CARBON DEPOSITION OVER TRANSITION-METAL ALLOYS .2. 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, less-than-or-equal-to 50 wt% Ni in [FeNi], or less-than-or-equal-to 35 wt% for Co in [FeCo] alloys, Fe3C 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 mug/cm2/min). If iron carbides were not favored then the rate was low, <30 and <5 mug/cm2/min for [FeNi] and Ni3C, [FeNi], respectively. Also, [FeCo] alloys near 45 wt% Co were exposed to various gas mixtures with a(C)s ranging from almost-equal-to 10 to almost-equal-to 35. It was determined that when iron carbides are among the thermodynamically favored solid phases, the rate of carbon deposition was high (80 to 100 mug/cm2/min). If iron carbides were not favored, then the rate was significantly lower, <30 and <10 mug/cm2 min for [FeCo] and Co2C, [FeCo], respectively. If the [FeCo] alloy foils were preoxidized then the rate of carbon deposition increased. For the [FeCo] system the increased rates of carbon deposition are attributed to an increase in the surface area of the ''catalyst'' as Fe3C forms and breaks up the foil's surface.