Effects of plasma treatment on bioactivity of TiO2 coatings

In this work, nano-TiO₂ powders were deposited on titanium alloy substrates by atmospheric plasma spraying, followed by plasma immersion ion implantation (PIII) using hydrogen, oxygen and ammonia gases. The bioactivities of PIII-treated TiO₂ coatings were evaluated by the formation of apatite on their surface after soaked in simulated body fluids (SBF) for a period of time. As-sprayed TiO₂ coating is composed of rutile, anatase and TiO_2−x (most of them is Ti₃O₅). After immersion in SBF for two weeks, the hydrogen PIII-treated TiO₂ coating can induce bone-like apatite formation on its surface but apatite cannot be formed on the surface of as-sprayed and oxygen, ammonia PIII-treated TiO₂ coatings. The results obtained indicated that a hydrogenated surface plays a very important role to induce bioactivity of TiO₂ coatings.     

Thermal fatigue characteristics of gray cast iron with non-smooth surface treated by laser alloying of Cr powder

In order to enhance thermal fatigue resistance of gray cast iron with non-smooth surface further, researches on laser alloying of Cr powder with different scanning speed were performed to change both the composition and microstructure of non-smooth unit. The results indicated that there was an optimal scanning speed (2 mm/s) which can contribute maximal Cr element in alloyed zone, and increasing laser scanning speed resulted in smaller non-smooth unit and more pores in it. The microstructure in alloyed layer was composed of pre-eutectic phases, ledeburite eutectic and chromium carbides of Cr₂₃C₆. Thermal fatigue resistance of non-smooth sample is better than that of smooth sample, and among non-smooth samples, sample treated by laser Cr alloying has superior resistance to thermal fatigue compared with laser melting treated sample, in addition, the resistance of laser alloying non-smooth samples increases with scanning speed slowing.     

Tribological and anti-corrosion properties of Ni-W-CeO2 coatings against molten glass

Composite plating was used to prepare Ni-W infused rare earth oxide CeO₂ composite coatings. The high temperature friction behavior and corrosion resistance of the coatings against molten glass were investigated by using a high temperature tribometer. A microhardness tester and an environmental scanning electron microscope equipped energy dispersive spectroscopy were employed to investigate the microhardness and the surface morphology of the composite coatings respectively. The results show that the brittle fracture, high temperature tribological properties and the corrosion resistance of Ni-W infused CeO₂ coatings are superior to those of a standard Ni-W coating. CeO₂ particles decrease the friction coefficient from nearly 0.5 to about 0.25 during the composite coatings sliding against the molten glass at about 973 K, and proper quantities of CeO₂ decrease the variation of the friction coefficient value. Furthermore, CeO₂ can improve the corrosion resistance of the Ni-W coatings at high temperature effectively.     

Relationship between the growth rate and Al incorporation of AlGaN by metalorganic chemical vapor deposition

The growth rate and its relationship with growth conditions of AlGaN alloy films by metalorganic chemical vapor deposition (MOCVD) are investigated. It is found that both parasitic reaction and competitive adsorption play important roles in determining the growth rate and Al incorporation in AlGaN. Low reactor pressure can weaken parasitic reactions, thus increasing the Al composition. In addition, a decrease of absolute amount of Ga atoms arriving on the substrate may lead to a lower Ga competitive power, and then a higher Al content in AlGaN film.     

Characterisation of organic-inorganic hybrid coatings deposited on aluminium substrates

Optically transparent silica-based single amorphous phase organic-inorganic hybrid coating materials with differing organic/inorganic ratios have been prepared by a novel patented sol-gel process and deposited using a simple angled flow coating method on commercially pure aluminium substrates. Transmission electron microscopy and scanning electron microscopy were used to characterise the microstructures of the hybrid materials and the quality of the interfaces between the coatings and the aluminium substrates. The scratch resistance, critical load to failure and failure modes of these hybrid coating materials were studied using a simple scratch test. The coatings produced were all shown to adhere well to the aluminium substrates and exhibit good levels of scratch resistance.     

Characterization and blood compatibility of TiCxN1 − x hard coating prepared by plasma electrolytic carbonitriding

A novel technique to form Ti(C, N) on titanium, named as plasma electrolytic carbonitriding (PEC/N) on cathode was successful used to prepare TiC_xN_1 − x coating. The structure, composition and morphology of the coating were characterized by XRD, XPS and SEM, respectively. The results indicated that TiC_0.3N_0.7 as a new species appears on the surface of the titanium plate, and the thickness of the coating with porous surface morphology increases with the treated time. The blood compatibility of the TiC_0.3N_0.7 coating was evaluated by haemolysis ratios, dynamic blood clotting test, plasma recalcification time and platelet adhesion. The results indicated that the blood compatibility of the plasma-treated titanium with TiC_xN_1 − x coating is significantly improved as compared to the original titanium. Additionally, the results derived from measurements of hardness and corrosion indicated that the coating has excellent mechanical and corrosion-resistant properties.     

Corrosion inhibition study of pure Al and some of its alloys in 1.0 M HCl solution by impedance technique

This paper describes the use of the potentiodynamic polarization and electrochemical impedance spectroscopy technique (EIS) in order to study the corrosion inhibition process of pure Al, (Al + 6%Cu) and (Al + 6%Si) alloys in 1.0 M HCl solution at the open circuit potential (OCP) in the temperature range 10-60 °C. Dodecyl phenol ethoxidate as a non-ionic surfactant (NS) inhibitor has been examined. The Nyquist diagrams consisted of a capacitive semicircle at high frequencies followed by a well defined inductive loop at low frequency values. The impedance measurements were interpreted according to a suitable equivalent circuit. The results obtained showed that the addition of the surfactant inhibits the hydrochloric acid corrosion of the three Al samples. The inhibition occurs through adsorption of the surfactant on the metal surface without modifying the mechanism of corrosion process. Potentiodynamic polarization measurements showed that the surfactant acts predominately as anodic inhibitor. The inhibition efficiency increases with an increase in the surfactant concentration, but decreases with an increase in temperature. Maximum inhibition is observed around its critical micelle concentration (CMC). The inhibition efficiency for the three Al samples decreases in the order: (Al + 6%Si) > (Al + 6%Cu) > Al. Kinetic-model and Frumkin adsorption isotherm fit well the experimental data. Thermodynamic functions for both dissolution and adsorption processes were determined.     

Microstructure and cavitation erosion characteristics of Al-Si alloy coating prepared by electrospark deposition

With a high-power electrospark deposition (ESD) processing unit, an Al-Si coating was prepared on ZL101 aluminum alloy substrate. The Al-Si coating was composed of many thin deposited layers with the thickness of 30-40 μm and showed peculiar microstructural characteristics. The eutectic Si phase, which was homogenously distributed in the coating, existed in the form of latticed morphology with characteristics of being composed of many fine (~ 50 nm size) spherical Si particles. Hardness tests showed that the microhardness of the coating was in the range of 100-110 HV with a small fluctuation, which implied that the deposited coating possess a uniform strength through the whole coating. After 4 hours' cavitation erosion tests, the cumulative volume loss of Al-Si coating (9.03 mm³) was only 31.5% of the substrate (28.66 mm³), which indicated that the Al-Si coating possessed better cavitation erosion resistance than ZL101 substrate. The excellent performance of Al-Si coating was mainly attributed to the peculiar microstructure and the formation of fine spherical Si particles.     

High sintering resistance of a novel thermal barrier coating

Sintering resistance of a novel thermal barrier coating Nd_xZr_1 − xO_y with Z dissolved in, where 0 < x < 0.5, 1.75 < y < 2 and Z is an oxide of a metal selected from Y, Mg, Ca, Hf and mixtures thereof, was studied. The coatings of Nd_xZr_1 − xO_y and typical 7YSZ were deposited by electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS). The samples with the coating system of EB-PVD Nd_xZr_1 − xO_y or 7YSZ overlaid onto a MCrAlY bond coat were cyclically sintered at 1107 °C for 706 hours. The freestanding coatings of EB-PVD Nd_xZr_1 − xO_y and 7YSZ were isothermally sintered at 1371 °C for 500 hours. The microstructure of EB-PVD Nd_xZr_1 − xO_y before and after the sintering was evaluated and compared with EB-PVD 7YSZ. The sintering resistance of freestanding APS Nd_xZr_1 − xO_y coating was also investigated after isothermal sintering at 1200 °C for 50 and 100 hours. The results demonstrated that the new coatings of Nd_xZr_1 − xO_y applied with both EB-PVD and APS have higher sintering resistance than EB-PVD and APS 7YSZ, respectively.     

Hot corrosion and oxidation behavior of a novel Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating

A new type of Pt + Hf-modified γ′-Ni₃Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al₂O₃ scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I (i.e. 900 °C/1652 °F) and Type II (i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na₂SO₄ salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.     

Deposition of Ti-6Al-4V using laser and wire, part I: Microstructural properties of single beads

In this paper, the microstructural characteristics of Ti-6Al-4V single beads and their dependence on process parameters are addressed. Single beads represent the smallest - and therefore fundamental - unit of additive manufactured components. Bead-on-plates and single beads are produced using a 3.5 kW Nd:YAG laser and Ti-6Al-4V wire. Within 71 sets of process parameters laser beam power, welding speed, and wire-feed speed are varied. The microstructures are investigated by light and scanning electron microscopy, and are divided into several zones according to the thermal history underwent. Microstructural differences of the beads are correlated to the process parameters and discussed. The experiments reveal fundamental microstructural and process characteristics which are helpful for the repeatable and predictable manufacturing and understanding of multi-layered components.     

Phase segregation and its effect on the adhesion of Cr-Al-N coatings on K38G alloy prepared by magnetron sputtering method

Cr_1 − xAl_xN (0 < x < 1) coatings were fabricated by a reactive magnetron sputtering method on a K38G alloy. The composition and microstructure of the coatings were investigated. Phase segregation of cubic AlN was considered in Cr_0.65Al_0.35N using X-ray diffraction analyses. This segregation of cubic AlN from CrAlN matrix might be induced by the high micro-stress. The critical failure load determined by scratch tests of the coating with c-AlN segregation was highest among all the coatings studied in the present work, which indicated that the coating has the best adhesion.     

Effect of Al-Si coating layer on the penetration and microstructures of ferritic stainless steel, 409L GTA welds

In this study, commercial 409L ferritic stainless steels and Al-8 wt% Si alloy-coated 409L ferritic stainless steels were subjected to gas tungsten arc welding and the effect of the coated layer on the penetration properties and microstructure were investigated. Full penetration was obtained with a welding current greater than 90 A and a welding speed lower than 0.52 m/min. At the full penetration condition, the bead width of Al-8 wt% Si alloy-coated 409L ferritic stainless steels was narrower than that of commercial 409L ferritic stainless steels. In these samples, a narrow and deep weld pool was considered to have formed under gas tungsten arc welding due to Marangoni convection with existing oxides in the Al-Si coating layer. The grain size of both materials was the largest in the fusion zone and decreased from near the heat-affected zone to the base metal. Especially, the grain size of the heat affected zone and the fusion zone of Al-8 wt% Si alloy-coated 409L ferritic stainless steels were small compared to those of commercial 409L ferritic stainless steels. The hardness was maximized in the fusion zone and decreased from the bond line to the base metal. The maximum hardness in the fusion zone was attributed to the re-precipitation of the TiN and Ti(C, N), which were present in the base metal with the finer grain size during the melting and solidification processes. However, the maximum hardness of Al-8 wt% Si alloy-coated 409L ferritic stainless steels was relatively high due to the formation of oxides such as Al₂O₃ and SiO₂ that originated from the dissolved elements from the coating layer.     

The bonding strength and corrosion resistance of aluminum alloy by anodizing treatment in a phosphoric acid modified boric acid/sulfuric acid bath

The process of phosphoric/boric/sulfuric acids anodizing was studied as a new pre-treatment for adhesive bonding of aluminum alloys. The microstructure and topography of the anodic films were examined using SEM and AFM, and the adhesive strength and corrosion behavior were studied with lap-shear test, wedge test and electrochemical technology. The results showed that by the process of phosphoric/boric/sulfuric acids anodizing a thicker film with high porosity and big pores can be obtained. The porous film was beneficial to improve the durability and lap-shear strength of the bonding joints. The thicker film can also provide better corrosion resistance. Compared with the films by boric/sulfuric acids anodizing and phosphoric acid anodizing, under humid and hot environments the phosphoric/boric/sulfuric acids anodic film showed better corrosion resistance, higher bonding strength and durability, and is a promising pre-treatment for adhesive bonding of aluminum alloys instead of the chromic acid anodizing process.     

Pulsed laser surface treatment of magnesium alloy: Correlation between thermal model and experimental observations

The effect of deposition of Al + Al₂O₃ on MRI 153 M Mg alloy processed using a pulsed Nd:YAG laser is presented in this study. A composite coating with metallurgical joint to the substrate was formed. The microstructure and phase constituents were characterized and correlated with the thermal predictions. The laser scan speed had an effect on the average melt depth and the amount of retained and/or reconstituted alumina in the final coating. The coating consisted of alumina particles and highly refined dendrites formed due to the extremely high cooling rates (of the order of 10⁸ K/s). The microhardness of the coating was higher and several fold improvement of wear resistance compared to the substrate was observed for the coatings. These microstructural features and physical properties were correlated with the effects predicted by a thermal model.     

Structural, optical and electrical properties of N-doped ZnO thin films prepared by thermal oxidation of pulsed filtered cathodic vacuum arc deposited ZnxNy films

In this study, N-doped ZnO thin films were fabricated by oxidation of Zn_xN_y films. The Zn_xN_y thin films were deposited on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) using metallic zinc wire (99.999%) as a cathode target in pure nitrogen plasma. The influence of oxidation temperature, on the electrical, structural and optical properties of N-doped ZnO films was investigated. P-type conduction was achieved for the N-doped ZnO obtained at 450 °C by oxidation of Zn_xN_y, with a resistivity of 16.1 Ω cm, hole concentration of 2.03 × 10¹⁶ cm⁻³ and Hall mobility of 19 cm²/V s. X-ray photoelectron spectroscopy (XPS) analysis confirmed the incorporation of N into the ZnO films. X-ray diffraction (XRD) pattern showed that the films as-deposited and oxidized at 350 °C were amorphous. However, the oxidized films in air atmosphere at 450-550 °C were polycrystalline without preferential orientation. In room temperature photoluminescence (PL) spectra, an ultraviolet (UV) peak was seen for all the samples. In addition, a broad deep level emission was observed.     

The effect of heat treatment temperature on the structure and barrier performance of a zirconia coating electrodeposited by pulse current

Zirconia coatings were prepared by heat-treating the electrodeposited zirconium hydroxide produced by pulse current on 316L stainless steel. The results showed that a coating with amorphous structure obtained after heat treating at 200 °C, reveals no corrosion barrier performance. Heat treating at 400 °C resulted in zirconia coating with nanocrystalline/amorphous structure, which reveals a strong corrosion barrier performance. This coating shows a high value of pore resistance and Warburg behaviour in electrochemical impedance spectroscopy. As the temperature was raised to 600 °C, an entirely nanocrystalline structure was found. However, this coating revealed muddy shape cracks, with inferior corrosion barrier performance.     

Enhanced corrosion resistance of carbon steel in normal sulfuric acid medium by some macrocyclic polyether compounds containing a 1,3,4-thiadiazole moiety: AC impedance and computational studies

We report here the use of macrocyclic polyether compounds containing a 1,3,4-thiadiazole moiety (n-MCTH) in the corrosion inhibition of C38 carbon steel in 0.5 M H₂SO₄ acid medium.

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The aim of this work is devoted to study the inhibition characteristics of these compounds for acid corrosion of C38 steel using electrochemical impedance spectroscopy (EIS). Data obtained from EIS show a frequency distribution and therefore a modeling element with frequency dispersion behaviour, a constant phase element (CPE) has been used. The experimental results obtained revealed that these compounds inhibited the steel corrosion in acid solution and the protection efficiency increased with increasing inhibitors concentration. The difference in their inhibitive action can be explained on the basis of the number of oxygen atoms present in the polyether ring which contribute to the chemisorption strength through the donor acceptor bond between the non bonding electron pair and the vacant orbital of metal surface. Adsorption of n-MCTH was found to follow the Langmuir’s adsorption isotherm. The thermodynamic functions of adsorption process were calculated and the interpretation of the results is given. These results are complemented with quantum chemical study in order to provide an explanation of the differences between the probed inhibitors. Correlation between the inhibition efficiency and the structure of these compounds are presented.     

Effect of Gd doping on magnetic, electric and dielectric properties of MgGdxFe2−xO4 ferrites processed by solid state reaction technique

MgGd_xFe_2−xO₄ (x = 0.0, 0.05, 0.1 and 0.15) ferrites, with improved dc resistivity, initial permeability, saturation magnetization, and extremely low relative loss factor, have been synthesized by solid state reaction technique. The microstructures, electric, dielectric and magnetic properties have been investigated by means of X-ray diffraction, Keithley 2611 system, impedance analyzer and VSM respectively. The addition of Gadolinium in Mg ferrite has been shown to play a crucial role in enhancing the electric, dielectric and magnetic properties. The dc resistivity is increased by two orders of magnitude as compared to Mg ferrite. Saturation magnetization has been increased by two times and remnant magnetization has been increased by more than three times due to the doping of Gd³⁺ ions in Mg ferrite. The relative loss factor was found to have very low values and is of the order of 10⁻⁴-10⁻⁵ in the frequency range 0.1-30 MHz. The variations of electric, dielectric and magnetic properties of the samples have been studied as a function of frequency and Gd³⁺ ions concentration measured at room temperature. High resistivity and improved magnetic properties can be correlated with better compositional stoichiometry and the replacement of Fe³⁺ ions by Gd³⁺ ions. The mechanisms responsible to these results have been discussed in this paper.