Two-phase nanocrystalline TiN/amorphous Si3N4 composite films were synthesized at room temperature by ion beam assisted deposition. Film bonding states and microstructure were investigated by x-ray photoelectron spectroscopy, x-ray diffraction, and transmission electron microscopy. Surface morphology was measured by atomic force microscopy and quantitatively analyzed using height–height correlation function. The mechanical properties were evaluated from nanoindentation measurements. The films consisted of nanometer-sized TiN crystallites embedded in an amorphous Si3N4 matrix. A maximum hardness of about 42 GPa was observed in a film with silicon content of 11.4 at. %. The improved mechanical properties of Ti–Si–N films with the addition of Si into TiN were attributed to their densified microstructure with development of fine grain size and reduced surface roughness. The combination of hardness and grain size was correlated with film phase composition and structural analyses. The results have been interpreted with the help of a Monte Carlo Potts model simulation. © 2004 American Institute of Physics.