Preparation of cobalt nanoparticles by direct current arc plasma evaporation method

In order to prepare cemented carbides with excellent mechanical properties, the cobalt nanoparticles were prepared by direct current (DC) arc plasma evaporation method for the first time and the effect of three adjustable parameters (current, filling pressure and the pressure ratio of hydrogen to argon (P_H2/P_Ar)) was investigated on the average diameter and the productivity through L₁₆(4³) orthogonal experiment in this paper. The crystalline structure, chemical composition, morphology, and particle size distribution of the samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray fluorescence analysis (XRF) and selected area electron diffraction (SAED), respectively. The results indicated that the cobalt nanoparticles achieved a high purity of 99.933 wt.% with a spherical shape, and the cobalt nanoparticles had a cubic crystal structure with a slight shrinkage in the lattice parameters. Both the average diameter and the productivity were increased with the increase of the three parameters, but the average diameter decreased when the value of P_H2/P_Ar varied from 2/3 to 1. Under different technical parameters, the average diameter varied from 28 nm to 70 nm and the productivity varied from 43 g/h to 324 g/h.     

Synthesis and properties of nanostructured dense LaB6 cathodes by arc plasma and reactive spark plasma sintering

Nanostructured polycrystalline LaB₆ ceramics were prepared by the reactive spark plasma sintering method, using boron nanopowders and LaH₂ powders with a particle size of about 30 nm synthesized by hydrogen dc arc plasma. The reaction mechanism of sintering, crystal structure, microstructure, grain orientations and properties of the materials were investigated using differential scanning calorimetry, X-ray diffraction, Neutron powder diffraction, Raman spectroscopy, transmission electron microscopy and electron backscattered diffraction. It is shown that nanostructured dense LaB₆ with a fibrous texture can be fabricated by SPS at a pressure of 80 MPa and temperature of 1300 °C for 5 min. Compared with the coarse polycrystalline LaB₆ prepared by traditional methods, the nanostructured LaB₆ bulk possesses both higher mechanical and higher thermionic emission properties. The Vickers hardness was 22.3 GPa, the flexural strength was 271.2 MPa and the maximum emission current density was 56.81 A cm^?2 at a cathode temperature of 1600 °C.     

On the effect of environment on spontaneous growth of lead whiskers from commercial brasses at room temperature

In this paper we report on a detailed statistics-based study of the effect of various environments – air, argon, vacuum, oil and water – on the room temperature spontaneous nucleation and growth of lead whiskers from hacksaw-cut surfaces of three leaded commercial brasses. The samples were initially held in the various environments for 166 h, before being stored in ambient air for a total of 1126 h. The environment was found to have a strong effect. The highest whisker density, ∼30,000 cm⁻², was observed for the sample initially kept under a mechanical (∼1 Pa) vacuum; the lowest, ∼3000 cm⁻², was observed for the sample initially immersed in water; the densities of the others were in between. The samples held in water only grew whiskers when removed from the water. Once exposed to air, a few whiskers grew at an average rate that, at 0.9 nm s⁻¹, was the highest measured. When the samples were exposed to air, after the initial storage in the different environments, both nucleation and growth of the whiskers were accelerated, before ultimately ceasing to grow roughly 400 h after creating the surfaces from which the whiskers grew. These results are consistent with a scenario in which oxygen and/or moisture diffusion down the Pb/brass interfaces results in a volume expansion that provides the driving force for whisker growth. The results also indicate that the oxygen levels needed for whisker growth can be as low as a few parts per million.     

Synthesis of high purity titanium silicon carbide from elemental powders using arc melting method

The objective of this study is to investigate the formation of Ti₃SiC₂ from Ti/Si/C powders using the arc melting method. The results show that the sample sintered at 80 s produced a near single-phase of Ti₃SiC₂ (99.2 wt.%) with a relative density of 88.9%. These results were confirmed by phase determination using XRD analysis and were supported with micrographs from FESEM/EDX analyses. The relative density and porosity of all samples were dependent on the formation of macropores in bulk samples and micropores in TiC_x grains. The proposed reaction mechanisms for the synthesis of Ti₃SiC₂ by arc melting is that Ti₃SiC₂ might be formed from TiC_x + Si, Ti₅Si₃C_x + C, and Ti₅Si₃C_x + TiC_x at early arcing time (≤ 10 s), while TiC_x + TiSi₂ take place at 15 s to 80 s. After 80 s, decomposition of Ti₃SiC₂ into TiC_x, TiSi₂ and C was observed.     

(Ti,Cr,Nb)CN coatings deposited on nitrided high-speed steel by cathodic arc method

The combined processes of plasma nitriding and cathodic arc deposition of (Ti,Cr,Nb)CN coatings were applied to HSS substrates. The nitrided layers, obtained in a mixture of H₂ (70%) and N₂ (30%) at two different temperatures (480 °C and 510 °C), were examined for the microhardness depth profiles. Characterization of the duplex coatings was performed by investigating elemental and phase composition, texture, hardness, friction and wear. XRD and XPS analyses revealed the formation of a mixture of a carbonitride fcc solid solution, in a dominant proportion, and metallic chromium. The film hardness was measured to be ~ 34 GPa. The duplex (Ti,Cr,Nb)CN coatings exhibited superior tribological behavior as compared to both nitrided layers and non-duplex coatings.     

High photoluminescence and photoswitch of bis(8-hydroxyquinoline) zinc nanoribbons

Highly luminescent bis(8-hydroxyquinoline) zinc nanoribbons with average width of 150 nm and thickness of 50 nm and length up to several hundreds of micrometers have been synthesized via a facile solvothermal method. The room-temperature photoluminescence spectrum of the products showed a strong and stable emission centered at 515 nm under excitation at 386 nm. The nanoribbons were found to be sensitive to light. The photocurrent enhanced by ca. 24 times under irradiation of an incandescence lamp with power density of 2.5 mW/cm². This conclusion was interesting and might found potential application in light-controlled devices in the future.     

Development of micro-plasma transferred arc (μ-PTA) wire deposition process for additive layer manufacturing applications

Micro-plasma transferred arc (μ-PTA) deposition process has potential to meet requirements of the meso-sized fabrication and repair of the high value components. This paper reports on the development of μ-PTA as cost effective and energy efficient alternative process for small sized deposition with an overall objective to repair and/or remanufacture the defective dies and molds. An experimental setup was developed to deposit 300 μm diameter wire of AISI P20 tool steel on the substrate of the same material which is one of the most commonly used materials for making the dies and molds used for various applications. Two stage experiments were conducted to indentify the important process parameters generating regular and smooth single bead geometry. The process was further explored for highest possible deposition rate for fabrication of straight walls through multi-layer deposition. The μ-PTA deposition process was found to be capable of fabricating straight walls having total wall width of 2.45 mm and effective wall width of 2.11 mm. The deposition efficiency was found to be 87% for the maximum deposition rate of 42 g/h. The microscopic examination and micro-hardness measurements revealed that the deposited wall is free from cracks, porosity, and inclusions. This study confirms the capability of μ-PTA for ALM in comparison to the existing high energy deposition processes used for meso-scale fabrication and repair applications of the dies and molds. This work confirms that μ-PTA wire deposition process offers the advantages of the laser based processes at much lower cost and more energy efficiency thus making it potential alternative process for repair and remanufacturing of the defective dies and molds. Use of finer wire can further reduce the deposition size enabling μ-PTA wire deposition process to fabricate the miniaturized parts.     

Production of carbon nanotubes by single-pulse discharge in air

The production of carbon nanotubes (CNTs) has various methods, such as arc discharge, laser ablation, chemical vapor deposition (CVD), template-directed synthesis. These methods generally require, besides catalyst particles, vacuum environment and special ambient gas to prevent carbon from high temperature oxidation. However in this paper, CNTs were successfully produced on selected locations under atmospheric environment and room temperature by micro electrodischarge method. The micro electrodischarge system was composed with transistor circuit to offer discharging time of microseconds and peak current of several ampere. The effect of peak current and discharging time on the production of CNTs was addressed. Experimental results show that the structure and quantity of CNTs is different with different processing parameters. Multi-wall CNTs with the outer diameter of 17 nm and inner diameter of 5 nm were produced using peak current of 2.5 A and discharging time of 1000 μs.     

Effect of different cooling rates on thermomechanically processed high-strength rebar steel

The present article is dealing with 0.2% C, 0.1% V and 0.02% Nb steel. Billets with 130 mm × 130 mm cross-section were austenitized and hold at 1080 °C. The billets were hot rolled to 22 mm bar diameter. Hot rolling was finished at 980–1000 °C. The final bars were air-cooled. On a parallel way, an experimental hot deformation investigation, on the same steel, was carried out at deformation temperature range 1200–800 °C with the same amount of deformation (97% reduction in area). However, cooling regimes after deformation were air cooling, water quenching to 600 °C followed by air cooling, and water quenching to room temperature. Microstructure investigation was done using both optical and scanning electron microscopes. Further evaluation was done using mechanical testing. The industrial trial has unsatisfied results with poorer yield strength with higher ultimate strength. Bainitic aggregates are detected in the hard phases islands. Air cooling after pilot hot deformation creates banded ferrite–pearlite microstructure with 9.11 μm ferrite grains. However, quick water quenching to 600 °C followed by air cooling develops tempered and softened coarse bainite phase. On the other hand, water quenching to room temperature develops fine bainite texture. Water quenching to 600 °C followed by air cooling is the best regime creating accepted mechanical properties.     

Effect of C2H2 gas flow rate on synthesis and characteristics of Ti–Si–C–N coating by cathodic arc plasma evaporation

The influences of C₂H₂ gas flow rate on the synthesis, microstructure, and mechanical properties of the Ti–Si–C–N films were investigated. Quaternary Ti–Si–C–N coatings were deposited on WC-Co substrates using Ti and TiSi (80:20 at.%) alloy target on a dual cathodic arc plasma evaporation system. The Ti–Si–C–N coatings were designed with Ti/TiN/TiSiN as an interlayer to enhance the adhesion strength between the top coating and substrate. The Ti–Si–C–N coatings were deposited under the mixture flow of N₂ and C₂H₂. Composition analysis showed that as the C₂H₂ gas flow increased, the Ti, Si and N contents decreased and the carbon content increased in the coatings. The results showed the maximum nanohardness of approximately 40 GPa with a friction coefficient of 0.7 was obtained at the carbon content of 28 at.% (C₂H₂ = 15 sccm). However, as the C₂H₂ gas flow rate increased from 15 to 40 sccm (carbon content from 25.2 to 56.3 at.%), both the hardness and friction coefficient reduced to 20 GPa and 0.3, respectively. Raman analysis indicated the microstructure of the deposited coating transformed from Ti–Si–C–N film to TiSi-containing diamond-like carbon films structure, which was strongly influenced by the C₂H₂ flow rate and is demarcated at a C₂H₂ flow of 20 sccm. The TiSi-containing diamond-like carbon films reveal low-friction and wear-resistant nature with an average friction coefficient between 0.3 and 0.4, lower than both TiSiN and Ti–Si–C–N films.     

Investigation the effect of B4C addition on properties of TZM alloy prepared by spark plasma sintering

TZM alloy is one of the most important molybdenum (Mo) based alloy which has a nominal composition containing 0.5–0.8 wt.% titanium (Ti), 0.08–0.1 wt.% zirconium (Zr) and 0.016–0.02 wt.% carbon (C). It is a possible candidate for high temperature applications in a variety of industries. However, the rapid oxidation of TZM alloys at high temperature in air is considered to be one of the drawback. In this study, TZM alloys with additions of 0–5 wt.% B₄C were prepared by spark plasma sintering (SPS) at 1420 °C utilizing 40 MPa pressure for 5 min under vacuum. The effects of B₄C addition on oxidation, densification behavior, microstructure, and mechanical properties were investigated. The TZM alloy with 5 wt.% B₄C have exhibited an approximately 66% reduction in mass loss under normal atmospheric conditions in oxidation tests made at 1000 °C for 60 min. And an increase from 1.9 GPa to 7.8 GPa has been determined in hardness of the alloy.     

Spatially resolved Langmuir probe measurements of a magnetically enhanced hollow cathode arc plasma

Hollow cathode arc discharges are efficient plasma sources and are applied in substrate pretreatment or plasma-activated deposition processes. In order to generate large volume homogeneous plasmas to guarantee uniformity of plasma activation and coating properties, in the presented configuration a ring-shaped anode is positioned coaxially around the hollow cathode tube. A magnetic field is applied, which is axial within the cathode tube and spreads out in the deposition chamber. In order to characterize the hollow cathode plasma, spatially resolved Langmuir probe measurements have been carried out. The charge carrier density maximum on the cathode tube axis reaches values up to 10¹³ cm^? 3. With increasing distance from the plasma source, the plasma density decreases and shows a smoother lateral profile. Maxwellian electron energy distribution functions are observed with spatially homogeneous electron temperatures in the range 1–4 eV. Increasing the chamber pressure leads to higher plasma densities and lower electron temperatures. Reduction of the gas flow through the hollow cathode tube results in a strong rise of the plasma density over two orders of magnitude. The magnetic field supports the low gas flow mode and leads to higher plasma densities, too. The results of the Langmuir probe measurements are discussed by means of the active zone model and are further related to optical emission measurements performed in the vicinity of the hollow cathode orifice.     

Hydrogen quantification of magnetron sputtered hydrogenated amorphous carbon (a-C:H) coatings produced at various bias voltages and their tribological behavior under different humidity levels

Hydrogenated amorphous carbon (a-C:H) films have extraordinary tribological properties under dry conditions since the C-atoms at the surface are hydratized and not available for any bonding with the opposing material. Under wet conditions water molecules are weakly absorbed by the a-C:H-coatings so the interaction between the coating surface and the tribological counterpart changes to a dipole-like interaction which is disadvantageous for the tribological performance. According to this, the hydrogen-content plays an important role in the wear and friction behavior of diamond-like carbon (DLC) coatings under different humid conditions.This work focuses on the quantification of the hydrogen content of differently bias a-C:H top layered coating systems and their influence on the tribological behavior under different humidity conditions. By means of a magnetron sputter device DLC-coating systems with an a-C:H-top layer have been deposited at bias voltages between ? 75 and ? 200 V. In order to quantify the hydrogen content of the layers Nuclear Reaction Resonance Analysis (NRRA) was used. In combination with the results of the tribological tests under different humid conditions using a ball-on-disk-tester, correlations between the hydrogen content, the bias voltage and the wear and friction performance were made. A clear relationship between the bias voltage and the hydrogen content has been observed, since the values decrease consistently from 27.2 at.% at ? 75 V to a minimum of 19.9 at.% at ? 200 V bias voltage. Furthermore the different humidity levels show a strong influence on the tribological performance, while the bias voltage effects mainly the wear and friction results of the samples tested under wet conditions.     

Fatigue crack growth of two pipeline steels in a pressurized hydrogen environment

Fatigue crack growth tests were conducted on two pipeline steel alloys, API 5L X52 and X100. Baseline tests were conducted in air, and those results were compared with tests conducted in pressurized hydrogen gas. All tests were run at (load ratio) R = 0.5 and a frequency of 1 Hz, except for one test on X100, run at 0.1 Hz. Tests were conducted at hydrogen pressures of 1.7 MPa, 7 MPa, 21 MPa, and 48 MPa. Fatigue crack growth rates for both X100 and X52 were significantly higher in a pressurized hydrogen environment than in air. This enhanced growth rate appears to correlate to pressure for X100 but may not for X52.     

Structural and mechanical properties of Cr–Al–O–N thin films grown by cathodic arc deposition

Coatings of (Cr_xAl_1?x)_δ(O_1?yN_y)_ξ with 0.33 ? x ? 0.96, 0 ? y ? 1 and 0.63 ? δ/ξ ? 1.30 were deposited using cathodic arc evaporation in N₂/O₂ reactive gas mixtures on 50 V negatively biased WC–10 wt.% Co substrates from different Cr and Al alloys with three different Cr/Al compositional ratios. For N₂ < 63% of the total gas, ternary (Cr,Al)₂O₃ films containing <1 at.% of N forms; as determined by elastic recoil detection analysis. Increasing the N₂ fraction to 75% and above results in formation of quaternary oxynitride films. Phase analyses of the films by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy show the predominance of cubic Cr–Al–N and cubic-(Cr,Al)₂O₃ solid solutions and secondary hexagonal α-(Cr,Al)₂O₃ solid solution. High Cr and Al contents result in films with higher roughness, while high N and O contents result in smoother surfaces. Nanoindentation hardness measurements showed that Al-rich oxide or nitride films have hardness values of 24–28 GPa, whereas the oxynitride films have a hardness of ～30 GPa, regardless of the Cr and Al contents. Metal cutting performance tests showed that the good wear properties are mainly correlated to the oxygen-rich coatings, regardless of the cubic or corundum fractions.     

Thermal behaviour of the consumable electrode in the vacuum arc remelting process

The thermal behaviour of the consumable electrode is studied using both experimental measurements and mathematical modelling. Infrared thermography measurements performed on an industrial furnace highlight qualitatively that the electrode height thermally affected by the electric arc is very small and depends on the melting rate. The vertical temperature gradient recorded at the base of the electrode varies between 350 K/cm and 650 K/cm, while the average measured superheat lies between 150 K and 200 K. A 2D mathematical model of the thermal behaviour of the consumable electrode during melting has been developed. Using the operating parameters as model inputs, it makes it possible to predict the melt rate and the evolution of the shape of the electrode base. The results of the model are successfully compared to melt rate experimental data obtained in a real VAR furnace. The thermal effect of cracks in the electrode is investigated and reveals a significant influence on the variation in melt rate.     

Multi-frequency ECT with AMR sensor

Multi-frequency Eddy current testing (ECT) system with high sensitive AMR sensor was developed. By subtracting the signals of two frequencies, the influence of lift off variance could be reduced. As a testing, a specimen of copper plate with grooves and slits was used to simulate the combustion chamber of liquid rocket and the defects. Three defects, with the width of 0.2 mm, the length of 4 mm and the depth of 0.2, 0.5, and 0.8 mm were made in the bottom of the grooves. Multi-frequency ECT experiments were done and the defects with the depth of 0.8 and 0.5 mm could be successfully detected.     

Quantitative assessment of intergranular damage due to PWR primary water exposure in structural Ni-based alloys

Two nickel-based alloys, alloy 718 and alloy 600, known to have different resistances to IGSCC, were exposed to a simulated PWR primary water environment at 360 °C for 1000 h. The intergranular oxidation damage was analyzed in detail using an original approach involving two characterization methods (Incremental Mechanical Polishing/Microcopy procedure and SIMS imaging) which yielded a tomographic analysis of the damage. Intergranular oxygen/oxide penetrations occurred either as connected or isolated penetrations deep under the external oxide/substrate interface as far as 10 μm for alloy 600 and only 4 μm for alloy 718. Therefore, assessing this damage precisely is essential to interpret IGSCC susceptibility.     

TEM analysis of the microstructure of thermal barrier coatings after isothermal oxidation

Thermal barrier coatings (TBCs) are extensively used to protect metallic components in applications where the operating conditions include an aggressive environment at high temperatures. The most important factor controlling TBC durability is the nucleation, and subsequent thickening, of a thermally grown oxide (TGO) layer which is formed during high-temperature oxidation. For this reason, the aim of this work is to analyse the TGO microstructure evolution during isothermal oxidation to explain the macroscopic oxidation behaviour. To this end, transmission electron microscopy (TEM) was used to evaluate the TGO fine microstructure. ZrO₂(Y₂O₃) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The TBCs were isothermally oxidized in air at 950 and 1050 °C for 24, 48, 72, 144 and 336 h and the principal differences in TGO composition were analysed. α-Al₂O₃ was the main TGO constituent in the TBC treated at 950 °C. On the other hand, Al was rapidly consumed in the TBC oxidized at 1050 °C leading to the formation of NiAl₂O₄ spinels, after 72 h exposure, and NiO, after 336 h. The TGO growth kinetics followed a power law, controlled by Al³⁺ diffusion, in the samples treated at 950 °C. However, two different power laws fitted the TGO growth kinetics in the coatings treated at 1050 °C as the diffusion of Ni²⁺ is relevant after 72 h exposure.     

Creep and microstructure of Nextel™ 720 fiber at elevated temperature in air and in steam

Creep of Nextel? 720 alumina–mullite fiber tows was investigated at 1100 and 1200 °C for tensile stresses of 100–400 MPa in air and in steam. Fiber microstructures were characterized after creep by transmission electron microscopy. At low stresses steam increased creep rates by up to an order of magnitude and reduced creep lifetimes. At high stresses creep rates in steam and air were similar. Cavitation was prevalent in steam but not in air. The creep-rupture data obtained at 1200 °C were analyzed in terms of a Monkman–Grant (MG) relationship. The MG parameters were independent of the test environment. Results reveal that the MG relationship can be used to predict creep rupture for Nextel? 720 fibers and composites reinforced with these fibers at 1200 °C in air and in steam. In steam the mullite in the Nextel? 720 fibers decomposed to porous alumina. Decomposition kinetics were linear and had an activation energy of ～200 kJ mol^?1. Intergranular films were not observed on alumina grain boundaries or alumina–mullite interphase boundaries after creep in steam. Creep mechanisms are discussed.