bluuebunny/crossref_metadata_2025_split huggingface dataset:
Strength of Ultrananocrystalline Diamond Thin films – Identification of Weibull Parameters
Authors: Peng B. , Espinosa H.D. , Moldovan N. , Xiao X. , Auciello O. , Carlisle J.A. , Gruen D.M. , Divan R.S. , Mancini D.C. , Gerbi J.E. , Birrell J. 
Published: June 2019

AbstractThe fracture strength of ultrananocrystalline diamond (UNCD) thin films, grown by microwaveplasma- enhanced chemical-vapor deposition (PECVD), was measured using the membrane deflection experiment (MDE) developed by Espinosa and coworkers. The data show that UNCD fracture strength appears to follow a Weibull distribution. Furthermore, we show that the Weibull parameters are highly dependent on the seeding process used in the growth of the films. When seeding was performed with micron-size diamond particles, using mechanical polishing of the substrate, the stress, resulting in a probability of failure of 67%, was found to be 1.74 GPa, and the Weibull modulus was 5.74. By contrast, when seeding was performed with nano-size diamond particles, using ultrasonic agitation, the stress, resulting in a probability of failure of 67%, increased to 4.13 GPa and the Weibull modulus was 10.76. The investigation highlights the role of microfabrication defects on material properties and reliability, as a function of seeding technique, when identical PECVD chemistry is employed. The parameters identified in this study are expected to aid the designer of MEMS/NEMS devices employing UNCD films.
Decoupling of the Chemical and Mechanical Surface Contributions in a Force Curve Measurement with AFM
Authors: Noel Olivier , Brogly Maurice , Castelein Gilles , Schultz Jacques 
Published: June 2019

AbstractAtomic Force Microscope (AFM) was used to perform surface force measurements in contact mode to investigate surface properties of model systems at the nanoscale. Model systems were considered and compared. The first one was related to systems of controlled chemical surface properties with identical mechanical properties (chemically modified silicon substrates with hydroxyl, amine, methyl and ester functional groups). The second one deals with model polymer networks (Cross-linked polydimethylsiloxane or PDMS) of controlled mechanical properties and identical surface chemistry. The third system consists in a model polymer network, whose surface is chemically controlled with the same groups as before with silicon substrates. The results show that the viscoelastic contribution is dominating in the adhesion force measurement. Finally, we propose a relationship (derived from the Gent and Schultz's one), which expresses the AFM adhesion force as a function of mechanical energy dissipated in the contact and the surface properties of the material.
Surface Mechanical Property Determination of Soft Materials Through an AFM Nanoindentation Experiment
Authors: Noel Olivier , Brogly Maurice , Castelein Gilles , Schultz Jacques 
Published: June 2019

AbstractIn this paper, we have studied the capability of the AFM to perform nanoindentation experiments. We mainly focused on the importance of a rigorous experimental procedure to get quantitative and reproducible data with the Atomic Force Microscopy (AFM). Systematic calibration procedure of AFM measurements, as well as a complete description of the mechanical behavior of the soft material is necessary before producing reliable quantitative data. In particular, the influences of the creep and of the strain rate have been studied. Then, this technique was used to probe model cross-linked polydimethylsiloxane (PDMS) and to extract their surface mechanical properties at the nanoscale. Young modulus of each sample was calculated by comparing different contact mechanics theories (Hertz and JKR theories and a power law expression). In conclusion, the contact mechanics relationships have to be redefined and adapted to soft materials. In particular, it appears necessary to consider the specific mechanical contribution of the polymers.
Analysis of the Elastic Modulus of a Thin Polymer Film
Authors: Roche S. , Bec S. , Loubet J.L. 
Published: June 2019

AbstractThis paper discusses the interaction between mechanical properties of a thin layer and those of its substrate. The underlined questions are: how the substrate's mechanical properties affect those of the film; how the layer's thickness affects its mechanical properties. Nanoindentation tests were performed on polymer films of different thicknesses deposited onto a silica substrate. The mechanical response of such a system is known to be a composite response (film+substrate). Our aim is to understand the elastic behavior of the film inside this structure.Model was used to estimate the elastic modulus of the film from the global measured value. For thin and “compliant” polymer layers deposited on “hard” substrates, an increase of the film elastic modulus was observed along indentation. We attribute this increase to the “anvil effect”: the bulk elastic modulus increases with the hydrostatic pressure which results from the compression of the film. In conclusion, one first interpretation is proposed to understand specific behavior of thin soft polymer layers on rigid substrates.
Increasing Initial Yield Stress at Small Length Scales
Authors: Spary I. , Bushby A.J. , Jennett N.M. , Pharr G.M. 
Published: June 2019

AbstractPlasticity size effects are well known in a wide variety of situations where either the material microstructure or a strain gradient exist at small length scales. Several theories have been developed to describe changes in the work hardening behaviour under these conditions but none that predict a change in the initial yield stress. Careful studies by Chaudhri et al and Pharr et al have unambiguously demonstrated plasticity size effects in ductile metals. In those experiments indentation stress-strain curves were generated using spherical indenters with radii ranging from a few micrometres to several hundred micrometres and these were compared to data from conventional compression tests. Large radius indenters produced a single indentation stress-strain curve independent of indenter radius with a power law hardening coefficient equivalent to that in the compression tests. However, the indentation stress-strain curves appeared at progressively higher pressures for smaller radius indenters. In this paper we model those experiments using finite element analysis methods. By inputting the uniaxial stress-strain data to the model (effectively, using von Mises criterion) the indentation stress-strain curves for the macro size indenters are reproduced. However, the model shows no length scale dependence for any size of indenter. We show that by off-setting the compression stress-strain curve by increasing the initial yield stress and inputting this data to the model, the indentation behaviour of the smaller radius indenters can be modelled. The increase in yield stress with decreasing indenter radius is demonstrated for Cu, Wand Ir and is shown to be consistent with the initiation of yielding over a finite volume.
Molecular-Dynamics Modelling of the Tensile Deformation of Helical Nanowires
Authors: Shintani K. , Kameoka S. 
Published: June 2019

AbstractDeformations of Au nanowires of helical structures under enforced elongation are addressed by the molecular-dynamics simulation. The embedded-atom method potential is employed for calculating the interaction between Au atoms. Model nanowires of the two kinds of helicities are prepared. Before elongation, a model nanowire is equilibrated at a specified temperature. Then, the Au atoms at one end of the nanowire are translationally moved in the axial direction. The simulation results show that a model nanowire can be elongated to form a single-atom chain of Au atoms under some circumstances.
Mechanical and Structural Properties of TiC and TiCN Thin Films Grown by RF Sputtering
Authors: Mani A. , Aubert P. , Khodja H. , Houdy P. 
Published: June 2019

AbstractTiC and TiCN thin films were deposited by RF sputtering from a TiC target. For TiC thin films, the various sputtering pressure were carried out in order to observe the influence of this parameter on structural and mechanical properties. The experimental results show that a pressure of 1 Pa is necessary to obtain stoichiometric films with <111> texture. Lower pressures induce the formation of distorted titanium carbide, while RBS spectra show that the Ti/C ratio is constant for all these samples. Both the compressive stress and the hardness exhibited a maximum value for the lowest pressures. For TiCN thin films, the composition and the hardness were investigated as function of the N2 partial pressure (PN2 = 3% to 70%).
Dual Tribological Behavior of a Nanolayered Ceramic: Ti3SiC2
Authors: Souchet Alexandra , Fontaine Julien , Belin Michel , Mogne Thierry Le , Loubet Jean-Luc , Barsoum Michel W. 
Published: June 2019

AbstractThe MAX phases are new, thermodynamically stable, nanolayered ternary carbides and nitrides. These materials have a big potential in tribological applications due to their structure, similar to graphite or molybdenum disulfide. For example, the friction coefficients of the basal planes of Ti3SiC2 have been shown to exhibit very low (<; 5 × 10-3) friction coefficients. The aim of this study is to better understand the tribological behavior of polycrystalline Ti3SiC2 against stainless steel. Experiments were conducted on a ball-on-flat tribometer (∼ 25°C and ∼ 30% relative humidity) that simultaneously measures friction coefficients and electrical contact resistance. Different stainless steel ball diameters and normal loads were used and resulted in contact pressures between 0.35 and 1.25 GPa. Two different tribological behaviors were observed, both with relatively low friction coefficients for ceramics. The first behavior, referred to as type I, is characterized by low friction coefficients (≍ 0.15); low wear and a transfer film containing titanium and carbon formed on the ball. The other behavior, type II, is characterized by friction coefficients that starts at ≍ 0.15, and then increases to about 0.4. At the end of the experiment, the ball is worn, and compacted wear debris containing iron can be found on the plane. The two behaviors seem to be independent of contact pressure, but are rather sensitive to normal applied load. The transition between these two regimes will be discussed.
Atomistic Simulation of Flow Stress and Dislocation-Interface Interaction in Thin Metal Films
Authors: Ege E. S. , Shen Y.-L. 
Published: June 2019

AbstractThis study focuses on the atomistics of interface-mediated plasticity in thin metallic films. Molecular statics simulations are carried out to model the tensile stress-strain response of the free-standing and substrate-bonded films. In the free-standing film dislocation glide readily occurs, inducing slip steps at both surfaces of the film. The existence of an interface with the substrate constrains the dislocation motion in the film and restricts the slip steps to only the free surface. The propensity of film plasticity is dictated by the capability of atoms to slide along the interface. The flow stress of the film can be correlated with the dislocation activities obtained from the simulation. The propensity of spreading of relatively energetic atoms associated with the dislocation near the interface is also discussed.
The Behavior of Copper Nanoparticle Chain Aggregates Under Strain – A Molecular Dynamics Approach
Authors: Dalis Adamos S. , Friedlander Sheldon K. 
Published: June 2019

AbstractNanoparticle chain aggregates (NCA) serve as reinforcing fillers that are combined with molecular polymers to produce nano-composite materials, e.g. carbon black in rubber. The reinforcing mechanism due to the incorporation of nanoparticle aggregates is not well understood. Molecular dynamics (MD) computer simulations are employed to investigate the behavior of nanoparticle chain aggregates under strain. The interaction potential used is that of Cu obtained with the embedded atom method (EAM). Three single-crystal Cu nanoparticles are placed in contact in two different configurations (linear and kinked) and the structures are initially relaxed with MD steps for 300 ps. We observe plastic deformation during the sintering process for very small particles (∼2.5 nm in diameter) at temperatures as low as 300 K. The relaxed configurations are then strained to the breaking point at strain rates in the order of 1 m/s. We identify mechanisms of strain accommodation that lead to nanoparticle plastic deformation and eventually fracture. The linear and the kinked configurations break at strains of 0.263 and 0.344 respectively, while the maximum stress is close to 4 GPa (strain rate: 0.625 m/s). Both structures fail at the low-angle grain boundaries developed during the sintering process, while the higher strain for fracture for the kinked configuration is associated with interface sliding not observed in the linear case.
Structure of Dislocation Cores in GaAs
Authors: Beckman S. P. , Chrzan D. C. 
Published: June 2019

AbstractThe atomic scale structures of the partial dislocation cores in GaAs are explored using ab initio electronic structure total energy techniques. The structure of the 30° partial dislocations are expected to be period doubled along the core. The structure of the 90° partial dislocations remains more uncertain, and here, an effort is made to predict which of two proposed reconstruction, double period or single period, is more stable. The relative energies of the two core structures are found to be equal, within the accuracy of the present calculations. It is suggested that at temperature, both core reconstructions will be present.
3D simulation of the dislocation dynamics in polycrystalline metal thin films
Authors: Blanckenhagen B. Von , Arzt E. , Gumbsch P. 
Published: June 2019

AbstractThe plastic deformation of polycrystalline fcc metal thin .lms with thicknesses of 1 μm and less is investigated by simulating the dynamics of discrete dislocations in a representative columnar grain. The simulations are based on the assumption that dislocation sources or multiplication sites are rare and that sources have to operate several times to generate appreciable plastic deformation. This model is thoroughly tested by calculating the response of randomly distributed dislocation sources to an applied stress and comparing the results with experimental data. Stress–strain curves, the influence of boundary conditions, dislocation densities, work hardening rates and their dependence on the film thickness as well as the dependence on grain orientation are studied. The agreement between simulation and experiment is good and many aspects of thin film plasticity can be understood with the assumption that small-scale plastic deformation is source controlled rather than mobility controlled.
Features of Semiconductors during Nanoindentation
Authors: Richter Asta , Wolf Bodo , Smith Roger , Günther Margita 
Published: June 2019

AbstractSi and InSb were subject to depth sensing multi-cycling nanoindentation. The load-depthcurves exhibited hysteresis loops which are explained in terms of pressure induced phase transformations. In order to study the impact of crystal distortions on phase transformation, the specimens were subject to boron implantation (ion energy 180 keV) of different implantation doses (1014 to 1017 ions/cm2) and indented without annealing. In InSb, the hysteresis loops disappeared after implantation of 1016 ions/cm2, and for Si with its stronger bonds, a dose of 3*1016/cm2 is required for the same effect. Indentation cycling with constant maximum load results in a sudden disappearance of the hysteresis loop after a small gradual loop area reduction during the first initial cycles.
Dislocation patterning in fatigued silicon single crystals
Authors: Legros M. , Ferry O. , Feiereisen J.-P. , Jacques A. , George A. 
Published: June 2019

AbstractTension compression fatigue tests and subsequent TEM observations were conducted on single crystalline silicon in a temperature and strain rate domain where lattice friction is still effective: 800-900°C and 1.5 to 6x10-4s-1. Samples oriented for single slip conditions were cyclically loaded under plastic strain amplitude control. For amplitudes ranging from 6x10-4 to 10-2, cyclic stress-strain curves exhibit two different stages of hardening and pass through a maximum before saturation is reached. TEM observations suggest that strain localization takes place near the maximum cyclic stress and beyond. Before mechanical saturation, edge dislocation dipoles sit mainly in thick rectilinear walls. Once the maximum stress is reached, these thick walls “condense” in much thinner walls that seem to carry out the imposed deformation while other regions become inactive. In this case, the dislocation structure anneals out and a loop structure is created from the dipolar walls.
Effects of Grain Boundary Constraint on the Constitutive Response of Tantalum Bicrystals
Authors: Ziegler A. , Campbell G. H. , Kumar M. , Stölken J. S. 
Published: June 2019

AbstractThe role of grain boundary constraint in strain localization, slip system activation, slip transmission, and the concomitant constitutive response was examined performing a series of uniaxial compression tests on tantalum bicrystals. Tantalum single crystals were diffusion bonded to form a (011) twist boundary and compressed along the [011] direction. The resulting threedimensional deformation was analyzed via volume reconstruction. With this technique, both the effective states of stress and strain over the cross-sectional area could be measured as a function of distance from the twist boundary, revealing a highly constrained grain boundary region. Post-test metallurgical characterization was performed using Electron Back-Scattered-Diffraction (EBSD) maps. The results, a spatial distribution of slip patterning and mapping of crystal rotation around the twist-boundary, were analyzed and compared to the known behavior of the individual single crystals. A rather large area near the grain boundary revealed no crystal rotation. Instead, patterns of alternating crystal rotation similar to single crystal experiments were found to be some distance away (~1mm) from the immediate grain boundary region, indicating the large length scale of the rotation free region.
Application of a Modified Jogged-Screw Model for Creep of Titanium Aluminides: Evaluation Of The Key Substructural Parameters
Authors: Karthikeyan Subramanian , Moon Junho , Viswanathan Gopal B. , Mills Michael J. 
Published: June 2019

AbstractA modification of the jogged-screw model has been adopted recently by the authors to explain observations of 1/2[110]-type jogged-screw dislocations in equiaxed Ti-48Al under creep conditions. The aim of this study has been to verify and validate the parameters and functional dependencies that have been assumed in this previous work. The original solution has been reformulated to take into account the finite length of the moving jog. This is a better approximation of the tall jog. The substructural model parameters have been further investigated in light of the Finite Length Moving Line (FLML) source approximation. The original model assumes that the critical jog height (beyond which the jog is not dragged) is inversely proportional to the applied stress. By accounting for the fact that there are three competing mechanisms (jog dragging, dipole dragging, dipole bypass) possible, we can arrive at a modified critical jog height. The critical jog height was found to be more strongly stress dependent than assumed previously. The original model assumes the jog spacing to be invariant over the stress range. However, dynamic simulation using a line tension model has shown that the jog spacing is inversely proportional to the applied stress. This has also been confirmed by TEM measurements of jog spacings over a range of stresses. Taylor's expression assumed previously to provide the dependence of dislocation density on the applied stress, has now been confirmed by actual dislocation density measurements. Combining all of these parameters and dependencies, derived both from experiment and theory, leads to an excellent prediction of creep rates and stress exponents. The further application of this model to other materials, and the important role of atomistic and dislocation dynamics simulations in its continued development is also discussed.
The Stability of Icosahedral Cd-Yb
Authors: Krauss Günter , Deloudi Sofia , Steiner Andrea , Steurer Walter , Ross Amy R. , Lograsso Thomas A. 
Published: June 2019

ABSTRACTThe stability of single-crystalline icosahedral Cd-Yb was investigated using X-ray diffraction methods in the temperature range 20 K ≤ T ≤ 900 K at ambient pressure and from ambient temperature to 873 K at about 9 GPa. Single-crystals remain stable at low temperatures and in the investigated HP-HT-regime. At high temperatures and ambient pressure, the quasicrystal decomposes. The application of mechanical stress at low temperatures yields to the same decomposition, the formation of Cd. A reaction of icosahedral Cd-Yb with traces of oxygen or water causing the decomposition seems reasonable, but a low-temperature instability of this binary quasi-crystal cannot be ruled out totally.
Effects of Ga- and In-doping on the Thermoelectric Properties in Ba-Ge Clathrate Compounds
Authors: Okamoto Norihiko L. , Inui Haruyuki 
Published: June 2019

ABSTRACTThe crystal structures and thermoelectric properties of Ba-Ge type-I clathrate compounds (Ba8GaXGe46-X and Ba8Ga16-YInYGe30) have been investigated as a function of Ga and In contents. Ba8GaXGe46-X alloys have a crystal structure that contains an ordered arrangement of Ge vacancies, forming a superstructure based on the normal type-I structure until X reaches 3, whereas they have the normal type-I structure when X exceeds 3. The dimensionless thermoelectric figure of merit (ZT) increases with the increase in the Ga content, exhibiting the highest value of 0.49 for Ba8Ga16Ge30. The power factor for Ba8Ga10In6Ge30 is 1.5 times that for Ba8Ga16Ge30 so that the In containing alloy exhibits a ZT value as high as 1.03 at 700°C.
Urbach Edge, Disorder, and Absorption On-set in a-Si:H
Authors: Cody G. D. 
Published: June 2019

AbstractThis paper presents a brief review of the research that began in the early '80s, continued through the '90s, and produced a “standard model” for the optical absorption edge of amorphous silicon. The research began as a response to the invention of a-Si:H solar cells by Carlson and Wronski at RCA laboratories in 1976, and the subsequent worldwide interest in the optical characterization of a-Si:H thin films. The immediate need was soon met, but the research continued as an effort to understand the physics of the optical absorption edge in a-Si:H, as well as to understand the differences between, and similarities to, the indirect optical absorption edge of c-Si. In this paper, we highlight the successes of this standard model, and briefly cover its experimental and theoretical development over the last 25 years. We summarize its current status, and suggest some experimental and theoretical opportunities for, and challenges to, what may now be called a standard model for the optical absorption edge of both a-Si:H and a-Ge:H.