Tribology of naturally occurring boric acid films on boron carbide

In this paper, we describe the formation and self-lubricating mechanisms of naturally occurring boric acid films on boron carbide (B4C) substrates. The sliding friction coefficients of yttria/partially stabilized zirconia pins against plain B4C substrates are quite high at 0.3-0.4, but are 6-10 times lower against the B4C substrates subjected to annealing at 800 degrees C. We determined that this low friction was the result of a thin boric acid film that forms naturally on the sliding surface. During annealing at 800 degrees C, the exposed surface of B4C undergoes oxidation and forms a layer of boron oxide. During cooling, this layer undergoes a secondary chemical reaction with moisture in the air to form a thin boric acid film that is responsible for the ultralow friction coefficients reported here. As reported in previous literature, the low-friction mechanism of boric acid is associated with its layered-triclinic-crystal structure. The atoms on each layer are closely packed and strongly bonded to each other but the layers are widely separated and are held together by van der Waals forces. During sliding, these atomic layers can align themselves parallel to the direction of relative motion and slide over one another with relative ease to provide the 0.03-0.05 friction coefficients reported here. Raman spectroscopy was used to elucidate the chemical bond structure of bulk boron carbide and lubricious boric acid film on sliding surfaces.