Interfacial reaction of Co-Fe films with SiO2 substrates

The interdiffusion reaction between Co1−xFex deposited films of various compositions (x = 0.27, 0.32 and 0.50) and an amorphous SiO2 substrate during annealing in vacuum at 800 °C was identified by analytical transmission electron microscopy. The reaction results in the formation of Fe2SiO4 mixed silicate of olivine structure as an interfacial phase. The following microstructural changes occurring during this reaction are inferred: (a) recrystallization of as-deposited films during the 800 °C annealing results in large grains of the body centered cubic Co-Fe solid solution; (b) metals diffuse into the SiO2 substrate and nucleate grains of the Fe2SiO4 silicate along the film/SiO2 interface; (c) silicon and oxygen partially released during the reaction, in turn, diffuse into an unreacted metallic film and form precipitates of the (Co,Fe)3O4 spinel phase and solid solution of Si in Co-Fe. To our best knowledge, the formation of silicates with olivine-type structure (known as fayalite for Fe) as products of the metal/SiO2 reaction has never been reported before. Thermodynamic evaluation of the reaction employing the semi-empirical CALPHAD (Calculation of Phase Diagrams) method supports the experimental findings, although the reaction requires an excess of oxygen.