Effect of doping (Mg,Mn,Zn) on the microstructure and mechanical properties of spark plasma sintered hydroxyapatites synthesized by mechanical alloying

Although hydroxyapatite based bioceramics exhibit excellent bioactivity, their poor mechanical properties limit their applications to primarily as non-load bearing components and surface coating. The current investigation is aimed to mitigate this problem. In the present study, nanocrystalline undoped and 5 mol% Mn, Mg and Zn-doped HAp powders are synthesized by mechanical alloying at room temperature. All as-milled samples are subsequently sintered at 950 degrees C using spark plasma sintering (SPS) process to retain the nanocrystalline grains during sintering. Microstructural characterization in terms of lattice imperfections and relative phase abundances in both as-milled and sintered samples are done by analyzing respective XRD patterns employing Rietveld's structure and microstructure refinement method. For all HAp compounds, similar to 97% of theoretical density has been achieved after powder compaction into pellets by SPS without significant grain growth. Mechanical properties such as, hardness, elastic modulus, fracture toughness and brittleness index of the sintered and polished pellets are measured using Vickers microhardness tester with varying load up to 1 kg. Significant improvements in the mechanical properties of doped HAp pellets are noticed with 220% increase in fracture toughness for Mn doped sample. The experimental results are interpreted by the changes in structure and microstructure parameters of both undoped and doped HAp samples.