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Plastic deformation of freestanding thin films: Experiments and modeling

Submitted by Anonymous (not verified) on
research
Plasticity
thin films
size scale dependent plasticity
Discrete dislocation modeling

This is a paper we recently published in JMPS on a study of the mechanical properties on thin films comparing experimental results with discrete dislocation simulations. It provides insight in the strengthening that occurs in thin metal films when surface or interface effects become important.

The abstract is below; the full paper can be downloaded from here

Abstract - Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress–strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank–Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.

MEMS Switch Reliability

Submitted by Qing Ma on
research
MEMS

It is well-recognized that MEMS switches, compared to their more traditional solid state counterparts, have several important advantages for wireless communications.  These include superior linearity, low insertion loss and high isolation.  Indeed, many potential applications have been investigated such as Tx/Rx antenna switching, frequency band selection, tunable matching networks for PA and antenna, tunable filters, and antenna reconfiguration. 

However, none of these applications have been materialized in high volume products to a large extent because of reliability concerns, particularly those related to the metal contacts.  The subject of the metal contact in a switch was studied extensively in the history of developing miniaturized switches, such as the reed switches for telecommunication applications.  While such studies are highly relevant, they do not address the issues encountered in the sub 100mN, low contact force regime in which most MEMS switches operate.  At such low forces, the contact resistance is extremely sensitive to even a trace amount of contamination on the contact surfaces.  Significant work was done to develop wafer cleaning processes and storage techniques for maintaining the cleanliness.  To preserve contact cleanliness over the switch service lifetime, several hermetic packaging technologies were developed and their effectiveness in protecting the contacts from contamination was examined.  

iMechanica is great.

Submitted by Zhijun Zheng on
students

iMechanica is great. I am glad to find this useful web and register an account with my full name.

I am a PhD candidate under the supervision of Prof. Jilin Yu in the Department of Modern Mechanics at University of Science and Technology of China (USTC). It is expected to get my PhD degree in July 2007. My PhD research work is mainly focused on the adhesive contact between elastic objects. Prior to this, I have ever interested in the reverse-Karman vortex street, the progressive buckling of tubes and the dynamic crushing of cellular structures. In these work, I gradually learnt the experimental, theoretical and numerical methods. For more details, please refer to my homepage at: http://mail.ustc.edu.cn/~zjzheng.

I am pursuing a postdoctoral position in the micro/nano mechanics field to continue my research career. It will be very appreciated if you could give me some advice or inform me some information about my application.

Microstructural evolution in elastically inhomogeneous systems

Submitted by Mogadalai Gururajan on
research
inhomogeneous elasticity
spectral techniques
phase field modelling

I am very happy to be part of iMechanica, and what best way to start than post some stuff that I have been doing recently. I received my PhD for a thesis I submitted to the Department of Materials Engineering (formerly Department of Metallurgy), Indian Institute of Science, Bangalore 560012 INDIA titled Elastic Inhomgeneity Effects on microstructures: a phase field study.

A mismatch in elastic moduli is the primary driving force for certain microstructural changes; for example, such a mismatch can result in rafting, phase inversion, and thin film instability.

My thesis is based on a phase field model, which is developed for the study of microstructural evolution in elastically inhomogeneous systems which evolve under prescribed traction boundary conditions; however, we show that it is also capable of simulating systems which are evolving under prescribed displacements.

The (iterative) Fourier based methodology that we adopt for the solution of the equation of mechanical equilibrium is characterised by comparing our numerical elastic solutions with corresponding analytical sharp interface results; in addition to being accurate, this solution methodology is also very efficient. We integrate this solution methodology into our phase field model, to study microstructural evolution in systems with dilatational misfit.

Are notes and textbooks a higher priority than journal clubs?

Submitted by Roberto Ballarini on
opinion
textbooks
lecture notes

I registered for iMechanica a few days ago, and found many postings instructive. Here is my first blog entry.

The topics being studied today by mechanicians are very difficult (what I often call "dirty problems"). In fact, often the mechanical theories (actually coupled mechanics, biology, chemistry) required to gain improved understanding are still in their infancy. Mechanicians that have entered fields such as mechanics of biological structures have gotten up to speed by paying the price (hopefully an enjoyable time on a learning curve) of reading large numbers of papers and discipline-based books. Many of these papers are cryptic and, while they may be of high scientific quality, they do not have significant pedagogical value to those entering the field (graduate students for example).

ES 240 project: Analysis of Resonance in Wine Glasses

Submitted by Danny Goodman on
ES 240
Fall 2006
solid mechanics
project

We studied in class the phenomenon of resonance in forced, damped oscillators.  The mass and stiffness of a one-dimensional oscillator give rise to a natural frequency of oscillations known as the resonance frequency.  With no damping, energy input at this frequency accumulates and the amplitude of vibrations increases.

The phenomenon of resonance generalizes to linear elastic materials with many more (ie infinite) degrees of freedom: energy input at a natural frequency of vibration will accumulate and result in increasing amplitude of vibration.  The natural frequency in this case is determined by material properties (ie Young's modulus) and the geometry and dimensions of the object (ie a wine glass).  With so many degrees of freedom, the resonance frequency of common objects may be impossible to calculate exactly and it may be necessary to use the finite element method to investigate resonance.

A method to analyze dislocation injection from sharp features in strained silicon structures

Submitted by Zhen Zhang on
industry
suo group research
strained silicon
dislocation nucleation
stress intensity factor

Stresses inevitably arise in a microelectronic device due to mismatch in coefficients of thermal expansion, mismatch in lattice constants, and growth of materials. Moreover, in the technology of strained silicon devices, stresses have been deliberately introduced to increase carrier mobility. A device usually contains sharp features like edges and corners, which may intensify stresses, inject dislocations into silicon, and fail the device. On the basis of singular stress fields near the sharp features, this letter describes a method to obtain conditions that avert dislocations.

ES 240 project: Deformation of the Sarcolemma

Submitted by Will Adams on
ES 240
Fall 2006
solid mechanics
project

The cardiac myocyte is the basic contractile unit of the heart. In addition to potentiating contraction through chemical and electrical means, each myocyte is a complex sensor that monitors the mechanics of the heart. Through largely unknown means, mechanical stimuli are transduced into biochemical information and responses. Such mechanotransduction has been implicated in the etiology of many cardiovascular pathologies [1]. One such mechanical parameter that the myocyte most likely monitors is the hydrostatic pressure in the myocardium.

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