Stress and Hardness of CrN x films
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Stress and Hardness of CrNx films G.C.A.M.Janssen, J.-D.Kamminga1, W.G.M.Sloof Department of Material Science and Technology, Delft University of Technology, the Netherlands, e-mail: [email protected]; 1 Netherlands Institute for Metals Research, Delft, the Netherlands
ABSTRACT Chromium nitride films CrNx with x ranging form 0 to 1 were deposited by reactive PVD. Both stress and hardness in the films are a function of the composition. The growth stress and hardness for the majority of the films can be related through the Hall Petch relation. It is shown that the hardest films fall outside this relation. It is also shown that the hardest films are nanocrystalline. It is argued that the hardness of these films is a consequence of the nanocrystallinity of these films. INTRODUCTION Chromium nitride coatings are succesfully applied in tribological systems where corrosion, oxidation and intense wear are expected [1]. Hardness of the coating is one of the key parameters to the succesful performance of the coating. Wear and corrosion resistant coatings are preferably in a state of compressive stress. The stress in the films at room temperature is the sum of the tensile growth stress generated at the column boundaries, the compressive growth stress generated by the ion bombardment on the growing film, and the thermal stress. Typically deposition conditions are chosen such that the compressive stress from the ion bombardment is larger than the tensile stress from the columnar growth, leading to corrosion and wear resistant films. In the work reported in this paper however conditions were chosen such that the tensile stress is still visible. It turns out that growth stress and hardness can be related through the Hall-Petch relation for most CrNx films. The hardest films obtained in this work fall outside that relation, indicating that the deformation mechanism for these films is different. EXPERIMENTAL Chromium nitride films were deposited on ball bearing steel and silicon in a Hauzer HC 139 PVD machine by reactive sputter deposition. The base pressure in the system was always below 1×10-6 mbar. In all experiments an argon flow of 115 sccm was led into the reaction chamber. The chamber was pumped by a 2200 l/s turbomolecular pump resulting in an argon partial pressure of 3×10-3 mbar. By sputter deposition in an argon/nitrogen mixture CrNx films are deposited, with x depending monotonically on the nitrogen flow. The nitrogen flow was varied from 0 to 150 sccm, resulting in films ranging from pure chromium to CrN. All depositions were done at 180°C. The samples performed a planetary motion in the chamber; i.e. the substrate table rotated around its axis and the samples were mounted on rods on the substrate table, which also rotated about their axes (Fig.1). All depositions were carried out for 75 minutes. The thickness of the resulting film decreases monotonically with increasing nitrogen flow during deposition. Pure chromium films were 1.7 micron thick, CrN films were 1.1 micron thick. V3.24.1
Fig.1. Schematic l
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