Significant impact of individual surface and modulation structure on mechanical properties of NbN/NbB2 multilayers

Abstract

The controllable growth of NbN/NbB2 multilayers at a constant modulation period of 10 nm with different modulation ratios (tNbN: tNbB2) was achieved through magnetron sputtering system. The X-ray diffraction (XRD) and X-ray reflectivity (XRR), cross-sectional transmission electron microscopy (TEM) and scanning electron microscopy (SEM) examinations indicated that in the alternately deposited monolayers of NbN and NbB2, a clear multilayer structure was obtained and the diffraction peaks were assigned to typical cubic structure of NbN and hexagonal structure of NbB2. For the cubic NbN, the combination of surface energy and strain energy affected NbN (111) and (200) texture in polycrystalline NbN layers. Moreover, the multilayers appeared a superhard effect due to mainly the preferential growth of NbB2 (001). The corresponding highest hardness and critical load reached 36.86 GPa and 87.93 mN, respectively, at a tNbN: tNbB2 of 1:4. The preferential growth of NbB2 (001) texture was investigated by first-principle. Through CESTEP calculation of the NbB2 (001) and (101) surface, the main texture of bulk NbB2, the (001)-B surface is the most stable surface. The calculated results revealed the nature of the preferential growth in NbN/NbB2 multilayers, which offered a profound understanding of its growth mechanism.