A comparative study on the direct and pulsed current electrodeposition of hydroxyapatite coatings on surgical grade stainless steel

摘要

Hydroxyapatite [Ca _(10)(PO _4) _6(OH) _2, (HAP)] coatings were developed on 316L stainless steel substrate from the electrolyte containing hydrogen peroxide (H _2O _2) with the concentration ranging from 600 to 3000ppm by both the direct and pulsed current electrodeposition methods. The effects of direct current density upon the addition of H _2O _2 into the electrolyte on the phase purity and morphology of the as-deposited coatings were reported. The influence of pulsed parameters such as peak current density and pulse on and off time on the deposit compositions was also examined and compared with direct continuous current deposition in relation to the crystallinity, microstructure and the corresponding phases. X-ray diffraction (XRD) and Fourier transform infrared spectroscopic (FT-IR) techniques were performed in order to assure the purity, phase compositions of the coating and the morphology of the coating were characterized by scanning electron microscopic (SEM) technique. The results showed that the coating consists of mixed phases of calcium phosphate (Ca-P) in the absence of H _2O _2 in the electrolytic bath. Whereas the addition of H _2O _2 lowers the deposition current with the formation of smooth and uniform layer comprised solely of HAP. It is highly beneficial to increase the peroxide concentration from 600 to 2000ppm for the deposition of pure HAP. While increasing the peroxide concentration to 3000ppm, the coating morphology is not uniform as evidenced from the SEM result. Moreover, the increased adhesion and crystallinity of the HAP coating were achieved by pulsed current electrodeposition method at lower current density with longer pulse off time. The results of pulsed electrodeposition show that the relaxation time of the pulse is beneficial for the growth of HAP because it allows the diffusion of ions from bulk solution to the surface of electrode and thus lowers the concentration polarization in the next pulse on time. The combination of pulsed electrodeposition and addition of H _2O _2 into the electrolyte promisingly improve the physico-chemical properties of HAP coating.