Crystallization and Composition of Ni-C/Ti Multilayer with Varied Ni-C Thickness

Ni-C/Ti are suitable for the components of neutron supermirrors with high reflectivity because of their excellent optical constant and smoother interfaces compared to Ni/Ti. In this paper, to investigate the mechanism of C doping to the interface, crystallization, and composition of a Ni-C/Ti multilayer with variable Ni-C thickness, four Ni-C/Ti multilayers were prepared by direct current magnetron sputtering, in which the thickness of the Ni-C layers was 1.5 nm, 2.5 nm, 3.5 nm, and 4.5 nm, respectively, and the thickness of the Ti layers was kept at 5 nm. The prepared samples were characterized by XRD, XPS, HRTEM, EDX, and SAED. The XRD and HRTEM results show that Ni-C layers in Ni-C/Ti multilayers translate from amorphous to polycrystal form, with their thickness increasing from 1.5 to 4.5 nm, and the crystallite size in Ni-C layers is equivalent to the layer thickness, respectively. The XPS, SAED, and EDX results illustrate that the enrichment position of C in Ni-C/Ti multilayers evolves from the Ni-C layers to the Ti layers as the respective Ni-C layer thickness increases from 2.5 to 4.5 nm. The enrichment position evolution of C in Ni-C/Ti multilayers could be due to the lower standard Gibbs free energy of TiC (−180.1 KJ/mol) compared with NiTi (−37.3 KJ/mol) and Ni3Ti (−35.9 KJ/mol) at 298 K.