Effect of mechanical alloying on the phase evolution, microstructure and bio-corrosion properties of a Mg/HA/TiO2/MgO nanocomposite

In this study, a magnesium-matrix nanocomposite comprising bioactive ceramics of Mg-substituted hydroxyapatite (HA), farringtonite (Mg3(PO4)2), perovskite (CaTiO3), geikielite (MgTiO3) and brucite (Mg(OH)2) was synthesised through mechanical alloying and annealing of a Mg–HA–TiO2–MgO mixture by mechanically induced self-propagating reactions. The phase evolution and microstructure were analysed using X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy. The results indicated that after 16 h of milling, the powder mixture exhibited a homogenous distribution of fine agglomerates composed of spherical particles with an average size of 83 nm. The crystallinity and thermal stability of the HA phase decreased with increasing milling time because of the substitution of Mg atoms in the HA structure. The mean crystallite size of the product phases was approximately 75 nm after 8 h of mechanical alloying, and these values increased slightly to 82 and 88 nm after 1 h of annealing at 500 and 630 °C, respectively. The bio-corrosion properties of the nanocomposites were investigated using electrochemical tests and immersion tests after different milling times and subsequent annealing. The electrochemical tests revealed that the corrosion potential (Ecorr) of the as-blended samples shifted towards a nobler direction from −1565 to −1457 mVSCE by increasing the amount of HA from 12.5 wt% to 25 wt%. After 16 h of milling, the corrosion resistance of the milled samples comprising 12.5 wt% and 25 wt% HA increased to 4.63 and 4.97 kΩ cm2, respectively. The milled samples annealed at 630 °C exhibited lower corrosion rates compared with those annealed at 500 °C. The present study suggests that the high-energy ball milling of the Mg–HA–TiO2–MgO mixture accompanied with post annealing is a promising option for controlling the rapid corrosion rate.