doubtful. Measurements derived

from a test may be misleading if they are obtained af-

ter an inappropriate test duration. An example is shown

in Fig. 13.19 for a steel/steel sliding couple. If the tests

had been stopped after 100 000 cycles, the wear rates

deduced from the tests would have been the same for

the dry (7% r.h.) and the humid (≈100% r.h.) condi-

tions. Only longer term tests reveal that in this system

the wear rate in fact depends strongly on the humidity

of the surrounding atmosphere. Atmospheric humidity

is a parameter which is often not controlled in tri-

bological tests, even though it is known to affect the

tribological behavior of some systems, especially ce-

ramics, very strongly. Figure 13.20 shows an example

of the inﬂuence of humidity on tribological behavior for

two ceramics, with very different results for tests un-

der winter (low humidity) and summer (high humidity)

conditions.

The speciﬁc wear rate of the steel/silicon carbide

couple under winter conditions was more than twelve

Volumetric wear W

(10

–3

)

Number of cycles n (10

)

500

100

0 400300200100

100Cr6

Δx = 160 μm

rel. humidity 7 %

rel. humidity 100 %

rel. humidity 50 %

Fig. 13.19 Example showing development of wear volume for

a steel/steel couple with the number of sliding cycles (reciprocat-

ing), for different relative humidities of the surrounding air

100Cr6 ball

against

Wear coefficient

(10

–6

/Nm)

SiC

5 % r.h.

“Winter” conditions

f 0.41 0.54

SiC

12.5

0.23 0.53

0.11

50 % r.h.

“Summer” conditions

22.6

13.2

1.8

1.4

SiC

Fig. 13.20 Example showing the strong effect of atmospheric hu-

midity on tribological behavior for two ceramic materials. Speciﬁc

wear rates and mean coefﬁcients of friction for reciprocating slid-

ing of a 100Cr6-steel ball on SiC and Si

discs in air with relative

humidities of 5 and 50%

times that of the steel/silicon nitride couple, whereas

with the higher summer humidity the speciﬁc wear

rate of the steel/silicon carbide couple was about one

tenth of that of the steel/silicon nitride couple. The dif-

ferences in friction coefﬁcient, in contrast, were quite

small.

In a reciprocating sliding test the length of the stroke

and the period of each cycle can inﬂuence the wear

rates if reactions with the environment, for example

with oxygen or water vapor, occur. Similar effects may

Part D 13.5

764 Part D Materials Performance Testing

also occur in continuous sliding on a rotating disc or

ring. For these types of test, the time for which the wear

scar surface is exposed to the atmosphere, out of contact

with the other triboelement, depends on the detailed ge-

ometry of the triboelements and the kinematics of their

relative motion.

The interpretation of the speciﬁc wear rate k derived

from a tribological test also needs care. This quantity is

deﬁned as the wear volume divided by the sliding dis-

tance and the normal force (Sect. 13.1.3). Speciﬁc wear

rates at higher normal forces can therefore be lower than

those at low normal forces, despite the total wear being

the same or higher. This may happen if a system can

sustain loads up to a certain limit without any change in

damage or wear rate.

Accelerated tribological tests, achieved by increas-

ing the load (or pressure) or velocity, for example in

order to estimate component life-time, run the danger

of encountering a transition in the wear mechanism,

which may be from a relatively low-wear to a high-

wear regime. Predicting system wear behavior based on

such an accelerated test can lead to errors of one or even

two orders of magnitude. Much more reliable informa-

tion can be provided by wear mapping, as described

in Sect. 13.2.4.

One reason for variability in friction and wear re-

sults is inhomogeneity of the test samples. Material

properties can vary between positions on a test sample,

and a test which involves a large area of the triboelement

will generally be more reproducible than one which

samples only a small area. Fretting tests, for example,

typically show lower reproducibility than tests involv-

ing continuous sliding, since a fretting test with a ball

on a ﬂat, with a very small amplitude of motion, may

be located close to a ﬂaw in the material, or on an es-

sentially ﬂaw-free region. In a continuous sliding test,

however, the response of the material will tend to be

averaged over a much larger region.

13.6 Characterization of Surfaces and Debris

Worn surfaces, and the debris resulting from wear, may

be examined for several reasons.

•

To study the evolution of wear during an experi-

ment, or during the life of a component in a practical

application

•

To compare features produced in a laboratory test

with those observed in a practical application

•

To identify mechanisms of wear

•

(by studying debris) To identify the source of debris

in a real-life application.

13.6.1 Sample Preparation

Samples for surface examination may vary greatly in

cleanliness. At one extreme are those which have been

prepared and tested under very clean laboratory con-

ditions, perhaps even in ultrahigh vacuum, and at the

other extreme, those which have been salvaged from

ﬁeld trials on heavy machinery operating under dirty

conditions.

After tribological testing under vacuum conditions,

or in a controlled atmosphere, the specimen may be

moved directly through a transfer port into a vacuum

chamber for instrumental examination (for example by

SEM or the various surface analytical techniques de-

scribed in Sect. 13.6.2 below) without being exposed

to air. In some experiments, the tribological tests may

be performed within a vacuum system so that imag-

ing and surface analysis can be carried out during or

immediately after the experiment.

However, most tribological tests (and most tribo-

logical applications of materials) occur in air, and the

presence of atmospheric oxygen and sometimes humid-

ity often plays an important role in the wear process.

Further exposure of the specimen to dry air will prob-

ably have little effect on the surface ﬁlm, although any

signiﬁcant corrosion that might occur in steel samples

should be avoided by keeping them dry (for example in

a desiccator).

Test samples are often contaminated with lubricant.

Wear debris may be present, as may corrosion products

and, in the case of samples retrieved from ﬁeld trials or

components exposed to wear in service, even extrane-

ous dirt introduced while dismantling the machine and

extracting the sample to be studied. These features pose

particular challenges, since on the one hand they may

obscure or confuse the examination of the worn surface,

but on the other hand, as in the case of wear debris,

they may be valuable in providing extra information

about the wear process. Although each case may present

individual problems, it is possible to provide some gen-

eral guidelines. The sample should be removed as soon

as possible after the test apparatus or machine has

Part D 13.6

Friction and Wear 13.6 Characterization of Surfaces and Debris 765

stopped, to avoid possible additional corrosion, which

might for example be enhanced by the condensation

of water in a system which cools below the dew point

after running at a higher temperature. Organic lubri-

cants can be removed with organic solvents, and visual

and low-magniﬁcation optical inspection should reveal

the presence of any gross solid contaminants. Corro-

sion products may not be readily distinguishable from

wear debris, and indeed in many cases the wear de-

bris will result from chemical reaction of the substrate

material with the environment; but any debris which is

localized in and around the worn region is more likely

to result from the wear process than material which is

more widely distributed on the sample and thus proba-

bly a product of general corrosion. When preparing the

surface, a balance must be struck regarding the amount

of debris to remove to allow the material surface to be

studied. Solvent cleaning in a gentle ultrasonic bath, for

example with ethanol, isopropanol or propanone, fol-

lowed by hot-air drying, should produce a surface which

is clean enough for microscopic examination, but such

a process may remove debris, which can provide valu-

able information about the wear mechanism. It is some-

times possible to retrieve such debris by careful ﬁltra-

tion of the solvent, and to examine the debris on the ﬁl-

ter, or to further classify it, for example by ferrography.

Replication involves the preservation of the sam-

ple’s surface topography by casting or molding a repli-

cating medium against the surface, and then removing

it. Careful experimental technique and the use of an

appropriate replicating medium can lead to excellent

results, and the reproduction of surface features on

a submicrometer scale. Replication allows techniques

such as SEM and proﬁlometry to be applied to regions

of large tribological contacts in the ﬁeld from which

conventional samples cannot be cut, or to which access

is very difﬁcult. It also allows a sequence of records

to be made from a single specimen at intervals during

a wear test, showing the evolution of surface features,

since it is a nondestructive technique. The ability to

build up a library of replicas of the same region of the

specimen during a test allows the investigator to subse-

quently study in detail the evolution of features which

only become known to be of particular interest at the

end of the test sequence.

Control experiments can be valuable in providing

assurance that the observations on the specimen are not

associated with artefacts introduced by the method of

specimen preparation.

In some cases, examination of the surface of a sam-

ple does not provide sufﬁcient information about the

processes involved in wear, and the subsurface material,

very close to the worn surface, must be studied. It may

also be valuable to measure mechanical properties by

micro- or nanoindentation in this region. Cross-sections

of samples can be prepared for microscopy by conven-

tional metallographic or ceramographic techniques, but

since the regions of interest in the tribological context

will be at or very close to the surface, special attention

must be paid to retaining features in this region, and

to avoiding the introduction of damage during prepara-

tion. The surface can be protected by applying metallic

coatings (such as electroplated nickel) to metal spec-

imens, or by using a hard embedding resin, preferably

containing a hard ﬁller (such as carbon or glass ﬁbers, or

ceramic particles), before the sample is cut perpendicu-

larly to the worn surface and then ground and polished.

Similar grinding and polishing rates should be aimed for

in the protective material and in the worn sample itself,

to achieve a perfectly plane cross-sectional surface for

examination. Ceramics are particularly difﬁcult to pro-

tect in this way, but edge protection may be achieved

by clamping two samples of the same ceramic together,

with the worn surface at the interface, before embedding

and sectioning the composite sample.

Taper sectioning may also be used to study near-

surface microstructure and reduce the effect of pol-

ishing artefacts close to the surface. This technique

involves the examination of a section cut and polished

at a shallow angle to the surface. For example, on a sec-

tion cut at an angle of 5.7

◦

to the surface, a distance

of 10 μm normal to the edge of the sample corresponds

to a depth beneath the surface of only 1 μm (since tan

(5.7

◦

) ≈0.1).

Focused ion beam (FIB) milling provides a powerful

technique to make cuts perpendicular to a worn surface,

either for subsequent examination of the near-surface

regions by SEM, or as a ﬁrst step in the production of

a sample for TEM. Since it is carried out on a ﬁne scale

and in regions which can be examined in detail by SEM

before the milling takes place, it is possible to study

the subsurface features associated with speciﬁc surface

areas of interest.

13.6.2 Microscopy, Proﬁlometry

and Microanalysis

Worn surfaces are often imaged in order to characterize

their surface topography and provide information about

the wear mechanism. In many cases, accurate diagnosis

of wear mechanism may not be possible from imaging

alone, since information on the local chemical composi-

Part D 13.6

766 Part D Materials Performance Testing

tion and microstructure of the near-surface material, as

well as subsurface defects such as cracks or pores, may

be required. Thus a wide range of techniques may be

employed to study tribological surfaces and subsurface

regions. These vary in the information they provide, the

accuracy of that information, and the dimensional scale

(in both lateral extent and depth into the surface) over

which they provide information.

Table 13.5 summarizes commonly used techniques

and their attributes. These include imaging techniques

and methods of microstructural and compositional anal-

ysis. Resolution and sensitivity ﬁgures are approximate

guides to relative performance, and there is often

a trade-off between resolution and sensitivity. Further

information on methods of surface chemical analysis

can be found in Sect. 6.1 of this volume, and informa-

tion on surface topography analysis in Sect. 6.2.

Scanning electron microscopy is very widely used

to examine worn surfaces and, through the use of dif-

ferent imaging modes, can provide both topographic

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) 1.5nm – – –

NCOP 1 μm 3nm – Topography

CP 0.5 μm 1nm – 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 1–2μm 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 50nm 0.1 Z

SSIMS 1 μm 2 monolayers 0.01% All Z

DSIMS 20 nm 10 monolayers <1 ppm All Z

SAM 5nm 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 1nm 1 monolayer 0.1–1% All Z

EBSP 100 nm 50nm – 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

(for example, from secondary electron detection) and

compositional (such as the mean atomic number from

back-scattered electrons) information. Stereo imaging,

which involves the capture of two images of the same

area of the surface tilted relative to each other by a small

angle, can be used to provide qualitative information

on topography, and the images can also be processed

by suitable software to yield quantitative topographic

maps. This method supplements the more traditional

methods of proﬁlometry, which use a stylus or optical

means to map the surface heights (Sect.13.6.2). Atomic

force microscopy can be used to explore topography at

the very ﬁnest level.

Low-vacuum and environmental SEMs can be

especially valuable for studying some of the contam-

inated and poorly conducting (such as polymer and

ceramic) samples encountered in tribological investiga-

tions. Ion channeling contrast can be used in a focused

ion beam (FIB) microscope to reveal phase distribu-

tions and deformed microstructures, and as mentioned

Part D 13.6

Friction and Wear References 767

in Sect. 13.6.1 the FIB can also be used to cut sections

beneath the specimen surface for microscopy.

13.6.3 Wear Debris Analysis

Debris poses special problems in examination; the ﬁrst

challenge is to obtain a representative sample. In a lubri-

cated system the technique of ferrography can be used

to separate and grade magnetic debris. In ferrography,

a suspension of wear debris ﬂows through a magnetic

ﬁeld gradient and the particles become separated and

distributed according to size. Filtration can be used to

separate nonmagnetic particles. The particles can then

be examined (for example by optical or scanning elec-

tron microscopy), chemically analyzed, and their sizes

and shapes characterized. Optical methods (such as

laser light scattering) are commonly used to determine

particle size distributions. Automated systems exist to

evaluate and describe particle shapes in wear debris, and

link them to wear mechanisms [13.25]. The methods of

microscopy and microanalysis outlined in Sect. 13.6.2

can be used for debris as well as for worn surfaces.

References

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wear and erosion (ASTM International, West Con-

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13.2 M.B. Peterson, W.O. Winer (Eds.): Glossary of terms

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1303

13.3 DIN standard 50322: Wear: Classiﬁcation of cate-

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13.4 I.M. Hutchings: Tribology: Friction and Wear of

Engineering Materials (Arnold, London 1992)

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13.22 American Society for Testing and Materials: Stan-

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Wear Testing of Separable Electrical Connector

Part D 13