Controllable phases evolution and properties of zinc-phosphate/strontium‑zinc-phosphate composite conversion coatings on Ti: Effect of temperature

Bioactive phosphate coatings based on hopeite (HP, Zn3(PO4)2·4H2O) and strontium‑zinc-phosphate (SZP, SrZn2(PO4)2) phases have attracted extensive attention in the surface modification of metallic implants. Nevertheless, further clarifications on the synergistic effect of HP and SZP, as well as the phase evolution between them are still needed. Herein, a chemical conversion technique was utilized to prepare composite phosphate coatings containing HP and SZP on titanium (Ti). The effect of phosphating temperature on the phase, properties, and deposition mechanism of the coatings was investigated. The results showed that the coating obtained at 50 °C was composed of HP with minor SZP crystals, while their relative contents were reversed at 80 °C. The elevated temperature also changed the crystal morphology of SZP from ellipsoidal to cubic. These variations could be attributed to the enhanced priority of Sr and Zn ions to form bimetallic-phosphate (SZP), which has a different lattice structure from HP crystal. In addition, the HP/SZP composite coating formed at 60 °C exhibited superior comprehensive properties like roughness, wettability, bonding strength, ion release, and corrosion resistance. Overall, this study elucidates the temperature-induced phase evolution and properties of HP/SZP composite coatings, which facilitates the optimal regulation of phase composition to achieve desired properties.