Search

A non-equiatomic interstitial-strengthened high entropy alloy (iHEA), Fe49.5Mn30Co10Cr10C0.5 (at.%), is manufactured by laser powder-bed fusion (LPBF) with stripe and chessboard scanning strategies. The present study highlights the correlation between the laser scanning strategies with resulting microstructure, textures, and anisotropic mechanical properties in as-built iHEA. The results show that the LPBF processed iHEA exhibits an excellent strength-ductility synergy due to the combined deformation mechanisms of dislocation slip, martensite phase transformation- and nano twinning-induced plasticity. The samples printed by the stripe scanning strategy show more evident mechanical anisotropy than that of the chessboard-scanned samples. The difference in the degree of mechanical anisotropy is mainly attributed to the heterogeneous grain morphology and crystallographic texture resulted from different scanning strategies.

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.

Wire and arc additive manufacturing (WAAM) has received increasing attention due to its low cost and high productivity compared to traditional techniques for manufacturing high strength steels. In the present work, WAAM was used to fabricate bimetallic laminated steel structure (BLSS) with enhanced strength-ductility synergy by alternately depositing 304 stainless steel (304 SS) and low carbon steel (LCS) on Q345 substrate. The microstructure and mechanical properties of additive manufactured BLSS were investigated in detail. The 304 SS layer is composed of martensite, different from the vermicular ferrite distributed in the austenite matrix of the single-304 SS structure. While the LCS layer has no obvious change compared to the single-LCS structure, except for the refinement of the blocky ferrite. Meanwhile, fine grains formed at the interface between the LCS and 304 SS layers. The differences in element content, phase constituent and grain size between different layers lead to changes in the mechanical properties of this BLSS along different loading directions. The tensile strength and elongation in the longitudinal (Y) direction reached 999.8 MPa and 28.2%, respectively, where the strength was almost twice that of single-material 304 SS and LCS structure with the elongation between them. Digital image correlation (DIC) was used to observe the strain distribution of the BLSS during tensile process, and a significant stress concentration was observed in the transversal (Z) direction, resulting in a decrease in its tensile properties compared to the Y direction. Fracture topography analysis of specimens with different loading directions revealed a brittle-ductile mixed fracture mode. Our work provides a feasible and flexible method for the fabrication of BLSS with superior mechanical properties, which have potential applications in fields such as architectural and civil engineering structures.

Ultrasonic spot welding (USW) has attracted increasing attention due to its high-throughput solid-state bonding mechanism, which shows great potential in the semiconductor and automotive industries for joining of metal sheets. However, the short welding cycle makes it challenging to effectively monitor the temperature history and deformation of the workpieces during the process. In this study, a three-dimensional (3D) finite element analysis model for USW of superelastic NiTi shape memory alloy (SMA) with Cu interlayer was developed using ansysworkbench. The thermal-stress coupled phenomena including the heat generation and stress distribution during the welding process were simulated and analyzed. First, a superelastic constitutive model for NiTi SMAs was constructed. The distribution of temperature and stress fields was then obtained by thermal-stress analysis using the direct coupling method, and the superelasticity of SMAs was observed. The simulation results showed that the highest temperature occurred in the center of the welding area during USW, which is proportional to the welding time and inversely proportional to the clamping pressure. In addition, the maximum stress occurred at the center of the contact surface between upper NiTi and Cu interlayer. After that, the validity of the simulation results was verified by setting up a thermocouple temperature measurement platform to collect the temperature data, which exhibited a good agreement with the simulated results. The simulation procedure demonstrates its potential to predict temperature and stress distributions during the USW process.

To improve the mechanical and tribological properties of diamond-like carbon (DLC) film on nitrile butadiene rubber, the DLC/Ti-C bilayer films are fabricated by a dual-target magnetron sputtering method. The morphological characterization results indicate that the roughness of DLC/Ti-C first decreases and then increases slightly with the increase of Ti content and surface roughness of all modified samples is lower than that of nitrile butadiene rubber (NBR). Raman spectra show that the change of Ti content in the Ti-C interlayer has no effect on the ratio of sp2 to sp3 bonds of surface DLC film. The nanoindentation results show that the Ti content significantly affects the hardness, Young’s modulus, hrex/hmax and elastic recovery (%). Notably, the DLC/Ti-C films with a Ti content of 12.32 ∼ 14.77 at. % show the lowest coefficient of friction (CoF, 0.1 ∼ 0.2) and superior wear resistance due to the increased transformation of sp2 into sp3 in the films. This study unravels that the DLC films' superior mechanical and tribologic performance could be realized by regulating the Ti content in the Ti-C interlayer.

NiTi thin sheets were ultrasonic spot welded with a Cu interlayer, where different welding vibration amplitudes were applied to study the influence on the surface and interface microstructural characteristics, phase transformation behavior and mechanical response of the joints, which aimed to enhance the joint performance by proper optimization of the process parameters. An excellent bonding interface was achieved when an optimized vibration amplitude was applied, with a recrystallized microstructure formed in the Cu foil side near the bonding interface, which helped to improve the mechanical performance of the joints. Joints made with vibration amplitude of 55 mu m had an improved strength compared to the NiTi base material.

The present study is dedicated to explore the effect of grain refinement on cracking resistance of hot-dip galvanized Zn–Al–Mg coatings on steel substrate. In this work, we demonstrate the enhancement of plastic deformation and cracking resistance by refining the microstructure (primary zinc grains) of the Zn–Al–Mg coatings. For this purpose, two types of Zn–Al–Mg coatings namely, fine grained and coarse grained microstructures are investigated utilizing in-situ scanning electron microscopy tensile tests. Electron backscatter diffraction technique is used to illuminate the deformation behavior at the scale of grains (and/or within grains). The results reveal that the coating with fine grained microstructure possesses higher ductility and cracking resistance, whereas the coating with coarse grain microstructure induces more transgranular cracking during deformation. Moreover, primary zinc grain refinement has been shown to decrease the fraction of coarse deformation twins that serve as undesirable sites of micro-cracking. In particular, both deformation mechanisms and cracking behavior are found to be grain size-dependent in these coatings.

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.

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.

Polarization curves and mass losses of SAF3207 hyper-duplex stainless steel under various conditions were measured. The damaged surfaces after erosion-corrosion tests were characterized by scanning electron microscopy. The results showed that an increase in flow velocity could enhance the electrochemical corrosion and consequently decrease the passivation properties of the steel. The erosion-corrosion damage of the samples increased substantially when the flow velocity exceeded the critical value of 4 m.s(-1). The mass loss rate increased as the sand content increased, reaching a maximum at 7wt% sand content, corresponding to the most severe electrochemical corrosion damage. When the sand content was increased further, however, the mass loss rate decreased and then tended stable. The mass loss was divided into incubation, sustained, and stationary periods, with a maximum mass loss rate of 12.97 g.h(-1).m(-2) after an erosion period of 2.5 h. The erosion-corrosion mechanism was investigated in detail.

Zn-Al-Mg coatings are important materials for the corrosion protection of steel sheets. However, susceptibility towards cracking limits the formability performance of these coatings. In this study, we focus on the effect of the underlying steel substrate on cracking behavior in these coatings. In order to elucidate this, a high-strength low-alloy (HSLA) steel substrate and an interstitial-free (IF) steel substrate are coated with two different ZnAlMg coatings with and without binary eutectic microstructures. Meticulous in-situ tensile and bending tests are conducted in a scanning electron microscope. To quantify the strain distribution and damage incidents, micro and macro digital image correlation techniques are utilized in order to illuminate the associated cracking causes across length scales. Furthermore, electron backscatter diffraction method is applied to study the role of crystallographic orientation on the cracking tendency. Crack opening and crack area fractions are correlated with the applied strain and bending angles. The findings denote that the discontinuous yielding (Luders banding) of the HSLA steel substrate generates substantial surface roughening and heterogeneous deformation in the coatings that facilitates cracking. In contrast, the IF steel induces a more uniform deformation within the coatings leading to much reduced crack size and crack area fraction. This study has resulted in a key element of a guideline towards crack-resistant and formable Mg-alloyed zinc coatings.

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.

Self-healing tribocoatings are being developed for aerospace applications to improve the lifetime andreduce the surface maintenance of components in motion. Here the tribo-induced self-healing behaviourof a WS 2 /a-C tribocoating has been evaluated for the first time by in-situ scanning electron microscopy(SEM) to evaluate the mechanisms of damage and self-recovery. In-situ SEM imaging reveals that scratchdamage results in coating brittle fracture and spalling, and that Hertzian pressure affects healing rateat early stages of sliding. WS 2 nanocrystallites, formed via atomic rearrangement at flexural interfaces,enable the healing of irregular damages and congruently offer superlubrication in vacuum. Such damagecontrol in tribo-service may make flawless coatings an unnecessary prerequisite in tribo-applications.

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%).

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.

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)

Recent wave tank experiments on a flap-type wave energy converter showed the occurrence of extreme wave loads, corresponding to slamming events in highly energetic seas. In this paper, we analyse pressure-impulse values from available pressure measurements, for a series of experimental slamming tests. Then, we devise a pressure-impulse model of the slamming of a flapping plate, including the effects caused by air entrapment near the plate. Using a double conformal-mapping technique, we map the original domain into a semi-infinite channel, by means of Gauss' hypergeometric functions. This allows us to express the pressure impulse as a superimposition of orthogonal eigenfunctions in the transformed space. The mathematical model is validated against the experimental data. Parametric analysis shows that the system is much more sensitive to the impact angle than to the initial wetted portion of the flap. Furthermore, the presence of an aerated region determines the pressure-impulse values to increase significantly at all points on the flap surface. (C) 2018 Elsevier Ltd. All rights reserved.

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.

Cracking limits the formability of Mg-alloyed zinc coatings on steel substrates. Unfavorable crystal orientations and brittle microstructural components serve as the main sources of cracks in these coatings. In this study, we overcome the deformation-induced cracking and substantially enhance the formability of Zn-Al-Mg coatings by controlling their crystallographic texture. To demonstrate this, in-situ scanning electron microscopy uniaxial tensile tests and thorough orientation image microscopy have been employed. Ultimately, we validate our findings by implementing quantitative plastic deformation-based criteria, namely local strain hardening exponent and Schmid factor distributions within the examined coating microstructures. The approach and findings of the present work considerably resolve the long-lasting cracking problem of these coatings.

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.

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.

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.

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

Bottom-hinged Oscillating Wave Surge Converters (OWSCs) are an efficient way of extracting power from ocean waves. In our previous studies, wave and OWSC interaction has been investigated via computational fluid dynamics (CFD) models. However, these models were highly time-consuming, and significant re-reflection was observed. The present work couples a Boussinesq wave model with a CFD model in order to extend the scope of the applications of the previous models. This model takes advantage of the Boussinesq wave model, which simulates the wave propagation effectively, and the CFD model, which provides the local flow details comprehensively. The model is validated by a comparison of the present results with those obtained with the pure CFD model and the experimental tank testing. The final objective of the present work is to simulate some events experienced and recorded by the full-scale prototype (Oyster 800 developed by Aquamarine Power) incorporating the real bathymetry at the Oyster 800 site.

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

A multi-pump, multi-piston power take-off wave energy converter ((MPPTO)-P-2 WEC) has been proposed for use with a novel renewable energy harvester termed the Ocean Grazer. The (MPPTO)-P-2 WEC utilizes wave motion to pump via buoys connected to pistons working fluid within a closed circuit and store it as potential energy that can be converted to electricity via turbines. This paper introduces the mechanical design and model-based performance prediction of a single-piston pump that constitutes the basic building block for the (MPPTO)-P-2 WEC. Results provide preliminary validation of aqueous lubrication as a viable means of reducing friction and wear, suggesting that water-based hydraulic fluids can prohibit solid contact at the piston-cylinder interface while reducing volumetric leakage, and allowing for an estimation of the energy extraction efficiency for the mechanical pumping system. Pending more thorough and extended tribological investigations using the methodology introduced in this paper, findings suggest that the overall system efficiency will be dictated by the hydrodynamics of the buoys actuating the pumping system. (C) 2016 Elsevier Ltd. All rights reserved.

The mechanisms governing the microstructural evolution during friction stir welding of Monel at different heat inputs and the corresponding mechanical properties were investigated. In the low heat input condition, both continuous and discontinuous dynamic recrystallizations caused the formation of fine grains with an average grain size of 1.7 mu m and closely random texture. A high heat input condition resulted in larger average grain size of 23.5 mu m with shear texture by only the continuous dynamic recrystallization. Low heat input led to the simultaneous increase of strength and elongation. The origins of the tensile behavior of the joints have been discussed. (C) 2019 Elsevier B.V. All rights reserved.

In this work, we report a high-vanadium hierarchical coating prepared on the surface of nodular cast iron substrate by a low-cost plasma transferred arc (PTA) surface alloying process. The coating consists of a graded layer with an alloyed zone (AZ) rich in submicron sized granular (V-Ti-Nb-Cr-Mo) composite carbides on top of intermediate melted zone characterized by refined ledeburite and martensite. The dense spherical particles in the AZ are FCC structured MC-type (M = V, Ti and Nb) carbides which tend to aggregate while M7C3 and M2C carbides nucleate on MC. The super-lattice V8C7 maintains its cube-on-cube orientation relationship with TiC. The hardness of the AZ is 9.6 +/- 1.0 GPa, similar to 4 times that of the substrate. Nano- and micro-indentations point at a superior strength-toughness in the AZ, where cracks are deflected and bridged by the spherical MC carbides in a compressive residue stress state. The fracture mode appears to be rather ductile in the AZ whereas brittle failure appears in both the melted zone and substrate. TEM and EDS results confirm that such a micro architecture design, assisted by the rapid solidification rate of the PTA process, concurrently activates various strengthening micromechanisms including the precipitation hardening and grain refinement.

WS2/a-C nanocomposite coatings were deposited by magnetron co-sputtering using WS2 and graphite targets. The microstructure and triboperformance of the coatings were scrutinized via microscopy (AFM, SEM, FIB, HRTEM), spectroscopy (XRD, XPS) and tribometry. Atomic WS2 platelets are randomly embedded in an amorphous carbon matrix of the as-deposited nanocomposite coating. HRTEM observations of tribofilm/transfer layer reveal that the sliding contact immediately reorients WS2 platelets parallel to the sliding interface and thereby leads to self-adaptive “frictionless” response. The coefficient of friction falls to 0.02 in dry air and reaches 0.10 in humid air, and is reversible as testing atmosphere cycles between dry air and humid air.

In situ micro-cantilever bending tests were carried out on resistance spot weldedDP1000-GI dual-phase high-strength steel in order to derive the mechanicalresponse of the welds. Notched micro-cantilevers were milled using focused ionbeam milling at the base metal, inter-critical, fine-grained and coarse-grainedheat affected zones, and fusion zone. It was shown that due to large plasticyielding, linear-elastic fracture mechanics are inapplicable. To evaluate thefracture toughness of different weld zones, cyclic loading was applied to trackthe crack size and the conditional fracture toughness of weld zones was measured using crack tip opening displacement and J-integral methods. It wasfound that micro-cantilever bending method provides insight to the fracturetoughness and local mechanical response of different weld zones. The resultsobtained can be used to make an accurate correlation between resistance spotwelding process, microstructure and mechanical response of DP1000-GI dualphase high-strength steel welds.

The design of a product component may require complex processing steps such as metal forming followed by a thermal treatment. The thermal treatment may improve the functional performance of the material itself, but may result in rather unwanted changes in the shape of the product. Here it is shown that Finite Element modeling of the various processes can assist in the design of a robust and accurate production process. The modeling approach presented allows a coupling between various complex material models, in such a way that full cold forming and thermal treatment processes are calculated. This coupling of material models is key for the design and concerns the novelty of the proposed approach. Cold forming by deep drawing is calculated whereby planar anisotropy is implemented. The thermal hardening treatment consists of three contributions: creep, thermal expansion and phase transformation. All models are based on experimental data, acquired from tensile and dilatometer tests, and are implemented into the material model either directly or by a simple fit. It is shown that the effects of a complete forming and heat treatment of a cup could be successfully calculated. The predicted cup shape change was compared to experiments, and shows excellent agreement., This research was carried out under the project number T63.3.12480 in the framework of the research program of the Materials innovation institute M2i, Delft, the Netherlands; and has been part of Press Perfect project, RFSR-CT-2012-00021 funded by the Research Fund for Coal & Steel.

The paper concentrates on in situ bending tests and analysis of the failure of materials exhibiting strong anisotropy. As a typical example, Ti2AlC microcantilevers (MCs) were scrutinized. The anisotropic flexural strength of Ti2AlC MCs was measured, and the strength of the strong orientation was found to be two and half times as high as that of the weak orientation. The transition of fracture strength and mechanisms could be evaluated. Our findings have generic implications for future study on the fracture behavior of layered materials. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The work by Greenwood and Williamson (GW) has initiated a simple but effective method of contact mechanics: statistical modeling based on the mechanical response of a single asperity. Two main assumptions of the original GW model are that the asperity response is purely elastic and that there is no interaction between asperities. However, as asperities lie on a continuous substrate, the deformation of one asperity will change the height of all other asperities through deformation of the substrate and will thus influence subsequent contact evolution. Moreover, a high asperity contact pressure will result in plasticity, which below tens of microns is size dependent, with smaller being harder. In this paper, the asperity interaction effect is taken into account through substrate deformation, while a size-dependent plasticity model is adopted for individual asperities. The intrinsic length in the strain gradient plasticity (SGP) theory is obtained by fitting to two-dimensional discrete dislocation plasticity simulations of the flattening of a single asperity. By utilizing the single asperity, response in three dimensions and taking asperity interaction into account, a statistical calculation of rough surface contact is performed. The effectiveness of the statistical model is addressed by comparison with full-detail finite element simulations of rough surface contact using SGP. Throughout the paper, our focus is on the difference of contact predictions based on size-dependent plasticity as compared to conventional size independent plasticity. (C) 2017 Elsevier Ltd. All rights reserved.

Experimental results are presented on silica deposition in a typical domestic heat exchanger during combustion of siloxane-containing gas as a model of biogas that is produced naturally during the anaerobic degradation of organic material in landfills and waste water treatment plants. A model of silica deposition is developed. The main objective is to demonstrate that the mass flux of silica to heat exchanger surfaces is not sensitive to details of particle coagulation process and particle size distribution. It is shown that the deposition flux of silica depends linearly on siloxane concentration in input air/gas mixture.

Tribological experiments on phenol-formaldehyde composite reinforced with polytetrafluoroethylene (PTFE) and glass fibers were performed against 100Cr6 steel and TiC/a-C:H thin film-coated 100Cr6 steel. In both cases, the coefficient of friction increases with increasing sliding distance until a steady-state value is reached. Although the steady-state values of the coefficient of friction are very close and ultralow, the wear rate of the PTFE composite liner at a long sliding distance (1,000 m) is reduced when the steel ball is coated with the TiC/a-C:H coating. This behavior is mainly attributed to the smoother surface after long sliding and the improved wear resistance of TiC/a-C:H coating. PTFE transfer films are evident on the surfaces of the hard counterparts. The average thickness of the transfer film on TiC/a-C:H-coated surfaces is about 3.8 nm. On the surface of uncoated steel ball, a continuous but non-uniform transfer film of around 13.9 nm average thickness was found.

A high-vanadium high speed steel (HVHSS) alloying layer was synthesized from pre-placed powders (V-Cr-Ti-Mo) on ductile iron (DI) substrate using plasma transferred arc (PTA) technique. The PTA-alloyed layer, characterized by microhardness, optical microscopy, XRD, EDS enabled SEM, TEM and pin-on-disk tribometry, consists of three main regions: top alloyed zone (TAZ), intermediate remelted zone (IRZ), and heat affected zone (HAZ) of the DI substrate. A large number of globular carbides particles with size smaller than 5 μm form in the TAZ through in-situ reactions between the alloying elements and graphite in the molten pool. Further microstructural characterizations indicate that the carbides are primarily vanadium carbide (VC), confirming the formation of the HVHSS layer. The maximum microhardness of the PTA-alloyed sample occurring at the subsurface is 950 HV0.2 which is 5 times that of the substrate. The HVHSS layer exhibits superior tribological performance in comparison to PTA-remelted DI, Mn13 steel and DI substrate. The enhanced performance is attributed to the formation of mixed hard-phases such as MC, M7C3, M23C6, martensite and grain refining through rapid solidification accompanying the PTA process. Keywords: Plasma transferred arc technique, Surface alloying, High-vanadium high-speed steel coating, Ductile iron, Microstructure, Wear properties

In this work, the wear and failure mechanism of polytetrafluoroethylene (PTFE)/SiO2/epoxy composites with a high concentration of SiO2 particles under dry sliding is examined. In the composite with 12.5 wt.% PTFE, a significant rise of the coefficient of friction (COF) appears after sliding over several kilometers and under a load of 60 N against balls made out of Al2O3 and steel. It is attributed to an accumulation of back-transferred Al2O3/steel and fractured SiO2 on the worn composite surfaces as well as a reduction of PTFE on worn SiO2 surfaces. The TiC/a-C:H coated ball yields the most stable COF. It is also observed that the loading capacity of the composite decreases with increasing PTFE concentration. Massive wear of the composites is seen after few kilometers sliding when the normal load rises above the loading capacity. The increased wear is due to a high concentration of PTFE which lowers the hardness as well as the compressive strength of the composites.

A novel wave energy converter, termed the Ocean Grazer, designed to extract energy from waves of varying profiles and energy contents has recently been proposed by the University of Groningen. The authors have performed preliminary modeling work to predict the behavior of the converter’s power take-off system, and constructed a proof-of-concept prototype to validate basic model predictions.Copyright © 2014 by ASME

International audience; This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.