Boron carbide is a good material for very hard coatings mechanical applications. Sintered bulk B4C is one of the hardest known materials (40 GPa), with a high Young's modulus and a very high chemical and thermal stability. In B4C film deposition, ion bombardment during film growth can deeply affect the material properties. Films were deposited by tuned RF magnetron sputtering from a sintered B4C target, under different conditions of ion bombardment. corresponding to substrate biases varying from +15 to -80 V. Homogeneous and stochiometric B4C films were obtained. Their mechanical properties: microhardness, Young's modulus, internal stress and adhesion have been measured by the dynamical nanoindentation method, by the beam bending method and by the microscratch method, respectively. As ion energy is increased, the stress of the films and the critical load increases, while both microhardness and Young's modulus have maximum values of 30 and 350 GPa respectively, for a bombarding ion energy of 50 eV.