Preparing hydroxyapatite-silicon composite suspensions with homogeneous distribution of multi-walled carbon nano-tubes for electrophoretic coating of NiTi bone implant and their effect on the surface morphology

- The stable composite suspensions of hydroxyapatite, silicon and multi-walled carbon nano-tubes was prepared using functionalization of and multi-walled carbon nano-tubes in HNO3 vapor and triethanolamine as dispersing agent.
- The zeta potential of composite suspensions is less than that of hydroxyapatite suspension.
- The silicon particles presence in suspension causes to decrease the charge carrier in suspension and current density during electrophoretic deposition.
- The orientation of multi-walled carbon nano-tubes to parallel direction of the applied electric field during electrophoretic deposition can facilitate their moving towards the cathode and increase current density.
- The more zeta potential of suspension, the lower roughness of coatings during electrophoretic deposition.

Preparing a stable suspension is a main step towards the electrophoretically depositing of homogeneous and dense composite coatings on NiTi for its biomedical application. In the present study, different composite suspensions of hydroxyapatite, silicon and multi-walled carbon nano-tubes were prepared using n-butanol and triethanolamine as media and dispersing agent, respectively. Multi-walled carbon nanotubes were first functionalized in the nitric acid vapor for 15 h at 175 °C, and then mixed into suspensions. Thermal desorption spectroscopy profiles indicate the formation of functional groups on multi-walled carbon nano-tubes. An excellent suspension stability can be achieved for different amounts of triethanolamine. The amount of triethanolamine can be increased by adding a second component to a stable hydroxyapatite suspension due to an electrostatic interaction between components in suspension. The stability of composite suspension is less than that of the hydroxyapatite suspension, due to density differences, which under the gravitational force promote the demixing. The scanning electron microscopy images of the coatings surface show that more dense coatings are developed on NiTi substrate using electrophoretic deposition and sintering at 850 °C in the simultaneous presence of silicon and multi-walled carbon nanotubes in the hydroxyapatite coatings. The atomic force microscopy results of the coatings surface represent that composite coatings of hydroxyapatite-20 wt.% silicon and hydroxyapatite-20 wt.% silicon-1 wt.% multi-walled carbon nano-tubes with low zeta potential have rougher surfaces.

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