Thin films of Ti-B-N with different N contents were deposited on Si(1 0 0) at room temperature by reactive unbalanced close-field dc-magnetron sputtering using three Ti targets and one TiB2 target in an Ar-N2 gas mixture. The effect of N content on bonding structure, microstructure, phase configuration, surface roughness and mechanical properties have been investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cross-sectional scanning electron microscopy (SEM), plan-view and cross-sectional high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and microindentation measurements. It was found that the N content significantly affected phase segregation and microstructure. The nitrogen-free TiB0.65 films showed an amorphous compound consisting of Ti and TiB2 (Ti-TiB2). After adding about 28 at.% N, Ti was preferentially bonded to N to form TiN, accompanying with formation of small amounts of TiB and BN bonds. At this stage they combined TiB2 to form a two-phase nanocomposite with microstructures comprising of nanocrystalline (nc-) TiN phase in nitrogen-containing amorphous (a-) TiB2 matrix. Addition of more N promoted formation of BN bonding at cost of TiB2, which resulted in formation of nanocomposite nc-TiN/a-(TiB2, BN) thin films. A small grain less than 8 nm in size was found at low N content, and the grain size increased with increasing N content. A low microhardness value of about 20 GPa was obtained in the amorphous Ti-TiB2 compound, and a maximum microhardness value of about 50 GPa was achieved in nc-TiN/a-TiB2. A decrease of microhardness took place after formation of BN (i.e. amorphous matrix composed by both TiB2 and BN) with further increasing N content, and a hardness value of about 35 GPa was followed at high N contents. The surface roughness strongly depended on the phase configuration. The higher the mole fraction of nanocrystalline TiN phase, the rougher the surface became. © 2006 Elsevier B.V. All rights reserved.