Tribological behaviour and lubrication performance of hexagonal boron nitride (h-BN) as a replacement for graphite in aluminium forming

In metal-forming processes friction leads to the generation of heat, adhesion and the pick-up of work material, tool wear, inhomogeneous deformations, defects and a poor surface quality. Therefore, the use of suitable lubricants is required, especially in the case of aluminium and aluminium alloys. With the increased complexity of the parts being formed, higher strength requirements and the introduction of new alloys, the forming lubricants are also being subjected to ever-increasing demands in terms of temperature stability, reduced friction, galling prevention and surface protection. Graphite is a well-known, solid lubricant that is successfully used in the cold and hot forming of aluminium. However, it leaves dark stains on the surface of the formed part, which means that additional grinding or polishing is required. One of the possible substitutes for graphite is hexagonal boron nitride (h-BN), also known as white graphite. In this work the possibility of replacing graphite with h-BN in Al-forming lubricants was investigated, including the influence of h-BN powder size and concentration on its tribological performance and the aluminium’s surface quality. The results of the investigation show that h-BN, as a solid lubricant, is capable of successfully replacing graphite in Al-forming processes, while at the same time maintaining a clean surface without staining. The study has shown that the tribological performance, including the friction and wear, the lubrication-film stability and the surface quality, very much depend on the powder size and the concentration. Under the investigated contact conditions, the best performance was obtained for 30-µm h-BN powder when added to the grease in a concentration between 10 and 20%.