Structure and apatite formation of microarc oxidized TiO2-based films before and after alkali-treatment by various alkali concentrations

Alkali-treatment was performed to modify the surface of the TiO₂-based film containing Ca and P prepared by microarc oxidation (MAO) technique for improving the apatite-forming ability of the MAO film. Before alkali-treatment, in the MAO film amorphous and crystalline regions with main composition of Ti, Ca, P and Al both were observed; and in the latter TiO₂ nanocrystals were randomly distributed in Ca- and P-doped matrix. After alkali-treatment, the surfaces of the MAO films become rough, and the Ca and P concentrations decrease with increasing the concentration of NaOH solution (1, 3 and 5 mol/L). When 5 mol/L NaOH solution was used, amorphous calcium titanate hydrogel was formed on the surface, showing a nanoflake-like morphology. In vitro experiment indicates that the ability to induce apatite formation of the alkali-treated MAO films increase with increasing the concentration of NaOH solution, which is associated with the formation of Ti-OH groups during the incubation process in a simulated body fluid.     

Microstructure and apatite-forming ability of the MAO-treated porous titanium

Porous titanium was treated by micro-arc oxidation (MAO) in the aqueous electrolytes containing 0.1 and 0.2 M NaOH. The microstructure (including morphology, phase component, element composition and chemical species) and in vitro apatite-forming ability of the oxidized films formed on the inner-pore walls of porous titanium were investigated. It is found that continuous thin films with pore sizes of 20-60 nm are formed in both electrolytes. The films consist of an amorphous TiO₂ outmost layer, a coexisted intermediate layer of amorphous TiO₂ and rutile, and a Ti₂O₃ bottom layer, and tightly bond to the titanium substrate without any cracks. In vitro bioactivity assessment shows that both MAO films possess high apatite-forming abilities. It is also found that, compared with the film formed in the 0.1 M NaOH-containing electrolyte, the film formed in the 0.2 M NaOH-containing electrolyte has a higher roughness and more nanopores which help shorten apatite induction time. It is expected the MAO-formed bioactive porous titanium will not only be beneficial to bone ingrowth into the porous structure, but also be beneficial to achieve a tough chemical bonding at the bone/implant interface.     

Structure of calcium titanate/titania bioceramic composite coatings on titanium alloy and apatite deposition on their surfaces in a simulated body fluid

In this study, TiO₂-based coatings containing Ca and P ions were prepared on titanium alloy surfaces by microarc oxidation (MAO). After soaking in aqueous NaOH solution and subsequent heat treatment at 700 and 800 °C, calcium titanate/titania bioceramic composite (CTBC) coatings were obtained. The results show that the outer layers (0–1.5 μm) of the CTBC coatings are mainly composed of Ca, Ti, O and Na constituents with a uniform distributions with increasing the depth near the surfaces. The surface phase compositions of the CTBC coating formed at 700 °C are anatase, rutile and CaTi₂₁O₃₈ phases, as well as a few CaTiO₃, while those of the CTBC coating formed at 800 °C are anatase, rutile and CaTiO₃. When incubated in a simulated body fluid (SBF), apatite was deposited on the CTBC coatings probably via formation of hydroxyl functionalized surface complexes on the CTBC coating surfaces by ionic exchanges between (Ca²⁺, Na⁺) ions of the CTBC coatings and H₃O⁺ ions in the SBF. The CTBC coating formed at 800 °C seems to facilitate the deposition of Ca and P probably due to the good crystallographic match between perovskite CaTiO₃ and HA on specific crystal planes.     

The composite films of gradient TiO2-based/nano-scale hydrophilic sodium titanate: Biomimetic apatite induction and cell response

Chemical treatment was performed to modify the surfaces of the TiO₂-based (TOB) film containing P for improving its bioactivity. The apatite-forming ability of the chemically treated TOB (C-TOB) film was enhanced due to the formation of hydroxyl-functionalized surface during the SBF immersion process. However, further heat treatment of the C-TOB films formed crystalline sodium titanate, showing poor ability to release Na⁺ ions, which does not facilitate the formation of hydroxyl-functionalized surface, thus lowering the apatite-forming ability. Firstly, amorphous Ca- and P-containing precipitates formed during the SBF immersion process, eventually transformed to crystalline biomimetic apatite, exhibiting a porous structure on two-scales of micron and nanometer levels. The preliminary cell experiment showed that the C-TOB film has good biocompatibility.     

Biomimetic apatite induction of P-containing titania formed by micro-arc oxidation before and after hydrothermal treatment

Phosphorous (P)-containing titania films were prepared by micro-arc oxidation (MAO) of titanium (Ti) in an electrolyte containing β-glycerol phosphate disodium salt pentahydrate (β-GP, C₃H₇Na₂O₆P^.5H₂O), and their apatite inducing ability in a simulated body fluid (SBF) was investigated. Macro-porous titania films were formed, consisting of only anatase phase, and the P content in the films increased up to 8 at.% with an increasing applied voltage. During hydrothermal treatment, the P in the films was diffused out to the surface and hydrolyzed to form the hydrogen phosphate (HPO₄²⁻) group. When immersed in SBF, no apatite was induced in any of the P-containing MAO specimens for up to 28 days. However, after a hydrothermal treatment at 250 °C, apatite was induced on the titania surfaces as early as 9 h immersion, and the entire exposed surface was covered with the apatite globules after 36 h immersion, which was much faster than the apatite induction on Ca-containing titania. The higher apatite-inducing ability of P-containing titania after hydrothermal treatment was believed to be due to the crystal structure (anatase) and presence of HPO₄²⁻ group on the surface.     

In vitro bioactivity and osteoblast response on chemically modified biomedical porous NiTi synthesized by capsule-free hot isostatic pressing

Porous biomedical NiTi with an average porosity of 56% and a general pore size of 50-800 μm was synthesized by capsule-free hot isostatic pressing (CF-HIP) with NH₄HCO₃ as a space holder. In order to enhance the surface bioactivity, the porous alloy was subjected to H₂O₂ to form a TiO₂ coating followed by a NaOH treatment. Scanning electron microscopy (SEM), thin film X-ray diffraction (TF-XRD), and X-ray photoelectron spectroscopy (XPS) revealed that a porous sodium titanate (Na₂TiO₃) film formed on the surface of the porous NiTi due to the chemical reaction between NaOH and pre-formed TiO₂ at the interface between the NaOH solution and porous NiTi. An apatite layer was formed on the film after immersion in simulated body fluids (SBF) at 37 °C while no apatite could be found on the surface of the untreated porous NiTi. Formation of the apatite layer indicates that the chemically treated porous NiTi possesses excellent bioactivity and this bodes well for applications in bone implants. In our preliminary cell culture tests, osteoblast cells were found to attach and proliferate better on the chemically treated samples compared to the untreated alloys.     

Effect of a bioactive glass-ceramic on the apatite nucleation on titanium surface modified by micro-arc oxidation

The aim of this work was to evaluate the ability of a bioactive glass-ceramic to induce the apatite nucleation on the titanium oxide layer produced by micro-arc oxidation. “In vitro” tests were carried out on a simulated body fluid solution in two different manners: one group was soaked in the SBF, while the other group was soaked together with the bioactive glass-ceramic. Results revealed that after 7 days, the specimens soaked in SBF were covered with an amorphous calcium phosphate layer, while the specimens soaked in SBF plus glass-ceramic formed a crystalline apatite layer, suggesting thus, that the glass-ceramic provides silanol groups that accelerated the hydroxyapatite apatite precipitation on the anodic TiO₂ layer.     

High-Temperature Oxidation Behavior of Sputtered IN 738 Nanocrystalline Coating

A sputtered nanocrystalline coating of IN 738 alloy was obtained by means of magnetron sputtering. The isothermal oxidation behavior at 800, 900, and 1000°C and the cyclic oxidation behavior at 950°C of the coating were studied in comparison with IN 738 cast alloy. The results indicated that a double external oxide scale was formed on the nanocrystalline coating at 900, 950, and 1000°C without internal oxide and nitride. The scale consisted in an outer mixture of Cr₂O₃, TiO₂, and NiCr₂O₄ and an inner, continuous Al₂O₃ layer, which offered good adhesive and protectiveness. However, at 800°C a continuous Al₂O₃ scale could not be formed during oxidation of nanocrystalline coating and aluminum was still oxidized internally.     

Synthesis and characterization of hardness-enhanced multilayer oxide films for high-temperature applications

Multilayer oxide coatings consisting of amorphous Al₂O₃ and crystalline TiO₂ nanolayers have been deposited using reactive pulsed d.c. magnetron sputtering at different partial pressures of oxygen. Hardness enhancement has been observed in oxide multilayer coatings with amorphous Al₂O₃ as the majority component. These coatings had greater hardness-to-modulus ratios and showed greater resistance to wear over monolithic Al₂O₃ and TiO₂ majority phase multilayers. Multilayer films retain their high hardness up to ~ 800 °C in air; some hardness enhancement in the Al₂O₃ majority phase multilayer coating remains even after 1 h of air annealing at 1000 °C. The hardness decrease at elevated temperatures is due to the roughening of interfaces between nanolayers, which can be attributed to the annealing-driven change of crystallographic texture of TiO₂ layers.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

纯Ti表面微弧氧化-碱热处理仿生陶瓷膜的制备及耐蚀性

采用微弧氧化-碱热处理在纯Ti表面制备了含有羟基磷灰石(HA)的仿生陶瓷膜。利用SEM,XRD和电化学工作站等手段研究了膜层的形貌、物相及其耐蚀性。结果表明:在乙酸钙-磷酸二氢钙电解液体系中微弧氧化(MAO),纯Ti表面形成一层含Ca和P的TiO2多孔陶瓷膜。经水热处理后,膜层表面的孔洞变小、致密性增加,膜层中还出现了鳞状、层片状以及针棒状的HA。在Hank's模拟体液中,MAO膜和微弧氧化-碱热处理(MAOAH)膜均表现出较好的耐蚀性。MAO膜经模拟体液腐蚀后,形成了缺钙型HA(Ca8.86(PO4)6(H2O2)2)和CaTiO3;而模拟体液中的阴离子与MAOAH膜层的氧化物作用使膜层孔洞直径和深度增加。     

Microstructure and bioactivity of Ca, P and Sr doped TiO2 coating formed on porous titanium by micro-arc oxidation

In the present study, bioactive coatings enriched with micro-pores and Ca-P-Sr elements were formed on pore walls of porous titanium by micro-arc oxidation (MAO). It is found that pore size plays a significant role on the MAO treatment of porous titanium. For samples with pore size smaller than 90 μm, whatever applied voltage and treatment time were employed, MAO coatings were formed only in the near surface region. As to the sample with average pore size of 150 μm, MAO coatings were formed on both outer and inner pore walls throughout the depth. Compared with the untreated one, the specific surface area dramatically increased about 460 times. Further studies found that pore size, thickness and amounts of O, Ca, P and Sr elements of the coating on the outer pore walls were obviously higher than those on the inner pore walls. Additionally, the coating on the outer pore walls was composed of anatase and rutile TiO₂ and other complex Ca-P-Sr phases, in comparison with anatase TiO₂ formed on the inner pore walls. In spite of the distinct features of coatings on the different locations of pore walls, MAO-treated porous titanium overall showed a good apatite-inducing ability.     

Effects of plasma treatment on bioactivity of TiO2 coatings

In this work, nano-TiO₂ powders were deposited on titanium alloy substrates by atmospheric plasma spraying, followed by plasma immersion ion implantation (PIII) using hydrogen, oxygen and ammonia gases. The bioactivities of PIII-treated TiO₂ coatings were evaluated by the formation of apatite on their surface after soaked in simulated body fluids (SBF) for a period of time. As-sprayed TiO₂ coating is composed of rutile, anatase and TiO_2−x (most of them is Ti₃O₅). After immersion in SBF for two weeks, the hydrogen PIII-treated TiO₂ coating can induce bone-like apatite formation on its surface but apatite cannot be formed on the surface of as-sprayed and oxygen, ammonia PIII-treated TiO₂ coatings. The results obtained indicated that a hydrogenated surface plays a very important role to induce bioactivity of TiO₂ coatings.     

In situ composite coating of titania-hydroxyapatite on commercially pure titanium by microwave processing

Microwave (MW) processing has been studied as an alternative method of hydroxyapatite (HA) based composite coatings on commercially pure titanium (CPTi) to enhance the bioactivity for orthopaedic and dental implant applications. The coating was formed by processing CPTi metal packed in HA and at 800 W microwave power for 22 min. The composition of the coating was found to be TiO₂ (rutile) as major phase along with HA as minor phase. The MW absorption of non-stoichiometric TiO₂ layer, which was grown during the initial hybrid heating, resulted in sintering of apatite particles interfacing them. The non-stoichiometric nature of TiO₂ was evident from the observed mid-gap bands in ultraviolet-visible diffusive reflectance (UV-VIS-DR) spectrum. The lamellar α structure of the substrate suggests that the processing temperature was above β transus of CPTi (1155 K). The oxygen stabilized α phase whose thickness increased with microwave processing time, was likely to be the reason for the increase in Young's Modulus and hardness of the substrate. The coating induced apatite precipitation in bioactivity test. The osteoblast cell adhesion test demonstrated cell spreading which is considered favourable for cell proliferation and differentiation. Thus, in situ composite coating of titania and HA on CPTi was obtained by a simple one-step process.     

Effects of the electric conditions of AC-type microarc oxidation and hydrothermal treatment solution on the characteristics of hydroxyapatite formed on titanium

This study examined the effects of the conditions for AC-type microarc oxidation (MAO) and the type of hydrothermal treatment solution on the characteristics of hydroxyapatite(HAp)-containing oxide films deposited on commercially pure titanium (CP-Ti). The MAO treatments were carried out in an electrolyte containing 0.2 M calcium acetate monohydrate and 0.02 M β-glycerophosphoric acid disodium salt pentahydrate (β-GP) using AC-type rectangular electric pulses at different voltages and frequencies. HAp formation on the surface of the MAO-treated group was induced by a hydrothermal treatment in either an alkaline solution to form HT-treated groups or a 0.002 M β-GP solution (pH = 11.0) to produce HTP-treated groups. A mixed crystalline structure consisting of anatase TiO₂, rutile TiO₂ and CaTiO₃ was observed on the MAO-treated groups treated with a low frequency and voltage. When the AC frequency was increased, anatase TiO₂ became the dominant crystalline structure and there was an even distribution of pores. HAp particles were formed more densely on the HTP-treated groups than on the HT-treated groups. Among the HTP groups, the groups fabricated at higher frequencies contained more evenly distributed and crystallized HAp crystallites.     

Corrosion behavior of Cr3C2-NiCr vacuum plasma sprayed coatings

The objective of the present work is to determine the influence of the heat treatment on the corrosion resistance of a Cr₃C₂-NiCr coating of 450 μm thickness, deposited by a vacuum plasma spray process (VPS) on a steel substrate. The post-heat treatment of the as-deposited coating was carried out in Ar at 400 °C and 800 °C, respectively. The coatings were characterized by means of an electron probe micro analyzer (EPMA) with wavelength dispersive X-ray spectrometers (WDS). It was found that no significant changes were produced as a consequence of the heat treatment carried out at 400 °C. Therefore, the corrosion experiments were conducted for the substrate, the as-deposited coating and the post-heat treated coating at 800 °C. Potentiodynamic polarization showed that the annealed coating at 800 °C has a better corrosion resistance than the as-deposited coating. The corrosion current density (I_corr) of this coating was approximately 3 and 4 times smaller than that corresponding to the as-deposited coating and steel substrate, respectively. This significant improvement of the corrosion behavior of the post-heat treated coating is mainly due to both the microstructural changes that take place in the coating and the diffusion of Ni into Fe at the coating-substrate interface, which ensures the presence of a metallurgical bond.     

Calcium-hydroxide slurry processing for bioactive calcium-titanate coating on titanium

The present study investigates the relationships between the treatment conditions and characteristics of the surface layer in a surface treatment process using calcium-hydroxide slurry. Furthermore, the appropriate treatment conditions for preparing the bioactive coating are also investigated by soaking the specimens in a simulated body fluid. The treatment process using calcium-hydroxide slurry is as follows: a titanium substrate is buried in calcium hydroxide slurry, and the slurry is then heated at 873 K in air. The calcium hydroxide slurry was prepared by mixing 1 g of calcium hydroxide powder per 1, 1.2, 1.5, or 2 mL of water. The slurry was heated to 873 K with various heating rates from 100 to 600 K h^− 1 and held at 873 K for 1 and 2 h. As the ratio of water in the slurry increased or the heating rate decreased, the atomic ratio of calcium to titanium in the surface layer decreased. A surface treatment using slurry containing a mixture of 1.0-mL water per 1.0-g calcium hydroxide powder, a heating rate of 600 K h^− 1, and a holding time of 2 h produces a crystallized stoichiometric calcium titanate layer on the titanium substrate. Treatment using slurry of a higher water ratio or a slower heating ratio results in the formation of a dioxide layer containing locally formed calcium titanate. On the crystallized calcium titanate layer, calcium phosphate is formed faster in a simulated body fluid than in the TiO₂ layer containing calcium titanate; therefore, the treatment conditions forming the crystallized calcium titanate layer are appropriate for the bioactive coating.     

The role of aluminum and titanium on the oxidation process of a nickel-base superalloy in steam at 800°C

The role of aluminum and titanium on the oxidation process of a nickel-base superalloy containing 18.89%Cr, 2.13%Al, and 2.41%Ti was investigated in steam at 800°C. A Cr₂O₃-rich scale was formed on the alloy surface. Aluminum formed only internal oxides below the Cr₂O₃-rich scale. On the other hand, titanium formed not only internal oxides but also oxides in the scale and the granular particles of TiO₂ outside the scale. Agglomeration of the TiO₂ particles also occurred. The oxidation behavior of aluminum and titanium was discussed from thermodynamic and kinetics aspects.     

Osteoblast growth behavior on micro-arc oxidized β-titanium alloy

β-titanium (β-Ti) alloys are known for their excellent physical properties and biocompatibility, and are therefore considered as next-generation metals for orthopedics and dental implants. To improve the osseous integration between β-Ti alloys and bone, this study develops a titanium dioxide (TiO₂) coating on the surface of β-Ti alloys by using micro-arc oxidation (MAO) technique. The anatase (A) rich and rutile (R) rich TiO₂ layer, were formed on β-Ti, respectively. In vitro tests were carried out using pre-osteoblast cell (MC3T3-E1) to determine biocompatibility and bone formation performance. Biocompatibility includes cell adhesion, cell proliferation, and alkaline phosphatase (ALP) activity, while the bone formation performance contains osteopontin (OPN), osteocalcin (OCN) and calcium content. Cell morphology was also observed. In addition, raw β-Ti, A rich TiO₂ and R rich TiO₂ were implanted into the distal femora of Japanese white rabbits for 4, 8, and 12 weeks to evaluate its in vivo performance.Experimental results show that TiO₂ coating can be grown on and well-adhered to β-Ti. The anatase phase formed under a low applied voltage (350 V), while the rutile phase formed under a high applied voltage (450 V), indicating that crystal structure is strongly influenced by applied voltage. A porous morphology was obtained in the TiO₂ coating regardless of the crystal structure and exhibited superior bone formation performance than β-Ti. In vivo analysis and in vitro test show similar trends. It is also noticeable that the R rich TiO₂ coating achieved better biocompatibility, osteogenesis performance. Therefore, a MAO-treated R rich TiO₂ coating can serve as a novel surface modification technique for β-Ti alloy implants.     

The corrosion of Nb-modified Ti3Al in a combustion gas with and without Na2SO4−NaCl deposits at 600–800°C

The corrosion behavior of a Nb-modified Ti₃Al intermetallic compound containing 11 at.% Nb in a simulated combustion gas with and without deposits of a Na₂SO₄–NaCl mixture was examined at 600–800°C for times up to four days. In the absence of salt deposits the corrosion rates were rather low and increased only slightly with temperature, producing very thin scales of mixed oxides of Ti, Al, and Nb without sulfides. The presence of the salt deposits produced higher weight gains during an initial stage of one to two days at 600 and 700°C, after which the reaction stopped. A more important and longlasting effect was observed instead at 800°C, when the kinetics of hot corrosion became nearly linear. The scales formed by hot corrosion were complex mixtures of various corrosion products at all temperatures and showed a porous outer region containing a mixture of unreacted salts with oxides (mainly TiO₂), an intermediate region of a mixture of variable composition of oxides of the three metals, and a TiO₂-rich layer beneath it. At 800°C the scales tended to form a thin, discontinuous Al₂O₃-rich layer in the middle and contained an additional innermost region presenting a large concentration of sulfur, very likely as Nb and Ti sulfides. The high rate of hot corrosion at 800°C is attributed to the appearance of sulfides in the inner region of the scale and to a more efficient scale fluxing.