Hybrid, oxide-ceramic coatings formed on titanium alloys by PEO-EPD processes

摘要

Introduction: Novel titanium alloys are considered as future materials to produce bone implants, thanks to their biocompatibility and corrosion resistance in body fluids. However, the titanium alloys surfaces are often modified by several electrochemical methods to enhance their bioactivity Modification by anodic oxidation is relatively easy and inexpensive. It results in homogeneous oxide layer characterised by good adhesion to the substrate. The surface morphology and chemical composition of the modified alloy surface may be controlled by the electrochemical parameters during plasma electrolytic process (PEO) and electrophoretic deposition (EPD). The oxide layer formed on the substrate is a conversion layer, characterized by very good adhesion and corrosion protection. EPD is usually applied to acid bioactive substances, especially on the top of the material surface. Materials and Methods: The hybrid coatings were formed by PEO coupled with EPD. The surfaces of Ti-15Mo (TM) and Ti-13Nb-13Zr (TNZ) alloys were anodised in suspensions of various concentrations of tricalcium phosphate (TCP, Ca_3(PO_4)_2) in 0.1 M Ca(H_2PO_2)_2. The PEO process was carried out at a current density of 100 and 150 mA cm-2, and voltage limit of 300 and 350 V, for TM and TNZ alloys, respectively. Various electrochemical parameters were used to determine the best conditions for the wollastonite particles deposition on the previously anodised titanium alloys surfaces. Finally, the process was carried out at 30 V during 60 or 90 min, using 5 g dm~(-3) suspensions of wollastonite, 25% vol. ethanol (C_2H_5OH); NH_3 aq. was used to adjust pH of the suspensions to 10 -11. Following this, polyglycol (6 ml dm~(-3)) was added as it acted as a dispersant to stabilize the suspension. The morphology, thickness, chemical composition, surface roughness and wettability of the coatings were examined. The cytocompability of the modified titanium alloys surface was assessed using osteoblast-like MG-63 cells. The electrochemical properties of the coatings were determined using potentiostatic measurement in Ringer solution at 37°C. The scratch-test was used to characterize adhesion of the coatings to the substrate. Results: Fig. 1. presents the representative SEM images of the coatings formed on the TM and TNZ alloy surface. Fig. 1. SEM images of the coatings formed on A) TM, and B) TNZ alloy surface. The particles of wollastonite were deposited at 30 V, during 60 min for TM, and 90 min for TNZ. TL-XRD analysis showed that the coatings were mainly composed by anatase (TiO2), tricalcium phosphate and wollastonite. The roughness of the surface was below 4 μm, and the oxide layers were hydrophilic (the water contact angle was between 50-70°). The adhesion and number of MG-63 osteoblast-like cells on the samples after 4 h, 24 h and 5 and 7 days of culture were investigated. The cells were well adhered on all of the investigated samples, and the representative images are presented in Fig. 2. Fig. 2. Fluorescence microscope images of on modified titanium alloys samples. Staining: nuclei by DAPI (blue) and cytoplasm proteins by eosin (red), bar = 100 μm. Conclusions: Various oxide-ceramic layers were formed on the titanium alloys substrates. The particles of wollastonite were deposited on the top of the TM alloy surface, but they were embedded much deeper in the oxide layer formed on the TNZ alloy. The surface modification improved the corrosion resistance of the titanium alloys in Ringers solution. Scratch testing and nanoidentation measurements showed that the coatings adhered well to the substrate. The layers were cytocompatible and promoted cell adhesion and proliferation.