| Mg based materials could not keep stable degradation in complex human environment due to the active chemical property.With the aim of slowing down the degradation rate and improving the biocompatibility,calcium phosphate(Ca Ph)coating is prepared on pure Mg surface employing electro-deposition in this paper.Meanwhile,surface pretreatments prior to Ca Ph deposition are performed to provide more sites for Ca Ph deposition,and as well as improving the interface combination between the coating and substrate.The surface state and the chemical environment between the substrate and electrolyte interface were controlled via adjusting the parameters during electro-deposition process.The effect of surface state,deposition temperature and time on the surface morphologies of as-deposited coatings was investigated.The formation condition of the Ca Ph coatings under different compositions was revealed,and the formation mechanism was discussed.The intrinsic correlation of the structural characteristics and corrosion behaviors under different parameters was studied,and the corresponding structural evolution,degradation law and impeding effect in corrosion of the Mg substrate were explained.Results indicated that surface pre-treatments had significant influence on the composition,morphologies and corrosion properties of the Ca Ph coatings.The hybrid coating offered best interfacial stability,thus the coating showed highest impedance modulus and best corrosion resistance in 120 h of immersion in SBF.The deposition temperature played a vital role in the morphologies,composition and thickness of the Ca Ph coatings.The coating prepared at 27℃ contained plate-like DCPD.Plate-like DCPD/bar-like octacalcium phosphate and cauliflower-like HA were fabricated at the temperature of 47℃ or 67℃,respectively.Single cauliflower-like HA was achieved when the deposition temperature was 87℃.The reason for the difference in coating composition was that,the dissolution of MAO layer,the ionization of water,the conversion from HPO42-to PO43-and other reactions were promoted with the increase of deposition temperature.The current density decreased first,then increased and finally dropped gradually during the whole deposition period.The three stages were related to the nucleation of Ca Ph phases,the dissolution of MAO layer and the continuous nucleation and growth of Ca Ph phases,respectively.Higher temperatures promoted the deposition of Ca Ph layer,and the thickness of the coating increased accordingly.The coating morphologies became looser with the increase of temperature,some cracks could be observed in Ca P-67 and Ca P-87 coatings.HA was first detected when the deposition temperature was 47℃ or 67℃,following the deposition of DCPD or OCP.A local alkaline area was formed as the dissolution of Mg O.HPO42-preferentially converted to PO43-,and promoted HA formation.The dissolution of Mg O was impeded since HA covered the surface.HPO42-could not be fully converted to PO43-because the content of OH-decreased,leading the appearance of DCPD or OCP,respectively.Uniform DCPD coating was vulnerable to failure due to its fast dissolution.The HA coating showed stable physiochemical property in SBF.However,the degradation of Mg substrate was impeded because of the low solubility of HA.Two types of multi-phase compositions,DCPD+HA and OCP+HA,contained both the degradable and bio-stable phases.The more compact morphology of DCPD and HA coating efficiently inhibited the penetration of SBF,and showed higher corrosion resistance.The DCPD and HA hybrid coating impeded the permeation of SBF in first 6h of immersion.Two time constants were observed at high and medium frequencies,which were attributed to the Ca Ph layer and MAO layer,respectively.After 24 h,the time constant relating to the electrochemical response o f double electric layer appeared at low frequency.Other Ca Ph coated samples showed three time constants in 120 h of impedance measurement.Bone-like apatite was detected on DCPD and HA coating after 90 days of immersion.Meanwhile,some corrosion areas appeared at the Mg substrate,suggesting the system degraded gradually.The Ca Ph coatings prepared under different conditions showed good apatite formation ability.The thickness of the induced HA layer increased with the increase in immersion time.The coating containing both DCPD+HA and single HA exhibited excellent cyto-activity and apatite formation ability in physiological environment.Live cell on the surface kept proliferation and differentiation,and the numbers continuously increased.Similar morphologies were observed after 21 days of cell culture,the induced apatite layer on the coating containing DCPD+HA surface showed a more compact structure. |
