William M. Mook

Superhard silicon nanospheres

Abstract Successful deposition and mechanical probing of nearly spherical, defect-free silicon nanospheres has been accomplished. The results show silicon at this length scale to be up to four times harder than bulk silicon. Detailed measurements of plasticity evolution and the corresponding hardening response in normally brittle silicon is possible in these small volumes. Based upon a proposed length scale related to the size of nanospheres in the 20– 50 nm radii range, a prediction of observed hardnesses in the range of 20– 50 GPa is made. The ramifications of this to computational materials science studies on identical volumes are discussed.

Superhard nanocrystalline silicon carbide films

Nanocrystalline silicon carbide films were deposited by thermal plasma chemical vapor deposition, with film growth rates on the order of 10μm∕min. Films were deposited on molybdenum substrates, with substrate temperature ranging from 750-1250 °C. The films are composed primarily of β-SiC nanocrystallites. Film mechanical properties were investigated by nanoindentation. As substrate temperature increased the average grain size, the crystalline fraction in the film, and the hardness all increased. For substrate temperatures above 1200 °C the average grain size equaled 10-20 nm, the crystalline fraction equaled 80-85 %, and the film hardness equaled approximately 50 GPa.

Fracturing a nanoparticle

Conventional wisdom and indirect studies suggest that the mechanical properties of nanoparticles can be considerably different than their bulk properties would predict. However, little is actually known about their mechanical behaviour because of the practical difficulties in investigating individual particles. Direct experimental studies of these properties require knowledge of the crystallographic orientation, size and microstructure of the nanoparticle in order to be complete. By deforming a single nanoparticle in the transmission electron microscope we have been able to determine each of these parameters of an isolated silicon nanoparticle a priori. With this approach, we could then directly examine dynamic deformation processes and demonstrate the first direct observation of plasticity-induced cleavage fracture of a silicon nanoparticle in compression.

Geometry and surface state effects on the mechanical response of Au nanostructures

A study of ultra-thin gold films and thin-walled nanoboxes has confirmed that length scales in terms of dislocation spacing can predict flow stress. Initial stages of deformation conform to linear hardening with average dislocation spacing controlled by the number of geometrically necessary dislocations in a pile-up. Later stages of deformation exhibit parabolic behavior with Taylor hardening interpreted in terms of a dislocation density described by the total line length of prismatic loops per unit volume. Comparisons of 20 and 40 nm thick planar films could be made to 205 nm high hollow gold nanoboxes with a wall thickness of 24 nm. These highly constrained, ultra-thin planar films demonstrated increased hardness from about 2 to 10 GPa with strains of 20 percent while less constrained nanoboxes increased from 0.8 to 4 GPa for the same strain magnitude.

Compression of Nanowires Using a Flat Indenter: Diametrical Elasticity Measurement

A new experimental approach for the characterization of the diametrical elastic modulus of individual nanowires is proposed by implementing a micro/nanoscale diametrical compression test geometry, using a flat punch indenter. A 250 nm diameter single crystal silicon nanowire is compressed inside of a scanning electron microscope. Since silicon is highly anisotropic, the wire crystal orientation in the compression axis is determined by electron backscatter diffraction. In order to analyze the load-displacement compression data, a two-dimensional analytical closed-form solution based on a classical contact model is proposed. The results of the analytical model are compared with those of finite element simulations and to the experimental diametrical compression results and show good agreement.

An Energy Balance Criterion for Nanoindentation-Induced Single and Multiple Dislocation Events

Small volume deformation can produce two types of plastic instability events. The first involves dislocation nucleation as a dislocation by dislocation event and occurs in nanoparticles or bulk single crystals deformed by atomic force microscopy or small nanoindenter forces. For the second instability event, this involves larger scale nanocontacts into single crystals or their films wherein multiple dislocations cooperate to form a large displacement excursion or load drop. With dislocation work, surface work, and stored elastic energy, one can account for the energy expended in both single and multiple dislocation events. This leads to an energy balance criterion which can model both the displacement excursion and load drop in either constant load or fixed displacement experiments. Nanoindentation of Fe-3% Si (100) crystals with various oxide film thicknesses supports the proposed approach.

Length-scale-based hardening model for ultra-small volumes

Understanding the hardening response of small volumes is necessary to completely explain the mechanical properties of thin films and nanostructures. This experimental study deals with the deformation and hardening response in gold and copper films ranging in thickness from 10 to 400 nm and silicon nanoparticles with particle diameters less than 100 nm. For very thin films of both gold and copper, it was found that hardness initially decreases from about 2.5 to 1.5 GPa with increasing penetration depth. Thereafter, an increase occurs with depths beyond about 5–10% of the film thickness. It is proposed that the observed minima are produced by two competing mechanisms. It is shown that for relatively deep penetrations, a dislocation back stress argument reasonably explains the material hardening behavior unrelated to any substrate composite effect. Then, for shallow contacts, a volume-to-surface length scale argument relating to an indentation size effect is hypothesized. A simple model based on the superposition of these two mechanisms provides a reasonable fit to the experimental nanoindentation data.

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.

Room Temperature Deformation Mechanisms of Alumina Particles Observed from In Situ Micro-compression and Atomistic Simulations

Aerosol deposition (AD) is a solid-state deposition technology that has been developed to fabricate ceramic coatings nominally at room temperature. Sub-micron ceramic particles accelerated by pressurized gas impact, deform, and consolidate on substrates under vacuum. Ceramic particle consolidation in AD coatings is highly dependent on particle deformation and bonding; these behaviors are not well understood. In this work, atomistic simulations and in situ micro-compressions in the scanning electron microscope, and the transmission electron microscope (TEM) were utilized to investigate fundamental mechanisms responsible for plastic deformation/fracture of particles under applied compression. Results showed that highly defective micron-sized alumina particles, initially containing numerous dislocations or a grain boundary, exhibited no observable shape change before fracture/fragmentation. Simulations and experimental results indicated that particles containing a grain boundary only accommodate low strain energy per unit volume before crack nucleation and propagation. In contrast, nearly defect-free, sub-micron, single crystal alumina particles exhibited plastic deformation and fracture without fragmentation. Dislocation nucleation/motion, significant plastic deformation, and shape change were observed. Simulation and TEM in situ micro-compression results indicated that nearly defect-free particles accommodate high strain energy per unit volume associated with dislocation plasticity before fracture. The identified deformation mechanisms provide insight into feedstock design for AD.

Atomic Scale Dynamics of Contact Formation in the Cross-Section of InGaAs Nanowire Channels

Alloyed and compound contacts between metal and semiconductor transistor channels enable self-aligned gate processes which play a significant role in transistor scaling. At nanoscale dimensions and for nanowire channels, prior experiments focused on reactions along the channel length, but the early stage of reaction in their cross sections remains unknown. Here, we report on the dynamics of the solid-state reaction between metal (Ni) and semiconductor (In0.53Ga0.47As), along the cross-section of nanowires that are 15 nm in width. Unlike planar structures where crystalline nickelide readily forms at conventional, low alloying temperatures, nanowires exhibit a solid-state amorphization step that can undergo a crystal regrowth step at elevated temperatures. In this study, we capture the layer-by-layer reaction mechanism and growth rate anisotropy using in situ transmission electron microscopy (TEM). Our kinetic model depicts this new, in-plane contact formation which could pave the way for engineered nanoscale transistors.