Microstructure of CrN coatings produced by PVD techniques

The surface of machine parts and moulds in plastic transformation processes may be exposed to corrosive agents evaporated from the plastics at elevated temperatures and to the abrasive action of hard fibres in the case of reinforced plastics. Nitrides of transition metals can be wear resistant but also resistant to many chemically aggressive environments. The microstructure and mechanical properties of chromium nitride based coatings were studied in order to analyse the potential of this material to be used as a protective coating of the surface of injection moulding or extruding machines. Chromium nitride based hard materials were deposited by reactive magnetron sputtering onto stainless steel substrates in the form of homogeneous coatings. The coating microstructure and morphology were studied by X-ray diffraction (XRD), optical and atomic force microscopy (AFM). The mechanical properties of these coatings were studied by scratch and nanoindentation testing and residual stress measurements. The coatings can be strongly textured, if unbiased during deposition. Optical image analysis revealed the density and size distribution of defects. Atomic force microscopy was used to identify these defects as pinholes, particulates and grains growing with different growth direction. The coating residual stress depends, of course, on the strain within the growing grains and therefore on the deposition conditions. But the residual stress was also influenced by the coating morphology, namely the coating density. The Young's moduli of CrN-PVD coatings depend on the measuring direction as well as on the coating morphology. Normal to the coating surface a Young's modulus of about 400 GPa can be found. Parallel to the substrate surface, the E-modulus was very much affected by the existence of voids or loosely packed CrN and varied between 100 and 300 GPa. Some interesting features were found such as the case of open porosity being frequently found on coatings with higher density and lower effective elastic modulus and coatings with lower density and higher effective elastic modulus do not necessarily show open porosity. (C) 1999 Elsevier Science S.A. All rights reserved.