Carbide formation in tungsten coatings on carbon-fibre reinforced carbon substrates

摘要

Tungsten coatings with molybdenum interlayer deposited on carbon-fibre reinforced carbon (CFC) substrates were selected as the first wall material for the divertor in the Wall Project at Joint European Torus (similar to the International Thermonuclear Experimental Reactor). For such a layered structure, diffusion of carbon from the CFC substrate towards the Mo and W deposits is expected during the operation of the reactor. As both molybdenum and tungsten form stable carbides, brittle compounds may form at the interface, thus strongly affecting the thermomechanical performance of the coated tiles. For the purpose of prediction of the operation time of such coated tiles, carbon diffusion and carbide formation kinetics need to be determined. In the present study, W/Mo/CFC samples were subjected to heat treatment at 1470 K for various annealing times. The Focused Ion Beam technique was used for sample preparation for electron microscopy examinations. Transmission electron microscopy observations supported with diffraction pattern analyses revealed the both W_2C and WC carbides in the W coating, as well as that of Mo_2C carbide in the Mo layer. The results were used to estimate the kinetics of coatings degradation.