Carbon deposition over transition metal alloys—II. Kinetics of deposition over 〈FeNi〉 and 〈FeCo〉 alloy foils

The effects of 〈FeNi〉 and 〈FeCo〉 alloy composition on the deposition of filamentous carbon was investigated. This was accomplished by comparing the relative rates of carbon formation over pure Fe, Ni, and Co foils along with their respective carbide(s), to those over 〈FeNi〉 and 〈FeCo〉 alloys and the associated metal phases. It was observed that the heat treatment to which a foil is subjected prior to carbon-depositing conditions can affect the rate of carbon deposition. Heat treatment causes grain nucleation and growth, decreases internal stresses, as well as allowing dislocation migration. These changes within the foil inhibit subsequent carbide formation, decreasing the rate of surface break-up, leading to longer induction periods. Alloy foils ranging from pure Fe to either pure Ni or Co were exposed to a reaction gas having an a ̂ C of 10. This a ̂ C is sufficiently high to favor solid carbon deposition and, depending on the alloy concentration, ≤ 50 wt% Ni in 〈FeNi〉, or ≤ 35 wt% for Co in 〈FeCo〉 alloys, Fe 3C is thermodynamically favored to form at these conditions. For the 〈FeNi〉 system it was determined that when iron carbides are among the thermodynamically favored solid phases, the rate of carbon deposition was high (75 to 100 μg/cm 2/min). If iron carbides were not favored then the rate was low,