| Summary: | Biocompatible <i>β</i>-Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> and mechanically stable <i>t-</i>ZrO<sub>2</sub> composites are currently being combined to overcome the demerits of the individual components. A series of five composites were synthesized using an aqueous precipitation technique. Their structural and mechanical stability was examined through X-ray diffraction, Rietveld refinement, FTIR, Raman spectroscopy, high-resolution scanning electron microscopy, and nanoindentation. The characterization results confirmed the formation of <i>β</i>-Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>–<i>t</i>-ZrO<sub>2</sub> composites at 1100 °C. Heat treatment above 900 °C resulted in the degradation of the composites because of cationic interdiffusion between Ca<sup>2+</sup> ions and O<sup>−2</sup> vacancy in Zr<sup>4+</sup> ions. Sequential thermal treatments correspond to four different fractional phases: calcium-deficient apatite, <i>β</i>-Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>, <i>t</i>-ZrO<sub>2</sub>, and <i>m</i>-ZrO<sub>2</sub>. The morphological features confirm in situ synthesis, which reveals abnormal grain growth with voids caused by the upsurge in ZrO<sub>2</sub> content. The mechanical stability data indicate significant variation in Young’s modulus and hardness throughout the composite.
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