Friction and anti-galling properties of hexagonal boron nitride (h-BN) in aluminium forming

In the present work the friction and anti-galling properties of the h-BN in a steel-aluminium contact have been investigated, focusing on the possibility of replacing graphite with the h-BN in the aluminium forming operations. Extruded AA2014 alloy, widely used for the production of complex parts, was used. Investigation included the effect of h-BN powder size and concentration on the level and stability of the coefficient of friction, galling resistance and the quality of the Al surface. Tribological and anti-galling properties at room and elevated temperature of 400 °C have been carried out on the load-scanner test rig. Graphite was used as a reference solid lubricant. In the case of the h-BN powder three different particle sizes were used, i.e., 0.5 µm, 5 µm and 30 µm, added to NGL class-2 industrial grease in the concentrations of 5%, 10% and 20%. Results show that white h-BN, as a solid lubricant, is capable of successfully replacing graphite and providing “clean” surface, but only under mild contact conditions. It has similar lamellar structure but inferior load-carrying capacity, with its lubrication and anti-galling properties very much depending on the powder size, concentration and temperature. Contrary to the graphite tribological properties of h-BN deteriorate with increased concentration but improve with temperature and powder size. Best results are obtained for concentration of 5% and 30 µm powder size.