Effect of transition layer on the performance of hydroxyapatite/titanium nitride coating developed on Ti-6Al-4V alloy by magnetron sputtering

A gradient transition multilayer hydroxyapatite/titanium nitride (HA/TiN) coating was prepared on the Ti-6Al-4V alloy by magnetron sputtering. The composition, surface topography, microstructure, adhesion strength and electrochemical properties of the as-deposited coatings were characterized by SEM/EDS, AFM, XRD, FT-IR and electrochemical workstation. The experimental results showed that the single TiN coating deposited at a partial pressure of nitrogen (N₂) of 0.08?Pa had the best internal stress and tribological performance, and its volume loss was only 0.89% of that of Ti-6Al-4V alloy. The introduction of the TiN transition layer greatly improved the wear resistance of the Ti-6Al-4V alloy, and the adhesion strength of the HA layer to the substrate increased from 6.50?±?0.5?N to 11.70?±?1.2?N, an increase of 56%. The HA/TiN coating surface consisted of uniform hemispherical particles with dense structure and invisible defects (micro-cracks and pores). For the HA surface layer, the crystal structure and active hydroxyl (-OH) group was restored after heat treatment. Potentiodynamic polarization experiments indicated that the HA/TiN coating achieved the lowest corrosion current density and the most positive corrosion potential compared to the single TiN layer and Ti-6Al-4V alloy. In summary, it can be conclude that the gradient transition layer can well improve the mechanical properties and electrochemical behavior of the titanium alloy, and largely ensuring the stability of the surface bioactive coating.     

Pulsed laser deposition of functionally gradient diamondlike carbon–metal nanocomposites

Diamondlike carbon thin films possess atomic smoothness, chemical inertness, and hardness properties close to those of diamond. Unfortunately, these films exhibit poor adhesion to metals and polymers used in medical prostheses. This paper presents the processing and characterization of diamondlike carbon–copper, diamondlike carbon–silver, diamondlike carbon–silicon, and diamondlike carbon–titanium nanocomposite films with enhanced adhesion to Ti–6Al–4V alloy substrates. Silver forms nanoparticle arrays within the diamondlike carbon matrix in diamondlike carbon–silver nanocomposite films. On the other hand, titanium forms layers of titanium carbide within the diamondlike carbon matrix in diamondlike carbon–titanium nanocomposite films. These films were characterized using electron energy loss spectroscopy, transmission electron microscopy, Raman spectroscopy, Rutherford backscattering spectrometry, nanoindentation, wear testing, and scratch adhesion testing. Diamondlike carbon–metal nanocomposite films have numerous potential medical applications, including use on the surfaces of medical prostheses.     

Adhesion properties of functionally gradient diamond composite films on medical and tool alloys

We have investigated the adhesion properties of microcrystalline diamond thin films on Ti-Al-V alloy, Co-Cr-Mo alloy and steel. Microcrystalline diamond possesses high hardness, a low coefficient of friction, extreme chemical inertness and biocompatibility; these properties can enhance the performance of metal alloys used in medical implants and in machine tools. We have adopted three methods for improving the adhesion of microcrystalline diamond to commonly used metal alloys: (1) by alloying the substrate surface to minimize graphitization; (2) by employing appropriate buffer layers between the diamond film and the substrate; and (3) by creating functionally gradient diamond-(titanium carbide, tungsten carbide, titanium nitride and aluminum nitride) composites. We have demonstrated that functionally gradient discontinuous buffer layers of titanium carbide, titanium nitride, aluminum nitride and tungsten carbide are able to control stress and graphitization in microcrystalline diamond thin films. This work on buffer layers and functionally gradient coatings should allow the development of more adherent crystalline diamond films for medical and tribological applications.     

羟基磷灰石/TiO2复合涂层在模拟脑脊液中生物相容性（英文）

应用阳极氧化法在Ti-6Al-4V钛合金（TC4）表面制备了多孔TiO2涂层,在TiO2涂层表面电沉积制备了羟基磷灰石（hydroxyapatite,HA）/TiO2复合涂层,用实验用人工脑脊液（artificial cerebrospinal fluid,ACSF）体液模拟人体的脑脊液,以TC4和TiO2涂层为对比,研究了HA/TiO2涂层在浸泡过程中发生的物理化学变化,考察了HA/TiO2复合涂层抑制钛合金中元素Al和V的析出情况。结果表明：3种样品随浸泡时间的延长遵循的生长规律为：HA成核→HA晶粒长大→HA晶粒相互团簇形成一体→涂层逐渐扩大覆盖到整个基体表面;TC4,TiO2以及HA/TiO2涂层在ACSF中都能够诱导HA的生成,表现出了良好的生物活性。检测浸泡后溶液中Al和V的浓度可知,阳极氧化法制备的TiO2涂层对于Al,V元素的析出起到了一定的抑制作用,能够进一步提高钛合金的生物相容性。

Abstract：

Porous TiO2 coating was prepared on Ti-6Al-4V titanium alloy （TC4） substrate by the potentiostatic anodic oxidation method,and hydroxyapatite （HA） coating was prepared on the surface of TiO2 coating by the electrodeposition method to form HA/TiO2 composite coating. By using artificial cerebrospinal fluid （ACSF） to simulate human cerebrospinal fluid,the physicochemical changes of the HA/TiO2 coatings when soaked in ACSF were studied and compared with TC4 and TiO2 coating. Its inhabitation effects on Al and V were also studied. The results show that these three samples follow such a growth pattern：HA nucleation forma-tion,crystal growth,agglomeration,coatings formation. The bioactivity of TC4,TiO2 coating and TiO2/HA composite coating can be induced by the formation of HA in ACSF. According to the concentration of Al and V in ACSF,the TiO2 coating formed by anodic oxidation could inhibit the element precipitation more or less,and enhance the biocompatibility of titanium alloy.     

Laser Assisted Nitriding of Ti-6Al-4V Alloy: Metallurgical and Electrochemical Properties

The surface properties of Ti-6Al-4V alloy, such as wear resistance, are inadequate for many applications. To improve the surface properties of the alloy, many techniques have been considered. One of the promising techniques is to form a nitride layer on the surface of the workpiece by a laser beam. In the present study, laser assisted nitriding of the Ti-6Al-4V alloy surface is carried out under a nitrogen gas flow environment. A CO₂ laser is used to irradiate the Ti-6Al-4V alloy surface while nitrogen is introduced co-axially with the laser beam onto the workpiece surface. The resulting surface cross section is examined metallurgically. SEM and XRD were carried out for material characterization. The study is extended to include the electrochemical response of the resulting surfaces. The surface morphology of the electrochemically treated workpieces are examined. It is found that in the laser treated region dendritic structures occur and TiN forms in the surface vicinity. The density of pit formation at the surface of the treated region reduces considerably.     

N+ ion implantation-induced cell attachment to CNx coating prepared by ion beam assisted deposition

Ion beam-assisted deposition (IBAD) was used to synthesize carbon nitride (CN _x ) coatings on Ti-6Al-4V alloy at room temperature at 100-eV NHn⁺ beam bombarding energy. Nitrogen ion implantation was also conducted on the prepared coatings. The effects of ion fluence on the chemical bonding structure of the coatings were characterized by XPS, and Raman and FTIR spectroscopic methods. The results showed that N⁺ ion implantation increased the N concentration of the prepared CN _x coatings. The cell attachment tests gave promising results that N⁺ ion-implanted CN _x coatings exhibited low macrophage attachment. The adhered fibroblasts showed normal cellular growth and morphology. Under a fluence of 5 × 10¹⁷ ion/cm², the CN _x coating exhibits more N concentration and sp³ bonds which may be responsible for the changes in the cell attachment.     

Effects of Tool Electrode on EDM Performance of Ti-6Al-4V

Due to widespread application of Ti-6Al-4V especially in automotive, aerospace, defense, and biomedical industries; machinability of this alloy is of immense importance. Very low thermal conductivity of Ti-6Al-4V is mainly responsible for its poor machinability. During Electro-Discharge Machining (EDM), conductivity of the workpiece as well as electrode plays a vital role affecting process performance. In this context, choice of electrode also influences the machining efficiency. Therefore, selection of an appropriate electrode to enhance performance of EDM on Ti-6Al-4V alloy is indeed necessary. In the present reporting, an experimental work has been carried out to investigate ease of machining of titanium alloy (Ti-6Al-4V) for electro-discharge machining using Tungsten and Copper (normal and cryogenically treated) electrodes. Experiments have been performed at different values of peak discharge current to study EDM performance on Ti-6Al-4V in terms of material removal efficiency, surface roughness, surface crack density and white layer thickness observed for the EDMed end product prepared from Ti-6Al-4V work material. Influence of peak discharge current on topography of the EDMed work surface has been examined. EDS analysis followed by micro-indentation hardness test has been performed to examine the extent of carbon enrichment onto the machined surface during EDM operation. XRD tests have been carried out to comprehend metallurgical aspects of the EDMed work surface which are supposed to be affected by the thermo-electrical phenomenon of EDM operation. Results obtained thereof, have been analyzed in detail to understand effects of electrode in persuading machinability of Ti-6Al-4V while machining though EDM route. It has been found that among three tool electrodes selected (Tungsten, normal Copper and cryogenically treated Copper), cryogenically treated Copper electrode has been found the most superior in purview of EDM performance.

     

Hemocompatibility of diamondlike carbon–metal composite thin films

We have investigated the hemocompatibility of diamondlike carbon–silver composite and diamondlike carbon–titanium composite thin films prepared using a multicomponent target pulsed laser deposition process. These materials were examined using transmission electron microscopy, Raman spectroscopy, nanoindentation, electrochemical charge transfer testing, and platelet adhesion testing. Cross-sectional transmission electron microscopy revealed that silver self-assembles into nanoparticle arrays within the diamondlike carbon matrix in the diamondlike carbon–silver composite film. On the other hand, titanium self-assembles into alternating nanometer-thick titanium carbide layers within the diamondlike carbon matrix in the diamondlike carbon–titanium composite film. The hemocompatibility of these materials was examined using electrochemical charge transfer testing and platelet adhesion testing. A few small, widely scattered crystals were observed on the surface of the unalloyed diamondlike carbon film exposed to platelet rich plasma. On the other hand, dense fibrin networks with densely aggregated platelets were observed on the surfaces of diamondlike carbon–silver and diamondlike carbon–titanium composite thin films exposed to platelet rich plasma. Electrochemical testing revealed that the time constant for the diamondlike carbon thin film (λ = 1) was significantly higher than those for the diamondlike carbon–silver and diamondlike carbon–titanium composite thin films. These results suggest possible uses for diamondlike carbon thin films and diamondlike carbon–metal composite thin films as coatings in next generation cardiovascular implants.     

Influence of firing conditions and production methods on fracture strength of titanium-based metal ceramic crowns

Objectives: This study evaluated the effect of argon atmosphere compared with vacuum during porcelain firing on the fracture strength of crowns made of porcelain and electron beam melted (EBM) Ti-6Al-4 V, cast commercially pure titanium or milled commercially pure titanium. Methods: Sixty crown copings of c. p. titanium, Ti-6Al-4 V alloy and porcelain were fabricated using three production techniques. The copings were fired either under vacuum or in an argon gas atmosphere. Specimens were subdivided into groups of cast c. p. titanium, milled c. p. titanium and EBM Ti-6Al-4 V which were further subdivided according to firing modes employing either vacuum or argon gas. The 60 specimens were subjected to cyclic preloading and thermocycling, and were then individually loaded until interface fractured. Differences between the group mean values were calculated using the one-way ANOVA and Tukey’s range test. Two fractured samples from each group were cut with a diamond blade and examined using SEM and EDS for visualization and chemical composition analysis of the fractured interface. Results: The highest mean fracture strength values, though not significant, were recorded for the groups fired in argon atmosphere, and the lowest mean fracture strength values were recorded for the groups fired in vacuum, with one exception. Comparing the two main groups of firing atmosphere, no significant difference could be documented. SEM and EDS analysis indicated clear differences in composition and structure between the groups included in the study. Conclusions: Firing in argon atmosphere does not significantly improve the fracture strength of porcelain bonded to titanium.     

Influence of internal diffusion barriers on carbon diffusion in pure titanium and Ti–6Al–4V during diamond deposition

Diamond coatings were deposited on pure titanium and Ti–6Al–4V, at a temperature in the range of 600–750 °C, in a microwave plasma from CH₄/H₂ and CO/H₂ mixtures. The influence on carbon diffusion of different intermediate layers, especially tungsten, niobium, titanium nitride and pure titanium previously deposited on titanium alloys by physical vapor deposition (PVD) is reported. These intermediate layers are always composed of at least two sub-layers: (1) an internal diffusion barrier and (2) an external titanium layer that allows some carbon diffusion to be maintained. After diamond deposition, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) observations coupled with energy-dispersive X-ray (EDX) analysis of the final multilayer systems allow us to determine the diffracting phases, their lattice parameters and the efficiency of the different barriers. The carbon diffusion coefficients in the titanium carbide phase and in the α-titanium solid solution are deduced from an experimental study carried out on pure titanium with or without an underlying diffusion barrier. The results are compared to the carbon diffusion in Ti–6Al–4V alloy. This work permitted us to calculate the carbon concentration profiles in both pure titanium and Ti–6Al–4V substrates.     

Microstructure evolution and bonding mechanism of ZrO2 ceramic and Ti-6Al-4V alloy joints brazed by Bi2O3-B2O3-ZnO glass paste

Ceramics and metal joinings have been widely employed in aerospace, dental implants, and the electronic packaging industry for fabricating multifunctional components. In this study, the 35Bi₂O₃-50B₂O₃-15ZnO (mol.%) glass has been employed for joining the ZrO₂ ceramic and Ti-6Al-4V alloy. The effect of brazing temperature on the microstructure evolution, mechanical properties, and bonding mechanism of brazed joints has been analyzed. The microstructure of the ZrO₂/glass/Ti-6Al-4V joints and the content of Bi₄B₂O₉, Bi₂O₃ and Bi₂₄B₂O₃₉ precipitated crystals in glass were found to be dependent on the brazing temperature. The reaction product of Bi₄Ti₃O₁₂ was identified in the glass/Ti-6Al-4V interface because of the chemical reaction between the oxidized layer of Ti-6Al-4V alloy and glass. A maximum shear strength as high as 48.8 ± 5.2 MPa was obtained. Our work, thus, demonstrates that the 35Bi₂O₃-50B₂O₃-15ZnO glass is an effective bonding material for joining ZrO₂ ceramic and Ti-6Al-4V alloy under low temperature in an ambient atmosphere.     

Effects of pH value and temperature on the corrosion behavior of a Ta2N nanoceramic coating in simulated polymer electrolyte membrane fuel cell environment

To improve the corrosion resistance and electrical conductivity of Ti-6Al-4V bipolar plates used in polymer electrolyte membrane fuel cells (PEMFCs), a novel electro-conductive Ta₂N nanoceramic coating was developed by reactive sputter-deposition using a double cathode glow discharge plasma technique. The microstructure of the coating consisted of fine equiaxed Ta₂N grains with an average grain size of ∼13 nm, which exhibited a strong (101) preferred orientation. To explore the influence of both pH values and temperatures on the corrosion resistance of the coating, the electrochemical behaviors and electronic properties of passive films grown on the Ta₂N coating were systematically investigated using different electrochemical techniques in simulated PEMFC operating environment. It was shown that either increasing the acidity or the temperatures of the solution, the corrosion potential (E_corr) decreased and the corrosion current density (i_corr) increased. At a given temperature or pH value, the Ta₂N coating had a higher E_corr and lower i_corr as compared with uncoated Ti-6Al-4V. The results of EIS measurements showed that with increasing temperature or acidity of the solution, the resistance of the passive film (R_p) formed on the Ta₂N coating decreased slightly, being of the order of magnitude of 10⁷ Ω cm², which was an order of magnitude higher than that of uncoated Ti-6Al-4V. The interfacial contact resistance (ICR) values were found to increase with increasing pH value or decreasing solution temperature, and the ICR values of the Ta₂N coating were markedly lower than that of uncoated Ti-6Al-4V, due to the thinner thickness of passive films. Furthermore, the Ta₂N-coated Ti-6Al-4V is more hydrophobic than bare Ti-6A1-4V, which was favorable for both the simplification of water management and improving corrosion resistance in PEMFC operating environment.     

New titanium alloys for blended elemental powder processing

Six powder metallurgy titanium alloys based on commercial compositions were prepared using blended elemental titanium sponge and master alloy powders. The compositions were Ti-6Al-4V; Ti-6Al-4V + 0.5Fe; Ti-6Al-4V + 0.5Ni; Ti-6Al-6V-2Zr + 1Fe + 1Cu; Ti-6Al-6Zr-6Mo and Ti-10V-2Fe-3Al. The powders were cold isostatically pressed at 400 MPa and sintered at 1238 °C for 4 hours. The as-sintered samples had ～ 94% of the theoretical density, uniaxial yield strengths from 710 to 880 MPa and reductions in area from 0.5 to 8%. After a hot isostatic pressing cycle of 900 °C and 100 MPa for 2 h, the samples had ～ 99% of the theoretical density, yield strengths from 840 to 1035 MPa and reductions in area from 5 to 15%.     

The role of the polymer/metal interphase and its residual stresses in the critical strain energy release rate (Gc) determined using a three-point flexure test

The critical strain energy release rate (G _c), the residual stresses (σ), Young's modulus (E), and the practical adhesion, characterized by ultimate parameters (F_max or d_max), of organic layers made of DGEBA epoxy monomer and IPDA diamine hardener were determined. The prepolymer (DGEBA-IPDA) was deposited both as thick coatings and as a mechanical stiffener onto degreased aluminum alloy (5754) or chemically etched titanium alloy (Ti-6Al-4V). During the three-point flexure test used as a practical adhesion test [this test is also called the double cantilever adhesion test (DCAT)], the failure may be regarded as a special case of crack growth. To understand the real gradient properties of the interphase, substrate, and bulk polymer properties, a three-layer model was developed for quantitative determination of the critical strain energy release rate (G_c). The particular characteristic of this model was to consider the residual stresses developed within the entire three-layered system, leading to an intrinsic parameter representing the practical adhesion between a polymer and a metallic substrate. Moreover, to determine the residual stresses generated in such three-layer systems, the gradient of interphase mechanical properties was considered. The maxima of residual stress intensities are found at the interphase/substrate interface, leading to an adhesional (interfacial) failure that is experimentally observed. The determination of the critical strain energy release rate by the three-point flexure test (DCAT) shows that residual stresses cannot be neglected. A comparison between the results obtained from the three-point flexure test (DCAT) and those obtained by the tapered double cantilever beam (TDCB) test is presented.     

A comparative study on the synthesis and properties of suspension and solution precursor plasma sprayed hydroxyapatite coatings

In the present study, hydroxyapatite (HAp) coatings were deposited on Ti-6Al-4V alloy by solution precursor plasma spray (SPPS) and suspension plasma spray (SPS) processes and the properties of the coatings were compared. The feedstock powder for SPS method was prepared by coprecipitation technique and characterized for phase and morphology. The obtained HAp coatings were characterized by X-ray diffractometry, Raman spectroscopy and FT-IR spectroscopy. The biocompatibility of the coatings was evaluated using osteoblast like cells. Both the SPS and SPPS hydroxyapatite coatings exhibited similar crystallinity. Interestingly, the HAp-SPS coating showed marginally higher biocompatibility compared to HAp-SPPS and control samples. The wear and corrosion behavior of these coatings was also studied in Hanks' medium. The hydroxyapatite coating fabricated from SPS technique exhibited better corrosion and wear resistance compared to SPPS coating.     

Development of hydroxyapatite coatings on laser textured 316 LSS and Ti-6Al-4V and its electrochemical behavior in SBF solution for orthopedic applications

The current work focused on the development of hydroxyapatite (HAP) coating on laser textured metallic implants using electrophoretic deposition. HAP was synthesized by sol-gel technique and its phase purity and surface morphology were confirmed by FT-IR, XRD and SEM analysis. 316 L SS and Ti-6Al-4V metal implants were polished and the surface was modified using Nd-YAG laser operating at a pulse interval of 10 ns at various overlapping rate of 0%, 25% and 50%. The laser treated surface was characterized for its surface roughness using surface profilometry and surface morphology. The surface roughness of the metallic implants was increased by increase in the overlapping rate. The prepared HAP powder was electrophoretically deposited on bare and laser textured Ti-6Al-4V and 316 L stainless steel followed by vacuum sintering at 300 °C for 2 h. Scratch analysis results showed an improvement in adhesion strength for the HAP coatings on laser treated specimens than untreated metal. Corrosion efficiency of the coated samples was studied in SBF solution using EIS and potentiodynamic polarization studies. The result from the corrosion experiments proved increased corrosion resistance property of laser textured coated samples when compared to bare alloy due to higher adhesion of HAP coating on the metal surface.     

Enhancement of the Ti-6Al-4V alloy corrosion resistance by applying CrN/CrAlN multilayer coating via Arc-PVD method

In this study, the Ti-6Al-4V substrate was coated by CrN-CrN/TiN-TiN and CrN/CrAlN multilayer coatings using the cathodic arc physical vapor deposition (Arc-PVD) method. The results of potentiodynamic polarization (PDP) have shown the lowest and highest corrosion current density belong to the double-layer (0.16 µA/Cm²) and TiN (0.51 µA/Cm²) samples, indicating the higher corrosion resistance of the double-layer coating. The field emission electron microscope (FESEM), X-ray diffraction pattern (XRD), open circuit potential (OCP), PDP, and electrochemical impedance spectroscopy (EIS) analysis were employed in order to characterize the coatings and evaluate their corrosion behavior. Finally, applying the double-layer coating resulted in the significant improvement of the protective behavior of the Ti-6Al-4V alloy, as compared to the sample coated with TiN in corrosive environments.     

Electrodeposition of fluorohydroxyapatite gradient coating on titanium alloy

In order to obtain the apatite implants which can meet the requirements for both bonding strength and bioactivity, the FHA(Ca₁₀(PO₄)₆OH_2−xF_x(x = 0 ~ 2)) gradient coating was prepared by electrophoretic deposition on the surface of titanium alloy (Ti–6Al–4V). The effects of fluorine substitution and gradient structure on the morphology, bonding strength, and bioactivity of the coating were investigated. The gradient coating has the dense inner layer and porous outer layer that were deposited by controlling the suspension concentration and the preparation process. Through controlling of the fluorine substitution gradient, the thermal expansion coefficient gradually changed to improve the bonding strength more than 30 Mpa. The inner layer with high fluorine content can improve the thermal stability of the coating and bonding strength between the coating and the substrate, while the proper content of fluorine in the outer layer improves the bioactivity of the gradient coating.     

XPS and EIS study of the passive film formed on orthopaedic Ti-6Al-7Nb alloy in Hank's physiological solution

The composition and structure of the passive film formed on Ti-6Al-7Nb alloy by electrochemical oxidation in Hank's physiological solution were studied using X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The oxide layer was predominantly TiO₂, but contained small amounts of suboxides TiO and Ti₂O₃ at potentials more negative than 0.75 V. At more positive potentials, TiO₂ was the only form. The formation of suboxides in the lower potential range is less pronounced than in Ti-6Al-4V alloy. The passive range in Hank's physiological solution is broad and extends up to 6.0 V. Aluminium oxide Al₂O₃, and niobium oxides, Nb₂O₅, and NbO and/or NbO₂, are incorporated in the passive layer. Angular resolved XPS analysis confirmed that they are located mainly at the outer oxide/solution interface of the TiO₂ matrix. The thickness of the oxide layer was dependent on the oxidation potential and, after oxidation at 5.75 V, it reached 9.4 nm. EIS measurements correlate well with the XPS data. The incorporation of the oxides of alloying elements into the TiO₂ layer is reflected in the increase in the outer layer resistance at high anodic potentials and longer immersion times. The consequences of this process are beneficial for the overall stability and high corrosion resistance of the Ti-6Al-7Nb alloy under physiological conditions.     

Dip Coating of Calcium Hydroxyapatite on Ti-6Al-4V Substrates

Ti-6Al-4V alloy is the most commonly used metallic material in the manufacture of orthopedic implants. The main inorganic phase of human bone is calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA). To achieve better biocompatibility with bone, metal implants made of Ti-6Al-4V are often coated with bioceramics. Dip-coating techniques scarcely are used to apply HA onto metallic implants. New dipping-solution recipes to be used for HA coatings are described in this work. Scanning electron microscopy and X-ray diffractometry have been used for sample characterization.