Effects of hydrogenation on solid state reactions in metallic multilayers

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We studied effects of hydrogenation on the process and kinetics of solid-state reactions across the interfaces in metallic multilayers such as Nb/Ni, Nb/Pd and V/Ni systems. In these systems, intermetallic compounds such as Ni_3Nb, Pd_3Nb and Ni_3V are formed along the interfaces of the multilayers when they are annealed at temperatures above 〜550℃ in vacuum, and the multiplayers become homogenized. The objective of this study is to examine whether the dissolved hydrogen influences the reaction processes and kinetics and to reveal their mechanisms.Metallic bilayers of the Nb/Ni, Nb/Pd and V/Ni systems, each layer thickness being 100 to 300 nm, were prepared on Si (111) substrates with Ar ion-beam sputtering at room temperature, and were annealed at temperatures between 200 to 600℃ for 0.5-3.0 h in vacuum and H_2 atmosphere of 5 atm. The phase structure and composition distribution along the thickness of each bilayer subjected to different heat treatments were measured by X-ray diffraction (XRD) and Auger Electron Spectroscopy Depth Profiling (AESDP) techniques, respectively.It was found that the interlayer reactions forming the intermetallic compounds were significantly enhanced by the hydrogenation in all the systems. The reaction temperatures forming the Ni_3Nb, Pd_3Nb and Ni_3V compounds were remarkably reduced from 〜550℃ (vacuum annealing) to 〜450℃ (hydrogen annealing). This means that the Nb/Ni, Nb/Pd and V/Ni bilayers or multilayers are thermally destabilized by hydrogenation, in contrast to Ti/Ni, Ti/Pd and Ti/Fe multilayers which are strongly stabilized by hydrogenation. A mechanism for the destabilization by hydrogenation in the former systems is enhancement of interdiffusion across the interface of the bilayers caused by superabundant vacancies formed in the Nb and V layers during annealing under H_2 atmosphere. This hydrogen enhancement of solid-state reactions will be useful for improvements of materials fabrication and processing engineering.