Influence of CaF2 on the 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- 1.5 P2O5- 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 765ºC, 679°C and 629°C for 1 hour and then heat treated at 890ºC, 860ºC and 824ºC for the HG 1-3 respectively, for another hour to form apatite. 10 discs per material were heat treated as previously described (765ºC, 679°C and 629°C) then heat treated for 1 hour at 1022°C, 987°C and 892°C for the HG 1-3 respectively, to form apatite-mullite. The heat treatment temperatures were obtained from differential thermal analysis data (DTA). 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 glass transition temperature (Tg) was observed to drop with increasing fluoride content. Conclusions: Increasing the CaF2 content increased the BFS and decreased the Tg of the glass-ceramic materials tested.