Dense carbon (C) and entangled carbon nanofiber (CNF) layers were deposited on nickel foils by decomposition of ethylene in presence of different H2 concentrations at 450 °C for different reaction times. Both C and CNF layer thicknesses increase with time, but samples pre-oxidized at 500 °C normally lead to thinner CNF layers and thicker C layers, as compared to samples pre-oxidized and reduced at 700 °C. The mechanical stability of CNFs decreases with growth time, especially for oxidized-reduced samples. The addition of H2 creates a maximum in the CNF thickness that coincides with a minimum in the C layer thickness, at 5% H2 for samples oxidized at 500 °C and at 20% H2 for samples oxidized-reduced at 700 °C. CNF layer stability increases with C layer thickness but decreases with CNF layer thickness. The ratio between the C layer thickness and the CNF thickness determines in the end the mechanical stability of the CNF layer.