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High tensile strength of sputter-deposited ZrB<sub>2</sub> ceramic thin films measured up to 1016 K
Sim, G.D.,Choi, Y.S.,Lee, D.,Oh, K.H.,Vlassak, J.J. Elsevier Science 2016 Acta materialia Vol.113 No.-
<P>We report the results of tensile experiments on 900 nm sputter-deposited films of ZrB2, performed at temperatures ranging from ambient to 1016 K. The ZrB2 samples were tested using micromachined sample frames with integrated micro-heaters. Transmission electron microscopy shows that both as deposited films and films heated above 1000 K consist of two distinct layers, an amorphous layer and a crystalline layer with a very fine columnar structure. The ZrB2 films have a room-temperature tensile strength in excess of 1.2 GPa, far exceeding the tensile strength of bulk ZrB2, but their stiffness is lower and decreases significantly with increasing temperature. The reduction in stiffness is attributed to the presence of the amorphous phase and the high density of transverse grain boundaries in the crystalline region. The fracture toughness of the ZrB2 films was measured by introducing pre-cracks in the tensile specimens with a focused ion beam, and was found to be 2.57 +/- 0.03 MPa root m, similar to values reported for bulk ZrB2. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>
High-throughput analysis of thin-film stresses using arrays of micromachined cantilever beams.
Kim, Hyun-Jong,Han, Jun-Hyun,Kaiser, Roy,Oh, Kyu Hwan,Vlassak, Joost J American Institute of Physics 2008 Review of scientific instruments Vol.79 No.4
<P>We report on a technique for making high-throughput residual stress measurements on thin films by means of micromachined cantilever beams and an array of parallel laser beams. In this technique, the film of interest is deposited onto a silicon substrate with micromachined cantilever beams. The residual stress in the film causes the beams to bend. The curvature of the beams, which is proportional to the residual stress in the film, is measured by scanning an array of parallel laser beams generated with a diffraction grating along the length of the beams. The reflections of the laser beams are captured using a digital camera. A heating stage enables measurement of the residual stress as a function of temperature. As the curvature of each beam is determined by the local stress in the film, the film stress can be mapped across the substrate. This feature makes the technique a useful tool for the combinatorial analysis of phase transformations in thin films, especially when combined with the use of films with lateral composition gradients. As an illustration, we apply the technique to evaluate the thermomechanical behavior of Fe-Pd binary alloys as a function of composition.</P>
Effects of stretching and cycling on the fatigue behavior of polymer-supported Ag thin films
Sim, G.D.,Lee, Y.S.,Lee, S.B.,Vlassak, J.J. Elsevier Sequoia 2013 Materials science & engineering. properties, micro Vol.575 No.-
The fatigue behavior of silver films on polyethylene-terephthalate substrates is studied for various levels of film thickness, pre-stretch, sample width, and applied strain range. Films with large pre-stretch have a shorter fatigue life, with failure caused by strain localization (films thicker than 100nm) or intergranular crack formation (100nm film). There is a significant effect of film thickness on how the strain range affects the fatigue life - we observe 'smaller is better' behavior for films subjected to a total strain range of Δε<SUB>t</SUB>=1.0%, while the opposite is true when the total strain range increases to 2.0%. We attribute this difference to a shift in failure mechanism with strain amplitude from typical fatigue failure to a more ductile-type failure. Our experimental results are well described by the Coffin-Manson relationship and a failure mechanism map is drawn based on the experimental results. Considering stretchability and long-term reliability, design suggestions are made to optimize the fatigue life of coatings subjected to uniaxial stretch and fatigue.
Inorganic islands on a highly stretchable polyimide substrate
Sun, Jeong-Yun,Lu, Nanshu,Yoon, Juil,Oh, Kyu-Hwan,Suo, Zhigang,Vlassak, Joost J. Cambridge University Press (Materials Research Soc 2009 Journal of materials research Vol.24 No.11
<P>For a flexible electronic device integrating inorganic materials on a polymer substrate, the polymer can deform substantially, but the inorganic materials usually fracture at small strains. This paper describes an approach to make such a device highly stretchable. A polyimide substrate is first coated with a thin layer of an elastomer, on top of which SiN<I>x</I> islands are fabricated. When the substrate is stretched to a large strain, the SiN<I>x</I> islands remain intact. Calculations confirm that the elastomer reduces the strain in the SiN<I>x</I> islands by orders of magnitude.</P>
Pharr, Matt,Choi, Yong Seok,Lee, Dongwoo,Oh, Kyu Hwan,Vlassak, Joost J. Elsevier 2016 Journal of Power Sources Vol.304 No.-
<P><B>Abstract</B></P> <P>We measure stresses that develop in sputter-deposited amorphous Ge thin films during electrochemical lithiation and delithiation. Amorphous Li<SUB>x</SUB>Ge electrodes are found to flow plastically at stresses that are significantly smaller than those of their amorphous Li<SUB>x</SUB>Si counterparts. The stress measurements allow for quantification of the elastic modulus of amorphous Li<SUB>x</SUB>Ge as a function of lithium concentration, indicating a much-reduced stiffness compared to pure Ge. Additionally, we observe that thinner films of Ge survive a cycle of lithiation and delithiation, whereas thicker films fracture. By monitoring the critical conditions for crack formation, the fracture energy is calculated using an analysis from fracture mechanics. The fracture energies are determined to be <I>Γ</I> = 8.0 J m<SUP>−2</SUP> for a-Li<SUB>0.3</SUB>Ge and <I>Γ</I> = 5.6 J m<SUP>−2</SUP> for a-Li<SUB>1.6</SUB>Ge. These values are similar to the fracture energy of pure Ge and are typical for brittle fracture. Despite being brittle, the ability of amorphous Li<SUB>x</SUB>Ge to flow at relatively small stresses during lithiation results in an enhanced ability of Ge electrodes to endure electrochemical cycling without fracture.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Stresses were measured in-situ in a-Li<SUB>x</SUB>Ge during electrochemical cycling. </LI> <LI> a-Li<SUB>x</SUB>Ge was found to flow plastically at significantly lower stresses than a-Li<SUB>x</SUB>Si. </LI> <LI> The elastic modulus was measured in a-Li<SUB>x</SUB>Ge as a function of lithium concentration. </LI> <LI> The fracture energy of a-Li<SUB>x</SUB>Ge was measured, indicating a brittle material. </LI> <LI> a-Li<SUB>x</SUB>Ge exhibits an unusual combination of plastic flow and brittle fracture. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>