W-B-N sputter-deposited thin films for mechanical application

The structure, composition and hardness of reactively sputtered W-B-N thin films were investigated by XRD, EPMA, and Vickers ultramicroindentation. emical composition of the films was changed from W80B20 to W42B9N49 by varying the partial pressure of N2. All the as-deposited coatings were amorphous, except the W60B17N23 film, which showed crystalline peaks, indexed as bcc α-W, overlapping the amorphous feature. This nanocomposite structure lead to a maximum as-deposited hardness of 36 GPa. atings were thermally annealed at 850 and 950 °C in Ar / H2 atmosphere. All the films crystallised with the formation of the bcc α-W and/or the fcc-W2N phases, depending on their B and N contents. A small increase in hardness was registered for 850 °C annealing temperature in comparison to the as-deposited state. However, an inverse trend was detected after post-annealing at 950 °C, 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.