Investigating the influence of epitaxial modulation on the evolution of superhardness of the VN/TiB2 multilayers

A series of the VN/TiB2 nanomultilayers with different modulation ratios (t(VN):t(TiB2)) and different modulation periods were synthesized via a magnetron sputtering system. The cross-sectional transmission electron microscopy (TEM) and x-ray diffraction (XRD) examinations indicated that in the alternately deposited monolayers of the VN and TiB2, due to the influence of the crystal (111)(VN) texture, TiB2 layer presented epitaxial growth on the surface of the VN layer when its t(VN):t(TiB2) was 5:1. Moreover, the formation of the TiB2 crystal promoted the growth of (200)(VN) and significantly improved the preferential growth of nanomultilayers. With decreasing t(VN):t(TiB2) to 1:7, the thin VN layer was crystallized under the introduction of crystalline TiB2 layers. A type of double epitaxial growth was observed to be a main reason for the coherent growth of the VN/TiB2 nanomultilayers within a certain thickness. Consequently, the multilayers appeared to have a corresponding superhard effect, which presented a corresponding anomalous enhancement of hardness and elastic modulus. The highest hardness and elastic modulus reached 41.8 and 492.4 GPa, respectively, at a 1:7 ratio of t(VN):t(TiB2). Residual stresses were also released due to the coherent growth in the interfaces. Meanwhile, the coherent growth model of the multilayer was used to explain the growth mechanism of the VN/TiB2 nanomultilayers in this study. (C) 2016 Elsevier B.V. All rights reserved.