Microprocesses in nickel accompanying metal dusting

Polycrystalline nickel was reacted at 680 °C with a carbon-supersaturated CO/H2/H2O gas. Graphite deposited on the metal surface as (a) very thin (20–200 nm) layers on a smooth metal surface, (b) thick (1–2 μm), relatively uniform layers which developed serrated incursions into the metal, and (c) nodules which stood proud of, and protruded into, the metal surface. The thin, benign graphite deposits formed on {0 0 2} nickel surfaces but thicker graphite layers grew on {1 1 1} surfaces. Thickening of that layer, and graphite nodule growth, were both associated with the inward growth of graphite basal planes parallel to nickel {1 1 1} and {1 1 3} planes located within the metal interior. Microanalysis showed that the graphite contained nickel, which was identified by dark-field imaging and selected-area diffraction as nanoparticulate metal. These nanoparticles are thought to have resulted from metal disintegration caused by the volume expansion accompanying graphite nucleation and growth.