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. Hexagonal boron nitride (BN). has a lamellar crystalline structure, in which the bonding between molecules within each layer is strong covalent, while the binding between layers is almost entirely by means of weak van der Waals forces. This structure is similar to that of graphite and molybdenum disulfide _MoS ., which are highly success- 2 ful solid lubricants, and the mechanism behind their effective lubricating performance is understood to be owing to easy shearing along the basal plane of the hexagonal crystalline structures. Then, BN once captured attention as a potential solid lubricant for general use because of this similarity, and evaluation was made of its tribological properties. This was consid ered to prove that BN has essentially the same lubricating mechanism as graphite. However, these and other results
showed that, as a solid lubricant, BN was inferior to graphite and MoS except for its high thermal stability and its white color. In its consequence, tribological interest in BN thereafter has practically been limited to lubrication for metalworking processes where lubrication at high temperatures andror cleanliness of working environments are of critical concern. BN introduced to the actually rubbing parts strongly adhered to steel surfaces, and works to slightly increase friction but drastically decrease wear. In sliding of bearing steel vs. itself, addition of BN as little as 1 wt.% results in the reduction of wear by more than an order of magnitude, and further reduction is found for higher concentrations. The coefficient of friction is slightly increased by the addition of BN, but small fluctuation in the friction traces is eliminated. Surface analysis has been conducted of the wear scars, and the following results have been obtained, which would be responsible for the different lubricating performance of BN in steelrsteel and steelriron combinations.
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