Oxidation stability of chromium aluminum oxynitride hard coatings

Abstract

Hard coatings like CrAlN deposited by physical vapor deposition are state of the art for wear and oxidation protection of cutting tools. Conducting direct current magnetron sputtering (dcMS)/High power pulsed magnetron sputtering (HPPMS) hybrid processes combines the advantages of HPPMS, like denser structures and smoother surfaces, with the high deposition rates of dcMS. The current study aims to increase the aluminum content in the reaction layer of CrAlON coatings by incorporating oxygen in CrAlN and therefore achieve an enhanced oxidation stability. For this purpose, CrAlON coatings with different oxygen contents were deposited by dcMS/HPPMS hybrid processes. The morphology of the coatings was investigated by scanning electron microscopy. The chemical composition of the bulk was measured by electron probe micro analysis and of the reaction layer by X-ray photoelectron spectroscopy. The phase composition of the coatings was investigated by X-ray diffraction and the elastic-static properties by nanoindentation. Furthermore, the coated cemented carbide samples were heat treated at T = 900 °C and T = 1000 °C for t = 0.5 h in ambient atmosphere. A low oxygen content of x(O) = 6.9 at.% of the oxynitride coating increased the aluminum content of the reaction layer to x(Al) = 17.8 at.% compared to x(Al) = 8.5 at.% for the nitride coating and to x(Al) ≤ 9.6 at.% for the oxynitride coatings with higher oxygen contents of x(O) ≥ 18.2 at.%. As shown by XRD, this enhanced the oxidation stability of the coating with a low oxygen content of x(O) = 6.9 at.% from T = 900 °C to T ≥ 1000 °C for t = 0.5 h.