Thin films of Ti-B-x-N-y were deposited on Si(100) at room temperature by reactive unbalanced dc' magnetron sputtering in an Ar-N-2 gas mixture. The effects of B content on microstructure and mechanical properties of these films have been analyzed using x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, x-ray diffraction, transmission electron microscopy, atomic force microscopy, micro-indentation measurements, and an optical interference method. Microstructure studies revealed that depending upon the amount of B addition, the films showed two- or three-phase nanocomposite structure. At B contents below about 10 at. %, the films consisted of mainly TiN.bondings with a,small amount of TiB and BN bondings. As the B content increased, TiB gradually transformed to TiB2 and the films consisted of nanocrystalline (nc-) TiN embedded in an amorphous (a-) TiB2 matrix. A maximum hardness of similar to 44 GPa was observed in a film with B content of 19 at., %. The improved mechanical properties of Ti-B-x-N-y films with the addition of B into TiN were attributed to their densified microstructure with development of fine grain size and different phase combination. The reduction in grain size has also been supported by means of a Monte Carlo simulation. When B contents reached similar to 42 at. % or above, an amorphous-like nanocomposite of nc-TiN/a-TiB2/a-BN was formed. The effect of thin a-TiB2 layer in stabilizing nc-TiN structure is also elucidated and explained on the basis of structural and thermodynamic stability. (c) 2005 American Vacuum Society.