In situ nanomechanical testing provides critical insight into the fundamental processes that lead to deformation phenomena in materials. Often, in situ tests are performed in relevant conditions such as high or low temperatures, tribological contact, gas environments, or under radiation exposure. Modern diffraction and imaging methods of materials under load provide high spatial resolution and enable extraction of quantitative mechanical data from local microstructure components or nano-sized objects. The articles in this issue cover recent advances in different types of in situ nanomechanical testing methods, spanning from dedicated nanomechanical testing platforms and microelectromechanical systems devices to deformation analyses via in situ diffraction and imaging methods. This includes scanning electron microscopy, advanced scanning transmission electron microscopy, electron diffraction, x-ray diffraction, and synchrotron techniques. Emerging areas such as in situ tribology enable novel insights into the origin of deformation mechanisms, while the evolution of microelectromechanical systems for controlled in situ testing provide opportunities for advanced control and loading strategies. Discussion on the current state of the art for in situ nanomechanical testing and future opportunities in imaging, strain sensing, and testing environments are also addressed.

Stresses in thermally grown SiO2 films on silicon have traditionally been determined by substrate curvature measurements. This technique is useful for studying stresses in thin films, but it cannot be used to investigate stresses generated in the substrate during film growth. In the present work, we used microelectromechanical systems-based microstrain gauge devices fabricated from single-crystal and polycrystalline silicon (henceforth silicon and polysilicon, respectively) to measure oxidation-induced stresses in both dry and wet oxidizing ambients. Our microstrain gauges had thicknesses on the micrometer scale, and were themselves used as the substrates to be oxidized. Stresses could be detected in both the SiO2 scales and the silicon and polysilicon substrates. In the SiO2 scales, the stresses were compressive and exhibited viscoelastic relaxation. The as-grown compressive stresses were greater for wet oxidation than they were for dry oxidation, and greater in scales grown on polysilicon than they were in scales grown on silicon. The viscosity of thermally grown SiO2 was the same whether scales formed by wet or dry oxidation, and the same for oxide scales on silicon and polysilicon. Significant compressive stresses were also generated in polysilicon during oxidation, but not in silicon.

The unique capability of rendering opaque specimens transparent with atomic resolution makes transmission electron microscopy (TEM) an indispensable toolfor microstructural and crystallographic analysis of materials. Conventional TEM specimens are placed on grids about 3 mm in diameter and 10–100 μm thick. Such stringent size restriction has precluded mechanical testing inside the TEM chamber.So far, in situ testing of nanoscale thin foils has been mostly qualitative. Micro-electro-mechanical systems (MEMS) offer an unprecedented level of miniaturization to realize sensors and actuators that can add TEM visualization to nano-mechanical characterization. We present a MEMS-based uniaxial tensile experiment setup that integrates nanoscale freestanding specimens with force and displacement sensors, which can be accommodated by a conventional TEM straining stage. In situ TEM testing on 100-nm-thick freestanding aluminum specimens (with simultaneous stress measurement) show limited dislocation activity in the grain interior and consequent brittle mode of fracture. Plasticity at this size scale is contributed by grain boundary dislocations and partial dislocations.

We have designed and fabricated a standardized specimen holder that allows the operation of a microelectromechanical system (MEMS) device inside a transmission electron microscope (TEM). The details of the design and fabrication processes of the holder are presented. The sample loading mechanism is simple and allows reliable electrical contact to eight signal lines on the device. Using a MEMS-based, nanojoule calorimeter, we performed rapid-heating experiments on Bi nanoparticles to demonstrate the functionality of the holder. We show that the heat capacity can be measured simultaneously with TEM observations. The size-dependent melting of Bi nanoparticles was observed simultaneously by nanocalorimetry and selected area diffraction measurements. We believe this approach will open up new experimental pathways to researchers, combining the speed and resolution of transmission electron microscopy with the flexibility, precision, and compactness of MEMS-based sensors and actuators.

The crystallization and phase transformations of amorphous NiTi thin films were studied using in situ transmission electron microscopy (TEM). These films were sputter-deposited onto micromachined silicon-nitride membranes and subjected to heating and cooling conditions. The microstructural evolution was monitored and recorded. Kinetic parameters such as the nucleation rate, growth rate, and area-fraction transformed were independently determined by noting the number of grains per frame and their change in size. Using the Johnson–Mehl–Avrami–Kolmogorov analysis, fitted kinetic parameters were determined and found to be consistent with TEM observations. To explore the compositional sensitivity of crystallization, samples near-equiatomic and slightly Ti-rich were studied with these methods. TEM micrographs show that equiatomic films exhibit polymorphic crystallization while samples that are slightly off-stoichiometry showed more complicated behavior.

The Lithographie, Galvanoformung, Abformung (LIGA) technique is important for making metal-based high-aspect-ratio microscale structures (HARMS) and microdevices derived from metal-based HARMS. Recently, molding replication of HARMS made of Pb, Zn, and Al has been demonstrated, advancing LIGA technology from the state where only polymer-based HARMS could be replicated by molding. This demonstration offers a potential means for economical fabrication of a wide variety of metal-based microdevices. Micromolding of a metal requires heating the metal to be molded to a significant fraction of its melting temperature. At high temperatures, the strength of the mold insert itself will typically decrease. The insert strength thus places a limit on the range of materials that can be molded. In this paper, micromolding and tensile experiments on Pb were carried out. A simple mechanics model of the micromolding process was developed. This model relates the stresses on the insert during micromolding primarily to the yield strength of the molded metal and frictional tractions on the sides of the insert. Reasonable agreement was obtained between the Pb experiments and the model predictions. Ramifications for other material systems are discussed.

The synthesis and characterization of crystalline tungsten disulphide (WS2) solid lubricant thin films grown by atomic layer deposition (ALD) using WF6 and H2S gas precursors was studied. A new catalytic route was established to promote nucleation and growth of WS2 films on silicon surfaces with native oxide. Scanning electron microscopy with energy dispersive spectroscopy and Raman spectroscopy were used to determine the film morphology, composition, and crystallinity. The films exhibited solid lubricating behavior with a steady-state friction coefficient of 0.04 in a dry nitrogen environment.

Micromachined fuel cells are among a class of microscale devices being explored for portable power generation. In this paper, we report processing and geometric design criteria for the fabrication of free-standing electrolyte membranes for microscale solid-oxide fuel cells. Submicron, dense, nanocrystalline yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) films were deposited onto silicon nitride membranes using electron-beam evaporation and sputter deposition. Selective silicon nitride removal leads to free-standing, square, electrolyte membranes with side dimensions as large as 1025 μm for YSZ and 525 μm for GDC, with high processing yields for YSZ. Residual stresses are tensile (+85 to +235 MPa) and compressive (–865 to -155 MPa) in as-deposited evaporated and sputtered films, respectively. Tensile evaporated films fail via brittle fracture during annealing at temperatures below 773 K; thermal limitations are dependent on the film thickness to membrane size aspect ratio. Sputtered films with compressive residual stresses show superior mechanical and thermal stability than evaporated films. Sputtered 1025-μm membranes survive annealing at 773 K, which leads to the generation of tensile stresses and brittle fracture at elevated temperatures (923 K).

Although crystalline silicon is not generally considered susceptible to fatigue crack growth, recent studies suggest that there may be fatigue processes in silicon micro-machined structures. In the present study, a micro-machined fracture specimen geometry was used to examine stable crack growth under fatigue loading. Crack length and loads were carefully monitored throughout the test to distinguish between environmentally assisted crack growth (stress corrosion) and mechanically induced fatigue-crack growth. Results revealed similar steplike crack extension versus time for the cyclic and monotonic tests. The fatigue crack-growth curve extracted from the crack extension data exhibited a nearly vertical slope with no evidence of accelerated crack-growth rates. Fracture surfaces for the monotonic and cyclic tests were similar, further suggesting that a true mechanical fatigue crack-growth mechanism did not occur. Explanations for the observed lack of fatigue crack growth are presented and discussed with respect to reported stress-life behavior.

The effect of environmental exposure on the fracture characteristics of two polymer–metal interfaces was measured using a four-point bend delamination test. Films of high-molecular-weight polymethylmethacrylate (PMMA) resin were spin-cast on metal-coated silicon wafer substrates, for which the metal was either titanium or aluminum. Sandwich beam specimens were fabricated and stored in one of the following conditions prior to the bending tests: vacuum desiccator at 25 °C, vacuum desiccator at 65 °C, distilled water at 25 °C, or distilled water at 65 °C. Load–displacement behavior measured during bending revealed significant differences between the delamination characteristics of the two PMMA–metal interfaces after vacuum exposure which were eliminated after the degrading effects of water exposure. Three distinctive load–displacement behaviors were observed: plateau, fracture at a single load; R-curve, fracture at increasing load with crack extension; and stick-slip, cycles of gradual load increase and sudden load drop with crack extension. PMMA–Al samples stored in vacuum desiccator at 25 °C exhibited R-curve fracture and the largest average crack driving force, GC, 12.2 (±0.5) J/m2, of all samples tested. After storage in 25 °C water, these PMMA–Al samples exhibited stick-slip fracture and GC decreased to 7.1 (±2.6) J/m2; storage in 65 °C water further decreased GC to 2.1 (±0.7) J/m2. PMMA–Ti samples exhibited stick-slip fracture after storage in vacuum desiccator at 25 °C, with an average GC of 8.0 (±2.6) J/m2; storage in 65 °C water resulted in a transition to plateau fracture and a decrease in GC to 1.6 (±0.3) J/m2. The initial difference and subsequent similarity are interpreted in terms of surface roughness and hydrolysis, respectively.