Μm Thick Layer Research Articles

Study of the notch fatigue behaviour under biaxial conditions of maraging steel produced by selective laser melting

The current work aims to characterise the fatigue behaviour of an additively manufactured maraging steel, AISI 18Ni300. This is a class of high-strength steel widely used in biomedical, aircraft, aerospace, offshore, and military industries thanks to its good performance in terms of strength, toughness, ductility, dimensional stability, and weldability. Laser-beam powder bed fusion (additive manufacturing) is used to fabricate this type of steel and endows it with properties that make it an excellent candidate for producing prosthetic parts, thereby facilitating a reduction in manufacturing material consumption, labour, and machining time. Given the wide range of loads biomedical components are often subjected to, the current study focused on the multiaxial behaviour of this type of steel. To this end, Fatemi–Socie (FS) and Smith–Watson–Topper (SWT) critical plane methods combined with the theory of critical distances were applied to predict the fatigue life and cracking orientation of this material, with and without notches, under three biaxial loading scenarios. Cylindrical specimens were used and these were fabricated on the base plate in the vertical orientation using a linear printing system equipped with a Nd:YAG fibre laser. The building strategy involved the deposition of 40 μm thick layers at a scan speed of 800 mm/s. The two critical plane models returned good results at low life cycle fatigue but the FS model obtained better results at high life cycle fatigue. Regarding the crack angles, SWT model produced the best predictions.

Effect of Nitriding Process on Wear Behavior of Dual Phase (Ferrite+Martensite) Ductile Cast Iron

In this study, it was aimed to improve the surface hardness by applying gas nitriding treatment to dual-phase ductile cast iron and to produce ductile cast irons with a good strength-ductility combination. Thus, it has been tried to develop a ductile cast iron with high wear resistance, strength, and ductility. Ductile cast iron specimens with a dual-phase microstructure consisting of ferrite + martensite were produced by oil quenching at 70 °C after intercritical austenitizing at 800 ⁰C in the ferrite + austenite zone. Instead of the tempering process, nitriding treatment was applied to the dual-phase ductile cast iron specimens at 560 °C for 540 minutes. The tempering process required to be applied to the martensitic structure was also performed by the nitriding process and a hard 10 µm thick nitride layer was obtained on the surface. Optical microscope and XRD analysis were performed for microstructural characterization of nitrided dual-phase ductile cast iron specimens. The wear properties were tested with a tribometer. For comparison reasons, microstructural characterization and wear tests were also performed on non-nitrided dual-phase ductile cast iron. In ductile cast iron, microstructures consist of a hard nitride layer (10 µm white layer) on the surface and a ferrite + tempered martensite (43% martensite volume fraction) structure in the center can be produced by oil quenching from intercritical austenitizing and nitriding treatment. Surface hardness of 510 HV in nitrided dual-phase ductile cast iron, 193 HV in dual-phase ductile cast iron, and 353 HV in austempered KGDD were obtained. The lowest weight loss and coefficient of friction were obtained in nitrided dual-phase ductile cast iron. The highest weight loss and friction coefficient values with wear were obtained in dual-phase ductile cast iron. The wear resistance of nitrided dual-phase ductile cast iron is 2 times higher than that of dual-phase ductile cast iron and austempered ductile cast iron.

Demonstration of Al0.3Ga0.7N quasi-vertical Schottky barrier diodes with average breakdown electric field over 2 MV cm−1

The Al0.3Ga0.7N quasi-vertical Schottky barrier diodes on sapphire have been fabricated. The Al0.3Ga0.7N SBDs exhibit an excellent rectification behavior with a turn-on voltage of 1.03 V, a high on/off ratio of ∼109 and a low ideality factor of 1.22. The Al0.3Ga0.7N SBDs also present a high breakdown voltage of 64 V with a 0.3 μm thick drift layer and a record high average breakdown electric field E av of 2.13 MV cm−1, which is the best achieved among vertical GaN, Al x Ga1−x N and AlN SBDs. Furthermore, the devices exhibit excellent thermal stability, showing great potential in high-voltage, high-power and high-temperature applications.

MOF-layer composite polyurethane membrane increasing both selectivity and permeability: Pushing commercial rubbery polymer membranes to be attractive for CO2 separation

We demonstrated a novel MOF-layer composite membrane consisting of three layers, based on a polyurethane/PIM layer and a UiO-66-NH2 layer to separate CO2/N2 mixture. A small amount of PIM (i.e., ≤10 wt% PIM in PU) could completely dissolve in PU. After blending it with PU, it could use its own porosity to increase the CO2 permeability of the PU/PIM polymer blend layer. UIO66-NH2 was prepared into a 10 μm thick MOF layer by spin coating. The densely packed MOF in this layer brings abundant adsorption sites to enrich CO2, thereby bringing high selectivity to the composite film. These unique two layers simultaneously increase permeability and CO2/N2 selectivity, and the PAN-UIO66-NH2-PU/PIM (10 wt%) composite membrane displayed a CO2 permeance of 333 Barrer and a CO2/N2 selectivity of 138, its performance can easily exceed the 2008 CO2/N2 upper bound. Moreover, the entanglement of its molecular chain with PIM slows down the aging, leading a more consistent selectivity performance over a sixty-day-ageing study period. This attractive separation performance of PAN-UIO66-NH2-PU/PIM provides an exciting platform for rubbery polymer membranes to economically separate CO2 and makes commercial PU an attractive option for large-scale industrial CO2 separation.

Fast-sweeping continuous wave quantum cascade laser operating in an external cavity with polygon mirror.

We report a continuous wave room temperature quantum cascade laser operating in an external cavity in the Littrow configuration with a 10-facet polygon mirror rotating at 24,000 RPM. The quantum cascade laser emission is swept across ∼1520 - 1625 cm-1 wavenumber range in less than ∼45 µs with a sweep repetition rate of 4 kHz. The measured maximum output power at the laser gain maximum, 15°C and 0.86 A driving current is ∼90 mW; the estimated average output power across the 45 µs wavenumber sweep is ∼50 mW. Through its sweep, the laser emits on the sequential Fabry-Perot longitudinal modes of the laser chip cavity with the mode separation of ∼0.5 cm-1. The linewidth of the emitting modes is less than ∼0.05 cm-1. Spectral measurements of the infrared absorption features of a 10 µm thick layer of acetophenone and water vapor in the air have demonstrated the capability of obtaining spectral data in less than 45 µs.

C-276 nickel alloy corrosion in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions

Thermal energy storage (TES) is a key component for the practical application of concentrating solar thermal power. Recently, a push for higher temperatures has increased interest in molten salts with melting points above 550 °C as phase change thermal energy storage media. Corrosion of a nickel superalloy (C-276) with a eutectic salt mixture (59.5 %wt Na 2 CO 3 /40.5 %wt NaCl) at 650 °C was analysed under both isothermal conditions and with thermal cycle between 600 and 650 °C. Corrosion rates of 400–500 μm/year under isothermal conditions and 200–300 μm/year under thermal cycling were determined by measuring thickness loss, and oxide growth was primarily responsible for the metal loss. Thermal cycling reduced corrosion thickness, however it remained intact with no cracking or buckling. Post-test analysis of the salt found that molybdenum, manganese and tungsten were most likely to be removed from the alloy. Isothermal conditions did not cause chromium to be removed from the alloy by the salt. However, chromium was present in the salt after thermal cycling, due to its removal from the oxide layer. Whilst the corrosion was above the maximum acceptable corrosion rate for a TES system, this study shows that oxide layers formed on alloy C-276 have protective capabilities in this salt, gaining maximum thickness after about 400 h and preventing the salt from attacking the metal directly. Longer testing with more samples is require to determine if the corrosion of this alloy is halted due to the oxide layer growth on the metal surface. • Alloy C-276's compatibility with Na 2 CO 3 /NaCl molten salt at 650 °C was assessed. • Tests were undertaken under both isothermal and thermal cycling conditions. • Cross sections were analysed using microscopy for corrosion thickness and structure. • Oxide growth was the main corrosion mechanism, forming 40–60 μm thick scale layers. • Analysis of the salt found Mo, Mn and W dissolved into the salt from the alloy.

AlGaN/GaN High Electron Mobility Transistors Grown by MOVPE on 3C-SiC/Si(111) for RF Applications

In this work, an AlGaN/GaN HEMT structure is grown on a 0.8 μm thick 3C-SiC layer on high resistivity Silicon substrate. The RF propagation losses are investigated and compared with the ones of epi-layers grown directly on Silicon and on 6H-SiC substrates. Short gate length transistors are fabricated using e-beam lithography. In spite of ohmic contact resistance of 0.6 Ω.mm, a saturated current density of 0.7 A/mm at a gate bias of +1V and a transconductance peak higher to 250 mS/mm for 75 nm T-shaped gate transistors are reached on structure with thick 3C-SiC template. Moreover, for the first time, transition frequencies fT/fmax of 60/98 GHz are reported on such 3C-SiC template.

Defect Reduction in Epilayers for SiC Trench MOSFETs by Enhanced Epitaxial Growth

The yield of high power 4H-SiC Trench-MOSFET devices, especially for those with large chip area, is largely dependent on the quality of the underlying epitaxial layers and therefore low densities of critical defects are of utmost importance. Different growth conditions for the deposition of epitaxial layers were investigated to reduce the impact of defects on electrical device performance. For this investigation, 12 μm thick n-type epitaxial layers were grown varying growth rates for the buffer and the drift layer in a warm-wall chemical vapor deposition reactor. The defects in the epitaxial layers were characterized utilizing surface microscopy as well as ultraviolet photoluminescence techniques. A quantitative comparison of surface defects and crystallographic defects between the different growth conditions was conducted with these methods. The impact of the growth conditions on the formation of critical defects is discussed in detail. The reduction of critical defects, which resulted in an increase of the predicted die yield, as well as an outlook on future investigations, is discussed.

Near-infrared photodetector performance of Cu2ZnSnS4 in the metal-semiconductor-metal configuration: Theoretical studies

Cu2ZnSnS4 (CZTS), an earth-abundant, nontoxic quaternary semiconductor material, is investigated with the possibility of it as a near-infrared MSM (metal-semiconductor-metal) photodetector. The metal-semiconductor-metal (MSM) device configuration, i.e., Au/CZTS/FTO, is probed numerically using one-dimensional macroscopic simulations to understand the detector response. The impacts of various CZTS absorber parameters such as thickness, doping concentration, and bulk defects are examined to achieve the minimum dark current and the maximum photocurrent for the optimized photodetector response. We observed that the simulated Au/CZTS/FTO device is suitable for the 750 nm wavelength region, showing 0.5 A W−1 responsivity and a very large photo to dark current ratio (~ 1012 or more) for a one μm thick CZTS absorber layer.

Over One Watt Output Power Terahertz Quantum Cascade Lasers by Using High Doping Concentration and Variable Barrier‐Well Height

A GaAs/AlGaAs‐based terahertz quantum cascade laser with a peak power of 1.31 W and average power of 52 mW is demonstrated. To realize a high output power, an effective way is to increase the electron density at the active region by using a high doping level. This work aims to optimize a design enabling to use a heavy doping level, by realigning the resonant tunneling which can be seriously degraded due to a band bending from this heavy local doping. With variable quantum well and barrier height, the design owns an improved parameter tuning freedoms, and a saturation of peak gain can be achieved at a relatively high doping concentration of 1.0 × 1017 cm−3, which is twice higher than the recently reported device. Nonequilibrium Green's function method including full carriers scattering effect is used to carry out the design optimization, mainly including the realignment of resonant tunneling and suppression of leakages via high‐energy confined levels. In the experiment, a 14 μm thick active layer for this design is grown and also large mesa size is developed. To ensure a quasi‐continuous wave (quasi‐CW) operation, the heat dissipation is also managed.

Discrete switching in the ion sliced lithium niobate thin films with thick dielectric layer

Domain growth and shape evolution during local switching under the action of biased conductive tip of scanning probe microscope were studied in Z + cut lithium niobate on insulator (LNOI) with 1 µm-thick SiO2 layer between LN film and grounded electrode. Piezoresponse force microscopy and confocal Raman microscopy were used for domain imaging. The domain shape evolution during switching was qualitatively reconstructed. The crucial role of discrete switching on the kinetics of domain growth was revealed. The stability of the switched domains was confirmed. The obtained knowledge will be used for creation of the periodical domain structures by local switching.

Direct Electroplating on Indium-Tin-Oxide-Coated Textured and Polished Silicon Substrates via Transition Metal Alloyed Interlayers

In this study Fe electroplating is used on polished or pyramidally textured indium-tin-oxide (ITO)-coated silicon substrates as an initial layer for subsequent Ni and Cu plating. Up to 15 μm thick Cu layers are deposited on the Ni/Fe intermediate layer. The adhesion strength of this metal layer stack is qualitatively shown by the scotch tape method and quantified by measuring the pull-off stress of the metal stack on ITO. 1.44 MPa and 1.28 MPa have to be applied to pull off the metal stack from polished and pyramidally textured substrates, respectively. Electrical contact properties are measured by circular transmission line model (CTLM) structures. Very low contact resistivity for electroplated metal on ITO of 6.5 × 10-7 Ω cm−2 and 9.0 × 10−7 Ω cm−2 with transfer lengths of 487 nm and 720 nm are determined for polished and pyramidally textured substrates, respectively.

Laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy with significantly enhanced wear resistance

• Gradient nanostructured layer on TiZrHfTaNb refractory high-entropy alloy was fabricated by laser surface remelting technique. • The average grain size in gradient nanostructured layer is refined from ∼ 200 µm in matrix to only ∼ 8 nm in the top surface. • The microhardness is increased from ∼ 240 HV in matrix to ∼ 650 HV in the top surface layer. • The decomposition-mediated gradient refinement mechanisms were revealed mainly by HRTEM. • The gradient nanostructure shows significantly enhanced wear resistance, reducing the wear rate with an order of magnitude. Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory high-entropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ∼100 µm-thick gradient nanostructured layer is dramatically refined from the original ∼200 µm to only ∼8 nm in the top surface layer. The microhardness is therefore gradually increased from ∼240 HV in matrix to ∼650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase body-centered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich body-centered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZrHf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.

Transformations of the Microstructure and Phase Compositions of Titanium Alloys during Ultrasonic Impact Treatment Part II: Ti-6Al-4V Titanium Alloy

Experimental and theoretical studies enabled the reveal of patterns of the microstructure formation in the surface layer of Ti-6Al-4V titanium alloy subjected to ultrasonic impact treatment. The mixed amorphous and nanocrystalline structure of the 200 nm thick uppermost surface layer of titanium dioxide TiO2 was demonstrated using transmission electron microscopy. The 5 µm thick intermediate layer containing nanocrystalline α grains, and the 50–60 µm thick lower layer containing fragmented α-Ti grains with retained β phase were also observed. The refinement of the β-Ti phase during ultrasonic impact treatment was accompanied by the formation of the orthorhombic (α″) martensitic phase. Molecular dynamics simulation of strains of a vanadium-doped titanium crystallite subjected to ultrasonic impact treatment revealed the formation of striped dislocation substructures as well as the development of reversible β→α phase transformations. Ab initio calculations of the atomic structure of V-doped Ti crystallites containing α, β or α″ phases of titanium were carried out. On the basis of the results of the experimental observations, a molecular dynamics simulation and ab initio calculations a mechanism was proposed, which associated the development of the strain-induced β→α″ phase transformations in Ti-6Al-4V alloy with the presence of oxygen. The role of the electronic subsystem in the development of the strain-induced phase transformations was discussed.

Alternative Route of Fracturing in GaN Films Formed by Nanowires Coalescence on Si Substrate

Self-assembled GaN nanowires (NWs) were used as buffer structures to grow GaN layers on Si(111). The coalescence of GaN NWs into a planar layer was studied by measuring the crystalline quality and mass density. While the mismatch strain was found eliminated by the GaN NWs, a pattern of cracks with the average spacing of 70 μm developed in the coalesced 1.8 μm thick GaN layer. The finite-element simulations indicated that the fracturing occurred during the postgrowth cooling of the sample due to the 55% mismatch of the thermal expansion coefficients of GaN and Si. Moreover, tensile stresses in the coalesced GaN layer originate from a pulling effect from the NWs, which bend during the cooling under the difference in displacement between the GaN layer and Si substrate. These results show a fundamentally distinct fracturing mechanism in planar GaN films on Si(111) intermediated by bending of NWs.

Thermal oxidation of a porous Ti[sbnd]23Nb alloy for wear related biomedical applications: Effect of oxidation duration

In this study, an attempt has been made for tailoring the surface features of an 18 vol% porosity containing Ti23Nb alloy manufactured via powder metallurgy method. The aim was enhancing the wear resistance without sacrificing the mechanical properties. For this purpose, porous Ti23Nb alloys having hardness of 280 HV0.025 were subjected to thermal oxidation (TO) at 600 °C in air for two different holding times. TO duration of 6 h, which introduced a ~ 0.5 μm thick TiO2-rutile type exterior oxide layer (OL), provided surface hardness of 620 HV0.025. Extension of TO duration to 60 h caused covering of the surfaces with a ~ 3.0 μm thick OL and increase of surface hardness to 980 HV0.025. As the result of the increased surface hardness, TO'ed. alloys showed superior sliding wear resistance coupled with a reduction in friction coefficient (COF) against alumina ball in 1.5× simulated body fluid (SBF) when compared to the as-sintered state. Despite having the highest surface hardness, the alloy TO'ed. for 60 h exhibited slightly higher wear loss and COF than the alloy TO'ed. for 6 h. More important than this, the alloy TO'ed. for 60 h collapsed at a compressive stress well below the yield strength of the alloy TO'ed. for 6 h, which exhibited almost similar compressive stress-strain curve with that of the as-sintered alloy. From this perspective 6 h appeared as a promising TO duration for the examined porous alloy to be used in biomedical applications as it preserves the mechanical properties while providing remarkable enhancement in the wear resistance.

Microstructure and mechanical performance of nickel coating on copper sheet via laser shock processing

A novel technique of laser shock processing (LSP) was proposed to consolidate Ni powder into coating at room temperature to protect Cu substrate. This is a combined process of shock compaction of powder, shock micro-welding of particle/particle and particle/substrate. The required pressures of each process were calculated and the effect of laser energy on the aspect of microstructure, surface morphology, and mechanical properties of coatings were investigated. Experimental results showed that a fully dense (relative density 99.81%), smooth and uniform Ni coating without oxidation was fabricated due to the high-speed, high-pressure, and cold processing characteristic of LSP. The coating/substrate interface presented curved convex feature by mechanical locking and a 1.2 μm thick atomic diffusion layer by metallic bonding under adiabatic shear instability. The relative density and mechanical performance of coating was improved with a proper increase of laser energy. The maximum hardness of coating reached 332 HV, presenting approximately three times increment of substrate. The wear rate of coating decreased sharply with increasing laser energy, with the minimum value being 20.7% of substrate. The improvement of mechanical properties was attributed to the coupled effect of void collapse by high-speed plastic or viscoplastic deformation and the shockwave-induced microstructure refinement.

The oxidation mechanisms of the Xe20+ ion-irradiated Cr coatings on Zr alloy coupons: Accelerated diffusion and internal oxidation

Protective coatings of fuel claddings inevitably suffer from irradiation damage under actual nuclear operating condition. Here, effect of ion irradiation on the high-temperature oxidation behavior was studied for the Cr coatings on Zr alloy coupons that had already experienced the irradiation of 5 MeV Xe20+ ions at room temperature with the fluences of 1 × 1013 - 1 × 1015 ions/cm2. Although the irradiation damage concentrated in the ~1 µm thick surface layer, the oxidation resistance of the coatings was drastically degraded after the irradiation. It was observed that the irradiation-induced structural defects migrated and aggregated into cavities in large columns, which greatly accelerated the inward-diffusion of O and the outward-diffusion of Cr and Zr. The oxides rapidly nucleated within the irradiated area in the low-oxygen-partial-pressure conditions. During the 1200 °C steam oxidation, the oxides scales could not remain structural integrity, because the formation of Zr(O) phases along the oxide-metal interfaces resulted into bubbles and cracks. Therefore, the oxidation process was very sensitive to the irradiation-induced structural defects, and the irradiated coatings mainly proceeded the internal oxidation mechanism.

Fabrication and characterisation of Frequency Selective Surface (FSS) for Terahertz Sensing Application

Abstract A frequency selective surface (FSS) structure based on a circular slot with 28 to 31 μm was fabricated using Electron Beam Lithography combined with wet etching technique on a 400 μm thick quartz substrate. The circular slot pattern, designed using the CST software, was transferred during the Electron Beam (EBL) exposure with 100 kV voltage acceleration, 2 nA of e-beam current, and line dosage of 1000 μC2. A 0.5 μm thick aluminium layer was deposited on a quartz substrate using the thermal evaporator technique. The aluminium thickness was confirmed by laser microscope and surface profiler measurement. The investigation shows that 20 seconds is the optimal etching time for producing the desired FSS circular slot structure compared to 40 and 60 seconds. The fabricated samples surface metrology was examined using an optical microscope, Field Emission Scanning Electron Microscope (FESEM) and Atomic Force Microscope (AFM). The material compositions of the samples were confirmed by using the Energy Dispersive X-ray (EDX) measurement. Results show that, on average, there are ±1.5 μm of tolerance produced for fabricated sample size on the circular slot pattern compared to the CST simulation results. FSS performance slightly shifted in the frequency from actual 1.80 THz to 1.79 THz, whereas the transmission magnitude has increased by 0.03 from 0.91 to 0.94.

Temperature dependent fatigue crack growth in electrical steel laminates with μm-thick epoxy layers

The effect of adhesive layer thickness on monotonic roll peel strength and fatigue crack-growth kinetics was investigated for electrical steel laminates with bond line thickness values ranging from 7 to 48 μm. Roll peel strength was increasing with epoxy layer thickness up to 20 μm. In contrast, roll peel strength was leveling at higher thickness values. The fatigue fracture mechanics approach in mode I delivered an equivalent ranking such as roll peel testing. However, energy-based fracture mechanics analysis was much more sensitive than monotonic roll peel testing, also at thicker bond lines. The different failure modes were correlated with the plastic deformation zone size developing at the propagating crack. In case of a constrained plastic zone, cohesive failure mainly within the epoxy layer was ascertained. In contrast, interfacial failure and interestingly also a better fatigue crack growth resistance was deduced for a weakly or non-constrained plastic zone. Regarding the temperature dependency, an enhanced fatigue crack resistance was obtained at 60 °C. At this temperature about 50 °C below the maximum of the loss factor of the epoxy adhesive, the effect of epoxy layer thickness was much smaller than at 23 °C. At testing temperatures slightly below or above the glass transition temperature, the fatigue crack propagation rate was a factor of 10 to 100 higher.