Direct Surface Modification of Zirconium by Reactive Plasma Processing at Elevated Temperatures

This paper investigates a surface modification process for Zr to improve its durability, in order to make use of Zr in a blanket module as an effective neutron multiplier with (n, 2n) reaction. Modified ceramic layers were successfully synthesized on Zr substrates by carburizing, nitriding and oxidizing using reactive plasma processes. The microstructural evolution during the plasma process is governed both by the kinetics of the diffusion and by the kinetics of the ordering to form the reaction products. In the case of carburizing, the growth rate of the carbide layer is restrained by the diffusion of C, which was clarified to exhibit high covalency with the neighboring Zr atoms by a first principles molecular orbital simulation, through the layer. By contrast, the diffusion of O which exhibits high ionicity rather than covalency with the Zr lattice is much faster than the rate of ordering to form the monoclinic ZrO2 phase. A 20 MeV electron beam pre-irradiation process was also conducted at ambient temperatures as a pre-treatment before the plasma process. The electron beam irradiation can influence the reaction behavior during the plasma process, depending on the process conditions.