[en] Mechanical properties of sintered silver nanoparticles are investigated via substrate curvature and nanoindentation methods. Substrate curvature measurements reveal that permanent microstructural changes occur during initial heating while subsequent annealing results in nearly elastic behavior of the thinner films. Thicker films were found to crack upon thermal treatment. The coefficient of thermal expansion was determined from linear slopes of curvature curves to be 1.9±0.097 ppm/ degree sign C, with elastic modulus and hardness determined via nanoindentation. Accounting for substrate effects, nanoindentation hardness and modulus remained constant for different film thicknesses and did not appear to be a function of annealing conditions. Hardness of 0.91 GPa and modulus of 110 GPa are somewhat lower than expected for a continuous nanocrystalline silver film, most likely due to porosity

[en] In situ mechanical tests were carried out to measure the tensile behavior of single-crystalline face-centered cubic (fcc) gold (Au) and body-centered cubic (bcc) molybdenum (Mo) nano-pillars with diameters between 250 and 1 μm, and to compare this with the compression results of these materials at the equivalent sizes. In Au, we observed similar tensile and compressive flow stresses at ∼10% strain although strain-hardening in tension is somewhat more pronounced than it is in compression. In Mo, the amount of strain-hardening in tension is significantly lower than that in compression, leading to a distinct tension-compression asymmetry in the flow stress at ∼5% strain. The dissimilarities between tensile and compressive behavior in both crystals are discussed in terms of sample geometry constraints and dislocation behavior in bcc crystals.

[en] In situ nanomechanical tests are carried out to investigate the tensile and compressive behavior of <0 0 1>-oriented body-centered cubic (bcc) metals W, Mo, Ta and Nb with nanometer dimensions. We find that the strength of these metals exhibits strong size dependence. The compressive size effect in Nb, as evaluated by the log-log slope of strength vs. nanopillar diameter, is -0.93, a factor of 2.1 greater than that for the other three metals W, Mo and Ta (-0.44). In tension, however, Ta and Nb show higher size effect slopes (-0.80 and -0.77) as compared with W and Mo (-0.58 and -0.43). We also report that while the yield strength of these metals is a strong function of size, the strain-hardening behavior does not present any size-dependent trends. We further discuss the effects of strain-rate on deformation behavior and provide transmission electron microscopy analysis of microstructural evolution in the same Mo nanopillar before and after compression.

[en] Size-dependent deformation behavior of niobium single-crystalline samples 200-900 nm in diameter is investigated by in situ compression and tension testing. Stress-strain curves in compression show discrete plastic flow associated with dislocation escapes at the surface. In tension, plastic flow is more continuous with pronounced strain hardening, and fracture occurs at the low true strain of ∼3%. Transmission electron microscopy analysis of the same 100 nm nanopillar before and after deformation provides insights into dislocation activity during compression deformation.

[en] We report results of uniaxial compression experiments on single-crystalline Cu nanopillars with nonzero initial dislocation densities produced without focused ion beam (FIB). Remarkably, we find the same power-law size-driven strengthening as FIB-fabricated face-centered cubic micropillars. TEM analysis reveals that initial dislocation density in our FIB-less pillars and those produced by FIB are on the order of 10¹⁴ m^-2 suggesting that mechanical response of nanoscale crystals is a stronger function of initial microstructure than of size regardless of fabrication method.

[en] We compare mechanical strength of f.c.c. gold and b.c.c. molybdenum single crystal pillars of sub-micrometer diameter in uniaxial compression tests. Both crystals show an increase of flow stress with decreasing diameter, but the change is more pronounced in Au than in Mo. The ratio between the observed maximum flow stress and the theoretical strength is much larger in Au pillars than in Mo pillars. Dislocation dynamics simulations also reveal different dislocation behavior in these two metals. While in a f.c.c. crystal a dislocation loop nucleated from the surface simply moves on its glide plane and exits the pillar, in a b.c.c. crystal it can generate multiple new dislocations due to the ease of screw dislocations to change slip planes. We postulate that this difference in dislocation behavior is the fundamental reason for the observed difference in the plastic deformation behavior of f.c.c. and b.c.c. pillars

[en] We investigate the mechanical behavior of 3D periodically architected metallic glass nanolattices, constructed from hollow beams of sputtered Zr-Ni-Al metallic glass. Nanolattices composed of beams with different wall thicknesses are fabricated by varying the sputter deposition time, resulting in nanolattices with median wall thicknesses of ∼88 nm, ∼57 nm, ∼38 nm, ∼30 nm, ∼20 nm, and ∼10 nm. Uniaxial compression experiments conducted inside a scanning electron microscope reveal a transition from brittle, catastrophic failure in thicker-walled nanolattices (median wall thicknesses of ∼88 and ∼57 nm) to deformable, gradual, layer-by-layer collapse in thinner-walled nanolattices (median wall thicknesses of ∼38 nm and less). As the nanolattice wall thickness is varied, large differences in deformability are manifested through the severity of strain bursts, nanolattice recovery after compression, and in-situ images obtained during compression experiments. We explain the brittle-to-deformable transition that occurs as the nanolattice wall thickness decreases in terms of the “smaller is more deformable” material size effect that arises in nano-sized metallic glasses. This work demonstrates that the nano-induced failure-suppression size effect that emerges in small-scale metallic glasses can be proliferated to larger-scale materials by the virtue of architecting.

[en] We have performed mechanical and electrical characterization of individual as-grown, vertically oriented carbon nanofibers (CNFs) using in situ techniques, where such high-aspect-ratio, nanoscale structures are of interest for three-dimensional (3D) electronics, in particular 3D nano-electro-mechanical-systems (NEMS). Nanoindentation and uniaxial compression tests conducted in an in situ nanomechanical instrument, SEMentor, suggest that the CNFs undergo severe bending prior to fracture, which always occurs close to the bottom rather than at the substrate-tube interface, suggesting that the CNFs are well adhered to the substrate. This is also consistent with bending tests on individual tubes which indicated that bending angles as large as ∼ 70⁰ could be accommodated elastically. In situ electrical transport measurements revealed that the CNFs grown on refractory metallic nitride buffer layers were conducting via the sidewalls, whereas those synthesized directly on Si were electrically unsuitable for low-voltage dc NEMS applications. Electrostatic actuation was also demonstrated with a nanoprobe in close proximity to a single CNF and suggests that such structures are attractive for nonvolatile memory applications. Since the magnitude of the actuation voltage is intimately dictated by the physical characteristics of the CNFs, such as diameter and length, we also addressed the ability to tune these parameters, to some extent, by adjusting the plasma-enhanced chemical vapor deposition growth parameters with this bottom-up synthesis approach.

[en] We report a focused-ion-beam-less fabrication route for producing 100 nm-diameter bi-material nano-pillars comprising single crystalline ∼〈1 1 0〉-oriented Fe and ∼〈1 1 1〉-oriented Cu. Uniaxial compression tests revealed that these pillars exhibit strengths around 1 GPa, without failure at the Cu–Fe interface. The pillars represent a prototyped metal–matrix nano-composite with highly controllable microstructure, high strengths, and strong metal–metal interfaces. The isolation and engineering of a single interface in a single pillar allow for fundamental insights gained into their deformation mechanisms.

[en] Classical laws of mechanics hold that mechanical properties are independent of sample size; however, results of experiments and molecular dynamics simulations indicate that crystals exhibit strong size effects at the sub-micron scale. In experimental studies, the size effect can be explained by strain gradients. Atomistic simulations suggest that the yield strength depends on the size even without strain gradients and scales with the sample size through a power relationship. We address these different approaches to the size dependence of mechanical properties. Results of uniaxial compression experiments on gold at the sub-micron scale, without stress/strain gradients, are presented here. Freestanding Au cylinders are created by two unique fabrication processes and subsequently compressed in the Nanoindenter with a flat punch. Compressive stress, strain, and stiffness of the pillars are determined. Test results indicate a significant flow stress increase, up to several GPa. These high strengths appear to be controlled by dislocation starvation, unique to small crystals