Study of the Initial Cycles Number Influence on the Retentivity of a Commercial Friction Modifier Using the Twin-disk Test

The rails and wheels are one of the most critical and expensive components of railway transport, and any problem could generate numerous consequences. The ability to manage friction at the wheel-rail interface currently represents one of the greatest challenges and one of the most powerful tools in railway engineering. The application of friction modifiers, lubricants, and friction enhancers to friction control have been the focus of extensive research of wheel-rail systems. The friction modifier (FM) provides a targeted coefficient of friction (COF) of 0.3-0.4. Currently, the laboratorial retentivity tests conducted by manufacturers do not describe the standards used, neither the surface integrity before the FM application. This work studies the influence in the retentivity of the different surface integrity states before application of FM using a twin-disk test. It will be possible to define the surface condition necessary for the application of FMs in retentivity tests, using a twin-disk tribometer. A commercial friction modifier was applied before the test (zero cycles) and after 500; 1,000; 1,500; 2,000; 3,000; 4,000; and 5,000 dry cycles, ending the tests when the coefficient of traction (COT) reached 0.4. To compare the retentivity of different FMs, a complete standardization of the twin-disk test is essential, applying the FM in a controlled amount, with over 2,000 dry cycles, and using a disk with controlled initial roughness.