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

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The remaining class of thermal wear mechanisms is that associated with thermoelas-

tic instability. The acronym TEI and the term, thermal mounding, are other names used

for these processes. These processes essentially involve the collapse of the real area of con-

tact to a few localized areas as a result of localized thermal expansion (117). These areas

are referred to as hot spots or patches. Once formed, these sites can initiate other forms of

wear, including other forms of thermal wear. The minimum number of hot spots is the

minimum number required for mechanical stability, which in some cases can be as little as

one. While not limited to these situations, TEI wear processes are often signiﬁcant in the

wear behavior of seals, electrical brushes, and brakes (28,108). In addition to wear, TEI

processes can direct ly cause leakage in seals as a result of increased separation between

surfaces. Wear scars associated with thermoelastic instability tend to exhibit localized heat-

affected and thermally distressed areas, that is, hot spots or patches. Examples of such wear

scars are shown in Fig. 3.57. The following scenario descri bes the evolution of these hot spots.

Assume that as a result of a nonuniform temperature distribution or nonhomogene-

ity in thermal properties, a region or regions in the app arent contact area begins to bulge

above the mean level of the surface. As a result of this tendency, these areas will absorb

more heat and experience increased wear. If conditions are such that the increase in heat

is dissipated fast enough and the differential wear rate is large enough, the bulge will not

form and conditions will tend to become stable and more uniform across the contact.

However, if the increase in heat results in still higher local temperature and the increased

wear rate is not high enough, the contact will become unstable. A bulge will form and con-

tinue to grow, until contact between the surfaces is limited to those regions. It has been

found that the onset of this unstable behavior can be related to speed. There is a critical

speed, n , above which a contact becomes unstable and thermal bulges or patches will

form and below which they do not form.

Unlike other types of thermal wear processes, which generally do not occur under

lubricated conditions, TEI can occur under lubricated conditions. While the local

collapse of a ﬂuid ﬁlm can lead to TEI behavior, less severe perturbations to the lubricant

ﬁlm can also cause the formation of thermal patches as a result of changes in viscous heat-

ing in the ﬂuid (118,119).

Studies have indicated that stable arrangements or groups of hot patches can occur,

each with their own critical speed. While stable, these groups are not necessarily stationary.

For example, with seals, hot patches have been found to slowly precess around the seal

(120). The critical speeds for the formation of these groups depend on the size and geometry

of the contacting members. In addition to these factors, n is also a function of the relative

conductivity of the surfaces, thermal and mechanical properties of the surface, wear, and

lubricant properties but not directly of load. The following two equations have been

obtained for n. Equation (3.91) is for an unlubricated system and Eq. (3.92) for a lubri-

cated system (117,118). Both are based on some limiting assumptions: no wear; a noncon-

ductive, ﬂat and rigid counterface; simple cup face seal co nﬁguration. However, they do

provide some insight concerning the signiﬁcance of some parameters affecting TEI behavior



4pk

Elmw

ð3:91Þ



2pz



1=2

ð3:92Þ

In both equations, w is the spacing between the hot patches. z is the mean ﬁlm thickness and

g is the viscosity of the lubricant, respectivel y. The lowest critical speed would occur for the

largest spacing between hot patches possible. For the cup conﬁguration assumed by these

models, this would be the circumference of the cup. The effects of wear and counterface con-

ductivity on the value of n  are signiﬁcant. The modeling results shown in Figs. 3.58, 3.59,

3.60, and 3.61 illustrate their signiﬁcance.

The effect of counterface conductivity has been modeled for a cup seal conﬁguration

(120). The results of this analysis are shown in Figs. 3.58 and 3.59. In Fig. 3.58, this effect is

demonstrated as a function of the ratio of conductivities of the two surfaces. In Fig. 3.59,

Figure 3.57 Examples of wear scars resulting from thermoelastic instability, TEI. The examples are

from seals used in different applications. ‘‘A’’–‘‘D’’ are metal seals. ‘‘E’’ and ‘‘F’’ are carbon seals.

The localized regions of damage and discoloration are the result of thermoelastic instability. (From

Ref. 189, reprinted with permission from Elsevier Sequoia S.A.)

Figure 3.58 Variation of the critical disturbance velocity for a less conductive body sliding against

a more conductive body. The properties of both bodies were assumed to be aluminum with the

exception that the less conductive body was assumed to have a hypothetical reduced conductivity.

(From Ref. 117, reprinted with permission from Elsevier Sequoia S.A.)

Figure 3.59 Effect of the thickness of a thin glass ﬁlm, z, on the critical disturbance velocity in alu-

minum. (From Ref. 117, reprinted with permission from Elsevier Sequoia S.A.)

Figure 3.60 The effect of wear on the critical disturbance velocity of a scraper. Larger values of B

result in greater amounts of heat going to the counterface. (From Ref. 117.)

Figure 3.61 The effect of wear rate on the critical disturbance velocity for unlubricated self-mated

steel and aluminum. (From Ref. 188, reprinted with permission from Elsevier Sequoia S.A.)

it is illustrated by the effect of thin insulating layer on n. It was also found in these

analyses that the motion of the hot spots is affected by the pa rtition of heat. The effect

of wear against a conductive counterface on n  was modeled for a ﬂat blade sliding against

a rotating drum (119–122). The normalized results of that model are shown in Fig. 3.60.

The results for a steel and aluminum blade are shown in Fig. 3.61. This graph shows that

high wear rates can signiﬁcantly increase n.

An important aspect of TEI thermal wear processes is that they can occur under con-

ditions where there is only a moderate rise in surface temperature, for example, TEI beha-

vior has been observed in situations where the temperature rise of metal surfaces is 100



or less (123). The other types of thermal mechanism typically require signiﬁcantly higher

temperatures. For these, the severity of thermal wear can be reduced by cooling and using

materials whose properties are less sensi tive to increases in temperature.

3.7. TRIBOFILM WEAR PROCESSES

Many investigators have identiﬁed triboﬁlms and their importance to wear and friction

behavior (22,107,124–133). Triboﬁlms are layers of compacted wear debris that form on

surfaces during sliding. Such ﬁlms are also called transfer ﬁlms, third-body ﬁlms or simply

third-bodies. The term transfer ﬁlm is commonly used when the composition of the mate-

rial in the layer is the same as the counterface. The term third-body is a generic term for

any interface layer or zone which has different material properties than the surfaces and

across which velocity differences are accommodated (134). When used to refer to a tribo-

ﬁlm, it generally implies a mixture of wear debris in the layer. Triboﬁlms act as a lubricant

layer between the surfaces, providing separation and accomm odating relative motion

between the two surfaces. Relative motion with these layers is accomplished by shear

within the layer or slip between the layer and the surface. Examples of triboﬁlms on wear

surfaces are shown in Figs. 3.62 and 3.63.

Triboﬁlm wear processes are wear processes in which mass loss from the surfaces or

tribosystem occurs through loss of material from triboﬁlms. As material is lost from these

ﬁlms fresh wear debris from the surface enters the layer to maintain the ﬁlm. This process

is illustrated in Fig. 3.64. Before being lost from the ﬁlm, debris material is circulated

within the layer and between the surfaces. When a stable ﬁlm is formed, equilibrium

requires that the amount of material entering into the layer is the same as lost from the

layer. Therefore, once a stable ﬁlm is formed, wear behavior can be described with the

same models and relationships used for debris-producing mechanisms, such as adhesion

or repeated-cycle deformation mechanisms, by considering the ﬁlm as a lubricant. Concep-

tually, if WR

is the wear rate of a debris-producing mechanism without a triboﬁlm pre-

sent, the wear rate, WR, with the triboﬁlm present is

WR ¼KWR

ð3:93Þ

where K is a proportionality constant, which can generally be incorporated into the

empirical wear coefﬁcients of the model.

Triboﬁlms have a signiﬁcant effect on wear behavior (22,124,135). Generally when

stable ﬁlms are form ed, a signiﬁcant reduction in wear rate is seen. Such an effect can

be seen in the data shown in Fig. 3.65. While such ﬁlms are frequently cited as being

key aspects in the wear of polymer–metal systems (124,125,129,130,135), such ﬁlms can

also occur in other sliding systems, for example metal–metal and metal–ceramic.

(87,126,130–133,136). Examples of these are shown in Fig. 3.63. Generally, it is the softer

material that will form the ﬁlm. It should be recognized that while the material in these

layers originates from the sliding members, the properties may be different since they

can experience high shear and deformation, as well as elevated temperature in the forma-

tion process.

Figure 3.62 Examples of transfer and third body ﬁlms formed during sliding between plastic and

metal surfaces. ‘‘A’’ and ‘‘B’’ show disrupted polymer transfer ﬁlms formed on a metal counterface.

‘‘C’’ shows a continuous polymer transfer ﬁlm formed on a metal counterface sliding against a fabric

reinforced plastic. ‘‘D’’ shows the initial stages and ‘‘E’’ the ﬁnal stages of the third-body ﬁlm formed

on the surface of the plastic in that case. (‘‘A’’ and ‘‘B’’ from Ref. 128, reprinted with permission

from Butterworth Heinemann Ltd.; ‘‘C’’ and ‘‘D’’ from Ref. 140, reprinted with permission from

ASME.)

Initially, the ﬁlms tend to form in patches but with continued sliding, the coverage

becomes more uniform. During this phase, the thickness of the deposition might change

as well. At some point, a stable ﬁlm with a characteristic thickness is established. Studies

have indicated that the more complete the coverage, the better the wear performance.

Figure 3.63 Examples of nonpolymer ﬁlm formation. ‘‘A’’ shows an autoradiographic micrograph

of the wear track on a Ni surface, sliding against a ferrite counterface. The dark region indicates the

existence of a ferrite layer on the surface of the Ni. ‘‘B’’ shows the graphite ﬁlm that is formed during

rolling contact between graphitic A1 counterfaces. ‘‘C’’ shows the transfer ﬁlm formed on a steel sur-

face in sliding contact with a TiN counterface. The Auger spectra shown in ‘‘D’’ conﬁrm the presence

of the ﬁlm. (‘‘A’’ from Ref. 133, reprinted with permission from Elsevier Sequoia S.A.; ‘‘B’’ from

Ref. 131, reprinted with permission from ASME; ‘‘C’’ and ‘‘D’’ from Ref. 190, reprinted with per-

mission from ASME.)

Stable and beneﬁcial triboﬁlms generally do not form under lubricated co nditions. This is

because lubricants tend to inhibit the adhesion of the wear debris to the surfaces and thus

inhibit ﬁlm formation (125,135). Because of this behavior, the wear rate of sliding system,

which beneﬁts from triboﬁlm formation, can increase with the introduction of a poor

lubricant. An example of this behavior is shown in Fig. 3.66.

Because of the effect that material properties have on attachment, the formation and

properties of triboﬁlms are characteristics of material pairs, not simply the wearing

material. For example, in tribosystems, where triboﬁlms are involved, differences in wear

have been observed with different cou nterface material (22,125,137–139) . In add ition to

this, several other factors have also been identiﬁed as being signiﬁcant in the formation

Figure 3.64 Schematic illustrating the ﬂow of material associated with triboﬁlm wear processes.

Figure 3.65 Reduction in plastic wear rate as a result of transfer ﬁlm formation for unlubricated

sliding against stainless steel. For ‘‘A’’, the stainless steel counterface is rough, 0.14 mm Ra, inhibiting

transfer ﬁlm formation. For ‘‘B’’, the counterface is smoother, less than 0.05 mm Ra, allowing trans-

fer ﬁlm formation. (From Ref. 125.)

and development of such ﬁlms. Roughness (22,125,127–130), load (124,140), speed

(124,127,130,136,141), and type of motion (126) have all been found to inﬂuen ce these

types of ﬁlms. Several of these studies were done in the con text of polyme r–metal sliding

systems, but there is no reason to indicate that these inﬂuences are limited to those

systems. These studies suggest that there are optimum conditions associated with several

of these parameters (128,129). An example of this for roughness is shown in Fig. 3.67. The

proposed explanation for such behavior is that a certain degree of roughness promotes

adhesion of the wear debris to the surface, in much the same way that it helps with the

adhesion of coatings and platings. On the other hand, too coarse a roughness would result

in larger wear debris, which would not adhere as well. In addition, a thicker ﬁlm would be

required to protect against the higher asperities and such ﬁlms would tend to be unstable.

These counter trends result in an optimum condition. Thus, some studies have concluded

that a harder counterface is preferred to a softer one in that the optimum roughness con-

dition will remain stable and not be altered by wear (125). It should be noted that the com-

plete dependency of roughness is probably not explained by these rudimentary concepts.

For example, it has been indicated that the ﬂatness of the asperity tips may also be a

factor (127).

The inﬂuence of speed on polymer ﬁlm formation is shown in Fig. 3.68. As for

roughness, there appears to be an optimum for speed as well. The reason proposed for this

behavior is that a certain degree of softening of the polymer surface has to occur for sig-

niﬁcant transfer to occur. At low speed, the temperature is too low for softening; at higher

speeds, however, the temperature increases and the ﬂow characteristics of the softened sur-

Figure 3.66 Changes in polymer wear for various stainless steel=polymer couples when water is

used as a lubricant. (From Ref. 135.)

face layer allow ﬁlm formation to occur. At still higher speeds, the temperature is so high

that the ﬂow characteristics would degrade and ﬁlm formation would not occur. As a con-

sequence, it as been proposed that an important material property for transfer ﬁlm forma-

tion is the rheological properties of the polymer (130). Similar concepts can be proposed

for the effect of load. Increased load will promote adhesion and will also increase tempera-

ture. Excessive load will tend to result in larger wear debris, higher temperature, and more

effectively remove material from the contact surfaces. Studies have shown that ﬁlm

Figure 3.67 The effect of counterface surface roughness on transfer ﬁlm formation in the case of

ﬁlled PTFE sliding against steel. (From Ref. 128.)

Figure 3.68 The effect of sliding speed on transfer ﬁlm formation in the case of a polymer=polymer

couple. (From Ref. 124).