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

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the test procedure calls for the testing of the reference material under those conditions that

are performed and reported.

Studies have been done to understand the effect of various parameters on the test

and to determine the level of control required. These studies also identiﬁed some of the

factors, which can inﬂuence this mode of wear (9). Among these are particle velocity,

abrasive or particle characteristics, particle ﬂux, and temperature. Differences between

the test and application conditions with respect to these parameters cause the test to

provide a relative ranking rather than absolute performance. However, the erosion value

generated in this test does provide some means of directly relating test and application.

This is in term s of amount of abrasive. The erosion value is deﬁned as wear per amount

of abrasive. Therefore, knowledge of the amount of abrasive experienced or the amount

Figure 9.13 Morphology of particle erosion wear scars for grazing- (‘‘A’’ and ‘‘B’’) and near-

normal (‘‘C’’ and ‘‘D’’) impact conditions. (From Ref. 115, reprinted with permission from ASTM.)

of abrasive per unit time in an application can provide an estimate of the wear or wear

rate in the application. This information can also aid in assessing the relative severity of

the test to the application and possible acceleration. Although additional aspects of

theparticlestreamsinthetwosituations

are also needed to address these elements

completely, such as type and velocity of the particles, and impingement angle.

In the standard tests methods for dry and wet sand and slurry abrasivity, the design

of the test apparatus is treated in more detai l than in the standard test method for this

Table 9.1 Standard Test Conditions of ASTM G76 Test Method for Solid Particle Erosion

Nozzle tube Dimensions ID 1.5 mm0.075 mm; minimum length 50 mm

Orientation Axis 90  2



with specimen surface

Position 10  1 mm from specimen surface

Test gas Dry air

Particle Composition Al

Size 50 mm

Shape Angular

Feed rate 2.0  0.5 g=min

Velocity 30  2m=sec

Flow Rate 8 L=min

Presssure 140 kPa (may be different)

Duration Minimum, 10 min; maximum, any acceptable

provided crater depth does not exceed 1 mm

Temperature 18–28



Figure 9.14 Wear curves obtained in the solid particle erosion test for 1020 steel at two different

velocities. (From Ref. 115.)

erosion test. This is because of the nature of the wear situation involved. In the former

two bodies pressed against one another under conditions of relative motion generate the

wear. In these types of situations the alignment, rigidity, and consistency of speed provi-

ded by structure have signiﬁcant inﬂuence on wear behavior. Hence, there is a need to

control the structure to a high degree in those cases. In the present erosion test, however,

the situation is obviously quite diff erent. The pressing together of two bodies does not

provide the wearing action but rather by the impingement of gas stream. As a consequence,

alignment and characteristics of this stream are the controlling factors. Such items as the

static alignment of the nozzle with respect to the wear spe cimen, the nozzle shape,

particle ﬂux, particle velocity, and other aspects of the stream are of importance. Since

these are recognized in the method and tolerances are speciﬁed, the design of the rest

of the apparatus is not critical.

This test also illustrates another aspect discussed in the general section on wear test-

ing, which is the need to develop unique methods to measure and control certain para-

meters. Examples of this in the gas jet test are the measurement techniques for abrasive

ﬂux and abrasive particle velocity. These are both signiﬁcant factors in the test and

required development. A method for the former is presented and references to techniques

developed for the latter are given in the ASTM standard, G76 (10–12).

9.2.5. Vibratory Cavitation Erosion Test

This test was developed to simulate the erosive wear caused by the formation and

collapse of cavitation bubbles in applications associated with high-speed hydrodynamic

systems. Surface s of hydraulic turbines, pumps, propellers, and hydrofoils are exposed to

this type of wear. This test has been used as a mechanism for studying cavitation erosion

and has been used successfully in the selec tion and ranking of materials for applications

wherethistypeofwearisofconcern.ThetestisillustratedinFig.9.17

. The cavitation

ﬁeld generated at the surface of a vibrating specimen immersed in a liquid is used to

generate wear. The wear surface of the specimen, which is submerged, is located close

to the surface of the liquid, and an ultrasonic horn is used to vibrate the specimen.

Figure 9.15 An illustration of the effect of erosion wear scar geometry on particle impact condi-

tions. As a result of wear, the angles of impact are no longer constant across the surface.

Figure 9.16 Data and information that are to be reported with the ASTM solid particle erosion

test.

A standard method for conducting this test is described in ASTM G32. The results of

tests conducted with this procedure typically repeat within 20%.

Like the solid particle erosion test, the standard cavitation erosion test method

focuses on the control in the immediate wear situation and not the overall apparatus.

In addition to specimen geometry, other inﬂuences on cavitation include the temperature

and properties of the ﬂuid, the frequency and amplitude of the vibration, and the pressure

above the liquid (13–21). Also, the method of specimen attachment can inﬂuence the cou-

pling of the ultrasonic vibration between the specimen and the horn and therefore effects

the cavitation. The standard method requires control of the frequency and amplitude of

the vibration, control of the specimen and the liquid, including its temperature, attachment

of the specimen to the vibrator, and pressure over the ﬂuid. Procedures and techniques for

implementing the test are given, as well as cautions regarding some of the common pro-

blems encountered with this type of test. Along with wear data, the method also requires

the recording and reporting of additional information, including specimen characteriza-

tion, test parameters (if the recommended standard ones are not used), identiﬁcation of

the liquid used, and observations of singular or unusual nature. The standard also speciﬁes

that tests on a reference material or materials should be done in any test program a s a

means of control. If nonstandard conditions are use d, it provides a means of relating test

conditions. Several reference materials are identiﬁed in the standard for this purpose.

The wear produced in this test is directly measured as mass loss. By using the density

of the wear surface and the dimensions of the standard wear specimen, mass loss is

Figure 9.17 Schematic of the vibratory cavitation test apparatus.

converted to a wear depth, which is used as the measure of wear performance. The stan-

dard method requires the reporting of the mass loss as well as the wear depth. The test

method involves the generation of a wear cu rve (i.e., wear depth vs. time) by interrupting

the test at appropriate intervals. Typical curves for wear and wear rate from this test are

showninFig.9.18

. While these curves are generally nonlinear, their shapes tend to vary.

Both the intervals and overall duration of the test are dependent on the materials being

evaluated and the test conditions. If possible, the duration should be long enough

so that a maximum wear rate is produced. If that is not possible, diff erent test times

should be used so that the materials compared reach the same depth of wear. The

intervals also should be selected so that a well-deﬁned curve can be established. In

practice, this means that the test can range from a few to 10 or more hours.

In several of the preceding tests, a wear curve was developed and ﬁtted to some

functional relationship. The best ﬁt was then used to develop a single value that was used

to compare wear resistance or quantify wear behavior. In the slurry abrasion test, for

example, this was the wear rate after 2 hr. While it would be desirable to do something

Figure 9.18 The general forms of the wear curve and the curve for erosion rate as a function of test

time obtained in the vibratory cavitation erosion test. (From Ref. 117.)

similar in the case of cavitation, there is no consistency of behavior be tween materials to

make such an ap proach generally feasible. While some mate rials exhibit a maximum ero-

sion or wear rate in the test, others do not. In addition, the shape of the wear curve

appears to be sensitive to the manner in which the test is conducted. This range of

behaviorcanbeseeninFig.9.19

. Thus, the standard requir es the reporting of the wear

curve itself and that material performance be compared in terms of the relationship of

one curve to the other. In any individual study, the wear curves or wear behavior might

be similar enough that a single parameter or value can effectively be used to rank

materials. In this case, the maximum erosion rate in the test or the terminal erosion rate

of the test might be useful indicators of relative wear behavior.

This graphic comparison of wear behavior used in with this test is not as precise or

desirable as the use of a single value to rank materials. However, it does illustrate the type

of comparison that can be used and may be necessary to use in other wear studies or

evaluations, where there is a wide range of behavior. In doing this type of comparison,

there are certain conditions that tend to lead to confusion and errors in evaluations.

Figure 9.19 Illustration of the two methods of comparison used with the vibratory cavitation

erosion test. When the materials exhibit a maximum erosion rate in the test, the maximum erosion

rate is used, as shown in ‘‘A’’. For the case when there is not a maximum erosion rate in the test, the

time used to develop the same wear depth is used, as shown in ‘‘B’’. (From Ref. 117.)

For example, curves can cross. In such a case, the relative rankings of the materials

change, depending on the region the ranking is done (e.g., before or after the intersection).

Another example is when one material shows a decreasing wear rate, while another shows

an increasing wear rate. This implies that an intersection is likely at some point, even if

it did not occur in the test. In cases like these, direct application of the results to a situation

is risky. In general, such results indicate that further study or additional information

is needed before the results can be applied. Frequently, this can be done by extending the

test long enough so that the overall behavior can be broken up into a short- and long-term

region, each being characterized by a particular level or amount of wear and morphology.

The concept is that these two regions can be related to initial and long range wear in

an application.

In effect, the ASTM standard for the cavitation test does that in terms of the guide-

lines for test duration, that is, test until the maximum erosion rate is achieved (long-term

behavior) or test to common wear depth. The magnitude of the wear depth that is used

would depend on the application that is being considered. These concepts are illustrated

in Fig. 9.19. In general, when the use of a graphic comparison is needed because of the

varied nature of the materials wear behavior, more extensive testing and careful consi-

deration of the application are required. An alternate approach is to use another test to

simulate the application, one in which the ambiguity does not occur.

9.2.6. Block-on-Ring Wear Test Using Wear Volume



ThebasicconﬁgurationofthistestisshowninFig.9.20. It is one of the more commonly

used test conﬁgurations to study sliding wear and to rank materials in terms of resistance



Another test method using this general conﬁguration and wear rate is discussed in Sec. 9.2.16.

Figure 9.20 Schematic of a block-on-ring sliding wear test. Transducer to measure friction is not

always part of apparatus.

to sliding wear. While both the block and ring can wear in this test, the test is primarily

used to evaluate the wear of the block material. This same test conﬁguration has been used

to evaluate lubricants (22). The methods of conducting the test, the data obtained, and the

methods of analysis used are different for lubricant evaluations and wear studies. How-

ever, many of the aspects associated with control are the same. The test itself can be con-

ducted under a variety of conditions of load, speed, lubrication, and even environments.

When this test is used to rank materials, the ring material is typically ﬁxed and the block

material is varied. However, the wear of the block, which tends to experience the most pro-

nounced wear in the test, can be inﬂuenced by the material of the ring. As a consequence,

when used to rank individual materials for an intended application, the ring material

should be one of the materials used in the application. If not, the correlation between test

rankings and ﬁeld performance is likely to be poor. Also, in relating wear behavior in the

test to wear be havior in an application, it is necessary to consider the wear on the block

and the ring, not just the wear on the block. When this is done and the test conditions

provide good simulation of an application, material rankings obtained with this test have

been found to correlate with ﬁeld experience (23).

An ASTM standard for wear testing using this type of test has been developed

(ASTM G77).



These provide guidelines for conducting the test and analyzing an d

reporting data. Interlaboratory test programs using the procedures of ASTM G77 have

indicated that the intralaboratory coefﬁcient of variation for the block wear volumes is

typically 20% for metals. The interlabora tory variations are larger, 30%. For some

materials and test conditions, coefﬁcients in the range of 10% have been obtained.

The coefﬁcient for ring volume tends to be signiﬁcantly higher than those obtained

for the block (e.g., two times higher). The coefﬁcients of variation can vary with mate-

rials and test parameters. For example, with some plastics and short test times intra

and interlaboratory coefﬁcients of variations in the range between 30% and 60% have

been found. For 10 longer tests, these coefﬁcients reduce to the order of 10%. The

variation associated with this test is partially the result of the sensitivity of this type

of wear to a large number of parameters. It is also the result of measurement accuracy.

The coefﬁcient of variation for the wid th of the wear scar on the block, which is

directly measured in the test and used to compute the volume, is signiﬁcantly less

(e.g., they are in the range 5–20%). However, for the geometries of the test, wear

volume is related to the square of the width, which results in larger coefﬁcients for this

measure. For the ring, wear volume is determined by measuring a small change in a

large mass. Because of the large variation associated with wear volumes in this test,

it is generally recommended that several replicates (e.g., three or four tests) be done

when using it to rank material pairs.

The basic test method is to press the block against the rotating ring and the wear on

both the block and ring is measured after a speciﬁed number of revolutions. On the block,

a cylindrical groove is generated as a result of the wear. The volume of the wear is deter-

mined by ﬁrst measuring the width of the groove and to use this to calculate the volume.

ThegeometricalrelationshipandtheequationareshowninFig.9.21

. The volume of wear

for the ring is determined by mass loss and converted to volume loss by means of the

density of the ring. While the standard test method does not specify a load or test dura-

tion, it does require a single load and number of revolut ions be used when evaluating



ASTM G176 is a speciﬁc version of this method for plastic.

materials. The number of revolutions and load used is to be reported with the wear volume

measurements. The wear volumes are then used to rank materials in terms of their

wear resistance. In addition, the standard test method requires friction measurements

at the beginning, during and at the end of the test.

Elements of control are a signiﬁcant portion of the standard test method. Principal

areas of concern are apparatus construction and design and specimen geometry and pre-

paration. Like the dry and wet sand tests, the overall construction of the apparatus can

inﬂuence the results, through such aspects as dynamic characteristics, stiffness, ability to

maintain alignment under dynamic conditions, to name a few. To control these factors,

speciﬁc dimensions and tolerances are speciﬁed for critical elements, such as concentricity

of the ring, specimen geometry, and bearings. Beyond this, the standard also speciﬁes

that a standard test be run to qualify a particular apparatus. This standard qualiﬁcation

test is found in ASTM’s D2714.

Since the test can be performed with a wide variety of materials with different surface

conditions, speciﬁc details regarding specim en preparation and cleaning are not provided.

It is pointed out that this mode of wear can be very sensitive to the presence of surface

contamination, surface composition, roughness, and oxide layers, and therefore these

elements need to be controlled. It is pointed out that characterization of surfaces by

suchtechniquesasscanningelectronmicroscopy(SEM)

and electron dispersive x-ray

(EDX) might be appropriate. If a lubricant is being used in the evaluations, appropriate

control of the lubricant and method of lubrication is also needed.

The standard also prescribes speciﬁc procedures to be followed in the test as an

additional means of providing control. For example, instructions as to how to handle

Figure 9.21 Method for determining the block wear volume from a measurement of the width of

the wear scar. (From Ref. 118.)