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Investigation of hBN powder lubricating characteristics of die steel H13–ceramic Si3N4 tribopair at 800 ℃

In the background that hot-working die steels have been widely used in the hot stamping process of high strength steels for manufacturing car bodies and parts, solid lubricant materials are promising for decreasing the friction and wear on die surfaces at high temperatures. In this work, hexagonal boron nitride was used to lubricate the sliding interface of ceramic Si3N4 against die steel H13 at 800 ℃, and its frictional behavior was investigated at different contact pressures and rotating speeds by pin-on-disk testing. Tribological characteristics of hexagonal boron nitride solid lubricant were analyzed through 3D laser scanning confocal microscope and energy-dispersive spectroscopy. The results show that the wear behavior of hexagonal boron nitride powder lubricating film featured two stages: gradual undermining and complete damage. The formation-broken dynamic cycle of partial lubricating film constantly occurred at the frictional interface until the film was completely destroyed. In a short period, the hexagonal boron nitride lubricant can reduce the friction coefficient efficiently compared with dry friction, and the die substrate showed basically no abrasion because a layer of plasticity accumulated on the die surface, which protected the substrate. In a certain range, the boundary-lubricating film became damaged rapidly with increasing load, and high speed was not conducive to the stability of the lubricating film.