We developed a novel in situ observation method associated with synchrotron radiation X-ray diffraction (XRD) that enables us to simultaneously monitor structural changes of materials, images at frictional interfaces, friction force and temperature with a time resolution on the order of tens of milliseconds. The in situ method was applied to study scuffing process of martensitic steel under a dry condition. The result shows that during scuffing, martensite to austenite phase transformation occurred with plastic flow. The generated austenite phase disappeared when the shear test was stopped. The austenite was present at a surface temperature lower than the nominal austenitisation temperature. After intermittent occurrences of the austenitisation with local plastic flow, the scuffing feature showed a larger amount of austenite, higher friction and greater plastic flow. The XRD spectra suggest that some metallurgical properties of the near-surface material of the steel may change at the scuffing-mode transition.
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