Ti-Al-Si-N films were deposited on WC-Co substrates by the hybrid coating system of arc ion plating method for Ti-Al sources and dc magnetron sputtering technique for Si incorporation. The synthesized Ti-Al-Si-N films were revealed as composites of solid-solution (Ti,Al,Si)N crystallites and amorphous Si3N4 by instrumental analyses such as x-ray diffraction, high-resolution transmission electron microscopy, and x-ray photoelectron spectroscopy. The Si addition in Ti-Al-N films affected the refinement and uniform distribution of crystallites by percolation phenomenon of amorphous silicon nitride similar to that of the Si effect in TiN film. The solubility limit of Si in the Ti0.76Al0.24N crystal is believed to be about 6 at. %. No free Si was observed due to the very high ionization rate of nitrogen gas in the arc plasma. As the Si content increased up to about 9 at. %, the hardness of Ti-Al-N film steeply increased from 30 GPa to about 50 GPa. Ti-Al-Si-N films having the maximum hardness showed the nanocomposite microstructure consisting of fine (Ti,Al,Si)N crystallites, about 8 nm in size, dispersed uniformly in the amorphous Si3N4 matrix. (C) 2003 American Vacuum Society.