CHAPTER 7 TRIBOLOGY SURFACE TECHNOLOGY Surface Engineering Friction
CHAPTER 7 TRIBOLOGY
SURFACE TECHNOLOGY Surface Engineering Friction Thin film Wear Surface Roughnes s Oil Bearing Lubrication
FRICTION � Friction is defined as the resistance to relative motion between two bodies in contact under a normal load. � When a solid body slides on another solid, it takes energy to put the body in motion & keep it moving. � Friction plays an important role in all metalworking & manufacturing processes because of the relative motion & forces that are always acting on tools, dies, & workpieces. � Friction dissipates energy generating heat, which can have detrimental effects on an operation impedes free movement at interfaces, friction can significantly affect the flow & deformation of materials in metalworking processes.
� However, friction is not always undesirable. Without friction, for example, rolling metals, clamping workpieces on machines, or holding drills in chucks would be impossible. � The most commonly accepted theories of friction is the adhesion theory & abrasion theory. **adhesion = rekatan ** abrasion = lelasan
1) Adhesion theory � Figure - schematic illustration of the interface of two bodies in contact showing real areas of contact at the asperities. � When two surfaces are brought into contact & a load is applied forcing them together, junctions are formed where asperities come into contact. The total area of contact of these junctions is a function of load & penetration hardness of the softer material. � Sliding motion between two bodies having such an interface is possible only if a tangential force is applied. This tangential force is the force required to shear the junctions & is called the
� The ratio of F to N is called the coefficient of friction (μ).
2) Abrasion theory �Abrasion theory, an asperity from a hard surface (such as a tool) penetrates & plows through a softer surface (workpiece) �Ploughing may either cause displacement of the material or produce small chips or slivers, as in cutting & abrasive processes. Formation of small chips or slivers on surface **sliver/chip = keratan/serpih **plow = bajak
2) Abrasion theory � Almost all the energy dissipated in overcoming friction is converted into heat (a small fraction becomes stored energy in the plastically deformed regions), raising the interface temperature. � Temperature increases with friction, speed, & low thermal conductivity & specific heat of the sliding materials. � The interface temperature may be high enough to soften & even melt the surfaces, as well as cause structural changes. � These phenomena, in turn, adversely affect the operations involved, causing surface damage.
REDUCING FRICTION…… �Friction can be reduced by; selecting materials that have low adhesion e. g carbides & ceramics use surface films & coatings Use lubricants, such as oils, or solid films, graphite interpose an adherent film between tool, die & workpiece. �The film minimizes adhesion & interactions of one surface to the other reduce friction
FRICTION MEASUREMENT �The coefficient of friction (μ) is usually determined experimentally, either during actual manufacturing processes or in simulated tests using small-scale specimens of various shapes. �The techniques used generally involve measurement of either forces or dimensional changes in the specimen.
� A flat ring is upset plastically two flat platens (figure Ring compression test between a) � As its height is reduced, the ring expands radially outward � If friction at the interfaces is zero, both the inner & outer diameters of the ring expand as if it were a solid disk. � With increasing friction however, the internal diameter becomes smaller (a) Ring compression test (b) Testing results on ring [1 to 4 increasing friction] � For a particular reduction in height, there is a critical friction value at which the internal diameter increases from the original if μ is lower & decreases if μ is higher � By measuring the change in specimen's internal diameter, the coefficient of friction can be
WEAR � Wear is commonly defined as the loss of material from contacting solid surfaces in relative motion. � In many cases, wear can be quantified by the loss of materials from surface. � Although wear generally alters the surface topography & may result in severe surface damage, it also has a beneficial effect. � It can reduce surface roughness by removing the peaks from asperities. � In general, the most important mechanism are: �adhesive wear �abrasive wear �Erosion /corrosion wear
(a) Adhesive wear � If a tangential force is applied to the model shown in this Figure, shearing can take place either at the original interface or along a path below or above it & causing adhesive wear. (see adhesive theory)
(a) original interface (b) along a path below or above it hard soft (c) Although wear fragment is attached to the harder component (upper member in the Fig. c, it eventually becomes detached during further rubbing at the interface & develops into a loose wear particle
Adhesive wear can be reduced by the following methods: � �Selecting materials that do not form strong adhesive bonds. �Using a harder material as one of the pair. �Using materials that oxidize more easily. �Applying hard coatings. Coating one surface with a soft material (such as tin, silver, lead, or cadmium) also is effective in reducing wear.
(b) Abrasive wear �Abrasive wear is the result of one very hard material cutting or ploughing grooves into a softer material. (see abrasion theory) �The harder material may be one of the rubbing surfaces or hard particles that have found their way between the mating surfaces. These may be ‘foreign’ particles or particles resulting from adhesive wear.
� The particles either may be present at the surface of a second material or may exist as loose particles between two surfaces. Abrasive wear, caused by either trapped or free-flying abrasives, produces troughs in the material, piling up asperities that may fracture into debris � Abrasive wear is also used for grinding operations to remove material intentionally. � In many automotive application (e. g. , gears, pistons & cyclinders) abrasive wear behavior is a major concern. � Reduce abrasive wear by, increase the hardness of materials (such as by heat treatment & microstructural changes) reduce normal load
(c) Fatigue wear � Fatigue wear operates on the principle that as sliding, rolling, or impacting occurs repeatedly under the same operating conditions, wherein which cause material near the surface experiences cyclic stresses which initiate cracks in these nearsurface regions. � Once the cracks are formed, more cycling can cause crack growth & the cracks can link up with each other as well as with the surface. � In doing so, a crack network is formed & loose debris is generated which can be removed from the surface by continued motion. � As a result, the surface appears unaffected for long periods before pitting & spalling suddenly occurs followed by rapid wear.
� Fatigue wear can be reduced by; � Lowering contact stresses � Reducing thermal cycling � Improving the quality of materials by removing impurities, inclusions & various other flaws that may act as local points for crack initiation
(d) Corrosive wear � Corrosive wear, also known as oxidation, or chemical wear, is caused by chemical or electrochemical reactions between the surface & the environment. � The fine corrosive products (CP) on the surface constitute the wear particles. � When the corrosive layer is destroyed or removed, as by sliding or abrasion, another layer begins to form, & the process of removal & corrosive-layer formation is repeated. � Corrosive � selecting wear can be reduced by: materials that will resist environmental attack � controlling the environment � reducing operating temperatures to lower the rate of chemical reaction.
(e) Other types of wear � Erosion is caused by loose abrasive particles abrading a surface. � Fretting corrosion occurs at interfaces that are subjected to very small reciprocal movements. � Impact wear is the removal (by impacting particles) of small amounts of material from a surface. **erosion = kakisan ** fretting = penggeselsuaian **reciprocal = ulang alik
Lubrication �A lubricant is a substance introduced to reduce friction between moving surfaces � It has been noted that the surfaces of tools, dies, & workpieces are usually subjected to various processes with a wide range of parameters. � In addition to select appropriate materials & controlling process parameters, we can also select metalworking fluids (lubricants) to effectively reduce friction & wear. � There are 4 regimes of lubrication that are generally of interest in manufacturing operations; � 1) Thick-film lubrication � 2) Thin-film lubrication � 3) Mixed lubrication � 4) Boundary lubrication
The types of lubrication regimes generally occurring metalworking operations. in 1) Thick-film lubrication � The surfaces are separated completely by a film of lubricant, & lubricant viscosity is an important factor. � Such films can develop in some regions of the workpiece in high-speed operations, & with highviscosity lubricants that become trapped at dieworkpiece interfaces. � A thick lubricant film generates a dull, grainy surface appearance on the workpiece, the degree of roughness depending on grain size
2) Thin-film lubrication � The lubricant film becomes thinner as the load between the die & workpiece increases or as the speed & viscosity of the metalworking fluid decreases � This condition increases friction & leads to slight wear.
3) Mixed lubrication � In mixed lubrication, a significant portion of the load is carried by the physical contact (boundary lubrication) of the two surfaces & the rest by the fluid film trapped in pockets, such as the valleys of asperities.
4) Boundary lubrication � In boundary lubrication, the load is supported by contacting surfaces covered with a boundary layer of lubricant, typically natural oils, fatty acids & soaps, thus preventing direct metal-to-metal contact & reducing wear.
� The functions of a metalworking fluid (lubricants); � Reduce friction reduce force & energy requirements reduce temperature rise � Reduce wear reducing seizure & galling � Improve material flow in tools, dies & molds � Act as a thermal barrier between the workpiece & its tool & die surfaces preventing workpiece cooling in hot-working processes. � Act as a release or parting agent — a substance which helps in the removal or ejection of parts from dies & molds. **galling = gahar
Metalworking fluids (Lubricants) 1) Oils maintain high film strength on the surface of a metal (as we can observe when trying to clean an oily surface) � The sources of oils can be mineral (petroleum or hydrocarbon), animal, or vegetable in nature � Oils may be compounded with any number of additives or with other oils � 2) Emulsions An emulsion is a mixture of two immiscible liquids (usually of oil & water in various proportions) along with additives � Emulsifiers are substances that prevent the dispersed droplets in a mixture from joining together (hence the term immiscible). � 3) Synthetic & semi-synthetic solutions Synthetic solutions are chemical fluids that contain inorganic & other chemicals dissolved in water; they do not contain any mineral oils � Various chemical agents are added to a particular solution to impart different properties. �
4) Soaps, greases & waxes � Soaps are typically reaction products of sodium or potassium salts with fatty acids. - Alkali soaps are soluble in water, but other metal soaps generally are insoluble - Soaps are effective boundary lubricants & can form thick film layers at die–workpiece interfaces � Greases are solid or semisolid lubricants & generally consist of soaps, mineral oil & various additives � Waxes may be of animal or plant (paraffin) origin. Compared to greases, they are less “greasy” & are more brittle
� Metalworking fluids usually are blended with various additives, such as the following � Oxidation inhibitors � Rust-preventing agents � Foam inhibitors � Wetting agents � Odor-controlling agents � Antiseptics � Sulfur, chlorine, & phosphorus are important oil additives. Known as extreme-pressure (EP) additives & used singly or in combination, they react chemically with metal surfaces and form adherent surface films of metallic sulfides and chlorides.
5) Solid Lubricants - has unique properties & characteristics a) Graphite � Graphite is weak in shear along its basal planes thus it has a low coefficient of friction in that direction � It can be an effective solid lubricant, particularly at elevated temperatures � However, friction is low only in the presence of air or moisture. � Graphite can be applied either by rubbing it on surfaces or by making it part of a colloidal (dispersion of small particles) suspension in a liquid carrier such as water, oil, or an alcohol.
b) Molybdenum disulfide This is a widely used lamellar solid lubricant; it is somewhat similar in appearance to graphite. � However, unlike graphite, it has a high friction coefficient in an ambient environment. � Oils commonly are used as carriers for molybdenum disulfide and as a lubricant at room temperature. � Molybdenum disulfide can be rubbed onto the surfaces of a workpiece. � c) Metallic & polymeric films Because of their low strength, thin layers of soft metals & polymer coatings also are used as solid lubricants. � Suitable metals include lead, indium, cadmium, tin, silver, polymers such as PTFE (Teflon), polyethylene, and methacrylates. � However, these coatings have limited applications because of their lack of strength under high contact stresses and at elevated �
d) Glasses � Although it is a solid material, glass becomes viscous at elevated temperatures & hence, can serve as a liquid lubricant. � Viscosity is a function of temperature (but not of pressure) & depends on the type of glass. � Poor thermal conductivity also makes glass attractive, since it acts as a thermal barrier between hot workpieces & relatively cool dies. � Glass lubrication typically is used in such applications as hot extrusion & forging. e) Fullerenes or buckyballs � These are carbon molecules in the shape of soccer balls. � When placed between sliding surfaces, these molecules act like tiny ball bearings. � They perform well as solid lubricants & are effective
graphite molybdenum
Selection of Metalworking Fluids Selecting a lubricant for a particular application & workpiece material involves consideration of several factors; 1) Particular manufacturing process 2) Workpiece material 3) Tool or die material 4) Processing parameters 5) Compatibility of the fluid with the tool/die materials & the workpiece material 6) Required surface preparation 7) Method of fluid application 8) Removal of the fluid & cleaning of the workpiece after processing 9) Contamination of the fluid by other lubricants, such a those used to lubricate machinery 10) Storage & maintenance of the fluid 11) Treatment of waste lubricant 12) Biological & environmental considerations 13) Cost involved in all of the aspects
Selection of Metalworking Fluids Selecting oil as lubricant need to investigate its viscosity, temperature & pressure characterics • Low viscosity can cause significantly detrimental effects, high friction, high wear Water-based lubricants very effective coolants, but as lubricant not as effective as oils Lubricants should; Not leave any harmful residues that could interfere with machinery operations Not stain or corrode the workpiece or equipment Be checked periodically for deterioration caused by bacterial growth, accumulation of oxides, chips, wear debris, & general degradation & breakdown due to temperature & time [presence of wear particles is particularly important, they cause damage to the system], proper inspection & filtering are thus essential
Selection of Metalworking Fluids � Biological & environmental considerations; � Contacting or inhaling some of the fluids may cause potential hazards inflammation of skin (dermatitis) & long term exposure to carcinogens � Improper disposal of the liquids may cause adverse effects on the environment � Laws & regulations concerning the manufacture, transportation, use, & disposal of the liquids has been developed develop environmentally safe fluids & equipment for their proper treatment, recycling & disposal of fluids, OSHA (Occupational Safety & Health Administration) NIOSH (National Institute for Occupational Safety and Health)
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