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