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

Подождите немного. Документ загружается.

where wear rates, wear coefﬁcients, and mechanisms are likely to be studied. How-

ever, tests have to be of sufﬁcient duration that all run-in and break-in behavior have

ceased.

In some wear situations, it is also possible that initial wear rates might be lower than

longer-term wear rates. This is generally the result of initial surface ﬁlms or layers, which

act as lubricants and are gradually worn away. Such an effect is more common with un-

lubricated tribosystems than with lubricated tribosystems.

The effect of break-in and run-in illustrates another general aspect of wear behavior

that needs to be recognized. This is that the immediate or current wear behavior can be

inﬂuenced by earlier wear. In addition to the effect of break-in and run-in on longer-term

wear, the inﬂuence of wear debris is another illustration. Since wear debris can be trapped

in the wear region and cause further wear, its charact eristics can inﬂuence current wear

Figure 4.3 The inﬂuence of materials on wear in different situations. ‘‘A’’, abrasion of borided

materials; ‘‘B’’, lubricated sliding against 52100 steel; ‘‘C’’, Au–Au sliding; ‘‘D’’, solid particle ero-

sion. (‘‘A’’ from Ref. 9; ‘‘B’’ from Ref. 4; ‘‘C’’ from Ref. 101; and ‘‘D’’ from Ref. 9.).

behavior, while the prior wear behavior will determine the characteristics of the debris.

For example, the occurrence of initial coarse debris as the result of a momentary overload

condition or lubrication failure might inhibit the development of a mild, stable wear region

that is more typical of the wear system. The momentary introduction of a small amount of

abrasive into a system might trigger such a sequence as well.

Wear behavior is frequently characterized as being mild or severe (14,16,22–25).

Mild wear is generally used to describe wear situations in which the wear rate is relatively

small and the features of the wear scar are ﬁne. Severe wear, on the other hand, is asso-

ciated with higher wear rates and scars with coarser features. For reference, Fig. 4.10

shows wear scars that are representative of mild and severe wear. Most mate rials can

exhibit both mild and severe behavior, depending on the speciﬁcs of the wear system in

which they are used. Frequently, the transition from mild to severe are abrupt. Figure 4.11

illustrates such a transition for polymers. Mild wear behavior is generally required for

engineering applications. In cases where severe wear behavior must be accepted, main-

tenance is high and lives are short.

Figure 4.3 (continued )

Figure 4.4 The effect of load. ‘‘A’’, ‘‘B’’, and ‘‘C’’, unlubricated sliding against steel; ‘‘D’’, general

sliding and rolling. (‘‘A’’ and ‘‘B’’ from Ref. 5; ‘‘C’’ from Ref. 9.)

Figure 4.4 (continued )

Figure 4.5 The effect of speed. ‘‘A’’, PTFE sliding against polyethylene; ‘‘B’’, unlubricated steel

against steel; ‘‘C’’, unlubricated iron against steel. (‘‘A’’ from Ref. 6; ‘‘B’’ from Ref. 13; ‘‘C’’ from

Ref. 9. ‘‘A’’ and ‘‘B’’ reprinted with permission from ASME. ‘‘C’’ reprinted with permission from

ASM International.)

Figure 4.6 The effect of ambient temperature. ‘‘A’’ and ‘‘B’’ show the effect in several cases of

unlubricated sliding between metal interfaces. (‘‘A’’ from Ref. 13; ‘‘B’’ from Ref. 14.)

Figure 4.7 Wear behavior as a function of duration. ‘‘A’’, ceramic=steel sliding; ‘‘B’’, steel=steel

sliding; ‘‘C’’, block-on-ring tests; ‘‘D’’, polymer composite=cermet sliding; ‘‘E’’, impact; ‘‘F’’, ero-

sion; ‘‘G’’ , slurry abrasion. (‘‘A’’ from Ref. 77; ‘‘B’’ from Ref. 10; ‘‘C’’ from Ref. 15; ‘‘D’’ from

Ref. 7; ‘‘E’’ from Ref. 78; ‘‘F’’ from Ref. 50; and ‘‘G’’ from Ref. 79.)

Figure 4.7 (continued )

There are several factors, which contribute to the general complex and varied

nature of wear behavior. One factor is the number of basic wear mechanisms.

Depending on the mechanism and the parameter considered, there are a mixture of lin-

ear and nonlinear relationships possible, as well as transitions in mechanisms. Conse-

quently, a wide variety of behaviors is to be expected for different wear situations.

A second factor is that wear mechanisms are not mutually exclusive and can interact

in different ways. A ﬁnal contributor to complex wear behavior is the modiﬁcations

that take place on the wearing surfaces. As is obvious from the examination of worn

surfaces, wear modiﬁes the surface in addition to removing the material. Modiﬁcations

to the topography are generally immediately apparent (e.g., scratches, pits, smearing,

etc.). While less obvious, the composition of the surfaces can also be modiﬁed, as well

as the mechanical properties of the surfaces. These changes to tribosurfaces are signiﬁ-

cant factors in the break-in behavior referred to previously. As a generalization, these

Figure 4.7 (continued )

Figure 4.8 Wear curve showing the effect of break-in behavior.

surface modiﬁcations can be inﬂuenced by a wide variety of parameters associated with

the wearing system (e.g., relative humidity, nature of the relative motion, active com-

ponents of a lubricant, etc.). Since the wear mechanisms are functions of the surface

parameters, the dependencies of surface modiﬁcations on this larger set of parameters

can result in more complex relationships for wear. In addition, since wear can inﬂuence

surface modiﬁcations, a compounding of effects can take place. Interactions and trends

with wear mechanisms, wear transitions, and modiﬁcations of tribosurfaces are

discussed in further detail in the following sections.

The complex nature and range of wear behavior possible can generally be simpliﬁed

and reduced to a practical level for engineering because of the limited range of tribosystem

parameters that need to be considered. However, the range of behavior shown in

Figs. 4.1–4.7, along with these observations regardi ng the many factors associated with

wear behavior, suggests the following. As an overview, it is appropriate to consider wear

behavior generally as nonlinear, with linear behavior possible under certa in conditions

and narrow ranges of parameters.

4.2. MECHANISM TRENDS

One factor that contributes to the complex nature of wear behavior is the possibility of

different wear mechanisms. Depending on the mechanism and the parameter considered,

there are a mixture of linear and nonlinear relationships possible, as well as transitions.

For example, the simple model for adhesive wear gives a linear-dependency on sliding,

while a model for fatigue wear gives a nonlinear dependency. In an abrasive wear situa-

tion, theory supports a transition in wear behavior when the abraded material becomes

harder than the abrasive. In addition, not all mechanisms depend on the same parameters

in the same way. For example, the model for corrosive wear indicates an explicit depen-

dency on sliding speed; the models for the other modes do not contain an explicit depen-

dency on speed. Conse quently, a wide variety of behaviors is to be expected for different

wear situations.

A contributing element to this complexity is that wear mechanisms are not mutually

exclusive. Frequently wear scar morphology indicates the simultaneous or parallel occur-

rence of more than one mechanism (16,26–29). An illustration of this is shown in Fig. 4.12.

In this sliding wear scar da mage features suggestive of both single-cycle deformation and

repeated-cycle deformation wear are present. The overall wear behavior of such a system

Figure 4.9 Wear rate behavior as a result of break-in behavior.