Mechanics of Materials

a. Microsystems for in situ materials characterization

Materials at the micrometer and submicrometer scale exhibit mechanical properties that are substantially different from bulk materials. With the increasing miniaturization of devices, an accurate characterization of micro/nanoscale materials is necessary to ensure their reliability and performance. Precise characterization is also essential for a fundamental understanding of mechanisms that govern size dependent material responses. In situ testing is especially attractive for micro/nanoscale samples because one can monitor the overall macroscopic response while simultaneously observing the underlying deformation mechanisms and thus establish the structure-property relationship. To overcome these key obstacles, I develop and utilize MEMS-based methods for in situ materials characterization.

Publications

W. Kang, J. Rajagopalan, and T. Saif, In Situ Uniaxial Mechanical Testing of Small Scale Materials - a Review, 2(4), 282-287, Nanoscience and Nanotechnology Letters, 2010.
W. Kang and T. Saif, A Novel Method for In Situ Uniaxial Tests at the Micro/Nano Scale–Part I: Theory, 19(6), 1309-1321, Journal of Microelectromechanical Systems, 2010.
W. Kang, J. Han and T. Saif, A Novel Method for In Situ Uniaxial Tests at the Micro/Nano Scale–Part II: Experiment, 19(6), 1322-1330, Journal of Microelectromechanical Systems, 2010.

b. Thermo-mechanical properties

Micro and nano scale devices operating at various temperatures are ubiquitous today, e.g, microturbines, micropower generators, thermo-actuators, microheaters, and

sensors/actuators in automobile and aerospace applications. At the micro/nanoscale, it is expected that the material response depends on size and temperature and, as a result, thermomechanical coupling of these small-scale materials becomes increasingly important. However, reliable thermomechanical characterization of these small scale material samples is particularly challenging, requiring accurate application and measurement of deformation (load or displacement) while simultaneously controlling and measuring specimen temperatures.

Brittle to ductile transition (BDT) in single crystal silicon is one of such properties relevant to these devises. Materials can be brittle or ductile depending on temperature. BDT happens over a small range of temperature. For bulk silicon, BDT is within a few degrees of 545degreeC. It is speculated that BDT temperature of silicon may decrease with size at nanoscale. However, recent experimental and computational studies have provided inconclusive evidence, and are often contradictory to each other. Potential reason for the controversy might originate from the lack of an in situ methodology that allows to vary both temperature and sample size. Here we resolve the controversy by carrying out in situ thermo-mechanical bending tests on SCS samples with concurrent control of these two key parameters. We unambiguously show that BDT temperature reduces with sample size. For example, BDT temperature decreases to 293 degreeC for sample size 720nm. We propose a mechanism based model to interrupt the experimental observations.

Publications (*: corresponding author, IF: 5-year Impact Factor >5)

W. Kang*, M. Merrill, and J. Wheeler, In Situ Thermomechanical Testing Methods for Micro/Nano-Scale Materials, 9 (8), 2666-2688, Nanoscale, 2017 (Featured as the cover article, IF: 7.9).
W. Kang and T. Saif, Size and Temperature Effect on Brittle-to-Ductile Transition in Single Crystal Silicon, 23(6), 713-719, Advanced Functional Materials, 2013 (IF: 11.8).
W. Kang and T. Saif, A SiC MEMS Apparatus for In Situ Uniaxial Testing of Micro/Nanomaterials at High Temperature, Journal of Micromechanics and Microengineering, 21(10), 105017, 2011 (IOP Select Paper).
W. Kang and T. Saif, In Situ Thermo-Mechanical Testing for Micro/Nanomaterials, 1(1), 13-16, MRS Communications, 2011.

c. Electro-mechanical properties

Electrically assisted deformation (EAD) is an emerging technique to enhance formability of metals by applying an electric current through them. Despite its increasing importance

in manufacturing applications, there is still an unresolved debate on the nature of the fundamental deformation mechanisms underlying EAD, mainly between electroplasticity (non-thermal effects) and resistive heating (thermal effects). This status is due to two critical challenges: 1) a lack of experimental techniques to directly observe fundamental mechanisms of material deformation during EAD, and 2) intrinsic coupling between electric current and Joule heating giving rise to unwanted thermally-activated mechanisms. To overcome these challenges, we have developed a microdevice-based electromechanical testing system (MEMTS) to characterize nanoscale metal specimens in transmission electron microscopes (TEM). Our studies reveal that MEMTS eliminates the effect of Joule heating on material deformation, a critical advantage over macroscopic experiments, owing to its unique scale. For example, a negligible change in temperature (<0.02 degreeC) is predicted at ~3500 A/mm². Utilizing the attractive features of MEMTS, we have directly investigated potential electron-dislocation interactions in single crystal copper (SCC) specimens that are simultaneously subjected to uniaxial loading and electric current density up to 5000 A/mm². Our in situ TEM studies indicate that for SCC electroplasticity does not play a key role as no differences in dislocation activities, such as depinning and movement, are observed.

Publications (+: mentees)

R. Reid⁺, A. Pique, and W. Kang*, A Novel Method for In Situ Electro-Mechanical Characterization of Nanoscale Specimens, DOI: 10.3791/55735, JoVE, 2017.
W. Kang*, I. Beniam, and S. Qidwai, In Situ Electron Microscopy Studies of Electro-Mechanical Behavior in Nanoscale Metals Using a Novel Microdevice-Based Method, 87, 095001, Review of Scientific Instruments, 2016.

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