Carbon deposition and filament growth on Fe, Co, and Ni foils using CH₄H₂H₂OCOCO₂ gas mixtures

Nonequilibrium phase diagrams have been used to systematically investigate carbon deposition and filament growth on Fe, Ni, and Co foils. All three metals were observed to have similar slow rates of carbon deposition in the metal phase field. The carbon formed was primarily amorphous and platelet in type and morphology. In regions where carbides were thermodynamically favored to form, carbon deposition rates were orders of magnitude higher for Fe and Co foils. In the carbide phase field, the rate of fractional weight gain was highest at the highest carbide decomposition temperature and became progressively lower as the carbide decomposition temperature dropped. The order was Fe ≫ Co ≫ Ni. Carbon deposition in the carbide region for Fe and Co produced filaments and amorphous carbon and showed little or no site preference. Carbon deposition on Ni was plane specific and in the carbide region was primarily filamentous in morphology. The primary source of carbon on Ni is CH₄, while on Fe and Co it appears to be CO. The role of carbides on these foils is hypothesized to be to increase surface area through surface break-up, and to help set up the mass flux gradient for filament growth.