The Influence Of Zn Substitution And Electrolytic Concentration On The Electrochemically Deposited Ca-P Coatings

Hydroxyapatite (HAP) and other calcium phosphates (Ca-Ps) were synthesized by electrochemical depositon (ECD) technique.The formation process and mechanism of these Ca-Ps were studied and discussed systematically and detailly in this paper. The main contents of this paper lie in the following three parts:1) Zn-substituted HAP coatings on titanium substrates were obtained by electrochemical deposition in the electrolytes (1.2mM [Ca(NO3)2+Zn(NO3)2] and 0.72mM NH4H2PO4).The influence of Zn on the micro-morphology,phase composition and crystallinity of as-obtained HAP crystals were studied. The results show that the presence of Zn in the electrolyte would deteriorate the crystallinity of HAP significantly and HAP crystals are not obtained when Zn fraction reach 25mol% (Zn/(Zn+Ca)). Moreover, as the Zn fraction increased, the regular hexagonal rod-like character of HAP crystals disappear gradually, and the bonding strength between coating and titanium substrate is improved gradually.2) The influence of electrolytic concentration on the phase composition and morphology of products were discussed. The SEM and XRD results show that regular hexagonal rod-like HAP crystals with high crystallinity are obtained when electrolytic concentrations are relatively low. As electrolytic concentrations increase, the crystallinity of HAP decreased, and the product gradually transform to octacalcium phosphate (OCP). The morphology changes from nano-scaled hexagonal rod-like HAP crystals to micro-scaled belt-like OCP crystals are also observed. In addition, the reason of this concentration-induced phenomenon is analyzed thermodynamically.3) We attempted to reveal the formation mechanism of Ca-Ps by electrochemical deposition in this paper, which is seldom reported by others. By controlling the depositing time, we observed the micro-morphologies changes of products at different stages.It is proposed that thin plate-like OCP were formed at the early stage when electrolytic concentration is low(c(Ca2+)=1.2×10-3mol/L), and it served as precursor that would transform to HAP through solid-solid transformation. At some ranges of electrolytic concentration (c(Ca2+)=2.4×10-3mol/L～3.6×10-3mol/L), the nucleation manner of HAP is correspond to so-called "non-classical nucleation theory" which is widely reported and discoved during crystallization processes in recent years. In this case, the final HAP crystals are formed by the aggregation and oriented attachment of primary nano-particles.When the electrolytic concentration is relatively high(c(Ca2+)>1.2×10-3mol/L),the obtained OCP products are formed through classical nucleation theory.