Boron nitride (BN) films with carbon (C) contents varying from 3–11.8 at. % were fabricated using dual-ion-beam deposition. Films with low C contents (≤6.2 at. %) consist mainly of the c-BN structure. The hardness of these films is as high as 43 GPa, and the coefficient of friction μ is described by the formula μ=0.14 W0.25, where W is the normal load in millinewtons. The critical load of damage is above 50 mN. Because of the existence of high internal stresses, the films peel off readily from the substrates, and delamination occurs immediately once a scratch track is produced. When more C is added, the formation of the c-BN structure is suppressed, while a graphite-like structure becomes dominant. The hardness and critical load drop to 8.6 GPa and 6.8 mN, respectively, when the C content is 11.8 at. %. The values of μ remain unchanged. The internal stresses are partially released, thereby leading to improved film adhesion. We have found an optimum C content of 7.2 at. %, at which both the mechanical and adhesion properties are satisfactory. © 1999 American Vacuum Society.