substrates by a dc unbalanced magnetron sputtering technique from a Ti-B-C composite target. The relationship between microstructures and mechanical properties was investigated in terms of the nanosized crystallites/amorphous system. The synthesized Ti-B-C-N films were characterized using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). These analyses revealed that our Ti-B-C-N films are composites of solid-solution (Ti, C, N)B-2 and Ti(C,N). crystallites distributed in an amorphous boron nitride (BN) phase including some of carbon, CNx, B2O3 components. The hardness of the Ti-B-C-N films increased with the increase of N content up to a maximum value of approximately 45 GPa at 10 at.. % N, with a subsequent decrease in hardness at higher N, content. This value is considerably higher than the hardness measured in our Ti-B-C films (similar to 35 GPa). The Ti-B-C-N(10 at.%) film also showed the highest H-3/E-*2 value (similar to 1.7 GPa) among the coatings produced. A systematic investigation of the microstructures and mechanical properties of Ti-B-C-N films prepared with various N contents is reported, in this paper. (c) 2005 American Vacuum Society.