Boron nitride as a lubricant additive

Hexagonal boron nitride (BN) has a graphite-like lamellar structure, but has been considered less effective than other solid lubricants except for high-temperature applications. The present paper describes a series of sliding experiments which show somewhat curious behavior of BN when added to lubricating oil, and discusses their results by comparing with the results of observation and analysis of sliding surfaces. In the case of sliding of bearing steel vs. itself, BN slightly increased the coefficient of friction, but dramatically decreased wear. Boron was found to remain on the surfaces, but the remnant was almost oxidized; it was some sort of oxide but not stoichiometric. If bearing steel was slid against cast iron, BN decreased the coefficient of friction, but the decrease in wear was less marked, and the remnant in this case was mostly BN. These results show that BN is effective in reducing wear if used as a lubricant additive. (C) 1999 Elsevier Science S.A. All rights reserved.