Effects of lubricant rheology and additive chemistry in the wear of Si 3N 4 sliding on steel

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The friction and wear characteristics of silicon nitride/steel sliding couples were examined with a pin-on-disk tribometer under geometry, speed and loads that simulate those encountered in the high-speed machining of steel. The tests were performed in dry sliding and under lubrication with pure paraffin and with paraffin containing 2% chlorinated paraffin or sulfurized olefin. In all conditions, the steel is transferred to the silicon nitride surface and the latter wears by fatigue-induced fracture during the removal of the transferred steel. In dry sliding, it was found that the wear of silicon nitride increases with the amount of frictional power dissipated. At high loads, however, high wear is caused by macroscopic fracture of the silicon. In the tribological conditions, chosen here to simulate those of high-speed cutting, lubricated sliding occurs in the mixed lubrication mode. Accordingly, pure paraffin causes a decrease in friction and wear that is more pronounced as the sliding speed increases or the load decreases. The EP additives used in this test reduce the friction coefficient further by boundary lubrication and decrease the wear rate of Si 3N 4 by another order of magnitude. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for the analyses of the worn surfaces. Tribochemical reactions with the additives occurred on the steel surfaces only. Iron sulfides and chlorides were formed on the steel disk and chlorates and sulfates were found on the steel covering the ceramic disk. These differences are explained by the differing temperatures of pin and disk.