Effect of microstructure on the adhesion strength of TiBx coating on Ti6Al4V substrate

TiB_x coatings were deposited on Ti6Al4V and Si (100) wafer substrates by D.C. magnetron sputtering with various target-to-substrate distances (T.S. distances) from 50 mm to 200 mm. The influence of T.S. distance on the microstructure, hardness and adhesion strength of TiB_x coatings and Ti6Al4V substrate system was investigated. Results showed that the microstructure of TiB_x coatings transformed from dense to fibre columnar grain with the increase in T.S. distance, whilst the hardness decreased from 20.9 GPa to 9.4 GPa. The Rockwell-C indentation adhesion strength grade was also improved from HF6 to HF1. An adhesion evaluation factor G, which is related to the mechanical properties and the microstructure of TiB_x coating, is proposed based on the test results. The adhesion strength increased with G, which corresponded well with the results of indentation test. The high-speed rubbing test with a sliding speed of 300 m/s was performed to check the Al-adhesion resistance of the TiB_x coating against Al–hBN seal coating.     

Tribocorrosion characteristics of CrMoSiCN/Ag coatings on Ti6Al4V alloys in seawater

CrMoSiCN/Ag coatings were deposited on Ti6Al4V alloys at the trimethylsilane flow of 15 sccm using closed-field unbalanced magnetron sputtering, and their microstructures were observed and analyzed using SEM, XRD, XPS and TEM, respectively. The coatings’ mechanical properties were measured using nano-indenter. The tribocorrosion characteristics of Ti6Al4V and CrMoSiCN/Ag coatings were investigated in seawater using tribocorrosion tester. The results revealed that the nanocomposite coatings consisted of (Cr, Mo)N solid solution, Ag nanocrystal and amorphous SiCN_x matrix. As the Ag target current increased to 1.0 A, a large amount of Ag nanoparticles were observed on the coating surface. The coating hardness initially increased to 21.0 ± 0.7 GPa at the Ag target current of 0.4 A and then declined. After the Ag element was added into coatings, their tribocorrosion characteristics were improved. The tribocorrosion characteristics of coatings were much better than those of Ti6Al4V. The tribocorrosion characteristics of CrMoSiCN/Ag coating at the Ag target current of 1.0 A were the best in seawater.     

Improving the adhesive,mechanical, tribological properties and corrosion resistance of reactive sputtered tantalum oxide coating on Ti6Al4V alloy via introducing multiple interlayers

Tantalum oxide film has become an investigation focus for surface modification materials in the biomedical field owing to its outstanding biocompatibility, anti-corrosion, and anti-wear performances. However, tantalum oxide films exhibit poor adhesion because of the mismatch between the properties of the film and the substrate. In this study, a novel multilayer tantalum oxide coating of Ta_mO_n/Ta_mO_n-TiO₂/TiO₂/Ti (code M-Ta_mO_n) was deposited on Ti6Al4V by magnetron sputtering with Ta_mO_n single-layer coating as control. The purpose of this work is to evaluate the influence of the introduced Ta_mO_n-TiO₂/TiO₂/Ti multi-interlayer on the microstructure, adhesive, mechanical, and anti-corrosion properties of reactive sputtered tantalum oxide coatings. The outcomes show that the Ta_mO_n-TiO₂/TiO₂/Ti intermediate layer improves the bonding strength between the Ta_mO_n layer and Ti6Al4V matrix from 17.83 N to over 50 N and enables the Ta_mO_n coating to have an increased H/E and H³/E² ratio, decreased friction coefficient and wear rate, raised potential, and reduced corrosion current density. The improved properties of the multilayer system are attributed to the positive effects of the inserted multiple interlayers in reducing the residual stress in the coating, coupling the mechanical performance between the layer and the substrate, blocking the continuous growth of penetrating defects in a film with columnar structure. These experimental results provide a workable route for improving the properties of the tantalum oxide coating on Ti6Al4V alloy for medical applications.     

Effect of carburizing treatment on microstructural,mechanical and tribological performances of Cr doped DLC coating deposited on Ti6Al4V alloy

Cr doped diamond-like carbon (DLC) coating was deposited on the carburized Ti6Al4V alloy (TA) by magnetron sputtering (MS). The physical and chemical characteristics of Cr doped DLC coating were obtained using scanning electron microscope, energy dispersive spectroscope, Raman spectrometer, X-ray photoelectron spectroscopy and Fourier transform infrared, and the mechanical property and adhesion force were analyzed by nanoindenter and scratch tester. The effects of carburizing treatment on the coefficient of friction (COF) and wear mechanism of Cr doped DLC coatings were investigated on a ball-on-disk tribometer. The results show that the adhesion force and mechanical property of Cr doped DLC coating deposited on the carburized TA are higher than those deposited on the original TA. The average coefficients of friction (COFs) of Cr doped DLC coatings deposited on the original and carburized TAs under the dry-friction condition are 0.157 and 0.143, respectively, showing that the carburizing treatment has the obvious effect of friction reduction for the Cr doped DLC coating. The wear mechanism of Cr doped DLC coating deposited on the carburized TA is combined action of abrasive wear and adhesive wear, which are contributed to the enhancement of mechanical property of Cr doped DLC coating by carburization treatment. Furthermore, the average COFs of Cr doped DLC coatings deposited on the original and carburized TAs under the oil-lubrication condition are 0.152 and 0.131, respectively, which are superior to those under the dry-fiction condition. The carburizing treatment promotes the formation of self-repairing carbonyl of Cr doped DLC coating, and the oil-wet characteristic of Cr doped DLC coating with the aromatic aldehyde of CO as the functional group plays the main role of friction reduction.     

Voltammetric stability of anodic films on the Ti6Al4V alloy in chloride medium

The biocompatibility and mechanical integrity of Ti and Ti6Al4V alloy can be affected by corrosion processes. This paper presents studies on the stability of anodic oxide films on Ti6Al4V and Ti in chloride medium. The oxides were grown potentiodynamically up to 8.0 V in the phosphate buffer saline (PBS) solution (pH 6.8) at 25 and 37 °C. The morphology of the obtained anodic oxides and the type of corrosion that occurred were analyzed by SEM–EDS. The Ti6Al4V alloy presented less corrosion resistance than pure Ti. Elemental analyses showed that the decrease of the alloy corrosion resistance is due mainly to the corrosion of Al.     

Electrochemical and structural evaluation of functionally graded bioglass-apatite composites electrophoretically deposited onto Ti6Al4V alloy

Titanium and its alloys are widely used as materials for implants, owing to their corrosion resistance, mechanical properties and excellent biocompatibility. However, clinical experience has shown that they are susceptible to localised corrosion in the human body causing the release of metal ions into the tissues surrounding the implants. Several incidences of clinical failures of such devices have demanded the application of biocompatible and corrosion resistant coatings and surface modification of the alloys. Coating metallic implants with bioactive materials is necessary to establish good interfacial bonds between the metal substrate and the bone. Hence, this work aimed at developing a bioglass-apatite (BG-HAP) graded coating on Ti6Al4V titanium alloy through electrophoretic deposition (EPD) technique. The coatings were characterized for their properties such as structural, electrochemical and mechanical stability. The electrochemical corrosion parameters such as corrosion potential (E_corr) (open circuit potential) and corrosion current density (I_corr) evaluated in simulated body fluid (SBF) have shown significant shifts towards noble direction for the graded bioglass-apatite coated specimens in comparison with uncoated Ti6Al4V alloy. Electrochemical impedance spectroscopic investigations revealed higher polarisation resistance and lower capacitance values for the coated specimens, evidencing the stable nature of the formed coatings. The results obtained in the present work demonstrate the suitability of the electrophoretic technique for the preparation of graded coating on Ti6Al4V substrates.     

The optical and electrical properties of ZnO:Al thin films deposited at low temperatures by RF magnetron sputtering

Several ZnO:Al thin films have been successfully deposited on glass substrates at different substrate temperatures by RF (radio frequency) magnetron sputtering method. Effects of the substrate temperatures on the optical and electrical properties of these ZnO:Al thin films were investigated. The UV–VIS–NIR spectra of the ZnO:Al thin films revealed that the average optical transmittances in the visible range are very high, up to 88%. X-ray diffraction results showed that crystallization of these films was improved at higher substrate temperature. The band gaps of ZnO:Al thin films deposited at 25 ℃, 150 ℃, 200 ℃, and 250 ℃ are 3.59 eV, 3.55 eV, 3.53 eV, and 3.48 eV, respectively. The Hall-effect measurement demonstrated that the electrical resistivity of the films decreased with the increase of the substrate temperature and the electrical resistivity reached 1.990×10^?3 Ω cm at 250 ℃.     

Structural evaluation,preliminary in vitro stability and electrochemical behavior of apatite coatings on Ti6Al4V substrates

Medical-grade alloys, such as Ti-6Al-4V, have been used for fixation of fractured bone and for the total replacement of defective bone. Their bioactivity could be improved by applying a bone-like apatite layer onto their surfaces. This, in turn, enhances their integration with the surrounding tissues upon implantation. In addition, the presence of a bioactive bone-like coating minimizes the likelihood of corrosion. Various methods are known for the formation of apatite coating onto Ti-6Al-4V, among which sputtering has shown its promise as a simple direct method. In the current work, a sputtering technique was used to develop a 300 nm-thick bone-like apatite layer onto Ti-6Al-4V. Structural composition, integrity and morphology of the as-coated and thermally treated coatings were investigated. Coated substrates were further evaluated after soaking them in a simulated body fluid (SBF) for up to 14 days. Results showed the formation of an amorphous apatite layer onto the alloy, that was further shown to partially crystallize upon heat treatment. As a result of SBF treatment, the apatite layer was found to remodel through a dissolution-precipitation mechanism due to its amorphous and non-stoichiometric nature, forming a smooth layer with better homogeneity and decreased surface roughness. Electrochemical analysis of the coated alloys showed the enhanced corrosion protection of the alloy surfaces by coating them with apatite. In addition, pre-grinding of the alloy surfaces before the formation of the coating was also found to improve the corrosion inhibition of the alloy surfaces in aqueous media.     

Laser deposition of compositionally graded titanium oxide on Ti6Al4V alloy

In this study, a laser cladding process was developed to deposit dense and well-adhered titanium single tracks on the surface of Ti6Al4V alloy with a compositional and microstructural gradient. CuO doped, freeze-dried anatase powder was specially formulated for this process. The addition of CuO resulted in stable melt pool with low viscosity, low surface tension and enhanced wettability with the substrate. Continuous titanium oxide single tracks were formed with a cross-sectional profile that was advantageous for coating deposition by means of multiple overlapping scan tracks. Rapid heating and cooling associated with laser cladding produced unique solidified microstructures with a compositional gradient. No structurally critical fractures were observed in the graded oxide layers, or at the coating/substrate interfaces. Furthermore, a transition zone of oxide/metal mixture was observed at the interface, increasing the effective bonding area between the coating and the substrate.     

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.     

Microstructures and mechanical properties of wear-resistant titanium oxide coatings deposited on Ti-6Al-4V alloy using laser cladding

In this work, a freeze-dried TiO₂ nano-sized powder was used as the coating material and single tracks of TiO_x coating were cladded on Ti-6Al-4V substrates using a diode laser. The microstructure, chemical composition, and mechanical properties of the coatings were characterized and their relationships were explored. Coatings with structural and compositional gradients formed under a laser energy density (LED) of 20 kJ/m, while coatings with a relatively homogeneous microstructure were obtained using a LED of 30 kJ/m. The microstructure evolution was controlled by the molten pool lifetime and the intensity of convective flow during laser processing. The elastic modulus of the graded coating showed a decreasing trend from the top coating surface to the interface while that of the homogeneous coating remained constant. Our results also demonstrated that the hardness and wear resistance of the oxide coatings were up to four and ten times higher than that of the substrate.     

Effect of Surface Texture on the Adhesion Performance of Laser Treated Ti6Al4V Alloy

The growing demand in lighter and safer structures generates the requirement of lighter joining strategies, particularly for lightweight metal alloys, composites, and also joining dissimilar materials together. Titanium alloys stand out as the conventional choice for materials for light weight structures. Adhesive bonding of titanium is an appealing route for joint design, also for the possibility of joining it with dissimilar materials. The realization of a strong joint depends not only on the joint design and type of adhesive, but also on the preparation of the adhering surface. Laser texturing presents advantages compared to common surface preparation processes in terms of eco-compatibility, energetic efficiency, ease of manufacturing, and repeatability. This work presents a preliminary investigation on laser texturing of Ti6Al4 V alloy with a pulsed fiber laser source with the aim to increase surface adhesion for bonding. Particularly, different surface textures are proposed, and laser machining strategies are developed. The results showed that laser texturing provided up to eightfold and 30% higher shear strength compared to plain and sand blasted surfaces, respectively. Failure analysis showed that a margin of improvement is still possible by adapting the surface texture for better cavity filling and reducing surface damage caused by the laser treatment.     

A potential method for electrochemical micromachining of titanium alloy Ti6Al4V

The micromachining of complex three-dimensional microstructures (bulk micromachining) on metals can be applied to fabricate many novel devices for micro electromechanical system (MEMS), which will greatly benefit the development of MEMS. In this paper, a new electrochemical micromachining method named the confined etchant layer technique (CELT) was explored on the micromachining of the titanium alloy. Micro-scale trapezoidal slots were replicated on titanium alloy by using a mold with the corresponding negative microstructures (trapezoidal teeth). The machining resolution reached 0.503 μm. The electrochemical mechanisms involved in the process are analyzed and the parameters that influenced the machining resolution are discussed. A. Attia is on leave from Physical Chemistry Department, National Research Centre, El-Tahrir St., Dokki, Cairo, Egypt.     

Effective corrosion protection of AA6061 aluminum alloy by sputtered Al-Ce coatings

Al-Ce coatings were deposited on silicon and AA6061 aluminum alloy substrates by DC magnetron sputtering using aluminum in combination with pure cerium targets. The materials were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and electrochemical impedance spectroscopy (EIS) in order to consider their application as high corrosion resistance coatings. The corrosion behavior of the films was studied using a NaCl aqueous solution (3.5 wt%). As for the characterization results, an apparent amorphous phase of aluminum oxide with small cerium compounds embedded in the matrix was detected by the X-ray diffraction patterns and HRTEM on the deposited films at 200 W and 4 Pa. At these conditions, AFM and SEM images evidenced crack-free coatings with low-roughness nanometric structures and columnar growth. EIS and Tafel results converged to indicate an inhibition of the corrosion reactions. The film displayed good stability in the aggressive medium and after 1 day of exposure underwent very little degradation. The variations in the impedance and Tafel characteristics were found to occur as a function of cerium content, which provokes important changes in the film protective properties.     

Growth evolution of CaB6 thin films deposited by DC magnetron sputtering

CaB₆ films were deposited by a DC magnetron sputtering method to explore the growth evolution systematically through changing sputtering time. The crystalline structure was characterized by XRD and GIXRD respectively, which showed that the films were anisotropic with nanocrystalline structure. The grain sizes increased with the deposition time, and a weak (100) texture appeared when the deposition time reached to 120 min. HRTEM was employed to demonstrate the crystalline structure. The surface morphology evolution of CaB₆ films was analyzed by AFM and FESEM. The results showed that the films were initially formed by fine columnar grains. With the deposition time extended, the films exhibited a dense columnar structure with faceted surfaces. The grain size, film thickness and crystallization degree all increased with the sputtering time.     

Investigation of Ti3AlC2 formation mechanism through diffusional reaction between carbon and Mo-modified Ti6Al4V

Carbon and Mo-coated Ti6Al4V alloy diffusion couple was used to investigate Mo-modified diffusion reaction between carbon and Ti6Al4V. Randomly dispersed carbide particles were observed in Ti6Al4V alloy after 900℃ exposure. Carbide particles were found evolving from defective TiC_x (x<1) to mixture of TiC_x and defective Ti₃AlC₂. Although Mo atoms were hardly detected in particles, their dilution effect along Ti alloy grain boundary (GB) is beneficial to carbon diffusion and carbide formation along GB. Based on high-resolution TEM (HRTEM) imaging, high density stacking faults (SFs) and nanotwins were observed in TiC_x particles which explain Raman activation of defects in TiC. Formation of SFs and nanotwins in TiC_x is attributed to carbon vacancies, which inversely promotes incoherent twin boundary (ITB) formation. Transformation from TiC_x to Ti₃AlC₂ is considered driven by Al indiffusion along ITBs. Intergrowth of TiC_x in defective Ti₃AlC₂ is the compromise to low Al concentration in Ti6Al4V.     

Structure and properties of Si and N co-doping on DLC film corrosion resistance

Three species of diamond-like carbon (DLC) film were systematically examined in NaCl solution for their anticorrosion properties. Si&N&H-DLC has better electrochemical characteristics and salt spray corrosion testing results than the substrate and two species DLC films in NaCl solution. Due to the successive growth of Si, N, and H-DLC, there is a well-bonded Si–N interface and the formation of Si oxides. The Si&N&H-DLC film exhibits extremely high charge transfer resistance, exceeding 10⁶ Ω/cm². A salt spray test shows that the Si&N&H-DLC film presents a lower rate in NaCl solution in comparison to the substrate and the other two species of DLC films. As a result, the Si&N&H-DLC film significantly improved the corrosion performance of the substrate.     

Corrosion behaviour of Ti-6Al-7Nb and Ti-6Al-4V ELI alloys in the simulated body fluid solution by electrochemical impedance spectroscopy

The corrosion behaviour of Ti-6Al-7Nb and Ti-6Al-4V ELI (extra low interstitial) was investigated as a function of immersion hours in simulated body fluid (SBF) condition, utilizing potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. Polarisation experiments were conducted after 0, 120, 240 and 360 h of immersion in SBF solution. From the polarisation curves, very low current densities were obtained for Ti-6Al-7Nb alloy compared to Ti-6Al-4V ELI, indicating a formation of stable passive layer. Impedance spectra were represented in the form of Bode plots and it was fitted using a non-linear least square (NLLS) fitting procedure, in which it exhibited a two time constant system suggesting the formation of two layers. The surface morphology of the titanium alloys have been characterized by SEM and EDAX measurements.     

Structure,mechanical and corrosion properties of DC reactive magnetron sputtered aluminum nitride (AlN) hard coatings on mild steel substrates

Aluminum nitride (AlN) coatings of about 2 μm thick were deposited on mild steel (MS) by means of direct current (DC) reactive magnetron sputtering. AlN coatings were prepared in an Ar + N₂ gas mixture and their crystal structure, microstructure, and topography were analyzed by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. XRD revealed that the films are polycrystalline in nature and have a hexagonal wurtzite structure with a predominant peak observed along the (002) plane. SEM and AFM images showed the presence of continuously covered pebble like spherical grains on the surface. These coatings showed lower coefficient of friction and excellent wear resistance compared to the bare MS substrate. The potentiodynamic polarization studies showed lower corrosion current density and higher polarization resistance for the AlN/MS structure than the uncoated MS substrate.     

Effect of deposition parameters on properties of TiO2 films deposited by reactive magnetron sputtering

TiO₂ films were grown onto unheated 5083 aluminum alloy substrates by reactive magnetron sputtering from a pure Ti target in Ar-O₂ gas mixture in different power, bias voltage, Ar/O₂ ratio and deposition time at room temperature. The effects of different deposition parameters on the structure and properties of TiO₂ films were investigated systematically by field emission scanning electron microscope (FESEM), atomic force microscope (AFM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), nanoindentation tests, electrochemical tests and antibacterial tests. The results show that power and bias voltage are two main factors to affect the structure and properties of TiO₂ films during the sputtering process. XRD results show that anatase phase is the main phase of the film, and the enhanced content of anatase phase with the increase of sputtering power and bias voltage. Nanoindentation tests exhibit that higher H/E (Hardness/Modulus) ratio can be achieved by depositing TiO₂ film. And the corrosion resistance and antifouling property are all improved after depositing TiO₂ film. 2# sample shows the optimal corrosion resistance, E_corr and I_corr are −0.27388 V and 3.7232 μA/cm², respectively. 1# sample exhibits excellent antibacterial property, the d ensity of bacteria is only 217 cell / mm², which is 484% higher than that of uncoated matrix.