Being a undergraduate student of Harbin Institute of Technology,China, I will get my bachelor degree in JUL 2012 majoring in Composite Materials and Engineering. Then I want to find an availible PhD position of 2012 FALL to further my study.

I have studied all the basic courses of materials and mechanics in my bachelor education. Also I have got good scores in both class and lab. And you can see details in my CV and PS attached. 

My research interest includes nanoscience, nanotechnology, magnetic materials, thin films, energy materials and mechanical engineering.

Email address: wyjhitedu@126.com   yongjie_wang@hit.edu.cn 

Abstract - Indentation experiments on very thin films are analyzed by employing a rigorous solution to model elastic substrate effects. Two cases are discussed: elastic indentations where film and substrate are anisotropic, and elasto-plastic indentations where significant material pile-up occurs. We demonstrate that the elastic modulus of a thin film can be accurately measured in both cases, even if there is significant elastic mismatch between film and substrate. 

This manuscript has been accepted for publication in Journal of Materials Research.  

Next generation advances in energy storage for nanoelectronics, micro and nanosensors among others, require capacitors fabricated at the nanoscale. High dielectric constant materials such as ferroelectrics are important candidates for those. Consider the following: the expected capacitance of a 2.7 nm SrTiO3 thin film is 1600 fFmicro-m-2. What is the likely value in reality? 258 fFmicrom-2! This dramatic drop in capacitance is attributed to the so-called "dead layer" effect.

Abstract – A data analysis procedure has been developed to estimate the contact area in an elasto-plastic indentation of a thin film bonded to a substrate. The procedure can be used to derive the elastic modulus and hardness of the film from the indentation load, displacement, and contact stiffness data at indentation depths that are a significant fraction of the film thickness. The analysis is based on Yu’s elastic solution for the contact of a rigid conical punch on a layered half-space and uses an approach similar to the Oliver-Pharr method for bulk materials. The methodology is demonstrated for both compliant films on stiff substrates and the reverse combination and shows improved accuracy over previous methods. 

We are pleased to inform you that Ebatco’s Nano Analysis and Testing Laboratory (NAT Lab) now is open to serve your nanoscale mechanical and tribological testing and analysis needs. The NAT Lab’s staff scientists have more than 10 years of nanomechanical and nanotribological testing, applications R&D and instrument development experience. The lab is equipped with state-of-the-art nanoindenters with nm-resolution in-situ SPM imaging capability and full lines of advanced options such as nanoDMA, modulus mapping, feedback control, heating/cooling stage, vacuum chuck, and environmental control and isolation chamber.

Please note that a post-doc position on stresses and adhesion in optical multilayers is open. The position is for 2-years shared between SVI (Paris) and Phymat (Poitiers).

Link to a virus-free MS Word description of the position:

http://www.saint-gobain-recherche.com/svi/en/image_merethif_position.html

For further details and applications (cover letter, CV, statement of research interests):

Dear Esteemed Researcher,

H. Mei, J.Y. Chung, H.-H. Yu, C.M. Stafford, and R. Huang, Buckling modes of elastic thin films on elastic substrates. Applied Physics Letters 90, 151902 (2007).

Two modes of thin film buckling are commonly observed, one with interface delamination (e.g., telephone cord blisters) and the other with no delamination (i.e., wrinkling). Which one would occur for your film?

This Letter gives a quantitative criterion for the selection of the buckling modes. An experiment with a polystyrene film on a PDMS substrate was described showing a transition of the buckling modes.

The SES 2007 Conference, Oct. 21-24, 2007, Texas A&M University campus in College Station, Texas, home to the George Bush Presidential Library and Museum.

 Symposium: Plasticity and Damage Size Effects at the Micron and Nano Length Scales

Microcracks form by a mixture of local thinning and intergranular fracture in a 170-nm-thick Cu film that is well bonded to a polyimide substrate and is stretched to a strain of 30%. Details can be found in this paper. A related forum topic can be found

In recent development of deformable electronics, it has been noticed that thin metal films often rupture at small tensile strains. Here we report experiments with Cu films deposited on polymeric substrates, and show that the rupture strains of the metal films are sensitive to their adhesion to the substrates. Well-bonded Cu films can sustain strains up to 10% without appreciable cracks, and up to 30% with discontinuous microcracks. By contrast, poorly bonded Cu films form channel cracks at strains about 2%. The cracks form by a mixture of strain localization and intergranular fracture. The films rupture at large strains when the localization is retarded by the adherent substrates.

Stress-induced voiding (SIV) is investigated in Cu-based, deep-submicron, dual damascene technology. Two failure modes are revealed by TEM failure analysis. For one mode, voids are formed under the via when the via connects a wide metal lead below it. For the via which is instead under a wide metal line, voids are formed right above the via bottom. The void source results from the supersaturated vacancies which develop when Cu is not properly annealed after electroplating and before being constrained by dielectrics. The driving force comes from the stress built up due to grain growth and the thermal expansion mismatch (CTE) between Cu interconnect and dielectrics. A diffusion model is introduced to investigate the voiding mechanism primarily for the vias connected to wide metal leads.

Dislocations are common in epitaxial systems. For a thin film epitaxially grown on a substrate with coherent interface, it may have spontaneously-formed dislocations when its thickness is larger than certain value, i.e. critical thickness. The presence of dislocations can have an adverse effect on electrical performance of semiconductor materials, providing easy diffusion paths for dopants to lead to short circuits, or recombination centers to reduce carrier density. And, formation of dislocations is one of the most observed mechanisms of relaxation of mismatch strain. However, in optoelectric applications, strain alters the electronic bandgap and band edge alignment, and should be maintained. So, controlling formation of dislocations is very important in the manufacture of microelectronic and optoelectronic devices.

This term paper will review some basic concepts and try to produce some understanding about the control dislocation formation.

Today low-k dielectric materials are integrated into computer chips to improve the operation speed and reduce the cross-talk noise. Due to weak mechanical properties of low-k dielectric materials, cohesive failure is subjected to occur. Channel cracking is one common mode of cohesive failure. In this term paper, several potential issues relevant to channel cracking of low-k dielectric thin films are reviewed. These issues include the well known substrate constrain effect; the concentration of crack driving force due to patterned structure, and the degrading of fracture toughness as scaling down the dielectric constant of the films. Some design rules of applying the low-k dielectric thin films are also discussed in this report.

For films or coatings deposited on substrate at high temperature, residual compressive stresses are often induced in the surface layers because of the mismatch in the thermal expansion coefficients. Under such compressive residual stresses, the surface thin film is susceptible to buckling-driven delamination. Various shapes of buckled region are observed, including long straight-sided blisters, circular and the ‘telephone cord’ blister.

Each student completes a term paper of selected topics that (a) addresses a phenomenon in thin film materials, and (b) involves analyses using mechanics. The project contributes 25% of the grade, distributed as follows:

• 5%: November 30 (Thursday). Post your title and abstract in iMechanica, formated as below

1. Title (EM 397 Term Paper: e.g., Dislocations in Epitaxial Thin Films).
2. Tags (EM 397, Fall 2006, University of Texas at Austin, thin films, term paper)
3. Body: (i) Describe the phenomenon. (ii) Explain how mechanics is relevant. (iii) Cite at least 1 journal article.

• 10%: December 12 Tuesday (2:00-4:00 pm). 30 minute presentation. Use power point slides.
• 10%: December 18 Monday.
  
  

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.

Selective oxidation induced void growth is observed in thermal barrier coating (TBC) systems used in gas turbines. These voids occur at the interface between the bond coat and the thermally grown oxide layer. In this article we develop the modeling framework to simulate microvoid growth due to coupled diffusion and creeping in binary alloys. We have implemented the modeling framework into an existing finite element program. The developed modeling framework and program is used to simulate microvoid growth driven by selective oxidation in a binary beta-NiAl alloy. Axisymmetric void growth due to the combined action of interdiffusion and creeping is simulated. The sharpness of the void and direction of creeping are considered as parameters in our study. Our simulations show that the voids dilate without any change in shape when creeping is equally likely in all the directions (isotropic). Void growth patterns similar to those observed in experiments are predicted when the creeping is restricted to occur only along the radial and tangential directions. A hemispherical void grows faster compared to a sharp void. The sharpness increases in the case of a sharp void and could lead to interactions with the neighboring voids leading to spallation of the thermally grown oxide layer as observed in experiments.