Douglas Stauffer

High Cycle Fatigue in the Transmission Electron Microscope

One of the most common causes of structural failure in metals is fatigue induced by cyclic loading. Historically, microstructure-level analysis of fatigue cracks has primarily been performed post mortem. However, such investigations do not directly reveal the internal structural processes at work near micro- and nanoscale fatigue cracks and thus do not provide direct evidence of active microstructural mechanisms. In this study, the tension-tension fatigue behavior of nanocrystalline Cu was monitored in real time at the nanoscale by utilizing a new capability for quantitative cyclic mechanical loading performed in situ in a transmission electron microscope (TEM). Controllable loads were applied at frequencies from one to several hundred hertz, enabling accumulations of 10(6) cycles within 1 h. The nanometer-scale spatial resolution of the TEM allows quantitative fatigue crack growth studies at very slow crack growth rates, measured here at ∼10(-12) m·cycle(-1). This represents an incipient threshold regime that is well below the tensile yield stress and near the minimum conditions for fatigue crack growth. Evidence of localized deformation and grain growth within 150 nm of the crack tip was observed by both standard imaging and precession electron diffraction orientation mapping. These observations begin to reveal with unprecedented detail the local microstructural processes that govern damage accumulation, crack nucleation, and crack propagation during fatigue loading in nanocrystalline Cu.

In Situ Electromechanical Study of ZnO Nanowires

One-dimensional structures such as nanowires and nanotubes are potential candidates for nanoelectronic, optoelectronic, piezoelectric devices, sensors, actuators, etc. Due to length scale effects and higher surface-to-volume ratios, nanostructures exhibit superior mechanical and electrical, as well as other length scale dependent properties [1,2]. To utilize these fundamental advantages, it is essential to investigate and understand their unique characteristics as a function of the material parameters. In spite of the great technological progress that has been made during the last decade to characterize nanostructured materials, comprehensive electromechanical characterization of a single individual nanowire is still a challenging task. In this study, a MEMS-based uniaxial nanotensile testing device E-PTP (electrical push-to-pull) with integrated four-probe electrical contacts was used for electromechanical characterization of a single ZnO nanowire, Fig. 1a and 1b.

High Plastic Strain of Silica Microparticles under Electron Beam Irradiation

The studies of irradiation damage in silica are of significant interest because of its application in nuclear reactors, nuclear waste containers, optical fibers, and semiconductor devices [1,2]. Although there are a number of publication showing the effect of electrons, ions, protons, alpha-particles irradiation on microstructural changes of silica, understanding deformation behavior under applied stress of irradiated sample is still lacking [2,3]. In this work, we investigate plastic flow and failure behavior of amorphous silica particles under compressive stress inside a scanning electron microscopy (SEM).

Connectivity between plasticity and brittle fracture: An overview from nanoindentation studies

Nanoindentation and scanning probe microscopy techniques applied to deforming thin films, nanospheres, nanowires, and nanocrystalline structures have uncovered new mechanical property phenomena dependent on size scale. This overview addresses several segments — those associated with measurement by nanoindentation, as well as the resulting properties of elasticity and plasticity, and fracture toughness of semi-brittle crystals. Specific to volumes in indentation or compression, it is shown that both pressure and scale effects can become dominant for both elasticity and plasticity at sizes less than 100nm. A strong inverse relationship between yield strength and activation volume for both single crystal and nanocrystalline structures is reviewed. Equally strong is a relationship between fracture toughness, the number of shielding dislocations accommodating indentation prior to fracture, and the basic mechanical and physical properties of semi-brittle solids. The link between fracture toughness and plasticity in these semi-brittle materials is shown to be the activation volume for dislocation nucleation in these strong solids.

A Combined Effect of Electron Beam and Stress on Plastic Flow of Amorphous Silica Microparticles

Radiation induced plastic flow in amorphous silica glass is an important subject in glass science and technology and have been studied for decades by many researchers using high energy ions and particles. However, the deformation behavior of such material irradiated by low energy electrons is not well understood. In comparison to heavier particles and ions, electrons have much higher penetration depths and therefore can generate uniform damage and structural changes throughout the sample. In this study, we investigate plastic flow of silica particles under a combined effect of compressive stress and electron beam inside a scanning electron microscopy. To prepare the particle samples for compression experiments, silica microparticles of diameter ~1 μm were mixed in water, ultrasonicated for 10 minutes, and dispersed on silicon substrates. In situ compression experiments were conducted using a PI 85 SEM PicoIndenter (Hysitron, Inc., Minneapolis, MN) with 5 μm flat punch diamond probe. TriboScan software was used to record and analyze load-displacement data. The load-displacement plots and real-time video of deformation were synchronized and captured during the experiment, which aided post-experimental analysis. Quasistatic compression experiments were conducted at Pmax = 0.05 mN, 1 mN and 4 mN under different beam intensities.

An experimental methodology for the in-situ observation of the time-dependent dielectric breakdown mechanism in Copper/low-k on-chip interconnect structures

This study captures the time-dependent dielectric breakdown kinetics in nanoscale Cu/low-k interconnect structures, applying in-situ transmission electron microscopy (TEM) imaging and post-mortem electron spectroscopic imaging (ESI). A “tip-to-tip” test structure and an experimental methodology were established to observe the localized damage mechanisms under a constant voltage stress as a function of time. In an interconnect structure with partly breached barriers, in-situ TEM imaging shows Cu nanoparticle formation, agglomeration and movement in porous organosilicate glasses. In a flawless interconnect structure, in-situ TEM imaging and ESI mapping show close to no evidence of Cu diffusion in the TDDB process. From the ESI mapping, only a narrow Cu trace is found at the SiCN/OSG interface. In both cases, when barriers are breached or still intact, the initial damage is observed at the top interface of M1 between SiCN and OSG.

Combining Orientation Mapping and In Situ TEM to Investigate High-Cycle Fatigue and Failure

Material fatigue proves to be a limiting factor in many engineering cases. Repeated cyclic loading, even at stresses well below the monotonic yield stress of the material, leads to the accumulation of microstructural damage, crack initiation, crack growth, and eventual failure. In terms of the number of loading cycles, the high cycle regime of >10 4 loading cycles is often of interest, although there are cases in which fatigue lifetimes may reach >>10 7 cycles. Cyclic loading experiments with bulk specimens can determine fatigue lifetimes, however, the crack initiation and early crack growth regimes are more difficult to examine experimentally, as these processes take place at the grain level, often inside of the specimen. These incipient processes are important to bulk material, as this is where the failure process begins. Furthermore, microand nanoscale devices are directly affected in this regime. Mechanical testing performed in situ inside of the transmission electron microscope provides the ability to see these processes in real time as they occur; however, to date no general capability for fatigue loading at frequencies above a few Hz exists. Here, the authors demonstrate a newly developed mechanical testing capability that allows controllable quantitative cyclic loading to be performed in situ inside of a transmission electron microscope at frequencies in the hundreds of Hz. This capability, built into a nanoindentation TEM holder, is capable of reaching 10 6 cycles within an hour. Here, observations of microstructural changes near propagating cracks in nanocrystalline Cu are presented.

Resolution Limits of Nanoindentation Testing

As material and device length scales decrease, there must be a corresponding increase in the instrumentation resolution for accurate measurements. For these small length scale systems, including thin films, fine grained structures, and matrix composites, nanoindentation experiments provide a proven method for mechanical property measurements. Additionally, when nanoindentation is combined with scanning probe microscopy, individual tests can be placed directly in the regions of interest. However, these tests do not have infinite resolution, as they are limited by the volume probed during a test and the resulting residual damage. Here, an investigation of elastic and plastic mechanical properties is made in relation to the lateral test spacing and the mechanically probed volume. The results clearly show that closely spaced tests having residual plasticity adversely affect neighboring tests, having both poor accuracy and precision in the measurement. This is in contrast to purely elastic tests, which can be closely spaced without affecting accuracy or precision.

Optimization of a dissimilar platinum to niobium microresistance weld: a structure–processing–property study

Dissimilar metal resistance spot welds, critical to the manufacture of medical devices, typically form brittle intermetallic compounds that are prone to failure. Here, a case study of biocompatible metals platinum and niobium using advanced analytical techniques is presented. It describes the variation of properties and microstructure using microresistance spot welding under four conditions, including a legacy process and processing conditions optimized by design of experiments. Adjustments to the electrode force, welding current, surface roughness, and pulse duration and exchanging the platinum anode contact for a cathode result in a joint with less porosity and greater uniformity in the thickness, chemistry, and microstructure of the fusion zone. The optimized microstructure contains fewer defects, with increased plasticity under deformation and a more uniform microstructure reducing the propensity for failure and variability between welds. Extensive analysis with optical, scanning electron, transmission electron microscopy coupled with nano- and micromechanical testing (such as micropillar compression) was used to characterize the weld zone.