NAT'L INST OF STAND 8. TECH

A111D7 QSfllSl

NBSIR 76-992

Studies of Deformation at

A. W. Ruff

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

February 1976

Final

NAONR-35-75

Prepared for Office of Naval Research

~ apartment of the Navy

lington, Va. 22217

/OO

NBSIR 76-992

STUDIES OF DEFORMATION AT

SLIDING WEAR TRACKS IN IRON

A. W. Ruff

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

February 1976

Final

NAONR-35-75

Prepared for Office of Naval Research

Department of the Navy

Arlington, Va. 22217

U.S. DEPARTMENT OF COMMERCE. Elliot L. Richardson, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Abstract

Uetermi nations of strains have been made on the surface and subsurface on specimens of high purity iron after different amounts of

sliding wear have taken place. The method involved the measurement of

loss of intensity (contrast) of particular electron channeling lines

obtained from small selected areas near the wear track. Through the

use of a calibration specimen deformed plastically to a range of strain

values, the channeling line contrast loss was related to plastic strain.

Strain maps lateral to the wear track and below the original surface were

obtained for different total sliding distances by removing controlled

thicknesses of iron using electropol ishing. In all cases the maximum

strain was found at the track center location at the surface and the

strains decreased steadily with depth below the track. At 50 g load

the strains vanished at about 40 ym depth. Significant strains were

found to exist outside the wear track boundaries. The results are compared with other studies previously reported. There was no indication

of a soft or less hardened surface layer in any of the specimens studied.

Introduction

An increasing amount of attention has been directed recently

toward the analysis of wear debris produced in wearing systems. The oppor

tunity of monitoring the condition of the mechanical system is available

through access to the lubricating systems and the debris contained there.

However, it is necessary that relations be drawn between the type of

debris recovered and the processes under way at the solid surfaces. It

is recognized that debris particle sizes and shapes may vary considerably

depending on the nature of the wear processes and on the environment

experienced by the surfaces. In considering the various aspects of

this overall problem that need to be examined quantitatively, one

important consideration appears to involve the stress distribution and

plastic deformation history associated with the material at the wear

track. It is known that these quantities vary from metal to metal and

that the microstructure of the material near the surface can have a

significant effect on the wear. This present study was concerned with the application of the selected area electron channeling

method to the measurement of plastic strains adjacent to and beneath

sliding wear tracks produced on iron surfaces. Some microhardness

determinations that were made adjacent to the wear tracks for comparative purposes will also be discussed.

Consideration has been given in the literature to various aspects

of the stress and plastic deformation present in bodies that are in

contact and are wearing. The treatments principally involved asperity

interactions^'^ and indentation loading^ rather than a complete

elastic-plastic analysis around the wearing contact. Experiments

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involving the variation of wear track width with load and deformation

patterns beneath a wear track have been reported."* Studies in sliding^

and rolling^ contact have concentrated on the developing strains and

the wear track width in steel, aluminum and copper. Recently, Suh and

coworkers^ have examined theoretically and experimentally the consequences of plastic deformation associated with sliding wear. Their

theory (the "delamination" theory) is concerned with the microstructural

pattern that develops around a wear track, and to some extent, explains

quantitative differences between the responses of different metals to

wear. Two significant features of their model are (i) a "soft" or low

dislocation density region near the surface of the wearing solid and

(ii) void and/or crack formation at characteristic distances below the

wearing surface. Some of the information derived from the present study

of subsurface strains is pertinent in connection with that theory.

Experimental Method

A constant velocity, sliding wear tester was constructed for use

in these studies. The instrument is shown in Fig. 1.. A motor driven

cam transmitted a reciprocating velocity of 2.0 cm/s to a platform

on which a small tray was fastened containing the test specimen. The

linear motion was adjustable up to 2 cm in length. A loading pin was

in contact with the specimen from above with a load determined by a

dead-weight loading system. A bearing ball was clamped at the end of

this pin in contact with the specimen. The loading pin was fastened

to an elastic beam containing two mounted strain gauges. The frictional force at the contact surface could be measured during the wear

test using this strain gauge signal. Tests of durations from 5 to 1000

cycles (up to 40 minutes) were conducted under either dry or oil

lubricated conditions.

The oil and wear debris could be recovered after a test by washing

the specimen and tray with a solvent. Filtration through a micrometer

pore sized filter (typical hole size 1 urn) enabled recovery of the

debris for later examination. Careful washing and cleaning (without

mechanical action) usually also left some debris on the specimen surface near the wear track.

Specimens were fabricated from electron beam melted iron sheet,

0.5 rmi thick with a total impurity content of 150 ppm (principally

10 ppm C, 110 ppm 0, 12 ppm N). Specimens were cut to size 1 cm x

2.5 cm and then annealed for 2 hours at 900°C in a small, evacuated,

sealed quartz tube. Prior to wear testing each sheet specimen was

electropol ished between 10 and 15 minutes in a solution of 6% perchloric

acid and 94% glacial acetic acid. In experiments involving strain

measurements below the worn surface, the same electropolishing procedure

was followed in order to remove material in a strain-free. manner. Under

the conditions used, an approximate depth of 25 ijm of iron was removed

from each side of the sheet in 14 minutes. Specimens were clamped

down around the edges within the specimen tray in the wear tester.

Particular care was taken not to mechanically damage the specimens

during installation (except for the edge regions as noted above).

The loading pin consisted of a 2.4 mm diameter type 52100 steel

ball affixed to the end of a rod on which the load was applied so that

it could not rotate. Balls were used for one or more tests depending

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on the amount of wear accumulated. In those tests involving oil lubrication, the specimen tray was filled with SAE 30 weight automotive oil

prior to testing. The oil had been previously filtered through a

1.2 ym filter to remove solid impurities.

Wear tests were conducted by mounting the el ectropol i shed specimen in the tray, adding oil if desired, loading the pin to the desired

value, starting the drive motor and carefully lowering the pin into

contact with the specimen. The initial few cycles of signal recorded

from the strain gauges monitoring the friction force were usually not

reliable but very shortly the friction coefficient could be reproducibly

determined.

The wear specimen and bearing ball were examined after each test

and after cleaning with a solvent using optical microscopy and scanning

electron microscopy (SEM) techniques. The SEM available had been modified for selected area electron channeling pattern (SACP) studies involving the rocking beam mode^'^° of operation. The specimen was always

oriented normal to the electron beam and operated in the emissive

(secondary and backscattered electron) mode. The minimum selected area

realizable was about 10 ym across (approximately circular) at a specimenlens distance of 2 mm. The angular extent of the SACP was about 15 degrees

under those conditions. The microscope was operated at 20 kV and a

specimen current of 2 x 10"^ amperes. A discussion of the SACP method

is given in the appendix.

Results and Discussion

A. Selected Area Channeling Patterns

Previous workers^" have used either the line width or the contrast associated with a particular channeling line as a measure of the

strains present in the channeling volume. It is known that channeling

lines of different hkl indices (corresponding to the hkl of the planes

involved in the channeling process) exhibit different broadening amounts

even though the different lines originate from the same volume of deformed crystal. The higher index lines are more sensitive to lattice

strain. Hence it is necessary to systematically employ the same type

of channeling line and to calibrate the effect of strain on the line

using a suitably deformed specimen. In this study the 220 (in a

few cases the 200) channeling line in iron was measured. The contrast

of the line was determined according to the relation

C = [I(max) - I(min) ]/I (avg) where I is the detected (amplified) signal level. Proper use of the gain and DC level controls on the SEM

permitted the contrast values to be determined quantitatively from

photographs of the signal trace on the recording screen. An example

of an SACP taken from one grain in an iron specimen is shown in Fig. 2a.

Superimposed on the pattern is one signal trace and two signal reference

levels used in measuring the contrast value. The upper reference corresponds to the maximum signal level (white) and the lower to the minimum

signal level (black). The vertical deflection gain control is used to

establish the difference between these two levels at a fixed value for

all measurements. The DC level control then is used to depress the signal level by an integral multiple of full scale while the signal gain

control is increased to maintain the signal between the reference levels.

It was possible to obtain reproducible contrast values over the range of

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0 to 10% by following this procedure, provided that each specimen was

mounted carefully in the SEM in a reproducible manner. Figure 2b shows

a set of signal traces from which the average contrast value of

7.2 ± 0.5% was determined in this example. (The DC level is displaced

by two full scale units in this example.) It was possible to obtain

the average contrast value for a particular region in a specimen in a

period of about 5 to 10 minutes using this method. Repeated measurements indicated that a reproducibility of about 10% in any determined

contrast value could be obtained.

B. Polycrystal 1 ine Iron Calibration Specimen

A rod, 3 rrni in diameter, of electron beam melted iron was cut to a

length 20.3 mm. Two flat surfaces were ground on opposite sides of the

cylinder, one parallel to the rod axis, the other at an angle of

about 7° to the axis. This resulted in a tapered specimen, 1.3 rnn thick

at one end and 1.7 mm thick at the other end. The specimen was then

electropol ished, sealed in an evacuated capsule and annealed for 2 hours at

940°C. The specimen was then deformed in compression between two steel

plates on which teflon films had been placed until a uniform thickness

of 1.3 mm was reached. Examples of the grdin and surface structures at

two locations along the specimen are shown in Fig. 3. SACP from two

different regions of the specimen are presented in Fig. 4. It was not

difficult to locate a sufficient number of grains with 220 channeling

bands prominent in their patterns in order to map out the contrast

gradient that resulted from the introduced strain gradient along the

bar length. The calculated strain gradient is shown in Fig. 5.

Uavidson^^ has previously employed a calibration specimen deformed in

tension in a study of deformation using SACP methods without, however,

conducting quantitative signal contrast measurements.

Measurements were conducted to determine the variability in contrast

within one channeling band from a given region in the specimen. In one

case 13 measurements were made at different locations along a 220 band

resulting in an average contrast value of C = 7.2 ± 1.1%. Another

region in a different grain resulted in a contrast value C = 6.2 ± 1.0%

for 14 measurements. [In these cases locations along the contrast band

where fine structures were present and produced anomalous variations in

line contrast were intentionally avoided. These fine structures, some

of which can be seen in Fig. 2a are, in fact, high index channeling lines.

In some grains both 200 band and 220 band measurements could be made

and compared. Some results given in Table I showed that the difference

in average contrast value between these two bands was usually small and

within the scatter found for one band. While this result would clearly

not be true for all channeling bands, it indicates that selected band

comparisons can be made. In this study only contrast values of 220 bands

were used in the strain analysis study. The results of SACP contrast

measurements in grains along the length of the calibration specimen are

given in Fig. 5. Contrast values below 1% were not measured due to

poor precision. These data seem reasonably well described by a linear

decrease of contrast along the length of the specimen in the direction

of increasing strain. The resulting strain vs. SACP contrast relation

is shown in Fig. 6. Strains greater than about 17% cannot reliably be

determined by this method. Previous studies of stainless steel^'^°

have shown that strains in excess of about 20% (tensile) could not be

reliably measured using SACP methods.

It should be emphasized that while the linear relation shown in

Fig. 6 between the SACP contrast and strain does adequately fit the

experimental data, it is without theoretical justification. In fact

the SACP can be recognized at strains greater than 20% but precise

measurements are difficult. Schulson^^ has presented a simple analysis

of the effect of strain on SACP lines that describes the strain in

terms of rotations of the lattice planes involved in channeling. This

analysis is summarized in the Appendix. Using this approach a calculated shear strain vs_. SACP contrast curve was obtained and is shown

in Fig. 6. The curve is normalized at C = 8.5% with the experimental

data. The agreement with the experimental data is not good particularly

for strains around 10%, however, many of the assumptions involved in

the analysis are not closely fulfilled in the experimental situation.

One gains from these considerations some idea of the potential accuracy

of the SACP method for strain determination.

C. Sliding Wear Experiments

A series of wear experiments were conducted using progressively

larger total sliding distances. The load in all experiments reported

here was 50 g and the sliding speed was 2.0 cm/sec. The specimen surfaces were all electropolished for about 10 minutes immediately before

wear testing. The oil lubricated tests will be described first.

The least sliding distance attempted was 0.17 m involving 5 machine

cycles. The nature of the contact varied greatly along the track and

in many cases the track appeared interri'pted by regions of noncontact

with the loading ball. The electropol ished surface did typically contain an undulating texture so that the degree of contact could be

expected to vary. A number of areas in and adjacent to the wear track

were selected and SACP obtained. The strains determined from these

using the contrast-strain relation (Fig. 6) are shown in Fig. 15 and

will be discussed later. Patterns were taken from areas of apparent

noncontact and showed that no additional strains were present in those

areas compared to undisturbed surface regions.

The sliding distance was then increased to 1.7m under the same

conditions in other respects. The typical wear track was continuous

(Fig. 7) but with small regions where the original surface could be

seen within the track. The grain structure of the specimen can be seen

in Fig. 7 as a result of channel ing-type of contrast due to an orientationdependent emissive signal contribution. The marked locations are

those from which the SACP were taken; two of these (A and E) are shown

in Fig. 8. The loss of contrast in the 220 band (arrow) comparing position A remote from the track with position E at the wear track center

is apparent. The strain values determined are discussed later.

Tests conducted at a sliding distance of 17 m showed that in all cases

the wear track was continuous and well defined except at the edges.

In certain regions where deep pits existed in the initial surface,

those features could still be recognized. Figure 9 shows a portion of

such a track (taken at high contrast to emphasize the grain and track

structure) with some interesting detail at the edges. When the accumulated deformation is still low, apparently the channeling contrast

difference between neighboring grains can be sufficient to permit

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identification of the grain boundary beneath the edge of the wear track,

that is beneath a thin layer of deformed material (see arrow. Fig. 9).

Figure 10 shows some detail near an initial pit in the track. Evidence

of rubbing in the sliding direction can be seen on the bottom of this

depression, probably due to debris particles momentarily trapped within

the depression and supporting some of the load. The fine, pebbled

appearance of the surface at the right of the depression (Fig. 10b)

was frequently observed and may be indicative of the oxide film at the

surface. SACP signal traces obtained from the center and the edge of

this wear track at a depth of 10 ym below the original surface are

shown in Figs. 11a and lib, respectively. The loss of contrast at the

220 band (arrow) within the wear track can be seen.

A representative portion of the wear track generated at a sliding

distance of 170 m is shown in Fig. 12. Under these conditions the track

edges are well defined and none of the original surface structure

remains to be seen in the track. Ridges of material oriented parallel

to the sliding direction are a principle feature of the wear track

appearance. This specimen was examined in the SEM before careful

cleaning and many debris particles remained on the surface. A more

detailed view of the end of the track is shown in Fig. 13. Many debris

particles remain in this area. Two of the larger particles are shown

in Fig. 13b (arrows). They appear flat and laminar in shape and are

about 7 ym in lateral dimensions. In the region to the left, there

is evidence for extensive plastic flow of the near-surface material in

the wear track region, a common observation in this study. It appears

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that the flat laminar particles are produced by extensive plastic flow

of the wear track surface material until a fragment eventually breaks

loose.

The appearance of wear tracks formed under dry conditions was similar to the appearance of tracks obtained in oil lubricated tests. Figure

14 shows a portion of the track in one dry sliding test of sliding

distance 17 m. The appearance can be compared with the track in Fig. 9.

Somewhat greater height associated with the ridge features in the track

is seen under dry conditions. SACP traces from the track edge and at

a location distant from the track edge are shown in Figs. 14b and 14c,

respectively. The 220 channeling band is marked.

Wear coefficient and wear volume values were determined in two

of the oil lubricated experiments through measurements on the ball

used in applying the load to the specimen. At 50 g load and 17 m distance, the wear volume was = 0.8 x 10" ^cm^ and the wear coefficient

was k = 3$7 • H/L « D = 1 .5 x 10"^ where H, L, and D are hardness,

load, and sliding distance, respectively. At 100 g load and 240 m

distance, ^ - 4.3 x lO'^cm^ and k = 2.7 x 10"^. Under dry conditions

both values were considerably greater.

D. Strain Measurements at Wear Tracks

Shortly after each wear test was completed, SACP were obtained

from the original surface at positions of various distances from the

wear track center. A grain was selected that was sufficiently large and

located across the track from which all the SACP were obtained. At

each location at least four values of contrast across the 220 band were

obtained and averaged. The associated strain value was determined from

I

the calibration curve (Fig. 6). The results are given in Fig. 15. A

conmon feature of the lateral strain distribution is the rapid decrease

in strain near and beyond the edge of the wear track. However, the

surface strains still extend condsiderable distances from the apparent

edges of the wear tracks (arrows in Fig. 15). For example, at a sliding

distance of 17 m under oil, the strain at the track edge (45 urn) was

about 11% and decreased to about 3% at twice that distance from the center (90 ym). In two cases the strain could be measured over the entire

track width and was found to increase steadily as the track center was

approached. Since strains greater than about 18% could not be measured

by this method it was not possible to map out the entire surface distribution of strains in all cases. However, it appears that for sliding

distances up to 17 m (and possibly beyond) the maximum strain values at

the surface are found at the track center location. Comparison of the

17 m sliding distance experiments under dry and oil lubricated conditions

reveals that larger strains and a broader distribution are produced under

dry conditions but otherwise the strain distribution is similar.

Measurements of the strain distribution with depth below the

original surface were conducted after electropol ishing for appropriate

times to remove a specific thickness of material. The original track

center was located with reference to adjacent grain boundary junction

locations. Some uncertainty in position probably arose due to the change

in grain boundary locations with depth through the specimen. However,

the use of two or three such reference locations in each case probably

kept that uncertainty to about ±10 urn. The results of two 17 m

sliding distance experiments are shown in Fig. 16. In both cases the

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measured strains extend further lateral to the wear track than its

edges (marked by arrow) even at subsurface locations. Near the track

center the maximum strains are found at the surface and decrease smoothly

with depth, vanishing at 30 ym and 40 urn under oil -lubricated and

dry conditions, respectively. However, outside of the track edge location it appears that the maximum strains are located subsurface, about

10 ym deep in these experiments. At any specific depth, however,

the maximum strain was found under the wear track center.

Recently Popov and Titukh^** have used X-ray diffraction methods

to study the stresses in several steels at various depths below the

surface of specimens that had been tested for abrasive wear resistance.

Measurements of diffraction line broadening were converted to dislocation

density values in the usual manner. Their results on one steel (type

270 Kh 12 BI: 2.7% C, 1.2% Cr, 0.35% Ni , 0.35% Mn, 0.4% Si, 0.3% P,

0.3% S) are shown for the austenite phase in Fig. 17a. The dislocation

density (p^) decreases rapidly with depth from the original surface

and is reduced by one order of magnitude at a depth of 15 ym. The other

alloys studied showed essentially the same behavior; those with larger

values of dislocation density at the surface also retained larger values

to greater depths in the material. In all cases the largest values of

were found at the specimen surface. Using the usual relation that

strain "^9^^^^ ^nd Kehs^^ data on iron relating p^^ and elongation,

equivalent strain values are plotted in Fig. 17b. These X-ray derived

results are clearly consistent with the SACP derived results of the

present study. It is probably not valid to compare independently

obtained data using these two methods even on the same material unless

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some consistent method of strain calibration is utilized. It is worth

noting that the X-ray measurements involve lattice planes nearly parallel

to the specimen surface in contrast to the SACP method which utilizes

lattice planes nearly perpendicular to the specimen surface.

While no results on the strains developed beneath sliding wear

tracks were provided, Ronay^^ has reported an increase in the track

width with increasing number of cycles as observed in this study.

Ronay further reported on apparent work-softening associated with

reciprocal rather than unidirectional sliding, an observation that

indicates the possible sensitivity of the developing deformation structure on the details of the wear and loading geometries.

Dautzenberg and Zaat^^ have reported on quantitative determinations

of deformation associated with sliding wear using a geometric method

that measures the change of grain shape with deformation in the material.

The deformation there represented accumulated plastic strain in the

specimens. They reported a smooth decrease of effective deformation with

increasing depth below the wear track in copper, down to a depth of about

100 ym. Note that the strains measured in the present work in iron by

the SACP method also represent total (compressive) plastic strain through

the use of a calibration specimen. It would be of interest to compare the

two methods quantitatively.

E. Microhardness Measurements Beneath the Wear Track in Cross Section

Many studies have been conducted using indentation microhardness

methods for the purpose of determining the level of deformation

locally in materials after wear (and other mechanical) tests. The

advantage of the microhardness method is its simplicity and speed.

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however, the interpretation of the results in terms of stress level

or other microstructural variables is not well understood. Nevertheless

relations between flow stress and indentation hardness, for example,

have been described. A wear specimen (17m sliding distance) was

cross-sectioned, metallographical ly prepared and el ectropol ished lightly

to remove preparation damage. Microhardness determinations were then

made using a Knoop indenter and a 10 g load at various distances from

the original surface both under the wear track center and remote from

the wear track. The long axis of the indenter was aligned parallel to

the surface trace. The results of these measurements are shown in

Fig. 18. The hardness values for the undisturbed specimen region were

nearly constant at a value of 100. However, at a distance of 5 ym

from the original surface the hardness abruptly decreased by nearly

50%. This decrease was clearly due to flow of material away from the

indentation site as a result of the loss of constraint as the free

surface is approached. That is, accurate microhardness values could

not be determined within about 10 ym of the free surface due to relaxation effects at that surface. Indentations under the wear track center

at different distances from the original surface showed that the hardness increased as the surface was approached. Measurements closer than

about 20 ym to the original surface were not attempted due to the demonstrated relaxation effects experienced there. The microhardness variation with depth below the wear track was in reasonable agreement with

the strain variation found here (Fig. 17) except that the hardness

method is not believed suitable for regions close to the free surface.

Microhardness measurements in aluminum after sliding wear testing

have been reported by Savitskii.^^ His results for undeformed aluminum

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1

show a small decrease with distance from the surface (on cross-sectioned

specimens) but measurements were not made any closer than 20 um to the

surface. However, on specimens that had been tested in sliding wear,

he reported measurements as close as 5 ym to the free surface. The

result in each case was a decrease in microhardness value near the surface and also at some distance away, that is, a peak in microhardness

was found at depths of 10 to 15 ym. While that peaking tendency has

been cited^° as evidence for a softening of the near surface region

of the wear track, it was not established that the microhardness measurements were unaffected by proximity to the free surface.

Tsuya^^ has reported microhardness values for copper tested in

sliding wear. He noted that the hardness rapidly decreased with depth

below the surface on cross sections at the wear track center. In

contrast to that. Kirk and Swanson^^ have recently reported a zone of

low microhardness very near the wear surface in copper also tested in

sliding wear. The latter work did not report hardness measurements near

an unworn surface so that the relaxing effect expected near the surface

cannot be eliminated from their data.

There is some disparity in the reported trend of microhardness

results with distance from a wear track surface and further careful

studies are indicated. However, it is well to keep in mind that such

measurements are complicated measures of the average resistance to indentation deformation in the material. Anisotropic effects in such

measurements also have been reported.

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F. Relation of Present Results to the Delamination Theory

One of the purposes of the present study was to improve the understanding of the strain patterns that develop beneath a sliding wear

track as a function of several important parameters. In order to study the

initial stages in the development, relatively low loads and a progression of

sliding distances were studied. There was no indication of severe wear

or large wear debris particles in any of the oil lubricated tests the

dry tests did show an increased wear coefficient into the mild wear regime.

Suh et al . ^° have reported dry wear test results on iron, iron with small

amounts of tungsten added, and several types of steel (1010, 1018, 1020).

Subsurface delamination was reported for many of these tests, at however

a higher wear coefficient than found in the present study (approximately

k = 0.4 to 3 X lO""* compared to k - 2 x 10"^ here). Thus we can regard

the present study as indicative of the early stages of wear and deformation. It is planned to continue this study to the heavy wear regime.

One unique feature of the delamination theory is the existence of

a soft or low-dislocation density region near the wearing surface. Suh

et al . ^° have evaluated the thickness h of that region assuming film-free

conditions at the surface. For copper they calculate h - lOum and for

Fe-3% Si, h - 0.1 urn. Applying that same procedure to iron where the

flow stress a = 20 MPa, the shear modulus G = 84 GPa, the

Burgers vector b = 2.5 x 10"® cm, and Poissons ratio v = 0.28 the thickness of the low dislocation density zone is calculated to be h - 0.12 urn.

Thus the effect of image forces on reducing the stress level and dislocation density in iron should be observable in the first 1000 A, approximately. The SACP obtained in this study are derived from electrons

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generated in the near surface layer, probably within 500 A of the surface.

(Note that backscattered primary electrons that penetrate to these depths

o will contribute to secondary electron emission from within 50 A of the

surface upon exiting.) Thus the SACP obtained from the as-worn surface

in iron are associated with the near-surface layer that is of interest

in the delamination theory. Apparently this layer shows a maximum strain

in the present experiments on iron rather than any softening or lower

hardness behavior. Consideration of the image formation process in SACP

o generation would suggest that softening in the nearest 100 A to the surface might not be detectible but that the uppermost 1000 K is precisely

the region from which the preponderant signal originates.

Summary

(1) The selected area channeling pattern method can be utilized to

measure strains associated with wear deformation both at the original surface and subsurface after electrochemically removing

material

.

(2) Strains up to nearly 20% can be measured in iron by this technique.

The use of proper signal averaging techniques along one particular

channeling band can lead to reproducible contrast measurements.

(3) The maximum strains associated with the wear track, under low

wear conditions and probably involving the early stages of wear

deformation, are always found at the surface and the track center.

At locations outside the track edge, the largest remaining strains

appear to be located below the original surface.

(4) The soft or low dislocation density zone postulated by the delamination theory (for film-free surfaces) was not found in these

experiments on iron. Calculation indicates that zone should be

o 1000 A deep, approximately, and detectible using this method.

Acknowledgments

Many fruitful technical discussions were held during the course of

this research with various colleagues, including Richard Miller, Nam Suh,

Vernon Westcott, Lewis Ives, and others at NBS, and their contributions

are gratefully acknowledged.

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Appendix A. Indentation Deformation

It was of interest to apply the SACP strain measurement method to

another mode of deformation than wear in order to further explore the

technique. An indentation experiment was conducted using a spherical

indenter of radius 0.80 mm and a vertical load of 3kg. The specimen

was prepared in the same manner as the wear test specimens prior to

Indentation. The specimen was backed by a hard steel plate during loading, however, clear evidence of the indention location could subsequently

be seen on the back surface of the specimen indicating that the specimen

thickness (approximately 0.4 mm) was insufficient for any bulk material

approximation. Figure Al shows the surface after indenting and indicates

the locations, A through F, at which SACP were obtained at the surface.

Three subsurface sections were subsequently obtained by electropol ishing.

These were at depths of 35, 70, 120 ym below the original surface. SACP

were obtained at these levels and the strain values determined using the

calibration curve (Fig. 6). The location of the indentation center

could be visually located at each level and further position reference

was possible using two triple grain junctions nearby. The lateral position uncertainty below the original surface is believed to be not more than

±20 pm. Figure A2 contains four SACP taken from the indicated locations at a depth of 70 urn from the original surface. The 220 channeling

band is vertical and its loss of contrast as the indentation location is

approached can be seen. The appearance of the specimen after electropolishing to the final level (120 ym deep) is seen in Fig. A3. The

grain around the indentation center (marked A) from which all SACP were

taken can be seen.

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The strain configuration determined in a vertical plane through the

indentation is shown in Fig. A4. A vertical -to-hori zontal expansion of

two was used in constructing this figure. The strain values circled are

in percentage and are placed at the locations where they were determined.

The indentation is shaded. Approximate strain contour lines have been

sketched in the figure. There is some degree of symmetry to the strain

pattern measured in that strains of about 15% are found 100 ym distant

from the indentation center both vertically below and lateral to it. At

a distance of 300 to 400 ym the strain values are essentially that of

the undeformed specimen. A tendency for an increase in subsurface strain

values is seen at some distance from the indentation. Underneath the

indentation, the strains decrease smoothly with depth. This same type

of distribution was found around the wear tracks discussed earlier.

The elastic solution to the indentation problem would predict a

maximum pressure of about 1000 MPa. The corresponding maximum shear

stress would be 300 MPa at a depth of 50 ym, considerably in excess of

the flow stress of this material of about 20 MPa. According to Shaw

and DeSalvo^^ the boundary of plastic behavior beneath the indenter

would extend at least about 220 ym deep. This is consistent with the

data in Fig. A4 that shows strains of 15% at a depth of 120 ym and strains

of 5% at a lateral distance of 220 ym.

-22-

Appendix B. Selected Area Electron Channeling Method

The technique for obtaining electron channeling patterns from

small areas of a crystalline specimen using the "rocking beam" approach

has been described previously > ^° Briefly, the excitation currents

through the final two lenses and the drive currents applied to the scanning

coils are controlled so that the beam focus on the specimen surface is

as nearly stationary (in position) as possible in the SACP mode . The

scan current causes the beam to sweep angularly about the electron

optical column axis. The angular range is adjustable from about 18°

down to smaller values. In the image mode (obtained by altering the

scan coil -current configuration) the beam sweeps in position describing

a rectangular raster on the specimen surface. It is possible to switch

simply from one mode to the other during examination of the specimen.

Figure Bla shows an image of an electropol i shed surface of iron taken

in the image mode at a magnification of about 150X (magnifications up

to 2000X can be used conveniently). Switching the operation to the SACP

mode produced the pattern shown in Fig. Bib, obtained from a region in

the center of the above area of lateral dimensions about 10 ym. The

pattern in this example is slightly enlarged to reveal the fine detail

associated with the different channeling lines. The information contained within the SACP can be considered an average over the microvolume

irradiated by the beam, although in fact the beam position does trace

a regular but complicated path throughout that volume during one scan

frame. For this and other reasons it is best to obtain several contrast

measurements from a channeling band of interest in one frame and then

average those determinations. This procedure also minimizes the effects

of high index bands that intrude on the band of interest.

-23-

Schulson^^ has presented a simplified analysis of channeling line

broadening that seems to agree well with some observations in the present

study. He considers the distorted regions of the microvolume involved

in the channeling process to be described by local rotations of the

channeling planes (similar to some treatments of X-ray diffraction from

deformed material). Using a two-beam approximation (incident and diffracted beam), the deviation parameter s which measures the deviation

from exact Bragg conditions is changed an amount

|ds| 4) • |g|

where g is the diffraction vector and other effects due to the deformation are neglected in comparison. The dynamical theory calculation of

the lattice distortion effect is done for various values of <^ and g and

the results are graphically displayed in reference 13. In order to

relate the calculated contrast curves to strain in the present study

it was assumed that the angular rotations <t> were proportional to the

local shear strain value e. The calculated contrast-strain curve was

fitted to the measured contrast value for e = 0. The result is shown in

Fig. 6 and was discussed previously.

Other attempts to calculate SACP line contrast (and width) values

for undistorted and also for distorted crystals have been reported.

Recently Spencer^** has completed multibeam calculations of SACP profiles in several different materials. He considered the effects of

dislocations directly on the channeling process and calculated SACP

contrast vs. dislocation density. The agreement with experiments in

stainless steel is fair. Additional experiments appear to be needed

as well as some improvements in the theory before close comparisons can

-24-

be made. The undistorted crystal contrast value of 9% measured^** for copper 220 bands was significantly higher than the best calculated value

of 4%, although a better agreement was found for gold.

-25-

Table I. Comparison of average contrast

value at channeling bands indicated

in different grains in iron.

200 band 220 band

Grain Average

Contrast (%)

Std.

Dev. (%)

Average

Contrast {%)

Std.

Dev. (%)

A 2.8 0.3 2.8 0.1

B 1.9 0.2 1.9 0.3

C 3.2 0.9 3.3 0.5

D 2.3 0.3 2.4 0.4

-26-

References

1. F. P. Bowden and D. Tabor, Friction, lubrication and wear: A

survey of work during the last decade, Brit. J. Appl . Phys. V7

(1966) 1521-1544o

2. E. Rabinowicz, Friction and Wear of Materials , Wiley, New York

(1965)o

3o Me C. Shaw and G, J. DeSalvo, On the plastic flow beneath a blunt

axi symmetric indenter. Trans. ASME, (1970) 480-494.

4. F. B. Bowden and D. Tabor, The friction and lubrication of solids , Oxford (1964).

5. M. Ronay, Dependence of track width on load and on the number of

passes in sliding contact. Wear (1971 ) 187-205.

6o V. M. Radhakrishnan and S. Ramanathan, Plastic deformation in rolling

contact. Wear 32 (1975) 211-221.

7. N. P. Suh, The delamination theory of wear. Wear 25^ (1973)

lll-124c

8. N. P. Suh, S. Jahanmir, E. P. Abrahamson, II, and A. P. L. Turner,

Further investigation of the delamination theory of wear, J. Lubr.

Tech. 96_ (1974) 631-637.

9. R. Stickler, C. W. Hughes and G. Ro Booker, Application of the

SA-ECD method to deformation studies, SEM/1971 (IITRI, 1971) 473-480.

10. J. P. Spencer, Go R. Booker, D. C. Joy, C. J. Humphreys, Electron

channeling patterns from deformed crystals, SEM/1974 (IITRI, 1974)

920-926.

11. D. E. Newbury, The origin, detection and uses of electron channelling

contrast, SEM/1974 (IITRI, 1974) 1047-1054.

12. U. L. Davidson, A method for quantifying electron channeling pattern degradation due to material deformation, SEM/1974 (IITRI, 1974)

927-934.

13. E. M. Schulson, SEM electron channeling line width (broadening)

and pattern degradation in alkali halide crystals, SEM/1971 (IITRI,

1971) 489-496.

14. V. S. Popov and Yu. I, Titukh, X-ray investigation of transformation in the surface of alloys during abrasive wear. Metal Science

and Heat Treatment }7_, (1975) 23-26.

-27-

15o A, S. Keh, in Direct Observation of Imperfections in Crystals , ed.

J. B, Newkirk and J. H, Wernick, Interscience (1962) 213-238.

16, M. Ronay, Dependence of track width on load and on the number of

passes in sliding contact, Wear 18 (1971) 187-205.

17. J. H. Dautzenberg and J. H. Zaat, Quantitative determination of

deformation by sliding wear. Wear 23^ (1973) 9-19.

ISo Jo J. Gilman, Relationship between impact yield stress and indentation hardness, J. Appl , Phys. 46 (1975) 1435-1436.

19 o K. B. Savitskii, cited in V. I. Kragel'skii, Friction and Wear , Butterworth, Washington (1965).

20. No Po Suh, S. Jahanmir and E. P. Abrahamson, II, The delami nation

theory of wear, ARPA Progress Report NR 229-011 (1974).

21. Y. Tsuya, The behavior of the layer damaged by friction, Bull.

Jap. Soc. PreCo Enggo 2 (1967) 214-221.

22. J. A. Kirk and T. Uo Swanson, Subsurface effects during sliding

wear, Wear 35 (1975) 63-67.

23. M. C. Shaw and G. J. DeSalvo, The role of elasticity in hardness

testing, Metals Engg. Quarterly 12. (1972) 1-7.

24o J. P. Spencer, "Diffraction Contrast in the Scanning Electron

Microscope," Ph. Do disertation, Univ. of Oxford (1974).

-28-

Figure Captions

Fig. 1 (a) View of sliding wear tester, strain gauge amplifier and

chart recordero (b) Details of constant velocity tester; cam £, push rod P^, specimen S_, load L^, strain gauges G^.

Fig. 2 (a) SACP from one grain in iron specimen. The 220 band is

prominent; one signal trace is superimposed, (b) A set of

signal traces at different locations on SACP showing variation

along the 220 bando

Fig. 3 (a) Polycrystall ine iron bar specimen, low strain end, after

deformation, (b) High strain end showing surface flow and

grain structure.

Fig. 4 (a) SACP from grain at low strain (<1%) location in iron bar

specimeno Note contrast at 220 band marked B. (b) SACP from

another grain at mid-strain (==13%) location. Note contrast

at 220 band marked.

Fig. 5 Calculated strain _vSo distance along calibration specimen.

Determinations of SACP contrast at indicated positions are

shown. Repeated measurements in two grains are also shown

(n = 6,14).

Fig. 6 Strain vs^. SACP contrast determined from calibration specimen

and calculated after Schulson.^^

Fig. 7 Portion of the wear track after 1.7 m sliding in oil. Locations of SACP obtained are shown.

Fig, 8 SACP at locations (a) A and (b) E in previous figure.

Superimposed signal traces show loss of contrast at high strain

location.

Fig. 9 Portion of wear track after 17 m sliding test in oil. Note

that grain boundaries are detectible beneath edge of track.

Fig. 10 Details at wear track in previous figure. Note deformation

marks in pre-existing cavity in track surface.

Fig. 11 SACP traces of 220 band from 17 m sliding test specimen at

(a) wear track center and (b) wear track edge locations,

at a depth of 10 pm.

Fig. 12 Portion of wear track after 170 m sliding distance in oil.

Some wear debris remains on the surface.

Fig, 13 Details at the wear track end in previous specimen. Two large

wear debris particles are indicated. Note extensive ridge

formation in wear track.

-29-

Fig, 14 (a) Wear track portion from 17 m dry sliding test. Traces from

SACP obtained at (b) track edge and (c) at one track width

distance.

Fig. 15 Strain determinations for various sliding distances and wear

track locations.

Fig. 16 Strain determinations at and beneath the original surface for

dry and oil -lubricated wear tests.

Fig. 17 Dislocation density and strain comparison between present work

and Ref. 14.

Fig. 18 Microhardness measurements on wear test specimen comparing

material at and distant from wear track.

Fig. Al Area of indentation mark on specimen surface and locations

from which SACP were obtained.

Fig. A2 Four SACP from indicated positions at a depth of 70 ym below

original surface.

Fig, A3 Area of indentation mark at a depth of 120 ym below the

original surfce after el ectropol ishing.

Fig. A4 Configuration of strains determined in a vertical plane through

the indentation mark. Solid lines are approximate strain

contours.

Fig. Bl (a) El ectropol ished surface of iron single crystal, (b) SACP

of specimen, angular range 5 deg., near (113) orientation.

-30-

1

Fig. 1. (a) View of sliding wear tester, strain gauge

amplifier and chart recorder, (b) Details of

constant velocity tester; cam C_, push rod P^, specimen S_, load U strain gauges G.

-31-

Fig. 2 (a) SACP from one grain in iron specimen. The

220 band is prominent; one signal trace is

superimposed, (b) A set of signal traces at

different locations on SACP showing variation

along the 220 band.

-32-

i iiffiftilffli^Mi tmi iiiia I III!

Fig. 3 (a) Polycrystal 1 i ne iron bar specimen, low strain

end, after deformation, (b) High strain end

showing surface flow and grain structure.

-33-

Fig. 4 (a) SACP from grain at low strain (<1%) location

in iron bar specimen. Note contrast at 220 band

marked B. (b) SACP from another grain at midstrain (-13%) location. Note contrast at 220

band marked.

-34-

o

(o/o) |SDJ|UO0 dOVS

-35-

-36-

Fig. 7 Portion of the wear track after 1.7 m sliding

in oil. Locations of SACP obtained are shown.

-37-

Fig. 8 SACP at locations (a) A and (b) E in previous

figure. Superimposed signal traces show loss

of contrast at high strain location.

-38-

\

ITT rtfrrihiffinii -< nft inim r ,-Tfi"f *^»'"^°'^>»'«-'

Fig. 9 Portion of wear track after 17 m sliding test

in oil. Note that grain boundaries are detectible beneath edge of track.

-39-
i;

I

i!
—' —->:«^^

Fig. 10 Details at wear track in previous -figure.

Note deformation marks in pre-existing cavity

in track surface.

-40-

n SACP traces of 220 band from 17 m sliding test

specimen at (a) wear track center and (b) wear

track edge locations at a depth of 10 ym.

"41-

J

•c-

-D

E

F V

Fig. 12 Portion of wear

distance in oil

.

the surface.

track after 170 m sliding

Some wear debris remains on

-42-

Fig. 13 Details at the wear track end in previous

specimen. Two large wear debris particles are

indicated. Note extensive ridge formation in

wear track.

-43-

-J

Fig. 14 (a) Wear track portion from 17 m dry sliding

test. Traces from SACP obtained at (b) track

edge and (c) at one track width distance.

-44-



Distance from Track Center {firr\)

Fiy. 15 Strain deteniii nati ons at and beneath the

original surface for dry and oil-lubricated

wear tests.

-46-

Depth { jJLm)

Fig, 17 Dislocation density and strain comparison

between present work and Ref. 14

-47-

0

O!

O!

k- u

o c

a>u

o

o

o $ w. E

o o «#-o

der mol c <u •

o <

I!0 §1CVJ ^

J L

lO

8CM E

-1 SI

a.©

c

u

Q. (O

(/) (U

«" E O) O

•M &-

«4- S_

<U C

+J C CT) O -r- oLOC C

0) «

in I

Qi -f- E J-a

to -M

</) (T3 OJ E

c

"O cn

J- c

rtJ -r- x: i- . o rtj J*: i- O- u

o E lO 1- o i. 2: o •->

00

cn

OlO o

o

o

(5oi) ssdupJDH doou>{

-48-

Fig. AT Area of indentation mark on specimen surface

and locations from which SACP were obtained.

-49-

Fig. A2 Four SACP from indicated positions at a depth

of 70 \im below original surface.

-50-

1

/I

Fig. A3 Area of indentation mark at a depth of 120 ym

below the original surface after electropol ishing,

-51-

(oit/) qidaa

•r- O

-a +J

•I- c

CO

ZJo

4-) C

o

o

i. -a +j

O) C on

-l-> -r- O) O) -o O) 4->

CO -M E C -r-

•r- JC X

<o cn o

S- =J -M O

<+- -M O

<V O fO

•I— I— CO

4-> CL O)

<tJ C

5- r— -i- =J fC 1

cn o

•- -I- -a

*+- -M -r- C S_ f—

o <u o o > oo

Q.

CL

O)scn

-52-

Fig. Bl (a) Electropolished surface of iron single

crystal, (b) SACP of specimen, angular range

5 deg., near (113) orientation.

-53-
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Attn: Mr. D. Scott

National cureau of Standards

Department of Comiiierce

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Institute of Ocean Science & Engineering

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The Catholic University of America

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U. S. Army Mobility Equipment Command

Directorate of Research, Development and Engineering

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Office of Secretary or Defense (luL)

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-61-

Ur.ivcrsity College of Swansea

Singleton Par,

Swansea, Wales SA28 PP

Attn: Prof. F. T. Barwall * .

j'lusso.chusotts Institute of Technology

Lincoln Laboratory

Cambridqe, ."'ass. 02139

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. Prof. N. Suh

Imperial Ccller;^ cf Science and Technology

Dept. of I'icch'j! ileal Engineering

Exiiibition Road

London, Engiand SV17

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Af^-SYEi;srJ\ KUI.LAGERFACRIKEN

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Associate Director

Technical information .

• Climax f-iOlybdenum Company

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Ccclo b0331

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"

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-63-

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Department of Ch6nistry

University of North Carolina

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Attn: Dr. T. Isenhower

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-&4-
I

!

t

!
Dr. F. S. Pettit

Materials Engineering & Research Laboratory

Pratt & Whitney Aircraft

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Division of iXaterials Research

National Science Foundation

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"

'

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"

-65-

Secuntv Classification DOCUMENT CONTROL DATA -R&D

I Securi tv c Jss si f ic at ion of title, body of abstr/ict and indexing annotnti on mu^t be entered when the overall report /s classified)

1. ORIGINATING ACTIVITY (Corporate author) Dr. A. W. Ruff, Metallurgy Division

National Bureau of Standards

Washington, D. C. 20234

2». REPORT SECURITY CLASSIFICATION

Unclassified

26. GROUP

3 REPORT TITLE STUDIES OF DEFORMATION AT SLIDING WEAR TRACKS IN IRON

4. DESCRIPTIVE NOTES (Type of report and inclusive dales) Final Report (6/75 to 12/75)

5. AUTHORISI (First name, middle initial, last name)

A. W. Ruff

6- REPORT DATE

February 1976

7a. TOTAL NO. OF PAGES

67

7b. NO. OF REFS

24

8a. CONTRACT OR GRANT NO. NAonr-35-75

6. PROJEC T NO. NBS 3120438

c.

d.

9a. ORIGINATOR'S REPORT NUM8ER(S)

NBSIR 76-992

96. O TH ER REPOR T NO(S) (Any other numbers that may be assigned

this report)

10. OlSTRISUTION STATEMENT

Distribution of this document is unlimited.

11. SUPPLEMENTARY NOTES 12. SPONSO RIN G Ml L 1 T AR Y ACTIVITY

Department of the Navy

Office of Naval Research

Arlington, VA 22217

13. ABSTRAC T Attn: Lz. K. Mi i ler

Determinations of strains have been made on the surface and subsurface on specimens

of high purity iron after different amounts of sliding wear have taken place. The

method involved the measurement of loss of intensity (contrast) of particular

electron channeling lines obtained from small selected areas near the wear track.

Through the use of a calibration specimen deformed plastically to a range of strain

values, the channeling line contrast loss was related to plastic strain. Strain

maps lateral to the wear track and Delow -che original surface were obtained for

different total sliding distances by removing controlled thicknesses of iron using

electropolishing. In all cases the maximum strain was found at the track center

location at the surface and the strains decreased steadily with depth below the

track. At 50 g load the strains vanished at about 40 ym depth. Significant strains

were f6und to exist outside the wear track boundaries. The results are compared

with other studies previously reported. There was no indication of a soft or less

hardened surface layer in any of the specimens studied.

DD /.r..l473 '"'^^ "

S/N 0 to 1 . 807- 680 1 Security Classification
Ii
* NBS-114A (BEV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUHLK ATION OR Rl-;PORT NO.

NBSIR 76-992

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLI-: AND SUinTTl^li

Studies of Deformation at Sliding Wear Tracks in Iron

5. Publication Date February 1 976

6. Performing Organization Code

7. AUTHOR(S)

A. W. Ruff

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

NAonr-35-75

12. Sponsoring Organization Name and Complete Address {Street, City, State, ZIP)

Department of the Navy

Office of Naval Research

Arlington, VA 22217

13. Type of Report & Period Covered pinflT r 1 iici 1 Rpnnrt ixcuur u 1/75 to 12/75

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Determinations of strains have been made on the surface and subsurface on specimens

of high purity iron after different amounts of sliding wear have taken place. The

method involved the measurement of loss of intensity (contrast) of particular

electron channeling lines obtained from small selected areas near the wear track.

Through the use of a calibration specimen deformed plastically to a range of strain

values, the channeling line contrast loss was related to plastic strain. Strain

maps lateral to the wear track and below the original surface were obtained for

different total sliding distances by removing controlled thicknesses of iron using

electropolishing. In all cases the maximum strain was found at the track center

location at the surface and the strains decreased steadily with depth below the

track. At 50 g load the strains vanished at about 40 ym depth. Significant strains were found to exist outside the wear track boundaries. The results are compared

with other studies previously reported. There was no indication of a soft or less

hardened surface layer in any of the specimens studied.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Electron channeling; iron; metals; plastic deformation; surfaces; wear.

18. AVAlLAniLITY [X_ Unlimited

1

' For Official Distribution. Do Not Release to NTIS

' Order From Sup. of Doc, U.S. Government Printing Office

Washington. D.C. 20402. SD Cat. No. CH

f^~^ Order I' rom National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

69

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$A.50

USCOMM-OC 29042-P74