Enhancing the oxidation resistance of graphite by with crack healing at an elevated temperature

摘要

The potential of reducing the oxidation of the supporting graphite components during normal and/or accident conditions in the Very High Temperature Reactor (VHTR) design has been studied. In this work efforts have been made to slow the oxidation process of the graphite with a thin SiC coating (similar to 10 mu m). Upon heating at >= 1173 K in air, the spallations and cracks were formed in the dense columnar structured SiC coating layer grown on the graphite with a functionally gradient electron beam physical vapor deposition (EB-PVD. In accordance with the formations of these defects, the sample was vigorously oxidized, leaving only the SiC coating layer. Then, efforts were made to heal the surface defects using additional EB-PVD with ion beam bombardment and chemical vapor deposition (CVD). The EB-PVD did not effectively heal the cracks. But, the CVD was more appropriate for crack healing, likely due to its excellent crack line filling capability with a high density and high aspect ratio. It took similar to 34 mu m for the 20% weight loss of the CVD crack healed sample in the oxidation test with annealing at 1173 K, while it took 8 min for the EB-PVD coated sample, which means it took 4 times longer at 1173 K for the same weight reduction in this experimental set-up. (C) 2016 Elsevier B.V. All rights reserved.