Characteristics Of Plasma Sprayed And Micro-Arc Oxidized Coatings On A Low-Modulus Titanium Alloy

Low-modulus Ti2448 alloy was modified by plasma spraying and microarc oxidation. Influence of the alloy on the coatings and modification methods and effect of the modification methods and the post treatment on the alloy were the key point of the investigation. At the same time, the microstructure, mechanical properties, and induction characteristics of apatite were examined. X-ray diffraction (XRD), Energy disperse spectroscopy (EDS), X-Ray photoelectron spectroscopy (SPS), Fourier transform infrared spectroscopy (FT-IR), Roman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRT) were utilized to analyze the morphologies and microstructure of the coatings and substrate. The mechanical properties, such as nano/micro-hardness, modulus and bond strength of the coatings were measured by nano/micro-hardness instruments and tensile tests. Immersion in a stimulated body fluid (SBF) was applied for evaluation of the bioactivity of the coatings before and/or after alkali treatments.The results showed the increase of the current and voltage of plasma spraying influenced the properties of the alloy substrate although it improved the mechanical properties of the HA coating. With the applied current 600 A and the applied voltage 45 V, amounts of martensites deposited from the alloy, especially the near surface about 100μm, accompanied by appearance of ultra-fine grains of and martensites. The martensitic transformation and recrystallization ofβphase caused the remarkable increase of modulus of the entire alloy with 5mm thickness as well as the increase of the nanohardness and modulus in the near surface of the alloy.A new nanostructured composite coating was prepared by plasma spraying under relatively lower applied current and voltage from ball milled HA/Ti composite powder. The coating consisted of CaTiO₃ with perovskite structure, decomposition products of HA, and titanium oxides and titanium phosphide. The influence of milling time and rotational speed on the microstructure and mechanical properties was investigated. The results showed that increasing milling time, rotational speed and the concentration of Ti in the composite powder can improve the microstructure and mechanical properties of the coatings. Moreover, the coatings can induce apatite deposition during immersion in SBF. The mechanism can be described as follows: on the one hand, the release of Ca and P from the surface of the coatings to SBF increased the degree of supersaturation of SBF with respect to Ca and P ions near the surface of the coatings; on the other hand, the exchange of Ca2+ ions and H3O+ ions resulted in formation of Ti-OH groups on the surface of the coatings. Ti-OH groups absorbed Ca²⁺ ions in SBF, and then Ca²⁺ ions absorbed PO₄^3- and CO32-ions. As a result, the necluses of apatites formed on the surface of the coatings. In addition, the plane ( 022) of CaTiO₃ with perovskite structure in the coatings has perfect match with the plane (0001) of HA, which may provide favorable sites for necluses formation. The ball milling time and rotational speed had less influence on the ability of apatite formation than Ti concentration in composite poweders. High Ti concentration has disadvantages to the ability of apatite formation.The MAO coatings containing Ca and P were prepared on the surfaces of T2448 alloy and c.p.Ti. Ti2448 alloy caused significantly influenced the microstructure, chemical composition, and induction ability of apatite of the MAO coating and alkali-treated coatings. (1) The MAO coating on Ti2448 (MAO-Ti2448) consisted of Nb₂O₅ and a small amount of SnO2 and ZrO2 besides TiO₂ with brookite structure. All these phases existed in the MAO-Ti2448 were nano crystals or amorphous phase. However, TiO₂ in the MAO coating on Ti (MAO-Ti) had high crystallinity and anatase structure. (2) Alkali-modified MAO-Ti2448 had lower concentration of Ca and Na than alkali-modified MAO-Ti. In addition, nano network microsturcture were not formed on the alkali-modified MAO-Ti2448. (3) Alkali-modified MAO-Ti2448 had lower apatite induction ability than modified MAO-Ti. (4) The apatite induction ability can be increased by increasing the concentration of NaOH.In addition, the influence of the MAO process on Ti2448 alloy substrate was analyzed. The result showed the MAO process had weak influence on the substrate and there was noαphase deposition from the matrix after the preparation of the MAO coating.