Oxidation kinetics of sputtered Cr–N hard coatings

Abstract

Hard coatings are nowadays successfully applied for applications where they are subjected to high temperatures. Although the oxidation threshold of hard coatings has been investigated by several authors, there is only limited information on the oxidation kinetics. Within this paper, the oxidation behavior and kinetics of sputtered Cr–N coatings were studied systematically. All coatings investigated were prepared by means of reactive unbalanced DC magnetron sputtering using a Cr target and argon/nitrogen discharges. The N/Cr ratio was varied systematically to achieve coatings with phase compositions of single phase Cr 2N or CrN, respectively. For the single phase CrN coatings, the macrostresses were adjusted to values between −211 and 400 MPa using different ion bombardment conditions to evaluate the stress influence. The oxidation behavior of coatings deposited onto hot-working tool steel was investigated using thermo-gravimetric analysis in an argon/oxygen atmosphere. Dynamical measurements were carried out to determine the oxidation threshold and to provide basic information on suitable temperatures (between 725 and 825°C) for isothermal measurements. The activation energies for oxidation calculated by means of Arrhenius plots range between 102 and 465 kJ mol −1 for the different Cr–N coatings. The oxidation products were characterized using scanning electron microscopy, energy-dispersive X-ray analysis and X-ray diffraction and were found to be Cr 2O 3. Based on the results obtained in this study it can be concluded that the activation energy for oxidation provides a suitable basis for comparison of the oxidation behavior of different hard coatings.