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This invited presentation reports an experimental approach to deposit flexible diamond-like carbon (DLC) films on rubber seals with plasma assisted chemical vapor deposition and an analytical model to describe the self-segmentation mechanism of the DLC films. By making use of the substantial thermal expansion mismatch between the DLC films and the rubber substrates, a dense network of cracks forms in the DLC films and contributes to flexibility. The size of the micro-segments can be controlled by tuning the temperature variation of the substrate during deposition through varying the substrate bias voltage. The formation mechanism of the crack network and its effect on the flexibility of DLC films coated on rubbers are presented. The influence of the size of DLC film microsegments on the frictional performance is further studied. The effect of viscoelasticity of the rubber substrate on the frictional behavior of the DLC film-coated system is scrutinized by tribo-tests and theoretical analysis. The importance of adhesive and hysteresis contributions to friction is revealed, and an overarching model is presented.

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.

Presented herein is the performance of a pontoon-type wave energy converter (WEC) similar to that of the ‘Floater Blanket’ WEC, which consists of a grid of interconnected floating elements. The current WEC design takes into account the elastic deformation of the structure under wave action and investigates its efficiency in generating wave energy as compared to its counterparts that achieve this via rigid body motions. The power take off (PTO) systems are installed at the seabed and attached to the WEC to generate energy via the vertical motions of the WEC’s structural modules. Three types of pontoon-type WEC designs are investigated, i.e. with hinges, with a reduced number of hinges and without hinges. In addition, the influence of the flexibility of the structural modules towards the energy generation is also investigated in order to assess its performance when behaving like an elastic material. The pontoon-type WEC’s structural modules with fewer or without hinge connections are preferable due to their ease in installation, while allowing for flexible deformation in the structural design would yield savings in material cost.Copyright © 2018 by ASME

This paper reports a versatile template-free method based on the hydrogen reduction of metallic salts for the synthesis of nanoporous Ni and alloys. The approach involves thermal decomposition and reduction of metallic precursors followed with metal cluster nucleation and ligament growth. Topological disordered porous architectures of metals with a controllable distribution of pore size and ligament size ranging from tens of nanometers to micrometers are synthesized. The reduction processes are scrutinized through X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The formation mechanism of the nanoporous metal is qualitatively explained. The as-prepared nanoporous Ni was tested as binder-free current collectors for nickel oxalate anodes of lithium ion batteries. The nanoporous Ni electrodes deliver enhanced reversible capacities and cyclic performances compared with commercial Ni foam. It is confirmed that this synthesis method has versatility not only because it is suitable for different types of metallic salts precursors but also for various other metals and alloys.

Polydimethylsiloxane (PDMS) is a silicone elastomer-based material that is used in various applications, including coatings, tubing, microfluidics, and medical implants. PDMS has been modified with hydrogel coatings to prevent fouling, which can be done through UV-mediated free radical polymerization using benzophenone. However, to the best of our knowledge, the properties of hydrogel coatings and their influence on the bulk properties of PDMS under various preparation conditions, such as the type and concentration of monomers, and UV treatment time, have never been investigated. Acrylate-based monomers were used to perform free radical polymerization on PDMS surfaces under various reaction conditions. This approach provides insights into the relationship between the hydrogel coating and bulk properties of PDMS. Altering the UV polymerization time and the monomer concentration resulted in different morphologies with different roughness and thickness of the hydrogel coating, as well as differences in the bulk material stiffness. The surface morphology of the coated PDMS was characterized by AFM. The cross section and thickness of the coatings were examined using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The dependence of coating development on the monomer type and concentration used was evaluated by surface hydrophilicity, as measured by water contact angle. Elongation-until-break analysis revealed that specific reaction conditions affected the bulk properties and made the coated PDMS brittle. Therefore, boundary conditions have been identified to enable high quality hydrogel coating formation without affecting the bulk properties of the material.

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 pulsed current electrochemical deposition of calcium phosphate (Ca-P) coatings on a titanium substrate was investigated in this study. The effects of applied voltage and H2O2 concentration in the electrolyte solution on the phase composition and coating morphology were studied using X-ray diffraction and scanning electron microscopy. At lower concentrations of H2O2, the coating consists of mixed phases of dicalcium phosphate dehydrate, octa-calcium phosphate, and hydroxyapatite, whereas increased H2O2 concentrations results in a dual phase of octa-calcium phosphate and hydroxyapatite being deposited. Furthermore, with increasing H2O2 concentration, the voltage must be reduced in order to avoid H2 evolution. The best conditions for Ca-P deposition were achieved at − 1.4 V and 1.5 wt% H2O2. The morphological changes at different deposition times as well as the crystallographic orientation of deposited crystals were studied using scanning electron microscopy and transmission electron microscopy. It was found that the crystal growth of Ca-P coatings is a time-dependent process. During the first stage of deposition (t = 1 min), the electrolyte is supersaturated and randomly oriented polycrystals of Ca-P nucleate and form nanoplates. During the second stage (t = 3 min), crystals grow slightly in a more oriented fashion and form micro-sized plates. During the third stage (t > 10 min), the deposited crystals grow in a highly directional manner and the morphology of the coating consists of elongated ribbon-like single crystals.

In recent years, wave energy converters (WECs) have received considerable attention as an efficient way to harvest alternative energy sources. Within this class of systems, point-absorbers are popular and have become one of the most widely used renewable energy harvest designs all over the world, at least in the preliminary R&D stage, with many relevant research works having been published as well. However, unlike the single buoy and PTO systems which already have a comprehensive research basis, the connection cable has received little attention. The traditional taut cable analysis approach, initiated from the needs of the oil&gas industry, has been applied for WEC investigations. However, this approach utilizes an essential assumption that the oscillating term (PTO force) is much smaller than the static term of the cable force (pre-tension) and could be neglected, which may not be proper for WEC applications. In this work, a conventional frequency domain model is utilized to test and verify the validity of the previously mentioned assumption by presenting the ratio between two force terms. Then the ratio could be applied in combination with sea state contours to reveal the critical state of the cable. Then, a fully nonlinear time domain method of a numerical solution of the point-absorber wave energy converter is presented. According to the critical states obtained from the frequency domain analysis, an improved model of a slack cable is proposed. Its influence on the energy extraction performance is investigated using the open source code — WEC-Sim. This work provides insight into simulating a proper model of the cable and how the design of the cable influences the WEC performance.

This work reports the process optimization of various electrospinning parameters to fabricate polyvinylidene fluoride based piezoelectric flexible nanofiber webs for passive sensing and energy harvesting applications. Process parameters like electrospinning voltage and drum speed have been taken into consideration while optimizing the electrospun nanofiber webs for maximizing their piezoelectric property. Finally, the optimized recipe is used to fabricate a flexible PVDF nanofiber energy harvester to demonstrate the energy harvesting capability of such nanofiber webs.

We report a new approach for the synthesis of three-dimensional nanoporous metals as binder-free current collectors for batteries. A versatile template-free method is presented which is based on the thermal treatment of metallic salts. The method involves thermal decomposition, hydrogen reduction of metallic precursors, and clusters nucleation and ligament growth. Topological disordered porous metal structures with controllable distribution of pore size and ligaments size ranging from tens nanometers to micrometers are controlled by varying the heating temperature and duration. The synthesis process has been scrutinized through XRD, SEM and TEM. The formation mechanism of nanoporous metals is qualitatively explained. As an application, the as-prepared nanoporous Ni is used as binder-free current collectors for nickel oxalates anodes. The newly designed electrodes with loading of nickel oxalates deliver excellent reversible capacities, good cycling performances and rate performances as anodes for lithium-ion batteries. It is anticipated that the synthesis method has versatility not only because it is suitable for different types of metallic salt precursors but also for synthesizing various metals and alloys such as Ni-Co alloys.

In the present study, the effect of resistance spot welding scheme (i.e. single and double pulse welding) on the mechanical behaviour of resistance spot-welded DP1000-GI steel is investigated. It is shown that double pulse welding at low welding current decreases the maximum cross-tension strength and mechanical energy absorption capability of the welds. The factors that lead to the lower mechanical performance of double pulse welds are scrutinised. Local residual stress mapping reveals that the compressive residual stress perpendicular to the plane of the pre-crack either decreases or is fully released at the weld edge of double pulse welds. Orientation imaging microscopy analyses show that the martensite formed in front of the pre-crack of double pulse weld has a lower fraction of high-angle grain boundaries and a coarser structure of Bain groups as opposed to the corresponding area of single pulse weld. Lower mechanical performance of double pulse welds produced at lower welding current is ascribed to the lower compressive residual stress normal to the plane of crack and the formation of martensitic structure in front of the pre-crack with a lower fraction of high-angle grain boundaries and coarser Bain groups.

In this work, a frequency-domain method for the numerical solution of the nonlinear dynamics of a wave energy converter with a pumping system is presented. To this end, a finite Fourier series is used to describe the nonlinear force components, i.e., the pumping force. The dynamics of the buoy and the piston are obtained by solving a set of linear motion equations. The numerical model is validated by comparing it with experimental results. The dynamic characteristics of the pumping system are investigated by performing a series of numerical simulations for several approximation orders under various wave conditions. The advantages and limitations of this method are also discussed in the paper. This work provides great insight into the mechanism of a WEC with a pumping unit under various sea states, which can guide the design of a multi-piston pump (MPP) system. This model will be applied to investigate the optimal configuration for the MPP system in the future.

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.

This thesis describes the synthesis, properties and applications of the nanostructured graphene with different dimensionalities from 3D foams to, 2D film, and 0D graphene quantum dots (GQDs), and from micron-porous to nanoporous. The applications of the nanostructured graphene include batteries, photoluminescence and cell imaging. Specifically, the thesis comprises the following contents: (1) Nanoporous metallic templates are crucial for the synthesis of nanoporous graphene. A novel method for the synthesis of nanoporous metals was developed and is suitable for industrial production. The growth mechanism, kinetics and microstructures of porous metallic templates were investigated. In addition, the porous metals were synthesized as binder-free current collectors for high-capacity electrodes of lithium-ion batteries. (2) A new solid-state-growth approach is developed for controllable synthesis of nanoporous graphene with interconnected tubular pores and tunable porosities at relatively low temperatures. Nanoporous graphene greatly enhanced the electrochemical performances of high-energy-density Li-S batteries. (3) Large-area graphene film is successfully synthesized at near room temperatures from conversion of amorphous carbon using metallic catalysts. The nucleation, growth process and growth kinetics of graphene were investigated. The results point at several attractive strategies for the facile synthesis of graphene-based carbon films for industrial applications. (4) GQDs were successfully exfoliated from abundant carbon feedstocks such as carbon black in liquid phases with the assistance of ultrasonication. The new approach is eco-friendly and promising for large-scale production. GQDs delivered photoluminescence and light absorption properties, which are firmly associated with their microstructures. The as-synthesized GQDs show good performances as fluorescence nanoprobes for bioimaging.

This letter reports that a pre-notched WS2/a-C tribocoating surface can be fully healed through sliding. The healed notches act as superior lubricant micro-reservoirs affording frictionless responses leading to an ultralow coefficient of friction (0.02). HR-TEM observations reveal WS2 (002) platelets in the healed notch parallel to the outer coating surface and to the V-notch/coating interface. Direct evidence of the rearrangement of WS2 at the curved interface elucidates that basal plane reorientation does not occur necessarily parallel to the counterpart sliding direction as was taken for granted in literature. The patchy tribofilm provides a new avenue for self-healing efficiency in tribology. This research reveals a new avenue for self-healing and repairing of damage in tribocoatings whereby one may lift the requirement of producing defect-free coatings for triboapplications.

In atomistic simulations, contact depends on the accurate detection of contacting atoms as well as their contact area. While it is common to define contact between atoms based on the so-called ‘contact distance’ where the interatomic potential energy reaches its minimum, this discounts, for example, temperature effects on atomic vibrations and, correspondingly, the spatial distributions of atoms. In the present study, classical molecular dynamics was used to investigate the definition and detection of contact between a spherical particle (made of either silver, lead, or platinum) and a flat substrate (silver). Total contact areas are estimated via three previously published methods for the detection of atoms in contact: the first method detects contacting atoms based on their potential energy values; the second method utilizes interatomic distances; and, the last method is based on the interacting potential energy values between the contacting atoms of the counterpart surfaces. Each method is examined in detail with our findings suggesting that the use of interatomic distances is the most suitable way to define and detect the real area of contact in atomistic simulations of normal contact. Our results suggest that the minimum of the radial distribution function after the first peak can be defined as the contact distance. Also, a temperature-dependent empirical equation is proposed to estimate the contact distance for Lennard-Jones-type potentials. Moreover, the atomic diameter is defined as the distance between the origin (i.e. the center of the reference atom) and the first peak in the radial distribution function, allowing for an estimation of the atomic contact area.

In this study several Au–WO3 nanocomposite coatings were deposited by reactive magnetron co-sputtering and characterized with respect to their chemical composition, structure, microstructure and mechanical and optical properties. The nanocomposites consist of a dual phase system of Au nanoparticles dispersed homogeneously in an amorphous WO3 matrix, with compositions ranging between 0 and 11 at.% Au. To enhance the surface plasmon resonance effect in the coatings, the growth of Au nanoparticles was stimulated by applying a thermal annealing treatment at different temperatures up to 500 °C. The annealing treatments promoted the crystalization of the WO3 matrix. The hardness of the coatings remained close to 4.5 GPa, irrespectively of the Au content and annealing temperature. As a result of the microstructural changes in both the nanoparticles and matrix, surface plasmon resonance effect was successfully enhanced and different colourations were obtained in the coatings.

The influence of surface reactions on the tribo-performance of steel, Al2O3 and Si3N4 balls sliding against polytetrafluoroethylene/SiO2/epoxy composites was investigated. Al2O3 ball were found to exhibit the best tribo-performance, namely a low coefficient of friction and the lowest wear rates of both the composites and the counterpart ball, when sliding against the PTFE filled composites. The difference in the tribo-performance of the Al2O3 ball and the Si3N4 ball can neither be attributed to the different morphology of the worn composite surfaces nor to the amount of PTFE transferred onto the wear surfaces. Instead we found that the friction is greatly reduced in the case of the Al2O3 ball because two fluoro-terminated surfaces are sliding over each other; in fact, the formation of Al F bonding was confirmed by X-ray photoelectron spectroscopy.

In this work, the tribological performance of PTFE filled SiO2 particles–epoxy composites is investigated. Under a load of 60 N (~140 MPa contact pressure), the optimum content of PTFE lies between 10 and 15 wt%, which yields an ultralow coefficient of friction (CoF) in conjunction with a low wear rate of the composite when dry sliding against bearing steel balls within 1000 m. With 12.5 wt% PTFE in the composite, a CoF around 0.095 and a wear rate as low as 8.4×10−7 mm3/Nm were measured up to a sliding distance of around 2000 m. After 2000 m, eventually the gradual accumulation of the fractured SiO2 particles and back-transferred steel on the worn composite surface leads to a significant increase of CoF. In the steady-state of sliding, smearing of the PTFE particles along the worn surface was observed together with fracturing of the SiO2 particles and cracking of the epoxy matrix. Successive EDS mapping shows the formation and evolution of a PTFE-containing third-body tribolayer on the worn surface of the composite. The thickness of the tribolayer was measured about 20–30 nm on the surface of SiO2 particles after sliding for more than 700 m.

Cu0-mediated living radical polymerization (Cu0-mediated LRP) was employed in this research for the synthesis of starch-g-polyacrylamide (St-g-PAM). The use of a controlled radical grafting technique is necessary, as compared to the traditional free-radical polymerization methods, in order to obtain a well-defined structure of the final product. This is in turn essential for studying the relationship between such structure and the end-properties. Waxy potato starch-based water-soluble macroinitiator was first synthesized by esterification with 2-bromopropionyl bromide in the mixture of dimethylacetamide and lithium chloride. With the obtained macroinitiator, St-g-PAM was homogeneously synthesized by aqueous Cu0-mediated LRP using CuBr/hexamethylated tris(2-aminoethyl)amine (Me6Tren) as catalyst. The successful synthesis of the macroinitiator and St-g-PAM was proved by NMR, FT-IR, SEM, XRD and TGA analysis. The molecular weight and polydispersity of PAM chains were analyzed by gel permeation chromatography (GPC) after hydrolyzing the starch backbone. Monomer conversion was monitored by gas chromatography (GC), on the basis of which the kinetics were determined. A preliminarily rheological study was performed on aqueous solutions of the prepared materials.

TiO2 is widely studied for its good photocatalysis activity. In this research, CO2 gas was used as carbon and oxygen source to prepare carbon-doped TiO2 (C-TiO2) films on stainless steel using reactive magnetron sputtering. In order to grasp the relationship between sputtering process, microstructure and mechanical and photocatalysis properties of C-TiO2 thin film, orthogonal experiment design combining related mechanical analysis and photo-degradation experiment was employed to find a set of optimal reactive sputtering parameters, including substrate temperature, sputtering power and CO2 gas flow rate. Nano-indentation and nano-scratch tests were employed to analyze the hardness, modules and adhesion strength of as-deposited C-TiO2 films, respectively. Taking hardness, modules and adhesion between the film and substrate as a reference of integrated mechanical properties, range analysis of as-prepared C:Ti-O films were done to search the optimal combination of sputtering parameters. The photocatalytic activities of as-prepared C-TiO2 films using methyl orange (MO) as colored dye indicator were also evaluated by measuring absorbance and calculating the MO’s decolorization rate under different UV light irradiation time. The result of photocatalysis experiment compared with a blank group proved that at the optimized reactive sputtering condition C-TiO2 films can achieve both good photocatalysis and mechanical property.

Layered transition metal dichalcogenides (TMD) such as WS2 are materials well-known for their solid lubrication properties in vacuum [1]. However, the lubricating property degrades through oxidation in moisture and it is also limited by its low hardness and low load-bearing capacity. The nanocomposite WSxC coating, namely WS2 lamellae embedded in an amorphous diamond-like carbon (DLC) matrix, is reported to have excellent tribological characteristics, with both low coefficient of friction and low wear rate in a wide range of humidities [2]. One important issue possibly impairing magnetron sputtering high-quality TMD layers is that the WSxC coating is usually substoichiometric, with x

Dynamic roughening is a common phenomenon observed in plasma processing of materials, including both film deposition and plasma etching. That is to say, the roughness of a growing/etching interface becomes higher and higher with the processing time or the film thickness/etching depth. In film growth, the competition between interface roughening and smoothening essentially determines the nano-/microstructure and consequently the properties of a deposited film. This lecture presents several new findings on the breakdown of dynamic roughening in thin film growth. With increasing energy flux of concurrent ion impingement during pulsed DC magnetron (reactively) sputtering, a transition from dynamic roughening to dynamic smoothening is observed in the growth behavior of TiC/a-C(:H) nanocomposite films. The nanocomposite films show a negative growth exponent and ultra-smoothness (RMS roughness ~0.2 nm at film thickness of 1.5 µm). Based on high resolution cross-sectional transmission electron microscopy observations, an amorphous front layer ~2 nm thick covers the nanocomposite film during growth and suppresses the influence of nanocrystallites on the roughness evolution of the nanocomposite films, which is a solid experimental proof of the impact-induced downhill flow model [1] and subplantation model [2]. We were able to predict the evolution of surface roughness based on a linear equation of surface growth which contains two diffusivity parameters that control the atomic mobility along the growing outer surface. The model is in good agreement with atomic force microscopy measurements of roughness evolution [3-6]. The result indicates a general solution of microstructure control in the deposition of thin films with (reactively) sputtering.

The use of Polyvinylidene Fluoride (PVDF) based piezoelectric nanofibers for sensing and actuation has been reported widely in the past. However, in most cases, PVDF piezoelectric nanofiber mats have been used for sensing applications. This work fundamentally characterizes a single electrospun PVDF nanofiber and demonstrates its application as a sensing element for nanoelectromechanical sensors (NEMS). PVDF nanofiber mats were spun by far field electrospinning (FFES) process and complete material characterization was conducted by means of scanning electron microscope (SEM) imaging, Raman Spectroscopy and FTIR spectroscopy. An optimized recipe was developed for spinning a single suspended nanofiber on a specially designed MEMS substrate which allows the nano-mechanical and electrical characterization of a single PVDF nanofiber. Electrical characterization is conducted using a single suspended nanofiber to determine the piezoelectric coefficient (d33) of the nanofiber to be -58.77 pm/V. Also the mechanical characterization conducted using a nanoindenter revealed a Young’s Modulus and hardness of 2.2 GPa and 0.1 GPa respectively. Finally, an application that utilizes the single PVDF nanofiber as a sensing element to form a NEMS flow sensor is demonstrated. The single nanofiber flow sensor is tested in presence of various oscillatory flow conditions. NRF (Natl Research Foundation, S’pore) Published version

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.

An inductive sensor developed by Philips ATC has been used to study in-situ the austenite (γ) to martensite (α′) phase transformation kinetics during tensile testing in an AISI 301 austenitic stainless steel. A correlation between the sensor output signal and the volume fraction of α′-martensite has been found by comparing the results to the ex-situ characterization by magnetization measurements, light optical microscopy, and X-ray diffraction. The sensor has allowed for the observation of the stepwise transformation behavior, a not-well-understood phenomena that takes place in large regions of the bulk material and that so far had only been observed by synchrotron X-ray diffraction., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)., Carola Celada-Casero acknowledges the financial support from Consejo Superior de Investigaciones Científicas (CSIC) in the form of a JAE-Predoc grant (JAEPre_2011_01167), co-funded by the European Social Fund. The authors deeply acknowledge the financial support from Research Fund for Coal for funding this research under the Contract RFCS-02-2015 (Grant Agreement No.: RFSR-CT-2012-00021) and the Spanish Ministerio de Economia y Competitividad (MINECO) through the form of a Coordinate Project (MAT2016-80875-C3-1-R). Authors are also grateful to Jose Antonio Jimenez (CENIM-CSIC), Julio Romero de Paz (CAI of Physical Techniques, UCM) and Marijke de Vries and Ronald van der Linden (Philips ATC) for the experimental support.

This paper presents a modular modeling framework for the Ocean Grazer’s Power Take-Off (PTO) system, which operates as an array of point-absorber type devices connected to a hydraulic system. The modeling is based on the port-Hamiltonian (PH) framework that enables energy-based analysis and control of the PTO system. Firstly, a modular model of a point-absorber hydraulic system, which represents the main building block of the PTO, is presented. The model consists of wave-mechanical and hydraulic subsystems that are interconnected with a transformer-type interconnection. Secondly, we show passivity of the point-absorber hydraulic element and the accumulation of potential energy, which is due to the novel pumping mechanism of the point-absorber. Finally, we illustrate these properties through simulation results.

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 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 μm in the AZ are formed via in-situ reactions between alloy powders and graphites in DI. Microstructural characterizations indicate that the carbides are primarily MC-type (M=V, Ti, Nb) carbides combined with mixed hard-phases such as M2C, M7C3, M23C6, and martensite. Disperse distribution of spherical, submicro-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 5 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 PTA-remelted DI.

Two welding schemes of single and double pulse were used for the resistance spot welding of DP1000 dual phase steel. The changes in the mechanical performance and variant pairing of martensite under two different welding conditions were scrutinized. It is demonstrated that, although both welds fail in pull-out failure mode, double pulse welding enhances cross-tension strength and energy absorption capability of the weld compared to single pulse welding process. Double pulse welding effectively changes the microstructure of the fusion zone. While the fusion zone of single pulse welds has a typical microstructure of columnar grains, the initial fusion zone of double pulse welds is subdivided into two zones: the outer recrystallized layer composing of martensitic microstructure formed within the equiaxed prior austenite grains, and the inner part of columnar grains resulted from the rapid solidification process of resistance spot welding. Orientation imaging microscopy results show that the recrystallized layer of double pulse welds exhibits a lower fraction of high angle grain boundaries and coarse structure of Bain groups. The coarse grained heat affected zone of double pulse welds has a higher fraction of high angle grain boundaries and finer structure of prior austenite grains and Bain groups compared to that of the single pulse welds.

Layered transition metal dichalcogenides (TMD) such as WS2 are materials well-known for their solid lubrication properties [1]. However, the lubricating property degrades through oxidation or moisture and it is also limited by its low hardness and low load-bearing capacity. In contrast amorphous diamond-like carbon (DLC) films are reported to have many features that contribute to excellent tribological characteristics, such as high hardness, anti-wear property with both low friction coefficient and low wear rate[2]. The present research aims at depositing WS2/a-C nanocomposite coatings by magnetron co-sputtering method. The effects of carbon content and argon flow rate on the microstructure and mechanical performance were investigated. The WS2/a-C nanocomposite tribocoating was scrutinized by electron microscopy and mechanical testing. Transmission electron microscopy reveals feathery WS2 platelets, randomly distributed in the amorphous carbon matrix. The nanocomposite coating turns out to be more amorphous-like with increasing carbon content. Nanoindentations tests show that the hardness and elastic modulus of the coating increase with increasing carbon addition while decreasing with a higher argon flow from 10 sccm to 25 sccm. Ball-on-disk tribotests (100Cr6 steel ball as a counterpart) show that the coefficient of friction can be as low as 0.017 in a dry environment (5% relative humidity). It reaches 0.15 in a high humidity surrounding and remains stable within 20000 sliding cycles.

It is well known that rough surfaces have asperities at a wide range of length scales; that contacting asperities may yield even under low loads; and that plasticity in crystalline metals is size dependent at length scales below tens of micrometers. The influence of size dependent plasticity on the contact and frictional properties, however, is still quite an open question. Here we present a statistical description of rough surface contact that incorporates size dependent plasticity through a simple strain gradient plasticity theory. The intrinsic material length scale in the theory is obtained by fitting to two-dimensional discrete dislocation plasticity simulations of the flattening of a single asperity. The statistical model is an extension of the Greenwood-Williamson theory incorporating the interaction between asperities. The effectiveness of the statistical model is addressed by comparison with full-detail finite element simulations of rough surface contact using strain gradient plasticity. One of the most noteworthy results of the analysis is that contact force and total contact area remain linear, as for elastic materials, but with a proportionality factor that depends on the ratio of the rms roughness and the material length scale.

This thesis presents a number of nanotribological problems investigated by means of classical molecular dynamics (MD) simulations, within the context of the applicability of continuum mechanics contact theories at the atomic scale. Along these lines, three different themes can be recognized herein: measuring the contact area in atomistic simulations, the applicability of continuum mechanics theories for describing nanocontacts, and the topography of rough surfaces at the atomic scale.

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.

A versatile method to fabricate taper-free micro-/nanopillars of large aspect ratio was developed with focused ion beam (FIB) cutting. The key features of the fabrication are a FIB with an incident angle of 90° to the long axis of the pillar that enables milling of the pillar sideways avoiding tapering and the FIB current can be reduced step by step so as to reduce possible radiation damage of the milled surface by Ga ions. A procedure to accurately determine the cross-section of each pillar was developed.

Nanocomposite coatings consisting of nanocrystallites embedded in an amorphous matrix can be tailored to exhibit unusual combination of properties such as high hardness and modulus combined with low friction and wear. The properties of the coatings are governed by parameters of the deposition plasma, which are a nontrivial function of the deposition process parameters. Energy flux onto the growing films provided by ionized species is one of the key parameters influencing the properties of coatings. This study is focused on the influence of ion flux on the properties of the growing nc-TiC/a-C:H thin film in order to achieve enhanced mechanical properties. Two different magnetic field configurations were used. The saturated ion current on the substrate was found to be seven times higher in the strongly unbalanced magnetic configuration as compared to well-balanced configuration. The deposition temperature was constant at ~ 320 °C, which is higher than the usual temperature for similar depositions of nc-TiC/a-C(:H) coatings. The structure and mechanical properties of the coatings prepared at low or high ion flux were studied and compared. It is concluded that the level of the impinging ion flux in DC magnetron sputtering is not a significant factor influencing the mechanical properties of the coatings in the presented setup.

Flexible hydrogenated amorphous carbon (a-C:H) thin film coated on rubbers has shown outstanding protection of rubber seals from friction and wear. This work concentrates on the potential advances of expanding thermal plasma (ETP) process for a high throughput deposition of a-C:H thin films in Ar/C2H2 plasma on nitrile butadiene rubber (NBR). The effect of the arc current on the microstructure, interfacial adhesion and tribological properties of the deposited a-C:H films on NBR is scrutinized. The temperature variation during the short ETP process is small and only yields sub-millimeter segmented a-C:H films, in consistence with a previously developed model. Increasing the arc current from 30 A to 75 A leads to smaller sizes of film patches (reduced from 940 mu m to 125 mu m), enhanced adhesion strength from 20.4 MPa to 91.8 MPa and higher hardness (from 5.5 GPa to 8.9 GPa) of the deposited a-C:H films on rubber. The tribological performance of the ETP a-C:H coated NBR is a compromise between the adhesion strength and hardness of the a-C:H films, with the coefficient of friction being reduced from about 1 (uncoated NBR rubber) to less than 0.25 of a-C:H film coated NBR. (C) 2014 Elsevier B.V. All rights reserved.

A new model has been developed to estimate the flow stress under hot deformation conditions up to the peak of the stress-strain curves. This model is derived from the general form of hyperbolic function by introducing an additional parameter to bring the results to a more acceptable level. Stress-strain curves and the critical strain of a '304 austenitic stainless steel' are determined with an average percentage error of 1.24. The model is also used to obtain an equation which has the ability of predicting the critical strain for the onset of dynamic recrystallization. (C) 2013 Elsevier Ltd. All rights reserved.

Tailored writing and specific positioning of molecules on nanostructures is a key step for creating functional materials and nano-optical devices, or interfaces for synthetic machines in various applications. We present a novel approach for the selective functionalization of patterned glass surfaces with functional probes of any nature. The presented strategy is optimized for imaging fluorophore labeled nanostructures for (single-molecule) fluorescence microscopy. The first step in the protocol is coating a glass surface, here a microscope cover slide, with a 60 nm thick diamond-like carbon film. Subsequently, the pattern is defined by either writing silicon oxide on the coating with a focused electron beam, or by etching the coating with a focused ion beam to expose the glass surface. Finally, the pattern is silanized and functionalized. We demonstrate the selective binding of organic fluorophores and imaging with high contrast, especially in total-internal-reflection mode. The presented approach is flexible and combines bottom-up assembly with high-resolution lithography on glass cover slides to precisely position and image functional molecules of any type.

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.

Layered transition metal dichalcogenides (TMD) such as WS2 are well-known materials for their solid lubricating properties [1]. However, the lubricating performance degrades through oxidation or moisture and it is also limited by its low load-bearing capacity. In contrast amorphous diamond-like carbon (DLC) films are reported to have several features which contribute to excellent tribological characteristics, such as high hardness, wear resistance with both low friction coefficient and low wear rate[2]. The coating processing parameters such as argon pressure may have a strong influence on the structure and properties of the TMD coating[3]. The present research aims at depositing WS2/a-C nanocomposite coatings on silicon wafers by magnetron co-sputtering method. The effect of argon flow rate on the composition, microstructure and mechanical performance of the nanocomposite coating was investigated. The WS2/a-C nanocomposite tribocoating was scrutinized by various microscopy techniques (AFM, HRSEM, and HRTEM) and mechanical testing. Results indicate that with increasing argon flow from 10 sccm to 25 sccm, the S/W stoichiometric ratio and deposition rate of the coating increase whereas the hardness and elastic modulus decrease. These findings indicate that the coating becomes looser and in fact it turns out becoming more columnar-like, as was also confirmed by the enhanced roughness. Ball-on-disk tribotests show that the coefficient of friction against 100 Cr6 steel ball as counterpart can be as low as 0.017 in a dry environment (5% relative humidity). It reaches around a value of 0.15 in a high humidity surrounding and remains rather stable within 20000 sliding cycles. Transmission electron microscopy reveals feathery WS2 platelets distributed in the amorphous carbon matrix, which account for the ultralow friction.

Sensor designs found in nature are optimal due to their evolution over millions of years, making them well-suited for sensing applications. However, replicating these complex, three-dimensional (3D), biomimetic designs in artificial and flexible sensors using conventional techniques such as lithography is challenging. In this paper, we introduce a new processing paradigm for the simplified fabrication of flexible sensors featuring complex and bioinspired structures. The proposed fabrication workflow entailed 3D-printing a metallic mold with complex and intricate 3D features such as a micropillar and a microchannel, casting polydimethylsiloxane (PDMS) inside the mold to obtain the desired structure, and drop-casting piezoresistive graphene nanoplatelets into the predesigned microchannel to form a flexible strain gauge. The graphene-on-PDMS strain gauge showed a high gauge factor of 37 as measured via cyclical tension-compression tests. The processing workflow was used to fabricate a flow sensor inspired by hair-like &lsquo, cilia&rsquo, sensors found in nature, which comprised a cilia-inspired pillar and a cantilever with a microchannel that housed the graphene strain gauge. The sensor showed good sensitivity against both tactile and water flow stimuli, with detection thresholds as low as 12 &micro, m in the former and 58 mm/s in the latter, demonstrating the feasibility of our method in developing flexible flow sensors.

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.

This paper focuses on the frictional behavior of flexible diamond-like carbon (DLC) films coated hydrogenated nitrile butadiene rubber. By making use of the substantial thermal mismatch between DLC film and rubber substrate, the DLC film splits during deposition into micro-segments that contribute to flexibility. The size of film micro-segments can be tuned by controlling the temperature variation of rubber substrate during deposition. The influence of the size of DLC film micro-segments on the frictional performance is studied. The effect of viscoelasticity of the rubber substrate on the frictional behavior of DLC film coated system is scrutinized with tribo-tests and theoretical analysis. The importance of adhesive and hysteresis contributions to friction is revealed, and an overarching model is presented.