Preparation of tricalcium phosphate/alumina composite nanoparticles and self-reinforcing composites by simultaneous precipitation

The composite nanoparticles and corresponding self-reinforcing composites comprising tricalcium phosphate (TCP) and alumina (Al₂O₃) were synthesized by simultaneous precipitation from the CaCl₂, AlCl₃ and (NH₄)₂HPO₄ aqueous solutions, using aqueous NH₄OH as precipitant. Influences of the precipitating media pH and the Ca/P atomic ratios on phase composition and morphology of the composites were investigated. Results showed that except for the major phases β-TCP and α-Al₂O₃, there was always a third minor phase in the calcined composites coprecipitated either in neutral or alkaline condition. Formation of β-TCP is, however, favored at pH 9.2, whereas more of the third phase, mainly AlPO₄, is formed under neutral condition. High Ca/P ratios suppress the formation of α-Al₂O₃ phase under alkaline precipitating condition, but the effect is less significant in neutral condition. TEM observation showed that the ‘as prepared’ composite particles are nano-sized but interconnected to form a network-like morphology. They were changed to a core-shell-like structure after calcination, while their nano-scale dimension was retained. FEGSEM analysis revealed that the α-Al₂O₃ phase in the sintered composite compacts was in the form of fibrils dispersed in the phosphate phases. These in situ formed fibrils impart a unique role in self-reinforcement of the sintered composites. Mechanical measurements showed that the incorporation of alumina reinforced β-TCP effectively: the flexural strength increased from 15 MPa of the pure β-TCP to 84 MPa of the composite with 40 wt% of α-Al₂O₃.