Multifunctional porous titanium oxide coating with apatite forming ability and photocatalytic activity on a titanium substrate formed by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) was used to make a multifunctional porous titanium oxide (TiO₂) coating on a titanium substrate. The key finding of this study is that a highly crystalline TiO₂ coating can be made by performing the PEO in an ammonium acetate (CH₃COONH₄) solution; the PEO coating was formed by alternating between rapid heating by spark discharges and quenching in the solution. The high crystallinity of the TiO₂ led to the surface having multiple functions, including apatite forming ability and photocatalytic activity. Hydroxyapatite formed on the PEO coating when it was soaked in simulated body fluid. The good apatite forming ability can be attributed to the high density of hydroxyl groups on the anatase and rutile phases in the coating. The degradation of methylene blue under ultraviolet radiation indicated that the coating had high photocatalytic activity.     

HFCVD of diamond and its application as electrode in aluminum electrolysis

The Hall-Heroult process of aluminum production is featured by the release of CO₂ as a result of the sacrificing graphite anode. Finding a proper inert anode active in oxygen evolution has been a research topic for electrochemists to have a green process of aluminum electrolysis. A thin layer of diamond coating on silicon substrate has shown some promises. The challenges for the project are two folds, fabricating stable and active diamond electrode, and controlling proper electrolysis condition so as not to degrade the diamond thin film. This paper reports the recent findings from the authors' lab dealing with both challenges.     

Experimental studies on electrical discharge wire cutting of Ni-rich NiTi shape memory alloy

The experimental investigation explores the effect of electrical discharge wire cutting (EDWC) variable parameters such as spark gap voltage, wire tension, pulse off time, wire feed rate, and pulse on time on the surface roughness, average cutting rate, and metallographic changes of Ni_55.95Ti_44.05 shape memory alloy (SMA). The spark gap voltage, pulse off time, and pulse on time have the significant effect on the surface roughness and average cutting rate, whereas wire tension and wire feed rate have the trifling effect. Ni_55.95Ti_44.05 SMA’s surface after EDWC is characterized by many discharge craters, microcracks, voids, and white layer of resolidified molten material. The elemental composition analysis of white layer using energy-dispersive spectroscopy divulges the deposition of the foreign element from the brass wire as well as the dielectric on the surface after EDWC. The machined surface as well as the wire electrode surface consists of various compounds of Ti, Ni, Zn, and Cu which have been identified by X-ray diffraction peak analysis.     

Spark Stage of Welding Arc Discharge Binding on an Aluminum Surface

The article presents a study of the spark stage of binding of the welding arc discharge on the surface of aluminum covered by oxide film. The experiments have shown the formation of cathode- and anodedirected leaders in a nonuniform field between the pin cathode (tungsten) and the plate (aluminum). It has been found that, if the cathode spot occurs against the background of uniform discharge glow, then the spark channel forms, as a rule, in two stages. At the first stage, a diffuse channel bound to the cathode spot that forms in the gap. At the second stage, a highly conductive contracted spark channel spreads from the cathode side along the diffuse channel; the brightness of that spark channel is comparable to that of the cathode plasma glow. From the color spectrum, it has been found that intensive aluminum emission takes place in the domain of binding spots already at the stage of avalanche-streamer spark formation. The estimated calculation of the heat flux rate in the binding spot of the spark discharge has shown values of 10⁶–10⁸ W/cm², i.e., comparable to laser heating parameters. The spark discharge exerts a significant thermal impact consisting of melting of the surface in the spark binding zone and the development of the recrystallization process of an amorphous film matrix in the zone of thermal effect. Electron diffraction through the thin film layer in the thermal effect zone has shown clear concentric rings corresponding to the polycrystal γ-phase of Al₂O₃. According to transmission electron microscope data, the average size of γ-phase grains in the surface film layer after impact by the spark is d = 8–15 nm, whereas the volume of the produced γ-phase is at least 70%. The stable thermodynamic α-phase in the melting zone has been fixed. By the moment of arc discharge excitation, the entire aluminum surface in the spark-binding zone has been free of oxide film.     

Synthesis of Boride Coating on Steel using High Energy Density Processes: Comparative Study of Evolution of Microstructure

Ultrahard titanium diboride (TiB₂) coatings are deposited on plain carbon steel substrate using two high energy density processes, viz. pulsed electrode surfacing (PES) and laser surface engineering (LSE). These two processes are entirely different in physical nature and hence result in dissimilar microstructures. In the present investigation, a comparative study has been made between the evolved microstructures. Both processes produced a surface layer which is adherent and metallurgically bonded to the substrate. PES produced relatively thinner and less uniform coating than LSE process. The PES coating was, however, homogeneous and very fine grained. The laser-assisted coating was “composite” in nature with TiB₂ particles embedded in Fe matrix. Mechanical characterization of these coatings has been performed using microhardness measurements.     

The effect of biological environment on the surface of titanium and plasma-sprayed layer of hydroxylapatite

The solubility of titanium samples with different surface coatings, i.e., hydroxylapatite (HA) powders, a two-layer coating of ZrO₅+HA on a titanium substrate in solution and of tooth implants after long-term functioning in the human organism, was studied. A minimum difference in solubility of titanium samples with different surface finishes (polished or grit blasted) was established. For the HA powders and coatings, the lowest solubility was observed with a coarse-grained HA–B powder and a coating made of that powder. Clinical tests of tooth implants after long implantation times were performed. A titanium implant (implantation 12 y), a titanium implant with a two-layer coating of ZrO₅+HA–A (implantation time 4 y) and a titanium implant with a two-layer coating of Al₅O₃+3% TiO₂)+ HA–A (implantation time 6 y) were studied. The results show that the titanium surface and HA–A layers were dissolved. Nevertheless, after 6 y implantation, total removal of HA–A coating from that part of implant set into the bone, was not observed.     

Cohesion and adhesion of nanoporous TiO2 coatings on titanium wires for photovoltaic applications

The working electrode of a dye-sensitized photovoltaic fiber is constituted of a porous TiO₂ coated titanium wire. The cohesion and adhesion of such a brittle oxide coating on the ductile metal substrate are identified as crucial factors in maintaining photovoltaic efficiency during textile manufacture and weaving operations. The influence of coating thickness on these factors has been investigated in the present work. The tensile mechanical characterization with in situ microscopic observations shows that two damaging processes are involved. For the smaller thickness, loss of adherence appears to be at the interface and inside the coating bulk. Cracks become visible in a random distribution in size and density and do not cross the entire coating circumference. Large patches of coating are still anchored on the wire. For the larger thickness no cohesive rupture in the coating bulk has been observed. The loss of adherence appears at the interface closed to the cracks and grows rapidly as the strain increases. Numerical investigations based on the finite element method permit to analyze the distribution and the combination of radial interfacial stress and circumferential coating stress and their influence on the observed damage.     

Pt-containing oxide layers on titanium and aluminum

Pt-containing films have been grown by exposing Al and Ti substrates to spark discharges in aqueous electrolytes containing Na₆P₆O₁₈ and H₂PtCl₆. The average platinum content in the bulk of the films on aluminum and titanium is within 0.08 and 0.5 at %, respectively. The platinum is concentrated on the surface and in the near-surface region of the films, in the state Pt⁰ in the films on aluminum, Pt²⁺ on the surface of the films on titanium, and Pt²⁺ and Pt⁰ in the near-surface region of the latter films. The surface layer of the films contains arrays of dispersed platinum-enriched particles ～1 μm in size.     

In-Situ TiB2–TiC–Al2O3 Composite Coating on Aluminum by Laser Surface Modification

To improve the surface hardness of aluminum, in-situ TiB₂–TiC–Al₂O₃ composite coating was deposited on it by pre-placed laser coating process using precursor mixture of (TiO₂ + B₄C) and (TiO₂ + B₄C + Al). Pulsed Nd:YAG laser was used to produce coating track by scanning a laser beam in overlapped condition. Multiple tracks again overlapped to get a wider coating area. Phase constituents and microstructure of the deposited coating were studied by XRD and FESEM analysis. Vickers micro-hardness tester was used to measure micro-hardness of the coating. Results indicate that, in appropriate laser processing condition, coating was obtained with metallurgical bonding to aluminum substrate. XRD and microstructure analysis confirms the formation of TiB₂, TiC, and Al₂O₃ in the coating layer through in-situ reaction of reactant powders. Micro-hardness of the coating was found appreciably higher in comparison to the as-received aluminum substrate, due to presence of hard ceramic particles produced during in-situ reaction and their grain refinement for rapid cooling.     

Selective Laser Post-Treatment on Titanium Cold Spray Coatings

The aim of the present work is to investigate the feasibility and effects of a selective postdeposition laser treatment on titanium coatings. Commercially pure titanium grade 2 powders were deposited by means of a cold spray process on aluminum alloy AA2024-T3 sheets. The surface treatment of the coating was realized using a 220 W diode laser. The influence of heat input and dimensional features of coating layer and substrate was assessed by an experimental campaign conducted following a design of experiments approach. Optical and scanning electron microscopy analysis of the microstructure of the deposited and treated material as well as microhardness measurements showed the formation of a compact layer of titanium oxide on the coating surface and the preservation of the temper state of the aluminum substrate.     

Effect of fibre surface morphology on stability of C/TiBx coatings in Ti–6Al–4V/σ (SM1240) fibre metal matrix composite

Abstract

Sigmafibres (SM1240) produced by a chemical vapour deposition process using a 15 μm tungsten wire corefor SiC deposition have a duplex coating of graphitic carbon and TiBx. Nodules present on the fibre surface are attributed to the deposition of the carbon coating over soot particles present on the substrate. Both the carbon and TiB_x coatings were stable in vacuum or air at temperatures up to 973 K. The nodules werefound to be sites of preferential attack by the titanium alloy matrix. The average number of nodules per fibre decreased more rapidly when the specimens were heated in air than in vacuum. It is suggested that the nodules may reduce the stability temperature of the coatings.

MST/2028     

Deposition and Analysis of Composite Coating on Aluminum Using Ti–B4C Powder Metallurgy Tools in EDM

The present work investigates the method of depositing a ceramic coating on the surface of aluminum by means of electrical discharge coating (EDC) in electrical discharge machining (EDM). The present study makes use of powder metallurgy (P/M) green compacts made of titanium, boron carbide, and aluminum (Ti + B₄C + Al) powder as the EDM tool for surface modification of aluminum workpieces. EDM process was carried out with different tool parameters like composition of the electrode material, compaction pressure of the green compacts, and different settings of the process variables like peak current and pulse duration setting. Responses observed were material deposition rate (MDR), tool wear rate (TWR), and average layer thickness (LT). Experiments were designed and carried out using Taguchi L₁₈ orthogonal array. The most influential parameter for responses MDR, TWR, and LT was found to be peak current (I_p) with a percentage contribution of 60.72%, 59.52%, and 42.09%, respectively. In addition, various other characterization techniques such as optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectrum (EDX) analysis were performed in order to judge different attributes of the deposited coating.     

The microstructure and mechanical properties of fluxless gas tungsten arc welding–brazing joints made between titanium and aluminum alloys

Butt joining of a titanium alloy to an aluminum alloy by gas tungsten arc welding–brazing using an Al-Si eutectic filler wire without flux is investigated. The butt joints have dual characteristics, being a welding on the aluminum side and a brazing on the titanium side. The thickness of the reaction layer varies with position in the titanium alloy interfacial area of the joint, ranging from 2 to 5 μm. At the upper part of interfacial area, the reaction layer includes only the rod-like TiAl₃ phase with 10 at.% dissolved Si. At the bottom of interfacial area, the reaction layer consists of the needle-like τ1 phases (Ti₇Al₅Si₁₂) and the block-like TiAl₃ phase. Hardness of the reaction layer near the welded seam/Ti alloy interface was as much as 400–500 HV. The highest tensile joint strength observed was 158 MPa. Tensile joint failure was by cracks initiating from the reaction layer at the bottom of the joint propagating into the welded seam at the upper part of the joint.     

Effects of duty ratio at high 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 2,000 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. Duty ratio influenced the composition and structure of the coatings. Many residual discharging channels on the coating surface showed that the spark discharge at 2,000 Hz was mainly attributable to the breakdown of the oxide film, which was suitable for the elements both from the electrolyte and from the substrate to join MPO process, and therefore, the coating was mainly composed of Al₂TiO₅. Because of the increase of the congregation and the adsorption of Al from the electrolyte with increasing duty ratio, the redundant Al on the electrode surface led to the formation of γ-Al₂O₃. And Al and Ti in the coating existed in the form of the reverse gradient distribution. Meantime, ceramic coatings improved the corrosion resistance of Ti alloy, and the coating surface morphology and thickness determined the coated samples prepared at D = 20% had the best corrosion resistance among the coated samples.     

Synthesis of nanocrystalline TiO2 thin films by liquid phase deposition technique and its application for photocatalytic degradation studies

A transparent, high purity titanium dioxide thin film composed of densely packed nanometer sized grains has been successfully deposited on a glass substrate at 30°C from an aqueous solution of TiO₂-HF with the addition of boric acid as a scavenger by liquid phase deposition technique. From X-ray diffraction measurement, the deposited film was found to be amorphous and turns crystalline at 500°C. The deposited film showed excellent adherence to the substrate and was characterized by homogeneous flat surface. TiO₂ thin films can be used as a photocatalyst to clean up organohalides, a class of compound in pesticides that pollute the ground water. Photocatalytic degradation experiments show that indanthrene golden orange dye undergoes degradation efficiently in presence of TiO₂ thin films by exposing its aqueous solution to ultraviolet light. The suitable surface structure and porosity increases the photocatalytic activity. It was also observed that hemin doped TiO₂ thin films break up organohalides at a surprisingly high rate under visible light.     

Sputtered Ni-Cr-based resistors terminated on sputtered aluminum and screeen-printed Pt-Ag conductors

The standard technology to fabricate thin film resistors is to deposit a layer of resistive material, usually by evaporation or sputtering, followed by thin layers of nickel and gold. Gold plating is then used to augment the thickness of the gold layer, typically to 1 μm. The three-layer structure is then selectively etched to yield resistors with terminations suitable for wire bonding and soldering. We have used a somewhat unconventional approach in which a film 1–2 μm thick of aluminum is first sputtered onto 99.5% Al₂O₃ substrates, then etched to produce the desired conductor pattern. Isotropic (wet) etching is used to produce sloping edges to contact better the Ni-Cr based resistor material which is sputter deposited and patterned using a lift-off technique.We have also combined the thin and thick film technologies by terminating our thin film resistors on Pt-Ag conductors which are screen printed onto the substrate prior to sputter deposition of the resistors.Aluminum terminations can be wire bonded, or, when soldering is required, coated with sputtered or plated nickel. When a layer of suitable adhesion promoter is sputtered between aluminum and nickel, adhesion of aluminum to nickel is greatly improved.The use of a sputtering target made of an Ni-Cr-based alloy has enabled us to obtain resistors in the 50 Ω/□ range which can be mass produced with near-zero temperature coefficient of resistance (TCR). Such resistors show stability within better than 0.02% after 1000 h at 398 K under load. Also reported are resistance spread and shift, due to annealing, as a function of aspect ratio (ratio between length and width of resistor) for both aluminum and Pt-Ag terminations, as well as TCR as a function of annealing temperature.     

A comparative study on the NO2 gas sensing properties of ZnO thin films, nanowires and nanorods

ZnO thin films were fabricated using the spin coating method, ZnO nanowires by cathodically induced sol-gel deposition by the means of an anodic aluminum oxide (AAO) template, and ZnO nanorods with the hydrothermal technique. For thin film preparation, a clear, homogeneous and stable ZnO solution was prepared by the sol-gel method using zinc acetate (ZnAc) precursor which was then coated on a glass substrate with a spin coater. Vertically aligned ZnO nanowires which were approximately 65 nm in diameter and 10 μm in length were grown in an AAO template by applying a cathodic voltage in aqueous zinc nitrate solution at room temperature. For fabrication of the ZnO nanorods, the sol-gel ZnO solution was coated on glass substrate by spin coating as a seed layer. Then ZnO nanorods were grown in zinc nitrate and hexamthylenetetramine aqueous solution. The ZnO nanorods are approximately 30 nm in diameter and 500 nm in length. The ZnO thin film, ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). The NO₂ gas sensing properties of ZnO thin films, nanowires and nanorods were investigated in a dark chamber at 200 °C in the concentration range of 100 ppb-10 ppm. It was found that the response times of both ZnO thin films and ZnO nanorods were approximately 30 s, and the sensor response was depended on shape and size of ZnO nanostructures and electrode configurations.     

Auger and X-ray photoelectron spectroscopic depth profiling techniques applied to ultra-thin titanium films

Auger electron spectroscopy and X-ray photoelectron spectroscopy in combination with sputter profiling techniques were employed to study ultra-thin (100–500 Å) titanium films. The composition of the films was studied as a function of substrate, deposition temperature and heating. Particular attention was given to the interfacial region.The data indicated that the films were a mixture of titanium and TiO₂. At no point was a pure titanium phase present. At a deposition temperature of 450°C the interface was composed of titanium, TiO₂, silicon and SiO₂ when quartz was the substrate and titanium, TiO₂, aluminum and Al₂O₃ when sapphire was the substrate. The silicon and aluminum resulted from reduction of the substrate. When the deposition temperature was approximately 25°C the reductive interaction was minimized.     

Effect of TiCl4 treatment on the photoelectrochemical properties of LaTiO2N electrodes for water splitting under visible light

A lanthanum titanium oxynitride (LaTiO₂N) electrode was studied as a visible-light driven photoelectrode for water splitting. The electrode was prepared by casting a LaTiO₂N powder on a fluorine-doped tin oxide glass substrate, followed by calcination under dinitrogen. The as-prepared electrode exhibited an anodic photocurrent based on water oxidation under visible-light irradiation (λ > 420 nm) in an electrolyte (Na₂SO₄) solution. This current was increased by post-treatment with titanium(IV) chloride (TiCl₄) solution. Scanning electron microscopy and X-ray photoelectron spectroscopy revealed that the titanium species introduced by the post-treatment were titanium oxide, and that they were embedded within LaTiO₂N particles. Resistance measurements of LaTiO₂N electrodes suggested that the increase in the electrode photocurrent after TiCl₄ treatment was due to the improvement of inter-particle electron transfer in the LaTiO₂N thin film.     

Growth of pseudocubic perovskite-type SrRuO3 thin films on quartz substrate using pulsed laser deposition method

Strontium ruthenium oxide (SrRuO₃) thin films have been grown using pulsed laser deposition technique on silicon, Pt coated silicon and quartz substrates. The effect of substrate temperatures on the structural, microstructure, and electrical properties of the SrRuO₃ films on quartz substrate has been investigated using XRD, SEM, AFM and four-probe method, respectively. The lowest resistivity at room temperature for the SrRuO₃ thin film on quartz substrate has been achieved at substrate temperature of 700 °C. Furthermore, the comparisons of SrRuO₃ thin films deposited on various substrates have been done with respect to structural, microstructural and electrical properties. XRD patterns exhibit that all thin films are a single phase, pseudo-cubic perovskite structure. Study of surface morphology shows that grain size and roughness varies with respect to substrate. It is observed that SrRuO₃ thin films yield larger grain size and root mean square roughness on Pt/Si substrate. Investigation of electrical properties shows that SrRuO₃ thin films can serve the purpose of the bottom electrode in dielectric and ferroelectric devices.