Preparation And Properties Of High Strength Calcium Silicate Bioceramics

As calcium silicate（CaSiO₃,CS）ceramics have excellent bioactivity,biocompatibility and biodegradability,they have been widely used in bone repair.However,due to poor sinterability,the mechanical strength and fracture toughness of CS ceramics are relatively low.So,it is difficult for CS ceramics to maintain mechanical stability under stress.Therefore,it will significantly promote the practical application of CS ceramics in clinical practice by seeking suitable modification technology to prepare calcium silicate ceramics matching the mechanical properties of bone implant components.In addition to the mechanical properties of bone repair materials,proper pore structure is also a necessary condition to achieve cell adhesion growth and bone tissue regeneration.Therefore,this paper first explored the appropriate sintering additive system,and improved the sinterability and mechanical properties of calcium silicate ceramics by adjusting the process parameters.Then,on this basis,appropriate pore-forming processes were introduced to prepare porous ceramics with ideal pore structure.The study on improving the mechanical properties of calcium silicate ceramics by sintering additives showed that B₂O₃-SiO₂ binary sintering additives could promote the sintering densification by liquid phase sintering,and induce the transformation of the morphology of calcium silicate grains from granular to rod-like,thus improving the mechanical properties of calcium silicate ceramics.By regulating various process parameters,we could find that increasing sintering additives content and sintering temperature led to the significant densification of matrix.Thus,the porosity decreased and mechanical properties increased.The relative composition of the additives affected the size of the rod-like grains mainly by regulating the viscosity of the high-temperature liquid phase.With the decrease of the relative content of B₂O₃ in the additives,the viscosity of the liquid phase increased,and the rod-like grains became finer and the aspect ratio was lower.β-CS ceramics reinforced by 5 wt.%B_92.5(B_92.5 representing the mass ratio of B₂O₃:SiO₂ as 92.5:7.5)sintered at 1000°C for 2 h revealed the best mechanical properties.The flexural strength and fracture toughness were 228.1 MPa and 1.2 MPa·m^1/2,respectively.The study on the preparation of porous calcium silicate ceramics by adding porogens showed that with the increase of the content of porogens,the pore connectivity and porosity of samples gradually increased,while the compressive strength decreased.The size of porogens didn’t influence the porosity significantly,but was mainly used to regulate the pore size of porous ceramics.In addition,the pore cliff of the porous ceramics with B₂O₃-SiO₂ sintering additives was composed of tightly lapped rod-like grains with the aspect ratio of about 4.9.This structure would improve the strength of porous ceramics with high porosity.The study on the preparation of porous calcium silicate ceramics by freeze-casting method showed that the porous ceramics prepared with water-based slurry showed parallel lamellae pore structure along the direction of temperature gradient.The influence of various process parameters was investigated.It could be found that,with the increase of the solid content of slurry,the size of lamellar pores reduced,the porosity of porous ceramics decreased,and the compression strength increased.The freezing temperature mainly regulated the lamellar pore size of the porous ceramics but had little effect on the porosity.As the freezing temperature lowed,the lamellar structure became finer,and the strength of samples increased.What’s more,adjusting the content of sintering additives and sintering temperature was beneficial to promote the densification of the pore wall and improve the mechanical properties of samples.On the basis of freeze-casting,this work also tried to regulate the three-dimensional pore structure of porous ceramics by introducing a second pore-forming process.By combining freeze-casting with 15μm microspheres as porogens,a hierarchical pore structure was obtained,which consisted of 10μm pore formed by decomposition of porogens and 200μm lamellar pore formed by sublimation of ice crystals.By combining the freeze-casting and 3D-printing resin template,a hierarchical pore structure with 3.2μm pores formed by the lapping of rod-like grains on the pore cliff,interlamellar pores of more than 10μm formed by ice crystals and 200μm three-dimensional connected macro channels formed by the template was constructed.