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.
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