bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spontaneous formation of nanometer-scale self-organized structures in Ag-Cu alloys under irradiation
Authors: Enrique Raúl A. , Bellon Pascal 
Published: April 2011

AbstractIon-beam irradiation can be used as a processing tool to synthesize metastable materials. A particular case is the preparation of solid solutions from immiscible alloys, which have been achieved for a whole range of systems. In this process, enhanced solute concentration is obtained through the local mixing induced by each irradiation event, which if occurring at a high enough frequency, can outweigh demixing by thermal diffusion. The resulting microstructure forms in far from equilibrium conditions, and theoretical results for these kind of driven alloys have shown that novel microstructures exhibiting self-organization can develop. To test these predictions, we prepare Ag-Cu multilayered thin films that we subject to 1 MeV Kr+-ion irradiation at temperatures ranging from room temperature to 225 °C, and characterize the specimens by x-ray diffraction, TEM and STEM. We observe two different phenomena occurring at different length scales: On the one hand, regardless of the irradiation temperature, grains grow under irradiation until reaching a size limited by film thickness (~200 nm). On the other hand, the distribution of species inside the grains is greatly affected by the irradiation temperature. At intermediate temperatures, a semi-coherent decomposition is observed at a nanometer scale. This nanometer-scale decomposition phenomenon appears as an evidence of patterning, and thus confirms on the possibility of using ion-beam irradiation as a route to synthesize nanostructured materials with novel magnetic and optical properties.
A Finite Element Study on the Nanoindentation of Thin Films
Authors: Chen Xi , Vlassak Joost J. 
Published: April 2011

ABSTRACTNanoindentation is a technique commonly used for measuring thin film mechanical properties such as hardness and stiffness. Typically, shallow indentations with contact depths less than 10-20% of the film thickness are used to ensure that measurements are not affected by the presence of the substrate. In this study, we have used the finite element method to investigate the effect of substrate and pile-up on hardness and stiffness measurements of thin film systems. We find that: i) for soft films on hard substrates, the hardness is independent of the substrate as long as the indentation depth is less than 50% of the film thickness; ii) as soon as the hardness exceeds that of the substrate, the substrate effect becomes significant, even for indentations as shallow as 5% of the film thickness; iii) if the film is at least 40 times harder than the substrate, the plastic zone is mostly confined to the substrate while the film conforms to the deformed substrate by bending. We define a substrate effect factor and construct a map that may be useful in the interpretation of indentation measurements on thin films. It is found that the yield stress mismatch is a key factor characterizing the hardness of thin film system, and the elastic mismatch is important when making stiffness measurements. The results obtained in this study are very useful when it is difficult to avoid the influence of the substrate on the measurements.
Epoxy/Aluminum Adhesion as Measured by Contact Mechanics (JKR) in the Presence of an Organic Contaminant
Authors: Woerdeman Dara L. , Emerson John A. , Giunta Rachel K. 
Published: April 2011

ABSTRACTThe JKR contact mechanics approach is employed to analyze the effects of surface contaminants on adhesive bonding, as well as quantify the level of contamination at which adhesive strength decreases. The contact adhesive forces between two surfaces, one being a soft hemisphere and the other being a hard plate, can readily be determined by applying an external compressive load to join the two surfaces, subsequently applying a tensile load to assess the energy dissipation mechanisms involved in the debonding process. In the present work, we monitor the interactions between a diglycidyl epoxy elastomer and an aluminum oxide substrate in the presence of an organic contaminant. Furthermore, we present a method by which surface contamination can be quantified using a single number, referred to as the adhesion hysteresis parameter, H.
Adhesion of a Rigid Cylinder to an Incompressible Film
Authors: Yang Fuqian , Zhang Xinzhong , Li J.C.M. 
Published: April 2011

ABSTRACTThe adhesion between a rigid cylindrical particle with a flat end of radius a and an incompressible elastic film of thickness h deposited on a rigid substrate was studied. The contact surfaces between the particle and the film and between the film and the substrate are either frictionless (slip) or perfectly bonded (stick). Using integral equations, the stress distribution in the contact area was solved and used to obtain the load required to press the particle onto the thin film. Using a thermodynamic method, the pull-off force to separate the particle from the film was obtained numerically and analytically. For a>>h, the pull-off force is proportional to a2/h1/2 if it is frictionless on both contact interfaces and is proportional to a3/h3/2 if it is frictionless between the particle and thin film and bonded between the thin film and the substrate. For a<h/2 the pull-off force is proportional to a3/2 independent of h and the boundary conditions. To verify this the self adhesion of PDMS [poly(dimethylsiloxane)] was determined experimentally and the a3/2 relation was confirmed. The self-adhesion of PDMS was found to increase with the square root of contact time suggesting molecular diffusion as the dominant mechanism.
Correlation of Nanoindentation and Conventional Mechanical Property Measurements
Authors: Rice Philip M. , Stoller Roger E. 
Published: April 2011

ABSTRACTA series of model ferritic alloys and two commercial steels were used to develop a correlation between tensile yield strength and nano-indentation hardness measurements. The NanoIndenterII® was used with loads as low as 0.05 gf (0.490 mN) and the results were compared with conventional Vickers microhardness measurements using 200 and 500 gf (1.96 and 4.90 N) loads. Two methods were used to obtain the nanohardness data: (1) constant displacement depth and (2) constant load. When the nanohardness data were corrected to account for the difference between projected and actual indenter contact area, good correlation between the Vickers and nanohardness measurements was obtained for hardness values between 0.7 and 3 GPa. The correlation based on constant nanoindentation load was slightly better than that based on constant nanoindentation displacement. Tensile property measurements were made on these same alloys, and the expected linear relationship between Vickers hardness and yield strength was found, leading to a correlation between measured changes in nanohardness and yield strength changes.
Plastic Zone Development Around Nanoindentations
Authors: Woodcock C. L. , Bahr D. F. , Moody N. R. 
Published: April 2011

ABSTRACTJohnson's cavity model relating indenter geometry and deformation resulting from elastic-plastic indentations is appropriate for a wide variety of materials. In the case of nanoindentations in single crystal BCC metals, limitations are reached when creep is not fully accounted for. Both the standard Berkovich and cube corner geometries show that the ratio of plastic zone radius to contact radius increases with the duration of time at the peak load. Indenter tip geometry is shown to play an important role in this phenomenon. Length scale phenomena, such as the indentation size effect, are also subject to various interpretations. The traditional definition of hardness does not produce similar trends with indentation length scale between the blunt Berkovich geometry and the sharper cube corner tip. However, the ratio of plastic zone radius to contact radius proves to be a tip geometry independent method of assessing the plasticity of these metals.
Nanoscratch Deformation Response of Carbon Nitride Thin Films
Authors: Charitidis C. , Patsalas P. , Logothetidis S. 
Published: April 2011

ABSTRACTThe mechanical performance of thin carbon nitride (CNx) films deposited onto Si(100) substrate by sputtering, was investigated in the load range of 2-20 mN using nanoindentation and nanoscratch techniques. The microstructure of these films was characterized by the inhomogeneous embedding of crystalline grains into a N-poor amorphous matrix. A load dependent transition was found in both the scratch, as well as the friction responses. Below 5 mN, nanoscratching showed completely elastic behavior of the film, while above 10 mN a mixed elastic-plastic behavior was identified. Testing under a normal load of 20 mN resulted in local grooving at the film surface; however, in-situ profiling of the scratch trace showed no evidence of film failure. Both the coefficient of friction (μ), as well as the plastic deformation of the films were found increasing with the normal load applied following an exponential low.
Effect of Film Thickness on the Nanoindentation Measurements of Hard Diamondlike Carbon Films Prepared by Pulsed Laser Deposition
Authors: Wei Q. , Sankar J. , Sharma A. K. , Narayan J. 
Published: April 2011

ABSTRACTWe have investigated the effect of film thickness on the nanoindentation measurements of hard diamondlike carbon (DLC) films. The DLC films were deposited on Si (100) substrates by pulsed excimer laser deposition (KrF, λ=248nm, duration=25 ns, energy density about 3.0 J/cm2, repetition rate 10 Hz) in high vacuum (∼5×10−7torr) at room temperature for various periods of time (from 5 minutes to 20 minutes). The thickness of the films was measured by optical profilometry, and ranges from 200 to 50 nm. The nanohardness and elastic modulus of these films were measured by a depth-sensing nanoindentation technique with NanoindenterXP. Preliminary experimental observations show that the nanoindentation results are a function of film thickness. Both the hardness and Young's modulus versus displacement curves of the 200nm thick film exhibit a peak at around tenth of the film thickness. The rest of the samples with smaller film thickness show plateaus, which are higher than the hardness and Young's modulus values of the substrate.
An Assessment of the Mechanical Strengths of Aluminide-based Thin Coatings
Authors: Li S.Y. , Ng H.P. , Ngan Alfonso H.W. 
Published: April 2011

ABSTRACTTitanium aluminide and nickel aluminide-based thin coatings were synthesized by magnetron sputtering from intermetallic TiAl and Ni3Al alloy targets on nickel substrates. Both types of aluminide coatings exhibited high surface hardness values that varied with the degree of heat treatment. The hardness of the coatings was investigated using micro- and nano- indentation techniques. In order to estimate the intrinsic strength of the films, the indentation size effects of the apparent hardness were analyzed by the Jönsson-Hogmark model and a model recently proposed by the authors. The analysis indicated that the strengths of the aluminide coatings may considerably exceed their strengths in bulk.
Mechanical Response of Diamond at Nanometer Scaes: Diamond Polishing and Atomic Force Microscopy
Authors: Pérez Ruben , Jarvis Murray R. , Payne Michael C. 
Published: April 2011

ABSTRACTTotal energy pseudopotential methods are used to study two different processes involving the mechanical interaction of diamond nanoasperities and diamond surfaces: the wear processes reponsible for diamond polishing, an the mechanical deformation of tip and surface during the operation of the Atomic Force Microscope in contact Mode (CM-AFM). The strong asymmetry in the rate of polishing between different dirctions onthe diamond (110) surface is explained in terms of on atomistic mechanism for nano-groove formation. The pst–polishing surface morphology and the nature of the polishing residue epredicted by this mechanism are consistent with experimental evidence. In the case of CM-FAM, our calculations show that a tip terminated in a single atom is able to sustain forces in excess of 30 nN. The magnitude of the normal force was unexpectedlyfound to be verye similar for th approach on top of an atom or on a hollow position on the surface. This behaviour is due to tip relaxations induced by the interaction with the surface. These forces are also rather insensitive to the chemical nature of the tip apex.
Effect of Surface Steps on Dislocation Structure During Nanoindentation
Authors: Zimmerman Jonathan A. , Klein Patrick A. , Foiles Stephen M. 
Published: April 2011

ABSTRACTNucleation of dislocations during nanoindentation is a prime example of how nanoscopic details play an important role in the evolution of macroscopic mechanical behavior. Experimental studies of nanoindentation show that surface irregularities, such as steps, alter the mechanical response during indentation. Atomistic simulations using the embedded atom method are used to examine the indentation of a Au(111) crystal with a surface step. The amount of force needed to nucleate dislocations decreases significantly for indentation close to the step. A recently formulated atomistic deformation metric, the slip vector, is used to quantify dislocation content. This metric is used with the local atomic stress to determine the critical resolved shear stress necessary for dislocation nucleation. Atomic stress fields are also compared with continuum calculations performed using non-linear elasticity with a constitutive relation based on the same interatomic potential. This constitutive model shows good agreement of indentation forces and stresses with the atomistic simulations.
Computational Nanotribology: SAMs for MEMS
Authors: Berry Rajiv J. , Wintrich Nicole L. , Bharadwaj Rishikesh K. , Schwartz Martin 
Published: April 2011

ABSTRACTSelf-assembled monolayers (SAMs) consisting of hydrocarbon chains attached to silica walls were evaluated computationally for their high temperature stability, life cycle and performance in microelectromechanical systems (MEMS). Ab initio calculations at sufficiently high level of theory were conducted on model compounds to predict the bond strengths holding the monolayer tethered to the MEMS device and relate them to its thermal stability. Non-equilibrium molecular dynamics (NEMD) simulations under sliding periodic boundary conditions were employed to compute the frictional force as a function of applied load. The NEMD trajectories were analyzed for the structure and chain dynamics of the SAMs and compared with NEMD and equilibrium MD results for the fluid. The significance of monolayer penetration depth, monolayer gauche fraction, wall thermostat characteristics and the size of the simulation box on the computed results were investigated.
Microstructural Changes to Xenon Naonoclusters in Aluminum under 1 MeV Electron Irradiation.
Authors: Donnelly S. E. , Birtcher R. C. , Allen C. W. , Furuya K. , Song M. , Mitsuishi K. 
Published: April 2011

ABSTRACTAluminum films containing solid Xe precipitates have been subjected to 1 MeV electron irradiation in a high-voltage electron microscope. High-resolution images have been recorded on videotape in order to monitor the changes to the system resulting from the passage of electrons through the film. Inspection of the video recordings reveals that complex, rapid processes occur under the electron beam. These include shape changes, the creation and movement of extended defects within the Xe lattice, movement of small clusters, coalescence of neighboring clusters and the apparent melting and resolidification of the Xe. An interpretation of many of the observations is presented in terms of the interaction of the nanoclusters with defects created in the aluminum by the high-energy electrons.
Inhomogeneous Swelling Near Grain Boundaries in Irradiated Materials: Cavity Nucleation and Growth Saturation Effects
Authors: Dudarev S. L. 
Published: April 2011

ABSTRACTThe effect of inhomogeneous nucleation and growth of cavities near grain boundaries illustrates the failure of the standard rate theory to describe the kinetics of phase transformations in irradiated materials under cascade damage conditions. The enhanced swelling observed near grain boundaries is believed to result from the competition between the diffusional growth of cavities and their shrinkage due to the interaction with mobile interstitial clusters. Swelling rates associated with the two processes behave in a radically different way as a function of the size of growing cavities. For a spatially homogeneous distribution of cavities this gives rise to the saturation of swelling in the limit of large irradiation doses.We investigate the evolution of the population of cavities nucleating and growing near a planar grain boundary. We show that a cavity growing near the boundary is able to reach a size that is substantially larger than the size of a cavity growing in the interior region of the grain. For a planar grain boundary the magnitude of swelling at maximum is found to be up to eight times higher than the magnitude of swelling in the grain interior.
The Effect of Bulk Composition on Swelling and Radiation-Induced Segregation in Austenitic Alloys
Authors: Allen T. R. , Cole J. I. , Dietz N. L. , Wang Y. , Was G. S. , Kenik E. A. 
Published: April 2011

ABSTRACTChanges in bulk composition are known to affect both radiation-induced segregation and microstructural development, including void swelling in austenitic stainless steel. In this work, three alloys (designations corresponding to wt%) have been studied: Fe-18Cr-8Ni alloy (bulk composition corresponding to 304 stainless steel), Fe-18Cr-40Ni (bulk composition corresponding to 330 stainless steel), and Fe-16Cr-13Ni (bulk composition corresponding to 316 stainless steel). Following irradiation with high-energy protons, the change in hardness and microstructure (void size distribution and grain boundary composition) due to irradiation was investigated. Increasing the bulk nickel concentration decreases void swelling, increases matrix hardening, and increases grain boundary chromium depletion and nickel enrichment. The analysis shows that decreases in lattice parameter and shear modulus due to radiation- induced segregation (RIS) correlate with decreased void swelling and a decreased susceptibility to irradiation assisted stress corrosion cracking (IASCC). Traditional thinking on IASCC assumed RIS was a contributing factor to cracking. It may, however, be that properly controlled RIS can be used to mitigating cracking.
Atomistic Mechanism of Nucleation and Growth of Voids in Cu Studied by Computer Simulation
Authors: Shimomura Y. , Mukouda I. 
Published: April 2011

ABSTRACTA TEM observation of fission neutron-irradiated copper at 300°C shows that the maximum size of stacking fault tetrahedra (sft) observed is 6 nm of edge length which corresponds to a cluster of 280 vacancies and the minimum size of voids is 2.2 nm in diameter which corresponds to a cluster of 470 vacancies. The result suggests that a vacancy cluster whose size is smaller than 300 vacancies grows to sft while a cluster whose size is larger than 500 vacancies relaxes to a void in 300°C-irradiated copper. A computer simulation of molecular dynamics (MD) with an isotropic EAM potential examined this model. It is found that a vacancy cluster that is smaller than 300 vacancy segregates to a (111) platelet and relaxes to an sft. Small vacancy clusters which are generated at damage cascade cores aggregate to spherically distributed vacancies for the size of more than 500 vacancies, and relax to several (111) platelets, which finally form a vacancy (111) polyhedron. Inside a polyhedral vacancy platelet, vacancies are confined and grow to a void at high temperature.
Mobility of Interstitial Clusters in HCP Zirconium
Authors: de Diego Nieves , Osetsky Yuri N. , Bacon David J. 
Published: April 2011

ABSTRACTClusters of self-interstitial atoms (SIAs) formed in displacement cascades in metals irradiated with energetic particles play an important role in microstructure evolution under irradiation. They have been studied in the fcc and bcc metals by atomic scale computer simulation and in this paper we present the first results of a similar study of SIA clusters in an hcp crystal. Clusters of 4 to 30 SIAs were modelled over a wide temperature range using molecular dynamics and a many-body Finnis-Sinclair type interatomic potential for Zr. The results show a qualitative similarity of the dynamic properties of clusters to those for cubic metals. In particular, all clusters larger than 4 SIAs exhibit fast thermally-activated one-dimensional glide, which is in a <1120> direction in the hcp case. Due to the crystallographic features of the hcp lattice, this mechanism leads to two-dimensional mass transport in basal planes. Smaller clusters (≤ 4 SIAs) exhibit behaviour peculiar to the hcp structure, however, for they can migrate two-dimensionally in the basal plane. The jump frequency, activation energy and correlation factors of clusters have been estimated and comparison drawn between the behaviour of SIA clusters in different structures.
Effect of Annealing and Microwave Hydrogen Plasma Treatment on Structural, Chemical, and Electronic Properties of Ion Irradiated Diamond Films
Authors: Laikhtman Alexander , Hoffman Alon 
Published: April 2011

ABSTRACTIn the present study we correlate between the secondary electron emission (SEE) of variously treated Xe+ ion-damaged diamond films and their bonding structure in the near-surface region as identified by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy. The 50 keV Xe+ ion bombardment of hydrogenated polycrystalline diamond films to a dose of 2×1015 cm−2 results in the transformation of near-surface diamond to sp2-bonded amorphous carbon, increased oxygen adsorption, shift of the electron affinity from negative to positive, and strong degradation of its electron emission properties, although it does not induce a pronounced depletion of hydrogen. Exposure of the ion-bombarded films to microwave (MW) hydrogen plasma treatment for 30 min produces negative electron affinity diamond surfaces, but only partially regenerates SEE properties, retains some imperfection in the near-surface atomic layers, as determined by NEXAFS, and the concentration of oxygen remains relatively high. Subsequent annealing to 610 °C produces oxygen-free diamond films and somewhat increases their SEE. Annealing to 1000 °C results in desorption of the surface hydrogen, formation of a positive electron affinity surfaces and drastically degrades their electron emission properties. Prolonged, up to three hours MW hydrogen plasma treatment of as-implanted diamond films gradually improves the crystal quality and results in further increase of SEE intensity. This treatment does not, however, substantially reduce the concentration of oxygen in the previously damaged diamond, indicating its bulk diffusion during or after ion bombardment. To fully recover electron emission properties it is necessary to both remove the defects and hydrogenate the diamond surfaces.
Surface and Bulk Microstructural Modifications in Amorphous Carbon Films after Post-Growth Low Energy Ion Beam Irradiation
Authors: Patsalas P. , Logothetidis S. 
Published: April 2011

ABSTRACTWe present the crystallization effects occurring in sputtered amorphous Carbon (a-C) thin films deposited on Si induced by post-growth low energy (0.5-1.5 keV) Ar+ ion beam irradiation (IBI). The a-C films after IBI have the form of an amorphous matrix with embedded crystalline regions. X-ray diffraction and Electron Microscopy measurements identified the crystalline phases of carbon and SiC. We study in detail the effects of ion energy and fluence on the crystallization process. It was found that low fluence (∼2×1016 ions/cm2) of ions with an optimum ion energy (∼1.5 keV) promoted the diamond formation. X-Ray Reflectivity (XRR) and Spectroscopic Ellipsometry were used to study the amorphous matrix. XRR discriminated the IBI induced surface and bulk effects through the density and the a-C surface roughness, showing surface smoothing to be more prominent for low energy IBI.