Compositional and microstructural design of highly bioactive P2O5–Na2O–CaO–SiO2 glass-ceramics

Abstract

Bioactive glasses having chemical compositions between 1Na2O–2CaO–3SiO2 (1N2C3S) and 1.5Na2O–1.5CaO–3SiO2 (1N1C2S) containing 0, 4 and 6wt.% P2O5 were crystallized through two stage thermal treatments. By carefully controlling these treatments we separately studied the effects on the mechanical properties of two important microstructural features not studied before, crystallized volume fraction and crystal size. Fracture strength, elastic modulus and indentation fracture toughness were measured as a function of crystallized volume fraction for a constant crystal size. Glass-ceramics with a crystalline volume fraction between 34% and 60% exhibited a three-fold improvement in fracture strength and an increase of 40% in indentation fracture toughness compared with the parent glass. For the optimal crystalline concentration (34% and 60%) these mechanical properties were then measured for different grain sizes, from 5 to 21μm. The glass-ceramic with the highest fracture strength and indentation fracture toughness was that with 34% crystallized volume fracture and 13μm crystals. Compared with the parent glass, the average fracture strength of this glass-ceramic was increased from 80 to 210MPa, and the fracture toughness from 0.60 to 0.95MPa.m1/2. The increase in indentation fracture toughness was analyzed using different theoretical models, which demonstrated that it is due to crack deflection. Fortunately, the elastic modulus E increased only slightly; from 60 to 70GPa (the elastic modulus of biomaterials should be as close as possible to that of cortical bone). In summary, the flexural strength of our best material (215MPa) is significantly greater than that of cortical bone and comparable with that of apatite–wollastonite (A/W) bioglass ceramics, with the advantage that it shows a much lower elastic modulus. These results thus provide a relevant guide for the design of bioactive glass-ceramics with improved microstructure.