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 o f filamentous carbon was investigated. This was accomplished by compar ing the relative rates of carbon formation over pure Fe, Ni, and Co fo ils 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-depositin g conditions can affect the rate of carbon deposition. Heat treatment causes grain nucleation and growth, decreases internal stresses, as we ll as allowing dislocation migration. These changes within the foil in hibit subsequent carbide formation, decreasing the rate of surface bre ak-up, leading to longer induction periods. Alloy foils ranging from p ure Fe to either pure Ni or Co were exposed to a reaction gas having a n a(C) of 10. This a(C) is sufficiently high to favor solid carbon dep osition 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 [FeC o] alloys, Fe3C is thermodynamically favored to form at these conditio ns. For the [FeNi] system it was determined that when iron carbides ar e among the thermodynamically favored solid phases, the rate of carbon deposition was high (75 to 100 mug/cm2/min). If iron carbides were no t 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 e xposed 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 a re among the thermodynamically favored solid phases, the rate of carbo n deposition was high (80 to 100 mug/cm2/min). If iron carbides were n ot favored, then the rate was significantly lower, <30 and <10 mug/cm2 min for [FeCo] and Co2C, [FeCo], respectively. If the [FeCo] alloy fo ils were preoxidized then the rate of carbon deposition increased. For the [FeCo] system the increased rates of carbon deposition are attrib uted to an increase in the surface area of the ''catalyst'' as Fe3C fo rms and breaks up the foil's surface.