Equipment variables and deposition conditions affecting the plasma oxidation of pyrocarbon coatings on HTGR fuel particles

Selective etching with an oxygen plasma is a promising technique for revealing the structure of the high-density pyrocarbon (PyC) coatings used on HTGR nuclear fuel particles and can be used to predict the fast-neutron damage resistance of the PyC. Several factors that affect either the etching rate or the etch structure were examined in this investigation. Particles with PyC coatings deposited from hydrocarbon MAPP gas (C3H4 isomers) under a variety of conditions were polished to the midplane and systematically plasma oxidized to determine the effect of the coating parameters on the observed etch structure. Identical coatings were then oxidized by use of different plasma parameters to determine the relation of the observed etch pattern to these parameters. The concentration of hydrocarbon gas supplied during PyC deposition significantly affected the etch structure of the coatings. The etching rate of the PyC coating was an increasing function of the radiofrequency power used to excite the plasma and of the temperature of the coatings during the oxidation process. The plasma etching rate also appears to be related to the density of the coating (as measured by the gradient column method) and its deposition temperature. The etch structures of the coatings change dramatically in certain cases as the etching time is increased. But the dependence on etching time varies for particles having different preparation histories and thus complicates the use of this technique to characterize PyC coatings on HTGR fuel particles. The etch structure is also affected by the distribution of pores in the coatings. High etching rates occurred in localized areas of the coatings where the original pore concentration was high