Simulative Friction and Wear Testing

Transcription

1 14 Simulative Friction and Wear Testing Peter J. Blau Tribomaterials Investigative Systems 14.1 Introduction 14.2 Defining the Problem 14.3 Selecting a Scale of Simulation Levels of Tribosimulation 14.4 Defining Field-Compatible Metrics 14.5 Selecting or Constructing the Test Apparatus 14.6 Conducting Baseline Testing Using Established Metrics and Refining Metrics as Needed 14.7 Case Studies An Oil Pump Gear Set with Several Wear Modes Wear of Gravure Rollers on Doctor Blades Scoring of Spur Gears Wear of Plastic Parts in an Optical Disk Drive Wear of Rotary Slitter Knife Blades Erosive Wear of Piping 14.8 Conclusions 14.1 Introduction The selection of lubricants, materials, or surface treatments for friction and wear-critical applications often involves validation or screening tests before final decisions are made. Testing is particularly valuable during the development of new machines for which operating conditions are much different than existing designs. An example of the latter might be a new design that cannot use off-the-shelf bearings or gears because the temperatures are too high or the surrounding environments are too corrosive. The steps involved in developing tribosimulations of current or newly designed systems are, with only one significant exception, essentially the same. The exception is that for an existing friction of wear problem, there is prior experience in the response of the materials and lubricants to the operating conditions. In a new design, however, there may be no direct prior experience in the behavior of candidate materials, although there may be some relevant experience from machinery of a similar kind. There are two types of tribosimulations: computer simulations and physical simulations. The computer simulation uses a virtual mechanical assembly that consists of several components defined in terms of a set of properties, spatial relationships, and assumed rules of interaction. Known or estimated properties of the materials and/or lubricants are provided to the model, and the expected responses of the virtual tribosystem to such variables as load cycles, deflections, temperature excursions, etc., are calculated. Such programs have been prepared by both academic researchers and industry engineers for tribological components like bearings, face seals, brakes, and gears. Component designers have also developed sophisticated design tools for automotive valve trains, engines, and pumping systems as well. The second type of tribosimulation is the physical simulation. Here, materials and lubricants are screened in an apparatus that is intended to provide the essential operating characteristics of the intended

2 application. An additional use of virtual and physical simulations is to observe how a certain material or lubricant might react in a situation when the opportunity to observe the material or lubricant is not practical. For example, bearings for use in unmanned orbiting satellites might be observed in an ultrahigh vacuum, space-like environmental chamber created in the laboratory for that purpose. This article focuses on the development and use of physical tribosimulations. It describes the process involved in developing and validating laboratory bench-scale simulations for friction and wear-critical machine components. The steps in developing a physical tribosimulation are similar in some respects to those used to set up a computer simulation. They are: 1. Define the nature of the friction or wear problem to be investigated 2. Select a scale of simulation 3. Define field-compatible metrics to use to assess the results of the simulation 4. Select or build the apparatus (the model, in the case of the computer simulation) 5. Conduct baseline tests to establish the repeatability and characteristics of the method 6. Analyze baseline test results using the established metrics 7. Refine as needed to achieve an acceptable engineering confidence level 14.2 Defining the Problem Defining the nature of the friction or wear problem is critical before any test method can be selected, developed, or successfully employed. Rushing ahead to testing without proper analysis of the problem is akin to sitting down at a table to play cards without knowing what game is to be played and what its rules are. At the first level of problem definition, one answers the following questions: 1. What are the physical attributes of the tribosystem? 2. Is it an open or closed tribosystem? 3. What form or forms of friction or wear are likely to be involved? 4. What quantitative measures, if any, are there to describe the behavior of the materials in the situation of interest? 5. What materials or lubricants are currently under consideration for this application? 6. What time and resources are available to use to solve the current problem? By defining the physical limits of the tribosystem, we are forced to make the first judgment. At what distance from the tribological interface will the surroundings still affect the tribological function? For example, if we limit our focus to a pair of wearing gears in isolation from their surroundings, we might inadvertently neglect externally produced mechanical vibrations, sources of abrasive debris, or temperature excursions from surrounding components as potential contributions to the gears wear environment. Thus our simulation could omit critical, wear life-limiting elements which come from the surroundings, elements which might in fact be just as important to include in the simulation as the imposed load on the gear teeth or the speed of relative rotation. Closed tribosystems involve, for example, recirculating lubricants or contact environments which are contained within a certain portion of the machine. Open tribosystems are more complicated and difficult to simulate because materials of unforeseen origin might enter the area of contact from sources external to the machine and cause strong friction or wear responses. An example of an open tribosystem is the bucket teeth on construction equipment that must dig through all kinds of dirt, soil, and rock under wet and dry, hot and cold conditions. The forms of friction or wear must be defined early. It is common to observe more than one type of wear or mechanical surface damage in different locations, even on the same part. Therefore, the locations and wear types most damaging to the satisfactory operation of the component must be identified and prioritized. It is possible that more than one type of test will be required to ensure that the proposed materials or lubricants will respond suitably to all the critical contact conditions on various surfaces of the component. For example, the side of a pump gear may slide against its case while the teeth of the same gear may experience contact fatigue worsened by a small amount of tangential slip.

3 TABLE 14.1 Elements of a TriboSystem Analysis (TSA) Element Tribosystem open or closed? Macro contact and conformity Microcontact and surface finish Type of relative motion Speed of relative motion Contact load and pressure Temperature Contact environment and lubrication Third bodies Type(s) of wear Performance degradation Experience Metrics Description Is the system open to the environment, like the teeth of earthmoving equipment, or closed, like a sealed recirculation system? To what degree do the shapes of the contacting bodies conform to one another? How can the geometry be described: convex curve-on-convex curve, flat-on-flat, particles-on-flat, etc.? Is the contact open on the edges or closed so as to confine wear debris? Surface texture of the interacting bodies roughness, waviness, and lay. How long does the initial finish persist in service? Is the motion unidirectional, reciprocating, intermittent, or continuous? What is the characteristic constant distance (stroke length, etc.)? Fretting or long-distance sliding? Sliding velocity or impingement velocity, if particles are involved. How is the load distributed on the surfaces? What is the magnitude of the normal force? How does it vary? Does the temperature change during operation or remain constant? Is frictional heating an issue? What chemical environment does the contact area experience? Is there a lubricant? What lubricant regime is present (dry, starved, boundary, mixed, hydrodynamic)? What are the lubricant characteristics? Are there contaminants in the lubricant? Is the lubricant agitated so as to entrain air? Are there vibrations or other mechanical contributions to negative performance? Are there particles involved in the wear, and if so, what are their characteristics? Are particles generated by wear or externally to the contact area? Using surface analysis and microscopy, what is/are the dominant form(s) of wear? To solve the problem, the negative manner in which wear or friction affect the operation must be defined. What are the current materials and lubricants, and which others have been tried? What quantitative measures are used to described the wear or friction of the subject component? In existing applications, identifying the type of wear involves examining field-worn parts which have been protected from the environment after having been removed from the machine. Surfaces of worn parts which have stood unprotected for some time may be corroded and difficult to analyze. Other complications include removing surface deposits of degraded or heat-altered lubricants without destroying the most telling clues as to the dominant mode of wear damage. In new designs, the engineer is placed in a position of speculating what the environment of the tribosystem will be, and adjusting the simulation appropriately. Some guidance can, however, be obtained by analyzing existing systems with similar mechanical, thermal, and chemical aspects in the key areas of tribocontact. TriboSystem Analysis (TSA) [3] is a means to define the operating conditions of a system to be simulated. It involves a systematic analysis, aspect-by-aspect, of the operating environment of the subject component(s). Table 14.1 lists key elements for analyzing a tribosystem. Often, not all of the operating parameters are known. Therefore it is doubly important to understand the characteristic wear features or other key aspects of the tribosystem that will help to define metrics for the simulation (see Section 14.4). Understanding the properties and behavior of the currently used materials, or those used in an application which has key operating aspects in common with the component of interest, is important. Knowing what materials have and have not worked in the given application is equally valuable because it could save a great deal of time and effort. Simply asking the questions embodied in a thorough tribosystem analysis can go a long way toward solving the problem. Verifying the answers to those questions with a second opinion or a measurement can also be helpful. Sometimes people incorrectly assume that certain operating conditions exist. Tribosimulations sometimes need to be sensitive enough to distinguish between different variants of the same material. For example, there are many ways to heat-treat steels. A wear problem may occur if a component supplier changes the heat treatment, perhaps for reasons of economy, or changes the material supplier without notifying the customer (see the later example of gear scoring). The simulation in that case must be sensitive enough to detect the differences due to different heat treatments. Detecting

4 small variations in friction or wear using short-term laboratory tests can be challenging, but it can be facilitated through the use of statistical methods for data analysis. Therefore, multiple tests of each material or lubricant combination are needed to establish the characteristics of the laboratory test method; particularly, if the test method is new and has no historical record of performance. Using ASTM and other standardized tests offers the benefit of having previous data available from tests conducted in the same manner Selecting a Scale of Simulation Field-trials evaluate materials and lubricants under actual operating environments. Sometime field trials involve monitoring components as they are used in normal service. These instrumented components can be subjected to certain prescribed patterns of use. For example, some automotive companies have instrumented the brakes of test vehicles and returned those vehicles to their owners for a period of time, treating the driver s behavior as a variable. In other cases, instrumented vehicles have been run in several cities, but over carefully prescribed driving routes. Both types of field tests provide valuable information, but interpreting the wealth of data that obtains in these situations is difficult and complicated due to the many uncontrolled variables present in the field. For example, test drivers on city routes might be forced to break rapidly to avoid hitting a child or a dog. They could wind up behind a slow moving vehicle. In truth, the more channels of information that are collected, the more challenging is the task of interpreting the results and determining the underlying relationships in material and lubricant behavior. Like the automotive and truck brakes in the foregoing example, a great many tribological components are not operated under steady-state conditions. Some machine parts may experience short start-up and slow-down intervals but spend most of their lives under constant conditions. A classic example is the piston ring in an internal combustion engine that experiences hydrodynamic fluid film breakdown at the ends of the stroke. Other interfaces may be in a constant state of change and never reach what might be termed steady-state. Perhaps the contact pressure is not constant, the contact velocity is not constant, the temperature is not constant, and the direction of relative motion is not constant. In addition, tribosystems tend to age. Lubricants change properties as their additives degrade on exposure to high temperatures, react with the surfaces and other chemical species in the environment, and become contaminated with wear particles. The degree to which these factors affect the validity of a tribosimulation s results is not easy to determine a priori, but they can be taken into account to some degree in a well-designed tribosimulation Levels of Tribosimulation For the purposes of this chapter, we shall consider four levels of tribosimulation. Figure 14.1 shows these levels, indicating the parallels between physical and computer tribosimulations. Generally, the more realistic the simulation, the more costly and complicated it becomes. Level 1 tribosimulations use full-scale machines, such as entire automobiles, trucks, aircraft, ships, manufacturing machinery, and consumer products. These machines may be instrumented to measure loads, temperatures, strains, power demands on motors, and vibrations. In physical simulations, fullscale machines are operated under controlled conditions, such as on a test stand or inside an environmental chamber. For example, an entire communications satellite might be placed in a high-vacuum chamber and its antennas caused to rotate to determine whether the bearing lubricants will perform effectively in space. A truck might be rolled onto a chassis dynamometer and its wheels subjected to various braking loads and cycles. However, even the high-level simulations of Level 1 might omit certain factors present in the field. For example, the effects of zero-gravity on the aforementioned satellite cannot be completely simulated in a vacuum chamber on Earth. The effects of random potholes, loose gravel, and uneven road surfaces cannot easily be simulated on a test stand, but recent developments in computercontrolled test stands are making such simulations increasingly realistic.

5 FIGURE 14.1 The levels of tribosimulation. Level 2 tribosimulations use subassemblies which are subjected to near-operating conditions. Examples include brake pads and rotor combinations on dynamometers, fluid pumps in closed-loop pump test rigs, and jet engines in engine test cells. Nearly as expensive as Level 1 tests, Level 2 tests offer additional control of the externally applied test parameters. At the same time, fewer of the field-associated effects on performance are faithfully simulated. For example, the effects of road-induced vibrations on piston motions, the vehicle-specific flow of air past brake components, and the introduction of environmental contaminants into wheel bearing grease may be omitted in subassembly tests. In dynamometer tests of break component materials it is common to apply a series of test stages in an attempt to simulate specific types of frictional phenomena, like fade effects at elevated temperatures. Even staged Level 2 tests as complex as these cannot totally simulate the full range of habits of individual drivers and driving conditions. In some cases, however, Level 2 tribosimulations can be very effective in screening materials or lubricants because the operating conditions of the system are more clearly known. For example, loopby-loop ballpoint pen testers can show how long the products will continue to write effectively and establish failure statistics for the entire pen, whose satisfactory performance depends on the ability of the point to deliver a clear, uniform line of writing fluid to the paper. One tribosimulation area of particular medical interest is that of computerized hip and knee joint testing. Attention here is given to mimicking the forces, types of motion, and impact loads to which bioimplants are subjected. This subject area can make effective use of both physical and virtual component Level 2 simulations. The selection of the fluids to simulate synovial fluid and to correlate with clinical results is an important issue. Material swelling in situ, in the case of polymeric materials, and the role of debris particles as they interact with the soft tissues surrounding the joints, are also of interest. Level 3 tribosimulations involve test rigs designed to test specific components, like bearings and gears. For example, bearing test rigs have been successful in developing empirical design and selection guidelines for rolling element bearings of many kinds. Multiple-station rigs, automated to take data or to ascertain critical failure conditions, like excessive heat or vibration, can be run unattended, enabling the compilation of lifetime statistics and related performance data for consumer products or machine components. Level 4 tribosimulations involve test coupons of simple shapes. Examples include pin-on-disk tests, block-on-ring tests, four-ball lubricant tests, dry-sand-rubber-wheel abrasion tests, and vibratory cavitation tests. These tests are described elsewhere in this volume and in the wear testing literature [e.g., ASM (1992, 1997)]. Their usefulness is based on their ability to simulate the key contact conditions of the components of interest. For example, a cam roller follower in the engine of a certain diesel engine might be simulated by two disks turning at different speeds to impart a desired degree of slip to the

6 TABLE 14.2 Common Metrics for Assessing Friction and Wear Situations Situation Tribological Problem Typical Metrics Field component High friction Seizure; galling or scuffing marks; power draw of a motor; overheating of bearings or slideways; irregular motions; excessive wear of bearing or sealing surfaces; unusual noises or vibrations; marring of a formed product s surface, as in metalworking; irregular speed fluctuations in a bearing Surface damage Scuffing marks; galling and other visual indications Wear Fluid leakage in a seal; loss of compression in a piston; erosive perforation of a pipe elbow; presence of wear particles in a lubricant; loss of fit between parts; excessive or unusual noise from gears or bearings; excessive or unusual vibrations; changes in the appearance of contact surfaces (abrasive grooves, scuff marks, etc.); signal drop-out in electrical contacts; loss of cutting performance of a tooling insert. Laboratory specimen High friction Friction force or torque measurements Surface damage Visual inspection or profilometric measurements Wear Weight loss; displacement relative to another specimen or reference plane, wear scar size; wear depth; wear volume calculated by surface measurements or by weight change; visual examination; changes in friction force, surface temperature, or vibrations as detected by sensors; surface reflection characteristics measured by sensors contact. Furthermore, the test disks could be supplied with a lubricant and heated to simulate engine conditions. The linkage between tribosimulation levels can be important to establish the validation of Level 3 and 4 tests as effective screening methods. For example, if a set of Level 4 rankings agrees with relative rankings of the same set of materials or lubricants in Level 3 tests, and the validity of Level 3 tests in a certain application has been confirmed, then the usefulness of Level 4 tests will be greatly extended Defining Field-Compatible Metrics A metric is a measurable quantity or unique observational feature that can be used to rank or otherwise distinguish the performance of a material or lubricant in a tribosystem. Table 14.2 lists a few metrics commonly used in the field and in tribology laboratories. Wear and friction measurement methods are described in more detail elsewhere in this volume and in ASM publications (ASM, 1992, 1997). Ideally, metrics for assessing friction or wear problems should be quantitative, accurate, straightforward to measure, and not subject to the investigator s judgment. In practice, however, some or all of the foregoing criteria cannot be met. Furthermore, as Table 14.2 shows, there is often a significant difference between the parameters or observations that can be made in the field and those that can be made in the laboratory. Therefore, an important issue in assessing the usefulness and validity of simulations involves arriving at a set of common metrics which laboratory investigators and field engineers can agree on. There is no point in developing a laboratory test if field engineers refuse to believe that the results of the tests are relevant or cannot relate the test results to what they observe. Some key wear and surface damage metrics, in order of preference, follow: 1. Quantitative measures, like weight loss or dimensional changes, that can be measured both in the field and in the laboratory 2. A series of reference materials that rank in similar order of merit in the field and laboratory tests. 3. Visual wear features that appear similar in field-worn and laboratory-generated surfaces. 4. Wear debris that looks similar in field and laboratory-collected samples.

7 14.5 Selecting or Constructing the Test Apparatus Well-equipped tribology laboratories usually contain a variety of testing machines, capable of testing for a range of tribological behavior. One or more of these machines might be suitable for conducting the given tribosimulation. Commercially manufactured wear testing machines may be a cost-effective alternatives to designing and building new, special-purpose machines for the problem at hand. But commercial machines should be used only if they serve the required purposes, including simulating specific wear of friction conditions. It may also be possible to modify an existing machine to perform the requisite simulation. It is beyond the scope of this chapter to provide guidance on the design and construction of friction and wear testing machines because there is an enormous number of alternative designs. Well-equipped tribology laboratories also contain auxiliary equipment for measuring and characterizing wear, such as precision microbalances, profiling instruments, hardness testers, and microscopes of various types. Special techniques and highly trained personnel experienced in cross-sectioning wear surfaces or performing surface chemical analysis populate a well-equipped tribology laboratory. When selecting or designing a friction or wear testing apparatus for a specific purpose, it is important that the reason for testing, and the metrics selected for validating the results, are firmly in mind. For example, if weight loss is a metric, specimen fixturing and handling procedures should be designed to avoid sources of error in mounting specimens, treating them after exposure to wear, and transferring them to the weighing system. If surface appearance is a metric, protection of wear features from demounting and handling artifacts must be a part of the procedure Conducting Baseline Testing Using Established Metrics and Refining Metrics as Needed Baseline tests with the materials and lubricants in current use, or believed to be the leading candidates for a new application, are helpful in establishing the repeatability and characteristics of the test method itself. It is worthwhile to conduct a series of replicated tests to establish the repeatability of the baseline conditions. Knowing this, the performance of other materials or lubricants can be analyzed to determine if it is statistically different from the normal scatter in the test results. One test per materials couple or set of test conditions does not provide very strong evidence on which to make engineering decisions. If wear is measured quantitatively, then the wear factors or other metrics from baseline tests can be used in the denominator of a figure of merit. For example, the lifetime of a baseline cutting tool material, expressed as the number of workpieces produced before tool replacement is required, is divided into the lifetime of another candidate tool material to give the relative lifetime of the candidate. The higher the number is above 1.0, the greater the wear benefits of substitution, other factors being equal. An alternative engineering metric might be the tooling cost per unit part machined. Sometimes suitable quantitative metrics cannot be found. It may then be possible to compare the appearance of the test specimen to a worn part to establish the success of the simulation. An example is given in It is likely that laboratory simulations will not be able to reproduce every aspect of the part operating conditions. Therefore, it is, in general, unreasonable to expect that precisely the same wear rates or metrics will be obtained in the laboratory and in the field. From the standpoint of screening, however, it is very important that candidate materials rank in very much the same order in the field and in the laboratory. Since there are many ways to measure wear in the laboratory (weight loss, wear profile, wear scar dimensions, etc.), there may be one metric that correlates better with the field wear results than another. Therefore if one method of laboratory wear measurement does not correlate well with field results, another may work better. The following section describes several laboratory simulations that used different kinds of metrics and testing procedures depending on the nature of the parts being simulated.

8 14.7 Case Studies Six case studies on the selection and use of simulative friction and wear tests are provided below. The first case, which is described by Blau (1998), involves an automotive application in which new simulative test fixtures and procedures were developed. In the latter five case studies, summarized from an article by Blau and Budinski (1999), existing or slightly modified ASTM test methods were used to solve industrial plant wear issues and product-related wear problems An Oil Pump Gear Set with Several Wear Modes An initiative was undertaken to replace certain steel fluid pump gears with a lighter-weight, aluminumbased alloy. One criterion for the acceptability of the new gear material was that it possess acceptable wear characteristics when substituted for the current steel. The gears were of a gerotor type in which a wedge of fluid is trapped between the teeth of eccentrically mounted inner and outer gears. As the gears turn, the fluid is forced between them, pressurized, and then out of a pocket in the pump housing. This type of pump is typical of automotive oil pumps and automatic transmission fluid pumps. Based on the results of a TSA, described in Section 14.2, and with input from both pump part makers and pump users, the two wear-critical areas were determined to be the teeth contact points and the flat gerotor gear faces which can rub intermittently against the inside faces of the housing. In the latter case, little or nothing was known about the surface contact pressures or loads. In addition, specimens of gears run in actual service and in full-sized gear pump test rigs were carefully examined by optical microscopy and cross-sectioned for subsurface study. Based on the relatively small quantities of candidate materials available for use in the material selection process, it was necessary to devise a simulative test method which used small, round disks, about 20 mm in diameter and 10 mm thick. These were about 1 /4 the diameter of the actual gear disks. To simplify the testing process and make the best use of limited materials, it was decided to use the same specimen dimensions for tests of both tooth wear and the flat face-on-casing wear. It was desired to use quantitative metrics to screen the various candidate materials, but as described later, some of the wear metrics turned out to be only semiquantitative. Several testing configurations were developed. Eventually, the wear simulation evolved into a configuration that emphasized tooth-to-tooth slip which resulted in combined adhesive and three-body abrasive wear and subsequent loss of the gear tooth profile. One disk was rigidly held vertically with its curved outer diameter, simulating the curvature of the tooth face. It was oscillated against the flat face of a second disk of the same material (Figure 14.2). Hot, lubricated tests were performed at temperatures similar to that of the application. The length of the test was determined by the time needed to produce wear features similar to those seen in actual gears. The width of the wear scar on the curved disk s outer diameter was measured and converted to a wear volume. This wear volume was normalized by dividing by the product of the applied load and the number of cycles to obtain a wear rate (mm 3 /N-cycle). The gear face-on-casing sliding wear mode was simulated by placing the flat faces of two disk specimens together in a thrust-washer-type geometry (Figure 14.3). Circular insets were machined into one or both disk specimens to produce an annular contact. The upper specimen was held fixed in a spring-loaded arrangement to assure good flat-on-flat seating with the lower rotating disk. The rotating disk was made of the candidate lightweight gear material and the upper was made of typical casting alloy. Tests were run with oil-coated surfaces. Each test consisted of four segments in which the test was stopped and oil was replenished on the contact surfaces. Weight losses and dimensional changes were unsatisfactory quantities for measuring the small amount of wear produced in this type of flat-on-flat test. Therefore, a semiquantitative method for determining the wear severity was used. This involved cataloging the types of wear damage, such as scuffing, abrasion, gouging, etc., and assigning several severity levels to each. Table 14.3 shows the wear damage rating scale. Each level was defined sufficiently well so that two people independently obtained the same numerical rankings on the same test specimens. The wear damage ratings for each disk specimen were determined,

9 FIGURE 14.2 Curve-on-flat geometry used to simulate tooth-on-tooth rubbing contact. FIGURE 14.3 Flat-on-flat geometry used to simulate gear-on-casing sliding. and then a composite rating for each couple was determined (the sum of the two specimen ratings). Each test was duplicated to establish the repeatability of the results, and to enhance the investigators confidence in the differences between the wear ratings of different material couples. Figure 14.4 compares the wear seen on an actual part with that produced in laboratory experiments of several candidate alloys. Results from these two kinds of simulative tests, coupled with full-scale pump rig tests at a manufacturer s facility, cost modeling, and alloy processing trials, were used to select the leading alloy and surface treatment for this application Wear of Gravure Rollers on Doctor Blades In a certain industrial coating process, dimpled cylinders (gravure rollers) are used to pick up and deliver a solution to another surface. These cylinders are cyclically wiped by steel doctor blades to remove the

10 TABLE 14.3 Wear Damage Rating (WDR) Scale Used to Assess Flat-on-Flat Wear Factor Severity Description Light abrasion 1.0 Faint, widely spaced grooves aligned parallel to the sliding direction. Grooves may not be continuous around the track and are similar in depth to the original grinding marks. Moderate abrasion 4.0 Multiple, parallel wear grooves extending across a substantial portion of the contact area. Some of the original surface finish visible between the abrasion grooves. Severe abrasion 6.0 Deep abrasive wear grooves across the entire contact face. Little or no trace of the original surface finish. Light scuffing 1.5 Polished-looking areas with little or no original surface finish within their boundaries. Scuffed area < 25% of the nominal contact area. Moderate scuffing 3.0 Scuffed area 25 to 75% of the nominal contact area. Scoring 5.0 Localized, relatively deep grooves ( depth of original machining marks), suggesting plowing by large hard particles. Pull-out 5.0 Removal of particles or entire grains from the surface. Regions of pull-out may be associated with scoring by the removal material. Delamination 5.0 Detachment of thin, flat platelets; typically associated with fatigue crack growth parallel to the free surface. Burning 5.0 Production of dark oxides or tarnishes suggestive of exposure to excessive frictional heating. Severe metallic wear 6.0 Significant plastic deformation accompanied by deep grooving. No traces of the original surface finish. Often accompanied by shiny flake-like wear particles. Microwelding 1.0 Presence of tiny flecks of highly adherent, transferred material from the opposing surface. Major transfer 2.0 Presence of relatively large particles or patches of highly adherent material from the opposing surface. excess coating material, and were experiencing unacceptable wear as a result. It was decided to try ionimplanting the surfaces of the rollers to improve their wear. ASTM standards G-99 (pin-on-disk test) was used to compare the implanted and unimplanted (current) materials. While the wear of the roller material was markedly improved, the wear of the doctor blade material increased to an unacceptable level. Therefore, it was decided that ion implantation would not be an acceptable solution in this case. While the pin-on-disk method was not an exact simulation of the doctor blade operating conditions, it was felt to be adequate to evaluate one potential solution for this wear problem, and to determine that alternative methods of surface engineering or materials substitution would be required Scoring of Spur Gears An expensive steel gear set in production equipment began to exhibit signs of significant scoring. It was learned that the supplier had modified his processing and that the hardness of the new gears varied from that of the previous sets. ASTM standard G-98 (the button-on-block galling test) was used to determine the critical level of Rockwell hardness to avoid the onset of galling. This ASTM test method is based on using visual observations to obtain a numerical metric; namely, the threshold stress for galling. Using observations of test coupon surfaces subjected to increasing levels of normal force, one assesses the normal pressure at which galling begins. It turned out that a difference of only 1 or 2 units on the Rockwell C hardness scale made the difference between steel gears that ran acceptably and those that did not. Costly future failures were therefore avoided by tightening the hardness specifications on the gear steels. Increasing the hardness of a material to improve its surface durability and wear resistance is a longstanding, intuitive notion that is not always substantiated by testing. That is because other factors, such as the type of wear being experienced, the material s fracture toughness, fatigue resistance, and chemical reactivity with the environment can also affect the surface response to contact conditions. In the present fortuitous example, the suitability of the steels for use as gear teeth could be directly correlated to their Rockwell hardness numbers with the help of a standardized test method that captured the essential elements of surface contact in the application.

11 FIGURE 14.4 (a) Wear features on a gear side face tested in a commercial producers testing facility under severe operating conditions. (b d) Different levels of wear damage observed on laboratory test specimens in the flat-onflat simulator test shown in Figure Wear of Plastic Parts in an Optical Disk Drive Not only wear, but the presence of wear products (debris particles) can seriously affect the performance of imaging and computer equipment. This was the case for contacting plastic parts in an optical disk drive. The ASTM G-133 reciprocating pin-on-flat test was used to screen plastic pairs for those which not only had the best material-to-material compatibility, but also produced the least harmful debris insofar as the surrounding machinery was concerned. Therefore, an additional metric was an observationally bases scale of the distribution of wear debris in the vicinity of the contact area on the pin and flat specimens Wear of Rotary Slitter Knife Blades Rotary slitter knives were used to cut plastic sheeting to size. The edges of the knives slid against one another repeatedly as they worked. Excessive wear led to unsatisfactory performance, costly equipment shutdowns, and product damage. In this case, the development of acceptable metrics for laboratory screening was complicated by the lack of a clear definition for the blade sharpness. A decrease in product cut-edge quality is the result of worn blades, but edge quality is difficult to quantify in a way that can be used in cost-effective laboratory tests. The ASTM G-83 crossed cylinders wear test was eventually selected to screen materials for knife blade applications. This test produces a concentrated contact at the intersection of two orthogonal cylindrical 2001 by CRC Press LLC

12 specimens. While not exhibiting the exact geometry of the application, the small, highly-loaded contact between test specimens contained enough of the essential elements of rotary slitter knife interactions to produce a useful screening test. Wear volume is used as the metric and is computed from the test materials densities and their weight losses. By examining a great deal of crossed-cylinders laboratory wear test data that would have been impractical to obtain on the production floor, it was discovered that at least one of the blades had to be composed of carbide material in order for the slitter knives to perform satisfactorily. Since more than one material combination with satisfactory wear rates was identified in the course of the testing campaign, it was possible to select the most affordable solution to the problem from among several alternatives Erosive Wear of Piping During the process of designing a new plant involving the piping of dicalcium phosphate, it was necessary to know what material would be the best choice for the piping. Issues were not only erosion resistance, but corrosion resistance as well. The G-32 solid particle impingement erosion test was selected. Several candidate materials were exposed to dicalcium phosphate and other erodants using a pressurized air jet apparatus, such as that prescribed in the standard. It was determined that a soft stainless steel would work adequately in this application, and the decision was made to use that material for construction. Significantly, higher hardness did not ensure wear resistance, as it did in the case described in Section Therefore, the selection of materials for wear applications based on properties like hardness depends on the type of wear involved and on other performance requirements Conclusions The development of simulative friction and wear tests requires an interdisciplinary approach, beginning with a tribosystem analysis to define the problem and to establish key metrics that can be used to test the validity of simulations. Laboratory simulations using either custom-designed apparatus or standard test methods can be successfully applied to save time and money in solving friction and wear problems. No single test method will solve all problems, and proper test selection is critical for success. Sometimes, more than one test method will be needed to establish an engineering solution; especially, if more than one form of wear or surface damage is present in the application of interest. References ASM Handbook (1992), Friction Lubrication and Wear Technology, 18, ASM International Materials Park, OH. ASM (1997), Source Book on Friction and Wear Testing, ASM International, Materials Park, OH. Blau, P.J. (1998), Development of bench-scale test methods for screening P/M aluminum alloys for wear resistance, in Powder Metallurgy Aluminum and Light Alloys for Automotive Applications, Jandeska, Jr., W.F. and Chernenkoff, R.A. (Eds.), Metal Powder Industries Federation, Princeton, NJ, 97. Blau, P.J. and Budinski, K.G. (1999), Use of ASTM standard wear tests for solving practical industrial wear problems, Wear, , 1159.