The influence of the initial near-surface microstructure and imposed stress level on the running-in characteristics of lubricated steel contacts

The tribological behavior of polished and lapped 56NiMoCrV7 and Ck45 disks with lubrication was studied to link the impact of initial near-surface microstructure to the running-in behavior of the systems. The tests were performed using a pin-on-disk tribometer with radionuclide technique to resolve ultra-low wear rates. For the running-in different stressing regimes were applied. Whereas the lapped disks developed low friction and wear as response to a high-power running-in, the polished disks had to be stressed by a step-wise increase in load to achieve a similar result. In addition, the duration of some load steps and the sliding velocity had to be adjusted in order to obtain a proper running-in. With the help of focused ion beam and transmission electron microscopy the response of the microstructure to the stressing conditions was investigated. It turned out that the quality of the running-in crucially depends on a subtle equilibrium between material strengthening and softening. Strengthening by finishing and running-in was a prerequisite for the formation of the third body, whereas softening resulted in scuffing. When it was possible to charge the near-surface material in relation to its strengthening capabilities, low wear rates in the nanometer per hour regime were obtained