Chronology of the microstructure evolution for pearlitic steel under unidirectional tribological loading

• Microstructural evolution in steel for a unidirectional contact was examined.
• FIB cross-sections reveal that grains first bent and elongated in sliding direction.
• A second, nanocrystalline layer formed out of the most bent and elongated grains.
• A HPT-based model describes the growth of the nanocrystalline layer.

The microstructure of the material under the contact strongly influences tribological performance and the ability to control this microstructure therefore is key for optimizing a material׳s surface properties for low friction and little wear. There is however a significant lack of knowledge about the elementary mechanisms of microstructure evolution under tribological load as well as their kinetics. To cover different stages of this evolution, pearlitic steel pellets sliding against steel 100Cr6 disks were characterized after different numbers of tribological acceleration-deceleration cycles. Scanning electron and focused ion beam microscopy were applied to monitor the microstructure changes. Two tribologically modified surface layers are found: one with bent grain boundaries and one of nanocrystalline nature. We hypothesize that the second layer is formed by breaking down the most bent regions of the first one when a critical grain boundary bending angle is reached and then consumes it from the top.

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