In this letter, we describe the formation and ultralow friction mechanisms 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 of 0.03 to 0.05 to sliding steel surfaces. At temperatures above 600 °C, B4C undergoes oxidation and forms a layer of boron oxide (B2O3) in the upper surface. During cooling to room temperature, the B2O3reacts with moisture in the air to form a secondary film, boric acid (H3BO3). The sliding friction coefficient of 440C steel balls against this film is 0.04, compared to 0.7 against the bare B4C surfaces. Mechanistically, we propose that the ultralow friction behavior of the heat‐treated B4C surface is due mainly to the layered‐crystal structure of the H3BO3film that forms on the sliding surface. ©1996 American Institute of Physics.