Transition metal nitride (TMN) coatings have been consistently attracting attention over the course of the last decades, due to their outstanding mechanical properties as well as their exceptional thermal and chemical resistances. One of the most restraining factors, however, that limits the fields of application of ceramic coatings in general is their low fracture toughness. Recently, the authors found that growing TMN coatings as ultra-thin heterogeneous layers stacked atop each other in a nanolayered structure has the potential to enhance the fracture toughness as compared to their monolithic counterparts.
The experimental work was performed in conjunction with density functional theory simulations, which predict an outstanding toughness enhancement for TiN/WN superlattice films in comparison with other common TMN coatings. Consequently, we synthesised TiN/WN multilayer films with bilayer periods ranging from 2 to 200 nm using DC reactive magnetron sputtering. Special attention was directed towards the growth of pure single-phase fcc-WN films, which required fine tuning of the processing conditions so as to avoid the appearance of other phases. The films were characterised using X-ray diffraction, scanning electron microscopy, and nanoindentation. The intrinsic fracture toughness was determined by performing single cantilever bending experiments on freestanding film material.
The experimental results, showing a dependence of hardness and fracture toughness on the bilayer period, are discussed in relation to the simulation findings. This study is part of an overarching project, which seeks to explore the influence of the difference in structural and mechanical variables between the involved TMNs on the occurrence and extent of the toughness enhancement in TMN superlattice systems.