Oxidation kinetics of sputtered Cr-N hard coatings

Hard coatings are nowadays successfully applied for applications where they are subjected to high temperatures. Although the oxidation threshold of ha rd coatings has been investigated by several authors. there is only limited information on the oxidation kinetics. Within this paper, the oxidation be havior 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 composi tions of single phase Cr2N or CrN, respectively, For the single phase CrN c oatings, the macrostresses were adjusted to values between -211 and 400 MPa using different ion bombardment conditions to evaluate the stress influenc e. The oxidation behavior of coatings deposited onto hot-working tool steel was investigated using thermo-gravimetric analysis in an argon/oxygen atmo sphere. Dynamical measurements were carried out to determine the oxidation threshold and to provide basic information on suitable temperatures (betwee n 725 and 825 degreesC) for isothermal measurements. The activation energie s for oxidation calculated by means of Arrhenius plots range between 102 an d 465 kJ mol(-1) for the different Cr-N coatings. The oxidation products we re characterized using scanning electron microscopy, energy-dispersive X-ra y analysis and X-ray diffraction and were found to be Cr2O3. Based on the r esults obtained in this study it can be concluded that the activation energ y for oxidation provides a suitable basis for comparison of the oxidation b ehavior of different hard coatings. (C) 2001 Elsevier Science BN. All right s reserved.