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The structure, composition and hardness of reactively sputtered W-B-N thin films were investigated by XRD, EPMA, and Vickers ultramicroindentation.The chemical composition of the films was changed from W 80 B 20 to W 42 B 9 N 49 by varying the partial pressure of N 2 . All the as-deposited coatings were amorphous, except the W 60 B 17 N 23 film, which showed crystalline peaks, indexed as bcc a-W, overlapping the amorphous feature. This nanocomposite structure lead to a maximum as-deposited hardness of 36 GPa.The coatings were thermally annealed at 850 and 950 8C in Ar/H 2 atmosphere. All the films crystallised with the formation of the bcc a-W and/or the fcc-W 2 N phases, depending on their B and N contents. A small increase in hardness was registered for 850 8C annealing temperature in comparison to the as-deposited state. However, an inverse trend was detected after post-annealing at 950 8C, which was more evident for the high N content films. This behaviour was attributed to the formation of a soft external layer of boron nitride poorly crystallised. D