Effect of periodicity on hardness and scratch resistance of CrN/NbN nanoscale multilayer coating deposited by cathodic arc technique

Nano-scaled multilayer CrN/NbN coatings were produced in an industrial-size cathodic arc physical vapor deposition (PVD) chamber, with three cathodes in alternate positions (Cr/Nb/Cr). Four multilayer NbN/CrN coatings were produced with different periodicities (20nm, 10nm, 7.5nm and 4nm) with total thickness from 25 to 30μm in all cases. The coatings were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM), which provided the lattice parameter in each of the constituent layers and structural analysis of the multilayers, respectively. The multilayer coating system is composed of CrN and NbN with similar structures, but with a lattice mismatch, which varies the fraction of the region with lattice strain depending on the periodicity. For the thicker individual layers, the adjustment of lattice parameter at the interfaces does not represent a predominant fraction of the entire structure, i.e. separate peaks of NbN and CrN can be distinguished in the XRD analysis. In the presence of low periodicity (lower than 10nm), the lattice of each constituent may be coherently strained and just one intermediate lattice (d-spacing) is detected for the NbN/CrN multilayer. Mechanical and tribological characterization was conducted by microhardness measurements using a 500mN load (Instrumented indentation) and scratch tests using a conical diamond indenter with a 0.1mm tip radius. From high to low periodicity, the multilayer NbN/CrN coatings showed increasing hardness, decreasing coefficient of friction and increasing resistance to cohesive failure, possibly due to a strong (200) texture for the coatings with 10nm periodicity or less, indicating a potential better performance of the lower periodicity multilayer coating systems in service.