NBSIR 74-474

Metallurgical Analysis of Wear

Particles and Wearing Surfaces

A. W. Ruff

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

April 1974

Final Report

NAONR-31-73

NR 229-014

Prepared for Department of the Navy

Office of Naval Research

Arlington, Virginia 22217

Naval Air Engineering Center

Philadelphia, Pa. 19112

NBSIR 74-474

METALLURGICAL ANALYSIS OF WEAR

PARTICLES AND MfEARING SURFACES

A. W. Ruff

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Wastiington, D. C. 20234

April 1974

Final Report

NAONR-31-73

NR 229-014

Reproduction in wtiole or in part is permitted for any purpose of the United States Government. Approved for public release; distribution

unlimited.

Prepared for Department of the Navy

Office of Naval Research

Arlington, Virginia 22217

Naval Air Engineering Center

Philadelphia, Pa. 19112

U. S. DEPARTMENT OF COMMERCE, Frwlericfc B. Oent. Secretary

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I 'MMLiNAriNG ACTIVITY (Corporate author) National Bureau of Standards

Washington, D. C. 20234

Attn: A. W. Ruff, Metallurgy Division

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H 1- r' o R T r I T L i; METALLURGICAL ANALYSIS OF WEAR PARTICLES AND WEARING SURFACES

4 ntsCRiPTivE NOTES (Type a I report and inclus i ve dates) Final Report

Aij THORIS) (First namo, middle initial, last name)

A. W. Ruff

r, RI-PORT OATE

April 1974

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nn. CONTRACT OR GRANT NO 9a. ORIGINATOR'S REPORT NUM8ERIS)

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NBSIR 74-474

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this report)

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distribution of this document is unlimited.

II SUPPLEMENTARY NOTES

11 ABSTRACT

12 SPONSORING MILITARY ACTIVITY Department of the Navy

Office of Naval Research

Arlington, Va. 22217

flff n. If V Mi n oir

The initial approach has been concerned primarily with the examination of

particles recovered by the Ferrographic technique from samples of lubricating

oils taken periodically during tests and service of bearings, gears, sliding

surfaces, etc., in which such experimental variables as lubricants, lubricant

additives, bearing materials, loads, etc., have been studied. Our examinations

have been conducted principally using both Scanning and Transmission Electron

Microscopy techniques, observing particle shapes, sizes, surface structures and

other parameters as functions of distance along the Ferrogram and determining a semi-quantitative elemental chemical analysis of selected and typical particles,

These electron microscope techniques have been used to characterize the wear

particles and associated surface degradation produced in the bearing and gear

tests conducted by others. They provide information on particles and surface

details too small for study by optical microscopy methods.

DD FORMI NO V «S S/N 0 10 1-807-680 1 1473 ^^^^^ in

Unclassified

Security Classification
Unclassified

Security Classification

1 4 KEY WO R D S LINK A LINK B LINK C ROLE W T ROLE W T ROLE W T

Electron Microscopy

Lubricated bearing systems

Particle analyses

Wear particles

DD .?o?„1473 <BACK) Unclassified

(PAGE' 2) Security Classification
Table of Contents

Page

Project Objective and Approach 1

1. Introduction 2

2. Experimental Details 3

3. Results and Discussion 4

3.1 Army Heavy-Lift Helicopter Gear Transmission 4

3.2 Ball Bearing Bench Test 4

3.3 Ball Bearing Bench Test 7

3.4 Single Ball Test 10

3.5 Bearing Ball Worn Surface 11

3.6 Bearing Ball Spalled Surface 12

4. Conclusions 12

5. Suggestions for Future Work 13

6. References 15
(PAGE' 2)
Objective

The objective of the program has been to characterize the wear particles

and surface degradation produced by wear in bearing and gear tests in

\ which the effects of several variables on failure of the wearing sur- faces has been examined. The information obtained has been correlated

with the results of allied studies conducted by others in an attempt to

develop an understanding of the processes producing wear and degradation

of metal surfaces in sliding, rubbing, rolling, and/or rotating contact

and the effects of lubricants, lubricant additives, bearing materials,

etc. on these processes. The characterization of the wear particles and

wearing surfaces should aid in the establishment of the interrelationships

between wear particle shape, size, size distribution, chemical compositions,

metallurgical structure, and surface damage prior to failure.

Approach

The initial approach has been concerned primarily with the examination

of particles recovered by the Ferrographic technique from samples of

lubricating oils taken periodically during tests and service of bearings,

gears, sliding surfaces, etc., in which such experimental variables as lubricants, lubricant additives, bearing materials, loads, etc., have

been studied. Our examinations have been conducted principally using

both Scanning and Transmission Electron Microscopy techniques, observing

particle shapes, sizes, surface structures and other parameters as functions of distance along the Ferrogram and determining a semiquantitative elemental chemical analysis of selected and typical particles.

These electron microscope techniques have been used to characterize the

wear particles and associated surface degradation produced in the bearing

and gear tests conducted by others. They provide information on particles

and surface details too small for study by optical microscopy methods.

After suitable techniques have been developed for examining and characterizing both the wear particles and the surfaces that produced them, these

methods will be used to examine and evaluate specimens removed or obtained

from programmed wear tests being conducted on associated programs.

-1-
1. Introduction

The study of wear debris particles recovered from the lubricating fluid

in wearing contact systems has received increasing emphasis in recent

years. Techniques for retrieving the wear particles, examining them, and

monitoring the amount produced have been described.''^ Unusual particle

morphologies have been reported and are proposed as possible indicators

of particular wear modes. ^"^ Comparisons are being made between the

results of spectrometric analysis of oil from wearing systems and

measures of the amount of particulate debris in that oil.^ Recently a wear theory has been proposed" that qualitatively predicts both the

shape of the wear particle produced under specified conditions and the

processes of plastic deformation and fracture that occur in the bulk

material. Clearly much can be learned about the basic processes of wear and debris formation and about the prospects for monitoring wear

in actual operating systems. Archard and Hirst^ referred sometime ago

to nrild and to sevara wear, distinguishing between a normal, slow

degredative process and an abnormal, rapid destruction. The wear

particle studies reported here include both contributions, and it is

well to keep in mind that significance should be principally attached

to observations of large, unusual wear particles.

This report concerns the examination and identification of particles

found on Ferrograms of selected oil samples and the determination of

suitable techniques for such identification. The Ferrograph instrument^

enables particle collection from the lubricating oil on the basis of

particle magnetic moment. It provides an initial selection of wear

particles by rejection of nonmagnetic particles, thus saving analytical

efforts. The resulting Ferrogram displays the wear particles collected

into "strings" or groups oriented transversely to the oil flow direction

as a result of transverse magnetic fields in the instrument. Deviations

in this selection process occur when nonmagnetic particles adhere to

magnetic particles in some manner and are thus collected. It is

necessary to recognize that some types of particles important in the wear process, for example nonmagnetic abrasives, may not be collected

reliably from the oil by this technique.

Particles having large magnetic moments are deposited first, near the

entrance end of the oil on the Ferrogram. This leads to the development

of a particle size gradient along the Ferrogram, larger particles depositing first. When a variety of types of material are present in the particulate collection, the sizing process is disturbed since large particles

of a material with a relatively low specific magnetic moment will deposit

further down the Ferrogram. For example, specific magnetic moment/unit

volume values for three relevant materials in steel bearing systems are:

Fe (171 m Tesla), Fe304 (48 mT), FesC (99 mT). Thus physical sizing of

particulates may scatter by at least factors of 4 or more along the

Ferrogram as a result of material differences. As we shall see, examination of strings of particles in the Ferrogram does show a considerable

size variation.

Initial results on this project have shown that considerable information can be developed from electron microscope studies of metal wear

particles. Data on particle size, shape, composition, degree of oxidation,
contamination and deformation can be obtained using either scanning or transmission electron microscopy, electron diffraction, and X-ray

emission analysis -- singly and in combination. Results of several

specific studies will be presented.

2. Experimental Details

Various techniques have been developed to permit electron microscope

evaluation of the particles on the Ferrograms. Initially, conventional

glass Ferrogram slides were over-coated with vacuum-deposited carbon

films (=^200A thick). That deposit provides sufficient electrical con- ductivity to prevent electric charge build-up during SEM study. Carbon

is an excellent coating material in that no interference is presented

for subsequent X-ray emission analysis of particulates. A

second technique was developed that involved stripping the particles

from the Ferrogram slide using a plastic replicating film and subsequently carbon-coating that film. The plastic film could then be dissolved

in a solvent leaving the particle strings intact on a thin carbon substrate.

That method has been used to prepare specimens for transmission electron

microscope and diffraction studies on the particle strings.

A third method used glass slides that were previously coated with a thin

vacuum deposited carbon layer. The oil sample was passed through the

Ferrograph with the precoated slide in place. The particles were deposited on the slide in the usual manner. The carbon under-coating

did affect the base-line optical density readings taken from the Ferrogram

and in some cases contributed to oil overflow of the barrier strips along

the sides of the Ferrogram slide. However, those problems do not appear

serious. After usual washing procedures, the Ferrogram was heated in some cases for 30 seconds on a laboratory hot plate in air to remove

residual oils and then examined directly in the SEM. While some charging of particles was seen in the SEM, it was found that sufficient

electrical conductivity was present to permit detailed examination of

the particles. Since no over-coating procedure was applied to these

Ferrograms, all original surface features and details on the particles

were preserved. Over-coating with another film (carbon or metal) was

still possible if it was necessary for reasons of protection or fixation.

Dislodgment of uncoated particles on these substrates during handling

was noted and could be reduced by over-coating.

Examinations of solid bearing surfaces were conducted on a few components

removed from service for various reasons. Further studies of this type are planned where the bearing surface wear conditions are carefully controlled.

Transmission electron microscopy and diffraction data were obtained using

a lOOkV electron beam with the specimen held in the conventional manner

within a cryogenical ly-cool ed enclosure to minimize specimen contamination.

The limiting particle thickness for sufficient transmission would be about

0.25ym. The scanning electron microscopy data were obtained at 20kV

beam potential with specimen currents in the range 0.1 to 10 nA and with

the specimen held conventionally in a goniometer mounting. A Si (Li)

X-ray detector system was available in the SEM with a FWHM of 170eV. The

specimen was usually rotated to face the X-ray detector and

specimen currents established at about 240 pA for X-ray analysis.

-3-
3. Results and Discussion

The results of examining wear particles obtained from a group of

different lubricated systems will be described.

3.1 Army Heavy-Lift Helicopter Gear Transmission

The transmission from which this sample was taken had failed after a few hours of operation. There were many foreign or unusual particles

detected on this Ferrogram that was produced at Trans-Sonics.

The discussion will consider the classes of particles observed,

originating at either gear or bearing surfaces. The entry deposit is

shown in Fig. 1. The Ferrogram 2273 has been overcoated with a layer

of vacuum-evaporated carbon. Some charging is seen at particles

within the heavy, initial deposit. Many particles larger than lOym are found near the entrance end of F-2273. Several small spheroidal

particles were found (Fig. lb). One of these is shown in Fig. 2a

and contained only iron according to the X-ray emission analysis. A

large faceted wear particle of unusual morphology is shown in Fig. 2b.

Further down the Ferrogram the usual transverse strings of small wear

particles are found, as shown in Fig. 3a. The character of these

small {<_ lym) rubbing wear particles is seen more clearly in Fig. 3b.

A large foreign flake-like particle that is not ferromagnetic has

settled there and subsequently the small wear particles were collected

in strings that overlay it. The iron peak within the X-ray spectrum

from the large flake is actually associated with these small wear particles that have originated from the gear surfaces.

The wear particle strings further down the Ferrogram are shown

in Fig. 4 (rotated from the previous orientation). The variation in

particle size within the strings is large. Many flake-like wear

particles can be identified. In this field one iron spheroid is

seen, as is another large foreign flake-like particle. Recently

Cummins et^aj^.^ described observations on wear debris from a spur

gear system. They reported principally small particles of thickness

0.2ijm or less (note the small particle strings here in Figs. 3 and

4) together with some few spherical particles. The needle-like

particles they observed were not found in the present sample. A large,

faceted wear particle such as shown in Fig. 2b may originate in a fatigue-cracking process in the manner described by Venkatesh and

Krishnamurty in their study^ of cracking in gear systems.

These observations will now be compared with those made on wear particles obtained from lubricated bearing systems.

3.2 Ball Bearing Bench Test

The oil sample was obtained from SKF and originated from a lubricating

system used during rolling bearing tests. The Ferrogram (F-2117) was prepared at Trans-Sonics using carbon precoated slides furnished

by us. The same oil had been used previously to prepare F-1851 that was examined and will be discussed later. Optical examination of

F-2117 at Trans-Sonics indicated the presence of cutting wear particles, spherical particles, and spalls. The entrance end of

-4-
the Ferrogram is shown in Fig. 5 at a relatively low-magnification

(200x) scanning electron micrograph covering an area of about 0.5mm

square. The remainder of the Ferrogram slide will not be discussed

here but contained relatively few strings of small (< lym)

particles. The study of F-1851 from this same oil concentrated

on smaller particle strings and will be discussed later. Figure 5 shows that some larger particles do not have sufficient conductivity

to the carbon-undercoated glass slide and acquire an electrical

charge during a slow beam scan, causing streaks on the photographs.

This was rarely noted and caused no significant problem. The

labeled areas in Fig. 5 are examined at greater resolution in the

following photographs.

The region A is shown at higher magnification in Fig. 6. Many

flake-like particles are seen, the larger ones being about lOym in

lateral dimensions and of the order of O.lym thick. Most of the

flakes are bent and corrugated with ragged edges and evident surface

features such as lines or bands. Many small particles, less than

lym in size are also seen. The larger flakes presumably fractured

or spalled away from the bearing material during contact wear and

stressing. They could subsequently have been further deformed

if recirculated through the bearing contact regions by the

lubricating oil and even fractured again to produce some of the

smaller particles. The expected EHD film thickness would be one-tenth

or less the size of these particles (order of lOy-inch = 0.3ym).

It is of interest to estimate the numbers of particles collected in

these strings. Cursory inspection of Fig. 6a indicates about 10^

particles in the field, ignoring particles below 0.5ym size. Examination of other subfields of Fig. 5 indicates that to within factors

of 3, the total particle count in Fig. 5 can be estimated at about

3 X 10>^ particles. This number should be kept in mind in connection

with the validity of conclusions drawn from individual particle

analysis and extended to the macro-system. One clear drawback of

the Ferrogram technique for microscopic evaluation is seen in Fig. 6.

It involves the tendency for particles to clump and overlay one another in the strings. This prevents accurate particle counts

and particle sizing and may affect other observations, i n particular

X-ray emission analyses. However, dilution and redispersion

techniques should be applicable where the particle densities are

too large.

Another region from Fig. 5 is shown in Fig. 7. One observes

substantial differences in contrast between the particles. The

particles showing lower secondary electron emission (grey images) are probably more heavily oxidized and hence contain less iron.

Some particles show a porous surface structure that is also suggestive of oxide material. It is recalled that the particles have

not been overcoated prior to examination, hence the existing surface

structures are preserved. Flake-like particles are observed together

with several rod-like particles.

-5-
The region E from Fig. 5 is shown in greater detail in Fig. 8. One

flake-like particle in this string has considerable surface structure

that includes several cracks and smaller, attached particles. A

spheroidal particle (2.4ym diameter) is also seen. Its size is

sufficiently large to contain nearly all the X-ray emitting volume

permitting meaningful X-ray emission analysis. However, fluorescence

effects associated with neighboring particles in such a collection

could be substantial and require correction. Analysis of this

spheroid showed only an iron peak in the X-ray emission spectra as shown. The large flake-like particle contains a significant amount

of iron and zinc.

Results of X-ray emission analyses on other particles are shown in

Fig. 9 taken from region F in Fig. 5. Two particle analyses are

shown, one from another spheroid about 2.5ym in diameter. and the

other from an irregularly shaped 3ym particle. The background

analysis was obtained from area c. Analysis was conducted using

a stationary electron probe, an electron beam potential of 20kV,

a beam current of 200pA, and a counting time of about 100 seconds.

Both particles are iron with no other elements present.

The background contains elemental emissions from oil residue material

on the substrate, principally Si, K, Ti and Zn (some of the Si

peak originates in the glass substrate).

It is important to recognize the limitations present in X-ray

emission analysis conducted on particles of widely different size,

shape and composition, collected into close physical contact.

Beaman and Isasi^O indicate many of the general considerations

including the use of energy-dispersive detection systems. Even

where only qualitative or semi-quanti tati ve information is desired,

great care must be taken to avoid adjacent particle interferences

by X-ray flourescence, absorption, and emission due to back-scattered

electrons. Point-probe analysis must avoid exciting small surface

contaminant particles o-n larger particles. Penetration through the

particle of interest into other particles below must be recognized

when present. This study generally has examined reasonably well isolated particles larger than 2ym in size and oriented favorably with

respect to the X-ray detector. Bayard'^ has determined the relation between particle (spherical) size and X-ray intensity (relative

to bulk) for aluminum particles. We have estimated using his

results and data on the X-ray distribution in depth for copper and

aluminum'O that Fe particles of diameter 0.5ym should yield an X-ray

intensity about 75% that of bulk. At a diameter of 1.5ym, the

intensity should exceed 95% of the bulk value. Techniques for true

quantitative analysis of small particles are described by Bayard''

and are under further development in several laboratories.

The region (G) in Fig. 5 is shown in more detail in Fig. 10. A small

string of seven particles is present containing a spheroid of 4.5ym

diameter. One of the particles has a chip-like appearance and another

(c) shows significantly lower image contrast. The X-ray spectra from

particles a, b, c and background d are shown in Fig. 10(c). The

spheroid and particle b are iron, however, particle c shows a weak
iron line and a significant Si, K, and Zn content. Since these three

elements are associated with oil residue, it appears that particle

c may be iron oxide that has absorbed a significant amount of oil.

Another string of particles from this sample is shown in Fig. 11 together with the particle X-ray analyses. The faceted spheroid

(a) is a chromium-containing iron as is particle (e). Particles

(b) and (f) contain principally iron. Particles (c) and (d) are

iron oxide particles containing significant oil residue as indicated

by the Si and K peaks. These results are summarized in Fig. 12

where the various X-ray peak heights are given. Note the variation

in electron emission among the particles saen in Fig. 12 in the

Y-modulation image (where emission is proportional to Y-def lection

of the signal trace)

.

In summary, the bearing test oil sample contains a wide distribution

of particle sizes and types, some results from which are summarized in

Table I. Many examples of flake-like particles are found (perhaps

about 25% of the total in the size range 1 to lOym. Some elongated,

rod-like particles are found. Four spheroidal particles were found

(from an estimated total of 3 x 10"^). Three of those (size 2.4ym,

2.5ym, and 4.5um) were analyzed by X-ray emission and found to contain

only iron. Contrast differences were found among the particles imaged

by secondary electron emission. They are believed associated in some cases with the iron/iron oxide ratio of the particles.

3.3 Ball Bearing Bench Test

A second study was conducted on wear particles from the SKF test of

52100 steel ball bearings. Rather than carbon-coat the Ferrogram-1851

,

we removed the particle strings from the glass substrate by extraction

replica techniques. This would permit high resolution imaging in a transmission electron microscope (TEM) and transmission electron

diffraction (TED) examination, particularly of the smaller particles.

A softened polymer film was pressed onto the Ferrogram, allowed to

harden, stripped off, and overcoated with carbon, and then the polymer

film was dissolved in solvent. The resulting carbon film contained

the partially embedded particle strings and was supported on a conventional copper grid that could be loaded directly into the TEM specimen holder. Figure 13a shows two strings located in one grid opening.

This same area is shown in the optical microscope photograph in Fig. 13b,

indicating the versatility in this preparation and mounting technique.

Two specific strings of particles will be discussed among several

that have been examined from F-1851. String S-j is shown in more

detail in the SEM photograph of Fig. 14. The particle size varies

from about 0.1 to 2ym; other strings contained larger particles. The

difference in emissive-mode contrast between particles is evident.

Some particles appear thin and flake-like, others are more equiaxial.

Some angular features are seen on certain particles. Data on Fe-Ka

X-ray emission have been obtained on the particles labeled A, B, and

C; particle C contains significantly less iron.

-7-
TABLE I.

SELECTED PARTICLE CHARACTERISTICS FROM F-2117:

LUBRICANT FROM ROLLING BEARING TESTS

Particle Peak Heights

(Ka - 1,000 cts)

Shape Size (ym) Identification Fe : Other

spheroid 3 alloy steel n : 2.7Cr

spheroid 2.4 iron 10

spheroid 2.5 iron 12

spheroid 4.5 iron 10

lump 3 iron 11 lump 3 iron oxide 6.8 : 5.2Si

1 ump 2 alloy steel oxide 4.8 : 2.3Cr : 2.6Si

1 ump 1.5 iron 8.4

lump 3 iron 12

lump 4 iron 11

1 ump 3 iron oxide 3.2 : 8.4Si

flake 3 iron oxide 3.2 : 7.4Si

flake 8 iron/zinc 2.6 : 3.6Zn

flake 10 iron 11

flake 10 iron 11

flake 2 iron 12

free spheroid 2.5 foreign 1.3 : 7.8Si : 4.5A1
Transmission electron micrographs of string S-j obtained at lOOkV

accelerating potentials are shown in Fig. 15. „Here satisfactory

penetration is obtained at thicknesses of 2000A or less for iron

particles. Thicker particles will appear black, only the outline

regions being thin enough for penetration. The labeled particles have

been studied by electron diffraction. Particles C, D, E, and H have a strong iron oxide diffraction pattern together with an a-iron diffraction pattern. These particles are 50% or more composed

of a mixture of two oxides, Fe203 and Fe304, the balance being iron.

Particle A has a lower but still significant iron oxide content;

it is relatively thin and flake-like. Particles B, F, and G are thick (greater than 0.5Mm), composed of iron with a covering of the

combined oxide mentioned above. Further information on particles

A and B is shown in Fig. 16. Two dark field images are shown using

different reflections, along with the bright field image and the

diffraction pattern from particle A. In dark field imaging only

crystalline regions oriented precisely for diffraction into a narrow angular region will appear bright or in contrast. Examination of the

images s^own indicates that particle A is composed of many small

(200-500A) crystal regions of both iron and iron oxide. It has the

structure expected from a deformed iron fragment that had recrystallized and partially oxidixed. The remaining (residual) strain level

is not high, as evidenced by the diffraction pattern. Particle B offers less information due to its thickness, however, its surface

appears covered with an oxide film or collection of oxide particles.

The fine structure in the dark field images suggest the latter

covering is more likely.

In summary, the string S] contains a number of different particle

types. Thinner particles may be solid iron, composed of many

crystallites (A), they may be a loose collection of much smaller iron

oxide particles (C), or they may be relatively thick iron particles

with an oxide coating (B). No carbide particles were found in this

string. A more detailed electron diffraction pattern is shown in

Fig. 17 with the principal lines for a-Fe and Fe304 identified. The

Fe203 pattern contains two additional lines that serve to distinguish

it. As long as sufficient diffraction intensity is obtained, solutions

of patterns from particles are straightforward.

Another string (S2) is shown in Fig. 18. Again considerable size and

shape variation among the particles is seen. Fine structural details

can be observed here in emissive mode imaging. The brightness differences between particles are seen again in this string. A portion

of this string is shown in the TEM micrograph composite of Fig. 19.

The labeled particles have been studied in diffraction. Particles

1, 3a, 3b, 9b, and 12 give principally iron oxide patterns. Particles

2, 4, 5, 7, 10, 13, 14, and 15 give strong a-Fe patterns. The

remaining particles studied showed both iron and iron oxide content

to a substantial amount. Some of the thinner particles (10, 13, 14, 15)

appear to be solid, partially-oxidized iron fragments. Other particles

(3a, 9a) appear to be more a collection of small fragments, principally

iron oxide.

-9-
These observations are rather similar to those made on string S]

and suggest that a sufficiently valid description of the (smaller) wear particles on F-1851 can be made on the basis discussed. Over

seventy electron diffraction patterns have been analyzed. About

one-half are mixtures of a-Fe and iron oxide, about one-quarter are predominantly iron, and the remainder are principally iron oxide

particles. No carbides have been found. Some foreign crystallites

outside of the particle strings have been identified. The vast

majority of particles in string groups in this Ferrogram are iron

fragments, partially or completely oxidized. The low incidence

of foreign particles within the strings illustrates the advantage

of the Ferrograph particle selection and display technique in studying

wear particles.

In a study of wear of cast iron in dry sliding, Takeuchi^^ reported

a variation in iron oxide amount detected by diffraction measurements,

dependent on sliding distance and presumably surface temperature.

For long accumulated wear distances, the wear debris was predominantly

iron oxide. Those observations are consistent with the variation in

amount of particle oxidation seen here. The absence of carbide

particles in the wear particle strings examined was surprising in view

of the known importance of carbide content in steels on their wear behavior (see, for example, Hurricks^-') . However, particles smaller

than O.Sym have not been systematically studied here by electron

diffraction. Carbides dislodged from the steel bearing surfaces may

fracture into rather small particles and thus escape detection.

Brief X-ray diffraction examination of a few wear debris samples has

also not indicated significant carbide content.

3.4 Single Ball Test

A series of oil samples from a test of an M50 steel (includes

4% Cr, 4% V) ball and race was obtained by Trans-Sonics and processed

to produce several Ferrograms. A silicon-bronze ball retainer was included in the mechanical testing device. The oil was MIL-L-23699

type held at a temperature of 300°F. The test involved a time-tofailure of 9h 55m and a ball Hertz stress of 500 kpsi maximum. At

failure the ball became driven into the retainer. The Ferrogram

(F-2119) was prepared using carbon precoated slides from an oil

sample taken at 8h time (lh ,55m prior to failure).

A low magnification view of the entrance end of this Ferrogram is

shown in Fig. 20 covering an area about 0.9 by 2.7mm. Three large

particle strings are seen and will be examined in more detail. The

remaining areas of the Ferrogram contained isolated particles or small

particle (< lym) strings and have not been examined closely. The

particles are in place on the precoated-carbon Ferrogram slide and

have not been overcoated.

Region A in Fig. 20 is shown again in Fig. 21. Several examples of

long ribbon-like particles are found in that string together with some flake-like particles. Both types have surface structures and markings

that suggest plastic deformation and slip bands. X-ray analysis

results are given. Figure 21 shows the region B in more detail. One

-10-
curled ribbon particle is seen in this string. Several small

particle strings are present in this area but also in the area of

region A. Comparison of Figs. 21a with 22a and Figs. 21b with 22b

shows the particle sizing capability of the Ferrograph instrument.

The larger particles predominate in region A, however, many smaller

particles (1 to 4ym) appear present in both regions. Those regions

are separated by 0.45mm along the Ferrogram. It seems that the

larger magnetic moment particles are deposited earlier on the

Ferrogram but that smaller moment particles may deposit either (i)

adjacent to larger particles (within their strings) near the entrance

end or (ii) further down the Ferrogram in strings of like-sized

particles. That "early deposition" behavior may result from- relatively

large magnetic field gradients within the incomplete, forming

particle strings.

The final region (c) studied in Fig. 20 is shown in detail in Fig. 23.

The larger flake-like particles are clearly present in this region

as is one large, 8ym diameter spheroid. The spheroid is remarkably

smooth and free of surface structures as compared to adjacent particles.

X-ray emission spectra from four of the particles are shown in Fig. 24.

Particle c produces a single iron peak, however, the spheroid produces

a weak iron peak together with strong silicon and aluminum peaks. None

of the other particles analyzed showed these Si and Al peaks. The

flake-like particle d contains significant amounts of Cr and Ni

,

an alloy steel. The globular particle e also contains Cr and Ni

together with a significant amount of copper, presumably resulting

from the silicon-bronze retainer. X-ray signal imaging studies were conducted on this string using the prominent Fe and Si K^^ lines.

Those X-ray scans proved the low-iron, high-silicon content of this

particular spheroid which may have originated as a nonmetallic

inclusion in the steel ball or race.

In summary, only one spheroid has been found among the particles from

this single ball test. It is relatively large and it contains significant amounts of silicon and aluminum. There is no appearance of a rough or cracked oxide covering on the sphere. Adjacent particles

in the string that show a greater roughness do not show any significant

silicon content. The ribbon-like particles found in this Ferrogram

are typical cutting-wear particles.

3.5 Bearing Ball Worn Surface

A disassembled bearing that had failed during a bench test at SKF

was examined to attempt to relate the particle debris obtained by

the Ferrograph from the lubricating oil with the bearing surface

structures and markings. An example is shown in Fig. 25 taken from one 52100 steel bearing ball. Some of the surface tracks seen in

Fig. 25a may remain from initial finishing operations, however, many

examples of deep wear gouges are present. Detailed examination

indicated that plastically deformed metal at the gouge rim can be

detached in the form of small particles (Fig. 25b). Larger particles

may also form in this way as indicated by the 3ym particle formed

in Fig. 26. Note the surface markings on the particle and the similarity

with the bulk surface markings. This particular gouge appears to be

-11-
abrasively formed and the particle is formed from the edge displaced

metal. The degree of plasticity of the surface material is apparent.

Endo and Kotani^^ have recently reported similar topographic observations on steel surfaces wearing under lubricated sliding conditions.

3.6 Bearing Ball Spa! led Surface

This ball was obtained from a failed bearing supplied by the Naval

Ships Engineering Center and previously studied at Trans-Sonics

.

A spall occurred on the ball surface whose appearance (Fig. 27a)

; is typical of that type of damage. Considerable subsurface cracking

is seen in this example extending away from the exposed fracture

surface. The fracture surface morphology suggests that some plastic

behavior occurred, perhaps after the spall fragment was produced.

Many leaf-like protrusions remain on the surface, perhaps as a source of additional wear particles during subsequent contacts

and motion. Several spheroidal particles were located on this surface,

and a stereo-pair of SEM photographs of them is shown in Fig. 28.

These spheres are not solid, metallic particles, however, since

the X-ray emission peak intensity is considerably less than that

obtained from the background and from other particles on the surface.

They appear to be composed of iron and organic constituents.

4. Conclusions

Several different approaches to the study of metal wear particles using

microscopy and diffraction techniques have been evaluated and illustrated

in this report. The principal findings are as follows:

(1) The Ferrograph technique can reliably extract ferromagnetic wear

particles from a lubricating oil sample and deposit the particles on

a substrate suitable for microscopic examination.

(2) The particles are deposited so that smaller, lower magnetic

moment particles are located further from the entrance end of the

Ferrogram. The physical sizes of the particles may deviate by a factor of 5 within one particle string, due to local magnetic field

gradients.

(3) Nonmagnetic particles are found randomly deposited on the

Ferrogram, generally not within the strings of magnetic particles.

However, examples have been found of particles containing a small

proportional amount of iron located in such strings. The iron content

may be transferred wear metal in some cases.

(4) The glass substrate Fgrrogram can be examined in the SEM after

overcoating with about 200A of carbon. Carbon precoated glass slides

can be used in the Ferrograph and examined without overcoating. This

preserves any original particle surface features. Care must be

taken with such preparations or particles may be dislodged during

handling. Plastic film extraction type replicas can be taken from

the Ferrograms in order to examine the particles in transmission

microscopes

.

-12-
(5) The particle density within the strings is too large for certain

analyses. Particle sizing and shape classifying would not be possible.

X-ray emission analysis is complicated by excitation or absorption in

neighboring particles. Techniques for recovering particles from a Ferrogram of interest, after dilution and redeposition of the particles

on suitable substrates can probably be developed.

(6) X-ray emission analysis of individual particles by electron

beam excitation can be conducted. Iron particles as small as 2ym

in diameter can be studied. Many precautions are noted that must be

considered in such studies. Semi -quantitative analysis requires cor- rections for substrate and neighboring particle effects.

(7) Considerable differences are noted in the inteasity of the emissive

mode images of wear particles in the strings. These differences are due to particle composition, density, and thickness. The differences

can be used in some cases to identify the particles.

(8) Transmission electron diffraction studies have revealed differences in the amount of oxidation of iron wear particles, presumably

related to different histories of the particles. Carbide particles

have not been detected but may be present in the size range <0.1ym

that has not been carefully examined.

(9) Spheroidal particles have been observed in nearly all oil samples

examined, including both gear wear and bearing wear samples.

Typically from one to five spheroids may be found on one Ferrogram,

i.e., a very small proportion of all wear particles. The surfaces of

those particles are relatively smooth in nearly all cases. X-ray

analysis has proven that the small (<5ym) spheroids usually produce

only an iron emission line. Larger spheroids have been identified

as having significant concentrations of other elements and may be

oxides, silicates, etc. Some of the larger (>5ym) spheroids may

be partially composed of organic materials.

(10) Comparison of gear wear with bearing wear particle collections

indicates that significant size differences exist in the particle

distributions. Gear wear particles are generally smaller. Quantitative information on the particle size and shape distributions is

requi red.

(11) Examination of worn bearing surfaces indicates that displaced

material involved in the process of wear particle formation can be

detected. The SEM reveals a surface morphology that suggests con- siderable plastic deformation near the surface. Signal processing

conditions that produce a maximum contrast level are essential in

examining surface morphologies using the SEM.

5. Suggestions for Future Work

It is necessary to characterize the bulk surfaces from which wear particles

originate. The bulk phases present and their size and structure should

correlate with the particle characteristics observed. Shape and size

classification of the wear particles should be undertaken using automated

-13-
image analysis techniques. It does not appear possible to conduct sizing,

counting, and shape classifying operations on the particles in the present

Ferrograms due to problems of clustering and overlap. However, redispersion and dilution of the particles on a substrate should be possible.

Unusual particle morphologies must be examined carefully, including ^ spherical and cutting-wear particle shapes. Residual stress studies on

the particles can be conducted using techniques of electron channeling

and electron transmission.

Acknowl edgments

Much of the work reported here could not have been accomplished without

the cooperation of V. C. Westcott, R. W. Wright, and J. L. Middleton of

Trans-Sonics , Inc. and Dr. H. Dalai of SKF Industries, Inc. The advice

and support of Lt. R. Miller, ONR, and P. Senholzi , NAEC, is gratefully

acknowledged. The author also appreciates the assistance of several

colleagues at NBS, particularly the efforts of Dr. Anna Fraker during a phase of the TEM study.

-14-
References

1. W. W. Seifert and V. C. Westcott, "A Method for the Study of Wear

Particles in Lubricating Oil," Wear 22_, 27 (1972).

2. V. C. Westcott and W. W. Seifert, "Investigation of Iron Content

of Lubricating Oil Using a Ferrograph and an Emission Spectrometer,"

Wear 23, 239 (1973).

3. D. Scott and G. H. Mills, "A Scanning Electron Microscope Study of

Fracture Phenomena Associated with Rolling Contact Surface Fatigue

Failure," Wear 16, 234 (1970).

4. E. Broszeit and F. J. Hess, "Discussion to 'A Scanning Electron

Microscope Study of Fracture Phenomena Associated with Rolling

Contact Surface Fatigue Failure,'" Wear r7 , 314 (1971).

5. D. Scott and G. H. Mills, "Spherical Debris - Its Occurrence, Formation and Significance in Rolling Contact Fatigue," Wear 24, 235

(1973).

6. Nam P. Suh, "The Del ami nation Theory of Wear," Wear 25, 111 (1973).

7. J. F. Archard and W. Hirst, "The Wear of Metals Under Unlubricated

Conditions," Proc. Roy. Soc. A236 , 397 (1956).

8. R. A. Cummins, E. D. Doyle and B. Rebecchi , "Wear Damage to Spur

Gears," Wear 27, 115 (1974).

9. V. C. Venkatesh and R. Krishnamurty, "Intergranular Cracking During

Pitting of Gears," Wear 25, 329 (1973).

10. D. R. Beaman and J. A. Isasi, "Electron Beam Microanalysis ," ASTM

publication STP 506 (1972).

11. M. Bayard in Microprobe Analysis , ed. C. A. Andersen (J. Wiley and

Sons, N. Y., 1973) p. 323.

12. E. Takeuchi, "The Mechanism of Wear of Spheroidal Graphite Cast

Iron in Dry Sliding," Wear 19, 267 (1972).

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

and Abrasive Wear Resistance of Steels. A Review," Wear 26, 285

(1973).

14. K. Endo and S. Kotani , "Observations of Steel Surfaces Under Lubricated

Wear," Wear 26, 239 (1973).

-15-

Fig, 1(a). Ferrogram F-2273, oil sample from helicopter

gear transmission box. Oil flow is downward. Initial

particle deposit region is shown. M = lOOx.

(b). Region near A above. Several spheroidal particles

are seen (arrows). M = 500x.
Fig. 2(a). Spheroidal particle shown above. X-ray

spectrum indicated only iron in spheroid. M = 2,000x.

(b). Region B near initial deposit showing large faceted,

iron wear particle. M = 500x.
Fig. 3(a). F-2273 region just below initial deposit. Oil

flow is downward. Note characteristic transverse strings

of small wear particles. M = lOOx.

(b). Region C showing large flake underneath small wear particle strings. X-ray spectrum from upper portion of

flake is shown. M = 1 ,000x.
Fig. 4. Region further down from initial deposit in

F-2273. Note spheroid at arrow. Particle size in strings

varies from 5ym to well below lym. Many flake-like

particles are found. M = 1 ,000x.
Fig. 5. Ferrogram #2117 from SKF service tank oil, lubricating bearing tests. Carbon-undercoated Ferrogram, scanning

electron micrograph, M = 200X. Region shown is at entrance

end, oil flow downward. Some particles (metal?) are charging

in electron beam due to insufficient electrical contact with

substrate. Regions labeled are shown in several subsequent

photographs for greater detail on particles.
Fig. 6(a). Region A from above. Many flake-like particles

are seen in size range 1 to 10 ym. M = 1,000X.

(b). Region near B from above. Surface structures

on particles near B appear as slip or deformation lines.

Flake above B and flake at left contain only iron.

M = 2,000X.
Fig. 7(a). Region D from above. Several small particles

strings are shown together with some isolated particles.

M = 1 ,000X.

(b). One string from above area containing particles

showing different emissive image contrast as previously reported. M = 4,000X.
Fig. 8(a). Particle string from Region E in above. Many

flake-like particles are seen with resolvable surface

structures. M = 1 ,000X.

(b). Details in above particle string. Note surface features on large flake at left. Spheroidal particle (a) of 2.4 ym diameter is

shown. X-ray analysis indicates (a) spheroid contains only iron and

(b) flake has reduced iron and significant zinc content. M = 5,000X.
Fig. 9. Region F from above.

String contains several particles,

one of spheroidal shape (b) of

2.5 ym diameter. X-ray emission

spectra are shown for particles

(a) and (b) and background region

(c). Both particles are iron.

M = 2,000X.
Fig. 10fa)%Particle string from Region G in Fig, 5. (slightly

outside area shown). Spheroidal particle diameter is

4.5 ym. Large blob gives typical background X-ray spectrum.

M = 1 ,000X.

(b). Details of above string. Note contrast differences

between particles. X-ray emission spectra were obtained

from labeled particles. Spheroidal particle is iron.

Particle (c) contains oil residue. M = 2,000X.
10(c). X-ray spectra from particles (a), (b), (c), and

background (d) .
a

C_3 ^

Fig. 11. Particle string from F-2117 with X-ray spectra as indicated
Particle

Shape Size Ident.

Peak Heights

- 1 ,000 cts

Fe Other

a - spheroid 3 alloy steel 1

.

1 . 0. ICr

b

- 1 ump 3 i ron 1

.

1

c

- flake 3 iron oxide 0. 3 0. 7Si

d - 1 ump 3 iron oxide 0. 7 0. 5Si

e

- 1 ump 2 alloy steel

oxi de

0. 5 0. 2Cr

f - 1 ump 1.5 i ro n 0. 8

0.2Si

Fig. 12. Particle string above showing intensity variations

in Y-modulation. Analysis results are shown.
Fiq. 13(a).

F-1851. One support grid opening containing two particle

strings on carbon film in SEM. M = 600x.

(b). String S] on carbon replica in the

optical microscope. M = 1 ,000x.


O CD S E

+-> -r-s- X! CU E >

rO O

-a

I— <u

O) T3

•f— "t— M- X

o

-E O)

•r- O

S- z J3

I— OJ -a E -1-

•1- Xo

-a

CU E

Cn O

ro S-

-O • .— E OJ OO n3 1

O

O) E •.- E O -l-J •I- S_

S- -r- ro

+-> Q.

00 E

E E O

.- O Sly) +-> QJ U O) I— CD 3

O r- CT

.,- (13 +-> CD Q.

S_ S- O

D- "D M1— O

CD

. -r- CD to 4- C7> .— "O ^ CD

. S- cn TC! +->

• r- "O fO
Fig. 17. Electron diffraction pattern from an area of partially oxidized

iron particles in one string. F-1851.
Xo

CM

-a>

S-OJ

l/l O

CM

+->

LD

CO

XoooO i/l u cu O +-) 2: s_

2: CD LU C

00

OE

c: ro

O) CM O

00 £=CU ai sSZ <D

•I- 14- S- H-
J
Fig. 20. Ferrogram 2119 from single ball test run. Initial particle deposit is at C. Flow is downward.

M = lOOx.
Fig- 21 (a) .Region A from above shows string containing

flake-like particles and several examples of ribbon-like

particles. M = 500X.

(b). Details of ribbon-like particles in above string.

Note apparent deformation markings on ribbon (a). Both

ribbons indicated are alloy steel with principal elements

Fe(9,300 cts), Cr(3,200 cts) and Ni (1,000 cts). Ribbon

(b) also contains Cu. M = 2,000X.
Fig. 22(a). Region B from above,

strings are present. M = 500X.

Several small particle

jb). Details of ribbon-like particle in above string

(a), containing Fe, Cr, and Ni . Blob (b) contains only Fe,

about 40% that of (a) plus oil residue and is probably iron

oxide. M = 2,000X.
Fig. 23(a). Particle strings in Region C of above. Flake

and ribbon-like particles and one spheroid (arrow) are seen.

M = 300X.

(b). Details of above string and spheroidal particle

of 8 ym diameter. M = 2,000X.
Fig. 24. Area of above with X-ray

emission analyses shown for labeled

particles and background (a). Spheroidal

particle (b) has considerable Si, Al , and

Fe content. Particle (c) is principally

iron while particle (d) has substantial

Cr and Ni content indicating alloy steel

particle. Particle (e) contains Cu in

addition. M = 1 ,000X.
Fig. 25(a). Surface of bearing ball, 11/16 inch diameter,

52100 steel, after bearing bench test. Wear and gouge

tracks are seen in SEM photo. M = 3,000x.

(b). Details of metal deformed over gouge rim. Several

particles appear to be forming. M = 12,000x.
Fig. 26. Particle formed at edge of gouge on ball surface.

Note surface wear markings on particle. M = 7,000x.
Fig. 27(a). Surface spall on ball from failed bearing

M = 60x.

(b). Edge of above spall showing fracture surface

features and spheroidal particles on surface. M = 240x
E

s-0<+- -a s

o

X

O) > 0

Q.

to

II 0) 0

> 2: o

fO Mra >> O

<u >

U1 szOlid

fO OJ

S- Q.

CD CO O Ol CD

+-> O

j:: <4- CL J- s- 13 O (/) OJ Cl

S-O) 0

4-> fO O)

to Q. SCO <u 2: +-> LU c: CO 0

+-> CO MO) O) 00

C\J 0 1

+-> -c: CD S- CD

n3

Ll_ Q. S-

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U.S. 0£Pr. Or COMM. 1, I'UIU.K: ATIO.X OR RHI'ORT NO. 2, Gov't Accession

3ISL!0C.i?..\PHIC DATA

SHEi^T NBSIR 74-474

3. Recipient's Accession No.

4. TiTi.t: AND subtitle:

Metallurgical Analysis of Wear Particles and Wearing

-J U 1 1 U C J

5. Fiiblicatiun D:uc April Ly/H6. Performing Organization Code

7. AUTHORfS)

A. W. Ruff

8. Performing Organ. Report No.

9. PERF0RM1.\G ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 0 i eiu\ Uo

11. Contract/ Grant No. NA ONR-31-73

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

Deoartmpnt of thp Navv

Office of Naval Research

Arlington, Virginia 22217

13. Type of Report & Period Covered

April 73 - April 74

Final Report

14. Sponsoring Agtrncy Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary ol most si^ificant intormation. l( document includes a significant

bibliography or literature survey, mention it here.)

Results are presented from a program involved in characterizing the wear particles

and surface degradation produced by wear in bearing and gear tests in which the

effects of several variables on failure of the wear-ing surfaces has been examined.

The information obtained has been correlated with the results of allied studies

conducted by others in an attempt to develop an understanding of the processes

producing wear and degradation of metal surfaces in sliding, rubbing, rolling, and/or

rotating contact and the effects of lubricants, lubricant additives, bearing

materials etc. on these processes. The characterization of the wear particles

and wearing surfaces should aid in the establishment of the interrelationships

between wear particle shape, size, size distribution, chemical compositions,,

metallurgical structure, and surface damage prior to failure.

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

Bearings; Electron diffraction; Electron microscopy; Gears; Lubrication; Particles;

Wear

18. AVAILABILITY I^X Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

1 For Otficial Distribution. Do Not Release to NTIS

UNCLASSIFIED

30

j_ Order From Sup. of Doc, U.S. Government

'Xn.shington, D.C. 20.t02, SD Car. No. CI 3 Printing Office 20 SECURITY CLASS

(THIS PAGE)

22. Price

r" ' Or ior From National Technical Information

' Sprin-jfield, Vir.-ini.^, .?2 151 Service (NTIS)

UNCLASSIFIED