《材料失效分析 Materials Failure Analysis》课程教学资源（参考书籍）Fundamentals of Wear Failures.pdf

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

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