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Int. J. Mech. Eng. & Rob. Res. 2014 Aadarsh Mishra, 2014

ISSN 2278 – 0149 www.ijmerr.com

Vol. 3, No. 3, July, 2014

© 2014 IJMERR. All Rights Reserved

Research Paper

MICROSTRUCTURAL ANALYSIS OF WEAR DEBRIS

Aadarsh Mishra1*

*Corresponding Author: Aadarsh Mishra � aadarshm9@gmail.com

Wear generates debris. The debris comes in a wide variety of sizes and shapes. Wear debristurns motor oil black. The black in used oil is like a pigment, it is nanometer-size colloidal metalparticles (and carbon) suspended in the oil.Wear debris represents loss of geometric accuracyof moving contacting parts. It can also foul orificesand close spaced parts. Although the totalmaterial lost as wear debris in a machine is minute comparedto the volume and weight of themoving parts, it can signal failure of gears or bearings, and expensiverepairs or warrantypayments.Until the Industrial Revolution and the development of steam and internal combustionengines, wear was taken for granted. When wear of machinery and tools became a problem,the development of wearresistantmaterials and the study of wear itself increased rapidly. Nowwe have a large body of informationdeveloped from experience and scientific investigation thatcan be used in the design and maintenanceof more reliable and economical machinery.

Keywords: Wear debris, Spectrographic oil analysis

INTRODUCTION

Early Wear Theory as related by Bowdon andTabor(1964) and Holm (1946) indicated thatreal surface contact at high spots in thesurface micro topography (asperities) andwear was caused by shearing off thesecontacts. The theory was not clear, however,on how the wear particles were separatedfrom contacting asperities. Under abrasivewear conditions, it is presumed that hardparticles embedded in one surface will plowthrough a softer counter face and producemicro chips. The debris thus generated lookslike as shown in the given Figure 1.

1 Department of Mechanical Engineering, Manipal Institute of Technology, Manipal University, Eshwar Nagar, Manipal, Karnataka-576104.

In general, the higher the hardness of ametal the more susceptible to cutting wear itis. Contact load between sliding surface doesnot have to be large for cutting wear to takeplace. Wear debris can also be generatedby plastic deformation.Hard asperities,plowing over a softer surface without cuttingwill produce ridges. Ridges can then beflattened by further contact. Extrusions or lipsare formed. These lips are broken off andbecome flat wear flakes. Another method forgenerating wear debris includes materialtransfer from one surface to another. Anasperity plowing through a surface will cold

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weld to the mating surface and the weld willshear off, leaving some of theasperity stuckto the mating surface. Anin situscanning elec-tron microscope (SEM) wear experiment byGlaeser (1981) demonstrated this process.

Figure 1: SEM Micrograph of Cutting-type Debris from Abrasive Wear

In circulating oil systems, the SOAP (spec-trographic oil analysis program) process ofdebris monitoring has proved valuable incertain applications. Instead of extracting thedebris from the oil, the whole oil sample issubjected to spectrographic analysis.

The following potential failures can bedetected in this way:

• Broken piston rings, worn or scuffedcylinder walls, and worn valve guides

• Worn ball or roller bearings and/orretainers

• Worn journal bearings

• Worn spline couplings

• Worn cams and followers

• Scored pistons in hydraulic pumps

Wear debris can be collected by allowingit to fall on a glass slide. The debris can thenbe fixed to the slide and examined under a

microscope. In a circulating oil system, a filterin the line can be used to collect debris.Debris can be washed out of the filter withsolvent, or the filter element, if of an organicmaterial, can be dissolved by an appropriatesolvent and the remaining metallic debriscollected.It must be assumed that debriscollected from a wear process is as it waswhen separated from a surface if one is todeduce how it was generated. This often isnot the case. Wear debris can go through acontact zone many times and in the processbe altered in size and shape. This is true inroller bearings, reciprocating flat-on-flatsliders, splines, gear couplings, and wetclutches. Ductile metals can be mashed outinto extremely thin flakes - thin enough totransmit electrons. When these submicronparticles are examined in a transmittingelectron microscope (TEM), they appeartransparent.

Figure 2: TEM Micrograph of a SubmicronWear Particle Embedded in a Gel

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Figure 3: SEM Micrograph ofStainless Steel Extrusions

EXPERIMENTAL PROCEDURE

Most of the wear debris formed by the abovemechanisms is visible using light microscopy.Submicron debris can be found in SEManalysis of material filtered from used oil. Ina series of experiments designedto observethe generation of wear debris underlubricated conditions, a block-on-ring weartester was used by Glaeser (1983). The blockwas Cu-3.2 wt% Al alloy and the ring wascarburized steel. A drop of silicone oil wasapplied to the ring so that it clung to thebottom of the ring.The drop was illuminatedwith a light source and was observed througha microscope. As the ring rotated, oil fromthe drop circulated through the block contactand back to the drop, carrying any debrisgenerated into the drop. At the beginning ofrotation, the oil drop began to fill with metalflakes that sparkled as they tumbled in thedrop. Later in the test, the drop becamecloudy. At this point,the experiment wasterminated and the drop removed foranalysis. The large wear debris wasremovedby centrifuging. The still cloudy oilwas then filtered through a Nucleoporepolycarbonate filter. The filter was mounted

on a carbon grid for an electron microscopeand the filter dissolved with a solvent. Theremaining material was then examined in atransmission electron microscope and wasfound to consist of equiaxed nano particlesembedded in a gel.EDX analysis of theparticle showed it to be 80% Cu, 1.6% Fe,and 2% Al. The balance was silicon.Thesilicone oil had formed a gel with thesubmicron metal particles. The large bronzeflakes that appearedinitially were from thewear-in process as the roughness in thebronze surface was removed. Heavy surfaceflow produced smeared layers. As theselayers were subjected to increasing numbersof rubbing cycles,particles broke off and weretrapped in the surface. These particles werereduced to very small particles bycomminution, as happens in the ball millingprocess.

SEM analysis revealed particle reductionin progress on the aluminum bronze block .The submicron particles produced reactedwith the oxidizing lubricant to form a gel whichclouded the oil drop. This type of wear is oftenfound in boundary-lubricated or very thin filmelasto hydrodynamic-lubricated contactingsurfaces.

Wear debris can be collected by allowingit to fall on a glass slide. The debris can thenbe fixed to the slide and examined under amicroscope. In a circulating oil system, a filterin the line can be used to collectdebris. Debriscan be washed out of the filter with solvent,or the filter element, if of an organic material,can be dissolved by an appropriate solventand the remaining metallic debris collected.The fluid with suspended metal particles isallowed to flow over a glass slide inclined at

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a low angle. The glass slide is held to a strongpermanent magnet in such a way thatamagnetic gradient is developed along thelength of the slide. In this way, the particlesare separated by size.Ferrous particlesseparate in strings, with the largest particlesat one end.Centrifuging suspensions of weardebris in oil-solvent solutions will separatemetal particles accordingto mass. Theparticles will be distributed in layers. Oncethe centrifuging is complete, the fluid isdecantedfrom the vessel and the wearparticles removed by washing out withsolvent. The solvent volume can be reducedby evaporation, the wear particles suspendedby ultrasound and the remaining fluid pouredouton a glass slide. The wear particles willbe well dispersed on the slide when thesolvent evaporates. If onewishes to separatethe particles by mass, the layers can beremoved by pipette and deposited onseparate slides. Wear tests will often leavewear debris on one of the wear specimens.For instance in a fretting test inwhich a ball isoscillated against a flat, debris will appear atthe edges of the wear scar. The debris canbepicked up with a cellulose acetate filmdampened with solvent. The film can thenbe dissolved, leavingthe debris free forexamination.

RESULTS AND DISCUSSION

Wear debris comes in all sizes and shapes.The way in which the debris has been formedcan be deduced from size, shape, andsurface texture. For instance, cutting weardebris is unique in having a curly and stringyshape. The presence of cutting wear debrisin an oil sample signals a severe wearproblem needing immediate attention.

Figure 4: Spherical Wear DebrisFrom Grinding Swarf

Figure 5: Rubber Abrasion Pattern

Use of wear debris as a diagnostic tool forthe health of operating machinery is difficult.Recognition of significant morphologicalcharacteristics, color, and chemical make upof the variety of particles and elements thatturn up in a sample requires experience. TheSOAP program has proven effective for truck,tank, and railroad diesels, hydraulic systems,gear boxes, and compressors. The diagnosisis based on the history of concentration ofspecific elements found in oil samples. The

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particles involved are under 10 µm in sizeand more likely to be of submicron size. Theyare detected by spectrographic methodswhich give quantitative results, usually inppm. The trends for several metals arecompared with the original oil sample takenat hour one. By experience, these trends (forinstance, sudden continuing increase in ironcontent) can be related to pending wearfailures of certain components.

Particles from a given sample of oil sepa-rated by ferrograph or other methods can beclassified as to size and a size-frequencydistribution plot made. Assuming that thelarger particles are related to severe wear,significant increases in their contentcompared with the running-in plot shouldsignal shut down and maintenance.

Some of the more easily recognizedcharacteristics of wear particles include:

• Wear-in conditions. Flat, fairly smoothparticles about 10 ?m or smaller formedfrom broken off lips and extrusionsproduced by machining.

• Cutting. Curled strings and crumpledflakes resulting from abrasive wearprocess.

• Deformation. Flat flakes with parallelstriations from breakout of deformedsurface material caused by heavysliding contact.

It must be assumed that debris collectedfrom a wear process is as it was whenseparated from a surface if one is to deducehow it was generated. This often is not thecase. Wear debris can go through a contactzone many times and in the process bealtered in size and shape. This is true in roller

bearings, reciprocating flat-on-flat sliders,splines, gear couplings, and wet clutches.Ductile metals can be mashed out intoextremely thin flakes - thin enough to transmitelectrons. When these submicron particlesare examined in a transmitting electronmicroscope (TEM), they appear transparent.

Figure 6: TEM Micrograph of an IronParticle Subjected to Ball Milling toSimulate Wear Debris Subject to TepeatedPassage Through a Rolling Contract

Under heavy load, surface features can beproduced that are peculiar to the surfacemorphology. The surface of an Al-Si alloy pinwhich had been run against a hardened diskhad developed “inclined shear plates” on thesurface. A particle, extracted from the weardebris showed the same morphology. There-fore, it was recognized as a piece of the pinsurface broken out during heavy sliding con-tact. Ferrographs will also collect non-magnetic and nonmetallic particles. They willtend to precipitate out of the fluid as it movesover the glass slide. The same analysis canbe performed on wear particles separatedfrom oil by other means (filters, centrifuging).Organic materials can be detected by viewingthe slide with transmitted light. Polarized lightwill show up crystalline minerals.

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CONCLUSION

Wear debris comes in many sizes andshapes. Study of wear debris morphology andchemistry can provide clues to the conditionof various lubricated parts in a machine andthe wear mechanisms extent, allthis withouttearing an engine down. By monitoring theamounts of given metals in a circulating oilsystem as a function of running time, one candetect the onset of a mechanical part(bearing, gear, piston ring, or seal) failure.This program (SOAP) has been successfullyused for a number of years forconditionmonitoring of engines.Individualwear particles can provide clues to the wearmechanism that produced them. Forinstance,a sudden increase in bronzeparticles from an engine with bronze bearingsmay be the result of ingestionof abrasivecontaminants. Just monitoring the increasein copper in the oil would not indicate thechangein wear mode. Bronze particle sizeand shape would suggest this. Failureanalyses can be enhanced bytheexamination of wear particle size and shape.

Researchers can use wear particleanalysis as a tool to follow the wear processas it changes with changesin operatingconditions (bearing load, change inenvironment, increased sliding velocity).Classificationof wear debris according to size,thickness, shape, color, and chemistry andrelating debris types to wearmechanisms orfailure of mechanical components has beenthe goal of debris atlases. However, caremustbe exercised in using this information

since the character of wear debris is ofteninfluenced by factorsother than wear modeor parts failure. Diagnostics with wear debrisrequires experience in interpreting andrecognizing significant characteristics.

ACKNOWLEDGMENT

I would also like to dedicate this researchwork to my father Late R S Mishra and motherKL Mishra.

REFERENCES

1. Anderson D P (1982), “Wear ParticleAtlas”, NAEC Report 92-163, Lakehurst,NJ.

2. Bowen E R, Scott D, Seifert W W andWestcott V C (1964), “Ferrography”,Tribology International.

3. Bowden F P and Tabor D (1964), “TheFriction and Lubrication of Solids”, Part1 (1950) and Part II, Oxford Press,London.

4. Davies J E (1986), “Spherical Debris inEngine Oil”, Wear.

5. Glaeser W A (1981), “Wear Experimentsin the Scanning Electron Microscope”,Wear.

6. Glaeser W A (1991), “Ball Mill Simulationof Wear Debris Attrition”, Proceedings,18th Leeds-Lyon SymposiumonTribology, Tribology Series 21, Elsevier.

7. Holm R (1946), “Electric Contacts,Almquist and Wicksells”, Uppsala,Sweden.