A thin film of boron carbonitride B(C,N) was formed on a mirror-polished Si(111) substrate at 600 degrees C under plasma from a gas mixture of NH3 + C2H2 + B The film consisted of B-enriched amorphous B(C,N). The critical load to detach the film from the substrate using a scratch tester was found to increase linearly as the RF power was raised from 100 to 400 W. The mean critical load was higher by a factor of approximately 3.0 for 400 W than for 100 W. The load, however, could not be determined clearly in the film prepared in the absence of plasma, because of the poor film hardness and adhesion to the substrate compared with the films deposited under plasma conditions. The B(C,N) film became harder and adhered strongly to the substrate as the RF power increased, forming B-C-Si bonds at the interface with the Si substrate and B-C bonds in the film.