A STUDY OF THE FORMATION AND SELF-LUBRICATION MECHANISMS OF BORIC-ACID FILMS ON BORIC OXIDE COATINGS

An investigation was made of the formation and self-lubrication mechanisms of boric acid films on boric oxide coatings prepared by vacuum evaporation. Measured friction coefficients of a steel ball sliding on a boric-oxide-coated steel disk and a sapphire ball sliding on a boric-oxide-coated alumina disk were 0.025-0.05 at steady state, depending on load and substrate material. This low friction was correlated with the formation of a lubricious boric acid film on boric oxide coatings exposed to open air. For the mechanism of self-lubrication, the layered triclinic crystal structure of boric acid was proposed. The atoms constituting each boric acid molecule are arrayed in closely packed and strongly bonded layers that are 0.318 nm apart and held together by weak forces, such as van der Waals'. It is hypothesized that, during sliding, these layers can align themselves parallel to the direction of relative motion and, once so aligned, can slide over one another with relative ease to provide low friction. Structural and chemical findings were included to substantiate the proposed solid lubrication mechanism.