Structural characterization, mechanical, and electrochemical studies of hydroxyapatite-titanium composite coating fabricated using electrophoretic deposition and reaction bonding process.

In the present work, hydroxyapatite (HA)-titanium (Ti, 20 wt%) composite coating was coated on NiTi alloy substrate by EPD (electrophoretic deposition) process. Before applying the coating, the HA powder was composed with Ti powder using a ball milling process. Influence of the ball milling time on morphology and phase structure of HA-Ti powder was evaluated using TEM and XRD analysis. After composing the HA particles with Ti, the HA-Ti composite powders were coated on the NiTi substrate by the EPD process in an n-butanol medium for 2 min, with the applied voltage of 60 V. XRD and SEM analysis were utilized to evaluate the phase analysis and morphology of the coatings. Mechanical and electrochemical characteristic of the coatings were also assessed using the micro-indentation, micro-scratch, and polarization tests, respectively. The results revealed that the milling process time had a significant influence on reaction bonds and optimum mixing time was 4 hr. Micro-hardness of the HA-Ti composite coating (304 HV) was substantially higher than the HA coating (72 HV). Also, as the HA coating was composed with Ti particles, the amount of force (in the micro-scratch test) required for detaching the coating from the NiTi substrate increased from 7.1 to 17.8 N. The polarization results showed that the HA-Ti composite coating had a higher electrochemical resistance compared with the HA coating. Corrosion resistance of the NiTi alloy coated with HA increased from 133 kΩ.cm to 2,720 kΩ.cm after composed with the Ti particles.