Bayer R.G. Mechanical Wear Fundamentals and Testing, Revised and Expanded

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formation can either decrease (124) or increase (140) with increasing load. Again, this

would suggest that an optimu m condition should exist for ﬁlm formation.

Geometrical and shape elements, which can affect the trapping and displ acement of

debris in the co ntact region, can also have an effect of triboﬁlm formation (128).

3.8. ABRASIVE WEAR

Abrasive wear is wear caused by hard particles and protuberances. Abrasion and erosion

are terms commonly used for abrasive wear situations. These two types of situations are

illustrated in Fig. 3.69. When two surfaces are involved, the wear situation is generally

referred to as abrasion. A distinction is usually made between two types of abrasion,

two-body and three-body abrasion , because of signiﬁcant differences in the wear behavior

associated with these two situations. Two-body abrasion is when the wear is caused by

protuberances on or hard particles ﬁxed to a surface. Three-body abrasion is when the

particles are not atta ched but between the surfaces (142). Filing, sanding, and grinding

would be examples of two -body abrasion, as well as wear caused by magnetic media

and paper; a rough, ﬁle-like metal surface sliding on a polymer surface would be another.

Examples of three-body abrasion would be wear caused by sand or grit in a bearing and

hard wear debris and abrasive slurries trapped between moving surfaces. The term erosion

is generally applied to abrasive wear situations when only one surface is involved. Slurry

erosion and solid particle erosion are common generic terms for such situations. Solid par-

Figure 3.69 Abrasive wear situations.

ticle erosion is when a stream of particles or ﬂuid containing particles impacts a surface,

causing wear. The wear caused by sand and grit in air streams are examples. An example

of slurry erosion would be the wear of pipes through which slurries are pumped. Examples

of abrasive wear scars are shown in Figs. 3.70 and 3.71.

In the following discussion of abrasive wear two-body abrasion by protuberances is

considered to be equivalent to two-body abrasion by hard particles or abrasive grains

attached to a surface.

Figure 3.70 Examples of abrasive wear. ‘‘A’’, two-body abrasion. ‘‘B’’, particle impingement. ‘‘C’’

and ‘‘D’’, three-body abrasion. (‘‘A’’, and ‘‘B’’ from Ref. 152 and ‘‘C’’ and ‘‘D’’ from Ref. 64; rep-

rinted with permission from ASME.)

When the abrasives are harder than the surface they are wearing, the dominant type

of wear mechanism in abrasive wear is single-cycle deformation, though repeated-cycle

deformation mechanisms, as well as chemical and thermal mechanisms may also be

involved (143). When the abrasives are softer than the surface they are wearing, the

dominant type of mechanism becomes repeated-cycle deformation (61,144). In abrasive

wear situations, the signiﬁcance of single-cycle deformation mechanisms does not

decrease with sliding or duration, as they typically do in nonabrasive wear situations.

Generally, these mechanisms remain the same unless there is a change with the charac-

teristics of the particles involved, such as changes in size, sharpness, or amount. Such

changes can take place as a result of particle wear and fracture and the accumulation

of particles within the contact area with time. The atmosphere and ﬂuid media in which

abrasive wear takes place is often a factor in the abrasive wear. Wear rates tend to be

higher when there is a chemical interaction with the wearing surface. This is generally

attributed to chemical wear mechanisms, the modiﬁcation of surface mechanical proper-

ties as a result of chemical interaction, that is, the Rebinder Effect (145), and synergistic

effects between wear and corrosion (146). Synergism between wear and corrosion results

from the fact that wear produces fresh surfaces, which are more readily oxidized. In turn,

this increased oxidation results in higher wear rates.

Figure 3.70 (continued )

It is generally found that one or more of the following equations can describe abra-

sion: (34,61,144,147–151)

V ¼ KPS ð3:94Þ

V ¼

KPS

ð3:95Þ

V ¼

KPS

ð3:96Þ

In these equations, V is wear volume, P is load, S is sliding distance, and p is hardness. K is

a wear coefﬁcient, which is determined empirically. Equation (3.94) is the most broadly

Figure 3.71 Expample of wear scars resulting from three-body abrasion. ‘‘A’’,‘‘B’’, and ‘‘C’’ illus-

trate various degrees of severe abrasive wear, while ‘‘D’’ is an example of mild abrasive wear. (‘‘A’’

from Ref. 148, ‘‘B’’ from Ref. 191, ‘‘C’’ from Ref. 192, and ‘‘D’’ from Ref. 34. ‘‘A’’ and ‘‘B’’ rep-

rinted with permission from ASME. ‘‘C’’ reprinted with permission from IBM. ‘‘D’’ reprinted with

permission from Elsevier Sequoia S.A.)

applicable one. It is applies to most materials systems, independent of the relative hardness

of the surface to the abrasives, and to two-body and three-body abrasion. In this equation,

the wear coefﬁcient is a function of the wearing material, the abrasives, the media or

environment in which the abrasio n takes place, and the freedom of the particles to move.

Equation (3.95), whi ch is the same as the equation used for single-cycle wear, Eq. (3.20), is

generally applic able to all material systems and types of abrasion when the abrasive is

harder than the wearing surface. As discussed in Sec. 3.3, K in this case tends to be depen-

dent only on material type, not individual materials. This is illustrated by the data in Figs.

3.20 and 3.72. Otherwise the dependencies are the same as with Eq. (3.93). The situation is

the same with Eq. (3.96). However, this equation applies to situations where the hardness

of the surface and abrasives is similar or when the surface is harder. When similar, limited

data indicate that n is around 10. When the surface is harder, n is around 5 (151). The

change in the hardness dependency between these two equations is the result of the change

in the type of dominant wear mechanism, that is, from single-cycle deformation to

repeated-cycle deformation.

Nominal values for K in Eq. (3.95) are given in Table 3.8 for a variety of conditions.

It can be seen that K ranges over several orders of magnitude and that some trends exist.

One trend that is evident is that two-body abrasive wear situations generally have higher

values of K than three-body conditions. The explanation for this is that in the three-body

situation, the abrasive grain is free to move and therefore may not always produce wear.

For example, it may roll and tumble along the surface instead of sliding and cutting

out a groove. Or it may align itself so that the bluntest proﬁle presents itself to the surface.

This concept is illustrated in Fig. 3.73.

A second trend that is illustrated by the data given in Table 3.8 is that the larger the

abrasive grain or particle, the larger the value of K. This same trend is also found in three-

body abrasion (152). In addition to the intuitive one that larger grains can form larger

Figure 3.72 The effect of hardness on the abrasive wear rate of pure metals. Data are for two-body

abrasion, when the abrasive is harder than the abraded surface. (From Ref. 193.)

groves, several reasons for this trend have been proposed. One of these is that surface

roughness and debris clogging effects become less signiﬁcant with larger grains. This is illu-

strated in Fig. 3.74. Another mechanism that has been proposed is that larger grains are

more likely to fracture with multiple engagements, forming new particles with sharp edges,

while smaller particles are likely to have their edges rounded by a wear process. Still

another possibility is that with naturally occurring abrasive particles it is frequently

difﬁcult to separate size and sharpness. Therefore it has been suggested that in certain

cases the larger particles may just be sharper. As with most aspects of wear all of these

effects probably contribute to the overall trend with some being more signiﬁcant than

others in particular situations.

While the precise reasons for the dependency on size is not known, there appears to

be a very deﬁnite relationship that ap plies to many situations (34,153). This trend is shown

Table 3.8 K Values for Abrasive Wear

Condition

Dry Lubricated

Two-body

File 5  10

2

1

New abrasive paper 10

2

2  10

2

Used abrasive paper 10

3

2  10

3

Coarse polishing 10

4

2  10

4

< 100 mm particles 10

2

> 100 mm particles 10

1

Nominal range, dry and lubricated: < 1to > 10

4

Three-body

Coarse particles 10

3

5  10

3

Fine particles 10

4

5  10

4

Nominal range, dry and lubricated: < 10

2

to 10

6

Figure 3.73 The effects of rolling and sliding actions in three-body abrasion.

in Fig. 3.75. It can be seen that in all cases there appears to be an almost linear relationshi p

between size and wear, up to approximately 100 mm, but above that size wear tends to be

independent of particle size. As with the general trend with size seen in Table 3.8, there is

no established explanation for the transition above 100 mm. However, it has been proposed

that in addition to the aspects mentioned earlier, particle loading could also play a role. As

size increases, the number of particles involved at any instant may change, probably

decreasing, which would tend to decrease the abrasive wear rate. As the number decreases,

the load per particle would increase, which woul d tend to increase wear associated with

each particle. These two effects would tend to offset each other. Under certain conditions,

they could cancel and stable wear behavior as a function of size could result.

Figure 3.74 The effect of the accumulation of wear debris in two-body abrasive wear.

Figure 3.75 The inﬂuence of abrasive particle size on wear. The data are for SiC particles. (From

Ref. 153.)

The third trend, which can be seen in the data given in Table 3.8, is the inﬂuence of

lubrication. Lubrication tends to increase abrasive wear. This is consistent with the effect

of lubrication on single-cycle deform ation wear discussed in Sec. 3.3, where two mechan-

isms are proposed for this. One is a change from plowing to cutting. Basically, cutting

results in more material removal than plowing or plastic deformation. By reducing, the

coefﬁcient of friction lubrication can increase the likelihood or amount of cutting taking

place by lowering the critical attack angle for cutting. The variation in critical attack angle

with the coefﬁcient of friction is shown in Fig. 3.76. The second way lubrication can affect

abrasive wear is the prevention of clogging by wear debris (154). While both are likely to

be involved in the case of abrasion, the primary mechanism for this trend is generally

accepted to be the accumulation of wear debris, which shares the load and protects the

surface from the abrasive grains. The presence of a liqui d lubricant at the interface helps

to ﬂush the wear debris from the interface and to reduce the shielding effect. The simplest

illustration of this behavior is the build-up of debris that occurs on polishing and

sanding papers and on ﬁles without lubrication. When these surfaces become sufﬁciently

contaminated, the effective abrasive action decreases.

Experiments with dry, silicon carbide abrasive paper show that the wear rate

decreases to 0 with time and that the effect occurred sooner with ﬁner grain papers

(155). While two effects probably contribute to the total behavior, that is, clogging of

Figure 3.76 Critical angle of attack for cutting as a function of the coefﬁcient of friction (based on

Eq. (3.27)).

the surface of the paper and wear of the grains, the latter point suggests clogging as being a

signiﬁcant factor. Finer grain paper would be more easily clogged than larger grain paper.

A common practice in ﬁling and polishing (sanding) is to use a lubricant to reduce clog-

ging of the paper or ﬁle.

The signiﬁcance of contamination by wear debris on the abrasive wear process is also

illustrated in a study of abrasive wear of polymers (156). The following type of relation-

ship was found for wear:

V ¼ Kx

ð3:97Þ

with n < 1. With n < 1 a decreasing wear rate occurs. Graphically this behavior is illu-

strated in Fig. 3.77 for several plastics and different values of n. This behavior was corre-

lated to the build-up of a polymer layer on the abrasive surface whi ch prevented some of

the abrasive grains from contacting the wear surface. As the layer became thicker, more

and more grains would be buried. This effect is illustrated in Fig. 3.78.

In addition to size, the wear coefﬁcients in the equations for abrasive wear are also

affected by other attributes of the particles. One is their sharpn ess or angularity. Wear

coefﬁcients generally are higher for angular particles than rounded particles. The number

of particles involved is also a factor. In general, there tends to be a saturation level in terms

of the number of particles, above which wear rate does not increase. This is generally

attributed to the fact that below a certain number of particles part of the load is supported

by asperity contact. This is shown in Fig. 3.79. Difference in particle friability and wear

resistance can also affect values of these coefﬁcients (34,157).

These same general trends also apply to the wear coefﬁcient when the surface is

harder than the abrasives, that is K in Eq. (3.95). However, the exact relationships can

be different. For example, with a harder surface, particle size is not a factor above

Figure 3.77 Effect of debris accumulation on abrasive wear. The equivalent value of the exponent

in Eq. (3.97) is shown on the graphs. (From Ref. 156, reprinted with permission from Elsevier

Science Publishers.)

10 mm, while for softer su rfaces, this only occurs above 100 mm (34,158). Difference in par-

ticle friability and wear resistance would also tend to be more important when the surfaces

are harder than when they are softer.

As shown previously in Fig. 3.20, a one-to-two order of magnitude reduction in wear

rate is typically found when the hardness of the abraded surface exceeds the hardness of

the abrasive. This is a very signiﬁcant fact for practical handling of abrasive wear situa-

tions. Basically, to achieve low wear rates in abrasive wear situations, the goal is to select

a material which is harder than the abrasives encountered.

In erosive situations, particles are not pressed against the surface as in abrasion; they

impact the wearing surface. The load between the particle and the surface is an impulse

load, which can be described in terms of the momentum and kinetic energy of the particle.

Figure 3.78 The effect of polymer ﬁlm formation on abrasive wear rate.

Figure 3.79 The effect of the amount of abrasives, a, on the abrasive wear coefﬁcient, showing a

saturation effect. (From Ref. 34.)