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

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oxidative or chemical wear process discus sed in the section on wear mechanisms. In this

case, an oxide or other type of reacted layer is formed on the surface, as a result of the

wear. In general, the properties of the reacted layer will be different from those of the par-

ent material or any initial oxide. Another way in which the chemical make-up of the tri-

bosurface can be modiﬁed is conceptually illustrated in Fig. 4.21. Wear fragments from the

counterface, or wear fragments from the reacted layer, are worked into the surface, form-

ing a composite structure. An example of this type of phenomenon in the case of impact

wear is shown in Fig. 4.22. Similar observations have been made for sliding wear (32–34).

Figure 4.20 (continued )

Structural changes can also take place as the result of the wearing action (e.g., plastic

deformation and ﬂow). In the case of metals, changes in dislocation density and grain size

at or near the surface are frequently observed in wearing situations. Figure 4.23 shows

some examples of this behavior. Frequently, these changes result in a harder, more brittle

surface. When hardness changes as a result of wear, it is generally found that wear beha-

vior is related to the modiﬁed hardness. This is also true, when hardness is affected by fric-

tional heating, as described further in this section.

Another example is the formation of a layer composed of extremely ﬁne grains and

ﬂow-like striations found in some sliding wear situations (Fig. 4.24). As is apparent in this

ﬁgure, the morphology of this layer is very suggestive of ﬂuid ﬂow, and shows both

laminar and turbulent characteristics in different regions. Careful examination of the

Figure 4.22 Example of mechanical mixing under impact conditions between a steel sphere coated

with a thin layer of NI (5000 A) and a Cu ﬂat. A cross-section through the wear scar in the Cu is

shown in ‘‘A’’. In ‘‘B’’, an EDX dot map of NI is shown for this region, conﬁrming the mixing.

(From Ref. 92, reprinted with permission from Elsevier Sequoia S.A.)

Figure 4.21 Mechanically mixed surface layer.

micrograph shows the formation of what appears to be an extrusion lip. Because of this lip

formation and the general characteristics of the layer, ﬁne grains, and ﬂow c haracteristics

(which are very similar to behavior seen in metal working operations), this mode of wear

has been referred to as extrusion wear (35).

Figure 4.23 Examples of the dislocation networks formed and grain size changes produced during

wear. ‘‘A’’ is the dislocation networks formed beneath the wear surface of a Cu specimen during ero-

sion. ‘‘B’’ shows the grain growth occurring in Cu in a small amplitude sliding wear situation. (‘‘A’’

from Ref. 93, reprinted with permission form ASTM; ‘‘B’’ from Ref. 94, reprinted with permission

from ASME.)

Figure 4.24 Example of subsurface ﬂow during wear: ‘‘A’’ for sliding, ‘‘B’’ for impact. (‘‘A’’ from

Ref. 95, reprinted with permission from ASM International; ‘‘B’’ from Ref. 92, reprinted with per-

mission from Elsevier Sequoia S.A.)

Preferential wearing of one phase of a multiphase material can also result in a change

in the composition of the surface and inﬂuence wear behavior. An example of this would

be the preferential removal of a so ft matrix around a hard ﬁller, grain, or particle (36).

This situation is illustrated in Fig. 4.25. This type of action can also produce topological

or roughness changes. A further way by which surface composition can change is shown in

Fig. 4.26. This is by preferential diffusion of certain elements (37), either to the surface of a

material or into the surface of the mating material. Solubility and temperature are strong

factors in this type of mechanism. This is frequently a factor in wear situations involving

high temperatures, such as in machine tool wear.

Up to this point, metals have been used to illustrate composition an d structural

changes in tribosurfaces; however, such changes are not conﬁned to this class of materials.

Similar changes can take place with other classes but they may be of different types, which

depend on the basic nature of the material. With polymers, for example, changes in both

the degree of crystallinity, chain length, and degree of cross-linking have been observed.

The formation of different polymer structures has also been observed to occur as a result

of wear (38–41).

Figure 4.25 Example of preferential wear of a soft matrix. This micrograph is of a coating, which

contained diamond particles in an Rh matrix, after it was worn by sliding against a paper surface

coated with magnetic ink. (From Ref. 36.)

A factor that has to be included with the consideration of tribosurfaces is surface

temperature. In addition to leading to thermal wear mechanisms, discus sed in Sec. 3.6, sur-

face temperatures can affect wear behavior in other ways. As indicated in that section,

there are several factors which inﬂuence surface temperature, such as the heat energy gen-

erated at the surface, the thermal conductivities of the materials, heat conduction paths

away from the interfaces, and ambient temperature. This is illustrated in Fig. 4.27.

Because the heat or thermal energy is generated in the surfa ce (e.g., frictional heating in

sliding), surface temperatures are generally higher than elsewhere in the materials, which

can inﬂuence the nature of the surface in two ways. One is simply related to the fact that

most material properties are temperature dependent. As a result, the surface will exhibit

material behavior appropriate to that elevated temperature. For example, the hardness

of metals can decrease with surface temperature. This type of effect is particular ly impor-

tant in the case of polymer wear. With polymers degradation in wear performance is gen-

erally observed with the surface temperatur es approaching and exceeding the glass

transition temperature (23,42,43). A secon d way in which surface temperature can inﬂu-

ence tribosurfaces is through the temperature dependencies that the surface modiﬁcation

processes have. Elevated temperature can increase reaction rates, inﬂuence phase changes,

increase diffusion, and enhance ﬂow characteristics of materials.

Figure 4.26 An example of a diffusion layer formed on a tool surface during machining. The dia-

gram shows the location of the diffusion layer on the tool surface. (From Ref. 96.)

In addition to these possibilities, removal of existing layers and the formation of new

layers or ﬁlms, such as, transfer and third-body ﬁlms, can modify tribo surfaces, and

in turn affect wear behavior (7,16,24). Typically, a change in the coefﬁcient friction

can be found with changes in oxide structure, removal of other ﬁlms, and the form ation

of triboﬁlms (44). Removal of existing ﬁlms and oxides is often a contributing factor

of break-in behavior (45). Many investigators have identiﬁed the formation of transfer

and third-body ﬁlms and their importance in wear and friction behavior

(6,7,18,26,33,38,39,42, 46–49). The formation of these triboﬁlms and related wear pro-

cesses are described in Sec. 3.7.

While it is convenient for discussion to consider separately the various ways tribosur-

faces can unde rgo changes as a result of wear, it must be recognized that under actual

wearing conditions these changes can be going on simultaneously and in an interactive

fashion. For example, a ﬂow layer might also contain a mixture of material from both sur-

faces, as well as oxides of these materials. An illustration of what typical conditions might

exist on worn surfaces is shown in Fig. 4.28.

4.4. WEAR TRANSITIONS

These changes to tribosurfaces, coupled with the possibility of different wear mechanisms,

can result in dramatic changes or transitions in wear behavior (44). The break-in pheno-

menon discussed previously is an example of such a transition. Break-in transitions tend

to be gradual and are usually attributed to the modiﬁcation of the surfaces by such mecha-

nisms as oxide formation, transfer ﬁlm formation, and asperity proﬁle modiﬁcation that

Figure 4.27 Factors affecting surface temperatures.

are associated with the wearing action. Transitions, which are associated with changes in

parameters, such as load, speed, relative humidity, or tempe rature, tend to be much more

abrupt and are frequently associated with a transition between mild and severe wear. In

either case, there can be changes in relative contributions and interactions of the several

possible wear mechanisms, along with changes in the characteristics of the wearing sur-

faces. To illustrate the general nature of such trans itions and the possible factors involved,

several examples will be considered.

During a break-in period, the wear rate is higher than after the break-in period. In

this sense, break-in behavior can be thought of as a transition from a severer to a milder

mode of wear. A wear curve, showing typic al break-in behavior, is presented in Fig. 4.29.

In this case, the wear is the wear of the plastic member of a cermet-plastic sliding wear

system. The micrographs in the ﬁgure show the appearance of the surfaces after the

break-in period. The break-in period in this case is associated with the formation of trans-

fer and third-body ﬁlms on the surfaces. Break-in behavior is not limited to polymer–metal

sliding systems but occurs for other systems and in other situations. However, the mechan-

ism involved may be different. For example, the fretting wear behavior of a metallic sys-

tem is plotted in Fig. 4.30. A break-in period is evident and oxide ﬁlm formations, along

with topological changes, are associated with this period. The appearance of the wear scar

in the break-in period and the stable period are different. These are shown in Fig. 4.31.

Another example for a metallic system is shown in Fig. 4.32. In this case, it is for a more

normal or gross sliding situation. However, the explanation is the same, oxide formation

with surface temperature being a driving factor. The insets show how the wear surface

appears in both regions.

In addition to the occurrence of break-in other transitions can occur as a function of

duration of the wearing actio n. An example of this is the behavior found in some four-ball

wear tests with lubricated metal pairs. This is shown in Fig. 4.33. In this particular case,

there appears to be several identiﬁable regions of wear behavior. The initial break-in pe r-

iod, a region of steady state wear, followed by a period of zero wear rate, ultimately leads to

a region of rapid or accelerated wear. The appearances of the worn surfaces in these

regions are different . The overall behavior is likely the result of a complex relationship

between ﬁlm formation, topological modiﬁcation, changing wear mechanisms, and lubri-

cant effects. A possible scenario is that oxide and other ﬁlms form in the two initial peri-

ods, along with micro-smoothing of the topography, which leads to a period of very low

wear rate. Ultimately, however, fatigue wear roughens the surface and disrupts the bene-

ﬁcial ﬁlm, leading to accelerated wear.

Figure 4.28 Possible state of a surface modiﬁed by wear.

Another example of a transition in wear behavior is shown in Fig. 4.34. This is for

sliding wear of a metal–polymer system. In this case, an increase in wear rate is seen well

beyond the break-in period, and is associated with the disruption of the third-bod y ﬁlm.

The data shown in the ﬁgure are for UHMWPE (ultra high molecular weight polyethy-

lene) and were obt ained in a thrust washer test. The same material, when tested in a

pin-on-disk conﬁguration, showed similar behavior (i.e., the occurrence of a transition

well after break-in) (11,28). The reason for the increased wear was identiﬁed with the start

of fatigue wear, as evidence by micro-cracks in the surface. These delayed contributions

from fatigue wear are the result of the incubation period associated with this mechanism.

Another example of transitional behavior is the PV Limit generally associated with

the wear of polymers in metal–polymer and polymer–polymer sliding systems. In this case,

Figure 4.29 (‘‘A’’) Wear curves for the plastic in a plastic-cermet sliding situation. The curves are

for increasing roughness of the cermet. The mircrographs show the discontinuous transfer ﬁlm on

the counterface (‘‘B’’) and the third-body ﬁlm formed on the plastic (‘‘C’’) during break-in that

results in the reduction of wear rate. (From Ref. 7, reprinted with permission from ASME.)

the transition is between mild and severe wear behavior. Sliding conditions above the PV

Limit result in too high a temperature for the polymer, softening it to the extent that accel-

erated wear occurs. This behavior is illustrated in Fig. 4.35, which shows the PV Limit

curve for a polyimide and the correspondi ng relationship between wear rate and surface

Figure 4.30 Break-in behavior in the case of fretting between two metals. (From Ref. 24.)

Figure 4.31 Examples of wear scar morphologies for the system referenced in Fig. 4.30. ‘‘A’’ illus-

trates wear scar morphology prior to the formation of a stable and continuous oxide ﬁlm. ‘‘B’’ illus-

trates the appearance after the formation of such a ﬁlm. (From Ref. 24, reprinted with permission

from Elsevier Sequoia S.A.)

Figure 4.32 Example of break-in behavior as a result of oxide formation. Micrograph ‘‘A’’ shows

the appearance of the wear scar prior to the formation of the oxide layer, and ‘‘B’’ shows the appear-

ance after. (From Ref. 14, reprinted with permission from ASME.)

Figure 4.33 Time-dependent wear behavior of a lubricated metal couple in a four-ball wear test.

(From Ref. 19.)