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. 1/29/2013
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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! 1/29/2013
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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.
The first recorded tribologist – pouring lubricant (water?) 1/29/2013
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 1/29/2013
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A more famous Tribologist – 500 years ago Sled Friction Test Geometry
Leonardo Da Vinci
4-Ball Test Geometry
ASTM D1894 – Stat ic and Kinetic COFs of Plastic Film & Sheeting
Ball Bearing 1/29/2013
ASTM D5183 - COF ASTM D2266, D2596 – EP ASTM D4172, D2783 - Wear 9
A more famous Tribologist – 500 years ago Sled Friction Test Geometry
Leonardo Da Vinci
4-Ball Test Geometry
ASTM D1894 – Stat ic 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 1/29/2013
ASTM D5183 - COF ASTM D2266, D2596 – EP ASTM D4172, D2783 - Wear 10
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 1/29/2013
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Individual Components
Gears
Brake & Clutch Pads 1/29/2013
Bearings 13
Assemblies or Products
Rock Climbing Shoes
Pocket Watch
Engines
Curling Stones 1/29/2013
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Manufacturing Processes
Turning Rolling
Stamping 1/29/2013
Grinding/Polishing 15
Construction/Exploration
Mine Slurry Pumps Excavator
Chunnel Digging Drill 1/29/2013
Oil Drilling Rig
Space Shuttle 16
Natural Phenomena Wear Water Erosion
Friction Wind Erosion
On/Off Stiction: Gecko Feet
Superhydrophobicity: Lotus Leaf
Plate Tectonics 1/29/2013
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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 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
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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. 1/29/2013
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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 1/29/2013
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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 1/29/2013
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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 1/29/2013
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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 1/29/2013
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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 1/29/2013
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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) 1/29/2013
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 1/29/2013
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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” 1/29/2013
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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.” 1/29/2013
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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.”… 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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Summ ary of Fri cti on Fundame ntals 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
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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 1/29/2013
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Lubrication Regimes: The Stribeck Curve
Thin Film, Mixed
Journal Bearing
y r a d n u o B
Thick Film
Speed*Viscosity Load 1/29/2013
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Lubrication Regimes: Boundary Lubrication – Solid Lubricants
• Solid Lubricants
• Compounds with Low Shear Stress • MoS , Graphite, WS , HBN 2 2 • 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 1/29/2013
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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. 1/29/2013
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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”. 1/29/2013
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Wear Fundamentals • Erosive Wear, Cavitation, Impact, Electro-arcing Dependency on particle size, shape, composition, of impingement,angle as well as ductility of “target”
Particle Classification
1 cm
“Fluting” Damage 1/29/2013
Steam Control Valve
Cavitation Damage 59
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 1/29/2013
• Debris typically gets rolled over, creating additional damage. 60
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 mm 2/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
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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. 1/29/2013
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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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