Crack-free, Oxidation-Immune Coatings for Carbons

Carbon materials have been proposed for the control rod structural materials of the next-generation nuclear power plant. Both graphite and carbon fiber carbon matrix [C/C] composites have many outstanding qualities for this application, but they are subject to rapid oxidation by air and water at both the baseline operating temperatures and the much higher temperatures that will occur during depressurized conduction cooldown (DCC) accident scenarios. DCC accompanied by an air/water ingress accident would rapidly and drastically alter their shape and function. Also, the long term oxidation of carbon control rods by the residual oxidizing gases in the helium coolant of the next generation nuclear plant (NGNP) has never been studied. The oxidation of both graphite and C/C composites can be mitigated by applying a silicon carbide (SiC) coating to the composite. However, due to thermal expansion mismatch between SiC and carbon, the coatings crack during cooling after their synthesis. At temperatures above ~1350 C, silica, created by oxidation of the SiC, flows to seal the cracks. At lower temperatures, though, the glass does not flow and oxidation is unchecked. In this project, TDA research will use a new low-temperature chemical route to produce functionally graded SiC coatings on carbon materials that greatly reduce the stress during synthesis, preventing crack formation during cooldown, and providing an oxidation-resistant coating that can function in both temperature regimes, as well as in the high neutron flux of the reactor core.Commercial Applications and Other Benefits: Aside from the use described in nuclear reactor cores, robust SiC coatings for C/C composites that extend their use temperatures in oxidizing environments will also find application in gas turbine blades, rocket exhaust nozzles and deflectors, and high-temperature manufacturing.