The Running-in Tribological Behavior of Pb-Free Brass and Its Effect on Microstructural Evolution

In this paper, the running-in tribological behavior of Pb-free brass (i.e., 64% copper and 36% Zinc) sliding against 100Cr6 under lubricated condition and its effect on grain refinement are investigated systematically. The sliding experiment is preformed using a tribometer based on in situ holography microscopy and radionuclide technique. After the tribological test, the friction pair is characterized using scanning electron microscopy and transmission electron microscopy in order to quantify the micrography, chemistry and microstructure. It is found that the curve of the friction coefficient exhibits two steep falls during the initial stage, while the real-time wear rates at these moments are relatively high. Then the test enters the dynamic stability stage due to equilibrium between the processes of strain hardening and recovery. The brass sliding experiment is carried out in the boundary lubrication region leading to dislocation movement and void formation due to plastic deformation. Moreover, an exceptional increase in two directional stacking faults is observed that extend and interdiffuse into each other. More importantly, as has been observed before, the coarse grains are considerably refined into nano-grains due to the accumulation of dislocation movement induced by repeated sliding test.