Enhanced adhesion of coating layers by Ion Beam Mixing: An application for nuclear hydrogen production
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Enhanced adhesion of coating layers by Ion Beam Mixing: An application for nuclear hydrogen production Jae-Won Park, Hyung-Jin Kim, Sunmog Yeo and Seong-Duk Hong Korea Atomic Energy Research Institute, Daejon-City, South.Korea ABSTRACT The bonding between two dissimilar materials has been a problem, partiularly in coating metals with non-metallic protective layer. In this work, it is demonstrated that a strong bonding between ceramics/metal can be achieved by mixing the atoms at the interface by ion-beam. Specifically, SiC coating on Hastelloy X was studied for a high temperature corrosion protection. Auger elemental mapping across the interface shows a far broader mixed region than the region expected by SRIM calculation, which is thought to be due to the thermal spike liquid state diffusion. The results showed that, although the thermal expansion coefficient of Hastelloy X is about three times higher than that of SiC, the film did not peel-off at above 900 oC confirming excellent adhesion. Instead, the SiC film was cracked along the grain boundary of the substrate above 700 oC. At above 900 oC, the film was crystallized forming islands on the substrate so that a considerable part of the substrate surface could be exposed to the corrosive environment. To cover the exposed area, it is suggested that the coating/IBM process should be repeated multiply. INTRODUCTION High Temperature Gas Cooled Reactor (HTGR) combined with the Iodine-Sulfur (IS) cycle has been regarded as the most efficient system for a mass production of hydrogen [1,2]. In the IS cycle, a process heat exchanger (PHE) comprised of channels for He and decomposed sulfuric acid gas (SO2/SO3/H2O) is needed. The material used for the sulfuric acid gas channels is subjected to such a severe corrosion environment, however, there is no suitable commercial metallic material available presently. For this reason, consideration has been given to surface modification of metallic materials . We selected Hastelloy X as the metallic substrate due to its good mechanical properties at a high temperatures and SiC as a corrosion inhibiting coating material (its corrosion resistance being due to the very strong covalent bonding between silicon and carbon [3,4]). This ceramic-coating-on-metal system certainly has considerable merit because that it does not hamper the manufacturabilty of the system as compared to a ceramic PHE system. The prime concern in such a combination is the adhesion between the film and the substrate at elevated temperatures due to the large difference in the thermal expansion coefficients. (CTE of Hastelloy X:16.6x10-6 at 980°C and CTE of SiC :5.0x10-6 at 1000°C). In this paper, excellent bonding between e-beam deposited SiC film and Hastelloy X metal is demonstrated by employing ion mixing. An Auger elemental mapping data obtained across the interface of the SiC film-Hastelly X substrate are discussed. The surface morphology change as a function of temperature was observed with SEM and optical microscopy and the formation of new phases at the interf
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