A dual ion beam system is used to produce hard nanocomposite TiN/Si 3N4 coatings on Si. Cross-sectional high resolution transmission electron microscopy analysis of the coatings shows that ion assistance causes microstructure to change from the non-assisted columnar form to one where there are small crystals present in an amorphous percolation network. For an unheated Si substrate, the microhardness increases with increasing ion-assist energy from 24 to 29 GPa, whereas for a deposition substrate at 400 °C, the microhardness values are 7-8 GPa or higher. The value of microhardness does not change even when coatings are annealed in vacuum at 1000 °C, showing that these coatings have high thermal stability. X-ray photoelectron spectroscopy data indicate that the -Ti-N-Si- bonds expected when the percolation network is formed are present only for substrate temperatures above 600 °C and that Ti-Si bonds form at lower temperature and during excess ion bombardment.