Elements of a Wear Test

General elements of a wear test are simulation, acceleration, specimen preparation, control, measurement, and reporting. Simulation is the most critical, but no element of the application should be overlooked. Wear and wear phenomena can be influenced by load, environment, geometry, motion, the wearing mediums and counterface, and other factors.

Simulation ensures that the behavior experienced in the test is the same as in the application. Given the complexity of wear and the current incomplete understanding of wear and its phenomena, test development is subject to trial and error and is dependent on the capability of the developer. The ideal test exactly duplicates a wear situation. Generally, this is not practical, but any modifications in the test procedure should be carefully evaluated to obtain the most useful wear data.

General knowledge and experience can aid in assessing the differences between test and application, but correlations between test and application should also be studied. The most helpful correlation in developing a test is comparison of the worn surface and wear debris produced in the test to those produced in the application. For example, the morphology of the scar, the presence or absence of oxidized surface layers, changes in the microstructure of the material, and wear debris size, shape, and composition can be compared. If major features of the wear scar and debris are different, valid simulation is unlikely. Wear mechanisms frequently result in characteristic wear particles.

Measurement. Common direct measures of wear are mass or weight loss, volume loss or displacement, and scar width, depth, or other geometrical measures. Examples of indirect measures are the time required to wear through a coating, the load required to cause severe wear, and the load required to change surface reflectance. The selection of variables to measure wear is often based on convenience, the nature of the wear specimens, available techniques, and the amount of wear.

Material wear behavior can be compared by determining a wear curve or measuring wear at a single point in the test. Wear behavior frequently is complex, and a wear curve provides more information and allows evaluation of more complex behavior than single-point measurement. For example, the wear behaviors of two materials in the same test are plotted as functions of the number of sliding cycles (Fig. 1).

Fig. 1 Wear behavior of two materials in the same test

Reporting. Wear is a system response. When wear data are reported, a description of the wearing system must be supplied, including apparatus, geometry of contact, type of motion, load, speed, environmental condition, condition of wearing mediums, description of materials, description of lubricant and lubrication, description of wear-in period, and unusual observations, such as evidence of transfer.

Surface Engineering of Cast Irons

J.R. Davis, Davis & Associates

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