The influence of calcium fluoride (CaF2) on biaxial flexural strength of apatite–mullite glass–ceramic materials

Objective The aim of this study was to evaluate the effect of varying the molar percentage of calcium fluoride (CaF2) on the biaxial flexural strength (BFS) of apatite–mullite glass–ceramics. Methods Three apatite–mullite glass–ceramic materials were produced based on a formula of 4.5SiO2–3Al2O3–1.5P2O5–3CaO–XCaF2, where X=0.5, 1 and 3 and called HG 1–3, respectively. These materials were used to produce discs of 2 mm thickness and 12 mm diameter using the lost-wax casting process. 30 discs per material were produced, 10 discs per material were cast and then left to bench cool (glass state), 10 discs per material were given a heat treatment at 765, 679 and 629 °C for 1 h and then heat treated at 890, 860 and 824 °C for the HG 1–3, respectively, for another hour to form apatite. Ten discs per material were heat treated as previously described (765, 679 and 629 °C) then heat treated for 1 h at 1022, 987 and 892 °C for the HG 1–3, respectively, to form apatite–mullite. The heat treatment temperatures were obtained from differential thermal analysis data. A lithium disilicate glass–ceramic was used as a control. Biaxial flexural strength (BFS) was determined using a LIoyd 2000R tester. Results Data showed that the BFS increased as the fluoride content increased, and the apatite–mullite samples had significantly higher BFS values than the as cast glass or apatite samples (p<0.05), with the control having significantly higher BFS values than all the HG glass–ceramic materials for every condition (p<0.05). The fictive glass transition temperature (Tg) was observed to drop with increasing fluoride content. Significance Increasing the CaF2 content increased the BFS and decreased the Tg of the glass–ceramic materials tested.