Coated Ti 6Al 4V Research Articles

Effect of inflammatory conditions and H2O2 on bare and coated Ti–6Al–4V surfaces: Corrosion behavior, metal ion release and Ca-P formation under long-term immersion in DMEM

The surface oxide film and calcium-phosphate (Ca-P) formation on Ti–6Al–4V during long-term immersion in biological environments play a decisive role for the biocompatibility of the implant. Hence, the aim of the study was to evaluate the corrosion resistance, metal ion release and Ca-P formation in DMEM under physiological conditions at pH values of 7.4 and in comparison under simulated inflammatory conditions with pH 5 and in presence of H2O2. Furthermore, the influence of the immersion conditions was investigated on different surface treatments: on bare Ti–6Al–4V, after anodization, and for TiO2 nanoparticle (NP) and hydroxyapatite (HA)-incorporated TiO2-NP coatings. In the absence of H2O2, the impedance response indicated a stable thin oxide film and Ca-P formation after 28 days or 56 days depending on the coating, while under inflammatory conditions the Ca-P formation on the surface is time-delayed and dissolution of the anodized oxide layer as well as selective etching of the β-phase and phase boundaries in case of the bare alloy occur. Electrochemical impedance spectroscopy (EIS), however, indicates a good general corrosion behavior in all cases. The quantities of Ti, Al and V released from the bare and coated Ti–6Al–4V alloy markedly increased with decreasing pH (pH≤5). Although the rapid increase of metal release was observed for all samples at pH 5, the quantities were significantly higher for the bare and anodized alloy than after coating with TiO2-NP or HA.

A nanocrystalline zirconium carbide coating as a functional corrosion-resistant barrier for polymer electrolyte membrane fuel cell application

A ZrC nanocrystalline coating is engineered onto a Ti–6Al–4V substrate using a double cathode glow discharge technique in order to improve the corrosion resistance and long-term stability of this alloy. The new coating exhibits an extremely dense, homogeneous microstructure composed of equiaxed grains with an average grain size of ∼12 nm and is well adhered on the surface of the substrate. The corrosion behaviour of the coating is systematically investigated using various electrochemical methods, including potentiodynamic, potentiostatic polarizations and electrochemical impedance spectroscopy (EIS), in a simulated polymer electrolyte membrane fuel cell (PEMFC) operating circumstances under different temperatures. The results show that with rising temperature, the corrosion potential (Ecorr) decreases and the corrosion current density (icorr) of the ZrC coated specimen increases, indicating that the corrosion resistance decreased with increasing temperature. However, at a given temperature, the ZrC-coated Ti–6Al–4V alloy has a higher Ecorr and lower icorr as compared to the bare substrate. The results of EIS measurements show that the values of the resistance for the ZrC coated Ti–6Al–4V alloy are three orders of magnitude larger than those of Ti–6A1–4V in the simulated PEMFC environment.

Friction surfacing of Ti–6Al–4V: Process characteristics and deposition behaviour at various rotational speeds

By the process of friction surfacing, coatings are generated from metallic materials at temperatures below their melting range. The high degree of deformation while depositing leads to grain refinement in the microstructure, which has a positive effect on the mechanical properties of the layer. The applicability of the process has been described for a large number of materials. The deposition of Ti–6Al–4V has been reported in one publication but was not systematically studied. Therefore, the main aims of the present work are to define the process parameter fields for the deposition of Ti–6Al–4V leading to flash and defect free coatings and associate them with geometric features of the deposited layer.This investigation has shown that Ti–6Al–4V coatings can be effectively deposited onto a Ti–6Al–4V substrate by friction surfacing. A wide range of process parameters was established in which coatings of high quality have been obtained. The consumption rate control has been implemented as an efficient mode for the deposition of Ti–6Al–4V coatings. Temperature measurements at the coating interface have been accomplished showing that the coating material has been deformed in the β-phase. Furthermore, the homogeneity of the coating surface has been established to be a function of the rotational speed. The coatings exhibited a defect-free bond at the interface with the substrate. Two process parameter ranges with respect to the flash formation have been established. One of them enables flash-free coatings and the other generates coatings with flash formation on the retreating side, which can be controlled by the rotational and deposition speeds. Moreover, an increase in the rotational speed has been shown to lead to an increase in the coating thickness and width as well as an increase in the deposition efficiency up to 39 %.

Ti/TiN/HA coating on Ti–6Al–4V for biomedical applications

An unbalanced physical vapor deposition (PVD) magnetron sputtering technique was used to deposit a titanium/titanium nitride/hydroxyapatite (Ti/TiN/HA) multilayer coating on Ti–6Al–4V. The Taguchi optimization method with an orthogonal array of L9 was utilized to determine the optimal conditions of coating for better surface integrity. The effects of the temperature, DC bias voltage, nitrogen flow rate, and DC power on the adhesion strength, coefficient of friction, surface hardness, and surface roughness were investigated. In achieving the optimized coating parameters, the signal to noise (S/N) response analysis method was implemented. Confirmation tests were carried out to demonstrate the improvement using the best combination of parameters obtained from the optimization process. The surface hardness of the Ti/TiN coated and uncoated specimens was enhanced by 14% and 48%, respectively. The adhesion strength of the coating to the substrate, coefficient of friction, and surface roughness were improved by 5.1%, 43%, and 10.52%, respectively. Finally, hydroxyapatite was successfully deposited atop Ti/TiN coated Ti–6Al–4V using a PVD sputtering technique to improve the biocompatibility of the substrate. The presence of Ti/TiN interfacial thin layers between the HA layer and substrate enhanced the adhesion strength of the HA coating on the substrate by 44.57% compared to the HA-coated substrate without the interfacial layers.

Improving corrosion resistance of Ti–6Al–4V alloy through plasma-assisted PVD deposited nitride coatings

TiN and TiAlN/TiAlCrN were deposited on the alloy Ti–6Al–4V by means of plasma-assisted PVD technique in order to improve its corrosion resistance in a 2M HCl solution at 25, 60, and 80°C. The corrosion behaviour was investigated using open-circuit potential vs. time measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. The uncoated Ti–6Al–4V alloy demonstrated an active behaviour with an active–passive transition in the HCl solution. The TiN and TiAlN/TiAlCrN coated Ti–6Al–4V alloy presented a passive behaviour. The corrosion resistance of the coated alloy was superior, which was evidenced by lower corrosion current densities and higher impedance values.

In vitro biological performance of minerals substituted hydroxyapatite coating by pulsed electrodeposition method

The present study deals with the optimization of minerals (Sr, Mg and Zn) substituted hydroxyapatite coatings (M-HAP) at different pulse on and off time (1 s, 2 s, 3 s and 4 s) by pulsed electrodeposition method. The formation of M-HAP coating was investigated using Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction studies (XRD). The morphological features and the content of Sr, Mg and Zn ions in M-HAP coated Ti–6Al–4V were investigated by high resolution scanning electron microscopy (HRSEM) and energy dispersive X-ray analysis (EDAX). The electrochemical studies were performed for M-HAP coated Ti–6Al–4V in simulated body fluid which exhibited better corrosion resistance at the prolonged pulse off time. The in vitro cell adhesion test revealed that the M-HAP coating is found appropriate for the formation of new cell growth which proves the enhanced biocompatible nature of the coating. Thus the M-HAP coating will serve as a potential candidate in orthopedic applications.

Exploring new W–B coating materials for the aqueous corrosion–wear protection of austenitic stainless steel

The material loss of metallic surfaces through corrosion–wear is a serious concern in many application sectors, ranging from bio-medical implants to marine, oil and gas field components to transport vehicle and nuclear reactor devices. In principle, self-passivating alloys, like stainless steels, can be protected from surface degradation caused by corrosion–wear through the application of protective thin, hard surface coatings. In this work the suitability of using W matrix coating materials supersaturated with varying levels of boron were applied to austenitic stainless steel substrates (Ortron 90) and assessed for this purpose. These materials were compared to a highly corrosion–wear resistant “datum” surface engineered material (CrN coated Ti–6Al–4V) in sliding contact tests against a chemically inert aluminium oxide ball, whilst immersed in 0.9% NaCl solution at 37°C. The work demonstrated that all the coated materials to be very much more resistant to material loss through corrosion–wear (by nearly an order of magnitude) compared to uncoated stainless steel, and two coatings, W–13%B and W–23%B coated Ortron 90 were similarly resistant as CrN coated Ti–6Al–4V. Three fundamental types of corrosion–wear were discovered that represented differing levels of passive film durability. The total material loss rate (TMLR) during corrosion–wear testing showed linear proportionality with the change in open circuit potential δOCP which obeyed the governing equation: TMLR=m δOCP+C.

Characterization and corrosion resistance of plasma sprayed HA and HA–SiO2 coatings on Ti–6Al–4V

The objective of this study is to evaluate corrosion resistance of plasma sprayed hydroxyapatite (HA) and hydroxyapatite–silicon oxide (SiO2) coated Ti–6Al–4V substrate. In HA–SiO2 coatings, 10wt.% SiO2 was mixed with HA. The feedstock and coatings were characterized by X-ray diffraction and scanning electron microscopy/energy dispersive X-ray spectroscopy. The corrosion resistance was determined for the uncoated and two coatings. The corrosion resistance of the Ti–6Al–4V was found more after the deposition of the HA+10wt.% SiO2 rather than after HA coating and uncoated. Both HA, as well as, HA+10wt.% SiO2 coatings were crack free after 24h dipping in Ringer's solution for electrochemical corrosion.

Improved Surface Wettability of Water by Applying SiC/Ti6Al4V Coatings on Carbon/Carbon Composites

SiC/Ti6Al4V coatings were applied on carbon/carbon composites to improve the surface wettability of water. SiC interlayers were preprepared by pack cementation to bond both the carbon/carbon composites and the Ti6Al4V, and then the Ti6Al4V coatings were applied by magnetron sputtering technique. The morphology and crystalline of the SiC/Ti6Al4V coatings were analyzed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The surface wettability of the coatings was tested by video-based contact angle measuring device. The results showed that SiC can serve as an interlayer between the carbon/carbon composites and Ti6Al4V. The SiC/Ti6Al4V coatings covered the carbon/carbon composites uniformly with spherical morphology. The coatings improved the surface wettability of carbon/carbon composites with the contact angle of water decreasing from 85.7 ± 4.1° to 26.5 ± 0.1°.

Ti6Al4V coating for carbon/carbon composites synthesized by magnetron sputtering: microstructure and in‐vitro biotests

In order to improve the cell compatibility of biomedical carbon/carbon composites, Ti6Al4V coatings were applied on carbon/carbon composites by magnetron sputtering technique. The microstructure and cell responses of the coatings were evaluated, and the bonding strength between the coatings and carbon/carbon composites was analyzed. The Ti6Al4V coatings modulated the surface morphology, decreased the surface roughness, improved the surface wettability and achieved a strong bonding strength to carbon/carbon composites. The cells showed larger cell extension and higher cell proliferation for the Ti6Al4V coatings than for the carbon/carbon composites. Copyright © 2012 John Wiley & Sons, Ltd.

Electrochemical investigation of chromium nanocarbide coated Ti–6Al–4V and Co–Cr–Mo alloy substrates

This study investigated the electrochemical behavior of chromium nano-carbide cermet coating applied on Ti–6Al–4V and Co–Cr–Mo alloys for potential application as wear and corrosion resistant bearing surfaces. The cermet coating consisted of a highly heterogeneous combination of carbides embedded in a metal matrix. The main factors studied were the effect of substrate (Ti–6Al–4V vs. Co–Cr–Mo), solution conditions (physiological vs. 1 M H 2O 2 of pH 2), time of immersion (1 vs. 24 h) and post coating treatments (passivation and gamma sterilization). The coatings were produced with high velocity oxygen fuel (HVOF) thermal spray technique at atmospheric conditions to a thickness of 250 μm then ground and polished to a finished thickness of 100 μm and gamma sterilized. Native Ti–6Al–4V and Co–Cr–Mo alloys were used as controls. The corrosion behavior was evaluated using potentiodynamic polarization, mechanical abrasion and electrochemical impedance spectroscopy under physiologically representative test solution conditions (phosphate buffered saline, pH 7.4, 37 °C) as well as harsh corrosion environments (pH ∼ 2, 1 M H 2O 2, T = 65 °C). Severe environmental conditions were used to assess how susceptible coatings are to conditions that derive from possible crevice-like environments, and the presence of inflammatory species like H 2O 2. SEM analysis was performed on the coating surface and cross-section. The results show that the corrosion current values of the coatings (0.4–4 μA/cm 2) were in a range similar to Co–Cr–Mo alloy. The heterogeneous microstructure of the coating influenced the corrosion performance. It was observed that the coating impedances for all groups decreased significantly in aggressive environments compared with neutral and also dropped over exposure time. The low frequency impedances of coatings were lower than controls. Among the coated samples, passivated nanocarbide coating on Co–Cr–Mo alloy displayed the least corrosion resistance. However, all the coated materials demonstrated higher corrosion resistance to mechanical abrasion compared to the native alloys.

Micro-abrasion wear testing of triode plasma diffusion and duplex treated Ti–6Al–4V alloy

In this paper micro-abrasion wear testing is used to evaluate the wear resistance of triode plasma diffusion-treated, single-layered TiN-, CrAlN-, and WC/C-coated and duplex-diffusion and coated Ti–6Al–4V under uniform three-body rolling abrasion. Nanoindentation, Knoop microhardness, mechanical surface profilometry, optical microscopy, scanning electron microscopy and atomic force microscopy, were used to characterise the surfaces under investigation. Optimum testing conditions for rolling abrasion were established by varying the test parameters and resultant severity of contact. Very low normal loads and high volume fractions of particles in the abrasive slurry are necessary to obtain predictable and reproducible results. Relatively coarse SiC abrasive particles, having a mean diameter of around 3μm, appear more suitable for micro-abrasion testing of the samples investigated, compared to finer Al2O3 particles. Problems associated with the measurement of the scar volume and subsequent calculation of the wear rate for hard coatings deposited on relatively soft metals like titanium are identified, and suitable testing and measurement techniques are suggested. Three-dimensional wear scar maps generated by mechanical stylus profilometry were used to measure the wear volumes. Under the test conditions used, wear coefficients can be determined from perforating and non-perforating tests, although perforating tests provide more consistent results. Triode plasma diffusion treatments, plasma-assisted (PA) PVD TiN and PAPVD CrAlN can reduce the specific wear rate of Ti–6Al–4V, while PACVD-based WC/C coatings do not provide suitable protection against abrasive wear. The combination of triode plasma oxynitriding diffusion treatments and PVD coatings to create duplex treatments can also lead to further reductions in the coating wear coefficient when compared to non-duplex coatings deposited on non-pretreated substrates.

Tribological properties of duplex plasma oxidised, nitrided and PVD coated Ti–6Al–4V

Sequential triode plasma oxidation and nitriding have been used to provide enhanced load support for physical vapour deposited (PVD) hard coatings. The diffusion process has been designed to maximise process efficiency and coating adhesion, thereby significantly improving the tribological properties of the Ti–6Al–4V alloy — particularly at high contact pressures. This has been demonstrated using unlubricated linear reciprocating-sliding ball-on-plate wear tests and micro-scratch adhesion testing. Also, surface micro-profilometry, nano/micro-indentation hardness testing, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and glow-discharge optical emission spectroscopy (GDOES) data are presented to corroborate the effect of the several plasma diffusion processes and duplex diffusion/coating combinations discussed here. The results presented show that the novel processing technique developed permits the use of oxygen diffusion in order to obtain relatively large case depths in shorter treatment times without compromising the adhesion strength of subsequently deposited PVD layers.

Fatigue behavior of PVD coated Ti–6Al–4V alloy

The aim of the present work is to identify an environmentally clean coating process that will present lower influence in the Ti–6Al–4V fatigue strength. Axial fatigue tests of Ti–6Al–4V alloy TiN, CrN and WC:H (tungsten containing diamond-like carbon) Physical Vapor Deposition (PVD) coated was evaluated. Decrease in fatigue life was observed for coated specimens in comparison to base metal. Scanning Electron Microscopy technique was used to observe crack origin sites and coating thickness. DLC coating provides higher Ti–6Al–4V alloy PVD coated fatigue strength due to lower defects presence and the chromium interlayer, which acts as a barrier to fatigue crack propagation.

Microstructure, micro-mechanical and tribological properties of the nc-WC/a-C nanocomposite coatings magnetron sputtered on non-hardened and oxygen hardened Ti–6Al–4V alloy

Nanocomposite nc-WC/a-C coatings with hydrogen-free amorphous carbon a-C matrix have been deposited onto the surface of a baseline (non-hardened) and oxygen hardened Ti–6Al–4V alloy by means of a reactive magnetron sputtering. Oxygen hardening of the Ti–6Al–4V alloy was performed by means of oxygen atoms soaking from the Ar+O2 atmosphere at 1173K with assistance of plasma glow discharge. Investigations of a micro/nanostructure as well as micro/nanohardness and tribological properties of the baseline and surface treated alloys were performed. The results of micro/nanostructural analyses showed that the coatings are composed of different tungsten carbides nanocrystals (up to 6nm in size) embedded in an amorphous matrix. Scratch tests revealed a better adhesion of the coatings to the oxygen hardened substrate, than to the baseline alloy. Oxygen hardening in plasma glow discharge effectively improved the surface properties of the investigated alloy. It was established that the nc-WC/a-C coating on oxygen hardened alloy has much better tribological properties than the same coating produced on baseline alloy. The applied duplex surface treatment combining the oxygen hardening with subsequent deposition of nc-WC/a-C coatings decreases the friction coefficient as well as essentially improves microhardness and wear resistance in dry sliding of the coated Ti–6Al–4V alloy. The results are promising from the point of view of potential applications of such nanocomposite coatings in aeronautics and automotive industry for the components exposed to sliding wear.

Relative performance of hydrogenated, argon-incorporated and nitrogen-incorporated diamond-like carbon coated Ti–6Al–4V samples under fretting wear loading

Hydrogenated diamond-like carbon (DLC) (H-DLC), argon-incorporated DLC (Ar-DLC) and nitrogen-incorporated DLC (N-DLC) coatings were deposited on flat rectangular Ti–6Al–4V samples. The DLC coatings were characterised by Raman spectroscopy and nanoindentation. Fretting wear tests were conducted on uncoated and DLC coated samples with an alumina ball as the counterbody. As the Ar-DLC and N-DLC coatings had relatively more sp 2 network compared to the H-DLC coating, they exhibited lower values of hardness and elastic modulus. At both loads of 4.9 N and 14.7 N, all DLC coated specimens showed lower values of tangential force coefficient (TFC), wear volume and specific wear rate compared to the uncoated samples. While the Ar-DLC coated sample exhibited the lowest TFC, wear volume and specific wear rate at 4.9 N load, the N-DLC coated specimen exhibited the lowest TFC, wear volume and specific wear rate at 14.7 N load.

Effect of WC–10%Co–4%Cr coating on the Ti–6Al–4V alloy fatigue strength

High strength/weight ratio and effective corrosion resistance are primary reasons to use titanium alloys replacing steel and aluminum in some aeronautical components. However, titanium alloys have poor tribological properties, which reduce devices performance under friction; making surface treatments a requirement to improve wear. Thermal spray coatings have attractive characteristics as high hardness and strong coating/substrate adhesion. Compared with thermal spray processes, the High Velocity Oxygen Fuel (HVOF) presents less porosity and oxide contents due to the lower flame temperature used in the process operation. Electroplated coatings used to improved abrasive wear and corrosion properties, affects negatively the fatigue strength, providing lower results than those for uncoated parts. To increase fatigue strength of coated materials, techniques as compressive residual stresses induced by shot peening are used. In this study the influence of WC–10%Co–4%Cr coating deposited by HVOF on the fatigue strength of Ti–6Al–4V alloy was evaluated. Comparison of fatigue strength of coated specimens and base material shows also a decrease when parts are coated. It was observed that the influence is more significant in high cycle fatigue tests. The shot peening prior to the thermal spray coating is an efficient surface treatment to improve fatigue resistance of coated Ti–6Al–4V. Scanning electron microscopy technique (SEM) was used to observe crack origin sites and thickness in all the coatings.

Fretting wear studies on diamond-like carbon coated Ti–6Al–4V

This work is concerned with the performance of diamond-like carbon (DLC) coated Ti–6Al–4V flat samples fretted against alumina ball in two environments—ambient air and simulated body fluid (Ringer solution). Hydrogenated DLC (H-DLC) coatings were prepared by radio frequency plasma enhanced chemical vapour deposition process. Raman scattering measurements were done on the coatings. Fretting wear tests were conducted at different normal loads using a fretting wear test rig working on scotch yoke mechanism. Wear scars on the tested specimens were observed using an optical microscope. Wear volume was calculated based on wear scar diameter measured along parallel and perpendicular to the fretting direction. Tangential force coefficient (TFC) values exhibited by DLC coated samples were lower than those exhibited by uncoated specimens in both ambient air and Ringer solution. Very low TFC values and low wear volume were observed for H-DLC coated samples tested in air at lower loads of 1.9 N and 4.9 N. It was attributed to occurrence of graphitisation and formation of transfer layer on alumina ball.

On the mechanical stability of porous coated press fit titanium implants: A finite element study of a pushout test

Pushout tests can be used to estimate the shear strength of the bone implant interface. Numerous such experimental studies have been published in the literature. Despite this researchers are still some way off with respect to the development of accurate numerical models to simulate implant stability.In the present work a specific experimental pushout study from the literature was simulated using two different bones implant interface models. The implant was a porous coated Ti–6Al–4V retrieved 4 weeks postoperatively from a dog model. The purpose was to find out which of the interface models could replicate the experimental results using physically meaningful input parameters.The results showed that a model based on partial bone ingrowth (ingrowth stability) is superior to an interface model based on friction and prestressing due to press fit (initial stability).Even though the present study is limited to a single experimental setup, the authors suggest that the presented methodology can be used to investigate implant stability from other experimental pushout models. This would eventually enhance the much needed understanding of the mechanical response of the bone implant interface and help to quantify how implant stability evolves with time.

Sliding wear characteristics of the diamond-like carbon films on alloy substrates

This study used an un-balanced magnetron sputtering system to deposit different thickness of diamond-like carbon (DLC) film on Co–Cr–Mo and Ti–6A1–4V alloy substrate. Wear test, Raman spectroscopy analysis, nano-indentation test and morphology of contact surface were conducted to evaluate the tribological performance of these films for bio-medical applications. The ring-on-disk wear testing shows that the thicker film leads to the lower friction coefficient and higher wear resistance. In the testing process, the fluctuating friction coefficient of DLC coated Co–Cr–Mo substrate was less than that of DLC coated Ti–6Al–4V substrate. Raman spectroscopy analysis and nano-indentation test provide the underlying mechanism for the enhancement of the wear resistance. The DLC coated Co–Cr–Mo substrate provides better wear resistance than the DLC coated Ti–6Al–4V substrate owing to better adhesion of DLC film to the Co–Cr–Mo substrate.