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738 Part D Materials Performance Testing

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Part D 12

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12.87 J.R. Davies (Ed.): Heat-Resistant Materials (ASM

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Part D 12

Corrosion References 741

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http://ie.jrc.cec.eu.int/facilities.html

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1981) p. 441

Part D 12

743

Friction and

13. Friction and Wear

Almost all mechanical systems, artiﬁcial or natural,

involve the relative motion of solid components.

Wherever two surfaces slide or roll against each

other, there will be frictional resistance, and

wear will occur. The response of materials to this

kind of interaction, often termed tribological,

depends not only on the precise nature of the

materials, but also on the detailed conditions of

the contact between them and of the motion.

Friction and wear are system responses, rather

than material properties. The measurement of

tribological behavior therefore poses particular

challenges, and a keen awareness of the factors

that inﬂuence friction and wear is essential.

This chapter provides deﬁnitions of the key

concepts in Sect. 13.1, and provides a rationale

for the design and selection of test methods

in Sect. 13.2. Various standard and other tribolog-

ical test methods are comprehensively reviewed

in Sect. 13.3, which is followed by descriptions of

methods used for the quantitative assessment

of both friction (Sect. 13.4) and wear (Sect. 13.5).

Methods used for characterizing worn surfaces and

wear debris are addressed in Sect. 13.6.

13.1 Deﬁnitions and Units ............................ 743

13.1.1 Deﬁnitions .................................. 744

13.1.2 Types of Wear .............................. 744

13.1.3 Units for Wear.............................. 745

13.2 Selection of Friction and Wear Tests ....... 747

13.2.1 Approach to Tribological Testing..... 747

13.2.2 Test Parameters ........................... 748

13.2.3 Interaction with Other Degradation

Mechanisms ................................ 750

13.2.4 Experimental Planning

and Presentation of Results........... 750

13.3 Tribological Test Methods...................... 751

13.3.1 Sliding Motion ............................. 751

13.3.2 Rolling Motion ............................. 751

13.3.3 Abrasion ..................................... 752

13.3.4 Erosion by Solid Particles............... 753

13.3.5 Scratch Testing............................. 753

13.4 Friction Measurement ........................... 754

13.4.1 Friction Force Measurement........... 754

13.4.2 Strain Gauge Load Cells

and Instrumentation .................... 754

13.4.3 Piezoelectric Load Sensors ............. 755

13.4.4 Other Force Transducers ................ 755

13.4.5 Sampling and Digitization Errors .... 756

13.4.6 Calibration .................................. 756

13.4.7 Presentation of Results ................. 758

13.5 Quantitative Assessment of Wear ........... 759

13.5.1 Direct and Indirect Quantities ........ 759

13.5.2 Mass Loss .................................... 759

13.5.3 Dimensional Change..................... 759

13.5.4 Volume Loss ................................ 760

13.5.5 Other Methods ............................. 762

13.5.6 Errors and Reproducibility

in Wear Testing ............................ 762

13.6 Characterization of Surfaces and Debris .. 764

13.6.1 Sample Preparation ...................... 764

13.6.2 Microscopy, Proﬁlometry

and Microanalysis ........................ 765

13.6.3 Wear Debris Analysis..................... 767

References .................................................. 767

13.1 Deﬁnitions and Units

Tribology is the science and technology of interacting

surfaces in relative motion, which includes the study of

friction, wear and lubrication. Friction and wear rep-

resent two key aspects of the tribological behavior of

materials. Like other measurements of material perfor-

mance, such as corrosion or biodegradation, both friction

and wear result from exposure of the material to a par-

ticular set of conditions. They do not therefore represent

intrinsic material properties, but must be measured and

deﬁned under well-characterized test conditions.

Part D 13

744 Part D Materials Performance Testing

13.1.1 Deﬁnitions

Wear can be deﬁned as damage to a solid surface, gener-

ally involving progressive loss of material, due to relative

motion between that surface and a contacting substance

or substances [13.1]. Materials in contact are subjected

to relative motion in many different applications. In

some cases this motion is intentional: for example in

rotating plain bearings, pistons sliding in cylinders, auto-

motive brake disks interacting with brake pads, or in the

processing of material by machining, forging or extru-

sion. It may also be unintentional, as in the small cyclic

displacements known as fretting which can cause wear

in certain structural joints under oscillating loading. If

hard particles are present, for example as contamination

in a lubricant, or intentionally as in abrasive machining,

then these particles will have a profound inﬂuence on the

resulting wear process.

The friction force is deﬁned as the force acting tan-

gentially to the interface resisting motion, when, under

the action of an external force, one body moves or tends

to move relative to another. The friction force F may

be associated with sliding motion or with pure rolling

motion of the bodies; in some practical engineering ap-

plications such as the contact between ball and track in

a ball-bearing or between a pair of mating gear teeth,

there may be more complex motion involving both slid-

ing and rolling.

The coefﬁcient of friction μ is a dimensionless num-

ber, deﬁned as the ratio F/N between the friction force F

and the normal force N acting to press the two bod-

ies together. The kinetic coefﬁcient of friction μ

is the

coefﬁcient of friction under conditions of macroscopic

relative motion between the two bodies, while the static

coefﬁcient of friction μ

is the coefﬁcient of friction

corresponding to the maximum friction force that must

be overcome to initiate macroscopic motion between

the two bodies. The coefﬁcient of friction is a conve-

nient method for reporting friction force, since in many

cases F is approximately linearly proportional to N over

quite large ranges of N. The equation

F = μN (13.1)

is sometimes called Amontons Law.

The value of μ can be expected to depend signiﬁ-

cantly on the precise composition, topography and his-

tory of the surfaces in contact, the environment to which

they are exposed, and the precise details of the load-

ing conditions. Although tables of coefﬁcients of friction

have been published, they should not be regarded as any-

thing more than general indications of relative values

under the speciﬁc conditions of measurement. The co-

efﬁcient of friction usually lies in the range from 0 to

1, although there is no fundamental reason why it need

do so. Negative values of μ may be thought of as being

rather artiﬁcial, since they result from adhesive forces

between the bodies involved which result in a tangential

force while the bodies are subjected to a tensile normal

force, and friction is perhaps not a helpful term to de-

scribe this phenomenon; but values of μ greater than 1

are physically quite reasonable, and can be encountered,

for example, in the interaction between a car tire and

a dry road surface, or in the sliding of certain ductile met-

als in the absence of oxygen.

A large number of specialist terms used in tri-

bology have been formally deﬁned, most recently by

ASTM [13.1]. An earlier compilation including many

other terms, as well as their translations into French, Ger-

man, Italian, Spanish, Japanese, Arabic and Portuguese,

was published by an international group [13.2]. A par-

ticularly useful term is triboelement, deﬁned as one of

the two or more solid bodies involved in a tribological

contact.

13.1.2 Types of Wear

It is important, especially when describing wear, to dis-

tinguish clearly between the nature of the relative motion

responsible for the wear and the physical mechanisms by

which the material is removed or displaced in wear. Wear

tests are designed to produce speciﬁc types of relative

motion, which can often lead to different mechanisms

of wear in different materials, or at different loads or

speeds. Methods that can be used to distinguish between

wear mechanisms are discussed in Sect. 13.6.

Figure 13.1 illustrates some common types of rel-

ative motion that can result in wear. These represent

idealized examples and can be further subdivided. For

example, sliding and rolling motions may be either con-

tinuous or interrupted, and may occur along the same

track on a rotating counterbody or on a continuously

fresh track. They may involve constant relative veloc-

ity (continuous sliding), or varying velocity (such as

reciprocating motion with perhaps sinusoidal or linear

velocity variation).

When analyzing the motion in a tribological contact,

the nature of the contact must also be examined. It is

helpful to consider how the contact region moves with

respect to the surfaces of the bodies in contact. For ex-

ample, in the simple example of a small block sliding on

a larger counterbody shown in Fig. 13.1, the contact re-

gion is ﬁxed relative to the block, which is in continuous

Part D 13.1

Friction and Wear 13.1 Deﬁnitions and Units 745

contact, whereas it moves along the larger triboelement,

so areas of this experience contact only for a certain pe-

riod. In the case of simple rolling shown in Fig. 13.1,

the contact region moves relative to the surfaces of both

triboelements, and areas on both of these surfaces are

therefore in intermittent contact.

In many practical situations there is a lubricant ﬁlm

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-

ment may result in impact. Repeated impact by a large

counterbody on an essentially single-contact region can

lead to wear, as can impact by a large number of small

particles or even liquid droplets, in which case the impact

sites are usually distributed over an area of the surface.

Wear can also result from the collapse close to a solid

surface of vapor or gas bubbles produced by local pres-

sure ﬂuctuations in a cavitating liquid.

The presence of small particles in the contact re-

gion, usually harder and thus less deformable than one

of the moving bodies, leads to a signiﬁcantly different

wear process to that for plain sliding. The most common

types of wear associated with hard particles in a slid-

ing contact are illustrated in Fig. 13.2. If the particles

are attached to one of the sliding bodies, either form-

ing part of it (for example, as hard discrete phases in

a softer matrix, or as particles ﬁxed to the surface, as in

abrasive paper) or embedded in the surface, then they

Sliding

Rolling

Sliding +

rolling

Impact (large body)

Particle impact

Liquid impact

Cavitation

≠ r

Fig. 13.1 Examples of tribological contacts involving rela-

tive motion of triboelements, as well as erosion by particle

impact, liquid impact or cavitation in a liquid

Sliding abrasion

(two body)

Rolling abrasion

(three body)

Scratching

Fig. 13.2 Examples of abrasive wear due to the presence of

hard particles in a sliding contact, and scratch damage

will be dragged over the counterbody in a process de-

scribed as sliding abrasion. This type of abrasion is often

termed two-body abrasion, since the particles effectively

form part of one of the two triboelements. If the particles

are free, and roll between the sliding bodies, then their

interaction with them is different and can be described

as rolling abrasion; this is also known as three-body

abrasion, since the particles form a distinct third body

which moves with respect to the two sliding bodies. The

terms two-body and three-body can lead to confusion,

since the motion of free particles in a sliding contact will

depend on the relative hardnesses of the surfaces and

on the applied pressure, and can vary between sliding

and rolling even in different regions of the contact. The

conditions of abrasive wear are sometimes described as

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

fracture.

If surface damage results from the action of a single

hard asperity, or a small number of relatively large asper-

ities, then it is often termed scratching, and scratch test-

ing can be used to characterize the response of a material

to this type of deformation (see Sect. 13.3.5). Scratching

is also shown schematically in Fig. 13.2.

13.1.3 Units for Wear

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,

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

a spherically-ended pin to form a ﬂat area on the pin

may be calculated from a measurement of the diameter

of the ﬂat area; volume changes in more complex geome-

tries may be derived computationally from the difference

between proﬁle traces recorded before and after wear.

Part D 13.1

746 Part D Materials Performance Testing

Measurement of mass loss is in principle comparatively

straightforward, although great care must be taken in the

experimental technique to reduce errors. Linear changes

in dimensions (for example, the reduction in length of

a pin specimen, or the change in clearance in a bearing)

are also in principle straightforward to measure, but are

prone to error and so careful experimental procedures are

needed. Measurements of mass change can be affected

by material transfer and oxidation; dimensional changes

can also be inﬂuenced by thermal expansion. These and

other aspects of experimental techniques are discussed

in Sect. 13.5.

In some cases material may be lost from both tri-

boelements, or signiﬁcant transfer of material may occur

between the triboelements, and particular care is then

needed in both measuring and describing the magni-

tude of wear. If wear is associated with the movement

of material within the surface of a single triboelement,

it will lead to changes in the topography of the sur-

face which can be detected by proﬁlometry, as discussed

in Sect. 13.6.2.

The amount of wear that occurs when two surfaces

slide over each other tends to increase with sliding dis-

tance, and hence with time. The wear rate can then be

deﬁned as the rate of material removal or dimensional

change per unit time, or per unit sliding distance. Be-

cause of the possibility of confusion, the meaning of the

term wear rate must always be deﬁned, and its units

stated. In the case of erosion the wear rate is usually

quoted as the mass or volume loss per unit time; for ero-

sion by solid particle impact, it may also be expressed

Wear by hard

particles

Unlubricated

sliding

Lubricated

sliding

Erosion

Abrasion

Severe

Mild

Boundary

EHL

–14

–4

–6

–8

–10

–12

–2

Fig. 13.3 Typical ranges of wear coefﬁcient K exhibited under

various tribological conditions: hydrodynamically lubricated (HL);

elastohydrodynamically lubricated (EHL); lubricated by a boundary

ﬁlm; unlubricated sliding; abrasive wear; and erosive wear

as the volume or mass loss per unit mass of impinging

particles.

Many quantitative models for sliding wear have been

developed, but one of the simplest, due to Archard, is

useful for comparing wear rates and material behavior

under different conditions. The Archard model leads to

the equation

Q = KW/H , (13.2)

where Q is the volume of material removed from the sur-

face by wear per unit sliding distance, W is the normal

load applied between the surfaces, and H is the indenta-

tion hardness of the softer surface. Many sliding systems

do show a dependence of wear on sliding distance which

is close to linear, and under some conditions also show

wear rates which are roughly proportional to normal

load. The constant K, usually termed the Archard wear

coefﬁcient, is dimensionless and always less than unity.

The value of K provides a means of comparing the sever-

ities of different wear processes. Equation (13.2) can be

applied to abrasive wear as well as to sliding wear; an

analogous equation can also be developed for erosion by

solid particle impact, from which a value of K can be

derived.

Figure 13.3 shows, very approximately, the range

of values of K seen in various types of wear. Under

unlubricated sliding conditions (so-called dry sliding),

K can be as high as 10

−2

, although it can also be as

low as 10

−6

. Often two distinct regimes of sliding wear

are distinguished, termed severe and mild. Not 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 associated with rel-

atively large particles of metallic debris, while in mild

wear the debris is ﬁner and formed of oxide particles.

In the case of ceramics, the severe wear regime is as-

sociated with brittle fracture, whereas mild wear results

from the removal of reacted (often hydrated) surface

material.

When hard particles are present and the wear pro-

cess involves abrasion (by sliding or rolling particles)

or erosion (by the impact of particles), then the highest

values of K occur; the relatively high efﬁciency of the

removal of material by abrasive or erosive wear explains

why these processes can also be usefully employed in

manufacturing.

The values of K that occur for unlubricated sliding,

or for wear by hard particles, are generally intolera-

bly high for practical engineering applications, and in

most tribological designs lubrication is used to reduce

Part D 13.1

Friction and Wear 13.2 Selection of Friction and Wear Tests 747

the wear rate; the effect of lubrication in reducing wear

is far more potent than its effect on friction, and the in-

crease in life which results from the reduction of wear

is generally much more important than the increase in

efﬁciency from the lower frictional losses. As Fig. 13.3

shows, even the least effective lubrication can reduce

the wear rate by several orders of magnitude, and as the

thickness of the lubricant ﬁlm is increased in the progres-

sion from boundary, to elastohydrodynamic (EHL)and

then to hydrodynamic lubrication (HL), the value of K

falls rapidly. In the hydrodynamically lubricated compo-

nents of a modern automotive engine, values of K as low

as 10

−19

are achieved.

The quantity Q/W(= K/H), given the symbol k,

and sometimes termed speciﬁc wear rate (UK) or wear

factor (USA), is also useful. The units in which k is ex-

pressed are usually mm

/Nm, representing the volume

lost (mm

) per unit sliding distance (m) per unit normal

load on the contact (N). These units are commonly used

when quoting experimentally measured rates of wear. It

is useful to note that for a material with a Vickers hard-

ness H of 1 GPa (≈ 100 HV), then the numerical value

of K (dimensionless) is the same as that of k (in units of

/Nm).

Although k (or K) is effectively constant over quite

large ranges of load or sliding speed in many cases of

sliding wear, in some instances sharp transitions can oc-

cur, and k may change by a factor of 100 or even 1000

for a relatively small change in the conditions. This be-

havior is associated with a change in the predominant

mechanism of material removal; for this reason it is al-

ways dangerous to extrapolate wear data to predict the

likely rate of wear in a system from data obtained under

different conditions.

13.2 Selection of Friction and Wear Tests

Friction and wear testing is often carried out to ensure

that good tribological performance is achieved from

materials. This information is required at different times

in the life cycles of products. Designers require data that

will enable them to design products that will not wear

out or have unacceptably low efﬁciency, or even fail

to function, due to frictional losses; process engineers

need equipment to make the products that will function

cost-effectively without premature failure from wear;

and those marketing the products need to know that they

will have good tribological performance. Other tests are

performed for more fundamental reasons to investigate

wear mechanisms or material performance, perhaps as

part of a program of material development.

Friction and wear are important in most industrial

sectors, but there is now also increasing attention paid to

the application of tribological testing to societal needs.

Traditionally, the evaluation of friction and wear has

been of major concern in areas such as mining and

drilling, transport power plant, power generation and

machine tools. Now there is increasing attention to ar-

eas such as the tribological performance of biomedical

implants and the degradation of decorative ﬁnishes. In

recent years there has also been much attention given to

the design and development of microelectromechanical

systems (MEMS) where friction and wear are strongly

inﬂuenced by the scale of these small components, ham-

pering the development of new devices.

13.2.1 Approach to Tribological Testing

There are many different approaches to friction and

wear testing, illustrated schematically in Fig. 13.4,that

differ in the scale and complexity of the element that

is being tested. Ultimately ﬁeld testing of the ﬁnal

product will always be required for ﬁnal proof of effec-

tiveness, but it can often be prohibitively expensive to

evaluate several different alternative materials or design

variations by this method. It also suffers from a lack

of control of the parameters that deﬁne the wear pro-

cesses that are occurring, so that an understanding of the

dependence of wear mechanisms on important factors

such as load and speed cannot be developed.

As Fig. 13.4 demonstrates, it is possible to break

down the complete product into its functional elements.

The ﬁnal stage of this process of abstraction is to use

laboratory tests that simulate a particular tribological

element of the complete product. Moving away from

ﬁeld trials towards laboratory testing, in general, re-

duces the costs of testing and increases the ability to

control the important factors that affect the magnitude

of friction and wear. This enables programs of testing

to be designed that allow for the cost-effective devel-

opment of materials and tribological systems through

better understanding of the tribological processes.

There are two main situations for which laboratory

tests are required. The ﬁrst is to provide information on

Part D 13.2