Formation of Cristobalite From Silicon Carbide

Development of oxide scale microstructure on single-crystal SiC

“…la shows an oxide film grown for 0.5 h. While it has skeletal formations of thin rosettes the film is amorphous, as the accompanying selected area diffraction (SAD) pattern in Fig. lb shows. (In view of following results, the rosettes are suspected to be incipient spherulltes or "crystallization centres", as suggested by Schuster and Gugel [2] .) Crystallization started early: in most samples the scale was fully devitrified to spherulitic cristobalite after 1 h at 1400 ~ C. An example is shown in Fig.…”

“…Much of the work on SiC oxidation was conducted on such granules, with consequent complication of the interpretation of results: such materials behave neither quite as individual single crystals nor as polycrystalline samples in oxidation and the results obtained were strongly dependent on the sample used. The coarser granules, essentially single crystals [2], oxidized differently from the powdery specimens whose kinetics were found to be strongly influenced by the granule size [3,4]. There were also contradictions concerning the polymorphic phase and the structural details of the oxide formed.…”

“…There were also contradictions concerning the polymorphic phase and the structural details of the oxide formed. Shuster and Gugel [2] provide a survey of these.…”

“…Nevertheless the works cited, excepting those of [2,8], dealt mainly with oxidation kinetics. Yet oxide scale microstructure can reveal useful information on, for instance, the diffusion process limiting oxidation [8] and phase changes during oxidation [2]. The latter acquires special importance in view of the claims that crystalline SiO2 may be less passivating than the vitreous form [4,9,10].…”

“…Single crystals of SiC grown under controlled conditions are now available and their oxidation is yielding interesting observations on, among other things, the disparate rates of oxygen adsorption on different crystallographic faces [5] and of oxidation on faces of different polarity [6] *; the effect of phase and polytypic modifications on the oxidation behaviour [2] ; the role of temperature and pressures on the active/passive nature of the oxidation [7] ; and the transition of the oxide scale from amorphous silica to cristobalite [2,8]. Nevertheless the works cited, excepting those of [2,8], dealt mainly with oxidation kinetics.…”