Tribology 101 – Introduction to
the Basics of Tribology
SJ Shaffer, Ph.D. – Bruker-TMT
Steven.shaffer@bruker-nano.com
Outline
• Origin/Definition of “Tribology” (Term and Field)
• Encompassing Fields
• Fundamentals of Tribology:
•
•
•
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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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The First Recorded Tribologist – 2400 B.C.
Transporting the statue of Ti – from a tomb at Saqqara, Egypt
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The First Recorded Tribologist – 2400 B.C.
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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
Sled Friction Test
Geometry
4-Ball Test Geometry
Leonardo Da Vinci
Ball Bearing
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A more famous Tribologist – 500 years ago
Sled Friction Test
Geometry
Leonardo Da Vinci
4-Ball Test Geometry
ASTM D1894 – Static
and Kinetic COFs of
Plastic Film & Sheeting
Ball Bearing
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ASTM D5183 - COF
ASTM D2266, D2596 – EP
ASTM D4172, D2783 - Wear
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A more famous Tribologist – 500 years ago
Sled Friction Test
Geometry
Leonardo Da Vinci
4-Ball Test Geometry
ASTM D1894 – Static
and Kinetic COFs of
Plastic Film & Sheeting
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.
Ball Bearing
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ASTM D5183 - COF
ASTM D2266, D2596 – EP
ASTM D4172, D2783 - Wear
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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
Gears
Brake & Clutch Pads
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Bearings
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Assemblies or Products
Rock Climbing
Shoes
Pocket Watch
Engines
Curling Stones
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Manufacturing Processes
Turning
Rolling
Stamping
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Grinding/Polishing
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Construction/Exploration
Mine Slurry Pumps
Excavator
Chunnel Digging Drill
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Oil Drilling Rig
Space Shuttle
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Natural Phenomena
Wear
Water Erosion
Friction
Wind Erosion
On/Off Stiction:
Gecko Feet
Superhydrophobicity:
Lotus Leaf
Plate Tectonics
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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
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•
•
•
•
•
•
•
•
•
•
•
•
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…
Lubricant
Adsorbed
Contaminants
Oxide
Surface Properties
“Disturbed Material”
Bulk Material
Properties –
“Handbook
values”
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The Surface is not Simple…
Lubricant
nms - µms
≈
mms - cms
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Adsorbed
Contaminants
Oxide
Surface Properties
“Disturbed Material”
Bulk Material
Properties –
“Handbook
values”
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Nor is it Flat!
Lubricant
Adsorbed
Contaminants
Oxide
Surface Properties
Disturbed Material
Bulk Material
Properties
All engineering surfaces have a roughness, and this
roughness plays an important role in tribology.
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Nor is it Flat!
Lubricant
Adsorbed
Contaminants
Oxide
Surface Properties
Disturbed Material
Bulk Material
Properties
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…
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
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We need to think about…
2 Aspects of a Surface:
Ground
• Physical - Surface Roughness
• Dictates Contact Area
• Dictates Contact Stresses
• Paths or Reservoirs for
Lubricants/debris
Bead Blasted
• Chemical - Intervening Layers
• Chemical Compatibility
• Shear Strength
• Lubricant Properties, e.g. Viscosity
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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-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?
When objects touch – there are forces between them.
Microscopic
forces of
molecular
Adhesion.
(includes electrostatic,
Van der Waals, metallic
bonds)
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Microscopic
forces of
mechanical
Abrasion.
(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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Friction Fundamentals – “The COF”
• 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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Friction Fundamentals – “The COF”
• Suppose a colleague wants to know:
“What is the
COF of steel?”
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Friction Fundamentals – “The COF”
• A: “Well, dear colleague, you can use from
0.1 to 0.6. Take your pick.
“What is the
COF of steel?”
• Is that close enough for your needs?”
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Friction Fundamentals – “The COF”
Well not really.
?
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Friction Fundamentals – “The COF”
Well not really.
?
• “Then I guess we’ll need a bit more
information.”
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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.”…
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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
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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
• 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
* - composite surface roughness = (rq12 + rq22)1/2
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Lubrication Regimes:
The Stribeck Curve
Thin Film,
Mixed
Boundary
Journal Bearing
Thick Film
Speed*Viscosity
Load
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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 anodizing
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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
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Wear
Fundamentals
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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
“The harder material
scratches the softer
material.”
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Wear Fundamentals
• Adhesive Wear, Galling, Scuffing
Begins as “local welding”
10 mm
Material “compatibility” is
important for adhesive
wear.
Galling of Stainless Steel Samples
Stacking fault energy,
crystal structure, natural
oxide formation all
influence adhesive wear.
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Wear Fundamentals
• Fretting/Fretting Corrosion
Small
amplitude
displacement
(< 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”.
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Wear Fundamentals
• Erosive Wear, Cavitation, Impact, Electro-arcing
Dependency on
particle size, shape,
composition, angle of
impingement, as well
as ductility of “target”
Particle Classification
1 cm
“Fluting” Damage
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Steam Control Valve
Cavitation Damage
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Wear Fundamentals
• Rolling Contact Fatigue, Spalling, Delamination
• Reversing sub-surface shear each
time the roller or ball passes over
the surface.
Propagation to surface of
sub-surface-initiated cracks
• Accumulation of these stresses
leads to subsurface crack
formation, usually at a
microstructural inhomogeneity.
• Cracks grow toward surface and
particle spalls off.
Spalled Bearing Inner Race
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• Debris typically gets rolled over,
creating additional damage.
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Wear Fundamentals
• Tribo-Corrosion
• Wear in the presence of corrosion
can have synergistic effect.
• Can happen with erosion or
sliding wear.
• Bio-tribo-corrosion is major area
Erosion-Corrosion
• Down-hole drilling environment is
another
• ASTM Method G119 – Standard
Guide for Determining Synergism
between Wear and Corrosion
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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
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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.
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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
Reciprocating Drive
Block-on-Ring Drive
Rotary Drive
Indentation & Scratch Testing
• Indentation & Scratch Tester
Scratch test example
• Large load range: nano & micro
• Wide variety of imaging options
•
(AFM, profiler, optical)
Indentation example
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