Evaluation of hexagonal boron nitride nano-sheets as a lubricant additive in water

The purpose of this work was to investigate the possible lubricating effects of aqueous dispersions of hexagonal boron nitride (h-BN) nano-sheets. The sheets consisted of 300 nm wide and sub-30 nm thick flakes, and concentrations containing 1, 0.05 or 0.01 wt% were synthesized without surfactants or with organic functionalization by means of bath sonication. Optical absorbance data for the h-BN dispersions revealed that nano-sheets remained stably dispersed in water for 30 days. The friction and wear of the clean aqueous dispersions were evaluated using a SiC ball sliding on a disk (Si wafer) arrangement. Results indicated that even small amounts of h-BN nano-sheets could enhance wear resistance and reduce friction coefficient. Also, 30 days after synthesis, the tribological properties of dispersions were not much degraded with time. Tribo-flims were observed to have formed on worn surfaces due to repeated exfoliation and deposition of h-BN nano-sheets during sliding. This process is believed responsible for the tribological improvements from aqueous dispersions. Accordingly, h-BN could be a promising “green” lubricant additive for water.