Boron carbide films were deposited by dc-magnetron sputtering and their composition, chemical bonding and mechanical properties were investigated as a function of the substrate bias voltage. The films are stoichiometric within the entire range of substrate bias voltages employed as determined by Rutherford backscattering spectrometry. As the substrate bias voltage increases, higher amounts of trapped Ar atoms are found in the films. A decrease of the number of B-C bonds is observed while the hardness decreases and the compressive stress increases. It is suggested that the trapped Ar atoms are in substitutional sites and act to inhibit chemical bonds within the amorphous network which leads to softer films. The trapped Ar atoms also induce local expansion of the amorphous network and a correlation between the amount of trapped Ar atoms and the magnitude of the compressive internal stress is suggested. (C) 2003 American Vacuum Society.