A comparative study of microstructure and properties of TiZrN/NbN and TiSiN/NbN nanolaminate coatings

As a superhard coating material, multilayer nanolaminates are one of the hot points in recent decades [1-3]. In nanolaminates, the downscaling of the multilayer architecture to the nanometric regime allows for further extending the functional benefits of coatings through the expression of synergistic properties arising from the small-scale heterogeneity of the coating, such as the establishment of stress gradients impeding dislocations and cracks propagation, a phenomenon often referred to as the superlattice-effect. Due to these distinctive phenomena, these materials are becoming popular in diverse applications that require high strength, increased ductility and fracture toughness, wear and oxidation resistance, shock resistance, biocompatibility, and certain optical properties. To our best knowledge, no report exists on the development and experimental analysis of TiZrN/NbN and TiSiN/NbN nanolaminates. In the attempt to fill this vacancy, here we report on the compare the microstructure and mechanical properties of cathodic-arc deposited TiZrN/NbN and TiSiN/NbN nanolaminates. We found that in the case of TiSiN/NbN nanolaminate the crystallographic structure of the modulation layer consisted of fcc-TiN+a-SiNx in TiSiN nanolayer and fcc-NbN+hcp-NbN-δ' in NbN nanolayer multiphases. The crystallographic structure of TiZrN/NbN nanolaminate was also multiphase: fcc-TiN+fcc-ZrN in TiZrN nanolayer and fcc-NbN+hcp-NbN-δ' in NbN nanolayer. From the low-magnification image of TiSiN/NbN nanolaminate, it was clear that no columnar growth structure can be observed, suggesting that the insertion of TiSiN nanolayers destructed the columnar growth structure, which made its cross-sectional morphology featureless. From the high-magnification image, it could be seen that TiSiN nanolayers exhibit an amorphous state, implying that the “template effect” of NbN nanolayers on TiSiN nanolayers disappears. The amorphous TiSiN nanolayers blocked the epitaxial growth structure between NbN nanolayers. At the low-magnification image of TiZrN/NbN nanolaminate, it was obviously clear that the coating was a columnar growth structure, indicating that the insertion of TiZrN nanolayers could not prevent the columnar growth structure. In the high-magnification image, it was clear that the lattice fringes continuously went through several modulation nanolayers, indicating that TiZrN nanolayers had partially transformed into the crystalline structure under the “template effect” of NbN nanolayers and grown coherently with NbN nanolayers. The mechanical properties were the following: for TiZrN/NbN nanolaminate the hardness was 40.6 GPa and the elastic moduli was 597 GPa, and for TiSiN/NbN nanolaminate the hardness was 40 GPa and the elastic moduli was 560 GPa. The above results suggest that with different constituent elements, the multilayer TiZrN/NbN and TiSiN/NbN nanolaminates exhibit different morphologies, which lead to different microstructures and mechanical properties.