Characterisation and wear properties of industrially produced nanoscaled CrN/NbN multilayer coating

Present work deals with morphological, microstructural, compositional and tribological characterisation of nanoscaledmultilayerCrN/NbNcoatingproduced by an industrial process presently in development phase. This coating has been applied on steel ring components used in textile plants subjected to contact erosion wear, at high frequency and low load, between the external surface of a ring and a bar where friction coefficient and corrosion resistance are critical. Nanoscaledmultilayer structures usually show both high hardness and better wear resistance, correlated with grain refinement, coherency strain hardening, inhibition of dislocation motion, together with an excellent corrosion resistance due to the interruption of coating columnar pinholes and to the combined metal element effect. In order to obtain multilayer structure a non-conventional technique has been set up, consisting of triggering alternatively on Cr or Nb cathodes with appropriate time constant so as to obtain couple of layers of about 5 nm each. In order to satisfy industrial requirements, the process was optimised using a commercially available Cathodic Arc PVD equipment, routinely used to produce conventional CrNcoatings. Microstructural and compositional properties were investigated and reported hereby. Low angle X-ray diffraction, Optical and Atomic Force Microscopy, Electron Probe Microscopy (SEM, TEM, SAD, EDS) and Focussed Ion Beam techniques has been used. Defects were also investigated, particularly microdroplets (shape, dimension, density, clustering and other process-sensitive features). Mechanical and tribological properties were characterized by micro and nano hardness measurements, scratch test, ball on ring, ball-cratering and residual stresses evaluation with X-ray diffraction (XRD) sin2ψ method. Multilayercoating shows higher H/E ratio, a clear tendency to delaminate during fracture and a different size distribution of microdroplets. As a consequence, CrN/NbNcoating results in a lower wear rate with respect to the CrNcoating (up to 30%) but only if a normal force dominated stress is applied. Finally, performances results (e.g. wear rate and degradation behaviour) obtained by operating in line two different sets of components (respectively CrN and CrN/NbN coated) are presented; lifetime of industriallyproducedmultilayer coated components has been elongated from 9 to 11 months.