Fundamentals of wear failures Raymond G. Bayer, Tribology Consultant Introduction WEAR is a persistent service condition in many engineering applications with important economic and technical consequences In terms of economics, the cost of abrasion wear has been estimated as ranging from 1 to 4% of the gross national product of an industrialized nation. The effect of abrasion is particularly evident in the industrial areas of agriculture, mining, mineral processing, and earth moving. Likewise, wear is a critical concern in many types of machine components; in fact, it is often a major factor in defining or limiting the suitable lifetime of a component. An important example is the wear of dies and molds Wear generally is manifested by a change in appearance and profile of a surface. Some examples illustrating these types of changes are shown in Fig. 1. Wear results from contact between a surface and a body or substance that is moving relative to it. Wear is progressive in that it increases with usage or increasing amounts of motion, and it ultimately results in the loss of material from a surface or the transfer of material between surfaces. Wear failures occur because of the sensitivity of a material or system to the surface changes caused by wear.Typically, it is the geometrical or profile aspects of these changes, such as a dimensional change, a change in shape, or residual thickness of a coating, that cause failure. However, a change in appearance and the nature of the wear damage also can be causes for failure. An example of the former would be situations where marring is a concern, such as with optical scanner windows, lens, and decorative finishes. Examples of the latter include valves, which can fail because of galling, and structural components, where cracks caused by wear can reduce fatigue life(Ref 1, 2). In addition to these differences, the same amount or degree of wear may or may not cause a wear failure; it is a function of the application. For example, dimensional changes in the range of several centimeters may not cause wear failure on excavator bucket teeth, but wear of a few micrometers might cause failure in some electromechanical devices. As a consequence of these differences there is no universal wear condition that can be used to define failure. The specific nature of the failure condition generally is an important factor in resolving or avoiding wear failures. It can affect not only the solutions to a wear problem but also the details of the approaches used to obtain a solution. While this is the case, there are some general considerations and approaches that can be of use in resolving or avoiding wear problems Thefileisdownloadedfromwww.bzfxw.com
Fundamentals of Wear Failures Raymond G. Bayer, Tribology Consultant Introduction WEAR is a persistent service condition in many engineering applications with important economic and technical consequences. In terms of economics, the cost of abrasion wear has been estimated as ranging from 1 to 4% of the gross national product of an industrialized nation. The effect of abrasion is particularly evident in the industrial areas of agriculture, mining, mineral processing, and earth moving. Likewise, wear is a critical concern in many types of machine components; in fact, it is often a major factor in defining or limiting the suitable lifetime of a component. An important example is the wear of dies and molds. Wear generally is manifested by a change in appearance and profile of a surface. Some examples illustrating these types of changes are shown in Fig. 1. Wear results from contact between a surface and a body or substance that is moving relative to it. Wear is progressive in that it increases with usage or increasing amounts of motion, and it ultimately results in the loss of material from a surface or the transfer of material between surfaces. Wear failures occur because of the sensitivity of a material or system to the surface changes caused by wear. Typically, it is the geometrical or profile aspects of these changes, such as a dimensional change, a change in shape, or residual thickness of a coating, that cause failure. However, a change in appearance and the nature of the wear damage also can be causes for failure. An example of the former would be situations where marring is a concern, such as with optical scanner windows, lens, and decorative finishes. Examples of the latter include valves, which can fail because of galling, and structural components, where cracks caused by wear can reduce fatigue life (Ref 1, 2). In addition to these differences, the same amount or degree of wear may or may not cause a wear failure; it is a function of the application. For example, dimensional changes in the range of several centimeters may not cause wear failure on excavator bucket teeth, but wear of a few micrometers might cause failure in some electromechanical devices. As a consequence of these differences, there is no universal wear condition that can be used to define failure. The specific nature of the failure condition generally is an important factor in resolving or avoiding wear failures. It can affect not only the solutions to a wear problem but also the details of the approaches used to obtain a solution. While this is the case, there are some general considerations and approaches that can be of use in resolving or avoiding wear problems. The file is downloaded from www.bzfxw.com
Fig 1 Examples of wear showing loss of material, changes in dimension, and changes in appearance .(a) Erosion damage on a butterfly valve component (b) Fretting damage on a friction band.(c) sliding wear on a cam follower The types of activities generally required for the resolutions of wear problems are Examining and characterizing the tribosystem Characterizing and modeling the wear process Obtaining and evaluating wear data Evaluating and verifying the solution While these activities roughly follow the sequence in the list, they generally are interwoven, and the overall approach is somewhat iterative in practice. For example, some modeling considerations might influence the details of the examination of the tribosystem or failure during the verification can lead to additional tribosystem examination and modeling. Brief descriptions of the need for and the nature of these types of activities are presented in the following sections. More detailed treatment of these activities can be found in Ref 3 and 4 References cited in this section 1. K. Budinski, Incipient Galling of Metals, Proc. Intl. Conf. On Wear of Materials, ASME, 1981, p 171- 185 2. C. Lutynski, G. Simansky, and A.J. McEvily, Fretting Fatigue of Ti-6Al-4V Alloy, Materials Evaluation Under Fretting ConditionS, STP 780, ASTM, 1982, p 150-1 3. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994 4. R.G. Bayer, Wear Analysis for Engineers, HNB Publishing, New York, 2002
Fig. 1 Examples of wear showing loss of material, changes in dimension, and changes in appearance. (a) Erosion damage on a butterfly valve component. (b) Fretting damage on a friction band. (c) Sliding wear on a cam follower The types of activities generally required for the resolutions of wear problems are: · Examining and characterizing the tribosystem · Characterizing and modeling the wear process · Obtaining and evaluating wear data · Evaluating and verifying the solution While these activities roughly follow the sequence in the list, they generally are interwoven, and the overall approach is somewhat iterative in practice. For example, some modeling considerations might influence the details of the examination of the tribosystem, or failure during the verification can lead to additional tribosystem examination and modeling. Brief descriptions of the need for and the nature of these types of activities are presented in the following sections. More detailed treatment of these activities can be found in Ref 3 and 4. References cited in this section 1. K. Budinski, Incipient Galling of Metals, Proc. Intl. Conf. On Wear of Materials, ASME, 1981, p 171– 185 2. C. Lutynski, G. Simansky, and A.J. McEvily, Fretting Fatigue of Ti-6Al-4V Alloy, Materials Evaluation Under Fretting Conditions, STP 780, ASTM, 1982, p 150–164 3. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994 4. R.G. Bayer, Wear Analysis for Engineers, HNB Publishing, New York, 2002
Fundamentals of wear Failures Raymond G. Bayer, Tribology Consultant Examination and Characterization of the Tribosystem Wear is a system teristic or phenomenon; it is not a materials property, Materials wear differently in different wear sit and different materials wear differently in the same situation. Therefore,it necessary to examine and characterize a number of different parameters, not simply the worn part. A tribosystem consists of all those elements that influence the wear process. The basic elements of a tribosystem Contacting material Geometrical parameters(shape, size, roughness) Relative motion · Type of lubrication Environment This tribosystem concept can be extended to include those elements or factors that affect the fundamental ones listed. In practice, it generally is appropriate to think of the tribosystem as at least extending to the mechanism device in which the wear occurs. The tribological aspect number (TAN) is a method for characterizing ibosystems(Ref 5). This system is useful in evaluating the elevance o of data and determining the most appropriate simulation test. The wear situation is described in terms of the contact velocity, contact area, contact pressure, and entry angle The purpose of the examination and characterization is to be able to define the tribosystem at the point of contact or wear site. It is necessary to define to some degree all of the basic parameters for that contact situation. The degree that is necessary generally is not the same for all the basic parameters. It depends on a number of factors. Fundamentally, it depends on the potential sensitivity of the wear to the various parameters in the particular service environment. It is influenced also by the detail that is needed to obtain a solution and the type of solution that is acceptable. For some engineering situations, a very crude description might be sufficient, such as describing the tribosystem as being a lightly loaded lubricated contact at low sliding speed in an ambient room environment. However, greater detail is always desirable and generally necessary Typically, magnitudes of most parameters, material identification, and details about the nature of the contact geometry, loading, and relative motion, are necessary The examination of the tribosystem should include also the inspection and measurement of the wear scars. The shape, morphology, and location of the wear scars provide important information generally needed to characterize the tribosystem and the wear process. Quantifying the amount of wear, particularly in terms of depth, generally is useful as well. The magnitude of the wear can support the characterization of the wear behavior and aid in the identification of a solution when used in conjunction with various models and analytical relationships. a generally good practice in examining wear scars is to examine them using several different methods, such as visual, low-power optical, and scanning electron microscopy (SEM). In many situations magnifications between 30 and a few hundred are most useful In addition to these methods for examining wear scars, a variety of other methods are often used. These procedures can and often do include methods to characterize materials, measure dimensions and surface roughnesses, determine loads, determine contact stresses. and determine environmental conditions. Some of these techniques are discussed in other articles in this Volume, as well as in Ref 3, 4, 6 In general, the amount of wear or root that results in the failure should be identified and defined. a criterion for acceptable wear also should be identified. Both pieces of information generally are important in developing an economical and practical solution. These factors should be determined as part of the examination and characterization process Thefileisdownloadedfromwww.bzfxw.com
Fundamentals of Wear Failures Raymond G. Bayer, Tribology Consultant Examination and Characterization of the Tribosystem Wear is a system characteristic or phenomenon; it is not a materials property. Materials wear differently in different wear situations, and different materials wear differently in the same situation. Therefore, it is necessary to examine and characterize a number of different parameters, not simply the worn part. A tribosystem consists of all those elements that influence the wear process. The basic elements of a tribosystem are: · Contacting materials · Geometrical parameters (shape, size, roughness) · Relative motion · Loading · Type of lubrication · Environment This tribosystem concept can be extended to include those elements or factors that affect the fundamental ones listed. In practice, it generally is appropriate to think of the tribosystem as at least extending to the mechanism or device in which the wear occurs. The tribological aspect number (TAN) is a method for characterizing tribosystems (Ref 5). This system is useful in evaluating the relevance of data and determining the most appropriate simulation test. The wear situation is described in terms of the contact velocity, contact area, contact pressure, and entry angle. The purpose of the examination and characterization is to be able to define the tribosystem at the point of contact or wear site. It is necessary to define to some degree all of the basic parameters for that contact situation. The degree that is necessary generally is not the same for all the basic parameters. It depends on a number of factors. Fundamentally, it depends on the potential sensitivity of the wear to the various parameters in the particular service environment. It is influenced also by the detail that is needed to obtain a solution and the type of solution that is acceptable. For some engineering situations, a very crude description might be sufficient, such as describing the tribosystem as being a lightly loaded, lubricated contact at low sliding speed in an ambient room environment. However, greater detail is always desirable and generally necessary. Typically, magnitudes of most parameters, material identification, and details about the nature of the contact geometry, loading, and relative motion, are necessary. The examination of the tribosystem should include also the inspection and measurement of the wear scars. The shape, morphology, and location of the wear scars provide important information generally needed to characterize the tribosystem and the wear process. Quantifying the amount of wear, particularly in terms of depth, generally is useful as well. The magnitude of the wear can support the characterization of the wear behavior and aid in the identification of a solution when used in conjunction with various models and analytical relationships. A generally good practice in examining wear scars is to examine them using several different methods, such as visual, low-power optical, and scanning electron microscopy (SEM). In many situations, magnifications between 30 and a few hundred are most useful. In addition to these methods for examining wear scars, a variety of other methods are often used. These procedures can and often do include methods to characterize materials, measure dimensions and surface roughnesses, determine loads, determine contact stresses, and determine environmental conditions. Some of these techniques are discussed in other articles in this Volume, as well as in Ref 3, 4, 6. In general, the amount of wear or root cause that results in the failure should be identified and defined. A criterion for acceptable wear also should be identified. Both pieces of information generally are important in developing an economical and practical solution. These factors should be determined as part of the examination and characterization process. The file is downloaded from www.bzfxw.com
References cited in this section 3. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994 R.G. Bayer, Wear Analysis for Engineers, HNB Publishing, New York, 2002 5. R. M. Voitik, Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers, Tribology: Wear Test Selection for Design and Application, STP 1199, A W. Ruff and R.G. Bayer, Ed, ASTM, 1993 6. P.J. Blau, Ed, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook, ASM International. 1992 Fundamentals of wear Failures Raymond G. Bayer, Tribology Consultant Characterization and Modeling of the wear Situation Wear behavior is different in different service environments. The significance of factors and parameters tends to change with the type of wear situation. Significant wear phenomena also tend to be different. As consequence, some form of characterization of the wear process is not only helpful but also generally needed to resolve or avoid future wear failures. The characterization enables the sorting and identification of appropriate information on wear behavior, model development, and selection of wear data. The most useful method of characterization is to classify the situation in terms of broad types of wear and then refine these in terms of specific operational features. The term abrasive wear is used to describe situations where the principal cause of the wear is scratching or cutting by abrasive particles. The term nonabrasive wear is used to describe all other wear situations involving contact between two solid bodies(for example, sliding). The term erosion is used where the wear is caused by a fluid, a stream of particles, or bubbles(in the case of cavitation), not by contact between two solid bodies. The operational classification for nonabrasive wear situations(Table 1)is directly based on the characterization of the elements of the tribosystem at the contact. The nominal type of motion, that is rolling, sliding and impact, and in the case of sliding, lubricated or nonlubricated wear, often are considered as major subcategories of nonabrasive wear situations. Some subcategories for abrasive wear and erosion tuations, and operational elements that are often significant, are given in Tables 2 and 3 Table 1 Operational attributes of nonabrasive wear ttribute Variations Motion Rolling With sli Without sli Impact With sli Without slip Sliding Unidirectional or reciprocator High Fretting or gross sliding Lubrication Type of lubricant Lubricant Heavy or light Constant or variable
References cited in this section 3. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994 4. R.G. Bayer, Wear Analysis for Engineers, HNB Publishing, New York, 2002 5. R.M. Voitik, Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers, Tribology: Wear Test Selection for Design and Application, STP 1199, A.W. Ruff and R.G. Bayer, Ed., ASTM, 1993 6. P.J. Blau, Ed., Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook, ASM International, 1992 Fundamentals of Wear Failures Raymond G. Bayer, Tribology Consultant Characterization and Modeling of the Wear Situation Wear behavior is different in different service environments. The significance of factors and parameters tends to change with the type of wear situation. Significant wear phenomena also tend to be different. As a consequence, some form of characterization of the wear process is not only helpful but also generally needed to resolve or avoid future wear failures. The characterization enables the sorting and identification of appropriate information on wear behavior, model development, and selection of wear data. The most useful method of characterization is to classify the situation in terms of broad types of wear and then refine these in terms of specific operational features. The term abrasive wear is used to describe situations where the principal cause of the wear is scratching or cutting by abrasive particles. The term nonabrasive wear is used to describe all other wear situations involving contact between two solid bodies (for example, sliding). The term erosion is used where the wear is caused by a fluid, a stream of particles, or bubbles (in the case of cavitation), not by contact between two solid bodies. The operational classification for nonabrasive wear situations (Table 1) is directly based on the characterization of the elements of the tribosystem at the contact. The nominal type of motion, that is rolling, sliding and impact, and in the case of sliding, lubricated or nonlubricated wear, often are considered as major subcategories of nonabrasive wear situations. Some subcategories for abrasive wear and erosion situations, and operational elements that are often significant, are given in Tables 2 and 3. Table 1 Operational attributes of nonabrasive wear Attribute Variations Motion Rolling With slip Without slip Impact With slip Without slip Unidirectional or reciprocating High speed or low speed Sliding Fretting or gross sliding Lubricated or not Type of lubricant Lubrication Lubricant Load Heavy or light Constant or variable
Impact or not Contact geometry point, line, or area Cont Contact stress Above or below yield Environment Hostile or nonhostile High temperature or low temperature With or without abrasive particles pH level Materials Type-to-type or dissimilar Table 2 Subcategories of abrasive wear Attribute Variations Number of bodies One-body (abrasive surface) or two-body (loose particles between I High stress or low stress Surface alteration Scratching, polishing, or Presence of fluid Dry abrasion or slurry abrasion(particles in fluid) Relative hardness of particles to Surface harder or softer than particles surface Table 3 Subcategories of erosion TY Variables Particle erosion Grazing or high angle of incidence I High or low temperature Cavitation erosion grazing or high angle of incidence I High or low temperature Corrosive or noncorrosive fluid Slurry erosion Grazing or high angle of incidence H Corrosive or noncorrosive fluid Droplet erosion Grazing or high angle of incidence High or low temperature Corrosive or noncorrosive fluid Jet erosion le of incidence High or low temperature Corrosive or noncorrosive fluid It often is desirable and helpful to augment this type of classification with a characterization of the principal wear mechanism or mechanisms involved. Generally, for engineering purposes, characterization in terms of broad generic types of wear mechanisms is adequate, and often, the most useful. Identification and characterization in terms of specific mechanisms generally are not as useful or needed since they can change with materials and other tribosystem parameters. Wear mechanisms generally can be grouped into six generic Adhesive mechanism Single-cycle deformation mechanisms Repeated-cycle deformation mechanisms Chemical mechanisms Thermal mechanisms Tribofilm mechanisms Thefileisdownloadedfromwww.bzfxw.com
Impact or not Contact geometry Point, line, or area Conforming or nonconforming Contact stress Above or below yield Hostile or nonhostile High temperature or low temperature With or without abrasive particles Environment pH level Materials Type-to-type or dissimilar Table 2 Subcategories of abrasive wear Attribute Variations Number of bodies One-body (abrasive surface) or two-body (loose particles between surfaces) Stress level High stress or low stress Surface alteration Scratching, polishing, or gouging Presence of fluid Dry abrasion or slurry abrasion (particles in fluid) Relative hardness of particles to surface Surface harder or softer than particles Table 3 Subcategories of erosion Type Variables Particle erosion Grazing or high angle of incidence High or low temperature Grazing or high angle of incidence High or low temperature Cavitation erosion Corrosive or noncorrosive fluid Grazing or high angle of incidence High or low temperature Slurry erosion Corrosive or noncorrosive fluid Grazing or high angle of incidence High or low temperature Droplet erosion Corrosive or noncorrosive fluid Grazing or high angle of incidence High or low temperature Jet erosion Corrosive or noncorrosive fluid It often is desirable and helpful to augment this type of classification with a characterization of the principal wear mechanism or mechanisms involved. Generally, for engineering purposes, characterization in terms of broad generic types of wear mechanisms is adequate, and often, the most useful. Identification and characterization in terms of specific mechanisms generally are not as useful or needed since they can change with materials and other tribosystem parameters. Wear mechanisms generally can be grouped into six generic types: · Adhesive mechanisms · Single-cycle deformation mechanisms · Repeated-cycle deformation mechanisms · Chemical mechanisms · Thermal mechanisms · Tribofilm mechanisms The file is downloaded from www.bzfxw.com
More than one type of mechanism can be involved in a wear situation. Also, these individual mechanisms can interact sequentially to form a more complex wear process. However, one mechanism generally is the controlling and primary mechanism. The relative importance or occurrence of individual mechanisms can change with changes in tribosystem parameters. Therefore, materials can exhibit transitions in wear behavior as a result of changes in other operational parameters, such as load, velocity, and friction(Ref 7). The wear map for unlubricated sliding between steels(Fig. 2)illustrates such transitions(Ref 8) Sliding velocity, v(m/s) Seizure 2103Me104 10-3(10-5) Wear 1010-4(106 106 10-5(10 Mild oxidational wear Martensite 10-6(10-8 formation Severe 103 n wea oxidational wear 10-9 10-5(10 Mild to severe 10 10-9 108 105 wear 10-9 10 03 Normalized velocity, v Fig. 2 Wear-mechanism map for unlubricated sliding of a steel couple. The normalized pressure is the contact pressure divided by hardness. The normalized velocity is the velocity multiplied by the ratio of the radius of the contact to the thermal diffusivity. The contour lines are lines of constant normalized wear rate. Pin-on-disk configuration Source: Ref 8 Except for adhesive and tribofilm mechanisms, these generic types are possible with all types of relative motion involving one or two solid surfaces. Adhesive and tribofilm mechanisms generally are limited to sliding contact between two solid surfaces. Adhesive wear mechanisms are those involving adhesion and transfer of material Single-cycle deformation mechanisms are mechanical processes, which occur as a result of a single engagement, such as plastic deformation or cutting. Repeated cyclic deformation mechanisms are mechanical processes, which require repeated engagements, such as fatigue or ratcheting. Chemical or oxidative mechanisms are wear processes in which the rate-controlling parameters are those associated with a chemical reaction at the surface, such as oxidation or corrosion. Similarly, thermal mechanisms are wear processes where the rate-controlling mechanism is associated with temperature. Tribofilm mechanisms are those that involve the formation of layers of wear debris on and between surfaces and the loss of material from these layers. Tribofilm mechanisms tend to be significant in unlubricated sliding situations, particularly between polymers and metals
More than one type of mechanism can be involved in a wear situation. Also, these individual mechanisms can interact sequentially to form a more complex wear process. However, one mechanism generally is the controlling and primary mechanism. The relative importance or occurrence of individual mechanisms can change with changes in tribosystem parameters. Therefore, materials can exhibit transitions in wear behavior as a result of changes in other operational parameters, such as load, velocity, and friction (Ref 7). The wear map for unlubricated sliding between steels (Fig. 2) illustrates such transitions (Ref 8). Fig. 2 Wear-mechanism map for unlubricated sliding of a steel couple. The normalized pressure is the contact pressure divided by hardness. The normalized velocity is the velocity multiplied by the ratio of the radius of the contact to the thermal diffusivity. The contour lines are lines of constant normalized wear rate. Pin-on-disk configuration. Source: Ref 8. Except for adhesive and tribofilm mechanisms, these generic types are possible with all types of relative motion involving one or two solid surfaces. Adhesive and tribofilm mechanisms generally are limited to sliding contact between two solid surfaces. Adhesive wear mechanisms are those involving adhesion and transfer of material. Single-cycle deformation mechanisms are mechanical processes, which occur as a result of a single engagement, such as plastic deformation or cutting. Repeated cyclic deformation mechanisms are mechanical processes, which require repeated engagements, such as fatigue or ratcheting. Chemical or oxidative mechanisms are wear processes in which the rate-controlling parameters are those associated with a chemical reaction at the surface, such as oxidation or corrosion. Similarly, thermal mechanisms are wear processes where the rate-controlling mechanism is associated with temperature. Tribofilm mechanisms are those that involve the formation of layers of wear debris on and between surfaces and the loss of material from these layers. Tribofilm mechanisms tend to be significant in unlubricated sliding situations, particularly between polymers and metals
Wear also can be classified in relative terms as mild and severe. This classification is based on the nature of the wear, not the amount. The differentiation is primarily in terms of the features of the wear scars and secondarily by wear rate. Coarse features and high wear rates are characteristics of severe wear. Examples of mild and severe sliding wear are shown in Fig 3. Most types of wear mechanisms have mild and severe forms, and most materials can exhibit both types of behavior. Transitions between these two states are often sharp and can be parameters of the tribosystem; speed, temperature, load, and lubrication are the most common. Severe wear behavior usually cannot be tolerated. Consequently, if the wear observed is severe, the minimum corrective action required to solve the wear problem is to make changes(by selecting more wear- resistant materials or by modifying other parameters, for example, shape and load)to achieve mild wear behavior. Because of the typical large difference in wear rates between these states, many times such changes are adequate, particularly in applications that are less sensitive to wear. 额姿 到“yA (b) Fig 3 Examples of mild and severe wear morphology(a)A lubricated sliding wear scar on steel in the mild wear regime.(b)The appearance of the same type of scar in the severe wear regime Generally, by characterizing the wear situation in these manners, there is sufficient information to formulate a model and to assess the significance of various parameters, including materials, which can be changed to resolve the wear problem Modeling is an essential element in resolving wear failures. The characterizations of the tribosystem and the wear situation provide the basis for models. Simple phenomenological models sometimes are adequate However, more complex models, using analytical relationships for wear behavior, always are superior and sometimes necessary. Through the modeling activity, reasons for the failure and measures needed to resolve the wear problem both are identified. Models are particularly useful when they involve analytical relations to identify the significance of parameters and to assess the significance of proposed changes. The incorporation of analytical relationships generally facilitates the identification of causes and minimizes the amount of testing associated with obtaining a solution An effective model for a wear situation may be as simple as concluding that, because of too much clearance fretting occurred. Based on this model, two solutions can be proposed. One solution is to select materials that are more resistant to fretting wear. The second is to change tolerances to reduce the clearance. This method is an example of a simple phenomenological model. A more elaborate model would be describing the wear situation as unlubricated mild sliding, impact, or rolling wear, and using analytical models for these wear situations to develop relationships between wear and design parameters. These relationships would then be used to evaluate the effects of changing different design parameters to achieve adequate wear behavior. Case studies which further illustrate this type of modeling and the use of these models, can be found in Ref 3 and 4 Thefileisdownloadedfromwww.bzfxw.com
Wear also can be classified in relative terms as mild and severe. This classification is based on the nature of the wear, not the amount. The differentiation is primarily in terms of the features of the wear scars and secondarily by wear rate. Coarse features and high wear rates are characteristics of severe wear. Examples of mild and severe sliding wear are shown in Fig. 3. Most types of wear mechanisms have mild and severe forms, and most materials can exhibit both types of behavior. Transitions between these two states are often sharp and can be associated with most parameters of the tribosystem; speed, temperature, load, and lubrication are the most common. Severe wear behavior usually cannot be tolerated. Consequently, if the wear observed is severe, the minimum corrective action required to solve the wear problem is to make changes (by selecting more wearresistant materials or by modifying other parameters, for example, shape and load) to achieve mild wear behavior. Because of the typical large difference in wear rates between these states, many times such changes are adequate, particularly in applications that are less sensitive to wear. Fig. 3 Examples of mild and severe wear morphology. (a) A lubricated sliding wear scar on steel in the mild wear regime. (b) The appearance of the same type of scar in the severe wear regime Generally, by characterizing the wear situation in these manners, there is sufficient information to formulate a model and to assess the significance of various parameters, including materials, which can be changed to resolve the wear problem. Modeling is an essential element in resolving wear failures. The characterizations of the tribosystem and the wear situation provide the basis for models. Simple phenomenological models sometimes are adequate. However, more complex models, using analytical relationships for wear behavior, always are superior and sometimes necessary. Through the modeling activity, reasons for the failure and measures needed to resolve the wear problem both are identified. Models are particularly useful when they involve analytical relations to identify the significance of parameters and to assess the significance of proposed changes. The incorporation of analytical relationships generally facilitates the identification of causes and minimizes the amount of testing associated with obtaining a solution. An effective model for a wear situation may be as simple as concluding that, because of too much clearance, fretting occurred. Based on this model, two solutions can be proposed. One solution is to select materials that are more resistant to fretting wear. The second is to change tolerances to reduce the clearance. This method is an example of a simple phenomenological model. A more elaborate model would be describing the wear situation as unlubricated mild sliding, impact, or rolling wear, and using analytical models for these wear situations to develop relationships between wear and design parameters. These relationships would then be used to evaluate the effects of changing different design parameters to achieve adequate wear behavior. Case studies, which further illustrate this type of modeling and the use of these models, can be found in Ref 3 and 4. The file is downloaded from www.bzfxw.com
References cited in this section 3. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994 R.G. Bayer, Wear Analysis for Engineers, HNB Publishing, New York, 2002 7. P.J. Blau, Friction and Wear Transitions of Materials, Noyes Publications, Park Ridge, NJ, 1989 8. S.C. Lim and M F. Ashby, Acta Metall., Vol 35 (No. 1), 1987, p 1-24 Fundamentals of wear Failures Raymond G. Bayer, Tribology Consultant Obtaining and Evaluating Wear Data No matter which type of model is developed, the resolution of the problem involves the use of material wear data. For phenomenological models, this is generally in the form of rankings or relative wear performance for different materials obtained from tests or prior experience. For models involving analytical relationships, this generally is in the form of values for empirical wear coefficients, associated with the underlying wear equations. All equations proposed for wear have such coefficients(Ref 9). Because of the wide range of wear behavior possible and the system nature of wear, the applicability of any data generally depends on how closely the conditions of the test used to obtain the data simulate those of the wear situation(Ref 5, 10). If the test produces a poor simulation, then the data are less applicable. This concept applies to wear data obtained from wear tests done for a specific situation, from material suppliers, or from the technical literature. Generally, the best simulation and, hence, the more relevant data are obtainable with wear tests done for the specific situation However, this is usually the least feasible in many engineering environments. On the other hand, vendor and technical literature data generally are based on tests that provide less simulation, and their relevance needs to be assessed in terms of the simulation that they provide. Some form of extrapolation is generally involved with the use of such data. Often it is the trends in such data that are important. the key to the simulation is to ensure that the appropriate wear mechanisms occur in both the test and the application a testing approach that may be used in conjunction with a phenomenological model is testing that simulates the wear situation and establishes a correlation. This approach is most often used for materials solutions to wear problems, but it can be used for other parameters, such as roughness, load, or lubrication. Such a test can then be used for rankings. With the use of a reference or central material, an estimate of the improvement in the wear situation can be obtained(Ref 9 The usefulness, methodology, and general types of laboratory, bench, and component tests are summarized in Ref 11 References cited in this section 5. R. M. Voitik, Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers, Tribology: Wear Test Selection for Design and Application, STP 1199, A W. Ruff and R.G. Bayer, Ed, ASTM, 1993 9. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994, Section C2 10. A W. Ruff and R.G. Bayer, Ed, Tribology: Wear Test Selection for Design and Application, STP 1199, ASTM. 1993 11. M. Anderson and F.E. Schmidt, Jr, Wear and Lubricant Testing, Chapter 25, ASTM Manual on Fuels, Lubricants, and Standards: Application and Interpretation, ASTM, 2002
References cited in this section 3. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994 4. R.G. Bayer, Wear Analysis for Engineers, HNB Publishing, New York, 2002 7. P.J. Blau, Friction and Wear Transitions of Materials, Noyes Publications, Park Ridge, NJ, 1989 8. S.C. Lim and M.F. Ashby, Acta Metall., Vol 35 (No. 1), 1987, p 1–24 Fundamentals of Wear Failures Raymond G. Bayer, Tribology Consultant Obtaining and Evaluating Wear Data No matter which type of model is developed, the resolution of the problem involves the use of material wear data. For phenomenological models, this is generally in the form of rankings or relative wear performance for different materials obtained from tests or prior experience. For models involving analytical relationships, this generally is in the form of values for empirical wear coefficients, associated with the underlying wear equations. All equations proposed for wear have such coefficients (Ref 9). Because of the wide range of wear behavior possible and the system nature of wear, the applicability of any data generally depends on how closely the conditions of the test used to obtain the data simulate those of the wear situation (Ref 5, 10). If the test produces a poor simulation, then the data are less applicable. This concept applies to wear data obtained from wear tests done for a specific situation, from material suppliers, or from the technical literature. Generally, the best simulation and, hence, the more relevant data are obtainable with wear tests done for the specific situation. However, this is usually the least feasible in many engineering environments. On the other hand, vendor and technical literature data generally are based on tests that provide less simulation, and their relevance needs to be assessed in terms of the simulation that they provide. Some form of extrapolation is generally involved with the use of such data. Often it is the trends in such data that are important. The key to the simulation is to ensure that the appropriate wear mechanisms occur in both the test and the application. A testing approach that may be used in conjunction with a phenomenological model is testing that simulates the wear situation and establishes a correlation. This approach is most often used for materials solutions to wear problems, but it can be used for other parameters, such as roughness, load, or lubrication. Such a test can then be used for rankings. With the use of a reference or central material, an estimate of the improvement in the wear situation can be obtained (Ref 9). The usefulness, methodology, and general types of laboratory, bench, and component tests are summarized in Ref 11. References cited in this section 5. R.M. Voitik, Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers, Tribology: Wear Test Selection for Design and Application, STP 1199, A.W. Ruff and R.G. Bayer, Ed., ASTM, 1993 9. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994, Section C2 10. A.W. Ruff and R.G. Bayer, Ed., Tribology: Wear Test Selection for Design and Application, STP 1199, ASTM, 1993 11. M. Anderson and F.E. Schmidt, Jr., Wear and Lubricant Testing, Chapter 25, ASTM Manual on Fuels, Lubricants, and Standards: Application and Interpretation, ASTM, 2002
Fundamentals of wear Failures Raymond G. Bayer, Tribology Consultant Evaluation and verification of solutions The development of a model and possible solutions to resolve or avoid a wear problem typically involves making assumptions, interpolating and extrapolating data, and often working with incomplete information about the tribosystem or conditions of operation. Because of the complex nature of wear behavior, it is important to provide some structure and control to the normal engineering practice of evaluating and verifying solutions and designs. Generally, this structure and control involve the characterization and control of hardware and operating conditions and identifying a procedure for monitoring wear. Ideally this procedure should involve the measurement of wear as a function of usage or exposure. This often facilitates, simplifies, and shortens the verification by allowing the determination of stable wear behavior and projecting improvement and life, as illustrated in Fig. 4. Frequently, the relationship between usage and a measure of wear often used in engineering, such as depth or width, is nonlinear; that is, doubling the amount of usage does not result in doubling the wear depth but results in something less than double. Because of the precision and variation typical of these measurements in engineering situations, it generally is adequate and preferable to select measurement intervals on a log or semi-log basis, such as decades or half decades. An example would be measurements after one, ten, one hundred five hundred, and a thousand hours of operation, rather than after one hundred. two hundred three hundred hours and so on 0.1 0.010 0.001 104 Number of cycles Fig 4 Example of a log-log plot of wear data used to identify stable wear behavior and projecting wear. a straight line in this type of plot is indicative of stable behavior. The characterization of the hardware and operating conditions serves two purposes. One, it insures that the conditions and materials are as they should be. The other purpose is that in the event that wear behavior is unsatisfactory, this information is needed for the continuation of the process to resolve the wear problem. All the tribosystem elements should be characterized. However, the principal parameters to characterize in most cases are material conditions, dimensions, roughness, and adjustments. If multiple tests can be done, or if different conditions can be included in a single test it is useful and desirable to sort the hardware into different ategories, particularly ones representing extremes, to determine the range of wear behavior that would be associated with these differences. This concept is particularly true when such differences are large and may be significant. The same approach should be used for operating conditions, when there are pronounced differences over the range of these parameters Thefileisdownloadedfromwww.bzfxw.com
Fundamentals of Wear Failures Raymond G. Bayer, Tribology Consultant Evaluation and Verification of Solutions The development of a model and possible solutions to resolve or avoid a wear problem typically involves making assumptions, interpolating and extrapolating data, and often working with incomplete information about the tribosystem or conditions of operation. Because of the complex nature of wear behavior, it is important to provide some structure and control to the normal engineering practice of evaluating and verifying solutions and designs. Generally, this structure and control involve the characterization and control of hardware and operating conditions and identifying a procedure for monitoring wear. Ideally this procedure should involve the measurement of wear as a function of usage or exposure. This often facilitates, simplifies, and shortens the verification by allowing the determination of stable wear behavior and projecting improvement and life, as illustrated in Fig. 4. Frequently, the relationship between usage and a measure of wear often used in engineering, such as depth or width, is nonlinear; that is, doubling the amount of usage does not result in doubling the wear depth but results in something less than double. Because of the precision and variation typical of these measurements in engineering situations, it generally is adequate and preferable to select measurement intervals on a log or semi-log basis, such as decades or half decades. An example would be measurements after one, ten, one hundred, five hundred, and a thousand hours of operation, rather than after one hundred, two hundred, three hundred hours, and so on. Fig. 4 Example of a log-log plot of wear data used to identify stable wear behavior and projecting wear. A straight line in this type of plot is indicative of stable behavior. The characterization of the hardware and operating conditions serves two purposes. One, it insures that the conditions and materials are as they should be. The other purpose is that in the event that wear behavior is unsatisfactory, this information is needed for the continuation of the process to resolve the wear problem. All the tribosystem elements should be characterized. However, the principal parameters to characterize in most cases are material conditions, dimensions, roughness, and adjustments. If multiple tests can be done, or if different conditions can be included in a single test, it is useful and desirable to sort the hardware into different categories, particularly ones representing extremes, to determine the range of wear behavior that would be associated with these differences. This concept is particularly true when such differences are large and may be significant. The same approach should be used for operating conditions, when there are pronounced differences over the range of these parameters. The file is downloaded from www.bzfxw.com
Fundamentals of wear Failures Raymond G. Bayer, Tribology Consultant Avoiding wear failures The ideal way of addressing a wear failure is by designing the mechanism initially to give adequate life(ref 3) This approach is sometimes referred to as wear design. This method is similar to that of resolving a wear failure and becomes identical to it once there are some testing results. With design, the initial step is not the examination of hardware, as is the case with a failure, but developing various scenarios about the operation of the device and what constitutes a wear failure. As a minimum, the initial design should follow good tribological design practices and be selected to ensure that severe wear is avoided. For some applications, this approach may be adequate For applications that are more sensitive to wear, analytical relationships for detailing the wear process may then be used to further refine and evaluate designs. Once hardware is built and some testing dor worn parts are available for examination, and the activities are identical to those that started with a wear failure It is essential to determine the root cause of the wear failure Wear failures and problems tend to be very individualistic in terms of wear behavior and what constitutes a failure. Because of this tendency, there is no one type of solution or model that can be used as the basis for a solution. There are, however, some common elements in approaches that are used to resolve or avoid wear problems, and these have been described. Combined, they provide a general methodology for approaching wear failures. A key aspect in resolving wear failures is to recognize that wear is a system property or characteristic nd not a materials property. While material changes are often involved in solutions to wear problems, this not the only way of resolving such problems. Often, changes in other parameters are adequate by themselves or required in conjunction with a material change. Very often a key to the resolution of a wear problem is by detailed examination of worn parts and studying the operation of the device. While not al ways necessary, the use of analytical relationships for analyzing wear failures and developing solutions is feasible, generally worthwhile, and recommended(Ref 4). Further information about techniques used to investigate wear failures, as well as information about failure modes and wear behavior in different situations, is presented in other articles of this section. Several of the references(Ref 3, 4, 5) provide additional information about wear behavior, testing, and the use of analytical relationships for wear that are useful in resolving and avoiding wear failures References cited in this section 3. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994 4. R.G. Bayer, Wear Analysis for Engineers, HNB Publishing, New York, 2002 5. R. M. Voitik, Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers, Tribology: Wear Test Selection for Design and Application, STP 1199, A W. Ruff and R G. Bayer, Ed, ASTM, 1993 Fundamentals of wear Failure Raymond G. Bayer, Tribology Consultant References 1. K. Budinski, Incipient Galling of Metals, Proc. Intl Conf On Wear of Materials, ASME, 1981, p 171
Fundamentals of Wear Failures Raymond G. Bayer, Tribology Consultant Avoiding Wear Failures The ideal way of addressing a wear failure is by designing the mechanism initially to give adequate life (Ref 3). This approach is sometimes referred to as wear design. This method is similar to that of resolving a wear failure and becomes identical to it once there are some testing results. With design, the initial step is not the examination of hardware, as is the case with a failure, but developing various scenarios about the operation of the device and what constitutes a wear failure. As a minimum, the initial design should follow good tribological design practices and be selected to ensure that severe wear is avoided. For some applications, this approach may be adequate. For applications that are more sensitive to wear, analytical relationships for detailing the wear process may then be used to further refine and evaluate designs. Once hardware is built and some testing done, worn parts are available for examination, and the activities are identical to those that started with a wear failure. It is essential to determine the root cause of the wear failure. Wear failures and problems tend to be very individualistic in terms of wear behavior and what constitutes a failure. Because of this tendency, there is no one type of solution or model that can be used as the basis for a solution. There are, however, some common elements in approaches that are used to resolve or avoid wear problems, and these have been described. Combined, they provide a general methodology for approaching wear failures. A key aspect in resolving wear failures is to recognize that wear is a system property or characteristic and not a materials property. While material changes are often involved in solutions to wear problems, this is not the only way of resolving such problems. Often, changes in other parameters are adequate by themselves or required in conjunction with a material change. Very often a key to the resolution of a wear problem is by detailed examination of worn parts and studying the operation of the device. While not always necessary, the use of analytical relationships for analyzing wear failures and developing solutions is feasible, generally worthwhile, and recommended (Ref 4). Further information about techniques used to investigate wear failures, as well as information about failure modes and wear behavior in different situations, is presented in other articles of this section. Several of the references (Ref 3, 4, 5) provide additional information about wear behavior, testing, and the use of analytical relationships for wear that are useful in resolving and avoiding wear failures. References cited in this section 3. R.G. Bayer, Mechanical Wear Prediction and Prevention, Marcel Dekker, 1994 4. R.G. Bayer, Wear Analysis for Engineers, HNB Publishing, New York, 2002 5. R.M. Voitik, Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers, Tribology: Wear Test Selection for Design and Application, STP 1199, A.W. Ruff and R.G. Bayer, Ed., ASTM, 1993 Fundamentals of Wear Failures Raymond G. Bayer, Tribology Consultant References 1. K. Budinski, Incipient Galling of Metals, Proc. Intl. Conf. On Wear of Materials, ASME, 1981, p 171– 185