The structural and mechanical properties of ion‐beam deposited (B0.5−xSix)N0.5 films (0≤x≤0.5) were characterized by x‐ray photoelectron spectroscopy, infrared absorption experiments, and nanoindentation tests. A single‐layer BN film (x=0) has 70 vol. % in cubic phase (c‐BN), and a hardness of 38 GPa. However, it peeled off very soon after deposition due to the high internal stress. If a buffer layer was deposited first, followed by a (B0.5−xSix)N0.5 film with x≊0.013, the whole configuration adhered very firmly to both quartz and silicon substrates. This improvement in adhesion was probably due to the formation of Si–N bonds, which served to release partly the stress inside the (B0.5−xSix)N0.5 films. Since the Si content was low, the film structure remained highly cubic, and there was no observable drop in hardness. For higher x, the cubic structure in (B0.5−xSix)N0.5 films disappeared rapidly and was replaced by a hexagonal structure. This structural change led to a rapid drop in hardness from 38 to 12 GPa. As x was further increased, more Si–N bonds were formed in the (B0.5−xSix)N0.5 layers. As a result, the hardness increased from the minimum value to a value ≊24 GPa. © 1996 American Institute of Physics.