Dynamic oxidation of a Fe2+-bearing calcium–magnesium–aluminosilicate glass: the effect of molecular structure on chemical diffusion and reaction morphology

Rutherford backscattering spectroscopy (RBS) and optical microscopy (OM) and transmission electron microscopy (TEM) have been used to characterize the oxidation process in a homogeneous, well-annealed, ferrous iron-bearing calcium–magnesium–aluminosilicate (Fe–CMAS) glass. Suites of specimens were exposed to oxidizing environments of air (pO2=0.21 atm) or of argon (pO2∼10−6 atm) within a time range 1–200 h and a temperature of ∼750°C (near the glass transition). Oxidation causes: (1) formation of crystalline Mg/Fe oxides on the free surface of the glass, and (2) an internal region that is depleted of divalent cations. In general, this morphology is unequivocal evidence of the oxidation being rate-limited and dominated by the chemical diffusion of divalent, network-modifying cations out of the glass; the cation flux is charge-compensated by an inward flux of electron holes (polarons). Specifics of the reaction morphology vary for the two oxidation atmospheres. In the case of the Ar environment, outfluxing Mg2+ and Fe2+ form discrete particles of crystalline (Mg,Fe)3O4 on the free surface of the glass; in air, a continuous surface film forms that contains cubic γ-Fe2O3. In both cases, nm-scale ferrites precipitate at an internal reaction front; the air-oxidized case sees the presence of a second front that changes the ferrite precipitates to γ-Fe2O3. The air-oxidized case also sees substantial outfluxing of Ca2+. Consistent with rate-limitation by chemical diffusion, parabolic reaction kinetics characterize the oxidation reaction. The different reaction morphologies seen for the different oxidizing environments demonstrate directly the applicability of the Modified Random Network (MRN) model to the structure of the oxidized (residual) glass. Removal of Mg2+ and Fe2+ in the case of Ar oxidation creates internal ferrite precipitates and a residual glass that retains interconnected channels for the network-modifying cations, hence the formation of discrete crystalline oxide precipitates on the surface. In air, the internal transformation of the ferrite to γ-Fe2O3, by requiring the outward flux of Ca2+, sees the collapse of the interconnected channels in the remnant glass, and so a continuous oxide film forms on the free surface. The threshold network-modifier-oxide content for the existence of the interconnected channels of modifiers in the CMAS residual glass is thus estimated as ∼10 vol.%.