Evaluation of adhesion and tribological behaviour of tantalum oxynitride thin films deposited by reactive magnetron sputtering onto steel substrates

Ta–O–N coatings (2–4 μm thick) were deposited on high-speed steel by DC reactive magnetron sputtering. The O2 / N2 ratio in the plasma was varied between 0.08 and 1.33. The (O + N) / Ta ratio in the Ta–O–N films varies between 1.27 and 2.78 and the oxygen fraction, f(O) = O / (O + N), between 0.06 and 0.96.The hardness decreases from 27 to 6 GPa with increasing (O + N) / Ta ratio in the films.The adhesion of the coatings to steel was evaluated by scratch tests. First cohesive failures occur at Lc1 = 1–4 N for all the films. Most films with 2.16 ≤ (O + N) / Ta ≤ 2.77 fail by gross interfacial shell-shaped spallation at Lc2 = 3–5 N. Films with highest (O + N) / Ta = 2.78 are very brittle and exhibit a large area interfacial spallation already at low loads. At high loads (Lc3 = 22–26 N) all films fail via large scale interfacial spallation inside the track, resulting in coating delamination.The tribological behaviour of the coatings in pin-on-disk tests is discussed with respect to the metalloid content in the Ta–O–N.