TRIBOLOGY OF NATURALLY-OCCURRING BORIC-ACID FILMS ON BORON-CARBIDE

In this paper, we describe the formation and self-lubricating mechanis ms of naturally occurring boric acid films on boron carbide (B4C) subs trates. The sliding friction coefficients of yttria/partially stabiliz ed 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 an nealing 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 t he air to form a thin boric acid film that is responsible for the ultr alow friction coefficients reported here. As reported in previous lite rature, the low-friction mechanism of boric acid is associated with it s layered-triclinic-crystal structure. The atoms on each layer are clo sely packed and strongly bonded to each other but the layers are widel y separated and are held together by van der Waals forces. During slid ing, these atomic layers can align themselves parallel to the directio n of relative motion and slide over one another with relative ease to provide the 0.03-0.05 friction coefficients reported here. Raman spect roscopy was used to elucidate the chemical bond structure of bulk boro n carbide and lubricious boric acid film on sliding surfaces.