Simulative Friction and Wear Testing

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 (mm3/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,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 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. 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 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 14.7.3. 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.