Mechanical performances and microstructural characteristics of plasma-sprayed bio-functionally gradient HA–ZrO2–Ti coatings

Bio-functionally gradient HA–ZrO2–Ti coatings on Ti6Al4V were fabricated through the plasma spraying process. The characteristics of gradient variations in physical performances, microstructure, chemical compositions, and fracture surface were investigated by means of nanoindentation, scanning electron microscopy with an energy dispersive spectroscopy and X-ray diffraction. The results showed that: (1) hardness and Young's modulus increased gradually from the Ti6Al4V substrate to the coating along the line vertical to the coating/Ti6Al4V interface; (2) microstructure and compositions changed gradually in the coatings, no distinct interface between adjacent layer of different compositions was observed by scanning electron microscopy; (3) the characteristic of the fracture surface demonstrated that the bio-functionally gradient coatings had the compositive mechanical properties such as intergranular cleavage and ductile tearing; and (4) as revealed by the X-ray diffraction results, the surface of the heat-treated coating was composed of crystal hydroxyapatite. According to ASTM C633-79 standard, the bonding strength of the two kinds of coatings were obtained and the value of heat-treated bio-functionally gradient coatings was 53.6 MPa, much higher than that of single HA coatings.