Effect of spark plasma sintering on the microstructure and in vitro behavior of plasma sprayed HA coatings

The crystalline phases and degree of crystallinity in plasma sprayed calcium phosphate coatings on Ti substrates are crucial factors that influence the biological interactions of the materials in vivo. In this study, plasma sprayed hydroxyapatite (HA) coatings underwent post-spray treatment by the spark plasma sintering (SPS) technique at 500°C, 600°C, and 700°C for duration of 5 and 30min. The activity of the HA coatings before and after SPS are evaluated in vitro in a simulated body fluid. The surface microstructure, crystallinity, and phase composition of each coating is characterized by scanning electron microscopy and X-ray diffractometry before, and after in vitro incubation. Results show that the plasma sprayed coatings treated for 5min in SPS demonstrated increased proportion of β-TCP phase with a preferred-orientation in the (214) plane, and the content of β-TCP phase corresponded to SPS temperature, up to 700°C. SPS treatment at 700°C for 30min enhanced the HA content in the plasma spray coating as well. The HA coatings treated in SPS for 5min revealed rapid surface morphological changes during in vitro incubation (up to 12 days), indicating that the surface activity is enhanced by the SPS treatment. The thickest apatite layer was found in the coating treated by SPS at 700°C for 5min.