Tribology Intro and Basics

8/10/2019 Tribology Intro and Basics

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Tribology 101 – Introduction tothe Basics of Tribology

SJ Shaffer, Ph.D. – [email protected]



Outline

• Origin/Definition of “Tribology” (Term and Field)

• Encompassing Fields• Fundamentals of Tribology:

• Surfaces in Contact• Friction• Lubrication• Wear

• Concluding Words• Upcoming Topics in Series

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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 .

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“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!

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5

The First Recorded Tribologist – 2400 B.C.

Transporting the statue of Ti – from a tomb at Saqqara, Egypt

Figure taken from“History of Tribology”,by Duncan Dowson.

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Transporting the statue of Ti – from a tomb at Saqqara, Egypt

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The first recorded tribologist – pouring lubricant (water?)in front of the sledge in the transport of the statue of Ti.

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A more famous Tribologist – 500 years ago

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Leonardo Da Vinci

Ball Bearing

4-Ball Test GeometrySled Friction TestGeometry




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Leonardo Da Vinci

Ball Bearing


ASTM D5183 - COF ASTM D2266, D2596 – EP ASTM D4172, D2783 - Wear

ASTM D1894 – Stat icand Kinetic COFs of

Plastic Film & Sheeting




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Leonardo Da Vinci

Ball Bearing


Two Observations:1. The areas in contact have no effect on

friction.2. If the load of an object is doubled, its

friction will also be doubled. ASTM D5183 - COF ASTM D2266, D2596 – EP ASTM D4172, D2783 - Wear

ASTM D1894 – Stat icand Kinetic COFs of

Plastic Film & Sheeting



Tribology 101 - Basics

Applications and Fields which

Encompass Modern Tribology

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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 Phenomena

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Individual Components

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Gears

BearingsBrake & Clutch Pads



Assemblies or Products

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Engines

Curling Stones

Rock ClimbingShoes

Pocket Watch



Manufacturing Processes

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Rolling

Stamping

Turning

Grinding/Polishing



Construction/Exploration

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Mine Slurry Pumps

Excavator

Chunnel Digging Drill

Oil Drilling Rig

Space Shuttle



Natural Phenomena

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Plate Tectonics

Wind ErosionWater Erosion

Wear Friction

On/Off Stiction:Gecko Feet

Super-hydrophobicity:

Lotus Leaf



Tribology 101 - Basics

In Parallel to these different Scales,

There are Many Areas of

Engineering and Industry which

have a Need to Use/Understand Tribology

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Tribology is also in Virtually every Area ofEngineering and Industry

• Aerospace• Agriculture• Automotive

• Engine: Piston ring/cylinder,Bearings, valve seats, injectors

• Brakes/clutch

• Tooling/Machining/Sheet metalforming

• Coatings Providers• Low Friction• Wear Resistant

• Thin Films or Hardfacings

• Cosmetics/Personal Care• Dental Implants• Energy

• Nuclear• Wind• Fossil

• Solar

• Fabric/Clothing• Flooring• Food Processing• Highway/Transportation

Depts.

• Lubricant Manufacturers• Medical Diagnostics• Medical Implants• Military• Pharmaceutical

• Shoe Manufacturers• Sports Equipment Companies• Universities/Educators

• Mechanical Engineering• Materials Science Engineering• Physics

• Chemistry1/29/2013 19



Commonality in Tribology

What 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?

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Commonality…

Every Application has:

Surfaces in Contact , andin Relative Motion

(e.g. sliding, rolling, impacting)

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Tribology Basics - Surfaces in Contact

So let’s begin by looking

closely at a surface…

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23

The Surface is not Simple…

Bulk Material

Properties –“Handbookvalues”

Surface Properties“Disturbed Material”

Oxide

AdsorbedContaminants

Lubricant

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The Surface is not Simple…

Bulk Material

Properties –“Handbookvalues”

Surface Properties“Disturbed Material”

Oxide


Lubricant

≈

mm s - cm s

nm s - µm s

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Nor is it Flat!

Bulk MaterialProperties

Surface PropertiesDisturbed Material

Oxide


Lubricant

All engineering surfaces have a roughness, and thisroughness plays an important role in tribology .

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Nor is it Flat!

Bulk MaterialProperties

Surface PropertiesDisturbed Material

Oxide


Lubricant

All 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.

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We need to think about…

• 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

2 Aspects of a Surface:

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• Paths or Reservoirs forLubricants/debris

• Chemical - Intervening Layers• Chemical Compatibility

• Shear Strength

• Lubricant Properties, e.g. Viscosity


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Ground

Bead Blasted






• Lubricant Paths or Reservoirs

• Chemical - Intervening Layers

• Chemical Compatibility• Shear Strength

• Lubricant Properties, e.g. Viscosity,EP or boundary-forming


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Surface Characterization

Variety 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 → Viscometry

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Tribology 101-Basics

Summary of Surfaces in Contact

• Tribo-Forces are Dictated by Interaction ofAsperities• Asperities have Mechanical and Chemical

Properties• Methods Exist to Characterize these

Properties

• Asperity Geometry and Distribution resultfrom Manufacturing Method, Handling andPrior Rubbing History

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FrictionFundamentals



33

Friction Fundamentals

Conceptual Definition of Friction

Friction is the resistance torelative motion between two

bodies in contact.

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Where does the resistance comefrom?

Microscopicforces of

molecular Adhesion .

Microscopicforces of

mechanical Abrasion .

When objects touch – there are forces between them.

(includes electrostatic,Van der Waals, metallic

bonds)

(includes elastic andplastic deformation)

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Where does friction come from?

Remember, there are also “contaminants” at the interface

Oxides,

Adsorbed films,

Adsorbed gases,

Foreign or“domestic” particles

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Friction Fundamentals – “The COF”

• The Coefficient of Friction: A simple

constant of proportionality.

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• The Coefficient of Friction: A simple

constant of proportionality.

• Or is it?

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38

Friction FundamentalsMeasuring Friction:

The Coefficient of Friction

Very Simple Relation:

F= µNN

F

µ = F/N = “COF”

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• Suppose a colleague wants to know:

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“What is the

COF of steel?”




• A: “Well, dear colleague, you can use from0.1 to 0.6. Take your pick.

• Is that close enough for your needs?”

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“What is the

COF of steel?”




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Well not really.

?




• “Then I guess we’ll need a bit moreinformation.”

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Well not really.

?




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 how

precisely you need the COF.”…431/29/2013



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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• …

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ll h d d



45

All things considered

It’s not so bad after all

Fortunately, 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.

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Summary of Friction Fundamentals



Summary of Friction Fundamentals The 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)Environment

Temperature, Humidity, Atmosphere (air, exhaust gases, vacuum)

Friction is NOT a Material Property

Friction is a “ System ” PropertyNo such thing as the COF of “steel”, or the COF of “rubber”

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LubricationFundamentals



Lubrication Fundamentals

• The role of a lubricant is to:

• Reduce Friction

• Prevent / Minimize Wear

• Transport Debris away from Interface

• Provide Cooling

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L b i i F d l



Lubrication Fundamentals:Lubrication Regimes, with liquid present

* - composite surface roughness = (r q12 + r q2

2)1/2

• In Liquid Lubrication, Regimes can be basedon: Fluid Film Thickness

• The Lambda Ratio is defined as the ratio of the

fluid 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

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Lubrication Regimes:The Stribeck Curve

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Speed*Viscosity

Load

Journal Bearing

Thick Film

Thin Film,Mixed

B o u n

d a r y



Lubrication Regimes:

Boundary Lubrication – Solid Lubricants

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• Solid Lubricants• Compounds with Low Shear Stress

• MoS2, Graphite, WS

2, HBN

• Behave like a “deck of cards”

• Bonded Films• DLC• Resin-bonded PTFE• Impregnated porous anodizing

S f L b i i



Summary of LubricationFundamentals:

• Key Factors in Lubricant Effectiveness• Fluid Shear Properties

• Viscosity, Viscosity Index

• Pressure-Viscosity Index• Chemistry

• Reactivity with the Surface• Boundary Film-Forming Properties• Extreme Pressure Constituents• Shear strength of solid lubricant or coating

• Thermal Conductivity/Heat Capacity

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WearFundamentals

W F d l



Wear Fundamentals

Conceptual Definition of Wear

Removal (or displacement) of material

from one body when subjected to contactand relative motion with another body.

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W F d l W M d



Wear Fundamentals - Wear Modes

6 Primary Wear Modes:

1. Abrasive Wear, Scratching

2. Adhesive Wear, Galling, Scuffing

3. Fretting/Fretting Corrosion4. Erosive Wear, Cavitation, Impact, Electro-arcing

5. Rolling Contact Fatigue, Spalling, Delamination

6. Tribo-Corrosion

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W F d t l



Wear Fundamentals

• Abrasive Wear, Scratching

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“The harder materialscratches the softermaterial.”

W F d t l



Wear Fundamentals

• Adhesive Wear, Galling, Scuffing

Galling of Stainless Steel Samples

10 mm

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Begins as “local welding”

Material “compatibility” isimportant for adhesivewear.

Stacking fault energy,crystal structure, naturaloxide formation allinfluence adhesive wear.

W F d t l



Wear Fundamentals

• Fretting/Fretting Corrosion

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• 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”.

Smallamplitudedisplacement(< 50 µm).

W F d t l



Wear Fundamentals

• Erosive Wear, Cavitation, Impact, Electro-arcing

Cavitation Damage

1 cm

Steam Control Valve

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“Fluting” Damage

Dependency onparticle size, shape,composition, angle ofimpingement, as wellas ductility of “target”

Particle Classification

Wear Fundamentals



Wear Fundamentals

• Rolling Contact Fatigue, Spalling, Delamination

Spalled Bearing Inner Race

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Propagation to surface ofsub-surface-initiated cracks

• Reversing sub-surface shear eachtime the roller or ball passes overthe surface.

• Accumulation of these stressesleads to subsurface crackformation, usually at amicrostructural inhomogeneity.

• Cracks grow toward surface andparticle spalls off.

• Debris typically gets rolled over,creating additional damage.

Wear Fundamentals



Wear Fundamentals

• Tribo-Corrosion

Erosion-Corrosion

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• Wear in the presence of corrosioncan have synergistic effect.

• Can happen with erosion or

sliding wear.• Bio-tribo-corrosion is major area

• Down-hole drilling environment isanother

• ASTM Method G119 – StandardGuide for Determining Synergismbetween Wear and Corrosion

Wear Assessment



Wear Assessment

• The Wear Coefficient, k• k → volume of material removed per unit load and sliding

distance

• Units of k are:• mm 3 /N ⋅m• Please do NOT reduce the units of k to mm 2 /N or 1/kPa• This 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 reciprocatingsliding

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Summary of Wear Fundamentals



Summary of Wear Fundamentals

• Like Friction, Wear is a System Property, NOT aMaterials Property

• There are several distinct wear regimes, though

some can operate simultaneously, or sequentially• Observed abrasive wear can results from initial

adhesive wear

• If you properly simulated the system and wearmode, the wear coefficient, k, can be used topredict lifetimes

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Some

Final Words forToday’s Webinar

Tribology Fundamentals



Tribology FundamentalsKey Concepts

1. 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 will

succeed, 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.1/29/2013 65



Means to AssessTribo-systems

Tribology & Mechanical Testing (TMT)



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 tests



Many different Tribology tests

Linear Stage Block-on-Ring Drive

Reciprocating Drive Rotary Drive

Indentation & Scratch Testing



Indentation & Scratch Testing

• Indentation & Scratch Tester

• Large load range: nano & micro• Wide variety of imaging options

• (AFM, profiler, optical)

Scratch test example

Indentation example



www.bruker.com

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Tribology Intro and Basics