Filamentary oxide formation on iron

• Main Author: Rhoades, B. L.
• Language: en
• Published: University of Canterbury. Mechanical Engineering 2014
• Online Access: http://hdl.handle.net/10092/9574

The oxidation of iron in air or oxygen at temperatures between 300°C and 800°C is accompanied by the growth of filamentary crystals from the outer oxide surface. Below 470°C the predominant crystal habit is pointed platelets of approximately 10 μm thickness and 1-4 μm length, while above 470°C ribbon-like whiskers of 5 μm to 2 μm thickness and lengths up to 30 μm are observed. The structure of these crystals has been examined by transmission electron microscopy and electron diffraction. Selected area electron diffraction spot patterns have been indexed using a rapid computer technique which has been developed to compare the geometry of diffraction patterns with reciprocal lattice planes of any crystal. Both platelets and whiskers are shown to consist of αFe₂O₃ with <101> structural rhombohedral growth direction. A feature of diffraction from the platelet crystals is the presence of continuous reciprocal lattice streaks, which indicate the existence of a high density of planar defects lying parallel to the (010) platelet face. Most of the whisker filaments consist of multiple crystals with a common <101> growth direction; and no evidence of lattice twist, to indicate the existence of screw dislocations, is provided by the diffraction patterns. The construction of a high temperature controlled atmosphere specimen chamber for the JEM 7A electron microscope has enabled the growth process of the oxide filaments to be directly observed. Specimen motion includes tilting to a limit of 15° about any axis in the plane of the specimen and continuous observation is possible at temperatures up to 1000°C and gas pressures up to 30 kPa. After an initial induction period both whisker and platelet crystals are preferentially nucleated on surface protruberances and exhibit a linear rate of growth with respect to time The linear growth is abruptly terminated at a variety of filament lengths. At 500°C a variation of growth rates of 1.6 μm/min to 0.1 μm/min is observed between adjacent filaments on a single specimen; and a maximum growth rate of 15 μm/min has been observed at approximately 800°C. Platelet crystals maintain a constant shape and broaden at the base as the length increases. Arguments are presented in support of the proposal that the growth of both platelets and whiskers occurs by transport of oxide molecules to the filament tips by surface diffusion over whisker and platelet sidewalls of high crystalline perfection; rate control being attributed to the generation and propagation of growth steps at the filament tips.