Carbon (C) at concentration up to 11.8 at.% was incorporated into ion-beam deposited cubic boron nitride (c-BN) films. Films with C content less than or equal to 6.2 at.% were found to contain about 78 vol.% of c-BN, and have high hardness (36-47 GPa) and high elastic modulus (250-320 Gpa). Due to the high internal stress in the films they peel off readily from the substrates. At higher C content, the cubic structure is disrupted, and the structural change is accompanied by drops in both the hardness and elastic modulus. However, film adhesion is greatly improved, possibly because of the partial release in internal stresses induced by the incorporated C atoms. The film with 7.2 at.% C exhibits good mechanical properties (hardness approximate to 34 GPa, elastic modulus = 240 GPa) as well as satisfactory adhesion to substrates, and is thus suitable for application as a hard coating material.