Contamination of the lubricant with water or chemicals can lead to lubricant decomposition, corrosion of contact surfaces or both. Contamination with abrasive substances or debris can cause abrasive wear, especially when the size of the contaminant particles is as thick or thicker than the lubricating film In internal-combustion engines, water, halide acids, sulfur acids, and products of partial combustion of fuel hydrocarbons are picked up by the lubricating oil. These contaminants can cause various undesirable chemical reactions with oil and metallic surfaces, resulting in the formation of varnish deposits, sludges, viscous emulsions in the oil, or in corrosive wear of engine components Because of their chemical nature, fixed oils are particularly susceptible to chemical alteration by alkalis. Alkalis cause saponification(formation of soap) by direct chemical reaction with the fatty acids in fixed oils. This reaction alters the nature of the lubricant and, consequently, its lubricating properties The viscosity of virtually all oils and greases is affected by temperature and pressure. almost any change in temperature significantly affects viscosity: the higher the temperature, the lower the viscosity. Increase of pressure increases viscosity, but the effect is seldom significant except at very high pressures a decrease in temperature of mineral oil from 100 to0C (212 to 32F)can increase viscosity by 100 times or more. The osity of fixed oils is also affected by temperature, but to a lesser extent than for mineral oils. For the above decrease in temperature range, lard oil will increase its viscosity by approximately 30 times fluid and free-flowing at operating temperatures from 95 to 205.C(200 to 400F), depending on the type of grease Greases that are solid or semisolid at room temperature gradually soften with increasing temperature and can become Oils containing substantial quantities of volatile compounds may lose these components by evaporation when operating temperatures are too high. This process not only alters the viscosity, but also upsets the chemical nature of the oil, thus changing other properties. In some instances, volatilization can take place within the lubricating film, as when frictional of the film, leading to adhesive or abrasive wear or, in severe cases, cavitation pitting of opposing surfacesying capacity heat is ineffectively removed by the circulating fluid. Bubble formation within the film reduces the load-carrying capacity Loss of Film. When boundary lubrication is provided by soft metallic soaps (iron stearate, for example), a rise in surface temperature can result in a marked increase in the coefficient of friction and the wear rate suddenly changes from mild to severe. The temperature at which this change occurs(transition temperature )is the point at which the soap desorbs from the metal surface and no longer provides a bonded, continuous surface film. Transition temperatures generally are within the range of 120 to 205C(250 to 400F), depending on the lubricant and the chemical composition of the metal substrate. Extreme-pressure lubricants function by reaction with the metal surface rather than by adsorption of components in the lubricant. They often are used as substitutes for soap-type boundary lubricants when operating temperatures exceed the transition temperature Prevention of Lubricant Failures. Often, lubricant failure can be traced to the selection of an inappropriate lubricant Petroleum lubricating oils are available in various formulations, and they have various special properties. When these properties cannot be obtained by conventional refining techniques or are obtainable only at a very high cost by refining, they are imparted to the lubricant by additives. Often, certain oil additives impart entirely new performance characteristics to the base oil In many instances, lubricant failures(and subsequent component failures) can be prevented or corrected by changes in design of the device or in design of the lubrication system. Starvation of a bearing caused by inadequate lubricant flow or by clogging of oil passages sometimes can be corrected by increasing the size of the passages. Often, an increase or decrease in clearance between sliding surfaces will enable the lubricant to function more effectively. Shields, covers, and seals sometimes can prevent lubricant contamination from external sources. In other instances, filtration or absorption devices can be incorporated into the system to remove unwanted contaminants Analysis of Wear Failures Analysis of wear failures depends greatly on knowledge of the service conditions under which the wear occurred However, many determining factors are involved, requiring careful examination, both macroscopic and microscopic Wear failures generally result from relatively long-time exposure, yet certain information obtained at the time the failure is discovered is useful in establishing the cause. For example, analyzing samples of the environment(especially the lubricant), sludge from the lubricating system, or a used oil filter can reveal the nature and amount of wear debris or abrasive in the system. Therefore, analyses of wear failures involve some techniqu other types of failure. As previously mentioned, it is necessary to consider the complete system involved in the failure. Examination of the worn parts alone can lead to erroneous conclusions and inadequate recommendations Background Information Determine the nature of the application and the specified manufacturing methods of the parts, including surface treatments or coatings Establish the design and the actual operating conditions: loads, speeds, temperatures, and so forth
Contamination of the lubricant with water or chemicals can lead to lubricant decomposition, corrosion of contact surfaces, or both. Contamination with abrasive substances or debris can cause abrasive wear, especially when the size of the contaminant particles is as thick or thicker than the lubricating film. In internal-combustion engines, water, halide acids, sulfur acids, and products of partial combustion of fuel hydrocarbons are picked up by the lubricating oil. These contaminants can cause various undesirable chemical reactions with oil and metallic surfaces, resulting in the formation of varnish deposits, sludges, viscous emulsions in the oil, or in corrosive wear of engine components. Because of their chemical nature, fixed oils are particularly susceptible to chemical alteration by alkalis. Alkalis cause saponification (formation of soap) by direct chemical reaction with the fatty acids in fixed oils. This reaction alters the nature of the lubricant and, consequently, its lubricating properties. The viscosity of virtually all oils and greases is affected by temperature and pressure. Almost any change in temperature significantly affects viscosity: the higher the temperature, the lower the viscosity. Increase of pressure increases viscosity, but the effect is seldom significant except at very high pressures. A decrease in temperature of mineral oil from 100 to 0 °C (212 to 32 °F) can increase viscosity by 100 times or more. The viscosity of fixed oils is also affected by temperature, but to a lesser extent than for mineral oils. For the above decrease in temperature range, lard oil will increase its viscosity by approximately 30 times. Greases that are solid or semisolid at room temperature gradually soften with increasing temperature and can become fluid and free-flowing at operating temperatures from 95 to 205 °C (200 to 400 °F), depending on the type of grease. Oils containing substantial quantities of volatile compounds may lose these components by evaporation when operating temperatures are too high. This process not only alters the viscosity, but also upsets the chemical nature of the oil, thus changing other properties. In some instances, volatilization can take place within the lubricating film, as when frictional heat is ineffectively removed by the circulating fluid. Bubble formation within the film reduces the load-carrying capacity of the film, leading to adhesive or abrasive wear or, in severe cases, cavitation pitting of opposing surfaces. Loss of Film. When boundary lubrication is provided by soft metallic soaps (iron stearate, for example), a rise in surface temperature can result in a marked increase in the coefficient of friction and the wear rate suddenly changes from mild to severe. The temperature at which this change occurs (transition temperature) is the point at which the soap desorbs from the metal surface and no longer provides a bonded, continuous surface film. Transition temperatures generally are within the range of 120 to 205 °C (250 to 400 °F), depending on the lubricant and the chemical composition of the metal substrate. Extreme-pressure lubricants function by reaction with the metal surface rather than by adsorption of components in the lubricant. They often are used as substitutes for soap-type boundary lubricants when operating temperatures exceed the transition temperature. Prevention of Lubricant Failures. Often, lubricant failure can be traced to the selection of an inappropriate lubricant. Petroleum lubricating oils are available in various formulations, and they have various special properties. When these properties cannot be obtained by conventional refining techniques or are obtainable only at a very high cost by refining, they are imparted to the lubricant by additives. Often, certain oil additives impart entirely new performance characteristics to the base oil. In many instances, lubricant failures (and subsequent component failures) can be prevented or corrected by changes in design of the device or in design of the lubrication system. Starvation of a bearing caused by inadequate lubricant flow or by clogging of oil passages sometimes can be corrected by increasing the size of the passages. Often, an increase or decrease in clearance between sliding surfaces will enable the lubricant to function more effectively. Shields, covers, and seals sometimes can prevent lubricant contamination from external sources. In other instances, filtration or absorption devices can be incorporated into the system to remove unwanted contaminants. Analysis of Wear Failures Analysis of wear failures depends greatly on knowledge of the service conditions under which the wear occurred. However, many determining factors are involved, requiring careful examination, both macroscopic and microscopic. Wear failures generally result from relatively long-time exposure, yet certain information obtained at the time the failure is discovered is useful in establishing the cause. For example, analyzing samples of the environment (especially the lubricant), sludge from the lubricating system, or a used oil filter can reveal the nature and amount of wear debris or abrasive in the system. Therefore, analyses of wear failures involve some techniques in other types of failure. As previously mentioned, it is necessary to consider the complete system involved in the failure. Examination of the worn parts alone can lead to erroneous conclusions and inadequate recommendations. Background Information · Determine the nature of the application and the specified manufacturing methods of the parts, including surface treatments or coatings. · Establish the design and the actual operating conditions: loads, speeds, temperatures, and so forth
Find out what lubricant, if any, was specified and how was it applied Establish the ambient environment and service conditions Determine whether the observed wear is normal and typical for the particular application, or if it is abnormal and Examination Define the relative motions in the system, including direction and veloci surface Define the surface configuration of both the worn surface and the original Define the force or pressure between the mating surfaces, or between the worn surface and the wear environment on both the macroscopic and microscopic scales Identify the actual materials in the worn part, the environment, the abrasive, the wear debris, and the lubricant Define the type and effectiveness of any lubricant: oil, grease, surface film, naturally occurring oxide layer adsorbed film. other or none Determine the wear rate and coefficient of friction Reach a conclusion as to the wear mechanism, or the combination of mechanisms: abrasive, adhesive, rolling- ontact fatigue, and so forth Devise a solution and formulate recommendations The following sections discuss some of the details and special methods that are involved in the analysis of wear failures Service Conditions. The first step in wear failure analysis is the initial identification of the type of wear or, if more than one type can be recognized, evaluation of the relative importance of each type as quantitatively as possible. This identification of the type or types of wear requires a detailed description of the service conditions based on close bservation and adequate experience. A casual and superficial description of service conditions will be of little value Descriptions of service conditions often are incomplete, thus imposing a serious handicap on the failure analyst, problem of a badly seized engine cylinder. Obviously, this is an instance of adhesive metal-to-metal wear or lubrica e especially if working in a laboratory remote from the service site. For instance, assume that an analyst must study the ear, because use of a suitable engine oil is implied. Furthermore, assume that during an oil change the system had flushed with a solvent such as kerosene to rinse out the old oil and had been left filled with solvent instead of new oil inadvertently. Also assume that a slow leak, resulting in loss of the solvent, was not detected during the operating period mmediately preceding seizure. The analyst probably would receive the damaged parts (cylinder block and pistons )after established, clear determination of the cause of failure would be extremely difficult or perhaps impossible could not be they had been removed from the engine, cleaned, and packed. If evidence of the substitute"lubricant could not be Another example could involve a rolling-element bearing that was running hot because of the lack of an adequate oil apply. The first thing maintenance people will often do when they find a hot bearing is add oil. So, when the bearing fails and is sent to the laboratory, it arrives showing clear signs of overheating, but is covered with oil Similarly, incomplete descriptions of service conditions can be misleading in analysis of abrasive wear. For example, in describing the source of abrasion that produces wear of mining and ore-handling equipment, generalized references to the ore, such as copper ore, are common. Such descriptions are too vague to be meaningful; the mineral being extracted usually has little effect on the abrasiveness of the mixture, whereas the bulk rock, or gangue, is the principal source of abrasive particles. Unless the gangue minerals are studied both qualitatively and quantitatively, a valid assessment of wear, whether normal or abnormal, is impossible In analysis of conventional lubricated wear, detailed description of the lubricant is essential and often must be upplemented by data regarding pressures applied to mating surfaces, operating temperatures, and surface conditions When corrosion is a factor in lubricated wear, determination of the temperature, degree of aeration, hydrogen-ion concentration, velocity of the lubricant, and the composition and concentration of the corrodent in the lubricant may be difficult. Other analysis complications include the presence of substances that inhibit or accelerate corrosion. In some pumps and compressors, the fluid being processed may leak into the lubrication system, contaminating the oil and causing it to become corrosive. So, it is important in such cases to know the composition of the process stream and to submit a sample of the lubricant for analysi Initial Examination and Measurements. Examination of a worn part generally begins with visual observation and measurement of dimensions, usually requiring micrometers, calipers, and standard or special gages. Observations of the amount and character of surface damage often must be made on a microscopic scale. An optical comparator, toolmakers microscope, recording profilometer, or other fine-scale measuring equipment may be required to assess adequately the amount of damage that has occurred Weighing a worn component or assembly and comparing its weight with that of an unused part can help define the amount of material lost. The difference in weight is a measure of the severity of the wear. This material is lost; it is transferred to an opposing surface by adhesive wear or eliminated by abrasive wear, erosion, cavitation, or combinations Thefileisdownloadedfromwww.bzfxw.com
· Find out what lubricant, if any, was specified and how was it applied. · Establish the ambient environment and service conditions. · Determine whether the observed wear is normal and typical for the particular application, or if it is abnormal and unique. Examination · Define the surface configuration of both the worn surface and the original surface. · Define the relative motions in the system, including direction and velocity. · Define the force or pressure between the mating surfaces, or between the worn surface and the wear environment on both the macroscopic and microscopic scales. · Identify the actual materials in the worn part, the environment, the abrasive, the wear debris, and the lubricant. · Define the type and effectiveness of any lubricant: oil, grease, surface film, naturally occurring oxide layer, adsorbed film, other, or none. · Determine the wear rate and coefficient of friction. · Reach a conclusion as to the wear mechanism, or the combination of mechanisms: abrasive, adhesive, rollingcontact fatigue, and so forth. · Devise a solution and formulate recommendations. The following sections discuss some of the details and special methods that are involved in the analysis of wear failures. Service Conditions. The first step in wear failure analysis is the initial identification of the type of wear or, if more than one type can be recognized, evaluation of the relative importance of each type as quantitatively as possible. This identification of the type or types of wear requires a detailed description of the service conditions based on close observation and adequate experience. A casual and superficial description of service conditions will be of little value. Descriptions of service conditions often are incomplete, thus imposing a serious handicap on the failure analyst, especially if working in a laboratory remote from the service site. For instance, assume that an analyst must study the problem of a badly seized engine cylinder. Obviously, this is an instance of adhesive metal-to-metal wear or lubricated wear, because use of a suitable engine oil is implied. Furthermore, assume that during an oil change the system had been flushed with a solvent such as kerosene to rinse out the old oil and had been left filled with solvent instead of new oil inadvertently. Also assume that a slow leak, resulting in loss of the solvent, was not detected during the operating period immediately preceding seizure. The analyst probably would receive the damaged parts (cylinder block and pistons) after they had been removed from the engine, cleaned, and packed. If evidence of the substitute “lubricant” could not be established, clear determination of the cause of failure would be extremely difficult or perhaps impossible. Another example could involve a rolling-element bearing that was running hot because of the lack of an adequate oil supply. The first thing maintenance people will often do when they find a hot bearing is add oil. So, when the bearing fails and is sent to the laboratory, it arrives showing clear signs of overheating, but is covered with oil. Similarly, incomplete descriptions of service conditions can be misleading in analysis of abrasive wear. For example, in describing the source of abrasion that produces wear of mining and ore-handling equipment, generalized references to the ore, such as copper ore, are common. Such descriptions are too vague to be meaningful; the mineral being extracted usually has little effect on the abrasiveness of the mixture, whereas the bulk rock, or gangue, is the principal source of abrasive particles. Unless the gangue minerals are studied both qualitatively and quantitatively, a valid assessment of wear, whether normal or abnormal, is impossible. In analysis of conventional lubricated wear, detailed description of the lubricant is essential and often must be supplemented by data regarding pressures applied to mating surfaces, operating temperatures, and surface conditions. When corrosion is a factor in lubricated wear, determination of the temperature, degree of aeration, hydrogen-ion concentration, velocity of the lubricant, and the composition and concentration of the corrodent in the lubricant may be difficult. Other analysis complications include the presence of substances that inhibit or accelerate corrosion. In some pumps and compressors, the fluid being processed may leak into the lubrication system, contaminating the oil and causing it to become corrosive. So, it is important in such cases to know the composition of the process stream and to submit a sample of the lubricant for analysis. Initial Examination and Measurements. Examination of a worn part generally begins with visual observation and measurement of dimensions, usually requiring micrometers, calipers, and standard or special gages. Observations of the amount and character of surface damage often must be made on a microscopic scale. An optical comparator, toolmaker's microscope, recording profilometer, or other fine-scale measuring equipment may be required to assess adequately the amount of damage that has occurred. Weighing a worn component or assembly and comparing its weight with that of an unused part can help define the amount of material lost. The difference in weight is a measure of the severity of the wear. This material is lost; it is transferred to an opposing surface by adhesive wear or eliminated by abrasive wear, erosion, cavitation, or combinations The file is downloaded from www.bzfxw.com
of these mechanisms. Weight-loss estimates also help define relative wear rates for two opposing surfaces that may be made of different materials or have been worn by different mechanisms Screening of abrasives or wear debris to determine the particle sizes and weight percentage of particles of each size is often helpful. The combination of determination of particle size with chemical analysis of the various screenings can be useful when one component in an abrasive mixture primarily causes wear, or when wear debris and an abrasive coexist in the wear environment. The combination of screening with microscopy often can reveal such details as progressive alteration of the size and shape of abrasive particles with time, as might occur in a ball mill Physical measurements can define the amount and location of wear damage, but they seldom provide enough information to establish either the mechanism or the cause of the damage Surface damage can range from polished or burnished conditions to removal of a relatively large volume of material Examination of the worn surface can provide much information, including The amount of material removed The type of damage(scratching, gouging, plowing, adhesion, pitting, corrosion, spalling, or simple penetration) The existence and character of surface films Whether certain constituents are being attacked preferentially The direction of relative motion between a worn surface and abrading particles Whether abrading particles have become embedded in the surface Because wear is a surface phenomenon, the original surface configuration of the components in contact influences wear by influencing resistance to relative motion Microscopy may be useful to study features of the worn surface including the configuration, distribution, direction of scratches or gouges, and indications of the preferential removal of specific constituents of the microstructure. Abrasive particles or wear debris should be studied under the microscope to observe their shape and the configuration of their edges (sharp or rounded) and to determine if they have fractured during the wear process. These particles can also be chemically analyzed in an SEM using eds to determine their nature and indicate their source Examination of the worn surface by light microscopy at magnifications up to about 100 diameters usually is required to detect uneven or abnormal wear patterns and to reveal the direction of relative movement between the worn surface and the opposing surface or abrasive. Sometimes, higher magnifications are required, and a SEm may be necessary to study areas of slight wear. These techniques are described further in Friction, Lubrication, and Wear Technology, Volume 18 of the asm Handbook. Direct observation at magnifications greater than about 50 diameters can be difficult if the part does not fit in the stage of a metallurgical microscope. Sectioning to remove a portion of the worn surface for direct observation precludes repair and reuse of the part. Some stereobinocular microscopes are capable of such magnifications. Replication is another technique that can be used for light-microscope or SEM observations of worn surfaces of large parts. Replication using plastic films or harder cast materials offers the additional advantage that a reproduction of the surface can be obtained at a remote site and carried back to the laboratory for detailed study Changes in surface configuration that occur during the wear process affect subsequent stages of wear. As mentioned earlier, each manufacturing method produces surfaces that are characteristic of that process, whether it is grinding machining, burnishing, or something else. The process of"wearing in, which involves progressive reduction in surface oughness by adhesive or abrasive wear of opposing surfaces, generally is followed by a period of relatively little wear The initial smoothing out of asperities, particularly in lubricated systems that operate under boundary lubrication, reduces the microscopic hills and valleys on the surface to a height about the same as the thickness of the lubricant film. The surfaces then ride on each other with no interference between peaks on the opposing surfaces, and wear essentially ceases In normal service, then, there will be a characteristic pattern of this break-in wear that will define the expected contact between mating surfaces In other instances, particularly if the initial surfaces are somewhat rougher or if boundary lubrication is ineffective adhesive wear may result in progressive surface roughening and eventual failure. If this process releases wear debris into the joint and the particle size of this debris exceeds the thickness of the lubricating film, combined adhesive and abrasive ear between the opposing surfaces and the wear debris can result in rapid deterioration Direction of Relative Motion. When only unidirectional sliding is involved, scratches or gouges produced on the worn surface are aligned with the direction of relative motion In a sleeve bearing, for example, the scratches should run circumferentially on the inner surface of the bearing and on the mating shaft. Scratches resulting from wear that are oriented in other directions indicate misalignment, vibration, or looseness. These factors can contribute to the severity of the wear In devices that undergo combined rolling and sliding, knowledge of the relative velocities and directions of rolling and sliding is necessary for definition of the wear mechanism. The direction of rolling is defined as the direction in which the point of contact moves. The direction of rolling is always opposite to the direction of rotation of a rolling element. On a given surface, a condition of positive sliding exists if the direction of sliding is the same as the direction of rolling
of these mechanisms. Weight-loss estimates also help define relative wear rates for two opposing surfaces that may be made of different materials or have been worn by different mechanisms. Screening of abrasives or wear debris to determine the particle sizes and weight percentage of particles of each size is often helpful. The combination of determination of particle size with chemical analysis of the various screenings can be useful when one component in an abrasive mixture primarily causes wear, or when wear debris and an abrasive coexist in the wear environment. The combination of screening with microscopy often can reveal such details as progressive alteration of the size and shape of abrasive particles with time, as might occur in a ball mill. Physical measurements can define the amount and location of wear damage, but they seldom provide enough information to establish either the mechanism or the cause of the damage. Surface damage can range from polished or burnished conditions to removal of a relatively large volume of material. Examination of the worn surface can provide much information, including: · The amount of material removed · The type of damage (scratching, gouging, plowing, adhesion, pitting, corrosion, spalling, or simple penetration) · The existence and character of surface films · Whether certain constituents are being attacked preferentially · The direction of relative motion between a worn surface and abrading particles · Whether abrading particles have become embedded in the surface Because wear is a surface phenomenon, the original surface configuration of the components in contact influences wear by influencing resistance to relative motion. Microscopy may be useful to study features of the worn surface including the configuration, distribution, direction of scratches or gouges, and indications of the preferential removal of specific constituents of the microstructure. Abrasive particles or wear debris should be studied under the microscope to observe their shape and the configuration of their edges (sharp or rounded) and to determine if they have fractured during the wear process. These particles can also be chemically analyzed in an SEM using EDS to determine their nature and indicate their source. Examination of the worn surface by light microscopy at magnifications up to about 100 diameters usually is required to detect uneven or abnormal wear patterns and to reveal the direction of relative movement between the worn surface and the opposing surface or abrasive. Sometimes, higher magnifications are required, and a SEM may be necessary to study areas of slight wear. These techniques are described further in Friction, Lubrication, and Wear Technology, Volume 18 of the ASM Handbook. Direct observation at magnifications greater than about 50 diameters can be difficult if the part does not fit in the stage of a metallurgical microscope. Sectioning to remove a portion of the worn surface for direct observation precludes repair and reuse of the part. Some stereobinocular microscopes are capable of such magnifications. Replication is another technique that can be used for light-microscope or SEM observations of worn surfaces of large parts. Replication using plastic films or harder cast materials offers the additional advantage that a reproduction of the surface can be obtained at a remote site and carried back to the laboratory for detailed study. Changes in surface configuration that occur during the wear process affect subsequent stages of wear. As mentioned earlier, each manufacturing method produces surfaces that are characteristic of that process, whether it is grinding, machining, burnishing, or something else. The process of “wearing in,” which involves progressive reduction in surface roughness by adhesive or abrasive wear of opposing surfaces, generally is followed by a period of relatively little wear. The initial smoothing out of asperities, particularly in lubricated systems that operate under boundary lubrication, reduces the microscopic hills and valleys on the surface to a height about the same as the thickness of the lubricant film. The surfaces then ride on each other with no interference between peaks on the opposing surfaces, and wear essentially ceases. In normal service, then, there will be a characteristic pattern of this break-in wear that will define the expected contact between mating surfaces. In other instances, particularly if the initial surfaces are somewhat rougher or if boundary lubrication is ineffective, adhesive wear may result in progressive surface roughening and eventual failure. If this process releases wear debris into the joint and the particle size of this debris exceeds the thickness of the lubricating film, combined adhesive and abrasive wear between the opposing surfaces and the wear debris can result in rapid deterioration. Direction of Relative Motion. When only unidirectional sliding is involved, scratches or gouges produced on the worn surface are aligned with the direction of relative motion. In a sleeve bearing, for example, the scratches should run circumferentially on the inner surface of the bearing and on the mating shaft. Scratches resulting from wear that are oriented in other directions indicate misalignment, vibration, or looseness. These factors can contribute to the severity of the wear. In devices that undergo combined rolling and sliding, knowledge of the relative velocities and directions of rolling and sliding is necessary for definition of the wear mechanism. The direction of rolling is defined as the direction in which the point of contact moves. The direction of rolling is always opposite to the direction of rotation of a rolling element. On a given surface, a condition of positive sliding exists if the direction of sliding is the same as the direction of rolling
Negative sliding occurs on the mating surface, where the directions of rolling and sliding are opposite. Most surface origin, rolling-contact fatigue failures originate in regions of negative sliding, because the shear stresses are usually more severe there than in regions of positive sliding. Negative sliding occurs on the dedenda of gear teeth, on the cam follower riding on a cam, and in other devices on the part that have the lower surface velocity in a rolling-sliding system Metallography. Once the worn surface has been characterized, it is appropriate to prepare metallographic sections through the wear damage and through comparable unworn areas. Because wear is a surface phenomenon, it is almost always necessary to use special techniques to avoid rounding of the edge of the metallographic specimen. One technique is nickel plating on the specimen before it is sectioned and mounted. Additions of powdered glass, hard steel shot, or pelletized alumina, to name a few examples, may be used in the mounting material to improve edge support The mounted specimen must be polished with care using polishing cloths with little nap, such as silk or nylon, so that the edge is not rounded For valid analyses of very thin surface layers, techniques such as taper sectioning are needed to allow metallographic observations and microhardness measurements. Taper sectioning involves sectioning the specimen at a shallow angle, and it produces a geometric magnification in the direction of sectioning of surface layers and coatings Etchants, in addition to preparing a specimen for the examination of microstructure, also reveal characteristics of the worn surface. Two features that can be revealed by etching a worn surface are phase transformations and metal transfer caused by localized adhesion to an opposing surface and the results of overheating caused by excessive friction. An example of a structure produced by overheating is the"white layer"(untempered martensite) that sometimes develops on steel or cast on under conditions of heavy sliding contact. Etching a worn surface also can help in detecting the selective removal of pecific constituents of the microstructure. It is very important that the worn surface be examined under the microscope and photographed before it is etched, because some topographic features may be easier to observe on the unetched ace Metallography will determine whether or not the initial microstructure of the worn part met specification, both in the base material and in any hardened case or surface coatings that may have been applied. It also will reveal existence of localized phase transformation, sheared or cold-worked surface layers, or, as described in the case study on chromium-plated cylinders, the presence of embedded abrasive particles Macroscopic and microscopic hardness testing indicate the resistance of a material to several wear mechanisms. Because harder materials are likely to cut or scratch softer materials, comparative hardness of two sliding surfaces may be important. Microhardness measurements on martensitic steels may indicate that frictional heat has overtempered the steel and, when used in conjunction with a tempering curve(a plot of hardness versus tempering temperature), can allow a rough estimate of surface temperature. In some failures, such as in case crushing, a microhardness traverse to verify the depth of the hard case will be needed. Bulk hardness measurements also can indicate whether or not a worn part was heat Effect of Microstructure and Hardness on Wear. The microstructural heterogeneity of a wear surface influences the wear process, because constituents such as carbides, inclusions, intermetallic compounds, and dispersed phases have properties different from those of the matrix. Hard microconstituents such as carbides make a metal extremely resistant to abrasive wear if they are closely spaced in a relatively hard matrix Matrix hardness is important to wear resistance. If hard microconstituents are dispersed widely in a matrix that is not hard enough to have good wear resistance of its own, the matrix may wear away rapidly, leaving the hard particles projecting from the surface, where they can cut into a mating surface. For this reason, under dry-sliding conditions, fine pearlite exhibits considerably better wear resistance than does coarse pearlite or a mixture of ferrite and pearlite X-ray and electron diffraction analyses disclose the structure of a crystalline solid. These techniques are particularly aluable for analyzing abrasives, wear debris, or surface films because they can identify compounds, not merely elements Modern diffraction units, with computerized data collection and analysis using search-match techniques, can help by identifying metals, corrosion products, abrasive minerals, and wear debris. Microstructural features such as retained austenite cannot always be seen by microscopic examination of an etched specimen; quantitative diffraction analysis can reveal the relative amounts of such unresolved constituents in the microstructure Chemical Analysis. One or more of the various techniques of chemical analysis-wet chemical analysis, spectroscopy, colorimetry, x-ray fluorescence, atomic absorption, EDS, or electron-beam microprobe analysis--usually is needed for properly analyzing wear failures. The actual compositions of the worn material, the wear debris, the abrasive, and the urface film must be known in order to devise solutions to most wear problems Although not so familiar to most failure analysts, a similar number of analytical methods are available for evaluating lubricants. If an ample quantity of lubricant is available, some of the bulk properties, such as viscosity, can be measured Identification of the oil, its additives, and contaminants can be done by Fourier transform infrared(FTir) analysis and inductively coupled photospectroscopy (ICP). An analysis of the lubricant can determine if the proper base stock and additives were present, and if any contamination or degradation has occurred in service Chemical analysis may be needed to establish or confirm the wear mechanism. The following example describes an instance in which adhesive wear caused by solid-phase welding between mating surfaces was verified by electron-beam microprobe analysis Thefileisdownloadedfromwww.bzfxw.com
Negative sliding occurs on the mating surface, where the directions of rolling and sliding are opposite. Most surfaceorigin, rolling-contact fatigue failures originate in regions of negative sliding, because the shear stresses are usually more severe there than in regions of positive sliding. Negative sliding occurs on the dedenda of gear teeth, on the cam follower riding on a cam, and in other devices on the part that have the lower surface velocity in a rolling-sliding system. Metallography. Once the worn surface has been characterized, it is appropriate to prepare metallographic sections through the wear damage and through comparable unworn areas. Because wear is a surface phenomenon, it is almost always necessary to use special techniques to avoid rounding of the edge of the metallographic specimen. One technique is nickel plating on the specimen before it is sectioned and mounted. Additions of powdered glass, hard steel shot, or pelletized alumina, to name a few examples, may be used in the mounting material to improve edge support. The mounted specimen must be polished with care using polishing cloths with little nap, such as silk or nylon, so that the edge is not rounded. For valid analyses of very thin surface layers, techniques such as taper sectioning are needed to allow metallographic observations and microhardness measurements. Taper sectioning involves sectioning the specimen at a shallow angle, and it produces a geometric magnification in the direction of sectioning of surface layers and coatings. Etchants, in addition to preparing a specimen for the examination of microstructure, also reveal characteristics of the worn surface. Two features that can be revealed by etching a worn surface are phase transformations and metal transfer caused by localized adhesion to an opposing surface and the results of overheating caused by excessive friction. An example of a structure produced by overheating is the “white layer” (untempered martensite) that sometimes develops on steel or cast iron under conditions of heavy sliding contact. Etching a worn surface also can help in detecting the selective removal of specific constituents of the microstructure. It is very important that the worn surface be examined under the microscope and photographed before it is etched, because some topographic features may be easier to observe on the unetched surface. Metallography will determine whether or not the initial microstructure of the worn part met specification, both in the base material and in any hardened case or surface coatings that may have been applied. It also will reveal existence of localized phase transformation, sheared or cold-worked surface layers, or, as described in the case study on chromium-plated cylinders, the presence of embedded abrasive particles. Macroscopic and microscopic hardness testing indicate the resistance of a material to several wear mechanisms. Because harder materials are likely to cut or scratch softer materials, comparative hardness of two sliding surfaces may be important. Microhardness measurements on martensitic steels may indicate that frictional heat has overtempered the steel and, when used in conjunction with a tempering curve (a plot of hardness versus tempering temperature), can allow a rough estimate of surface temperature. In some failures, such as in case crushing, a microhardness traverse to verify the depth of the hard case will be needed. Bulk hardness measurements also can indicate whether or not a worn part was heat treated correctly. Effect of Microstructure and Hardness on Wear. The microstructural heterogeneity of a wear surface influences the wear process, because constituents such as carbides, inclusions, intermetallic compounds, and dispersed phases have properties different from those of the matrix. Hard microconstituents such as carbides make a metal extremely resistant to abrasive wear if they are closely spaced in a relatively hard matrix. Matrix hardness is important to wear resistance. If hard microconstituents are dispersed widely in a matrix that is not hard enough to have good wear resistance of its own, the matrix may wear away rapidly, leaving the hard particles projecting from the surface, where they can cut into a mating surface. For this reason, under dry-sliding conditions, fine pearlite exhibits considerably better wear resistance than does coarse pearlite or a mixture of ferrite and pearlite. X-ray and electron diffraction analyses disclose the structure of a crystalline solid. These techniques are particularly valuable for analyzing abrasives, wear debris, or surface films because they can identify compounds, not merely elements. Modern diffraction units, with computerized data collection and analysis using search-match techniques, can help by identifying metals, corrosion products, abrasive minerals, and wear debris. Microstructural features such as retained austenite cannot always be seen by microscopic examination of an etched specimen; quantitative diffraction analysis can reveal the relative amounts of such unresolved constituents in the microstructure. Chemical Analysis. One or more of the various techniques of chemical analysis—wet chemical analysis, spectroscopy, colorimetry, x-ray fluorescence, atomic absorption, EDS, or electron-beam microprobe analysis—usually is needed for properly analyzing wear failures. The actual compositions of the worn material, the wear debris, the abrasive, and the surface film must be known in order to devise solutions to most wear problems. Although not so familiar to most failure analysts, a similar number of analytical methods are available for evaluating lubricants. If an ample quantity of lubricant is available, some of the bulk properties, such as viscosity, can be measured. Identification of the oil, its additives, and contaminants can be done by Fourier transform infrared (FTIR) analysis and inductively coupled photospectroscopy (ICP). An analysis of the lubricant can determine if the proper base stock and additives were present, and if any contamination or degradation has occurred in service. Chemical analysis may be needed to establish or confirm the wear mechanism. The following example describes an instance in which adhesive wear caused by solid-phase welding between mating surfaces was verified by electron-beam microprobe analysis. 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Geological studies of soils or similar abrasive mixtures of minerals are mandatory in analyzing wear of tillage tools, earthmoving equipment, and ore-handling devices. The abrasive characteristics of sandy soils are different from those of coarse, rocky soils, clay, or fine silt. Particle hardness and shape gagged or rounded)are important. The degree of compaction determines the amount of pressure that forces abrasive soil particles against a surface that is being dragged through the mixture. In addition, moisture content is important because it determines, in part, the temperature, lubricity and cohesiveness of the mixture. Many ores, slags, and similar bulk materials are extremely abrasive to equipment for handling and moving materials; geological analysis can define the abrasive character of these aggregates Practices in Failure Analysis Formulating Conclusions and Report writing At a certain stage in every investigation, the evidence revealed by the examinations and tests is analyzed and premises are formulated Obviously, many investigations will not become a series of clear-cut stages. If the probable cause of failure is apparent early in the examination, the pattern and extent of subsequent investigation will be directed toward confirmation of the probable cause and the elimination of other possibilities. Other investigations will follow a logical series of stages, as outlined in this article, and the findings at each stage will determine the manner in which the investigation proceeds. As new facts modify first impressions, different hypotheses of failure will develop and will be retained or abandoned as dictated by the findings. Where extensive laboratory facilities are available to the investigator, maximum effort will be devoted to amassing the results of mechanical tests, chemical analysis, fractography, and microscopy before the formulation of preliminary conclusions is attempted. Finally, in those investigations in which the cause of failure is particularly elusive, a search through reports of similar instances may be required to suggest possible clues Some of the work performed during the course of an investigation may be thought to be unnecessary. It is important, however, to distinguish between work that is unnecessary and that which does not bear fruitful results. During an examination, it is to be expected that some of the work done will not assist directly in determining the cause of failure; nevertheless, negative evidence may be helpful in excluding some potential auses of failure from consideration. Sometimes the evidence that excludes possible failure causes may be stronger than the available evidence supporting the probable cause for a failure On the other hand, any tendency to curtail work essential to an investigation should be guarded against. In some instances, it is possible to form an opinion regarding the cause of failure from a single aspect of the investigation, such as visual examination of a fracture surface or examination of a single metallographic specimen. However, before final conclusions are reached, supplementary data confirming the original opinion if available, should be sought. Total dependence on the conclusions that can be drawn from a single specimen. such as a metallographic section, may be readily challenged unless a history of similar failures can be drawn upon. Yet, circumstances sometimes dictate that a conclusion be formulated on limited data. This is especially true in forensic investigations because of the usually limited extent of the problem and the need to preserve as much evidence as possible. Also, of course, economic limitations are al ways present. It is rarely justified for the failure analyst to conduct every type of test and investigation possible The following checklist, which is in the form of a series of questions, has been proposed as an aid in analyzing the evidence derived from examinations and tests and in formulating conclusions. The questions are also helpful in calling attention to details of the overall investigation that may have been overlooked Has the failure sequence been established? If the failure involved cracking or fracture, have the initiation sites been determined? Did cracks initiate at the surface or below the surface? Was cracking associated with a stress concentration? low long was the crack present? What was the level or magnitude of the loads? What was the type of loading: static, cyclic, or intermittent? Is the fracture surface consistent with the type of loading assumed in the design? What was the failure mechanism?
Geological studies of soils or similar abrasive mixtures of minerals are mandatory in analyzing wear of tillage tools, earthmoving equipment, and ore-handling devices. The abrasive characteristics of sandy soils are different from those of coarse, rocky soils, clay, or fine silt. Particle hardness and shape (jagged or rounded) are important. The degree of compaction determines the amount of pressure that forces abrasive soil particles against a surface that is being dragged through the mixture. In addition, moisture content is important because it determines, in part, the temperature, lubricity, and cohesiveness of the mixture. Many ores, slags, and similar bulk materials are extremely abrasive to equipment for handling and moving materials; geological analysis can define the abrasive character of these aggregates. Practices in Failure Analysis Formulating Conclusions and Report Writing At a certain stage in every investigation, the evidence revealed by the examinations and tests is analyzed and premises are formulated. Obviously, many investigations will not become a series of clear-cut stages. If the probable cause of failure is apparent early in the examination, the pattern and extent of subsequent investigation will be directed toward confirmation of the probable cause and the elimination of other possibilities. Other investigations will follow a logical series of stages, as outlined in this article, and the findings at each stage will determine the manner in which the investigation proceeds. As new facts modify first impressions, different hypotheses of failure will develop and will be retained or abandoned as dictated by the findings. Where extensive laboratory facilities are available to the investigator, maximum effort will be devoted to amassing the results of mechanical tests, chemical analysis, fractography, and microscopy before the formulation of preliminary conclusions is attempted. Finally, in those investigations in which the cause of failure is particularly elusive, a search through reports of similar instances may be required to suggest possible clues. Some of the work performed during the course of an investigation may be thought to be unnecessary. It is important, however, to distinguish between work that is unnecessary and that which does not bear fruitful results. During an examination, it is to be expected that some of the work done will not assist directly in determining the cause of failure; nevertheless, negative evidence may be helpful in excluding some potential causes of failure from consideration. Sometimes the evidence that excludes possible failure causes may be stronger than the available evidence supporting the probable cause for a failure. On the other hand, any tendency to curtail work essential to an investigation should be guarded against. In some instances, it is possible to form an opinion regarding the cause of failure from a single aspect of the investigation, such as visual examination of a fracture surface or examination of a single metallographic specimen. However, before final conclusions are reached, supplementary data confirming the original opinion, if available, should be sought. Total dependence on the conclusions that can be drawn from a single specimen, such as a metallographic section, may be readily challenged unless a history of similar failures can be drawn upon. Yet, circumstances sometimes dictate that a conclusion be formulated on limited data. This is especially true in forensic investigations because of the usually limited extent of the problem and the need to preserve as much evidence as possible. Also, of course, economic limitations are always present. It is rarely justified for the failure analyst to conduct every type of test and investigation possible. The following checklist, which is in the form of a series of questions, has been proposed as an aid in analyzing the evidence derived from examinations and tests and in formulating conclusions. The questions are also helpful in calling attention to details of the overall investigation that may have been overlooked: · Has the failure sequence been established? · If the failure involved cracking or fracture, have the initiation sites been determined? · Did cracks initiate at the surface or below the surface? · Was cracking associated with a stress concentration? · How long was the crack present? · What was the level or magnitude of the loads? · What was the type of loading: static, cyclic, or intermittent? · Is the fracture surface consistent with the type of loading assumed in the design? · What was the failure mechanism?