Ti-Si-N-O nanocomposite coatings with different contents of oxygen were deposited by a combined dc/rf reactive unbalanced magnetron sputtering process in an Ar+N-2+O-2 mixture atmosphere. The composition, structure, mechanical, and tribological properties of the as-deposited coatings were analyzed by energy dispersive analysis of x-rays, x-ray diffraction (XRD), nanoindentation, and pin-on-disk tribometer experiments, respectively. It was found that in the range of lower oxygen content with atomic ratio of O/N <= 0.72, the tribological properties of the Ti-Si-N-O coatings are evidently improved, in comparison with the coating without oxygen incorporation. At O/N=0.72, the friction coefficient and wear rate of the as-deposited coatings are reduced to 20% and 45%, respectively. Meanwhile, however, their hardness was not reduced, but, on the contrary, slightly increased. With increasing oxygen content further to O/N >= 0.72, coating hardness decreased significantly. The friction coefficient of the as-deposited coatings decreased monotonously with the increase of oxygen content in the whole composition range investigated. The wear rate of the coatings exhibited a minimum value at around O/N=0.72. In the lower range of O/N, wear rate decreased significantly due to the lubricant effect of oxygen incorporation, while in the higher range of O/N, wear rate increased gradually due to the weakening of coating hardness. XRD patterns revealed that the as-deposited coatings were mainly crystallized in cubic TiN phase, accompanied with minority of rutile structure titania in the case of higher oxygen incorporation. (c) 2006 American Vacuum Society.