NBSIR 75-640 ^^"^

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 1, 1975

Interim Report for Period October 1 through December 31, 1974

Contract 4-8049

Prepared for Department of the Navy

Naval Air Engineering Center

Philadelphia, Pa. 19112

NBSIR 75-640

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 1, 1975

Interim Report for Period October 1 through December 31, 1974

Contract 4-8049

Prepared for Department of the Navy

Naval Air Engineering Center

Philadelphia, Pa. 19112

1 .

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

I. OBJECTIVE

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

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

which the effect 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.

II. 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 projects.
III. RESULTS AND DISCUSSION

A. Wear Debris Analysis

1. Diesel Engine Oil Sample (F-3319)

An unusual wear debris specimen was examined in detail following

preparation and study by Trans-Sonics . The specimen was obtained

from a diesel engine and was found to contain relatively large, bright

yellow particles in addition to the usual iron wear particles. Other

mineral-appearing particles were also noticed in the Ferrograms that were prepared. Figure 1 shows an area from one Ferrogram containing

several large yellow colored particles as viewed in white, reflected

light. The Ferrogram was coated with a thin layer of deposited carbon in a vacuum evaporator as previously described and then examined

in the scanning electron microscope. Figure 2 shows the entry deposit

region (oil flow is downward). Figure 3 provides detail in the region

indicated in Fig. 2. Particle A is the original particle of interest.

X-ray microanalysis was conducted on particle A and several other

particles on the Ferrogram. The spectrum from particle A is shown in

Fig. 4 to consist of principally sulfur and chromium. The small sili- con contribution probably originates from the glass slide substrate.

Particle C (actually several small particles) is the usual iron wear

fragment found in particle strings on the Ferrogram as confirmed by

the spectrum shown in Fig. 5. Relative analyses of all particles

labeled in Fig. 3 are as follows:

Particle A: 3S + ICr

C: Fe + trace S

D: IS + 3Ca

F: IS + 2Ca + 3Fe

' G: IS + ICa

H: IS + IK + 3Ca

Sulfur was found as a principal constituent of most of the larger

particles analyzed. It probably was introduced into the system through

the use of sulfur-containing diesel fuel and oils. Spectrometric

analysis of this oil indicated the following elemental content in parts

per million: B = 1.2, Si = 88, Ag = 5.8, Fe = 8.1, Cr = 23, Cu = 13,
Pb = 9.7, Sn = 5.0, Al = 23, Zn = 42, Ca = 248. An unusually high

Ca content was found in several of the individual particles analyzed.

Figure 6 shows another area near the initial deposit point, also

marked in Fig. 2. The larger particles in this area also were found

to contain a significant amount of sulfur. In view of the location

of these sulfur-containing particles, at the end of or imbedded within

magnetic particle strings, it is clear that these sulfur-containing

particles contain a significant amount of iron or smaller iron wear

particles. It is not clear whether actual sulfur-iron compounds are involved here or whether a physical aggregation process occurs. In

view of the possible increased use of higher sulfur-containing fuels

and lubricants in our technology, the influence of sulfur in wear

debris analysis (if any) needs examination.

2. ONR Lubricating Oil Sample (F-3356)

This specimen was provided through Trans-Sonics after an inquiry into

what information might be deduced from a wear debris particle examination.

The actual mechanical system involved was not revealed. The results

are presented here as unusual and illustrative. Figure 7 is a reflected,

white light photomicrograph of one region of the Ferrogram after heating

to 330°C. That treatment caused a blue coloration of the iron and steel

debris and is one principal analytical step taken in the examination

of many Ferrograms from mixed-metal systems. Note that the particle

labeled A (and several others) have a distinctively different color.

The Ferrogram was carbon-coated by vacuum deposition and examined in

the SEM. Figure 8 shows the area near the entry deposit containing

particle A. Several unusually large flake particles are seen, as are numerous larger, equi-axed particles, all generally oriented with the

iron wear particle strings. X-ray microanalysis indicated that

particle A was composed of chromium, probably originating in a Cr-plated

portion of the mechanical system. Figure 9 shows the emission spectrum

for A. Particle gave a very similar, Cr-rich spectrum. Particle

and others of similar appearance were rich in calcium and gave evidence

of containing significant oil residue. The region of the particle

-3-
string to the right of particle C in Fig. 8 is shown in Fig. 10.

Several small flake particles are seen here along with two 4 pm

diameter spheroids. Analysis of the spheroids indicated only an iron content. Many other spheroids were observed in this Ferrogram.

Further down from the initial deposit, a nonmagnetic copper particle

was found and is shown as D in Fig. 11. Note that the smaller iron

particles still deposit in a laterally oriented string-like configuration, quite distinct from the larger particles nearby. That difference

suggests the nonferromagnetic composition as verified in the case

of particle £.

B. Ferroqraph Evaluation

Studies have continued into the behavior of the 5 urn diameter nickel

impregnated, porous silica spheres as ferromagnetic particles in oil.

Samples of these particles are passed through the Ferrograph in order

to evaluate the instrument characteristics. An experiment currently

in progress involves a mixture of nickel -coated and uncoated 5 ym

spheres. The uncoated spheres should pass through the Ferrograph

while the nickel-coated spheres are magnetically trapped on the substrate. Figure 12 shows the entire deposit region (F-3420) at low

magnification in an optical microscope. A gradual decrease in deposit

density in the direction of the oil flow is seen. The initial deposit

is very dense. The number of spheres in the strings appears to decrease

steadily toward the bottom of the Ferrogram. Measurements of the nickel

content of the spheres along the slide are in progress to see whether

those spheres that reach the bottom of the slide systematically have

lower Ni content (note that the sphere size is reasonably constant at

5 ± 1 nm). Ideally all spheres of the same size and Ni content should

deposit closely in the same region. Preliminary but close examination

of the strings indicates relatively few uncoated Si02 spheres. Figure 13

shows an area near the initial deposit and three transparent, uncoated

spheres are seen in strings (arrows). More commonly the nonferromagnetic,

uncoated spheres are found to deposit randomly on the slide as seen at

A. These studies should provide useful information on the behavior of

nonmetallic, abrasive particles in oil samples that subsequently undergo

Ferrographic analysis and also on nonmagnetic metallic particles. We

-4-

1.
plan next to examine the fluid that passes through the Ferrograph

to determine the. particles contained therein.

Another experiment has been conducted using only nickel-coated 5 \im

spheres. The mechanism of lateral growth of the particle strings as additional particles pass over them was of interest. Figure 14 shows

a scanning electron micrograph of the entry deposit region of that

Ferrogram (F-2897). Note again the gradual decrease in particle density

along the flow direction. Figure 15 indicates some detail adjacent

to the entry deposit. The usual process of deposit thickening appears

to involve the addition of a second row of particles on the substrate

adjacent to the first row. Another particle row can then deposit

on top of the two lower rows, etc. Lateral lengthening and thickening

appear to go on together. The unusual background contrast near the

strings in this example is due to residual oil left on the Ferrogram.

The carbon film deposit does not adhere in the regions of this oil and

particle charging in the electron beam can occur. This problem is

most serious in any Ferrogram examination since the debris particles

are not fixed to the substrate in such cases and can be dislodged and

lost. Figure 16 illustrates this effect in this sample. Locations

of several short strings of particles are shown to be empty (A) and

displaced, single spheres are seen at random adjacent locations.

A mixed sample of nickel-coated spheres of 5 ym and 0.7 ym diameter

is also being studied. The emphasis here concerns the growth and

characteristics of the particle strings when a bimodal size distribution of particles is present. An area near the entry deposit in

F-2902 is seen in Fig. 17. The particle strings here are all rather

short and unconnected due to a lower particle density and coverage.

Figure 18 shows some details near two strings. The small spheres

appear in clusters in and on the end of many strings; they also appear

in isolated locations, usually clustered in groups of five or more.

It is not expected that the 0.7 ym spheres should deposit consistently

in the same strings with the 5 ym spheres unless they are clustered

into larger effective sized units in the oil. Further experiments

will examine the tendency to cluster and methods for better dispersion.

-5-
Another string of mixed size spheres is shown in detail in Fig. 19.

Some small clusters of 0.7 pm spheres are seen at the ends and along

this string. Clearly small submicron wear debris would behave in

a similar manner relative to large particles.

C. Particle Microanalysis

Further studies are in progress on the reliability of X-ray microanalysis

methods applied to particle collections in the micron size range. The

particles labeled in Fig. 19 were examined under constant electron beam

exposure and geometrical conditions. The Ni-Ka X-ray line emission was measured in each case using the SEM X-ray detector system. The results

are given below.

Particle 1 2 3 4 5 6 7 8 9 A B C IR 9R

Time for

10"^ counts

108 87 79 70 77 104 76 60 58 350 112 360 99 53

(sec.)

Particles A, B, C are actually clusters of the smaller spheres. The

variation in Ni count rate for the 5 pm spheres is due partially to

statistical variations (about 6%) and partially to a real variation in

Ni content of the spheres. The observed average value is 80 ± 18 sec,

a variation of about 25%. The string was rotated 180 degrees in the

SEM and the end particles reanalyzed. The values (IR, 9R) are not

significantly different than the original values, indicating that electron

excitation from adjacent particles is not a significant factor. The

Ni emission from the cluster of small spheres B is reasonably close to

that from the larger spheres suggesting a similar Ni content. The

clusters A and C appear thinner and penetration of the electron beam

would be more significant.

A previous study of the Ni -coated 5 urn spheres, selected to be isolated

from other spheres on the substrate, revealed the variation in Ni content

shown in Fig, 20, also about 25%. Calculations and experiments on the

effect of penetration of small particles by the electron beam and subsequent X-ray anission have been reported last quarter. Figure 21 shows

the results for iron (nickel results are nearly identical). No correction

for X-ray emission is needed for the 5 pm spheres. However, a factor
of about 0.3 is indicated for correction at 0.7 ym diameter. Considering

particle clusters A and C in Fig. 19, the corrected time for 10 counts

would be (350)(0.3) - 100 sec, which is reasonably consistent with

the values for the 5 pm spheres.

Acknowledgments

The assistance of Paul Boyer in obtaining and reporting these results

is appreciated. The silica sphere samples were provided through the

kind cooperation of Drs. J. P. Wolf III and R. K. Ider, E. I. DuPont,

Company.

-7-
Figure Captions

Fig. 1. Optical reflected light photograph of diesel engine oil

Ferrogram-331 9.

Fig. 2. Scanning electron micrograph of entry region in F-3319.

Fig. 3. Details in F-3319, compare with Fig. 1.

Fig. 4. X-ray spectrum from particle A in Fig. 3.

Fig. 5. X-ray spectrum from particles C in Fig. 3.

Fig. 6. Details in another area of Fig. 2.

Fig. 7. Optical reflected light photograph of F-3355 after heating

to 330° C.

Fig. 8. Scanning electron micrograph of region in Fig. 7.

Fig. 9. X-ray spectrum from particle A in Fig. 8.

Fig. 10. Particle string containing tv/o iron spheroids adjacent to area of Fig. 8.

Fig. 11. Region of F-3356 downstream from entry deposit. Particle D^ is principally copper.

Fig. 12. Ferrogram-3420 deposit; optical micrograph of 5 ym Ni/Si02

spheres.

Fig. 13. Optical photograph of strings near entry deposit. Note transparent spheres marked.

Fig. 14. Entry deposit of F-2897, scanning electron micrograph,

collection of 5 \jm Ni/Si02 spheres.

Fig. 15. Details of particle strings near entry deposit.

Fig. 16. Instability of particle strings due to oil contamination.

Fig. 17. Scanning electron micrograph near entry deposit of F-2902,

containing 5 um and 0.7 pm Ni/SiO^ spheres.

Fig. 18. Details of particle strings in F-2902.

Fig. 19. String of both sized spheres in F-2902. X-ray data obtained from

marked spheres (see text).

Fig. 20. Number of 5 pm Ni/Si02 spheres giving rise to fixed Ni-K^^

counts vs. count time. Indicates uniformity of nickel content

in selected, isolated spheres.

Fig. 21. Experimental data and calculated curve showing effect of particle

size on X-ray emission intensity for iron (nickel nearly identical)

USCOMM-NBS-OC
Fiqure 1

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Figure 7

Figure 8
Figure 10
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Figure 14
Figure 15

Figure 16

Ki.qure 19
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'bulk 05

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FlQure 21
u s Dt PT. OF COMM. ]. J^J M Ll( ! A 1 ION OK RKI'OUT NO. 2. Gov't AcccsHion

tMULIUO K A rM lU UAIA No. SHEF.T NBSIR 75-640

3. Recipient *(i Accessiori No.

4. I 1 1 I I-; AND SUMIITIJ-:

Metallurgical Analysis of Wear Particles

and Wparina Surfaces

5. Publication Date

6. Performirig Or/^nni/uiion ("ode

/. A I) 1 IH'Kto I A. W. Ruff

8. Kerlorming Orf-nn. Report No.

9. PliRrOHMING ORC.ANI/.ATION NAMK AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WA^rllNwTOn, U.t. 20^J4

10. Proj. cr /Task/Work Unit No. 3120437

1 l.'(-<»iHracl/Grnnt No.

4^8049

12. Spuii-^orm^. (>/:;inizati<>ii Name nnd Complete Address (Stfet, City, State, ZIP)

Department of the Navy

'Java! Air Engineering Center

pm laaeipmat ra. i^iiz

13. Type of Report & l\ rioJ

( .<>ver<;d Interim Report

10/1/74 - lP/31/74

14. Sponsoring Af>ency Code

15. SirpPl.l-MI-NTARY NOTKS

16. AHS I IIAC T (A 200-word or fens factual auawnary of moat nidpUicanl tnformation. II document Includoa a algntiicant

biblinfirBphy or literature aurvey, motion II here.)

The objective of the project has been to characterize the wear particles and

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

of several variables on failure of the wear surfaces has been examined. The

information obtained has been correlated with the results of allied studies con- ducted by others in an attempt to develop an understanding of the processes

producing wear and degradation of metal surfaces on 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. ttOKDS (Nix to twelve enlrlna; olphalK'lical order; capltolizc only IhtJ lirnt letter ol the lirnl key word unions a propor

/xiiiir,- ni-iuirttled by itemicolonn) , Bearings; debris; electron microscopy; lubrication; metals; particle analysis; wear.

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