NBSIR 75-645 6^)

Study of the Initial Stages of

Wear by Electron Channeling

Microscopy

A. W. Ruff

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

January 1975

Interim Report for Period 1 May 1974 through 31 December 1974

NAonr-33-74

Prepared for Department of the Navy

Office of Naval Research

Arlington, Virginia 22217

NBSIR 75-645

STUDY OF THE INITIAL STAGES OF

WEAR BY ELECTRON CHANNELING

MICROSCOPY

A. W. Ruff

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

January 1975

Interim Report for Period 1 May 1974 through 31 December 1974

NAonr-33-74

Prepared for Department of the Navy

Office of Naval Research

Arlington, Virginia 22217

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director
Abstract

Studies of the initial stages of sliding wear are in progress.

Both lubricated and unlubricated conditions will be involved, at low

sliding speeds. Abrasive particles of various hardness values and sizes

will be introduced into the bearing system. Selected area electron

channeling patterns are being obtained from the vicinity of the wear

track enabling direct determinations of the strains introduced during

the wear process. Iron single and polycrystall ine specimens are

being studied. Wear debris particles are being collected and examined

in order to determine the relationships between particle characteristics and the wear processes that occur.
1 . Introduction

One of the principal causes of material degredation and device

failure is wear. Most stressed, lubricated, bearing contacts are

designed to accommodate a minimum amount of normal wear, depending

on the materials of construction and the application. However, severe

or abnormal wear will occur sometimes and can lead to unanticipated,

sudden failures. Abnormal wear can involve fatigue cracking, corrosion,

foreign particle abrasion, and other physical processes [1-3], In

high performance equipment, e.g., aircraft turbine engines, sudden

failure of a single component by abnormal wear does occur and can

lead to larger scale disaster.

There is a need to develop techniques for monitoring the wear

taking place in mechanical systems, in order to anticipate sudden

failure and avoid it by remedial actions. One technique currently

being explored for wear monitoring involves the collection and examination of wear particle debris [4]. The wear debris can be obtained

from the lubricating fluids of the mechanical system without disturbing the operating cycle. It is likely that some information on the

modes of wear taking place can be obtained from examination of wear

debris. The onset of abnormal or disasterous wear modes may be determined very early in the development of those processes. In order to

conduct such investigations, it is essential to gain a background of

information on the various mechanisms of wear, the type and rate of

wear debris formation, and any other characteristics associated with

different wear modes that may prove to be useful indicators. It is

1
important to conduct such background studies on well characterized

and controlled systems so that complicated, mixed-mode wearing conditions are avoided [5].

This project is concerned with the initial stages of contact

wear, involving both lubricated and nonlubricated systems under lowload conditions [6]. Wear will be studied under noncontaminated condi

tions, involving, however, normal oxide films on the bearing surfaces,

and also under conditions where abrasive particles are present in

the bearing region. The abrasives will range from soft to hard,

relative to the metal surfaces, and will be carefully controlled in

size and density. The new technique of selected area electron channel

ing pattern (SACP) generation will be used to monitor the developing

strains in the vicinity of the wear track [7,8]. This technique

involves the channeling of incident electrons parallel to dense

crystal planes to various depths in a bulk specimen. Detection of

backscattered or secondary emitted electrons in the scanning electron

microscope (SEM) that channel back out to the specimen surface will

provide information on the strain distribution and lattice orientation

in the region under study. Lateral dimensions of the region studied

-3 -1 are as small as 10 um and the depth can vary from 10 to 10 ym.

Strains introduced into the specimen as a result of wear and plastic

deformation will disturb the interplanar spacing and introduce losses

into the electron channeling process. Since the SACP can be obtained

in a matter of a few seconds of observation in the SEM, it is possible

to monitor both the topographic, surface features as they develop

during wear and the growth of deformation strains. This interim

2
report will describe the initial results obtained on the project

involving studies of iron surfaces after brief wear exposures and the

determination of SACP's at various amounts of deformation.

2. Experimental

A constant velocity, sliding wear tester was constructed for use

on this project. A photograph of the tester and associated equipment

is shown in Fig. la. A strain gauge amplifier and chart recorder are

used to monitor the frictional forces at the specimen surface-wear

pin interface during testing. Details of the tester are shown in

Fig. lb. A constant velocity, motor driven cam causes a reciprocating

velocity of 2.0 cm/sec. in a dovetail slide on which a sheet specimen

is fixed. The linear motion can extend up to 2 cm in length. A pin

bears on the specimen surface from above with a load determined by a

dead-weight loading system. Current tests involve loads between 50g

and 300g. The loading pin is fastened to an elastic beam strip containing two mounted resistance strain gauges. The driven displacement

of the sheet specimen causes a frictional force on the pin and hence

on the beam strain gauges, whose response can be calibrated directly

in terms of force. Thus the result of changing conditions during wear

testing, e.g., the addition of abrasive particles to the lubricating

oit, can be followed in detail with each sliding cycle. Since a small

number of cycle tests are being conducted at present, the lubricating

fluid is simply placed on the sheet specimen surface initially.

The shape of the end of the loading pin used so far has been

either hemispherical or flat with slightly rounded edges. Small

3
bearing balls have been used in some tests and were affixed to the

pin by cementing into a conical depression. The sheet specimens are

generally also cemented to the specimen platform. With the present

motor speed of 23 RPM, tests from 1 cycle to 1,000 cycles duration

can be conveniently conducted.

The sheet specimens and pin end surfaces are examined after

testing in a commercial SEM, usually at 20 kV electron beam energies.

The oil and wear debris are recovered from the sheet surface by

washing prior to specimen examination. The system is designed so

that tests can be interrupted for specimen examination and subsequently

continued to completion. Channeling patterns are obtained in the

rocking beam mode from the specimen oriented at normal electron beam

incidence and operated in the emissive image mode. The minimum

selected area realizable for our equipment is about 10 pm across at

a working distance of about 3 mm. The angular extent of the SACP

is about 18 deg. under these conditions.

3. Results and Discussion

A. Iron Single Crystal Surface

The initial studies were conducted on an iron single crystal.

The crystal had been grown previously by the strain anneal method,

then carefully cut and polished to provide a flat surface of about

2

1 cm area near the (113) orientation. The specimen purity is about

99.9% Fe. Prior to testing, the surface was electropol i shed in a

solution of 6% perchloric acid in acetic acid for about 30 minutes

at 20 volts across the cell. A micrograph showing the initial smooth

4
but wavy surface appearance is given in Fig. 2a. An enlarged SACP

from this surface is shown in Fig. 2b. Considerable fine structure

is present in this pattern, arising from high index channeling bands,

indicating a high degree of crystal perfection.

The crystal surface was next lightly scratched in one area with

a steel point. That area is shown in Fig. 3a. A typical cutting

wear track was formed with considerable material displacement produced

into lips adjacent to the track center. Large plastic strains would be

involved in such deformation and large residual strains would be

expected [9,10]. SACP's taken from the area of the scratches showed

considerable pattern broadening associated with large residual strains.

SACP's obtained from areas adjacent to the cutting scratches showed

little evidence of broadening.

A second experiment was conducted on this crystal surface involving

three sliding wear strokes taken under light (- 25g) load, oil lubricated, with abrasive alumina particles about 1 ym diameter present.

Figure 3b shows some details associated with these wear tracks. The

deeper tracks extending across the field of view appear to involve

small lips of material much like the larger cutting scratches above.

Many of the alumina induced scratches end within this field by becoming

Increasingly shallow and narrow as though the load carried by the

particle involved was decreasing to zero. This may indicate that the

clearance between the bearing pin and surface Increased locally due

to surface flatness variations which could easily exceed 1 ym (40 y-in)

peak-to-peak in this sample.

1

5
A lower magnification view of this wear track is seen in Fig. 4a.

The track edge appears at the right, region ]_ and the track center at

region b. The amount of abrasion increases toward the track center.

SACP's taken from the numbered regions are shown in Fig. 5. The loss

of pattern definition and channeling line sharpness is clear as one

probes into the wear track. Variations of strain in the track are also

found as indicated by the pattern obtained from region 4. A further

comparison of regions J_ and 3 is shown in Fig. 6. Here the SACP's are

enlarged by scanning a smaller angular interval. Further, the detected

electron signal along one line trace through the pattern is also

shown. The increased signal contrast present in the SACP from the

unworn region J_ is seen in the trace of Fig. 6a. Quantitative measurement of signal contrast is possible using this line scan technique.

Calibration of the SACP signal contrast in terms of strain in the

specimen through the use of predeformed specimens would permit a

direct determination of local strain, the principal thrust of this

project. Finally, note the details at the ends of several of the

deeper wear tracks in Fig. 4b. It appears that alumina particles

have suddenly ceased the abrasion action, possibly due to fragmentation

or exiting from the edge of the slider pin. Some alumina particle

can be seen in this field of view.

B . Polycrystal 1 i ne Iron Calibration Specimen

An iron rod, 3 mm in diameter, purity 99.998% Fe, was cut to a

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

of the cylinder, one parallel to the rod axis, the other inclined at

6
about 7° to the axis. This resulted in a tapered flat specimen,

1.3 mm thick at one end and 1.7 mm thick at the other end. The

specimen was carefully electropolished, sealed into an evacuated

capsule, annealed for 2 hours at 940°C, cooled and reexamined.

High quality SACP's were obtained from grains along the length of the

specimen and indicated that the electropol ishing and annealing treatments had left a low residual strain level. The tapered specimen was

then deformed in compression between two teflon plates to a uniform

thickness of 1.3 mm. Hence, one end of the specimen was reduced

0.4 mm in thickness while the opposite end remained undeformed. The

distribution of calculated strain along the bar length is shown below.

T

1 ~1 i 1 ~T 1 1 TT

Figure 7 shows the grain structures observed at either end of the

specimen and Fig. 8 shows two SACP's obtained from grains at the low

strain end and the middle region of the bar. The analysis of patterns

obtained from randomly selected grains along this strain gradient has

not yet been completed. However, it is clear so far that regions

7
containing strains up to about 18% produce SACP's on which measurements

can be conducted. Patterns from the high strain end of the bar

(e = 24%) could not be obtained and hence represent the limit to which

the SACP technique can be extended in high purity iron. The extensive

plastic flow that has taken place at the high strain end of the bar is

suggested by the observed grain structures (Fig. 7b).

C. Polycrystal 1 i ne Iron Wear Studies

Studies have begun of the initial stages of wear on polycrystal 1 i ne

iron surfaces using the sliding wear tester. Experiments conducted so

far have involved carefully prepared, electropol ished surfaces on

99.998%Fe sheets, under clean oil-lubricated conditions and also

under dry conditions. One particular test will be described here

illustrating the type of experiments in progress. A steel bearing

ball, 2.4 mm diameter, was loaded with lOOg, bearing on the iron sheet

specimen. Figure 9 indicates the worn contact area on the ball at low

and high magnification. Several scratch patterns are seen on the flat

portion of the sphere surface due to some interruptions of this test

and reorientation of the pin relative to the direction of motion.

Comparison of the unworn surface (Fig. 9b) with the flat region indicates

the surface changes that have taken place. Scratch grooves are

commonly found of a width approximately 0.1 ijm even though considerable

care was taken to exclude abrasive material contamination (note that

the lubricating oil used was not subjected to additional filtering,

however).

8
Several areas from the wear track on the electropol ished iron

sheet surface are shown in Figs. 10 and 11. Details typically found

at the edge of the wear track are seen in Fig. 10b. Material is

plastically deformed under the slider contact, moved to the edge of

that contact and presumably breaks free to contribute to the wear

debris. The unworn electropol ished surface is shown in the lower

region of Fig. 10b. The central region of the wear track after

400 cycles (Fig. 11a) still retains areas where contact has been avoided

and the electropolished surface structure remains. The end region of

the wear track contains a considerable amount of debris and deformed

metal as seen in Fig. lib. Since a substantial oxide film was initially

present on this specimen, it seems likely that some of the debris here

may be largely iron oxide. Particles whose lateral dimensions are as

small as 0.5 ym are seen in this region of the wear track.

4. Sunmary and Plans

A sliding wear tester has been developed that permits the initial

stages of wear involving selected materials to be investigated. The

device can impose loads up to 300g in a pin-plate geometry, under dry

or lubricated, constant velocity conditions. The design facilitates

studies of single crystal and other specially prepared surfaces.

Selected area electron channeling patterns have been obtained adjacent to and across a lightly loaded abrasive wear track on an iron

crystal surface. The loss of pattern definition (increase in line

broadening) associated with increasing residual strains due to the wear

processes was observed. Calibration experiments have been conducted

9
that will lead to a relation between SACP characteristics and wearinduced strains. Studies of wear tracks in high purity iron specimens

have begun, involving observations of the surface structures, wear

debris and strains produced in the first 700 cycles. Considerable

surface deformation and flow has been observed. Wear debris is

readily produced from lips of material formed by surface flow. The

introduction of abrasive particles (e.g., alumina) leads to the

development of a considerably different surface structure. However,

lips of material are still present adjacent to abrasive scratches and

appear to contribute some of the wear debris.

The next phase of the project will complete the determination

of the SACP-residual strain relationship. From that point, strain

maps will be determined around and beneath wear tracks produced under

carefully controlled conditions. Oil-lubricated load-bearing contacts

will be involved, including the introduction of characterized abrasive

particles of various hardness and mean size values. The role of

surface oxide films in the developing wear process will be examined,

including the effect of oxide film thickness. Wear debris particles

will be collected during these studies in order to correlate the

debris characteristics with the wear modes involved.

10
References

E. Rabinowicz, "Friction and Wear of Materials" (Wiley, N.Y.

1965).

P. L. Hurricks, "Some Metallurgical Factors Controlling the

Adhesive and Abrasive Wear Resistance of Steels: A Review,"

Wear 26, 285 (1973).

M. A. Moore, "A Review of Two-Body Abrasive Wear," Wear 27, 1 (1974).

A. W. Ruff, "Metallurgical Analysis of Wear Particles and Wearing

Surfaces," National Bureau of Standards Report NBSIR 74-474

(1974).

F. Wilson and T. S. Eyre, "Metal lographic Aspects of Wear," Metals

and Materials 3, 86 (1969).

C. Pritchard, "Abrasion Under Low Bearing Pressures," Wear 1_7,

185 (1971).

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

SA-ECP Method to Deformation Studies," SEM/1971 (IITRI, 1971)

p. 473.

S. M. Davidson and G. R. Booker, "Effects of Dislocations on SEM Channeling Patterns," Proc. EMAG Inst. Physics (1971).

R. Feder and P. Chaudhari , "Transmission Electron Microscopy of

Wear Tracks," Wear 19, 109 (1972).

H. Kawabe, S. Ido, A. Ohtsuka, and T. Yamada, "Electron Microscope

Observation of Scratch-Deformed Surface Layer of Aluminum Single

Crystals," Bull. Japan Soc. Prec. Engg. 6, 95 (1972).

11
Figure Captions

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 L, strain gauges G.

Fig. 2 (a) Electropol ished surface of iron single crystal, (b)

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

Fig. 3 (a) Cluster of scratches on iron single crystal surface due

to steel cutting edge, (b) Scratches on iron surface from

1 \im alumina particle abrasion.

Fig. 4 (a) Area of wear track on el ectropol i shed iron single crystal

surface. Region 1 is right edge of track and region 5 is

approximate center of track, (b) Details of wear scratches on surface. Note differences in track-end characteristics.

Fig. 5 SACP's from numbered regions in Fig. 4a, angular range 18 deg.

Regions 1 through 5 show progressive Increase in deformation

due to wear.

Fig. 6 (a) SACP from undeformed region of single crystal surface,

angular Interval 3 deg. Upper line trace shows signal intensity

across SACP at location of lower, straight line trace, (b)

SACP from deformed region of crystal showing loss of signal across prominent channeling bands.

Fig. 7 (a) Polycrystal line Iron bar specimen, low strain end, after

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

grain structure.

Fig. 8 (a) SACP from grain at low strain (<1%) location in iron bar

specimen. Note contrast at 110 band marked B. (b) SACP from

another grain at midstrain (-13%) location. Note contrast at

no band marked.

Fig. 9 (a) Worn region on 52100 steel bearing ball after 700 cycle

wear test on iron sheet specimen, (b) Detail of worn region.

Fig. 10 (a) Wear track on high purity el ectropol i shed iron sheet

specimen after 400 cycles under oil. (b) Details at track

edge.

Fig. 11 (a) Central region of wear track showing small unworn areas surrounded by deformed areas, (b) End of slider wear track

showing displaced material (oxides?) and wear debris.

USCOMM-NBS-DC
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 l^, strain gauges G^.
'4&.

Fig. 2. (a) Electropolished surface of iron single

crystal, (b) SACP of specimen, angular range

5 deg., near (113) orientation.
Fig. 3. (a) Cluster of scratches on iron single crystal

surface due to steel cutting edge, (b) Scratches

on iron surface from 1 ym alumina particle

abrasion.
(a) Area of wear track on electropol ished iron

single crystal surface. Region j_ is right edge

of track and region 5^ is approximate center of

track, (b) Details of wear scratches on surface.

Note differences in track-end characteristics.

Fig, 6. (a) SACP from undeformed region of single crystal

surface, angular interval 3 deg. Upper line

trace shows signal intensity across SACP at

location of lower, straight line trace, (b)

SACP from deformed region of crystal showing

loss of signal across prominent channeling bands.
Fig. 7. (a) Polycrystall i ne iron bar specimen, low strain

end, after deformation, (b) High strain end

showing surface flow and grain structure.
Fig. 8. (a) SACP from grain at low strain (<1%) location

in iron bar specimen. Note contrast at 110 band

marked B. (b) SACP from another grain at midstrain (-13%) location. Note contrast at 110

band marked.
Fig. 9. (a) Worn region on 52100 steel bearing ball after

700 cycle wear test on iron sheet specimen, (b)

Detail of worn region.
Fig. 10. (a) Wear track on high purity el ectropol i shed

iron sheet specimen after 400 cycles under oil.

(b) Details at track edge.
Fig. 11. (a) Central region of wear track showing small

unworn areas surrounded by deformed areas, (b)

End of slider wear track showing displaced

material (oxides?) and wear debris.
U S, Ut.i 1. OF COMM. I. injlll.lc A 1 U)N OK KHPOKT NO. 2. Cov't Accession

BiCLIOGRAPHIC DATA No. SHEET NBSIR 75.^645

3. Hci ipicnt's Accession No.

4. 11 vi.i: AND suH rn ijstudy of the Initial Stages of Wear

by Electron Channeling Microscopy

5. Publication D.ntc

6. Performing Organization Code

7, AUTIIOK(.S)

A. W, Ruff

8. Performing Organ. Report No.

9. PKHFOKMING ()K(.AN17.AT10N NAMh AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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11. ConI raci /C>r.int No.

NAonr-33-74

12. Spoil .(irm;; Or^ .in i / .H i ( m N.imc M\d Complete Addreaa (Stt99t, City, State, ZIP)

Department of the Navy

Office of Naval Research

Arlington, Virginia 22217

13. 1 ypc of Kcpori & I'l riod

Ci)V( rt'il Interim Report —5/1/74 1.2/31/74

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16. AUS I |<A( 1 (A 200-word or leas (actual summary of moat aigfiificant information. If documertt includes m ai/fnificant

hi bl iof^rnphy or literature survey, mention it here.)

Studies of the initial stages of sliding wear are in progress. Both lubricated

and unlubricated conditions will be involved, at low sliding speeds. Abrasive

particles of various hardness values and sizes will be introduced into the bearing

system. Selected area electron channeling patterns are being obtained from the

vicinity of the wear track enabling direct determinations of the strains introduced

during the wear process. Iron single and polycrystalline specimens are being

studied. Wear debris particles are being collected and examined in order to

determine the relationships between particles characteristics and the wear

processes that occur.

17. Kl'Y WOKDS (six to twelve entries; alphabetical order; capitalize 'only Iho first loiter of the first key word unles.-i a proper

nuiuc; sepfirated by semicolons

)

Abrasives; debris; electron channeling; iron; mechanical strain; metals; wear.

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