Micro-scratch and corrosion behavior of functionally graded HA-TiO2 nanostructured composite coatings fabricated by electrophoretic deposition

• Utilizing EPD for the fabrication of functionally graded HA-TiO2 coatings.
• Highest critical distances of crack initiation and delamination for FG sample.
• Highest critical contact pressures were estimated for FG sample.
• Highest fracture toughness of 0.932 MPa m1/2 was achieved for FG sample.
• Enhancement of corrosion resistance for FG sample.

In the present study, functionally graded coatings of HA/TiO2 nanoparticles and HA-TiO2 nanocomposite coatings with 0, 10 and 20 wt% of TiO2 were fabricated by electrophoretic deposition on Ti–6Al–4V substrate. The functionally graded structure of HA/TiO2 coatings was formed by gradual addition of HA suspension into the deposition cell containing TiO2 nanoparticles. Micro-scratch test results showed the highest critical distances of crack initiation and delamination, normal load before failure and critical contact pressures for functionally graded coating. It was observed that the improvement of adhesion strength and fracture toughness of functionally graded coatings would be due to the reduction of thermal expansion coefficient mismatch between Ti–6Al–4V substrate and HA. The results of potentiodynamic polarization measurements showed that the graded structure of the coating could efficiently increase the corrosion resistance of substrate.