Structural rearrangement of a polycrystalline iron surface induced by carbon precipitation

The morphology of carbon precipitated at a polycrystalline iron surface, and the associated metal structural changes, have been examined by in situ hot stage scanning electron microscopy. Carbon is dissolved in the metal by reaction with methane at T ~ 750°C, and is precipitated by cooling the specimen below the Fe-C eutectoid temperature. The preparation of a clean thermally stable surface is described, along with the structural changes occurring in the CH₄ atmosphere during carbon dissolution. Precipitation on cooling is accompanied by a significant reorganisation of the metal surface structure. Facetted regions with step heights in the 100–300 Å range are smoothed out, the surface becoming covered with a laminar carbon film. There is no evidence of enhanced activity at grain boundary intersections. Intermediate stages in the complete precipitation process have been identified. The rate of lateral growth of the precipitated carbon overlayer is found to be limited by surface diffusion. The effect of adsorbed sulphur and oxygen impurities has been examined by scanning Auger microanalysis. The separation of particulate material from the surface, in a size range up to ~ 1.6 μm dia., has been observed if the specimen is cycled repeatedly between dissolution and precipitation temperatures.