FORMATION OF ULTRALOW FRICTION SURFACE-FILMS ON BORON-CARBIDE

In this letter, we describe the formation and ultralow friction mechan isms of a surface film on boron carbide (B4C). This film results from sequential reactions between B4C and oxygen and between the resulting boron oxide (B2O3) and moisture; it can afford friction coefficients o f 0.03 to 0.05 to sliding steel surfaces. At temperatures above 600 de grees C, B4C undergoes oxidation and forms a layer of boron oxide (B2O 3) in the upper surface. During cooling to room temperature, the B2O3 reacts with moisture in the air to form a secondary him, boric acid (H 3BO3) The sliding friction coefficient of 440C steel balls against thi s film is 0.04, compared to 0.7 against the bare B4C surfaces. Mechani stically, we propose that the ultralow friction behavior of the heat-t reated B4C surface is due mainly to the layered-crystal structure of t he H3BO3 film that forms on the sliding surface. (C) 1996 American Ins titute of Physics.