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

Abstract

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 (∼10μm). Upon heating at≥1173K 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∼34min for the 20% weight loss of the CVD crack healed sample in the oxidation test with annealing at 1173K, while it took∼8min for the EB-PVD coated sample, which means it took ∼4 times longer at 1173K for the same weight reduction in this experimental set-up.