Preparation And Characterization Of Bioactive Phosphate Chemical Conversion Coatings On Titanium And Its Alloys

The initial failure of titanium(Ti)-based implants and surface coatings on them has been one of the most urgent problems need to be solved.It is effective to prepare a surface modified coating on Ti for enhancing the long-term clinical efficacy of Ti implants.Calcium phosphates with similar chemical composition to inorganic components of natural bone and teeth,have been widely used in clinic due to their bioactivity and biocompatibility.Zinc,as a trace element in vertebrates,has been demonstrated to induce bone formation in tissue culture.Therefore,coating Ti with an thin layer of bioactive Zn/Ca phosphates can be an effective approach to improve bone integrations.Phosphate chemical conversion(PCC)technology is one of the most effective and simplest way to produce a coating on metal through complicated interaction between metal and solution.Over the past twenty years,it has generated a great deal of interest in biomedical field due to many advantages like short period of coating formation,easy processing operation,suitability of irregular metal surfaces and splendid adherence between coating and metal.This research has been focused on the fabrication of bioacitve PCC coatings on Ti.The aim of the research was to prepare bioactive PCC coatings on Ti with micro/nano-structures and biocompatible phases,such as Hopeite(Zn3(PO4)2·4H2O),Scholzite(CaZn2(PO4)2·2H2O),Brushite(CaHPO4·2H2O)and Hydroxyapatite(Ca10(PO4)6(OH)2,HA).The surface microstructure and phase composition of the coatings were analyzed by FE-SEM,XRD,XPS,FTIR and AFM,along with the bonding strength,surface wettability and corrosion resistance.The formation initiation and mechanism of the coatings on Ti was explored.The optimized processing parameters were obtained through comparative analysis of characterizations of the coatings treated with various process conditions.The associated effects of the coatings upon bacteria and osteoblasts responses were also investigated.The results revealed that galvanic coupling with Fe initiated the coating formation on Ti via PCC method.The formation rate and properties of the coating were improved by the assistance of Fe2+ ions and ultrasonic.The kinetics of coating formation on Ti with galvanic coupled Fe included six stages.The crystallization occurred on the surface of Fe first by the electrochemical attack of anodic Fe in the acidic bath.As the coating grew,the anode and cathode of the galvanic couple changed and the crystals precipitated on Ti.The coating was progressing until the dynamic balance of dissolution and formation of the coating was built.Raise the temperature and treatment time appropriately was beneficial to the coating formation and refinement.Zn-P coating was fabricated at lower pH levels(2.50-3.25)while Ca-P coating needed higher pH(above 3.50).The optimum Zn-P and Ca-Zn-P coating could be obtained at 55 ? after immersion for 30 min in the solution,while the Ca-P coating with fibrous crystals and nano-thickness could be derived at 60 ? after 30 min immersion in the solution with pH 5.00.The drying process did not mean much to the phase composition but could change the surface morphology of the coating.Generally,the complete and uniform coating was prepared following the rinsing then drying process.Compared with pure Ti,the Ca-Zn-P coating prepared on Ti-6Al-4V has a higher fraction of Zn element,which presented more dense and fine structure with higher bonding strength and corrosion resistance.In addition,a complete and continuous coating could be fabricated on the threaded surface of Ti dental implant.Four different types of zinc and calcium phosphate coatings were fabricated on Ti.The phase compositions of Zn-P,Ca-P and Ca-Zn-P coating were mainly hopeite(Zn3(PO4)2·4H20),brushite(CaHP04·2H20)and scholzite(CaZn2(PO4)2·2H20),respectively.Moreover,brushite and scholzite could be converted to HA with treatment of alkali and Ca-EDTA solution,respectively.The coatings belong to lightweight class with thickness of 5-20 ?m,Ca-Zn-P coating was the thinnest among them.The surface of the coatings was generally rough(Rq = 70-140 nm),and Zn-P coating was the coarsest one.The coatings presented high hydrophilicity with the contact angle of 15-52°,and the Ca-P-OH coating obtained from alkali treatment of Ca-P coating was the most hydrophilic.The scratch tests showed that the bonding strength between the coatings and metal were 55-70 N.The electrochemical tests suggested that the corrosion resistance of the coatings were favorable in simulated body fluid and 0.9%NaCl solution.The antibacterial studies demonstrated that the Ca-Zn-P coatings could inhibit the growth of both Gram-positive(Staphylococcus aureus)and Gram-negative(Pseudomonas aeruginosa)bacteria.The in vitro evaluations suggested that the coatings were non-cytotoxic and could enhance adhesion,proliferation and differentiation of osteoblasts compared with bare Ti,especially the Ca-Zn-P coating.Therefore,the conversion coatings fabricated on Ti presented well bioactive and biocompatibility as well as antibacterial performance.It suggested that the conversion coatings,especially the Ca-Zn-P coating might be a promising option in hard tissue replacement for early osteogenesis.