Tribology: Friction, Lubrication, and Wear (PDF)

LESSON 1 for lubricants, which enables standardization of TRIBOLOGY lubricant viscosity grades across industries. derived from the greek word “tribos”, Introduction of elastohydrodynamic...

LESSON 1 for lubricants, which enables standardization of TRIBOLOGY lubricant viscosity grades across industries. derived from the greek word “tribos”, Introduction of elastohydrodynamic lubrication which means rubbing. (1949): Tribology is the science and engineering of Hersey and Tabor developed the theory of understanding friction, lubrication and wear elastohydrodynamic lubrication (EHL), which phenomena for interacting surfaces in relative describes the lubrication of heavily loaded contacts motion. where elastic deformation of the surfaces plays a significant role. HISTORY OF TRIBOLOGY Discovery of superlubricity (1996): Although the term tribology was only coined Researchers at IBM discovered the phenomenon of in 1966 by Peter Jost, humans have recognized superlubricity, where friction practically vanishes the concept in some form for thousands of years. between atomically smooth surfaces sliding against The earliest mention of tribology is actually in the each other under certain conditions. This finding Bible where it talks about Noah building the ark and opened up new possibilities for reducing friction sealing it internally and externally with and wear. asphalt/bitumen. Advancements in surface engineering and The period of early civilization (post 3500 coatings: The development of advanced coatings BC), saw many advances in the field of tribology and surface treatments, such as diamond-like including stone sockets, lubricants to reduce carbon (DLC) coatings and nanocomposite friction, and different surface materials to reduce coatings, has significantly improved the tribological wear. Possible lubricants used were water, mud, performance of materials, reducing friction and and rendered fat from sheep or cows. wear in various applications. Progress in computational tribology: Using KEY MILESTONES computer simulations and modeling techniques has Leonardo da Vinci's observations (1493): expanded our understanding of tribological Leonardo da Vinci made significant observations phenomena and allowed for more efficient design about friction and lubrication, including the concept and optimization of lubricated systems, including of reducing friction by using a layer of lubricant predicting wear and friction behavior. between moving surfaces. ​ Guillaume Amontons' laws of friction (1699): Guillaume Amontons formulated empirical laws of APPLICATIONS friction, known as Amontons' laws, which Tribological considerations, such as surface established the relationship between frictional force roughness, material compatibility, and contact and applied load or contact area. stresses, should be noticed in the design, Invention of the ball bearing (1794): manufacture, and use of anything that is in contact Philosopher and mathematician J.W. Lund with another object. developed the concept of the ball bearing, which revolutionized machinery design by reducing The applications of tribology include; friction and enabling smooth rotational motion. ​ Individual components (gears, bearings, Establishment of the field of tribology (1966): brakes, etc.) The term "tribology" was coined by Peter Jost in a ​ Assemblies (engines, pocket watches, etc.) report commissioned by the British government, ​ Manufacturing processes (rolling, turning, highlighting the economic and industrial importance grinding, stamping, etc.) of understanding friction, wear, and lubrication. ​ Construction (mine slurry pumps, oil drilling Introduction of the Stribeck curve (1902): rig, excavator, etc.) Richard Stribeck presented a graphical ​ Natural phenomena (water/wind erosion, representation of the relationship between friction, plate tectonics, etc.) lubrication regime, and lubricant properties, known as the Stribeck curve. It provided a framework for FIELDS OF TRIBOLOGY understanding the lubrication regimes: boundary, -​ Classical Tribology mixed, and hydrodynamic. As the name suggests, Classical Tribology focuses Development of the ISO viscosity classification on friction and wear in machine elements system (1921): rolling-element bearings, gears, plain bearings, The International Organization for Standardization brakes, clutches, wheels, etc. – as well as (ISO) established a viscosity classification system manufacturing processes.  -​ Biotribology of outer space, particularly due to extreme With Biotribology, research focuses on lubrication in temperature fluctuations. biological systems such as human hip and knee -​ Open System Tribology joints. In fact, one of the most striking examples of Open System Tribology studies tribological systems Biotribology is with total hip exposed to and affected by the natural replacements, which “replace the body’s natural environment. ball-and-socket joint with a very smooth metallic (stainless steel or cobalt-chromium alloy) ball at the FRICTION head of the femur, articulating in a cup in the pelvis The force that resists motion when the surface of made from ultra-high molecular weight one object comes in contact with another surface. polyethylene” (Hutching, “Fifty Years of Tribology”). -​ Green Tribology Which was also introduced by Peter H. Jost, looks at minimizing the environmental impact, including ways to reduce tribological losses by using technologies with minimal impact on the environment. -​ Geotribology With Geotribology, the focus is on studying friction, wear and lubrication of geological systems such as KINDS OF FRICTION faults and glaciers (Wikipedia). As a new facet of -​ STATIC FRICTION, which occurs when two Tribology, Geotribology is gaining momentum in the objects are NOT MOVING RELATIVE TO scientific world particularly in its abilities to analyze EACH OTHER (i.e., chair on the ground) fault slips. -​ ROLLING FRICTION, which occurs when -​ Nanotribology two objects MOVE RELATIVE TO EACH With the development and commercialization of OTHER AND ONE “ROLLS” ON THE microelectromechanical systems (MEMS) and OTHER (i.e., car’s wheel on the ground) nanoelectromechanical systems (NEMS), the field -​ KINETIC FRICTION, which occurs when of Nanotribology has emerged as a strong focus. two objects MOVE RELATIVE TO EACH This application studies tribological phenomena at OTHER AND RUB TOGETHER (i.e., a a nanoscopic scale, which refers to structures with person sliding down a slide) a length scale applicable to nanotechnology. -​ SLIDING FRICTION, which occurs when Nanotribology has gotten a boost in its research two objects RUB AGAINST EACH OTHER since the invention of Atomic Force Microscopy (i.e., placing a book flat on a table and (AFM), which is a high-resolution form of moving it around) scanning probe microscopy (SPM). -​ Tribotronics WEAR Another industry application is Tribotronics, which is Is the gradual removal, damaging or displacement a facet of research combining machine elements of material at solid surfaces. and electronic components to create active tribological systems and increase a machine’s efficiency and lifetime. -​ Computational Tribology With Computational Tribology, the aim is modeling the behavior of tribological systems by combining several disciplines such as contact mechanics (i.e., the study of the deformation of solids that touch each other at one or more points), fracture mechanics (i.e., the study of the proliferation of cracks in materials) and computational fluid dynamics (i.e., the study of using numerical analysis and data structures to solve and analyze problems involving fluid flows). -​ Space Tribology It looks at tribological systems with the ability to operate under the harsh environmental conditions TYPES OF WEAR TRIBOLOGICAL TEST METHODS -​ ABRASIVE WEAR ​ Testing tribological properties of materials It occurs when a hard, rough surface slides across involved building the entire system to run a softer surface. specialized tests such as field tests, component -​ ADHESIVE WEAR tests, and model tests. Due to unwanted displacement and attachment of ​ Tribology testing should always be designed wear debris from one surface to another. and carried out to meet a defined need, and it is -​ FRETTING WEAR essential as it results in vital information regarding Occurs due to repeated cyclical rubbing between any failure mechanisms of mechanical two surfaces. components. -​ EROSIVE WEAR It occurs when solid or liquid particles impinge against the surface of an object. -​ SURFACE FATIGUE Occurs when the surface of a material is weakened by cyclic loading; and Corrosion/Oxidation Wear, which occurs due to chemical reactions between worn materials and a corroding medium. LUBRICATION The process, or technique employed to reduce TRIBOLOGIST wear of one or both surfaces in proximity by -​ Analyze and interpret data from tribology inter=posing a substance called “lubricant” experiments and real-world applications to between moving surfaces to prevent metal-to-metal understand wear mechanisms and lubricant contact. performance. -​ Collaborate with cross-functional teams CLASSIFICATION OF LUBRICANTS including chemists, engineers, and product LIQUID managers to develop new products and -​ Mineral Oil improve existing ones. -​ Vegetable Oil -​ Animal Oil LESSON 2 SOLID LUBRICANTS -​ Graphite What is lubrication? -​ Nitride Lubrication is the process, or technique -​ Hexagonal boron employed to reduce the wear of one or both -​ Molybdenum disulfide surfaces nearby by inter-posing a substance called -​ Tungsten disulfide “lubricants” between moving surfaces to prevent SEMI-SOLID metal-to-metal contact. -​ Grease GASEOUS What is Petroleum? -​ Air -​ Contains a chain of HYDROCARBONS -​ Nitrogen -​ It is a thick black sticky substance that -​ Helium comes from fossils which are remains of buried plants and animals far beneath the FUNCTION OF LUBRICANTS ground through time. Lubricants have several functions, including -​ Also called as “CRUDE OIL” reducing wear, protecting equipment from -​ The word petroleum comes from LATIN corrosion, controlling temperature and words PETRA & OLEUM meaning ROCK contamination, transmitting power, and providing a OIL. fluid seal. PETROLEUM & NATURAL GAS FORMATION Petroleum and natural gas were formed from marine microorganisms. These were covered by layers of silt and sand 300-400 million years ago. Over millions of years, the remains were buried deeper and deeper. They are pictured 100 million years condensers for further cooling before they are ago. transferred to storage tanks. The enormous heat and pressure turned the remains into oil and natural gas. Now, oil and CONSTITUENT OF LUBRICANTS natural gas deposits are found underground and -​ Lubricants are generally composed of most can be extracted via drilling through the layers of of the base oil 90% by volume plus a variety sand, silt, and rock. of additives to impart desirable characteristics. -​ Generally, lubricants are based on one type of base oil. -​ Mixtures of the base oils also are used to meet performance requirements. BASE OIL + ADDITIVES = OIL LUBRICANTS ADDITIVES These are chemical compounds that improve the lubricant performance of base oil (or oil “base stock”) to impart desirable new characteristics. PETROLEUM EXPLORATION TO REFINERY 1. Oil Detection - Locating oil deep beneath the earth’s surface. 2. Drilling - Creation of oil well. 3. Production - Oil extraction from the oil reservoir COMMON ADDITIVES PER MACHINE to the collection plant. 4. Transportation - Oil transportation from the production site to the Refinery by (road, rail, sea, or pipeline). 5. Refinery - Refining petroleum to different components by distillation process. LESSON 3 PROPERTIES OF LUBRICANTS PETROLEUM REFINERY VISCOSITY The hydrocarbons (compounds) differ in -​ Is a measure of fluid’s resistance flow. their hydrocarbon chain lengths, resulting in -​ The viscosity of liquids decreases rapidly different boiling points: The longer the chain, the with an increase in temperature, and the higher the boiling point. Refiners lump certain viscosity of gases increases with an compounds into increase in temperature. groups called fractions. -​ Thus, upon heating, liquids flow more easily, The oldest and most common way to whereas gases flow more slowly. separate them is a fractional distillation: basically, -​ Also, viscosity does not change as the you heat up (boil) crude oil (in the fractional amount of matter changes therefore it is an distillation column). As the vapor rises through the intensive property. trays in the column, it cools down and condenses to form a liquid. The various liquid fractions are collected in trays and may then be passed to LUBRICANTS VISCOSITY OXIDATION STABILITY Key factors to consider: A lubricant’s ability to resist chemically combining Speed with oxygen. Load -​ Acidity Temperature -​ Corrosion -​ Insoluble -​ Increase Viscosity -​ Sludge, Tars & Varnish -​ Higher impurities Sulfur, Aromatics VISCOSITY INDEX AMERICAL PETROLEUM INSTITUTE (API) BASE OIL CATEGORIES VISCOSITY CLASSIFICATION SAE – Society of Automotive Engineers AGMA – American Gear Manufacturers Association FLASH POINT ISO – International Standards Organization -​ The temperature at which a lubricant flashes in a test flame. This helps evaluate SPEED AND LOAD EFFECTS ON VISCOSITY a lubricant’s fire resistance. SELECTION -​ The flash point (typically 225 degrees C or 440 degrees F for mineral oils) is an indication of the safety hazard of lubricant with respect to fire and explosion. FIRE POINT -​ It is the lowest temperature at which the oil burns continuously. -​ The fire point also must be high in lubricating oil, so that oil does not burn in service. -​ The fire point is the temperature at which the oil sparks and flame are sustained. The most common mismatch of lubricant to a *Flash point explains the lowest temperature for machine application is the selection of the wrong ignition, but the ignition may not be sustained for viscosity. some time. The fire point is always slightly higher *The safest way to determine correct viscosity is to than the flash point. refer to the OEM Manual. OEM – Original Equipment Manufacturer POUR POINT LESSON 4 -​ The lowest temperature at which a lubricant FRICTION IN THE ENGINE will flow as a liquid. This property is Factors Affecting Friction important for determining whether a 1.​ On the nature of the two surfaces that lubricant is suitable for cold climates or are in contact operating conditions. ​ When the surface is smooth, the friction *The oil changes from a liquid state to a plastic or between the two reduces as there is not much solid-state when subjected to a lower temperature. interlocking of irregularities. While the surface is Temperature in some cases the oil starts to solidify rough, friction increases. which makes it appear cloudy. The temperature at 2.​ On the force that is acting on these which this occurs is identified as the CLOUD surfaces POINT. ​ Friction increases when the force is applied along with the irregularities.​ THERMAL STABILITY ​ A lubricant’s ability to resist decomposition when exposed to temperature changes. ​ This property helps predict how a lubricant will change physically and chemically when exposed to temperatures outside of normal operating conditions. DEMULSIBILITY ​ A lubricant’s ability to separate water from the liquid. This property is important in applications where the hydraulic system is exposed to water, such as marine environments or construction sites. EMULSIFICATION ​ A lubricating oil, when mixed with water, loses its lubricating properties. The emulsification number is an index of the tendency of an oil to emulsify with water. What Causes Friction? CORROSION PREVENTION -​ Molecular adhesion (asperities sticking ​ A lubricant’s ability to prevent corrosion. where they touch) ​ Corrosion costs companies billions of ​ When two surfaces touch, electromagnetic dollars each year. Much of this loss is due to the forces make molecules and atoms of both surfaces corrosion of iron and steel. When exposed to start to bond to each other at or near the contact moisture and oxygen, iron and steel will react, points. Every time a contact point is broken another forming an oxide. This oxide does not firmly adhere one is made, so friction is always there (persistent). to the surface of the metal and will flake off, -​ Surface roughness (asperities colliding) causing pitting. Extensive pitting eventually results ​ As one surface slides across another the in weakness and disintegration of the metal, asperities can hit each other, causing abrasion by leading to failure. breaking the tips of the asperities. This requires effort, opposing the motive effort. OILINESS -​ Ploughing (a hard material digging in to ​ A lubricant’s property that allows it to give a soft material) lower coefficients of friction than another fluid of the ​ Some materials are soft. When a hard same viscosity. material is moved across a soft one the hard object ​ This property is especially important at slow can dig in, causing the softer material to deform, speeds or high loads. adding to the friction. ​ Oiliness is a property of lubricants that describes how well a fluid reduces friction between surfaces. It’s related to a lubricant’s ability to wet and stick to a surface. Disadvantages of Friction -​ Richard Stribeck along with Mayo Hersey In most of running machines, friction is undesirable are accredited as the pioneer researchers of due to; the 20th century, who introduced Stribeck 1. Energy loss - Power is required elsewhere. curve by extending their work on the topic of 2. Leading to wear of vital parts - High friction friction within the railway industry; however means high wear. the results of the research were in 3. Deteriorating performance due to heat coherence with the previous researchers. generation - High friction leads to unnecessary heat generation and greater need for cooling. ​ For a contact of two fluid-lubricated surfaces, the Stribeck curve shows the relationship between the so-called Hersey TOTAL ENGINE FRICTION number, a dimensionless lubrication -​ Total engine friction is the energy lost to parameter, and the friction coefficient. friction in an engine, which is the difference ​ The Hersey number is the dimensionless between brake horsepower and indicated number obtained from the velocity (m/s) horsepower. times the dynamic viscosity (Pa∙s = N∙s/m2), -​ Total engine friction is the sum of all the divided by the load per unit length of friction in an engine, including the friction in bearing (N/m) the moving parts and the power required to run auxiliaries. What is the Coefficient of Friction? -​ It's caused by the resistance to motion of Coefficient of friction, μ the engine's moving parts. A number typically between 0 and 1 that describes the roughness between two surfaces, How can friction losses in passenger cars be where 0 is slippery and 1 is very rough. reduced? A unitless ratio of the frictional force to the There are various approaches to reducing friction normal force. losses affecting a The static friction coefficient μs is for surfaces road vehicle, some of them are: that are not sliding, while kinetic μk is for sliding ​ Improving the mechanical properties by surfaces. improved design of bearings, shafts, gears, cylinders and pistons ​ Use of low friction materials and alloys ​ Improved lubrication oil and lubrication systems ​ Use of friction reducing and surface enhancing coatings and additives ​ Improved tyres with low rolling resistance Stribeck Curve -​ Stribeck curve is a fundamental and one of the most widely known concepts in tribology and lubrication fields. -​ The concept represents the friction behavior in lubricated contacts as a function of viscosity of the lubricant, entrainment speed and roughness (sometimes, just roughness).  Factors that affect the Stribeck curve include: ​ Viscosity: The lubricant viscosity affects the coefficient of friction, especially at higher speeds. ​ Surface roughness: Increased surface roughness pushes the curve up and to the right. ​ Load: Increased load pushes the curve up and to the right. LUBRICATION REGIMES Boundary lubrication -​ Solid surfaces come into direct contact, load supported mainly by surface asperities, high friction. -​ This occurs at start-up, shutdown, low speed or high load conditions. Mixed lubrication -​ Some asperity contact, load supported by both asperities and the liquid lubricant. -​ Boundary lubrication is dramatically reduced as sliding speed increases, creating a wedge of lubricant film between the surfaces in motion. Hydrodynamic lubrication -​ Negligible asperity contact, load supported mainly by hydrodynamic pressure. -​ This lubrication regime condition occurs after a machine has begun to rotate and the speeds and loads are such that a wedge of oil has been formed between the shaft and bearing surfaces. This wedge of oil lifts the shaft away from the bearing surface so there is little risk of asperity contact. This is a desirable condition to avoid friction and wear.