Mechanical and tribological properties of CNx films deposited by reactive magnetron sputtering

The hardness, elasticity, wear rate and friction coefficient of carbon nitride (CNx) films of defined microstructure and composition are presented. CNx films were deposited by dc reactive magnetron sputtering from a C target in N2/Ar plasma. Films were grown on Si (0 0 1), Ni, and HSS substrates to thickness of ∼0.5 μm at a total pressure of 3 mTorr with the N2 fraction varied from 0 to 1, and the substrate temperature TS, varied from ambient to 350°C. The mechanical and tribological properties of the coatings were evaluated by nanoindentation and dry ball-on-disk test. For CNx (0≤x≤0.35) films deposited below 200°C (amorphous structure), the elastic recovery and hardness do not change significantly with increasing N concentration, however, the friction coefficient increases from 0.19 to 0.45, while the coating wear rate is low. For CNx (0≤x≤0.15) films grown at TS=350°C, where a transition from a graphite-like to a “fullerene-like” phase occurs, a dramatic increase in hardness and elasticity is observed. Furthermore, the rms surface roughness decreases from 15.0 to 0.4 nm. For 0.15≤x≤0.20, CNx films deposited at TS=350°C (fullerene-like phase) exhibit a smooth surface, high hardness and elasticity (∼90% recovery), and a coefficient of friction against hard steel of ∼0.25. For all substrates, film friction coefficient tends to increase as the nitrogen content in the film is increased. Results also indicate the formation of a transfer layer which improves the tribological properties of the films.