High plastically deformed sub-surface tribozones in sliding experiments

Tribological systems in forming applications often work under severe conditions as high contact pressure, sliding velocities and temperature resulting in different wear phenomena and changes in microstructural and mechanical properties. In this work, the material response of austenitic and martensitic tribo-partners was investigated after severe loading condition in a forming tribometer. Main focus was laid on the understanding of the microstructure modification in near-surface zone on a mesoscopic range and the changes of mechanical properties due to friction energy transformation within the tribomaterials. Tribotests were performed using 1.4301 (304) workpiece and 1.2343 (H11) tool material under constant contact pressure of 390 MPa under varying sliding velocity of 0.025 m/s and 0.125 m/s, respectively. Investigation of near-surface zones was done by optical microscopy and HV0.05 micro-hardness depth profiles combined with detailed High Resolution Scanning Electron Microscopy (HRSEM) analysis and Electron Back Scatter Diffraction (EBSD) measurements in depths from contact surface of 30 µm, 70 µm, 130 µm and 220 µm. The results showed that sub-surface zones correspond to refinement partly due to dynamic recrystallization and/or to phase transformation; whereas the final properties depend on the dominating mechanism. New grains and a strong increase of large angle grain boundaries (LAGB) were observed which may increase plasticity and reduce failure by fracture. Finally, for the applied variation of testing conditions, no formation of friction martensite on the tribologically stressed materials was observed.