Structure, stress and hardness of sputter deposited nanocomposite W-Si-N coatings

Tungsten-silicon-nitride (W-Si-N) coatings with thicknesses in the range 1.7∼3.0 μm were deposited onto Si(100) substrates using closed-field unbalanced magnetron sputtering in Ar–N2 gas mixture at 500 °C. The effects of N2 partial flow rate and silicon target current on the phase composition, microstructure, residual stress and hardness of the coatings were studied with X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), optical interference and micro-indentation. The coatings have a phase composition based on bcc W when the nitrogen flow rate is 20 sccm (Ar 10 sccm), and based on fcc W2N at the nitrogen flow rate 30 sccm (Ar 10 sccm). Cross-sectional SEM shows that the coating fracture changes from columnar structure to glassy appearance with the increase of silicon target current. AFM reveals that the surface roughness of the coatings decreases with the silicon target current. The compressive residual stress is less than 2.3 GPa for all coatings. The hardness of the coatings is 30∼50 GPa with the maximum at the silicon target current 0.8 or 1.2 A. The highest hardness 49.2 ± 3.8 GPa is obtained for the coating possessing the nanocrystalline W2N/amorphous Si3N4 composite structure identified by TEM analysis.