Many engineering tests start with a run-in procedure, as it is commonly accepted that running-in at some fraction of the target load is beneficial due to a reduction in surface roughness. However, the choice of load and duration is often based on historical context or the experience and philosophies of the testing personnel. The objectives of this study were to (1) evaluate the effects of contact pressure and initial composite roughness on the surface evolution during the initial stage of testing, and (2) use this information to determine if there is an optimal load and time for the run-in portion of a test. These tests were conducted with a disc-type machine using carburized steel specimens with a circumferential lay direction and oil with anti-wear additives under specific conditions of slide-to-roll ratio, entraining velocity, oil spin-off temperature, and oil/additive package. The specimen surface was inspected using non-contact profilometery every minute for the first 10 minutes of testing, with a subsequently increasing inspection interval for a total of 150 minutes. The data demonstrate that the surface roughness reduces within the first few minutes of testing and remains stable for a period of time, which is dependent on the pressure and initial composite roughness. Additionally, hardness measurements indicate that hardness gains are occurring during the test, but over a longer timeframe.
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