NBSIR 74-437

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

January 1974

Quarterly Report

October-December 1973

Prepared for Department of the Navy

Naval Air Engineering Center

Office of Naval Research

Philadelphia. Pa. 19112

NBSIR 74-437

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

January 1974

Quarterly Report

October-December 1973

Prepared for Department of the Navy

Naval Air Engineering Center

Office of Naval Research

Philadelphia, Pa. 19112

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

NATIONAL BUREAU OF STANDARDS

Date: January 24, 1974

Memorandum to Author: A. W. Ruff

(Thru Division Chief)

The Washington Editorial Review Board has approved and/or noted

the attached manuscript for issuance as an NBS Interagency /Internal

Report. (See the January 16, 1973 revision of Section 5 of the NBS

Manual for Scientific and Technical Communications .)

Any required security or sponsoring agency release should be obtained

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Washington Editorial Review Board

Author (s) : A. W. Ruff

Title of manuscript: Metallurgical Analysis of Wear Particles and Wearing

Surfaces

NBSIR number: 74-437 - ONR

Metallurgical Analysis of Wear Particles and Wearing Surfaces

A. W. Ruff

Metallurgy Division, National Bureau of Standards

Report: Oil Analysis Program/Contract NAonr-31-73

Sponsor: Naval Air Engineering Center/Office of Naval Research

Period: October 1 to December 31, 1973

Objective

The objective of the program is to characterize the v/ear particles

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

(conducted by others) in which the effects of several variables on failure

of the wearing surfaces have been examined. The information obtained will

be 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

surfaces will aid in the establishment of the interrelationships between wear

particle shape, size, size distribution, chemical compositions, metallurgical

structure, and surface damage prior to surface failure.

RESULTS

Introduction

Previously obtained 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. The wear particles are obtained from

the lubricating fluid by the Ferrograph instrument (for a description, see

the previous quarterly meeting report of October 19, 1973). The resulting

-1-
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. Nonmagnetic particles are generally not

collected as the oil traverses the Ferrograph. Thus the instrument serves

to select wear particles (from ferrous systems) and to a large extent

ignores other particulates . 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 particles important in

the wear process, for example nonmagnetic abrasives, may not be collected

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 size gradient along the Ferrogram, larger particles depositing first.

When a variety of material types are present in the particulate collection,

the sizing process is disturbed since large particles of a material with

low specific magnetic moment will deposit further down the Ferrogram.

Specific magnetic moment/unit volume values for three relevant materials

are: Fe (171 m Tesla ), Fe^O^ (48 mT), Fe3C (99 mT). Thus physical sizing

may scatter by factors of 4 or more in a collection of particulates of

these materials. As we shall see, examination of particles in strings on

the Ferrograms shows a considerable size variation.

A new technique was developed to assist in scanning electron microscope

(SEM) evaluation of the Ferrograms. Previously we had taken conventional

Ferrogram (glass) slides and over-coated them with vacuum deposited carbon

in order to obtain sufficient electrical conductivity. Carbon is an

excellent coating material in that no interference is presented for subsequent

X-ray emission analysis of the metallic particulates. An alternative technique

-2-
was developed that involved stripping the particles from the Ferrogram

using a plastic film and subsequently carbon coating that film. The film

could then be dissolved in a solvent leaving the particle strings intact

on a thin carbon substrate. That method is necessary for transmission

electron microscope and diffraction studies.

During this period we have examined several Ferrograms made using

glass slides previously coated with thin vacuum deposited carbon layers.

The oil sample is passed through the Ferrograph with the precoated slide in

place. The particles are deposited on the slide in the usual manner. The

carbon under-coating does affect the base-line optical density readings of

the Ferrogram and in some cases contributed to oil overflow of the barrier

strips along the sides of the Ferrogram slide. However, these problems do

not appear serious. After usual washing procedures, the Ferrogram was

heated for 30 seconds on a laboratory hot plate to remove residual oils and

then examined directly in the SEM. While some particle charging was seen

in the SEM, it was found that sufficient electrical conductivity was present

to permit detailed examination of the particles. No overcoating procedure

was applied to these Ferrograms, and hence all observed surface features

and details on the particles are original. This new technique should be

of value whenever future SEM examination of a Ferrogram is anticipated.

Overcoating with another film (carbon or metal) is still possible should

it be necessary for some reason. All the results presented in this report

were obtained from uncoated Ferrograms, prepared using carbon-undercoated

glass slides.

Two particular oil samples have been examined. One oil sample was

obtained through Dr. Dalai at SKF and originated from a service tank system

that had lubricated rolling bearing tests. The Ferrogram (F-2117) was

prepared at Transonics using carbon precoated slides furnished by us. The

-3-
same oil had been used previously to prepare F-1851 that was examined and

reported on during the proceeding quarter-year period. Those results should

be directly comparable with those reported here. The second oil sample

was obtained through Transonics from Pratt and Whitney Corporation. The

oil was MIL-L-23699 and had lubricated a single ball tester. The test

involved a time-to-failure of 9h 55m, an oil temperature of 300°F, and a

ball Hertz stress of 500 kpsi maximum. The bearing materials were not

specified. The Ferrogram (F-2119) was prepared using carbon precoated

slides from an oil sample taken at 8h time (Ih 55m prior to failure).

Ferrogram 2117 Examination

Optical examination of F-2117 at Transonics indicated the presence

of cutting wear particles, spherical particles and spalls. The entrance

end of the Ferrogram is shown in Fig. 1 at a relatively low magnification

(200x) 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 previous study of F-1851 from

this same oil concentrated on smaller particle strings and those results

probably are applicable to the remainder of F-2117 slide. Figure 1 shows

that some larger particles do not have sufficient conductivity to the carbonundercoated 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. 1 are examined

at greater resolution in the following photographs. All the SEM work was

conducted at 20kV beam potential, beam currents in the range 50 to 200pA,

emissive mode imaging, and with the specimen tilted about 30 degrees

toward the detector (located in the upward direction in each photograph).

Under these conditions the SEM can resolve details down to 50CA°. Thus

-4-
we should obtain fine detail on particles lym (10,000A ) in size and

above. Smaller particles than these can still be detected and some

features determined.

The region A in Fig. 1 is shown at higher magnification in Fig. 2.

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 structures 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

2

these strings. Cursory inspection of Fig. 2a indicates about 10 particles

in the field, ignoring particles below 0.5ym size. Examination of other

subfields of Fig. 1 indicates that to within factors of 3, the total particle

3

count in Fig. 1 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. 2. It involves the tendency for particles to clump and overlay

one another in the strings. This prevents accurate particle counts and may

bias other observations, in particular X-ray emission analyses.

Another region from Fig. 1 is shown in Fig. 3. A spheroidal particle

of diameter 1.7ym is found in one string. One also observes substantial

differences in contrast between the particles shown in Fig. 3b, as was noted

-5-
in the previous project report. The particles showing lower secondary

electron emission (grey images), at the right side of the string and just

below "C," are probably more heavily oxidized in accordance with the

previous results. Both 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. '

:

Another region (D) is shown in Fig. 4a and 4b. Flake-like particles

are observed in those strings together with several rod-like particles.

Note the electron emission differences between the particles in Fig. 4b.

The region E from Fig. 1 is shown in greater detail in Fig. 5. One flakelike particle in this string has considerable surface structure that includes

several cracks and smaller, attached particles. A spheroidal particle

(2.4ijm diameter) is also found. Its size is sufficiently large to permit

meaningful X-ray emission analysis in that it would contain nearly all the

X-ray emitting volume. Fluorescence effects associated with neighboring

particles in such a collection could be substantial and require correction.

Analysis of this spheroid showed, however, only an iron peak in the X-ray

emission spectra.

An example of the results of an X-ray emission analysis is shown in

Fig. 6 taken from region F in Fig. 1. Two particle analyses are shown,

one from another spheroid about Z.Bym in diameter and the other from an

irregularly shaped 3ym particle. The background analysis was obtained

from area c. A stationary electron probe was used for analysis, an electron

beam potential of ZOkV, beam current of 200pA, and a counting time of about

100 seconds. Both particles are iron with no other indicated elements

present. The background contains elemental emissions from oil residue

material on the substrate, as reported previously.

-6-
The final region (G) presented in detail from Fig. 1 is shown in

Fig. 7. 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. 7C. 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.

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

of particle sizes and types. 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 4.5ym) were analyzed by X-ray emission and found to contain only

iron. Contrast differences were found among the particles imaged by secondary

electron emission as previously reported for F-1851 from this same oil. They

are believed associated in some cases with the iron/iron oxide ratio of

the particles. Questions concerning the origin of the spheroids, flake-like

particles, and other particles may be resolved by associated examination

of the worn surfaces and further studies under more carefully controlled

wearing conditions. It does not appear possible to conduct sizing, counting,

and shape classifying operations on the particles in the Ferrogram due to

problems of clustering and overlap. However, redispersion of the particles

on the substrate should be possible and could produce a much more "dilute"

array for such analysis.

-7-
Ferrogram 2119 Analysis

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

in Fig. 8 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 were not examined closely. The particles are in place on the precoatedcarbon Ferrogram slide and have not been overcoated.

Region A in Fig. 8 is shown again in Fig. 9. Several examples of

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

plastic deformation and slip bands. Figure 10 shows the region B in more

detail. One 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. 9a with 10a and Figs. 9b with 10b shows the particle

sizing capability of the Ferrograph instrument in this example. The larger

particles predominate in region A, however, many smaller particles (1 to

4ijm) 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 (A) studied in Fig. 8 is shown in detail in Fig. 11.

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

one large, Sym diameter spheroid. The spheroid is remarkably smooth and

free of surface structures as compared to adjacent particles. X-ray emission

-8-
spectra from four of the particles are shown in Fig. 12. Particle c

produces the usual lone iron peak, however, the spheroid produces a weak

iron peak together with a strong silicon peak. None of the other particles

analyzed showed a prominent Si peak. The flake-like particle d contains

significant amounts of Cr and Ni , suggesting an alloy steel. The globular

particle e also contains Cr and Ni together with a significant amount of

copper.

X-ray signal imaging studies were conducted on this string using the

prominent Fe and Si lines. Fig. 13a shows the area of Fig. 12 using

the Fe signal. The particles are predominantly iron while the silicon

is located principally in the background. Certain particles in this field

show a reduced iron content. Fig. 14a is a higher magnification view of

the particles near the spheroid. The Fe K and Si K scans in Figs. 14b a a and 14c, respectively, prove the low iron-high silicon content of this

particular spheroid.

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

F-2119. It is over twice as large as those found in the F-2n7 examination

and it contains a significant amount of silicon. This may be due to a

thicker oxide covering on the spheroid in this case, leading to more substantial oil absorption and oil residue content. However, there is no

appearance of a rough or cracked oxide covering on the sphere, and adjacent

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

significant silicon image (Fig. 14c). Analysis through the volume of the

spheroid may be required to resolve this matter. The ribbon-like particles

found in this Ferrogram were not observed to the same extent in F-2117,

nor were any alloy steel particles seen there. Correlation of these

particle analysis results with the bulk material surfaces and structures

is required.

-9-
Future Plans

It is of interest to examine the bulk materials from which these

particles originated. The phases present and their size and structures,

may correlate in a useful way with the particle observations made here.

Further effort is intended toward shape classification of the string

particles, and toward an increased understanding of the particle history

as evidenced by their present mi crostructure. Unusual particle morphologies

will be examined, such as spherical and cutting-wear particle shapes.

-10-
j

j N8S-1UA IREV. 7-73)

U.S. DEPT. OF CC.VV. BIBLIOGRAPHIC DATA

SHEET

1. PUHLIC.ATION' OR REPORT NO.

NBSIR 74-437

2. Gov't Accession

No.

3. Recipient's Accession .\'o.

4. TITLE .-WD SLH I ITLE

Metallurgical Analysis of Wear Particles and

Wearing Surfaces

5. Publication Dace January 1974

6. Performing Organization Code

7. AUTHOR: ^> A. W. Ruff

8. Performing Orcan. Report No. 1 NBSIR 74-437

9. PHRFOR.MLNG ORGA.NIZATION \A.ME A.\'D ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect- Task. V.ork Unit No. 1 3120108

11. Contract, Grant No. NAonr-31-73

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

Naval Air Engineering Center

Office of Naval Research

Philadelphia, Pennsylvania 19112

13. Type of Report Oi: Period

Covered

Quarterly Report

Oct. -Dec. 1973

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^iticant information. If document includes a significant

bibliography or literature survey, mention it here.)

The objective of the program is to characterize the wear parotides and surface

degradation produced by wear in bearing and gear life tests (conducted by others)

in which the effects of several variables on failure of the wearing surfaces have

been examined. The information obtained will be 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 surfaces will aid in the establishment of the interrelationships

between wear parricle shape, size, size distribution, chemical compositions, , metallurgical structure^ and surface damage prior to surface failure. j

17. KEY UORDS (s,x to twelve entries; alphabetical order; capital i ze only the first letter of the first key word unless a proper name; separated by semicolons)

Lubrication; Metals; Particles; Surfaces; Wear

«

18. AVAILABILITY Unlimited

XX For Official Distribution. Do Not Release to NTIS

19.

> >

SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGf

10

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