Abstract

The crystallization behaviour of a glass in the SiO2–CaO–F system was analyzed using differential scanning calorimetry (DSC), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Three crystalline phases were detected according to ICDD patterns. The first phase formed at 583°C was identified as CaF2. The morphology was spherulitic with a diameter of approximately 100 nm. The second phase was formed at 664°C. It was identified as calcium fluoride silicate Ca2SiO2F2 (ICDD 35-0002). SEM investigation showed that the crystals were spherulitic with a diameter smaller than 100 nm. The crystals were precipitated in the volume of the glass and homogeneously distributed. As a third phase, cristobalite crystallized at 895°C. The simultaneous release of calcium and fluorine ions from the vitreous glass in lactate buffer solution at pH 4.0, simulating an acidic oral environment, was investigated using X-ray photoelectron spectroscopy (XPS). The release of calcium and fluorine ions is of special interest for dental applications. The atomic ratios of the components Si, Ca and F at the glass surface after different leaching periods were determined. In order to investigate the leaching process, concentration profiles were measured using ion beam sputtering with Ar+-ions. The dependence of the atomic ratios of Si, Ca and F on the sputter time was determined in order to measure the depth of the leaching layers. Most probably, the release of calcium and fluoride was controlled by a surface layer rich in calcium and flourine ions which dissolved with increasing leaching time. After 2 min leaching, a fluoride- rich surface layer measuring approximately 10 nm was detected. The atomic ratios of Si, Ca and F were different from the bulk composition ratios in a surface reaction layer of 800 nm thickness. After 30 min leaching time, a calcium- and fluoride-rich surface layer approximately 50 nm thick was formed. The bulk composition was reached at a depth of approximately 500 nm. The main component in the surface layer, after 12 days leaching in acidic environment, was silicon.

Keywords: calcium fluoride silicate, calcium release, crystallization, dental application