Ti-C-x-N-y thin films with different amounts of C incorporated into TiN0.87 were deposited on Si(100) substrates at 500 degrees C by reactive unbalanced dc magnetron sputtering. Their phase configuration, nanostructure, and mechanical behavior were investigated by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and microindentation measurements. The results indicated that the atomic ratio of (C + N)/Ti played a crucial role in phase configuration, nanostructure evolution, and mechanical behavior. When the ratio was less than one, a nanocrystalline (nc-) Ti(C,N), solid solution was formed by dissolution of C into the TiN lattice. Both microhardness and residual compressive stress values increased with an increase of C content. When the C reached saturation, precipitation of small amounts of SP2 amorphous (a-) phase appeared with more C incorporation. Further in-crease of C content (up to similar to 19 at. % C) made the amorphous phase fully wet nanocrystallites, which resulted in the formation of nanocomposite thin films of similar to 5 nm nc-Ti(C,N) natiocrystallites separated by an similar to 0.5 nm amorphous phase comprised mainly of sp(2) disordered C, graphite, and minor CNx. Thicker amorphous matrices and smaller sized grains followed when C content was further increased. The formation of nanocomposite structure greatly decreased both hardness and residual stress values of thin films. A hardness maximum was believed to be obtained at nc-Ti(C, N) solid solution containing the maximum C amount. Enhancement of the hardness value was attributed to solid solution effect and high residual stress value. (c) 2007 American Vacuum Society.