Fluorine deficient layer at the surface of transparent glass-ceramics with CaF2 nanocrystals

Optically transparent glass-ceramics with CaF₂ nanocrystals (diameter: ～30 nm) for 25CaF₂–5CaO–20Al₂O₃–50SiO₂ were fabricated using a conventional glass crystallization method in an electric furnace in air and the dispersion state of CaF₂ nanocrystals at the surface was examined from transmission electron microscope observations combined with focused ion beam sample preparations. It was demonstrated that a fluorine deficient layer of about 150–200 nm is formed during the heating at temperatures above the glass transition temperature in the surface region and thus CaF₂ nanocrystals are not present in the surface region. The reason for the formation of such fluorine deficient layers is due to the removal of some fluorine from the surface of bulk glass during heating. The present study gives new information on the crystal growth behavior of fluoride nanocrystals in oxyfluoride glass-ceramics.