Our study (1) reported on the deformation response of nanocrystalline Ni during in situ dark-field transmission electron microscopy (DFTEM) straining experiments and showed what we view as direct and compelling evidence of grain boundary-mediated plasticity. Based on their analysis of the limited experimental data we presented, however, Chen and Yan (2) propose that the reported contrast changes more likely resulted from grain growth caused by electron irradiation and applied stress rather than from plastic deformation. Here, we give specific reasons why their assertions are incorrect and discuss how the measurement approaches they have used are inappropriate. Additionally, we present further evidence that supports our original conclusions. The method Chen and Yan employed to measure displacement merely probes the in-plane (two-dimensional) components of incremental strain occurring during the very short time interval shown [figure 3 in (1)] instead of the accumulated strain. As we noted explicitly in the supporting online material in (1), the loading was applied by pulsing the displacement manually. After each small displacement pulse, the monitored area always moved significantly within or even out of the field of view. Clear images could be obtained only when the sample position stabilized within the field of view, and at that time severe deformation was nearly complete. Thus, little incremental strain occurs during this short image sequence [figure 3 in (1)], as one might expect. We believe that the images shown in figure 3 of (1) are particularly valuable in understanding deformation in nanocrystalline materials. In general, the formation process of grain agglomerates simply occurred too fast to be recorded clearly. Moreover, instead of remaining constant after formation, the sizes of the grain agglomerates changed in a rather irregular manner in responding to the deformation and fracture process (see, for example, Fig. 1, B to D). This indicates that strong grain boundary-related activity occurred inside the grain agglomerates. Figure 3 in (1), a short (0.5 s) extract from more than 6 hours of videotaped experimentation (imaged ahead of cracks), not only reveals the formation process of a grain agglomerate, but also shows conclusive evidence for grain rotation and excludes the effect of overall sample rotation. It should be noted that other small grains still exhibit some minor contrast changes in figure 3 in (1). Hence, using them as reference points yields measurements that may not be accurate to {+-}1 nm [as Chen and Yan (2) claim in their analysis] and limits the accuracy of their conclusions. Chen and Yan also claim that no deformation has occurred, yet simultaneously state that the analysis has a deformation measurement error of 0.5%. This is simply not consistent; even small strains of this order may cause plastic deformation. In contrast with previous in situ TEM experiments (3-5), the special sample design adopted in our investigation (1) ensured that all deformation was primarily concentrated in a bandlike area ahead of the propagating crack. We found that these grain agglomerates were observed only in this bandlike thinning area as a response to the applied loads (Fig. 1B). No similar phenomena were detected under the electron beam alone or in stressed areas apart from the main deformation area, and these phenomena have not been reported during in situ observations of this same material made by other researchers (5). Subsequent cracks were always observed to follow this deformation area upon further displacement pulses (Fig. 1, C and D). This clearly indicates that the enlarged agglomerates do not result simply from electron irradiation plus stress, but rather from stress-induced deformation. In their comment, Chen and Yan claimed a linear relation between 'grain' area and time based on their measurements made from figure 3 in (1) and claimed that these measurements are exactly consistent with the classical grain growth equation. However, as we noted (1), the growth in size of this agglomerate is not isotropic and occurs in an irregular manner. For example, after bright contrast emerged from a grain about 6 nm in diameter, it remained well defined in size as a single, approximately equiaxed grain until t = 0.1 s (fig. S1). We have reproduced the 'grain growth' plot of Chen and Yan (Fig. 2) using our entire video image sequence (fig. S1). Clearly, the growth in area of the agglomerate is not consistent with linear grain growth. (Unfortunately, only a portion of these data could be included in the original paper for reasons of space.) Notably, Chen and Yan did not apply a similar 'grain growth' analysis to nearby grains; this would have yielded no information in support of their argument, as those grains show essentially no growth.
The plastic behavior of crystalline materials is mainly controlled by the nucleation and motion of lattice dislocations. We report in situ dynamic transmission electron microscope observations of nanocrystalline nickel films with an average grain size of about 10 nanometers, which show that grain boundary-mediated processes have become a prominent deformation mode. Additionally, trapped lattice dislocations are observed in individual grains following deformation. This change in the deformation mode arises from the grain size-dependent competition between the deformation controlled by nucleation and motion of dislocations and the deformation controlled by diffusion-assisted grain boundary processes.
Multiple scattering effects in ERD measurements are studied by comparing two Monte Carlo simulation codes, representing different approaches to obtain acceptable statistics, to experimental spectra measured from a HfO2 sample with a time-of-flight-ERD setup. The results show that both codes can reproduce the absolute detection yields and the energy distributions in an adequate way. The effect of the choice of the interatomic potential in multiple scattering effects is also studied. Finally the capabilities of the MC simulations in the design of new measurement setups are demonstrated by simulating the recoil energy spectra from a WCxNy sample with a low energy heavy ion beam. Published by Elsevier B.V.
We give the results of a study using Monte Carlo ion interaction codes to simulate and optimize elastic recoil detection analysis for {sup 3}He buildup in tritide films. Two different codes were used. The primary tool was MCERD, written especially for simulating ion beam analysis using optimizations and enhancements for greatly increasing the probabilities for the creation and the detection of recoil atoms. MPTRIM, an implementation of the TRIMRC code for a massively parallel computer, was also used for comparison and for determination of absolute yield. This study was undertaken because of a need for high-resolution depth profiling of 3He and near-surface light impurities (e.g. oxygen) in metal hydride films containing tritium.
Thin-film mechanical properties can be measured using nanoindentation combined with detailed finite element modeling. This technique was used for a study of very fine grained Ni films, formed using pulsed-laser deposition on fused silica, sapphire, and Ni substrates. The grain sizes in the films were characterized by electron microscopy, and the mechanical properties were determined by ultra-low load indentation, analyzed using finite element modeling to separate the mechanical properties of the thin layers from those of the substrates. Some Ni films were deposited at high temperature or annealed after deposition to enlarge the grain sizes. The observed hardnesses and grain sizes in these thin Ni films are consistent with the empirical Hall-Petch relationship for grain sizes ranging from a few micrometers to as small as 10 nm, suggesting that deformation occurs preferentially by dislocation movement even in such nanometer-size grains.
The Microsystems Subgrid Physics project is intended to address gaps between developing high-performance modeling and simulation capabilities and microdomain specific physics. The initial effort has focused on incorporating electrostatic excitations, adhesive surface interactions, and scale dependent material and thermal properties into existing modeling capabilities. Developments related to each of these efforts are summarized, and sample applications are presented. While detailed models of the relevant physics are still being developed, a general modeling framework is emerging that can be extended to incorporate evolving material and surface interaction modules.
Ion implantation of O and Al were used to form nanometer-size precipitates of NiO or Al{sub 2}O{sub 3} in the near-surface of Ni. The yield strengths of the treated layers were determined by nanoindentation testing in conjunction with finite-element modeling. The strengths range up to {approximately}5 GPa, substantially above values for hard bearing steels. These results agree quantitatively with predictions of dispersion-hardening theory based on the precipitate microstructures observed by transmission electron microscopy. Such surface hardening by ion implantation may be beneficial for Ni components in micro-electromechanical systems.
Very fine-grained Ni and Cu films were formed using pulsed laser deposition onto fused silica substrates. The grain sizes in the films were characterized by electron microscopy, and the mechanical properties were determined by ultra-low load indentation, with finite-element modeling used to evaluate the properties of the layers separately from those of the substrate. Some Ni films were also examined after annealing to 350 and 450 °C to enlarge the grain sizes. These preliminary results show that the observed hardnesses are consistent with a simple extension of the Hall-Petch relationship to grain sizes as small as 11 nm for Ni and 32 nm for Cu.
The authors have developed a new experimental approach for measuring hysteresis in the adhesion between micromachined surfaces. By accurately modeling the deformations in cantilever beams that are subject to combined interfacial adhesion and applied electrostatic forces, they determine adhesion energies for advancing and receding contacts. They draw on this new method to examine adhesion hysteresis for silane coated micromachined structures and found significant hysteresis for surfaces that were exposed to high relative humidity (RH) conditions. Atomic force microscopy studies of these surfaces showed spontaneous formation of agglomerates that they interpreted as silages that have irreversibly transformed from uniform surface layers at low RH to isolated vesicles at high RH. They used contact deformation models to show that the compliance of these vesicles could reasonably account for the adhesion hysteresis that develops at high RH as the surfaces are forced into contact by an externally applied load.
Due to extreme surface to volume ratios, adhesion and friction are critical properties for reliability of Microelectromechanical Systems (MEMS), but are not well understood. In this LDRD the authors established test structures, metrology and numerical modeling to conduct studies on adhesion and friction in MEMS. They then concentrated on measuring the effect of environment on MEMS adhesion. Polycrystalline silicon (polysilicon) is the primary material of interest in MEMS because of its integrated circuit process compatibility, low stress, high strength and conformal deposition nature. A plethora of useful micromachined device concepts have been demonstrated using Sandia National Laboratories' sophisticated in-house capabilities. One drawback to polysilicon is that in air the surface oxidizes, is high energy and is hydrophilic (i.e., it wets easily). This can lead to catastrophic failure because surface forces can cause MEMS parts that are brought into contact to adhere rather than perform their intended function. A fundamental concern is how environmental constituents such as water will affect adhesion energies in MEMS. The authors first demonstrated an accurate method to measure adhesion as reported in Chapter 1. In Chapter 2 through 5, they then studied the effect of water on adhesion depending on the surface condition (hydrophilic or hydrophobic). As described in Chapter 2, they find that adhesion energy of hydrophilic MEMS surfaces is high and increases exponentially with relative humidity (RH). Surface roughness is the controlling mechanism for this relationship. Adhesion can be reduced by several orders of magnitude by silane coupling agents applied via solution processing. They decrease the surface energy and render the surface hydrophobic (i.e. does not wet easily). However, only a molecular monolayer coats the surface. In Chapters 3-5 the authors map out the extent to which the monolayer reduces adhesion versus RH. They find that adhesion is independent of RH up to a threshold value, depending on the coating chemistry. The mechanism for the adhesion increase beyond this threshold value is that the coupling agent reconfigures from a surface to a bulk phase (Chapter 3). To investigate the details of how the adhesion increase occurs, the authors developed the mechanics for adhesion hysteresis measurements. These revealed that near-crack tip compression is the underlying cause of the adhesion increase (Chapter 4). A vacuum deposition chamber for silane coupling agent deposition was constructed. Results indicate that vapor deposited coatings are less susceptible to degradation at high RH (Chapter 5). To address issues relating to surfaces in relative motion, a new test structure to measure friction was developed. In contrast to other surface micromachined friction test structures, uniform apparent pressure is applied in the frictional contact zone (Chapter 6). The test structure will enable friction studies over a large pressure and dynamic range. In this LDRD project, the authors established an infrastructure for MEMS adhesion and friction metrology. They then characterized in detail the performance of hydrophilic and hydrophobic films under humid conditions, and determined mechanisms which limit this performance. These studies contribute to a fundamental understanding for MEMS reliability design rules. They also provide valuable data for MEMS packaging requirements.