The effect of calcium fluoride (CaF2) on the chemical solubility of an apatite–mullite glass–ceramic material

Objective To assess the effect of varying CaF2 on the chemical solubility of apatite–mullite glass–ceramic (G–C) materials in both the glassy and crystallized states. Methods Apatite–mullite forming glasses used in this study are ionomer cement derivatives based on the general formula (4.5SiO2–3Al2O3–1.5P2O5–3CaO–XCaF2). Six glass formulations were produced where X=0.5, 1, 1.5, 2, 2.5 and 3, and called HG1–6, respectively. Batches were melted in covered silliminite crucibles in a furnace overnight at 1050 °C, then at 1450 °C for 2 h, before quenching in water. The six glass compositions were analyzed using differential thermal analysis (DTA), X-ray diffraction (XRD) and X-ray fluorescence spectrometry (XRF). Thirty discs (2 mm thick and 12 mm diameter) were produced per glass using the lost wax casting technique. Ten were left as cast and 10 heat treated to either apatite or apatite–mullite. Solubility testing was carried out according to International Standard BS EN ISO 6872 1999 and the mass difference in solubility calculated as μg/cm2. A lithium disilicate G–C system was used as a control material. Results All compositions formed glasses and on heat treatment could form apatite and apatite–mullite. The as-cast glass samples were the most soluble followed by the apatite samples. The apatite–mullite G–C was significantly less soluble than the other two phases (p<0.05) for all six compositions. The control material was significantly less soluble than all the HG glass–ceramic compositions for every phase (p<0.05). Decreasing the CaF2 content (3–0.5 mol%) led to a decrease in solubility, without affecting the ability of the material to form apatite and apatite–mullite phases. Significance Increasing the CaF2 content increases the chemical solubility for the glass, apatite G–C and apatite–mullite G–C phases. The solubility values obtained show that all the compositions, as cast and heat treated would be suitable for use as core ceramics.