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

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from a single impact. The smaller the area, the greater resistance the paint system has to

this type of wear.

The test for friction-induced damage involves dragging a roun ded slider across a

painted surface, under condition that will cause peeling (70). A schematic of the test appa-

ratusforthistestisshowninFig.9.63

.Figure9.64provides an illustration of the method.

During the test the friction force is monitored and a curve is generated of friction force vs.

slidingdistance,asillustratedinFig.9.65

.Theareaunderneaththiscurveis the energy

expended in the test. The wear resistance of the coated is rated by the ratio of the pealed

area to the energy expended. The smaller this value, the more wear resistance the paint.

Figure9.66

shows a comparison of several coatings as a function of temperature using this

parameter.

Figure 9.62 Diagram of apparatus used to simulate stone impact. ‘‘A’’ air reservoir; ‘‘B’’ fast act-

ing valve; ‘‘C’’ breech (sabot holder); ‘‘D’’ barrel. ‘‘E’’ muzzle valve, ‘‘F’’ velocity measuring system.;

‘‘G’’ insulated muzzle; ‘‘H’’ target mount. (From Ref. 68, reprinted with permission from Elsevier

Sequoia S.A.)

Figure 9.63 Diagram of apparatus used to simulate friction induced damage of paint surfaces.

(From Ref. 70, reprinted with permission from Elsevier Sequoia S.A.)

9.2.19. Scratch Test

Scratch tests are primarily used to investigate the effect of various material parameters

with respect to single-cycle deformation processes (71). Typically, these tests involve press-

ing a hard, sharp stylus against a ﬂat specimen of the material to be evaluated and e ither

moving the stylus along the surface or moving the surfa ce underneath the stylus. The

groove is measured and used to characterize the wear resistance. Larger grooves corre-

spond to lower resistance. Styluses with angular and rounded tips are used. In addition

to these sliding scratch tests, pendulum apparatuses are also used. In these, a stylus is

attached to a pendulum, which is dropped with a know amount of energy. The stylus

is set to engage the surface and move across it, creating a groove. The pendulum is then

captured at its peak and before it can swing back across the surface.

Wear behavior in these tests is often additionally characterized in terms of two other

parameters. One is the ratio of the volume of material removed to the volume of material

displaced. This ratio is sometimes called the removal coefﬁcient, degree of wear, or abra-

sive fraction. The second is speciﬁc grooving energy, which is the energy dissipated in

Figure 9.64 Phenomenological model for the friction induced damage test. The specimen moves to

the left with the counterface in its stationary position. Step A: normal load is applied and motion

initiated. Step B: onset of damage. Step C: conclusion of test, resulting in friction induced damage.

sþ denotes tension; s denotes compression; t denotes shear. (From Ref. 70, reprinted with

permission from Elsevier Sequoia S.A.)

Figure 9.65 Examples of force–distance curves obtained in the test for friction induced damage.

Areas under the curves are the energy lost or dissipated in the test. (From Ref. 70, reprinted with

permission from Elsevier Sequoia S.A.)

Figure 9.66 Example of the comparison of painted surfaces in terms of their resistance to friction

induced damage. Resistance is inversely related to the ratio of the damaged area to the energy dis-

sipated measured in the test. (From Ref. 70, reprinted with permission from Elsevier Sequoia S.A.)

forming per unit of mass removed. For sliding scratch test, this requir es obtaining a force–

displacement curve, such as the one used in the test for friction induced wear of painted

surface (Sec. 9.2.18). With the pendulum type of test, the energy dissipated can be

determined from the difference in the initial and ﬁnal height of the pendulum.

These types of tests are also used for two other purposes related to wear. One is to

measure adhesive strength of coatings. The other is to obtain a scratch hardness number

for materials (72). For determining adhesive strength of coatings, the load at which

debonding is observed to occur is determined by performing tests at increasing loads. This

load is then directly used to compare materials or converted to a more fundamental mea-

sure of bond strength. In a scratch hardness test, the width of the groove is measured and

used to compute a hardness value. As with an indentation hardness test, hardness in

a scratch test is deﬁned as the ratio of the load to the area supporting the load. This is

illustratedforascratchtestandindentationtestinFig.9.67

. Values vary with the shape

of the stylus. In scratch tests, there are two general shapes typically used for this purpose.

Shapes with circular cross-sections, such as cones, spheres, and parabolas, are one type.

Square-base pyramid shapes are the other type. The equation for scratch hardness using

the former geometries is

ð9:20Þ

For the latter,

ð9:21Þ

In these equations, N is the load and b is the width of the groove.

9.2.20. Wear Tests for Coatings

In general, the wear resistance of coatings is evaluated using the same tests that are used

with bulk materials. While this is the case, test parameters are usually different when

these tests are used to evaluate coatings. Generally, test parameters are milder so that

wear in the test can be limited to the coating. Depending on thickness, typical geom-

etrical methods for measuring wear can be used. Mass loss methods are also possible,

Figure 9.67 Illustration of a scratch hardness test using a conical stylus. ‘‘A’’ illustrates the test

when pure cutting is involved; no buildup of ridges. ‘‘B’’ illustrates the test when plowing or defor-

mation also occurs; there is the ridge formation. Note that part of the load, N, is supported by the

ridge. (From Ref. 71, reprinted with permission from ASM International.)

if densities are known. For thin coatings, special techniques might be required. An alter-

native to using wear or wear rate as the measure of wear behavior, the life of the coating

in the test is often used to rank and compare coatings. The life can be identiﬁed by

examination of the surface for presence of the co ating after tests of different exposure

or duration. Frequently, the life can also be determined by monitoring the friction

during the test. With wear-through, there is often a ch ange in the coefﬁcient friction

that can be detected by such measurements. Additional information on unique aspects

of wear tests with coatings and examples of wear tests for coatings can be found in Refs.

73 and 74.

9.3. OPERATIONAL WEAR TESTS

The examples of phenomenological wear tests discussed in the prior section illustrate some

of the main attributes of that category of wear tests. One is that the phenomenological

tests tend to address major or generic wear situations with the result that actual test con-

ﬁgurations are noticeably different than the practical devices or conﬁgurations. Another is

that the focus tends to be on the ranking or the determining of appropriate wear coefﬁ-

cients or parameters of materials and material pairs. As will be seen, operational wear tests

tend to focus on the wear situation associated with individual devices or applications.

While they are also used to rank and select materials, operational tests frequently

allow the effect of other parameters associated with the application to be evaluated. In

addition, with operational wear tests, there may be several potential wear sites and situa-

tions. As a result, the wear mechanisms involved might change as the conditions of the test

change. These aspects, as well as some of the more general aspects of wear testing, will be

illustrated by the consideration of several examples of these types of tests.

9.3.1. Jaw Crusher Gouging Abrasion Test

This test utilizes a jaw crusher to evaluate the wear resistance to what is termed gouging

abrasion. This is a coarse form of abrasion in which macro-gouges and -grooves are pro-

ducedinasingleaction.Figure9.68

shows an example of this type of wear. Fracture of the

abrasives is also a common feature for this type of abrasion. Jaw crushers, which are used

for the crushing of ore an d stone in mining operations, are examples of applications in

which this type of wear occurs. The test is a replica of this type of application and can

be done with a jaw crusher. This is the primary reason for its classiﬁcation as an opera-

tional test. The test also has a phenomenological aspect as well. For example, rankings

from this type of test have been applied to earth moving equipment, which experience

similar wear but under different conditions (75). Because of the similarity of the wearing

action, this type of apparatus and test has been used successfully to rank coatings for ice-

breaker hulls (76). A standard method (ASTM G81) has been developed for this type of

apparatus when used to crush rock.

ThetestconﬁgurationisshowninFig.9.69

. Basically, a jaw crusher consists of a

pair of jaws, one stationary and the other articulated. The material to be crushed is fed

between these two jaws and is squeezed by the action of the movable jaw against the sta-

tionary one. In the wear test, two pairs of wear plates are mounted on these jaws; one

member of the pair is a reference material and the other is the material to be tested. They

are mounted in such a manner that the reference and test specimens oppose one another,

asshowninFig.9.70

. The test basically consists of crushing a minimum amount of

rock in a series of steps. At the end of the series, the wear plates are removed and the wear

determined by mass loss, which is then converted to volume loss. A wear ratio is estab-

lished for both pairs, dividing the volume of wear of the test specimen by that of the refer-

ence specimen. The two values of this ratio (i.e., the one for the movable and the one for

the stationary jaws) are averaged. It is this average wear ratio that is used to rank materials

against a reference material. A value of less than 1 means improved performance over the

reference material, whereas a value of greater than 1 means poorer performance.

Figure 9.69 Schematic of a jaw crusher apparatus. (From Ref. 125, reprinted with permission

from ASTM.)

Figure 9.68 Example of gouging abrasion. ‘‘A’’ shows the morphology of the worn surface of a

Hadﬁeld steel from a jaw crusher application. ‘‘B’’ shows the morphology of the wear surface of cast

iron bucket teeth. (From Ref. 124, reprinted with permission from ASME.)

A single apparatus is not speciﬁed in the standard method. However, key dimensions

and tolerance are given, as well as guidelines to monitor performance. One key dimension

is the minimum jaw opening, which is speciﬁed to be 3.2 mm and the jaws are to be re-

adjusted to this value after crushi ng 225 kg of rock. The minimum amount of rock to

be crushed is 900 kg and twice that amount is recommended for the evaluation of very

wear resistant materials. Because of wear, this dimension is re-adjusted several times dur-

ing the cou rse of a test. As a means of control and calibration, it is also suggested that

three tests be ru n sequentially with all the test plates being reference material. If the aver-

age wear ratios for the last two tests of this sequence vary by more than 3%, the apparatus

should be examined for signs of deterioration and lack of conformance to the speciﬁ-

cations. It is also recommended that a single test of this type be performed after every

six or so normal tests to monitor performance of the apparatus. Finally, it is recommended

that the crushed size of the rock (i.e., after it has gone through the jaw crusher) be moni-

tored. If the size changes, then state of the apparatus and the consistency of the rock used

should be examined and any variations or degradations be addressed.

Speciﬁc reference materials or rock to be crushed are not identiﬁed or used in the

standard test procedure but the signiﬁcant attributes of both are identiﬁ ed. For example,

the reference material should have unifor m and consistent properties. The size of the rock

to be crushed is a key factor and is speciﬁed. It must be precrushed to a particle size 25–

50 mm and it should be hard and tough. A morainal rock of a speciﬁc composition is given

as an example of an appropriate material to be used in the test. The test method requires

that the rock and reference material used should be reported, along with an adequate

description of their properties, when report ing the rankings obtained with this test.

However, while absolute amounts of wear vary with the type of rock crushed, it has been

found that similar rankings are obtained with different types of rock. This is a conse-

quence of using an index based on relative performance to a reference material, which

tends to reduce the effect of different rock properties on results.

The standard test method also addresses technique, specimen preparation and

cleaning procedures. Because of the gross nature of this wear process, there is less con-

cern wi th cleaning, surface preparation, and surface control than with many other wear

tests. Cleaning is part of the test method mainly to insure accurate mass change data.

Figure 9.70 Position of reference and test specimens in the ASTM jaw crusher test. (From Ref.

125, reprinted with permission from ASTM.)

For the same reason, surface preparation is required to insure that initial rust or slag

layers are removed.

When the test is used as an operational test for a jaw crusher, the rankings can be

used directly in establishing absolute performance. This requires knowing the life of the

reference material in that application. The lives of other materials in that application

can be determined by multiplying the life of the reference material by the reciprocal of

the index. This illustrates an advantage that many operational tests can provide (i.e., direct

scaling of ﬁeld performance).

However, the test does not directly provide a means of determining absolute wear

performance in an y application involving gouging abrasion. It simply provides a rank-

ing for gouging abrasion resistance with respect to an arbitrary refer ence material,

based on an operational type of test (e.g., crushing of a particular amount of rock).

It does not provide the value of a wear coefﬁcient that can provide a relationship

between wear and parameters as load, type of rock crushed, or the amount of rock

crushed. Such a type of relationship is needed to provide a means of quantitatively

relating the test result to an application. Values of these test parameters can be factors

affecting the absolute difference in the relative performance of materials found in the

test and in the application. Consequently, the correlation of the test rankings to relative

performance in an application other needs to be addressed on a case-by-case basis. An

alternate way of stating this is that the sensitivity of the test with respect to an applica-

tion needs to be established on a case-by-case basis. For example, large test differences

might correspond to negligible differences in an application. Alternatively with some

applications, a small difference in test results might correspond to a very large differ-

ence in the application. However, reproducibility of this type of test is good. When

this test is performed in the manner described in ASTM G81, variation should be

within 5%.

9.3.2. Cylindrical Abrasivity Test

This test is one of several that have been used to determine the abrasivity of magnetic

tapes.AnacronymforthisparticulartestisSCAT

, which stands for SpinPhysics Cylind-

rical Abrasivity Test (77,78). This test is quite similar in concept to a rod wear test devel-

opedinthe1960s(79).ThebasicconﬁgurationofthetestisshowninFig.9.71

. The test

Figure 9.71 The basic conﬁguration of the rod wear test and the SCAT wear tests.

utilizes a commercially available tape drive, replacing the normal magnetic head with a

cylindrical wear specimen. The similarity of the contact between the tape and a recording

headandthetapeandacylindricalspecimenwithsomewearisshowninFig.9.72

. The

basic test consists of running the tape across the surface of the cylinder for 10 hr and mea-

suring the depth of wear produced during that period. The depth rate of wear is used as a

measure of the abrasivity of the tape. This rate is used as a ﬁgure of merit for tape abra-

sivity, rather than as an absolute measure. This is because it is determined for only

one representative condition and the methodology of the test does not address the effects

that differences in various operational parameters, such as tension, speed, etc., can have

on actual wear performance.

AsummaryoftheparametersusedinthetestisgiveninTable9.4

. The load

between the tape and the cylinder is determined by the tension in the tape and wrap

Figure 9.72 ‘‘A’’ illustrates the contact conﬁguration between the head and the tape in a typical

application. ‘‘B’’ shows the contact conﬁguration between a worn cylinder and the tape in the SCAT

and rod tests.

Table 9.4 Parameters in the SCAT Test

Wear bar Material Spinalloy

Dimensions

Shape Cylinder 1.5 in. long, 0.250 in. diameter

Finish 1 min average

Support Accuracy Azimuth and tilts  1 min of arc

Test transport Drive Honeywell Model 7600

Tape

Wrap 5



each side of test bar

Tension 8 oz.=in. of tape width

Speed 60 in.=sec

Width 0.5 or 1 in.

Environment Moisture Relative humidity to be controlled within 2% of the desired value

Duration Test time 10 hr

Source: Ref. 77.

angle of the tape around the cylinder. The size, roughness, and composition of the

cylindrical wear specimen were selected to be representative of magnetic heads. The test

allows the use of standard reels of either 1=2

-or1

-wide tapes. All the mechanical and

material parameters have to be controlled for reproducibility. In addition, since moist-

ure can have an effect on the wear between heads and tapes, a tolerance is placed on

the relative humidity under which the test is conducted. The actual temperature and

humidity of the test should be given with the ﬁgure of merit that is determined in

the test.

The test utilizes a novel way of measuring wear depth, involving a form of break-in.

The basic cylinder of Spinallo y, a head material made by SpinPhysics, is installed in the

apparatus. With a sample of the tape, a small ﬂat spot or window is developed on the

cylinder. This window is the wear region for the test. The cylinder is then removed and

a series of micro-hardness ind entations are placed in this window, using a diamond pyra-

mid indenter. The cylinder is then coated with a sputtered coating of ceramic to ﬁll the

indentations. This helps maintain the edges of the indentations during wear and enables

small amou nts of wear to be determined. The cylinder is remounted in the apparatus

and a sample of the tape to be tested is used to remove the ceramic coating in the window

area. The cylinder is removed and the diagonal of the diamond indentations is measured

and recorded as the initial values. The cylinder is then replaced, an unused tape sample is

mounted, and the test performed. The cylinder is removed at the end of the test and the

diagonals of the indentations are measured again. The depth of wear, d, is determ ined

by the following equation:

d ¼ 0:1428 ðD

 D

Þð9:22Þ

where D

and D

are the initial and ﬁnal diagonal measurements. Eq. (9.22) is based on the

geometry of the diamond micro-hardness indenter used. This technique is illustrated in

Fig.9.73

The SCAT test provides an opportunity to compare directly an operational

test and a phenomenological test used for the same purpose. A phenomenological

test has also been used to rank magnetic tapes in terms of their abrasivity (79). The

testutilizesaball–planecontactsituation(Fig.9.74

). A sphere is used to press the moving

magnetic tape against a ﬂat wear specimen. This produces a spherical-shaped wear scar

in the ﬂat wear specimen. The volume of this wear scar can be determined from pro-

ﬁlometer measurements and the use of geometrical relationships, as has been described

with other test methods (see Secs. 9.2.6, 9.2.8, 9.2.11, and 9.2.12). The test provides a

Figure 9.73 The use of microhardness indentations in determining wear depth in the SCAT test.