Microstructure investigation of plasma sprayed HA/Ti6Al4V composites by TEM

Forming bio-functional composites between hydroxyapatite (HA) and a metallic material can be an effective means of overcoming the poor mechanical properties of pure HA and the poor bond strength between HA coating and metallic prosthesis. In this study, HA/Ti6Al4V composite coatings were obtained through plasma spraying of HA/Ti6Al4V composite powders of 75–106-μm sizes onto Ti6Al4V substrates. Experimental results demonstrated that the bond strengths of these composite coatings with Ti6Al4V substrates were significantly enhanced over pure HA coatings. Transmission electron microscopy (TEM) analysis revealed that the as-sprayed coatings contained a large amount of amorphous phases. Within and adjacent to the amorphous regions, fine HA crystals (<50 nm) were formed during rapid solidification. Large HA crystals (>1 μm) were also observed in other areas. Molten Ti6Al4V alloy formed fine grains (<50 nm) adjacent to amorphous calcium phosphate. The good adherence among the phases as shown in the TEM images was believed to be due to the good wetting characteristics when the ceramic and metallic phases were concomitantly molten in the plasma stream. Ti6Al4V alloy with the main phase of α′ was homogeneously distributed in the coatings. Occasionally a small amount of oxide in the Ti6Al4V fine grain area was formed due to Ti6Al4V oxidation as detected from the electron diffraction pattern. After the coatings were heat treated at 600°C, crystalline HA material was formed from the amorphous regions. For this reason, the amount of amorphous phase was significantly reduced and could only be observed in certain local areas.