Surface phenomena of HA/TiN coatings on the nanotubular-structured beta Ti-29Nb-5Zr alloy for biomaterials

Surface phenomena of HA/TiN coatings on the nanotubular-structured beta Ti-29Nb-5Zr alloy for biomaterials have been investigated by several experimental methods. The nanotubular structure was formed by anodizing the Ti-29Nb-5Zr alloy in 1 M H₃PO₄ electrolytes with 1.0 wt.% NaF at room temperature. Hydroxyapatite (HA)/titanium nitride (TiN) films were deposited on Ti-29Nb-5Zr alloy specimens using a magnetron sputtering system. The HA target was made of human tooth-ash by sintering at 1300 °C for 1 h, and the HA target had an average Ca/P ratio of 1.9. The HA/TiN depositions were performed, using the pure HA target, on Ti-29Nb-5Zr alloy following the initial deposition of a TiN buffer layer coating. Microstructures and nanotubular morphology of the coated alloy specimens were examined by FE-SEM, EDX, XRD, and XPS. The Ti-29Nb-5Zr alloy substrate had small grain size and preferred orientation along the drawing direction. The HA/TiN coating was stable with a uniform morphology at the tips of the nanotubes.     

Improved biological performance of low modulus Ti-24Nb-4Zr-7.9Sn implants due to surface modification by anodic oxidation

Dental implants are usually made from commercially pure titanium or titanium alloys. The purpose of this study was to evaluate the influence of surface treatment to low modulus Ti-24Nb-4Zr-7.9Sn (TNZS) on cell and bone responses. The TNZS alloy samples were modified using anodic oxidation (AD). Surface oxide properties were characterized by using various surface analytic techniques, involving scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS), X-ray diffractometry (XRD) and surface profilometer. During the AD treatment, porous titanium oxide layer was formed and Ca ions were incorporated into the oxide layer. The viability and morphology of osteoblasts on Ca-incorporated TNZS were studied. The bone responses of Ca-incorporated TNZS were evaluated by pull-out tests and morphological analysis after implantation in rabbit tibiae. The non-treated Ti and TNZS samples were used as the control. Significant increases in cell viability and pull-out forces (p < 0.05) were observed for Ca-incorporated TNZS implants compared with those for the control groups. Porous structures supplied positive guidance cues for osteoblasts to attach. The enhanced cell and bone responses to Ca-incorporated TNZS implants could be explained by the surface chemistry and microtopography.     

Electrochemical and surface behavior of hydyroxyapatite/Ti film on nanotubular Ti-35Nb-xZr alloys

In this paper, we investigated the electrochemical and surface behavior of hydroxyapatite (HA)/Ti films on the nanotubular Ti-35Nb-xZr alloy. The Ti-35Nb-xZr ternary alloys with 3-10 wt.% Zr content were made by an arc melting method. The nanotubular oxide layers were developed on the Ti-35Nb-xZr alloys by an anodic oxidation method in 1 M H₃PO₄ electrolyte containing 0.8 wt% NaF at room temperature. The HA/Ti composite films on the nanotubular oxide surfaces were deposited by a magnetron sputtering method. Their surface characteristics were analyzed by field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS) and an X-ray diffractometer (XRD). The corrosion behavior of the specimens was examined through potentiodynamic and AC impedance tests in 0.9% NaCl solution. From the results, the Ti-35Nb-xZr alloys showed a solely β phase microstructure that resulted from the addition of Zr. The nanotubular structure formed with a diameter of about 200 nm, and the HA/Ti thin film was deposited on the nanotubular structure. The HA/Ti thin film-coated nanotubular Ti-35Nb-xZr alloys showed good corrosion resistance in 0.9% NaCl solution.     

Fabrication of Zr and Zr-N surface alloying layers and hardness improvement of Ti-6Al-4V alloy by plasma surface alloying technique

Titanium alloys are very attractive materials because they have high specific strength, excellent corrosion and erosion resistance in many active environments. However, their low hardness values and poor tribological properties require improvement of their surface properties. The present study is concerned with the fabrication of Zr and Zr-N alloying layers in the surfaces of Ti-6Al-4V substrates by plasma surface alloying technique. The microstructure, chemical composition and hardness of the surface alloying layers were analyzed to understand the mechanisms of surface alloying and hardness improvement. The Zr and Zr-N surface alloying layers formed were homogeneous and compact, in which the surface alloying elements all displayed gradient distributions. The Zr and Zr-N surface alloying layers all enhanced the surface hardness of Ti-6Al-4V alloy. Zr-N surface alloying resulted in greater improvement in hardness and the maximum microhardness of (1.37 ± 0.04) × 10³ HK was obtained at the subsurface, which was much higher than that of the untreated Ti-6Al-4V alloy. The Zr-N surface alloying layer consisted of an outer nitride layer and an inner diffusion zone of Zr and N, and its very high hardness owed to the formation of the nitride layer. The mechanism of hardness improvement of Zr surface alloyed Ti-6Al-4V alloy was solid solution strengthening.     

Effects of TiN coating on the corrosion of nanostructured Ti-30Ta-xZr alloys for dental implants

Electrochemical characteristics of a titanium nitride (TiN)-coated/nanotube-formed Ti-Ta-Zr alloy for biomaterials have been researched by using the magnetic sputter and electrochemical methods. Ti-30Ta-xZr (x = 3, 7 and 15 wt%) alloys were prepared by arc melting and heat treated for 24 h at 1000 °C in an argon atmosphere and then water quenching. The formation of oxide nanotubes was achieved by anodizing a Ti-30Ta-xZr alloy in H₃PO₄ electrolytes containing small amounts of fluoride ions at room temperature. Anodization was carried out using a scanning potentiostat, and all experiments were conducted at room temperature. The microstructure and morphology of nanotube arrays were characterized by optical microscopy (OM), field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The TiN coatings were obtained by the radio-frequency (RF) magnetron sputtering technique. The depositions were performed from pure Ti targets on Ti-30Ta-xZr alloys substrates. The corrosion properties of the specimens were examined using potentiodynamic test in a 0.9% NaCl solution by using potentiostat. The microstructures of Ti-30Ta-xZr alloys were changed from an equiaxed to a needle-like structure with increasing Zr content. The interspace between the nanotubes was approximately 20, 80 and 200 nm for Zr contents of 3, 7 and 15 wt%, respectively. The corrosion resistance of the TiN-coated on the anodized Ti-30Ta-xZr alloys was higher than that of the untreated Ti alloys, indicating a better protective effect.     

Surface characteristics of TiN/ZrN coated nanotubular structure on the Ti-35Ta-xHf alloy for bio-implant applications

In this study, we investigated the surface characteristics of the TiN/ZrN-coated nanotubular structure on Ti-35Ta-xHf ternary alloys for bio-implant applications. These ternary alloys contained from 3 wt.% to 15 wt.% Hf contents and were manufactured in an arc-melting furnace. The Ti-35Ta-xHf alloys were heat treated in Ar atmosphere at 1000 °C for 24 h, followed by water quenching. Formation of the nanotubular structure was achieved by an electrochemical method in 1 M H₃PO₄ electrolytes containing 0.8 wt.% NaF. The TiN coating and ZrN coating were subsequently prepared by DC-sputtering on the nanotubular surface. Microstructures and nanotubular morphology of the alloys were examined by FE-SEM, EDX and XRD. The microstructure showed a duplex (α′′ + β) phase structure. Traces of martensite disappeared with increasing Hf content, and the Ti-35Nb-15Hf alloy had an entirely equiaxed structure of β phase. This research has shown that highly ordered, high aspect ratio, and nanotubular morphology surface oxide layers can be formed on the ternary titanium alloys by anodization. The TiN and ZrN coatings formed on the nanotubular surfaces were uniform and stable. The top of the nanotube layers was uniformly covered with the ZrN film compared to the TiN film when the Ti-35Ta-xHf alloys had high Hf content.     

Surface microstructure of the oxide protective layers grown on vanadium-free Ti alloys for use in biomedical applications

In this work, we present a surface study by SFM (scanning force microscopy) of three new Ti alloys of composition (in wt%) Ti-7Nb-6Al, Ti-13Nb-13Zr and Ti-15Zr-4Nb, developed for biomedical applications. V was not included in these alloys since this element has been reported to be cytotoxic. The surface of these materials has been modified by a thermal treatment in air at 750 °C for different times. As a consequence of this treatment an oxide layer develops on the surface, resulting in both an improvement of the corrosion resistance and an increase of the roughness, which enhances the adhesion of the tissue cells to the implant. SFM has been used to characterize the surface structure and topography of the oxide layers grown on the three alloys. The surface roughness analysis obtained by SFM points to a correlation between the mean square roughness, the thickness of the oxide layer, and the α-phase/β-phase ratio in the base material.     

Hydrogen peroxide treatment on Ti-6Al-4V alloy: A promising surface modification technique for orthopaedic application

Ti-6Al-4V alloy was treated with various concentrations (5 wt.%, 15 wt.% and 25 wt.%) of hydrogen peroxide (H₂O₂) and then heat treated to produce an anatase titania layer. The surface modified substrates were immersed in simulated body fluid (SBF) solution for the growth of an apatite layer on the surface and the formed apatite layer was characterized using various surface characterization techniques. The results revealed that titania layer with anatase nature was observed for all H₂O₂ treated Ti-6Al-4V alloy, irrespective of the H₂O₂ concentrations. Ti-6Al-4V alloy treated with 15 wt.% and 25 wt.% of H₂O₂ induced apatite formation, however 5 wt.% of H₂O₂ treated Ti-6Al-4V failed to form apatite layer on the surface. The electrochemical behaviour of H₂O₂ treated specimens in SBF solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy. Ti-6Al-4V alloy treated with 25 wt.% of H₂O₂ solution exhibited low current density and high charge transfer resistance values compared to specimens treated with other concentrations of H₂O₂ and untreated Ti-6Al-4V alloy.     

A study on wear resistance and microcrack of the Ti3Al/TiAl + TiC ceramic layer deposited by laser cladding on Ti-6Al-4V alloy

Laser cladding of the Al + TiC alloy powder on Ti-6Al-4V alloy can form the Ti₃Al/TiAl + TiC ceramic layer. In this study, TiC particle-dispersed Ti₃Al/TiAl matrix ceramic layer on the Ti-6Al-4V alloy by laser cladding has been researched by means of X-ray diffraction, scanning electron microscope, electron probe micro-analyzer, energy dispersive spectrometer. The main difference from the earlier reports is that Ti₃Al/TiAl has been chosen as the matrix of the composite coating. The wear resistance of the Al + 30 wt.% TiC and the Al + 40 wt.% TiC cladding layer was approximately 2 times greater than that of the Ti-6Al-4V substrate due to the reinforcement of the Ti₃Al/TiAl + TiC hard phases. However, when the TiC mass percent was above 40 wt.%, the thermal stress value was greater than the materials yield strength limit in the ceramic layer, the microcrack was present and its wear resistance decreased.     

Phase analysis and magnetocaloric properties of Zr substituted Gd-Si-Ge alloys

The structure, microstructure, magneto-structural transition and magnetocaloric effect have been investigated in series of (Gd_5−xZr_x)Si₂Ge₂ alloys with 0≤x≥0.20. X-ray powder diffraction analysis revealed the presence of orthorhombic structure for Zr containing alloys at room temperature in contrast to the monoclinic structure observed in the parent Gd₅Si₂Ge₂ alloy. The microstructural studies reveal that, low Zr addition (x≤0.1) resulted in low volume fraction of detrimental Gd₅Si₃-type secondary phase compared to that present in the parent alloy. All the Zr containing alloys have shown the presence of only second order magnetic transition unlike the parent alloy showing both first order structural and second order magnetic transition. A moderate (ΔS)_M value of −5.5 J/kg K was obtained for the x=0.05 alloy at an enhanced operating temperature of 292 K compared to −7.8 J/kg K at 274 K of the parent alloy for an applied field of 2 T. The interesting feature of Zr (x=0.05) containing alloy is the wide operating temperature range of ∼25 K than that of ∼10-12 K for the parent, which resulted in enhanced net refrigerant capacity of 103 J/kg compared to that of 53 J/kg for the parent alloy.     

Characterization of sp carbon produced by plasma deposition on gamma-TiAl alloys

Surfaces of two gamma-TiAl alloys, Ti-47%Al-2%Nb-2%Cr (MJ12) and Ti-47%Al-2%Nb-2%Mn + 0.8%TiB₂ (MJ47), were modified by acetylene plasma deposition at −3 kV bias voltage for 0.5-4 h. By using GIXRD and SAED, C (n-diamond), TiC, Al, AlTi, AlTi₂, AlTi₃, Al_0.64Ti_0.36 and Al₂Ti were detected on both alloys. Additional TiB₂ was detected on MJ47. XPS and Raman analyses revealed the presence of sp³ and sp² carbon deposited on the alloy surfaces with their binding energies of 283.9-284.8 eV for MJ12 and 283.9-285.0 eV for MJ47. Both sp³ and sp² contents were increased with the increase in the exposure times. The increasing rate of the first was less than that of the second, due to the stress developed in the films. Moiré fringe and crystallographic planes were detected using TEM. Knoop hardness of the deposited alloys, influenced by sp³ carbon, was increased with the increase in the exposure time. Those of MJ12 and MJ47 with 4 h deposition are 1.88 and 1.57 times of the corresponding untreated alloys, respectively.     

Structural and superconducting properties of ion beam sputtered Nb thin films and Nb/Cu bilayers

We present the results of a study of structural and superconducting properties of polycrystalline Nb thin films (200 Å, 300 Å, 400 Å, 700 Å and 1000 Å) and Nb/Cu bilayers (300 Å/300 Å and 400 Å/300 Å) prepared on Si substrates by ion beam sputtering at room temperature. The thicknesses, roughnesses at the surfaces and interfaces were determined by X-ray reflectivity whereas the grain sizes were determined from grazing incidence X-ray diffraction and transmission electron microscopic studies. The superconducting transition temperature (T_C) of Nb thin films are smaller than T_C of bulk Nb. The Nb-200 Å sample does not show T_C down to 2.3 K. The average size of the grains varies from 42 Å for Nb-200 Å sample to 69 Å for Nb-1000 Å sample. Our results show that the T_C in these polycrystalline films is not only limited by its thickness but also by the size of the grains. The Nb films deposited in situ on the Cu layer (Nb/Cu) show a marginal increase in average sizes of the grains as compare to their respective values in Nb films of same thicknesses. As a result a marginal increase in T_C of these films is also observed. The maximum decrease in T_C due to oxygen intake during deposition should be about 0.5 K from its bulk value (9.28 K). We have attributed the large decrease in T_C in our case on the basis of decrease in the Debye temperature and density of states at the Fermi level for Nb thin films as compared to their respective values for bulk Nb.     

Structure, band offsets and photochemistry at epitaxial α-Cr2O3/α-Fe2O3 heterojunctions

We test the hypothesis that electron-hole pair separation following light absorption enhances photochemistry at oxide/oxide heterojunctions which exhibit a type II or staggered band alignment. We have used hole-mediated photodecomposition of trimethyl acetic acid chemisorbed on surfaces of heterojunctions made from epitaxial α-Cr₂O₃ on α-Fe₂O₃(0001) to monitor the effect of UV light of wavelength 385 nm (3.2 eV) in promoting photodissociation. Absorption of photons of energies between the bandgaps of α-Cr₂O₃ (E_g = 4.8 eV) and α-Fe₂O₃ (E_g = 2.1 eV) is expected to be strong only in the α-Fe₂O₃ layer. The staggered band alignment should then promote the segregation of holes (electrons) to the α-Cr₂O₃ (α-Fe₂O₃) layer. Surprisingly, we find that the α-Cr₂O₃ surface alone promotes photodissociation of the molecule at hν = 3.2 eV, and that any effect of the staggered band alignment, if present, is masked. We propose that the inherent photoactivity of the α-Cr₂O₃(0001) surface results from the creation of bound excitons in the surface which destabilize the chemisorption bond in the molecule, resulting in photodecomposition.     

The effect of different temperature annealing on phase relation of LaFe11.5Si1.5 and the magnetocaloric effects of La0.8Ce0.2Fe11.5−xCoxSi1.5 alloys

The phase relation of LaFe_11.5Si_1.5 alloys annealed at different high-temperature from 1223 K (5 h) to 1673 K (0.5 h) has been studied. The powder X-ray diffraction (XRD) patterns show that large amount of 1:13 phase begins to form in the matrix alloy consisting of α-Fe and LaFeSi phases when the annealing temperature is 1423 K. In the temperature range from 1423  to 1523 K, α-Fe and LaFeSi phases rapidly decrease to form 1:13 phase, and LaFeSi phase is rarely observed in the XRD pattern of LaFe_11.5Si_1.5 alloy annealed at 1523 K. With annealing temperature increasing from 1573  to 1673 K, the LaFeSi phase is detected again in the LaFe_11.5Si_1.5 alloy, and there is La₅Si₃ phase when the annealing temperature reaches 1673 K. There almost is no change in the XRD patterns of LaFe_11.5Si_1.5 alloys annealed at 1523 K for 3-5 h. According to this result, the La_0.8Ce_0.2Fe_11.5−xCo_xSi_1.5 (0≤×≤0.7) alloys are annealed at 1523 K (3 h). The analysis of XRD patterns shows that La_0.8Ce_0.2Fe_11.5xCo_xSi_1.5 alloys consist of the NaZn₁₃-type main phase and α-Fe impurity phase. With the increase of Co content from x=0 to 0.7, the Curie temperature T_C increases from 180 to 266 K. Because the increase of Co content can weaken the itinerant electron metamagnetic transition, the order of the magnetic transition at T_C changes from first to second-order between x=0.3 and 0.5. Although the magnetic entropy change decreases from 34.9 to 6.8 J/kg K with increasing Co concentration at a low magnetic field of 0-2 T, the thermal and magnetic hysteresis loss reduces remarkably, which is very important for the magnetic refrigerant near room temperature.     

Surface modification of γ-TiAl alloys by acetylene plasma deposition

Surfaces of two γ-TiAl alloys, Ti-47 at% Al-2at% Nb-2 at% Cr (MJ12) and Ti-47 at% Al-2 at% Nb-2 at% Mn + 0.8 at% TiB₂ (MJ47), have been modified by acetylene plasma deposition at bias voltages of −4, −5 and −6 kV for 3.6 × 10³ s (1 h) and 1.44 × 10⁴ s (4 h). Knoop hardness (HK) of the alloys is increased with the increase of bias voltage and prolonged time for the deposition. HK of MJ12 and MJ47 deposited at −6 kV for 1.44 × 10⁴ s is, respectively, 3.36 and 3.32 times as hard as the untreated alloys. SEM and AFM analyses show that the deposited alloys compose of a number of nano-dots which reflect their surface properties. The phases analyzed by XRD are in accord with the elements analyzed by EDX.     

Effects of duty ratio at low frequency on growth mechanism of micro-plasma oxidation ceramic coatings on Ti alloy

The aim of this work is to study the effects of duty ratio on the growth mechanism of the ceramic coatings on Ti-6Al-4V alloy prepared by pulsed single-polar MPO at 50 Hz in NaAlO₂ solution. The phase composition of the coatings was studied by X-ray diffraction, and the morphology and the element distribution in the coating were examined through scanning electron microscopy and energy dispersive spectroscopy. The thickness of the coatings was measured by eddy current coating thickness gauge. The corrosion resistance of the coated samples was examined by linear sweep voltammetry technique in 3.5% NaCl solution. The changes of the duty ratio (D) of the anode process led to the changes of the mode of the spark discharge during the pulsed single-polar MPO process, which further influenced the structure and the morphology of the ceramic coatings. The coatings prepared at D = 10% were composed of a large amount of Al₂TiO₅ and a little γ-Al₂O₃ while the coatings prepared at D = 45% were mainly composed of α-Al₂O₃ and γ-Al₂O₃. The coating thickness and the roughness were both increased with the increasing D due to the formation of Al₂O₃. The formation of Al₂TiO₅ resulted from the spark discharge due to the breakdown of the oxide film, while the formation of Al₂O₃ resulted from the spark discharge due to the breakdown of the vapor envelope. The ceramic coatings improved the corrosion resistance of Ti-6Al-4V alloy. And the surface morphology and the coating thickness determined the corrosion resistance of the coated samples prepared at D = 45% was better than that of the coated samples prepared at D = 10%.     

Influence of heat treatment on the structure and magnetic properties of the Co/Zr and Co/Hf multilayer amorphous films

The influence of annealing on the structure and magnetic properties of amorphous Co/Zr and Co/Hf multilayer films was studied with particular attention to the dependence of the magnetic properties, thermal stability and crystallization process on layer composition and thickness. The temperature at which crystallization commences increases from 400 to 460 °C as the layer thickness d_Zr or d_Hf increases from 6 to 18 Å, and decreases from 450 to 400 °C as d_Co increases from 12 to 18 Å. Multilayers containing 19–60 at% Zr were studied. The specific magnetization was found to increase even below the temperature at which crystallization commences. Our data are compared with non-multilayer Co–Zr amorphous films and rapidly quenched metallic glasses.     

In vitro corrosion behavior of Ti-O film deposited on fluoride-treated Mg-Zn-Y-Nd alloy

In this paper, a new composite coating was fabricated on magnesium alloy by a two-step approach, to improve the corrosion resistance and biocompatibility of Mg-Zn-Y-Nd alloy. First, fluoride conversion layer was synthesized on magnesium alloy surface by immersion treatment in hydrofluoric acid and then, Ti-O film was deposited on the preceding fluoride layer by magnetron sputtering. FE-SEM images revealed a smooth and uniform surface consisting of aggregated nano-particles with average size of 100 nm, and a total coating thickness of ∼1.5 μm, including an outer Ti-O film of ∼250 nm. The surface EDS and XRD data indicated that the composite coating was mainly composed of crystalline magnesium fluoride (MgF₂), and non-crystalline Ti-O. Potentiodynamic polarization tests revealed that the composite coated sample have a corrosion potential (E_corr) of −1.60 V and a corrosion current density (I_corr) of 0.17 μA/cm², which improved by 100 mV and reduced by two orders of magnitude, compared with the sample only coated by Ti-O. EIS results showed a polarization resistance of 3.98 kΩ cm² for the Ti-O coated sample and 0.42 kΩ cm² for the composite coated sample, giving an improvement of about 100 times. After 72 h immersion in SBF, widespread damage and deep corrosion holes were observed on the Ti-O coated sample surface, while the integrity of composite coating remained well after 7 d. In brief, the data suggested that single Ti-O film on degradable magnesium alloys was apt to become failure prematurely in corrosion environment. Ti-O film deposited on fluoride-treated magnesium alloys might potentially meet the requirements for future clinical magnesium alloy stent application.     

Effect of surface roughness on the development of protective Al2O3 on Fe-10Al (at.%) alloys containing 0-10 at.% Cr

The effect of alloy surface roughness, achieved by different degrees of surface polishing, on the development of protective alumina layer on Fe-10 at.% Al alloys containing 0, 5, and 10 at.% Cr was investigated during oxidation at 1000 °C in 0.1 MPa oxygen. For alloys that are not strong Al₂O₃ formers (Fe-10Al and Fe-5Cr-10Al), the rougher surfaces increased Fe incorporation into the overall surface layer. On the Fe-10Al, more iron oxides were formed in a uniform layer of mixed aluminum- and iron-oxides since the layer was thicker. On the Fe-5Cr-10Al, more iron-rich nodules developed on an otherwise thin Al₂O₃ surface layer. These nodules nucleated preferentially along surface scratch marks but not on alloy grain boundaries. For the strong Al₂O₃-forming Fe-10Cr-10Al alloy, protective alumina surface layers were observed regardless of the surface roughness. These results indicate that the formation of a protective Al₂O₃ layer on Fe-Cr-Al surfaces is not dictated by Al diffusion to the surface. More cold-worked surfaces caused an enhanced Fe diffusion, hence produced more Fe-rich oxides during the early stage of oxidation.