Tribology 101 – Introduction To The Basics Of Tribology
Tribology 101 – Introduction tothe Basics of TribologySJ Shaffer, Ph.D. – Bruker-TMTSteven.shaffer@bruker-nano.com
Outline Origin/Definition of “Tribology” (Term and Field) Encompassing Fields Fundamentals of Tribology: Surfaces in ContactFrictionLubricationWear Concluding Words Upcoming Topics in Series1/29/20132
What is Tribology ? Tribology comes from the Greek word, “tribos”,meaning “rubbing” or “to rub” And from the suffix, “ology” means “the study of” Therefore, Tribology is the study of rubbing,or “the study of things that rub”. This includes the fields of: Friction, Lubrication, and Wear.1/29/20133
“Tribology” is a new word Coined by Dr. H. Peter Jost in England in1966 “The Jost Report”, provided to the British Parliament –Ministry for Education and Science, indicated “Potentialsavings of over 515 million per year ( 800 million) forindustry by better application of tribological principles andpractices.”But Tribology is not a new field!1/29/20134
The First Recorded Tribologist – 2400 B.C.Transporting the statue of Ti – from a tomb at Saqqara, EgyptFigure taken from“History of Tribology”,by Duncan Dowson.1/29/20135
The First Recorded Tribologist – 2400 B.C.Transporting the statue of Ti – from a tomb at Saqqara, Egypt1/29/20136
The First Recorded Tribologist – 2400 B.C.1/29/2013The first recorded tribologist – pouring lubricant (water?)in front of the sledge in the transport of the statue of Ti.7
A more famous Tribologist – 500 years agoSled Friction TestGeometry4-Ball Test GeometryLeonardo Da VinciBall Bearing1/29/20138
A more famous Tribologist – 500 years agoSled Friction TestGeometryLeonardo Da Vinci4-Ball Test GeometryASTM D1894 – Staticand Kinetic COFs ofPlastic Film & SheetingBall Bearing1/29/2013ASTM D5183 - COFASTM D2266, D2596 – EPASTM D4172, D2783 - Wear9
A more famous Tribologist – 500 years agoSled Friction TestGeometryLeonardo Da Vinci4-Ball Test GeometryASTM D1894 – Staticand Kinetic COFs ofPlastic Film & SheetingTwo Observations:1. The areas in contact have no effect onfriction.2. If the load of an object is doubled, itsfriction will also be doubled.Ball Bearing1/29/2013ASTM D5183 - COFASTM D2266, D2596 – EPASTM D4172, D2783 - Wear10
Tribology 101 - BasicsApplications and Fields whichEncompass Modern Tribology1/29/201311
Tribology is All Around Us,In Applications from Simple to Complexand Scales from Small to Large Individual Components Assemblies or Products Manufacturing Processes Construction/Exploration Natural Phenomena1/29/201312
Individual ComponentsGearsBrake & Clutch Pads1/29/2013Bearings13
Assemblies or ProductsRock ClimbingShoesPocket WatchEnginesCurling Stones1/29/201314
Manufacturing olishing15
Construction/ExplorationMine Slurry PumpsExcavatorChunnel Digging Drill1/29/2013Oil Drilling RigSpace Shuttle16
Natural PhenomenaWearWater ErosionFrictionWind ErosionOn/Off Stiction:Gecko FeetSuperhydrophobicity:Lotus LeafPlate Tectonics1/29/201317
Tribology 101 - BasicsIn Parallel to these different Scales,There are Many Areas ofEngineering and Industry whichhave a Need to Use/Understand Tribology1/29/201318
Tribology is also in Virtually every Area ofEngineering and Industry Aerospace Agriculture Automotive Engine: Piston ring/cylinder,Bearings, valve seats, injectorsBrakes/clutchTooling/Machining/Sheet metalforming Coatings Providers Low FrictionWear Resistant Thin Films or Hardfacings Cosmetics/Personal Care Dental Implants Energy NuclearWindFossilSolar1/29/2013 Fabric/ClothingFlooringFood ProcessingHighway/TransportationDepts.Lubricant ManufacturersMedical DiagnosticsMedical ImplantsMilitaryPharmaceuticalShoe ManufacturersSports Equipment CompaniesUniversities/Educators Mechanical EngineeringMaterials Science EngineeringPhysicsChemistry19
Commonality in TribologyWhat do All These Diverse Fieldsand Applications have in Common?What do we need to think about asengineers and scientists when wedesign products or friction/wearexperiments?1/29/201320
Commonality Every Application has:Surfaces in Contact, andin Relative Motion(e.g. sliding, rolling, impacting)1/29/201321
Tribology Basics - Surfaces in ContactSo let’s begin by lookingclosely at a surface 1/29/201322
The Surface is not Simple LubricantAdsorbedContaminantsOxideSurface Properties“Disturbed Material”Bulk MaterialProperties –“Handbookvalues”1/29/201323
The Surface is not Simple Lubricantnms - µms mms - cms1/29/2013AdsorbedContaminantsOxideSurface Properties“Disturbed Material”Bulk MaterialProperties –“Handbookvalues”24
Nor is it Flat!LubricantAdsorbedContaminantsOxideSurface PropertiesDisturbed MaterialBulk MaterialPropertiesAll engineering surfaces have a roughness, and thisroughness plays an important role in tribology.1/29/201325
Nor is it Flat!LubricantAdsorbedContaminantsOxideSurface PropertiesDisturbed MaterialBulk MaterialPropertiesAll engineering surfaces have a roughness, and thisroughness plays an important role in tribology.Surface Roughness comes from all prior history of thepart: Manufacturing, handling and prior use in application.1/29/201326
We need to think about 2 Aspects of a Surface: Physical - Surface Roughness Dictates Contact Area Dictates Contact Stresses Lubricant Paths or Reservoirs Chemical - Intervening Layers Chemical Compatibility Shear Strength Lubricant Properties, e.g. Viscosity1/29/201327
We need to think about 2 Aspects of a Surface:Ground Physical - Surface Roughness Dictates Contact Area Dictates Contact Stresses Paths or Reservoirs forLubricants/debrisBead Blasted Chemical - Intervening Layers Chemical Compatibility Shear Strength Lubricant Properties, e.g. Viscosity1/29/201328
We need to think about 2 Aspects of a Surface: Physical - Surface Roughness Dictates Contact Area Dictates Contact Stresses Lubricant Paths or Reservoirs Chemical - Intervening Layers Chemical Compatibility Shear Strength Lubricant Properties, e.g. Viscosity,EP or boundary-forming1/29/201329
Surface CharacterizationVariety of Methods available, if needed Physical Characterization Roughness Macro – Waviness and Form (CMM) Micro – Surface Roughness– Stylus Profilometers (contact)– Optical Profilometers (non-contact)– AFM (sub-micron) Hardness Indent, Scratch Chemical Characterization Infrared, XPS, Raman, Auger Lubricant Shear properties Viscometry1/29/201330
Tribology 101-BasicsSummary of Surfaces in Contact Tribo-Forces are Dictated by Interaction ofAsperities Asperities have Mechanical and ChemicalProperties Methods Exist to Characterize theseProperties Asperity Geometry and Distribution resultfrom Manufacturing Method, Handling andPrior Rubbing History1/29/201331
FrictionFundamentals1/29/201332
Friction FundamentalsConceptual Definition of FrictionFriction is the resistance torelative motion between twobodies in contact.1/29/201333
Where does the resistance comefrom?When objects touch – there are forces between them.Microscopicforces ofmolecularAdhesion.(includes electrostatic,Van der Waals, metallicbonds)1/29/2013Microscopicforces ofmechanicalAbrasion.(includes elastic andplastic deformation)34
Where does friction come from?Remember, there are also “contaminants” at the interfaceOxides,Adsorbed films,Adsorbed gases,Foreign or“domestic” particles1/29/201335
Friction Fundamentals – “The COF” The Coefficient of Friction: A simpleconstant of proportionality.1/29/201336
Friction Fundamentals – “The COF” The Coefficient of Friction: A simpleconstant of proportionality. Or is it?1/29/201337
Friction FundamentalsMeasuring Friction:The Coefficient of FrictionVery Simple Relation:F µNNFµ F/N “COF”1/29/201338
Friction Fundamentals – “The COF” Suppose a colleague wants to know:“What is theCOF of steel?”1/29/201339
Friction Fundamentals – “The COF” A: “Well, dear colleague, you can use from0.1 to 0.6. Take your pick.“What is theCOF of steel?” Is that close enough for your needs?”1/29/201340
Friction Fundamentals – “The COF”Well not really.?1/29/201341
Friction Fundamentals – “The COF”Well not really.? “Then I guess we’ll need a bit moreinformation.”1/29/201342
Friction Fundamentals – “The COF”What we need to know “What steel? Stainless steel: 304, 316 , a 400-series or hardened 17-4PH or the like?Carbon steel: if so is it pearlitic or martensitic?Tool Steel? “Well I need to use it in water, so stainless steel, I guess.” “What is the function? “What is the mechanism?” “I’m designing a gear-driven mechanism, and I need to size the motor, assuming somefrictional loss in the gears, so I need the COF.” “Gears Then, it needs to be hardened. How about the driven gear, what’s its material?” “The same, I suppose.” “I’m not sure that’s a good idea, depending on the contact stress, sliding velocity andsurface finish. Do you know these parameters yet?” “Not yet, I’ll probably use standard values from my gear design handbook.” “OK, I gather you need low friction, how about lubricant or use of a lubricious coating, arethese permitted in the design?” ”A coating is OK, but I don’t think a liquid lubricant is permitted in this application.” “OK, a coating then. How long will it need to last?” “For the life of the mechanism. Can’t you just tell me the COF?” Really, I need more information, because I’ll likely need to run a test, depending on howprecisely you need the COF.” 1/29/201343
All things considered,The COF is Somewhat Complicated Surface roughness plays a role Lubricant plays a role Surface chemistry plays a role Contact Stress plays a role Contact geometry plays a role Environment plays a role Temperature plays a role Sliding speed plays a role 1/29/201344
All things consideredIt’s not so bad after allFortunately, while it appears complicated,friction is relatively easy to measure,(Only two things: Normal Load and Friction Force)But, we have to measure it under the rightconditions.1/29/201345
Summary of Friction FundamentalsThe equation is simple, but measuring it correctlyrequires care:When assessing a system’s tribology need, we must consider:Materials, Coating, LubricantContact Area, Geometry, StressSurface RoughnessesSliding SpeedSliding Mode (unidirectional, reciprocating, multidirectional)Duty Cycle (continuous contact, intermittent contact)EnvironmentTemperature, Humidity,Atmosphere (air, exhaust gases, vacuum)Friction is NOT a Material PropertyFriction is a “System” PropertyNo such thing as the COF of “steel”, or the COF of “rubber”1/29/201346
LubricationFundamentals1/29/201347
Lubrication Fundamentals The role of a lubricant is to: Reduce Friction Prevent / Minimize Wear Transport Debris away from Interface Provide Cooling1/29/201348
Lubrication Fundamentals:Lubrication Regimes, with liquid present In Liquid Lubrication, Regimes can be basedon: Fluid Film Thickness The Lambda Ratio is defined as the ratio of thefluid film thickness to the composite surfaceroughness* λ 3 full film (thick film) lubrication,hydrodynamics 1.2 λ 3 mixed or thin film lubrication λ 1.2 boundary lubrication* - composite surface roughness (rq12 rq22)1/21/29/201349
Lubrication Regimes:The Stribeck CurveThin Film,MixedBoundaryJournal BearingThick FilmSpeed*ViscosityLoad1/29/201350
Lubrication Regimes:Boundary Lubrication – Solid Lubricants Solid Lubricants Compounds with Low Shear Stress MoS2, Graphite, WS2, HBN Behave like a “deck of cards” Bonded Films DLC Resin-bonded PTFE Impregnated porous anodizing1/29/201351
Summary of LubricationFundamentals: Key Factors in Lubricant Effectiveness Fluid Shear Properties Viscosity, Viscosity Index Pressure-Viscosity Index Chemistry Reactivity with the SurfaceBoundary Film-Forming PropertiesExtreme Pressure ConstituentsShear strength of solid lubricant or coating Thermal Conductivity/Heat Capacity1/29/201352
WearFundamentals1/29/201353
Wear FundamentalsConceptual Definition of WearRemoval (or displacement) of materialfrom one body when subjected to contactand relative motion with another body.1/29/201354
Wear Fundamentals - Wear Modes6 Primary Wear Modes:1. Abrasive Wear, Scratching2. Adhesive Wear, Galling, Scuffing3. Fretting/Fretting Corrosion4. Erosive Wear, Cavitation, Impact, Electro-arcing5. Rolling Contact Fatigue, Spalling, Delamination6. Tribo-Corrosion1/29/201355
Wear Fundamentals Abrasive Wear, Scratching“The harder materialscratches the softermaterial.”1/29/201356
Wear Fundamentals Adhesive Wear, Galling, ScuffingBegins as “local welding”10 mmMaterial “compatibility” isimportant for adhesivewear.Galling of Stainless Steel SamplesStacking fault energy,crystal structure, naturaloxide formation allinfluence adhesive wear.1/29/201357
Wear Fundamentals Fretting/Fretting CorrosionSmallamplitudedisplacement( 50 µm). Experiments generally have zones of no-slip,and slip. Small adhesive pull-outs occur at the boundary. Often these oxidize, so sometimes called“fretting corrosion”.1/29/201358
Wear Fundamentals Erosive Wear, Cavitation, Impact, Electro-arcingDependency onparticle size, shape,composition, angle ofimpingement, as wellas ductility of “target”Particle Classification1 cm“Fluting” Damage1/29/2013Steam Control ValveCavitation Damage59
Wear Fundamentals Rolling Contact Fatigue, Spalling, Delamination Reversing sub-surface shear eachtime the roller or ball passes overthe surface.Propagation to surface ofsub-surface-initiated cracks Accumulation of these stressesleads to subsurface crackformation, usually at amicrostructural inhomogeneity. Cracks grow toward surface andparticle spalls off.Spalled Bearing Inner Race1/29/2013 Debris typically gets rolled over,creating additional damage.60
Wear Fundamentals Tribo-Corrosion Wear in the presence of corrosioncan have synergistic effect. Can happen with erosion orsliding wear. Bio-tribo-corrosion is major areaErosion-Corrosion Down-hole drilling environment isanother ASTM Method G119 – StandardGuide for Determining Synergismbetween Wear and Corrosion1/29/201361
Wear Assessment The Wear Coefficient, k k volume of material removed per unit load and slidingdistance Units of k are: mm3/N mPlease do NOT reduce the units of k to mm2/N or 1/kPaThis has no physical meaning k can be used to predict component lifetimes, providing thetribosystem does not change wear modes Duty cycle and directionality can influence wear Start-stop can be much more damaging than continuousmotion Unidirectional sliding is very different from reciprocatingsliding1/29/201362
Summary of Wear Fundamentals Like Friction, Wear is a System Property, NOT aMaterials Property There are several distinct wear regimes, thoughsome can operate simultaneously, or sequentially Observed abrasive wear can results from initialadhesive wear If you properly simulated the system and wearmode, the wear coefficient, k, can be used topredict lifetimes1/29/201363
SomeFinal Words forToday’s Webinar1/29/201364
Tribology FundamentalsKey Concepts1. COF is not a material property, it is a systemproperty.2. Wear Rate or wear resistance depends on the wearmode, which is a function of the Tribosystem.3. If we properly characterize and understand theTribosytem, the odds are better that we willsucceed, because we can make the right choice formaterials, contact geometry and chemistry, andmake the appropriate measurements to give usthe answer we seek for our design.1/29/201365
Means to AssessTribo-systems
Tribology & Mechanical Testing (TMT) Universal platform for Tribology studies: Wear, Friction,. when 2surfaces meet. Large load range Wide variety of environments (corrosion, HT, liquid) Wide variety of configurations (rotating & translating motions)
Many different Tribology testsLinear StageReciprocating DriveBlock-on-Ring DriveRotary Drive
Indentation & Scratch Testing Indentation & Scratch TesterScratch test example Large load range: nano & micro Wide variety of imaging options (AFM, profiler, optical)Indentation example
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Tribology 101 – Introduction to the Basics of Tribology SJ Shaffer, Ph.D. – Bruker-TMT . Steven.shaffer@bruker-nano.com
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