Spherulitic crystallization of apatite-mullite glass-ceramics: Mechanisms of formation and implications for fracture properties

Apatite-mullite glass-ceramics crystallize from glass of generic composition SiO₂ Al₂O₃ P₂O ₅ CaO CaF₂ to form an osseoconductive apatite phase existing as spherulites within a mullite matrix. To further investigate the formation of these apatite spherulites, two glass-ceramic systems, both known to form apatite and mullite, were heat treated to varying degrees of crystallinity. Microscopical investigation and mechanical testing was performed on the samples. The results allow us to show the effect of these spherulites on the mechanical properties of the material and elucidate evidence for a previously hypothesized mechanism describing their formation. Mechanical testing was used to determine the effect that this has on the indentation fracture toughness, K_{Ic, idt}. Following heat treatment and fracture testing, samples were prepared and viewed using optical imaging to determine crack interactions with the spherulites. Results showed that these crystal/glass boundaries have a significant influence on the mechanics of crack propagation and as such, K_{Ic, idt} is unreliable for partially cerammed glasses due to the differences between results obtained from indentation measurements performed on the glass, boundary and crystal. Following full ceramming of the glass, a marked increase in K_{Ic, idt} above that of the glass was observed.