Surface roughness control in nanolaminate coatings of chromium and tungsten nitrides

Frictional forces arising during machining operations of cutting tools generate high temperatures at the tool-workpiece interface, promoting tool-wear phenomena. Surface roughness is acknowledged to contribute significantly to friction generation, and its control is therefore of paramount importance. Structural confinement of growth defects in multilayer coating architectures with individual layers thickness in the nanometric regime can help reduce surface roughness development. Here, a set of nanolaminate coatings consisting in an alternated repetition of cubic chromium nitride (CrN) and cubic tungsten nitride (c-WN) layers is fabricated via unbalanced DC magnetron Sputtering. The individual layers thickness range is between 100 and 10 nm. The reduction of surface roughness as a function of individual layers thickness is demonstrated by semi-contact Atomic Force Microscopy (AFM). Scherrer analysis performed on X-Ray Diffraction (XRD) crystallographic signals shows that the reduction in surface roughness is correlated to a reduction in crystallite size, modulated by the nanolaminate architecture. Conformity and uniformity of multilayer coatings is observed via FIB-SEM cross-section micrographs. The relative contribution of CrN and c-WN layers to the overall roughness development is investigated through a SEM cross-section micrograph-based analysis. The microstructural and topographical features of nanostructured CrN/c-WN multilayer coatings are correlated to the observations from the cross-section analysis and discussed in terms of the energy transfer to the growing surface during each deposition step.