Search

This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

This study reports on the ultralubricity of a high-temperature resilient nanocomposite WS2/a-C tribocoating. The coefficient of friction of this coating remains at around 0.02 independently of a thermal treatment up to ∼500 °C, as confirmed by high-temperature tribotests. Moreover, the coating annealed at 450 °C keeps exhibiting a similar ultralubricity when cooled back down to room temperature and tested there, implying a tribological self-adaptation over a broad temperature range. High-resolution TEM observations of the tribofilms on the wear track unveil that WS2 nanoplatelets form dynamically via atomic rearrangement and extend via unfaulting geometrical defects (bound by partial climb dislocations). The (002) basal planes of the WS2 nanoplatelets, reoriented parallel to the tribo-sliding direction, contribute to a sustainable ultralubricity. The declining triboperformance beyond 500 °C is associated with sulfur loss rather than the transformation of WS2 into inferior WO3 via oxidation as suggested earlier. This self-adaptive WS2/a-C tribocoating holds promise for a constant ultralubrication with excellent thermal performance.

Recent advances in 3D printing technology have enabled unprecedented design freedom across an ever-expanding portfolio of materials. However, direct 3D printing of soft polymeric materials such as polydimethylsiloxane (PDMS) is challenging, especially for structural complexities such as high-aspect ratio (>20) structures, 3D microfluidic channels (∼150 μm diameter), and biomimetic microstructures. This work presents a novel processing method entailing 3D printing of a thin-walled sacrificial metallic mold, soft polymer casting, and acidic etching of the mold. The proposed workflow enables the facile fabrication of various complex, bioinspired PDMS structures (e.g., 3D double helical microfluidic channels embedded inside high-aspect ratio pillars) that are difficult or impossible to fabricate using currently available techniques. The microfluidic channels are further infused with conductive graphene nanoplatelet ink to realize two flexible piezoresistive microelectromechanical (MEMS) sensors (a bioinspired flow/tactile sensor and a dome-like force sensor) with embedded sensing elements. The MEMS force sensor is integrated into a Philips 9000 series electric shaver to demonstrate its application in "smart"consumer products in the future. Aided by current trends in industrialization and miniaturization in metal 3D printing, the proposed workflow shows promise as a low-temperature, scalable, and cleanroom-free technique of fabricating complex, soft polymeric, biomimetic structures, and embedded MEMS sensors.

Surface roughness of laser powder bed fusion (L-PBF) printed overhang regions is a major contributor to deteriorated shape accuracy/surface quality. This study investigates the mechanisms behind the evolution of surface roughness (Ra) in overhang regions. The evolution of surface morphology is the result of a combination of border track contour, powder adhesion, warp deformation, and dross formation, which is strongly related to the overhang angle (θ). When 0° ≤ θ ≤ 15°, the overhang angle does not affect Ra significantly since only a small area of the melt pool boundaries contacts the powder bed resulting in slight powder adhesion. When 15° < θ ≤ 50°, powder adhesion is enhanced by the melt pool sinking and the increased contact area between the melt pool boundary and powder bed. When θ > 50°, large waviness of the overhang contour, adhesion of powder clusters, severe warp deformation and dross formation increase Ra sharply.

In the present work, the effects of laser surfacing aiming at modifying the surface roughness on NiTi sheets prior to the application of ultrasonic welding (USW) were investigated. Three different configurations joining original and laser surfaced specimens were performed: original/original (referred as O/O), original/treated (referred as O/T), and treated/treated (referred as T/T). The influence of surface roughness on the interface formation, diffusion, and mechanical properties was investigated. It is observed that when both bonding surfaces becomes rougher (T/T configuration), the joint strength is the highest, followed by both smooth bonding surfaces (O/O configuration), and the strength of the joint is the lowest when only one of the bonding surfaces was roughened (O/T configuration), which is related to the degree of plastic deformation at the joining interface. The main joining mechanism of NiTi to the Al interlayer was a metallic bonding caused by shear plastic deformation and formation and growth of micro welds at the joining interfaces. Laser surfacing facilitates the metallic bonding, which is directly reflected in the change of the thickness of the Al interlayer after USW. This also helps to produce a mechanical interlocking at the interface, although there is no significant difference in the elemental diffusion. Interfacial failure occurred in all joints tested under different surface contact conditions and exhibited ductile-like fracture characteristics.

Three-dimensional interconnected nanoporous graphene (NPG) microfoams and nanofoams are developed via a new approach of solid-state catalytic growth. Both NPG microfoam and nanofoam exhibit similar nanoporous structures that contain close tubular pores and open non-tubular pores but with different particle sizes. As electrochemical reactors for sulfur cathodes, sulfur is encapsulated inside the tubular pores. It is found that NPG nanoreactors can enhance the electrochemical performances in comparison with NPG microreactors, including improved reversible capacities, cyclic performances and rate performances in particular. The electrochemical impedance spectroscopy analysis reveals that NPG nanoreactors facilitate Li+ transportation and decrease the charge-transfer resistance in comparison with the microreactors, promoting the redox kinetics of multi-step conversions between sulfur and lithium sulfides. This work demonstrates a significant particle size effect of nanoporous graphene on the Li–S electrochemistry and can be useful for designing Li–S batteries as well as other electrochemical energy storage systems.

Chloridazon has been a widely used herbicide during the past decades, especially in sugar-beet cultivation. UV-induced degradation of chloridazon leads to the formation of desphenyl counterparts, i.e. desphenyl-chloridazon and methyl-desphenyl-chloridazon. Even if accumulation of these residues in natural waters is far from alarming, a low-cost effective and environmentally friendly adsorbent, capable of binding chloridazon and its degradation products is desirable to reduce their concentration in water even further below legal limits. Here we show that pillared smectite clay, prepared by cation exchange of sodium with copper complexed, cage-shaped polyhedral oligomeric silsesquioxane (Cu2+@POSS) could be a promising candidate for this purpose. X-ray diffraction and high resolution transmission electron microscopy evidenced a homogeneous layered structure where the interlayer spacing is enlarged by 7.1 ± 0.2 Å (the diameter of Cu2+@POSS) with respect to the pristine clay. Exposure of this pillared smectite clay to chloridazon and its metabolites in water showed that Cu2+@POSS intercalation significantly improved its adsorption capacity. In addition, after several thermal regeneration cycles, Cu2+@POSS_SWy-2 still exhibited excellent adsorption properties. These findings demonstrate that smectite clay pillared with copper complexed polyhedral oligosilsesquioxane is a promising environmentally friendly and relatively low cost material for herbicide waste remediation.

Biocompatible antimicrobial coatings may enhance the function of many orthopedic implants by combating infection. Hydroxyapatite is a choice mineral for such a coating as it is native to bone and silver would be a possible antimicrobial agent as it is also commonly used in biomedical applications. The aim of the research is to develop a silver-containing calcium phosphate (Ag/Ca-P) coating via electrochemical deposition on titanium substrates as this allows for controlled coating buildup on complex shapes and porous surfaces. Two different deposition approaches are explored: one-step Ag/Ca-P(1) deposition coatings, containing silver ions as microsized silver phosphate particles embedded in the Ca-P matrix; and via a two-step method (Ag/Ca-P(2)) where silver is deposited as metallic silver nanoparticle on the Ca-P coating. The Ag/Ca-P(1) coating displays a bacterial reduction of 76.1 +/- 8.3% via Ag-ion leaching. The Ag/Ca-P(2) coating displays a bacterial reduction of 83.7 +/- 4.5% via contact killing. Interestingly, by preincubation in phosphate-buffered saline solution, bacterial reduction improves to 97.6 +/- 2.7 and 99.7 +/- 0.4% for Ag/Ca-P(1) and Ag/Ca-P(2) coatings, respectively, due to leaching of formed AgClx(x-1)- species. The biocompatibility evaluation indicates that the Ag/Ca-P(1) coating is cytotoxic towards osteoblasts while the Ag/Ca-P(2) coating shows excellent compatibility. The electrochemical deposition of highly bactericidal coatings with excellent biocompatibility will enable us to coat future bone implants even with complex or porous structures.

Titanium dioxide film as a stable material plays an important role in photocatalytic degradation of pollutants. One of the ways to improve photocatalytic efficiency is to increase the active sites on the semiconductor surface. Photolithography is a manufacturing technique for controlling precisely micrographics on surface. Here grating-structured, square-structured and hexagon-structured TiO2 films were prepared by photolithography and the effects of various surface structures on photocatalysis were studied. It was demonstrated that the photocatalytic activity of TiO2 film was not always improved as the surface area increased. The micro-patterned surface would also impede the mass transfer in the process of photocatalysis. (C) 2019 Elsevier B.V. All rights reserved.

The effect of the initial structure on the microstructure and texture of the friction stir welded Monel 400 alloy joints was investigated. For this aim, Monel 400 alloy of two initially treated conditions, annealed and rolled, was studied. Orientation imaging microscopy with a step size of 70nm was used to characterise the microstructures of the joints. The mechanical properties were evaluated using Vickers hardness and nanoindentation tests. In the joint of the initial annealed alloy, continuous dynamic recrystallisation governed the grain structure formation. However, in the case of the initial rolled base material, discontinuous dynamic recrystallisation also occurred to cause more random texture, smaller grains, much more high-angle grain boundaries and higher dislocation densities. In the stir zone, the hardness and yielding strength increased respectively from 177 HV to 192 HV and 215.9 to 238.8MPa by changing the base metal from the initial annealed to rolled condition. In addition, the microstructural features of the different zones containing the texture components and grain boundaries were scrutinised thoroughly.

Mg-alloyed zinc coatings suffer from early cracking during deformation due to the combined influences of detrimental constituents (e.g., Zn/MgZn2 binary eutectic), unfavorable crystal orientations and incompatible local plastic deformation. This study addresses the crack resistance and deformation mechanisms of a ZnAlMg coating, in the absence of binary eutectic, by using correlative in-situ scanning electron microscopy tensile tests, electron backscatter diffraction, scanning transmission electron microscopy and micro digital image correlation methods. The plasticity and strengthening mechanisms of the binary eutectic-free ZnAlMg coating are thoroughly unravelled via theoretical and experimental approaches including dislocation slip trace analysis, Taylor factor calculations, quantitative dislocation density determination and slip/strain transfer across the grain boundaries. It has been found that the microstructure of the coating, comprising of primary zinc and ternary eutectic, effectively accommodates the applied strain without cracking till fracture of steel substrate. The absence of binary eutectic and the activation of abundant deformation mechanisms in the coating microstructure, promote local compatible plastic deformation and inhibit early damage. Effective slip transfer is unveiled by activation of pyramidal slip in addition to primary dislocation slip systems. The present findings deliver significant insights and solution for improvement in the formability of Mg-alloyed zinc coatings on steel sheets by tailoring the microstructure.

In this work, metal foam current collectors (CCs, i.e. nickel [Ni] and copper [Cu]) were treated by thermal oxidation to create a lithiophilic oxide surface that exhibited enhanced electrochemical performances of lithium anodes such as the cyclic stability of Coulombic efficiency at different current densities for various capacities compared to pristine CCs. The oxidized CCs facilitated much increased diffusivities of ions for lithium growth than pristine CCs. It was found that an inhomogeneous solid electrolyte interphase (SEI) formed on pristine CCs while a uniform SEI formed on oxidized CCs. Uniform lithium (Li) deposition can be achieved on oxidized CCs owing to the lithiophilic oxide surface containing metal nanoparticles and the ionic compound lithium oxide (Li 2O) matrix that led to a uniform SEI film and many nucleation sites. In addition, the porous and non-porous composite anodes exhibited different electrochemical performances. The porous composite anodes showed initial lower voltage hysteresis but shorter lifetime with carbonate-based electrolyte than non-porous composite anodes. The porous composite anodes showed better rate performances in full-cell measurements while the non-porous composite anodes displayed better stability. The interfacial modification of porous hosts by lithiated oxides and the effects of porous structure on battery performances can be also useful for designing other electrodes (e.g. sodium [Na], potassium [K], zinc [Zn]).

By employing carbon dioxide as one source of reaction gases, carbon-doped Ti–O films were fabricated via reactive magnetron sputtering deposition. The chemical bonding configurations and microstructure of the films were analyzed by Raman spectrum and SEM, respectively. The effect of pH on the photocatalytic activities of the films was determined via evaluation of the decolorization rate of methyl orange under alkali, acid and neutrality conditions using UV light irradiation. Electrochemical impedance spectroscopy and potentiodynamic polarization tests were employed to determine the anti-corrosion properties. Compared with the C-free Ti–O film, the C-doped Ti–O films exhibit superior corrosion resistance. Furthermore, the results of the photodegradation experiment suggest that the C-doped Ti–O films have excellent photocatalytic activities and, for methyl orange, those with higher carbon content exhibit hyper-photodegradative effect under the alkali condition.

The growing demand for flexible, ultrasensitive, squeezable, skin-mountable and wearable sensors tailored to the requirements of personalized health care monitoring has fueled the necessity to explore novel nanomaterial-polymer composite-based sensors. Herein, we report a sensitive, 3D squeezable graphene-polydimethylsiloxane (PDMS) foam-based piezoresistive sensor realized by infusing multi-layered graphene nanoparticles into a sugar scaffolded porous PDMS foam structure. Static and dynamic compressive strain testing of the resulting piezoresistive foams sensors revealed two linear response regions with an average gauge factor of 2.87 ~ 8.77 over a strain range of 0-50 %. Furthermore, the dynamic stimulus-response revealed the ability of the sensors to effectively track dynamic pressure up to a frequency of 70 Hz. In addition, the sensors displayed a high stability over 36000 cycles of cyclic compressive loading and 100 cycles of complete human gait motion. The 3D sensing foams were applied to experimentally demonstrate accurate human gait monitoring through both simulated gait models and real-time gait characterization experiments. The real-time gait experiments conducted demonstrate that the information of the pressure profile obtained at three locations in the shoe sole could not only differentiate between different kinds of human gait including walking and running, but also identify possible fall conditions. This work also demonstrates the capability of the sensors to differentiate between foot anatomies, such as a flat foot (low central arch) and a medium arch foot which is biomechanically more efficient. Furthermore, the sensors were able to sense various basic joint movement responses demonstrating their suitability for personalized healthcare applications.

© 2019 Elsevier B.V. There is an indispensable need for fluid flow rate and direction sensors in various medical, industrial and environmental applications. Besides the critical demands on sensing range of flow parameters (such as rate, velocity, direction and temperature), the properties of different target gases or liquids to be sensed pose challenges to the development of reliable, inexpensive and low powered sensors. This paper presents an overview of the work done on design and development of Microelectromechanical system (MEMS)-based flow sensors in recent years. In spite of using some similar principles, diverse production methods, analysis strategies, and different sensing materials, MEMS flow sensors can be broadly categorized into three main types, namely thermal sensors, piezoresistive sensors and piezoelectric sensors. Additionally, some key challenges and future prospects for the use of the MEMS flow sensors are discussed briefly.

Calcium phosphate (Ca P) coatings were electrochemically deposited on titanium substrates. By increasing the electrodeposition time (from 1 to 30 min), the coating thickness increases but also the surface morphology of the Ca P coatings is greatly affected going from smooth to plate-like, featuring elongated plates, ribbon-like and finally sharp needle structures. Micro-stretch tests reveal that, regardless of the coating morphology and thickness, the electrodeposited Ca P coatings have strong adhesion with the titanium substrates and their failure mode is cohesive failure. The effects of different morphologies on cellular behavior such as adhesion, viability, proliferation, and osteogenic gene expression were studied. The surface morphology of Ca P coatings has a remarkable effect on cell attachment, proliferation, and viability. A smooth surface results in better adhesion of the cells, whereas the presence of sharp needles and ribbons on rough surfaces restricts cell adhesion and consequently cell proliferation and viability. The improved cell adhesion and viability on the smoother surface can be attributed to the higher contact area between the cell and the coating, while the needle-like morphology inflicts damage to the cells by physically disrupting the cell wall. There is no significant difference in the level of osteoblast gene expression when osteosarcoma cells are cultured on coatings with different morphologies. Our study provides crucial insights into the optimum electrodeposition procedures for Ca P coating formation leading to both good cell-material interaction and sufficient mechanical properties. This can be achieved with relatively thin coatings produced by short electrodeposition times.

Barreling compression test is an important tool for characterizing the mechanical responses of deformed material. This test also serves for studying the interfacial friction between the tool and workpiece, based on the only available method of Avitzur’s “limit analysis of disc and strip forging”. This method was found to be satisfactory for low friction conditions; however, it has remained to be examined in depth. In the present work, Avitzur model is thoroughly investigated, and its applicability and limitations are discussed. Along these lines, the kinematics of Avitzur model are presented to be used in describing both the barreling parameter of the model and the profile of the deformed sample. Following an exhaustive examination on the barreling parameters of various deformed samples, one of the methods is identified to be valid for evaluating friction factor up to m ≤ 0.5. Moreover, the results clearly show that the model has its highest accuracy as long as the side-surface folding phenomenon is negligible ( ≤ 0.25%).

The multi-nozzle electrospinning is under extensive investigations because it is an easy way to enhance the productivity and also feasible to produce special structure fibers such as core-shell fibers and to fabricate composite fibers of those polymers that cannot form blend solution in common solvent. Control over the multi-nozzle electrospinning fibers deposition has attracted increasing attentions. The most common method was to use the auxiliary electrode. However, the concentrated effect of the works of control multi-nozzle electrospinning deposit was inconspicuous. To enhance the controlling of multi-nozzle electrospinning deposition, a set-up based oppositely charged electrospinning was designed. In this set-up the air flow was used to transport neutralized nanofibers. This electrospinning method was named oppositely charged and air auxiliary electrospinning (OCAAES). The capacity of OCAAES in deposition area and pattern controlling were investigated. By the OCAAES, concentrated and several patterned nanofibers deposition were fabricated. Results showed that nanofiber deposition area and pattern of multi-nozzle electrospinning could be controlled actively, and nanofiber deposition could be fabricated in a quick thickening rate.

WSxC(H) coatings were deposited on single crystal silicon(100) wafers by magnetron co-sputtering and reactive sputtering at various target-substrate distances. Upon increasing the distance, the stoichiometric S/W ratio increases from 0.51 to 1.89. Also, the porosity of coatings gradually augments and a columnar microstructure tends to form. Preferential sulfur resputtering rather than contaminations primarily accounts for the low S/W ratio. TEM reveals randomly oriented WS2(002) platelets in the WSxC coatings when deposited at a large distance, which is supported by XRD. The composite coatings exhibit a decreasing hardness and elastic modulus with increasing target-substrate distance. The triboperformance is strongly affected by the coating composition, the target-substrate distance and the testing environment. Cross-sectional TEM of formed tribofilms reveals an obvious reorientation of WS2(002) basal planes parallel to the plane of sliding, leading to an ultralow friction.

This paper reports a three-dimensional (3D) stochastic bicontinuous micron-porous graphene foam (3D-MPGF) developed as lightweight binder-free current collectors for sulfur cathodes of lithium-sulfur batteries. 3D-MPGF is synthesized by a facile process that originally combines the synthesis of porous metals by the reduction of metallic salts and chemical vapor deposition (CVD) growth of graphene in a continuous route. 3D-MPGF presents micron-porous structure with both interconnected tubular pores and nontubular pores of sizes from hundreds nanometers to several microns. By adjusting CVD time, the thickness of graphene wall is tunable from few atomic layers to ten layers. Raman results prove a high crystalline of 3D-MPGF. Attributed to the low density and high quality, 3D-MPGF can be used as promising lightweight binder-free current collectors. The 3D-MPGF loaded with S of 2.5 mg cm−2 exhibited an ultrahigh initial capacity of 844 mAh g−1 (of electrode), and maintain at 400 mAh g−1 after 50 cycles at 0.1C (167 mA g−1). With increasing the loading of S, the electrodes present higher areal capacities. When the loading of S is 13 mg cm−2, the areal capacity of 3D-MPGF/S reaches 5.9 mAh cm−2 after 50 cycles at 0.1C. The use of 3D micron-porous graphene foam proves considerably enhanced gravimetric capacity densities (of overall electrode), which can be a direction not only for batteries but also for other energy storage devices.

This research investigated the ablation process of commercial stainless steels with ultrashort pulsed laser. Square hollows were ablated on stainless steel sheets using a picosecond laser with pulse duration of 0.25, 1 and 10 ps, respectively, and fluence ranging from 0.125 J/cm(2) to 5 J/cm(2). For each setting the surface quality and ablation efficiency were determined by optical microscopy and scanning electron microscopy (SEM). Processing windows producing high quality surfaces at high ablation efficiency were found at a fluence around 0.75 J/cm(2) for the two shorter pulse lengths tested. Individual cones and periodic cone-like protrusions were found in the low fluence regime ( 0.875 J/cm(2)), respectively, both of which make the ablated surface rough. Emphasizing on the individual cones, it is found that the cones are caused by inclusions in the base materials, attributed to a higher fluence threshold required for ceramic-like inclusions. This novel theory explaining the creation mechanism is verified by multiple analysis methods like SEM and energy dispersive X-ray spectroscopy and stop-motion SEM imaging.

Self-healing composites based on epoxy resin containing poly(methyl methacrylate) (PMMA) microcapsules filled with healing agents were prepared. The effect of healing agents microcapsules on mechanical properties and self-healing behavior of epoxy composites were investigated in this work. Epoxy and mercaptan as curing agent were microencapsulated in PMMA shell as two-component healing agent through internal phase separation method, and then, epoxy composites containing 2.5, 5, 7.5 and 10 wt% of healing agent microcapsules were prepared. The fracture toughness and healing efficiency of these composites were measured using a tapered double cantilever beam (TDCB) specimen. The results indicated that about 80% healing efficiency was achieved with 10 wt% PMMA microcapsules at room temperature after 24 h. The tensile strength of the epoxy with 2.5 wt% PMMA microcapsules increased initially and then decreased gradually with increasing microcapsules content up to 10 wt% PMMA microcapsules. DSC results also indicated that this system has good potential for spontaneous self-healing performance at room temperature.

In this study mid-frequency pulsed-DC magnetron sputtering was used to deposit W-B-C coatings. The effect of coating composition and the deposition parameters on the structure, mechanical and tribological properties of the coatings was investigated. The broad diffraction peaks in the X-ray diffractogram indicated the amorphous to nanocrystalline nature of the coatings. The hardness of the coatings ranged from 22.6 to 26.2 GPa. The coatings prepared at the elevated temperature of 500 degrees C exhibited greater hardness compared to the coatings prepared at the ambient temperature. The amount of W in the coatings was the main parameter influencing the friction coefficient with a higher W content leading to a higher friction coefficient. The coatings prepared at 500 degrees C showed lower wear in tribological measurements due to their higher hardness compared to the coatings prepared at ambient temperature. The friction coefficient decreased as the testing temperature increased beyond 500 degrees C. XRD analyses after the high temperature tests have shown that this decrease was due to the formation of W-O Magneli phases, which acted as a solid-state lubricant.

Pure copper and Cu-Zn plates were friction stir welded under the same condition to evaluate the effect of stacking fault energy on the microstructural evolution and mechanical properties of the joints. For this aim, microstructure and texture of the joints were systematically characterized by electron backscattered diffraction and transmission electron microscopy. Moreover, to study the mechanical properties of the different microstructural zones of the joints, nanoindentation tests were employed. The results showed that in pure copper, continuous dynamic recrystallization was the only restoration mechanism for the formation of new grains. By adding zinc into copper, namely decreasing stacking fault energy, both continuous and discontinuous dynamic recrystallization mechanisms occurred. To this end, the effect of stacking fault energy on the restoration mechanisms has been summarized by schematic models. Moreover, the effect of the restoration mechanisms on the yield strength and strain hardening behavior of the joints has been scrutinized. Keywords: Friction stir welding, Stacking fault energy, Dynamic recrystallization, Electron backscattered diffraction (EBSD), Transmission electron microscopy (TEM)

Flow sensors found in animals often feature soft and slender structures (e.g. fish neuromasts, insect hairs, mammalian stereociliary bundles, etc) that bend in response to the slightest flow disturbances in their surroundings and heighten the animal’s vigilance with respect to prey and/or predators. However, fabrication of bioinspired flow sensors that mimic the material properties (e.g. low elastic modulus) and geometries (e.g. high-aspect ratio (HAR) structures) of their biological counterparts remains a challenge. In this work, we develop a facile and low-cost method of fabricating HAR cantilever flow sensors inspired by the mechanotransductory flow sensing principles found in nature. The proposed workflow entails high-resolution 3D printing to fabricate the master mould, replica moulding to create HAR polydimethylsiloxane (PDMS) cantilevers (thickness = 0.5–1 mm, width = 3 mm, aspect ratio = 20) with microfluidic channel (150 μm wide × 90 μm deep) imprints, and finally graphene nanoplatelet ink drop-casting into the microfluidic channels to create a piezoresistive strain gauge near the cantilever’s fixed end. The piezoresistive flow sensors were tested in controlled airflow (0–9 m s−1) inside a wind tunnel where they displayed high sensitivities of up to 5.8 kΩ m s−1, low hysteresis (11% of full-scale deflection), and good repeatability. The sensor output showed a second order dependence on airflow velocity and agreed well with analytical and finite element model predictions. Further, the sensor was also excited inside a water tank using an oscillating dipole where it was able to sense oscillatory flow velocities as low as 16–30 μm s−1 at an excitation frequency of 15 Hz. The methods presented in this work can enable facile and rapid prototyping of flexible HAR structures that can find applications as functional biomimetic flow sensors and/or physical models which can be used to explain biological phenomena.

The fabrication of horizontal interior circular channels poses some unique challenges to the laser powder bed fusion (L-PBF) process. The engineering challenge is to be able to print horizontal interior channels using L-PBF without using support structures, while the scientific challenge is to predict the shape deviation in the horizontal channel. This paper studies the geometric fidelity (roundness and shape deviation) of L-PBF printed horizontal interior circular channels (diameters 1−3 mm) by developing experiment-based regression models and a preliminary computational fluid dynamics (CFD) simulation model. The roundness error is found to be affected by the shape/size of the melt pool, thermal stresses, beam offset, and the slicing algorithm. It is recommended that to decrease the roundness error, in addition to choosing a proper beam offset, the width/depth of the melt pool should be minimized by minimizing the volumetric energy density (smaller laser power or higher scanning speed). Shape deviation in overhanging structures is determined by the thermo-mechanical driven molten flow in the melt pool. Hanging structures with irregular profiles (dross) are formed due to the sinking of the melt pool on an unconsolidated powder bed under the effect of gravity, surface tension, and poor thermal conductivity. (Partially) unmelted powder randomly adheres to the edges of the melt pool enlarging the hanging structure and roughening the profile. Small laser power or large scanning speed benefits reducing the roundness error and hang-diameter ratio. 0° or 45° rotational linear scanning strategy can be selected for minimizing the roundness error or the hang-diameter ratio, respectively.

This work reports a facile method of fabricatingultralight weight (density of 0.305 g/cm3) and squeezablemicroporous graphene-PDMS piezoresistive sensors for humangait monitoring applications. The sensor reported in this workdemonstrates piezoresistivity by utilizing the conductive domaindiscontinuity mechanism demonstrated by multilayer graphenenanoflakes populating the inner pore walls of microporousPDMS sponges. Quasi-static compressive strain characterizationexperiments conducted on the sensor revealed a linear responsewith a gauge factor of 8.77 for compressive strains up to 9.5%.Two identical graphene-PDMS sponge sensors embedded into apair of soft shoe-soles were used to demonstrate comprehensivereal-time gait monitoring, which includes pressure profiling ofthe heels of both the legs.

This study reports a “high temperature fast annealing” approach to tailor the microstructure of ZnMg–Zn bilayer coatings through a diffusion-driven phase transformation and to improve the adhesion strength and corrosion resistance, simultaneously. Selection of the appropriate annealing condition, 250 °C for 3 min, promotes the formation of MgZn2 on the topmost surface of the coating and Mg2Zn11 at the interface of ZnMg/Zn. This results to an increase of the adhesion strength from 65 MPa in the as-deposited condition to 82 MPa after annealing as well as a reduction in the corrosion current density from 0.91 to 0.52 μA/cm2, indicating enhanced corrosion resistance. The diffusion of the elements at high temperatures is also modeled to predict the stability region of phases during the annealing treatment. An excellent correlation is obtained between simulation and the experimental results.

The interface between materials and cells plays a critical role in many biomedical applications. Inspired by the hierarchical architecture of collagen, most abundant structure in the extracellular matrix (ECM), a multiscale hierarchical topography is designed to mimic the collagen nano/micro hierarchical topography. It is hypothesized that the ECM topography affects osteogenesis of human mesenchymal stem cells but until now, it cannot be studied without the biochemical and mechanical influences of the ECM. The multiscale hierarchical topography is achieved by innovatively using sequentially aligned topography preparation via a silicone stretch-oxidation-release method and imprinting lithography. The anisotropically hierarchical topography influences stem cell morphology, orientation, and osteogenic differentiation. Intriguingly, the design resembling that of assembled collagen, exhibits the highest degree of osteogenesis. The hierarchical topotaxis effects are further exemplified by the enhanced vinculin expression, cell contractility, and more pronounced nuclear translocation of Yes-associated protein with the collagen-mimicking topography, indicative for enhanced osteogenesis. The developed multiscale hierarchical system provides insights into the importance of specific biological ECM-like topography by decoupling the biochemical influence. Various diseases, cancer, osteoarthritis, and fibrosis display impaired ECM structures, and therefore this system may have a great potential for tissue engineering approaches and developing in vitro disease models.

This paper reports on the factors governing the mechanical properties of resistance spot welded hot dip galvanized DP1000 under tensile-shear and cross-tension loading. In particular the effects of chemical composition on the microstructural evolution and mechanical properties of DP1000 resistance spot welds are studied thoroughly, by comparison of a higher and lower carbon alloying approach. It is shown that DP1000 steel with higher carbon content attains a martensitic microstructure in the weld nugget with smaller prior austenite grains and finer block sizes. The intervariant boundary fraction analysis also reveals that DP1000 steel containing lower carbon content shows stronger variant selection as the fraction of variants belonging to the same Bain group is higher for this steel. Intervariant plane distribution also reveals that the most of intervariant boundaries for both steels terminated at or near {011} slip planes. Mechanical testing of the welds reveals that the steel with higher carbon content shows a better mechanical performance in tensile-shear test, whereas the DP steel with a lower carbon content exhibits higher maximum load of cross-tension test. The key factors controlling the mechanical response of resistance spot welds during two different mechanical tests are explored via nanoindentation, slit-milling method combined with digital image correlation and micro-cantilever bending. It is demonstrated that the strength and/or hardness of the weld nugget is the key parameter governing the tensile-shear strength of the spot welds, while the fracture toughness of the weld is the dominant parameter that determines the cross-tension strength.

This work demonstrates the application of electrospun single and bundled carbon nanofibers (CNFs) as piezoresistive sensing elements in flexible and ultralightweight sensors. Material, electrical, and nanomechanical characterizations were conducted on the CNFs to understand the effect of the critical synthesis parameter—the pyrolyzation temperature on the morphological, structural, and electrical properties. The mechanism of conductive path change under the influence of external stress was hypothesized to explain the piezoresistive behavior observed in the CNF bundles. Quasi-static tensile strain characterization of the CNF bundle-based flexible strain sensor showed a linear response with an average gauge factor of 11.14 (for tensile strains up to 50%). Furthermore, conductive graphitic domain discontinuity model was invoked to explain the piezoresistivity originating in a single isolated electrospun CNF. Finally, a single piezoresistive CNF was utilized as a sensing element in an NEMS flow sensor to demonstrate air flow sensing in the range of 5–35 m/s.

In the present study, ZnMg-Zn bilayer coatings with different Mg concentrations and layer thicknesses are deposited on steel substrates by a thermal evaporation process. Thermodynamic calculations reveal that the work of adhesion at the ZnMg/Zn interface (~1.6 J/m2) is lower than that at the Zn/Steel interface (~3 J/m2). This indicates that the ZnMg/Zn interface is inherently weaker than the interface between Zn and steel substrate. The interfacial adhesion strength quantified by the scratch test shows that the adhesion strength at the ZnMg/Zn interface decreases with increasing the Mg content and reaches 66 MPa at 16.5 wt% Mg. It is found that the presence of interfacial defects largely decreases the adhesion strength compared to a defect-free coating. Meanwhile, it is also concluded that the interfacial adhesion strength at the ZnMg/Zn interface does not depend on the thickness of Zn interlayer. The results of the present investigation show that the interfacial adhesion strength is not the only governing parameter for the adhesion performance of the ZnMg-Zn bilayer coatings in forming process, but the thickness of the layers as well as interfacial defect density also play important roles in the adhesion performance. Keywords: ZnMg coatings, Physical vapor deposition, Adhesion performance, Interfacial adhesion strength, Scratch test

In-depth investigation of the microscale deformation behavior of ZnAlMg coatings is essential to reveal the origin and mechanism of cracking in these coatings. In this work anisotropic microstructural damage and cracking of multiphase Zn1.8Al1.8Mg alloy coatings produced by hot-dip galvanization process on a steel substrate have been studied extensively. Nanoindentation coupled with orientation image microscopy (OIM) is utilized to determine the local micro ductility/strength of the existing phases as well as the orientation dependent micromechanical properties of primary zinc grains. Plastic deformation and damage behavior of the coating are evaluated through in-situ tensile/bending tests, micro-digital image correlation and in-situ OIM analyses. Stress distribution fields and nucleation sites of cracks within the coating microstructure are investigated using extended finite element method. Three quantitative plastic deformation-based criteria are revealed to correlate the coating microstructure to micro-mechanical properties to comprehend the cracking phenomenon. In particular, the binary eutectic is identified as the most detrimental constituent for compatible plastic deformation. Local strain hardening exponent and Schmid factor of primary zinc grains are found to play a significant role in clarifying the cracking behavior. The results of this study are considered as an important step towards designing microstructure controlled ZnAlMg coatings with superior formability. Keywords: ZnAlMg coatings, Microstructural damage, Cracking behavior, In-situ test, Digital image correlation, OIM

This study investigates the self-healing capacity of a WS2/amorphous carbon (a-C) tribocoating. It is found that prenotches up to 45 µm wide in the WS2/a-C coating surface can be completely healed under the stimulus of sliding operations. The in situ tribotest of 100, 500, 2000, and 6000 laps confirms a dynamic filling of tribofilms that patch the voids and prenotched damages. The stabilized coefficient of friction (CoF) remains at an ultralow value down to 0.02, independent of the prenotched damage at the top of coating. The sites of notched damage in fact act as lubricant reservoirs to accumulate the otherwise “wasted” debris, which are restored as a superlubricant by the sliding operation. High resolution transmission electron microscopy reveals that WS2 (002) nanoplatelets in the healed notch are parallel to the top coating surface but conformal to the coating/notch interface. The patchy tribofilm holds excellent promise for the self-repair of damages in the field of tribology.

Applied catalysis / B 272, 118950 (2020). doi:10.1016/j.apcatb.2020.118950, Published by Elsevier, Amsterdam

This work presents the development of a piezoresistive flow sensor-based low-cost point-of-care lung monitoring device. The objective of the study is to find an affordable and accessible solution for pulmonary health diagnosis and monitoring. The proposed biomedical device consists of an ergonomically designed cylindrical mouthpiece, a flexible piezoresistive sensor, and an electronics unit for detection, signal processing, and transmission of the derived signal. The flexible piezoresistive sensor acts as a transducer for the device by exhibiting a change in resistance when subjected to the airflow from the user’s breath. Preliminary tests for lung monitoring such as peak expiratory flow rate test and breathing frequency tests were conducted using the device developed in the current study in order to identify its detection, health monitoring and telemedicine capabilities.

To promote the adhesion strength between diamond-like carbon (DLC) films and nitrile butadiene rubber (NBR) substrates, titanium-doped carbon (Ti-C) films prepared by dual-target magnetron sputtering under varied substrate bias voltages were used as an interlayer on the rough NBR. The surface topography and structure of Ti-C films were investigated by atomic force microscopy (AFM) and Raman spectra. Raman analysis indicates that the increase of substrate bias voltage leads to an increase of the number or the size of sp2 clusters in the Ti-C interlayer. The adhesion strength and tribological properties of DLC films coated on NBR substrate were scrutinized by a scratch tester and a ball-on-disk tribometer, respectively. It was found that DLC film with a Ti-C interlayer at a certain bias voltage exhibited superior wear resistance with a low coefficient of friction (CoF) during the sliding of 6000 laps. No clear damages in the coatings were observed from the wear tracks. Besides, the scratch test also revealed a reliable adhesion when the interlayer was prepared at −150 V, as confirmed by a scratch crack width of ~50 μm as compared to that of the pure DLC film increasing to ~ 120 μm. Therefore, a Ti-C interlayer could significantly enhance the adhesion and wear resistance of DLC thin films deposited on NBR.

A method for a significant improvement of the self-healing efficiency of Cr2AlC ceramic via pre-oxidation is presented. Micro-notches were made with the focused ion beam technique on specimens with a preexisting oxides scale followed by an oxidation healing treatment. An interesting finding is that in the former case the notches are more effectively healed by producing a dense Al2O3 layer than the counterpart specimens without a pre-existing scale. An explanation for this phenomenon is provided based transmission electron microscopy revealing a transition layer between Cr2AlC and Al2O3 with a gradient distribution of chemical composition. (C) 2018 Elsevier B.V. All rights reserved.

Powder bed fusion (PBF) is ideally suited to build complex and near-net-shaped metallic structures such as conformal cooling channel networks in injection molds. However, warpage occurring due to the residual stresses inherent to this process can lead to shape deviation in the internal channels and needs to be minimized. In this research, a novel analytical model based on the Euler-Bernoulli beam bending theory was developed to estimate the residual stress-induced deformation of internal channels printed horizontally using PBF. The model was used to predict the shape deviation for three different shapes of channel cross sections (circular, elliptical, and diamond-shaped), and showed very good agreement with the experimentally determined shapes of nine different internal channels (three cases per cross-sectional shape). Further, the model predictions were used to compensate for the shape deviation in the design stage, resulting in a reduction in root mean square (RMS) deviation of the circular channel by a factor of 2. The proposed approach is thus expected to be a useful tool to generate design-for-AM guidelines for the additive manufacturing of overhangs and internal channels.

In this work we developed a solid-state-growth approach for the synthesis of 3D interconnected bicontinuous nanoporous graphene (NPG) with the aid of nanoporous Ni templates at low temperatures (below 800 degrees C). The as-synthesized nanoporous graphene comprises both interconnected tubular pores and non-tubular pores. The layer thickness of NPG can be controlled from monolayer to multilayer. The porosity and tubular pore sizes of bicontinuous NPG could be adjusted from tens of nanometres to hundreds of nanometers by varying the ligament size of nanoporous Ni templates. Three types of NPG with average tubular pore sizes of 1000 nm, 160 nm, 50 nm were synthesized. NPG can also be made in the forms of powders up to macroscopic foams depending on the shape and size of the Ni templates. The specific surface area can reach 555 m(2) g(-1). By encapsulation of sulfur in the tubular pores, the NPG-sulfur composites exhibited superb electrochemical performances such as enhanced capacities and cycling performances. The capacitive and cycling performances presented great improvements with decreasing the tubular pore size from 1000 to 50 nm and increasing the pore volume.

In this work, large-area graphene-based carbon films were synthesized through a fast and low-temperature method. The processing route is illustrated on a free surface of Ni catalyst film by vacuum thermal processing of amorphous carbon. Key in the novel approach is that the synthesis is done at low temperatures, i.e. below 350 °C, and within a time as short as 1 min. The nucleation and growth of graphene on the free surface of nickel and along the interface between Ni film and SiO2 substrate are investigated by using a thin film Ni-C-Ni sandwiched structure on a SiO2/Si substrate. Raman spectroscopy demonstrates that the graphene-based carbon films consist of graphitic carbon rich of defects. HR-TEM observations reveal that the graphene-based carbon film grown on the top free surface is composed of thin multilayer graphene segments (3–6 atomic layers) and thick multilayer graphene segments (more than atomic 10 layers), covering the entire surface of Ni film over a large area. Growth parameters such as growth time, growth temperature and carbon/Ni ratio are reported in detail for a control of graphene growth kinetics. The results point at several attractive strategies for the facile synthesis of graphene-based carbon films for industrial applications. Keywords: Graphene film, Growth behavior, Low temperature, Nickel, Microscopy

This article addresses the question what the influence is of size and shape of glass both particles and fibers, as well as carbon fibers on the abrasiveness of polytetrafluoroethylene (PTFE)/epoxy composites sliding against steel balls. A size-dependence appears, i.e. large size glass particles (approximate to 80 mu m, L) are more abrasive than medium-size (approximate to 50 mu m, M) and small-size (approximate to 27 mu m, S) glass particles. Longer glass fibers (submilimeter) are more abrasive. Nevertheless, longer glass fibers still enhance the intrinsic mechanical properties of the composites. Also, carbon fibers reinforced composites yield much better mechanical properties than the glass fibers reinforced composites, but much more abrasive wear on the steel ball due to less surface-adhered polymer layers on fibers. Mechanical properties are enhanced by an increasing amount of glass fillers and by adding a small concentration of nanofillers. POLYM. COMPOS., 39:698-707, 2018. (c) 2016 Society of Plastics Engineers

A graded high-vanadium alloy composite coating was synthesized from premixed powders (V, Cr, Ti, Mo, Nb) on ductile iron (DI) substrate via atmospheric plasma arc surface alloying process. The resulted cross-section microstructure is divided into three distinct zones: upper alloyed zone (AZ) rich with spherical primary carbides, middle melted zone (MZ) with fine white iron structure and lower heat affected zone (HAZ). Spherical or bulk-like primary carbides with diameter < 1 μmin the AZ are formed via in-situ reactions between alloy powders and graphite in DI. Microstructural characterizations indicate that the carbides are primarily MC-type (M=V, Ti, Nb) carbides combined with mixed hardphases such as M 2C, M7C3, M23C6, and martensite. Disperse distribution of spherical, submicron-sized metal carbides in an austenite/ledeburite matrix render the graded coating hard-yet-tough. The maximum microhardness of the upper alloyed zone is 950 HV0.2, which is five times that of the substrate. Significant plastic deformation with no cracking in the micro-indentations points to a high toughness. The graded high-vanadium alloy composite coating exhibits superior tribological performance in comparison to Mn13 steel and plasma transferred arc remelted DI.

WS2/a-C coatings with various carbon contents (0-65 at.%) were deposited on silicon wafers by magnetron co-sputtering under different Ar flow rates. Increasing the argon flow rate increases the chemical stoichiometric S/W ratio, but the coating gradually becomes porous and columnar-like. The S/W stoichiometry is less influenced by the variation in carbon concentration. TEM and XRD confirm well crystallized basal (002) planes in the sputter deposited pure WS2 or low-carbon WS2/a-C coatings. The hardness of the composite coatings increases with increasing carbon content or decreasing Ar flow rate. The coating at 40 at.% C exhibits the highest hardness (10.6 GPa). Tribotests show that, together with a ultralow wear rate of 10(-7) mm(3) m(-1) N-1, the coefficient of friction can be as low as 0.02 in dry air (5% R-H) and around 0.15 in moisture (55% R-H) and remains stable within a sliding distance of 1000 rn. The wear resistance is strongly affected by the carbon concentration, deposition pressure and the testing atmospheres.

The statistical properties of rough surfaces at the nanoscale are studied using classical molecular dynamics. Atomistic fcc blocks of gold are generated with different crystal orientations, and with rough surfaces having the same root-mean-square roughness with variable power spectral density (PSD) slopes and roll-off wavenumbers. The process of rough surface generation may result in thermodynamically unstable sharp spatial features. In order to relax the structure, the blocks are equilibrated at 300 K . It is found that all surfaces experience changes in their roughness, regardless of their crystal orientation or temperature, although the changes are only temperature-independent when T ≤ 0.4 T m . Based on the analyzed PSDs of the equilibrated substrates, two methods for generating pseudo-stable rough surfaces are introduced and discussed.

A novel high-vanadium based hard composite coating was synthesized from premixed powders (V, Cr, Mo, Ti, Nb) on ductile iron (DI) substrate via atmospheric plasma beam surface alloying process. The graded coating can be divided into three distinct zones: upper alloyed zone (AZ) rich with spherical primary and eutectic submicron carbides, middle melted zone (MZ) with fine white iron structure embedded with high-carbon martensite and lower heat affected zone (HAZ) where martensite/ledeburite double shells were substantially formed. Spherical or bulk-like primary carbides with diameter

This paper reports on the microstructural evolution of resistance spot welded 1000 MPa dual phase steel under two different welding conditions, and their relation to the mechanical performance and failure mechanisms. It is shown that a double pulse weld scheme leads to an enhancement in cross-tension strength compared to single pulse welding. The second pulse subdivides the initial fusion zone of the first pulse into two zones. The inner part is solidified with a columnar structure after the second pulse, whereas with the second pulse the outer layer becomes recrystallized (named as Rex-zone) leading to the formation of an equiaxed structure of prior austenite grains. Characteristics of martensite formed in the Rex-zone and coarse-grained heat affected zone, where the crack initiated and propagated, were investigated using orientation imaging microscopy. It was found that a change in welding scheme from single to double pulse effectively alters the characteristics of martensitic microstructure of weld zones. The results obtained demonstrate that the Rex-zone has a lower fraction of high-angle grain boundaries and coarser structure of Bain groups as opposed to the coarse-grained heat affected zone with large fraction of high-angle grain boundaries and finer Bain groups. Besides, double pulse welding creates softer sub-critical heat affected zone which reduces stress concentration at the nugget edge during cross-tension test. The better mechanical performance of double pulse weld is attributed to the significant softening at sub-critical heat affected zone, formation of thick Rex-zone with lower residual strain and high fraction of high-angle grain boundaries and finer Bain groups in the coarse grained heat affected zone. Keywords: Dual phase steel, Resistance spot welding, Cross-tension test, Orientation imaging microscopy, Martensite

The importance of the often omitted running-in phase of tribological testing was studied on hard nc-TiC/a-C:H coating system selected as a representative of protective tribological coatings. The tribological measurements were periodically interrupted and the counterpart ball was replaced by a new one after every interrupted test to simulate machining applications, where the coatings are repeatedly in contact with a new material. The changes in the coefficient of friction and the wear rate of the counterpart ball were found to be dependent mainly on the roughness of the coating at any stage. It was found out that after substantial smoothening of the wear track of the measured coating, the coefficient of friction stabilized at the steady-state value attained in an uninterrupted long tribotest. This steady state value was reached at the very beginning of the test and initial instabilities in the coefficient of friction were no longer observed. It was also concluded that only after the nc-TiC/a-C:H coating gets smoothened enough within the wear track, the steady-state coefficient of friction can describe even applications where the counterpart is being periodically replaced by a new one.

Based on both linear and nonlinear estimations of work hardening rate versus strain curves, two mathematical models have been developed to predict the stress-strain curves under hot working conditions up to the peak stress. The models were validated for a mechanically alloyed Al6063/0.75Al2O3/0.75Y2O3 nanocomposite compressed under different hot forming conditions. The predicted results from both models are found to be in accord with the experimental flow stress curves up to the peak. For the present system, the linear model is found to more accurately predict the flow stress (with an average error of 0.81%) relative to the nonlinear model (with an average error of 2.06%).

The effect of quench polish quench (QPQ) nitriding temperature on the microstructure and wear resistance of SAF2906 duplex stainless steel was investigated. Results showed the surface of the nitrided samples was composed of an oxidized layer, a loose compound layer, a compact compound layer, and a diffusion layer. The oxidized layer was composed of Fe3O4. The main phases of the loose compound layer were CrN, &alpha, N, Fe2&ndash, 3N, and Fe3O4. The compact compound layer was composed of CrN, &alpha, N, and Fe2&ndash, 3N. In the diffusion layer, CrN and expanded austenite (S) were the main phases. The nitrided layer thickness increased from 20 to 41 &mu, m with an increasing temperature of 570 to 610 &deg, C. When the nitriding temperature was above 590 &deg, C, the precipitates in the diffusion layer became coarsened, and their morphologies gradually changed from spherical particulate to rod-like and flocculent-like. Tribotests showed the cumulative mass loss of QPQ-treated samples was much lower than that of the substrate. The cumulative mass loss of the samples nitrided at 610 &deg, C was higher than that at 570 &deg, C during the first 29 h. When the test time was over 29 h, the former was lower than the latter.