Spark plasma sintered hydroxyapatite-yttria stabilized zirconia composites

Hydroxyapatite (HA) ceramics have been well known to have good bioactivity, i.e. bone bonding ability. However, their mechanical properties need to be improved for some biomedical applications. One of the commonly used methods to improve the mechanical properties has been the production of HA-based composites. Nevertheless, conventional pressure less sintering tends to result in poor densification and severe decomposition of the HA phase. In this study, hydroxyapatite-yttria stabilized zirconia (YSZ) composites were fabricated by means of spark plasma sintering (SPS) technique. A relative density of up to 93% theoretical density of the HA-YSZ composites was achieved after spark plasma sintering at 1200°C for only 5min. Only slight decomposition of HA to alpha-tricalcium phosphate (α-TCP) was observed at the sintering temperature of 1150°C. Zirconia second phase remained to be tetragonal ZrO₂ phase even when the sintering temperature was as high as 1250°C. It was also found that the equiaxial YSZ grains were uniformly dispersed in the HA matrices. The HA grain size in the HA-YSZ composites was much smaller than that in the pure hydroxyapatite monoliths. Furthermore, the HA-YSZ composites sintered at 1200°C showed microhardness of about 9GPa and Young's modulus of up to 160GPa, considerably higher than those of the pure HA ceramics. The bending strength of the composites reached 200MPa under the current sintering conditions, which was nearly one time higher than that of the pure HA ceramics.