ABSTRACTThis paper studies the effects of bilayer thickness and chromium content on the microstructure and mechanical properties of nanolayered TiAlN/CrN thin films. By rotation of samples holder and control of targets activity, a variety of multilayers and chemically modulated thin films were grown on WC-Co substrates using cathodic arc PVD. Conventional and high resolution TEM showed that aluminum contributes to refinement of structure while chromium favors the formation of coarse columnar morphology. Consequently, TiAlN layers periodically interrupt the formation of columns in CrN layers in multilayer films, while in chemically modulated samples the columns are not interrupted leading thus to the formation of strongly columnar films. Both Cr content and bilayer thickness contribute to hardness enhancement. Effect of Cr arises from the formation of hard fcc-(CrAl)N phase to the detriment of softer wurtzite-like hcp-AlN. The contribution of bilayer thickness is explained by the grain refinement based on Hall-Petch effect and the formation of highly stressed columnar structures with (111) preferred orientation. Such structural modifications strongly influence crack modes and morphologies as observed using AFM and FIB cross-section of indents. Thin bilayer films exhibit well-organized straight cracks parallel to the contact edge between indenter and film, while large bilayer films show a network of discontinuous irregular mud cracks attributed to grain boundary sliding. Refinement of structure favours crack meandering and branching that prevents the propagation of large cracks with more dramatic effects.
Microstructural and micro-mechanical investigation of cathodic arc evaporated ZrN/TiN multilayer coatings with varying bilayer thickness
