Post on 15-May-2020
transcript
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:
• Surfaces in Contact • Friction • Lubrication • Wear
• Concluding Words • Upcoming Topics in Series
1/29/2013 2
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.
1/29/2013 3
“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!
1/29/2013 4
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.
1/29/2013
The First Recorded Tribologist – 2400 B.C.
Transporting the statue of Ti – from a tomb at Saqqara, Egypt
1/29/2013 6
7
The First Recorded Tribologist – 2400 B.C.
The first recorded tribologist – pouring lubricant (water?) in front of the sledge in the transport of the statue of Ti.
1/29/2013
A more famous Tribologist – 500 years ago
1/29/2013 8
Leonardo Da Vinci
Ball Bearing
4-Ball Test Geometry Sled Friction Test Geometry
A more famous Tribologist – 500 years ago
1/29/2013 9
Leonardo Da Vinci
Ball Bearing
4-Ball Test Geometry Sled Friction Test Geometry
ASTM D5183 - COF ASTM D2266, D2596 – EP ASTM D4172, D2783 - Wear
ASTM D1894 – Static and Kinetic COFs of Plastic Film & Sheeting
A more famous Tribologist – 500 years ago
1/29/2013 10
Leonardo Da Vinci
Ball Bearing
4-Ball Test Geometry Sled Friction Test Geometry
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 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
1/29/2013 12
Construction/Exploration
1/29/2013 16
Mine Slurry Pumps
Excavator
Chunnel Digging Drill
Oil Drilling Rig
Space Shuttle
Natural Phenomena
1/29/2013 17
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
1/29/2013 18
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
1/29/2013 19
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?
1/29/2013 20
Commonality…
Every Application has:
Surfaces in Contact, and
in Relative Motion
(e.g. sliding, rolling, impacting)
1/29/2013 21
23
The Surface is not Simple…
Bulk Material Properties – “Handbook
values”
Surface Properties “Disturbed Material”
Oxide
Adsorbed Contaminants
Lubricant
1/29/2013
24
The Surface is not Simple…
Bulk Material Properties – “Handbook
values”
Surface Properties “Disturbed Material”
Oxide
Adsorbed Contaminants
Lubricant
≈
mms - cms
nms - µms
1/29/2013
25
Nor is it Flat!
Bulk Material Properties
Surface Properties Disturbed Material
Oxide
Adsorbed Contaminants
Lubricant
All engineering surfaces have a roughness, and this roughness plays an important role in tribology.
1/29/2013
26
Nor is it Flat!
Bulk Material Properties
Surface Properties Disturbed Material
Oxide
Adsorbed Contaminants
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.
1/29/2013
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:
1/29/2013 27
We need to think about…
• Physical - Surface Roughness • Dictates Contact Area
• Dictates Contact Stresses
• Paths or Reservoirs for Lubricants/debris
• Chemical - Intervening Layers • Chemical Compatibility
• Shear Strength
• Lubricant Properties, e.g. Viscosity
2 Aspects of a Surface:
1/29/2013 28
Ground
Bead Blasted
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, EP or boundary-forming
2 Aspects of a Surface:
1/29/2013 29
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
1/29/2013 30
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
1/29/2013 31
33
Friction Fundamentals Conceptual Definition of Friction
Friction is the resistance to relative motion between two
bodies in contact.
1/29/2013
34
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)
1/29/2013
35
Where does friction come from?
Remember, there are also “contaminants” at the interface
Oxides,
Adsorbed films,
Adsorbed gases,
Foreign or “domestic” particles
1/29/2013
Friction Fundamentals – “The COF”
• The Coefficient of Friction: A simple
constant of proportionality.
36 1/29/2013
Friction Fundamentals – “The COF”
• The Coefficient of Friction: A simple
constant of proportionality.
• Or is it?
37 1/29/2013
38
Friction Fundamentals Measuring Friction:
The Coefficient of Friction
Very Simple Relation:
F=µN N
F
µ = F/N = “COF”
1/29/2013
Friction Fundamentals – “The COF”
• Suppose a colleague wants to know:
39 1/29/2013
“What is the
COF of steel?”
Friction Fundamentals – “The COF”
• A: “Well, dear colleague, you can use from 0.1 to 0.6. Take your pick.
• Is that close enough for your needs?”
40 1/29/2013
“What is the
COF of steel?”
Friction Fundamentals – “The COF”
• “Then I guess we’ll need a bit more
information.”
42 1/29/2013
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
44
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 • …
1/29/2013
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 right
conditions.
1/29/2013
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”
1/29/2013 46
Lubrication Fundamentals
• The role of a lubricant is to: • Reduce Friction
• Prevent / Minimize Wear
• Transport Debris away from Interface
• Provide Cooling
1/29/2013 48
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
1/29/2013 49
Lubrication Regimes: The Stribeck Curve
1/29/2013 50
Speed*Viscosity Load
Journal Bearing
Thick Film
Thin Film, Mixed
Bou
ndar
y
Lubrication Regimes: Boundary Lubrication – Solid Lubricants
1/29/2013 51
• 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
Wear Fundamentals Conceptual Definition of Wear
Removal (or displacement) of material from one body when subjected to contact and relative motion with another body.
1/29/2013 54
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
1/29/2013 55
Wear Fundamentals
• Abrasive Wear, Scratching
1/29/2013 56
“The harder material scratches the softer material.”
Wear Fundamentals
• Adhesive Wear, Galling, Scuffing
Galling of Stainless Steel Samples
10 mm
1/29/2013 57
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
1/29/2013 58
• 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
1/29/2013 59
“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
1/29/2013 60
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
1/29/2013 61
• 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
1/29/2013 62
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
1/29/2013 63
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
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)
Indentation & Scratch Testing
• Indentation & Scratch Tester
• Large load range: nano & micro • Wide variety of imaging options
• (AFM, profiler, optical)
Scratch test example
Indentation example