CHARACTERIZATION OF LOW-TEMPERATURE CRN AND TIN (PVD) HARD COATINGS

The plasma beam sputtering process was used to prepare CrN and TiN hard coatings on steel, sapphire and alumina Superstrate substrates at a temperature of 200 degrees C. The microstructural characteristics, coating morphology, interfacial properties, microhardness and internal, stresses were studied for coatings 3 mu m thick. Their oxidation behaviour in an oxygen flow in a tube furnace was studied at temperatures up to 600 degrees C for TiN and up to 800 degrees C for CrN coatings. The initial stage of oxidation of TiN and CrN coatings 350 nm thick was also studied by continuous in-situ electrical resistivity measurements. This measurement technique offers high reproducibility and accuracy, so can be used to study oxidation for all types of hard coating, and especially for new multilayer and duplex coatings. Weight gain measurements and Auger electron spectroscopy depth profile analyses of oxidized stoichiometric CrN coatings showed that oxidation in an oxygen flow at 800 degrees C for 4 h induced the growth of a stable Cr2O3 film. The thickness of this oxide film was only 15% of the total (3 mu m) coating thickness. The surface morphology and small roughness changes - as also observed after a long-term test at 800 degrees C for 200 h in oxygen - indicated that a stoichiometric CrN coating with the measured properties can be successfully used as a hard and oxidation-resistant coating in industrial practice, even if the working temperatures are higher than 750 degrees C.