Study On Structure And Properties Of Calcium Phosphate Coating On Titanium Alloy Prepared By Hydrothermal Synthesis

Calcium phosphate based coating material on the surface of titanium alloy combines the excellent mechanical properties of titanium alloy with the good biological activity and biocompatibility of calcium phosphate material,and has become a hot spot in the research of biomedical materials.However,the bioactive coating on the surface of titanium alloy prepared by the conventional methods has problems such as low bonding strength and easy detachment delamination due to physical bonding,which leads to limitation in clinical application.To this end,the hydrothermal reaction was used to form an intermediate transition layer of calcium titanate or titanium phosphate on the surface of titanium alloy,and then they were treated hydrothermally in a solution containing calcium and/or phosphate ions to form a calcium phosphate-based bioactive coating,improving the bonding behavior between the coating and the titanium alloy matrix,and expanding its range of applications.Based on the design idea of gradient materials,the calcium phosphate based coating on titanium alloy surface was conducted here.Firstly,the surface of the titanium alloy was activated in a hot alkaline solution,and then an intermediate reaction layer containing calcium or phosphorus was prepared by pre-calcification and pre-phosphorization.Finally,a calcium phosphate-based coating was prepared by hydrothermal reaction.HA coating on titanium alloy surface mainly composed of a chemical bond between the coating and the substrate was successfully prepared inthe present paper.XRD,SEM,EDS and other testing techniques were used to study and analyze the effects of hydrothermal conditions on the phase composition,crystal structure and uniformity both of intermediates and final products,and the mechanism and crystal nucleation growth of HA coatings prepared by hydrothermal reaction were also elucidated.The results show that the optimum surface activation conditions in the hot alkali treatment process were:hydrothermal temperature of 180°C,NaOH solution concentration of 3mol/L and reaction time of 6 h.The active layer prepared was mainly columnar structure Na2Ti6O13,and the coating was uniform and dense,with no or only a small amount of bulk sodium titanate particles.In the pre-calcification stage,when the hydrothermal temperature was 100°C,the reaction time was 6 h,and the CaCl₂ solution concentration was 0.5 mol/L,Ca²⁺could completely replace the Na⁺on the surface of the sodium titanate layer to form a uniform and flat calcium titanate reaction layer.The coating had a thickness of 5⁸?m with a porous network structure.At the same time,the calcium titanate crystals showed a columnar morphology,with a length between 100 and 500 nm.For the pre-phosphating treatment process,when the activated titanium alloy substrate was treated in a 2.5mol/L reaction solution at180°C for 12 h,no large amount of agglomerates were generated,and the pre-phosphating reaction layer was completely covered on the surface of the substrate.XRD analysis showed that such coating was mainly covered by a large number of irregular spheres formed by Ti?HPO₄?₂ and NaH₂PO₄.During the preparation process of the calcium phosphate based coating,when the reaction solution was composed of a mixed solution of Ca?NO₃?₂·4H₂O and NH4H2PO4,with a calcium to phosphorus ratio of 1.67,the growth of the HA crystals was more favorable.For the pre-calcinated titanium alloy substrate,when the reaction temperature was low and the reaction time was short,the obtained calcium phosphate coating was hardly found to completely cover on the surface of the titanium alloy,and the main phases were CaPO₃?OH?and Ca₄P₆O₁₉,among which CaPO₃?OH?showed a flaky morphology,Ca₄P₆O₁₉ being spherical;when the reaction temperature was high and the reaction time was long,the main crystalline phase of the obtained coating was HA and Ca₂P₂O₇ crystals,wherein the Ca₂P₂O₇ crystal showed a massive particle.When the rection was conducted in a acidic calcium phosphate solution,HA was easily formed on the surface of the coating,giving a good morphology.When the solution was alkaline,although the surface of the coating was found to be composed by HA and Ca₂P₂O₇ crystals,the coating was mainly formed by a deposited particles that were formed directly in the solution,giving a poor bonding behaviour.Therefore,the optimum preparation conditions for the HA coating were:a reaction temperature of 180°C,a reaction time of 12 h,and a pH of the calcium-phosphorus mixed solution of 3.The prepared HA coating was composed of needle-like crystals on a titanium alloy substrate,having a thickness of about 8?m.For the pre-phosphorized titanium alloy matrix,the increase of hydrothermal temperature was beneficial to the deposition rate of phosphate.When the reaction temperature was 160°C,the formation of Ca₂P₂O₇ crystal was more favorable,and the reaction temperature of180°C favored the formation of Ca₃P₅O₁₄)₂ and Ca₄H?PO₄?₃·2H₂O crystals,and the amount of Ca₂P₂O₇ and Ca₄H?PO₄?₃·2H₂O phases decreases with the increasing of the pH value of the solution.Therefore,the preparation of HA coating on the titanium alloy was best performed by hot alkali treatment and pre-calcification as surface treatment technology.