Spark plasma sintered superplastic deformed transparent ultrafine hydroxyapatite nanoceramics

Abstract

Hydroxyapatites (HAs) are used as bioceramics for artificial bone substitutes because of their good biocompatibility. In this study, highly transparent ultrafine HA ceramics with a mean grain size of 170 nm were synthesised by spark plasma sintering at 900–1000°C and 80 MPa. Phase analysis revealed the presence of a pure HA phase even after sintering at 1000°C. The sintered body was almost fully dense (>99%). The total forward transmission was >70% at 400 nm, and it approached the theoretical value of 89% in the infrared wavelength range. The HA ceramics contained several intragranular voids of 5–10 nm. An evaluation of the superplastic flow behaviour of this sintered HA sample at 950–1050°C revealed superplasticity with a maximum elongation and initial strain rate of 486% and 1.0x10^− 4 s^− 1 respectively at 1000°C. The deformed microstructure of HA indicated activated dislocation motion assisted grain boundary sliding to be the major mechanism of superplastic flow with stress exponent values ranging from 4 to 5 based on interface reaction controlled creep. Surprisingly, no dislocations were observed by transmission electron microscopy.

Keywords:

• Spark plasma sintering
• Transparent
• Hydroxyapatite
• Superplasticity
• Deformation mechanism

Acknowledgements

This work was supported by the 2015 Yeungnam University Research Grant. Many thanks to Professor Amiya K. Mukherjee at the University of California, Davis, USA, for his valuable comments on dislocation movement in superplastically deformed nanocrystalline materials.