tribology 101 – introduction to the basics of tribology

Tribology 101 – Introduction to the Basics of Tribology SJ Shaffer, Ph.D. – Bruker-TMT [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 in 1966 • “The Jost Report”, provided to the British Parliament –

Ministry for Education and Science, indicated… “Potential savings of over £515 million per year ($800 million) for industry by better application of tribological principles and practices.”

But… Tribology is not a new field!

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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 Geometry Sled Friction Test Geometry


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Leonardo Da Vinci

Ball Bearing


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

ASTM D1894 – Static and 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 – Static and 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 Complex and 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

Bearings Brake & Clutch Pads

Assemblies or Products

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Engines

Curling Stones

Rock Climbing Shoes

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 Erosion Water 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 of Engineering and Industry

• Aerospace • Agriculture • Automotive

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

• Brakes/clutch • Tooling/Machining/Sheet metal

forming • 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 • Chemistry

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Commonality in Tribology

What do All These Diverse Fields and Applications have in Common? What do we need to think about as engineers and scientists when we design products or friction/wear experiments?

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Commonality…

Every Application has:

Surfaces in Contact, and

in 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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The Surface is not Simple…

Bulk Material Properties – “Handbook

values”

Surface Properties “Disturbed Material”

Oxide

Adsorbed Contaminants

Lubricant

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The Surface is not Simple…

Bulk Material Properties – “Handbook

values”

Surface Properties “Disturbed Material”

Oxide


Lubricant

≈

mms - cms

nms - µms

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Nor is it Flat!

Bulk Material Properties

Surface Properties Disturbed Material

Oxide


Lubricant

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

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Nor is it Flat!

Bulk Material Properties

Surface Properties Disturbed Material

Oxide


Lubricant

All engineering surfaces have a roughness, and this roughness plays an important role in tribology. Surface Roughness comes from all prior history of the part: 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 for Lubricants/debris


• Shear Strength

• Lubricant Properties, e.g. Viscosity


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Ground

Bead Blasted




• Lubricant Paths or Reservoirs


• 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 of Asperities • Asperities have Mechanical and Chemical

Properties • Methods Exist to Characterize these

Properties

• Asperity Geometry and Distribution result from Manufacturing Method, Handling and Prior Rubbing History

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Friction Fundamentals

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Friction Fundamentals Conceptual Definition of Friction

Friction is the resistance to relative motion between two

bodies in contact.

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Where does the resistance come from?

Microscopic forces of molecular Adhesion.

Microscopic forces of

mechanical Abrasion.

When objects touch – there are forces between them.

(includes electrostatic, Van der Waals, metallic

bonds)

(includes elastic and plastic 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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Friction Fundamentals Measuring Friction:

The Coefficient of Friction

Very Simple Relation:

F=µN N

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 from 0.1 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 more

information.”

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Well not really.

?

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 some

frictional 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 and

surface 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, are

these 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.”… 43 1/29/2013

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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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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 right

conditions.

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Summary of Friction Fundamentals The equation is simple, but measuring it correctly requires care:

When assessing a system’s tribology need, we must consider: Materials, Coating, Lubricant Contact Area, Geometry, Stress Surface Roughnesses Sliding Speed Sliding 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” Property No such thing as the COF of “steel”, or the COF of “rubber”

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Lubrication Fundamentals

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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Lubrication Fundamentals: Lubrication Regimes, with liquid present

* - composite surface roughness = (rq12 + rq2

2)1/2

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

• The Lambda Ratio is defined as the ratio of the fluid film thickness to the composite surface roughness*

• 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

Bou

ndar

y

Lubrication Regimes: Boundary Lubrication – Solid Lubricants

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• 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 anodizing

Summary of Lubrication Fundamentals:

• 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 1/29/2013 52

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Wear Fundamentals

Wear Fundamentals Conceptual Definition of Wear

Removal (or displacement) of material from one body when subjected to contact and relative motion with another body.

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Wear Fundamentals - Wear Modes

6 Primary Wear Modes:

1. Abrasive Wear, Scratching

2. Adhesive Wear, Galling, Scuffing

3. Fretting/Fretting Corrosion

4. Erosive Wear, Cavitation, Impact, Electro-arcing

5. Rolling Contact Fatigue, Spalling, Delamination

6. Tribo-Corrosion

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Wear Fundamentals

• Abrasive Wear, Scratching

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“The harder material scratches the softer material.”

Wear Fundamentals

• Adhesive Wear, Galling, Scuffing

Galling of Stainless Steel Samples

10 mm

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Begins as “local welding” Material “compatibility” is important for adhesive wear.

Stacking fault energy, crystal structure, natural oxide formation all influence adhesive wear.

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”.

Small amplitude displacement (< 50 µm).

Wear Fundamentals

• Erosive Wear, Cavitation, Impact, Electro-arcing

Cavitation Damage

1 cm

Steam Control Valve

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“Fluting” Damage

Dependency on particle size, shape, composition, angle of impingement, as well as ductility of “target”

Particle Classification

Wear Fundamentals

• Rolling Contact Fatigue, Spalling, Delamination

Spalled Bearing Inner Race

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Propagation to surface of sub-surface-initiated cracks

• Reversing sub-surface shear each time the roller or ball passes over the surface.

• Accumulation of these stresses leads to subsurface crack formation, usually at a microstructural inhomogeneity.

• Cracks grow toward surface and particle spalls off.

• Debris typically gets rolled over, creating additional damage.

Wear Fundamentals

• Tribo-Corrosion

Erosion-Corrosion

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• Wear in the presence of corrosion can have synergistic effect.

• Can happen with erosion or sliding wear.

• Bio-tribo-corrosion is major area

• Down-hole drilling environment is another

• ASTM Method G119 – Standard Guide for Determining Synergism between Wear and Corrosion

Wear Assessment

• The Wear Coefficient, k

• k → volume of material removed per unit load and sliding distance

• Units of k are: • mm3/N⋅m • Please do NOT reduce the units of k to mm2/N or 1/kPa • This has no physical meaning

• k can be used to predict component lifetimes, providing the tribosystem does not change wear modes

• Duty cycle and directionality can influence wear

• Start-stop can be much more damaging than continuous motion

• Unidirectional sliding is very different from reciprocating sliding

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Summary of Wear Fundamentals

• Like Friction, Wear is a System Property, NOT a Materials 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 wear mode, the wear coefficient, k, can be used to predict lifetimes

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Some Final Words for

Today’s Webinar

Tribology Fundamentals Key Concepts

1.COF is not a material property, it is a system property.

2.Wear Rate or wear resistance depends on the wear mode, which is a function of the Tribosystem.

3.If we properly characterize and understand the Tribosytem, the odds are better that we will succeed, because we can make the right choice for materials, contact geometry and chemistry, and make the appropriate measurements to give us the answer we seek for our design. 1/29/2013 65

Means to Assess Tribo-systems

Tribology & Mechanical Testing (TMT)

• Universal platform for Tribology studies: Wear, Friction,.. when 2 surfaces meet.

• Large load range • Wide variety of environments (corrosion, HT, liquid) • Wide variety of configurations (rotating & translating motions)

Many different Tribology tests

Linear Stage Block-on-Ring Drive

Reciprocating Drive Rotary Drive

Indentation & Scratch Testing

• Indentation & Scratch Tester

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

• (AFM, profiler, optical)

Scratch test example

Indentation example

tribology 101 – introduction to the basics of tribology

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