Resultant effect of two different methods of incorporating metals in the hydroxyapatite structure on its mechanical...

Metal deficiencies in synthetic hydroxyapatite (Hap), is one factor among others behind its poor mechanical reliability. Hence, compensating these deficiencies has been suggested as a possible way to improve these properties. Several studies have shown that the incorporation of metals or metal oxides appear to increase the fracture toughness and lower the brittleness of synthetic Hap. In this study, we examined the resultant effect of two different methods of incorporating metals into the Hap structure on its mechanical properties. The first method consisted of incorporating metals by replacing some calcium in the Hap structure via substitution. The second method consisted of modifying Hap by preparing (metal oxide)/Hap composites. Thus Nano powders of pure Hap, substituted Hap (Ca_10-xM_x(PO₄)₆(OH)₂), and (MO)_x /Hap composites (M = Mg, Zn) with x = 0.0, 0.1, and 0.3 were prepared by a simple low temperature technique. The (ZnO)_x/Hap nanocomposites were prepared by precipitating Zinc oxide nanoparticles in a nano Hap slurry, while the (MgO)_x/Hap composites were prepared by precipitating Hap nanoparticles in a diluted nano MgO slurry. XRD, FTIR, and SEM analysis of all prepared materials identified Hap as the only crystalline phase present, exhibiting a uniform morphology with particles in the 40-100 nm size range. EDAX analysis clearly shows the presence of Ca, P, and O in pure hydroxyapatite, Ca, P, O, Mg and Zn in both substituted Hap and (MO)_x/Hap composites. The mechanical testing consisted of evaluating Breaking force, work of fracture, and brittleness/ductility of Hap, substituted Hap, and (MO)_x/Hap composites. Our study clearly shows that the mechanical properties of Hap are much more improved by modifying Hap with metal oxides than by calcium substitution for the same quantity and type of incorporated metal. Optimum mechanical strength was obtained for the (MO)_x/Hap composites with a 0.1:1.0 MO:Hap mole ratio (M = Mg, Zn).