Effect of a-C:H:SiO_x coating thickness on corrosion resistance and zeta potential level of Ti-6Al-4V

In this work, a-C:H:SiO_x coatings with thicknesses of 300–1400 nm were synthesized by plasma assisted chemical vapor deposition on the surface of titanium alloy Ti-6Al-4 V. The thickness dependence of the roughness of the coatings was determined by laser profilometry. This dependence is described by a parabolic function. The filtration potential method showed that in an acidic environment (pH = 3), an increase in coating thickness is accompanied by a decrease in zeta potential from +40 to −24 mV. In a more neutral environment (pH = 6–7.8), the zeta potential of the coatings varies from −30 to −40 mV. Potentiodynamic polarization curves of the samples, analyzed by extrapolation using the Tafel function method, show that an increase in coating thickness contributes to an increase in corrosion potential from 62 to 330 mV and a decrease in corrosion rate from 1.9 × 10⁻⁵ to 4.0 × 10⁻⁶ mm/year. Impedance spectroscopy shows that charge transfer resistance (R_ct) and impedance modulus (|Z|_{0.1 Hz}) reach maximum values for the thickest a-C:H:SiO_x coatings. This indicates the formation of dielectric surface layers that hinder charge transfer in the electrical double layer.