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 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 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 5 The First Recorded Tribologist – 2400 B.C. Transporting the statue of Ti – from a tomb at Saqqara, Egypt 1/29/2013 6 The First Recorded Tribologist – 2400 B.C. 1/29/2013 The first recorded tribologist – pouring lubricant (water?) in front of the sledge in the transport of the statue of Ti. 7 A more famous Tribologist – 500 years ago Sled Friction Test Geometry 4-Ball Test Geometry Leonardo Da Vinci Ball Bearing 1/29/2013 8 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 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 – 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 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 1/29/2013 11 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 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 14 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 17 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 1/29/2013 • • • • • • • • • • • • 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 20 Commonality… Every Application has: Surfaces in Contact, and in Relative Motion (e.g. sliding, rolling, impacting) 1/29/2013 21 Tribology Basics - Surfaces in Contact So let’s begin by looking closely at a surface… 1/29/2013 22 The Surface is not Simple… Lubricant Adsorbed Contaminants Oxide Surface Properties “Disturbed Material” Bulk Material Properties – “Handbook values” 1/29/2013 23 The Surface is not Simple… Lubricant nms - µms ≈ mms - cms 1/29/2013 Adsorbed Contaminants Oxide Surface Properties “Disturbed Material” Bulk Material Properties – “Handbook values” 24 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. 1/29/2013 25 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 26 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 27 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 28 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 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 Friction Fundamentals 1/29/2013 32 Friction Fundamentals Conceptual Definition of Friction Friction is the resistance to relative motion between two bodies in contact. 1/29/2013 33 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) 34 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 35 Friction Fundamentals – “The COF” • The Coefficient of Friction: A simple constant of proportionality. 1/29/2013 36 Friction Fundamentals – “The COF” • The Coefficient of Friction: A simple constant of proportionality. • Or is it? 1/29/2013 37 Friction Fundamentals Measuring Friction: The Coefficient of Friction Very Simple Relation: F=µN N F µ = F/N = “COF” 1/29/2013 38 Friction Fundamentals – “The COF” • Suppose a colleague wants to know: “What is the COF of steel?” 1/29/2013 39 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?” 1/29/2013 40 Friction Fundamentals – “The COF” Well not really. ? 1/29/2013 41 Friction Fundamentals – “The COF” Well not really. ? • “Then I guess we’ll need a bit more information.” 1/29/2013 42 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 43 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 44 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 45 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 1/29/2013 47 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 • 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 49 Lubrication Regimes: The Stribeck Curve Thin Film, Mixed Boundary Journal Bearing Thick Film Speed*Viscosity Load 1/29/2013 50 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 1/29/2013 51 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 1/29/2013 53 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 “The harder material scratches the softer material.” 1/29/2013 56 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 57 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 58 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 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 1/29/2013 61 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 Some Final Words for Today’s Webinar 1/29/2013 64 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 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 www.bruker.com © Copyright Bruker Corporation. All rights reserved.