Development of hydroxyapatite/calcium silicate composites addressed to the design of load-bearing bone scaffolds

This work deals with the preparation of bioactive ceramic composites to be employed for the development of load-bearing bone substitutes, made of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA) and bioactive dicalcium silicate (Ca₂SiO₄, C₂S) as a reinforcing phase. The composite materials were prepared by Fast Hot-Pressing (FHP), which allowed the rapid sintering of monolithic ceramics at temperatures up to 1500 ^{{ring operator}}C, well above the commonly adopted temperatures for the consolidation of hydroxyapatite (1200-1300 ^{{ring operator}}C). The purpose was to achieve the grain coalescence of both HA and the strengthening phase, so that to obtain a homogeneous ceramic material characterized by controlled phase composition and improved mechanical strength; the dwell time was reduced as much as possible to prevent HA decomposition and excessive grain growth. The most remarkable result, in terms of phase composition, was the absence of any secondary phases in the final ceramics other than HA and C₂S, even after sintering at 1500 ^{{ring operator}}C. The flexure strength of the composite materials was found to be much higher than that of HA alone. Further mechanical characterization was also carried out on HA and composites, sintered in different conditions, to evaluate the elastic properties and fracture toughness, and properties close to those of mineral bone were found. These preliminary results confirmed that composites of HA and Ca₂SiO₄ are promising for the development of bioactive load-bearing ceramic bone substitutes with controlled phase composition.