Mechanisms of reversible stretchability of thin metal films on elastomeric substrates

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(Fig. 1). a. SEM micrograph of 25 nm thick Au film on PDMS. b. and C. Normalized electrical resistance of the metal film under cyclic elongation. (Fig. 2.) a. Macroscopic visualization of microcracks. b. and c. schematic of the film pattened with the cracks, d. and e. deflection of the substrate in the plane (Au film on a hard substrate), and out of the plane (Au film on a soft substrate), respectively. Both cases are under a relative elongation of 20%.

Wiki entry by : Dongwoo Lee, AP225 Fall 2010.

Paper in this Wiki : Stéphanie P. Lacour, Donald Chan, Sigurd Wagner, Teng Li and Zhigang Suo, Mechanisms of reversible stretchability of thin metal films on elastomeric substrates, APPLIED PHYSICS LETTERS 88, 204103 (2006)


Summary

The paper describes the mechanism of the large elongation of the electrical conductive metal on an elastomer substrate with reversibility. A gold film was deposited on a PDMS substrate and tensile tests were performed for hundreds of cycles for tens of hours. The preparation and result of the experiment is described in the fig. 1. It turned out that there are two main reasons that make 32% stretching of Au film possible : (i) the microscale structure (micro cracks) of the film (ii) the compliance of the substrate. To be specific, when the tri-branched cracks on the Au film were elongated by deflecting and twisting out of plane, the stress concentration decreases allowing the film to stretch for a long distance. The compliance of elastomer substrate allows the Au film to deforms out of plane. This mechanism was demonstrated with both FEA and macro scale experiment as shown in the fig. 2.


Soft Matter Discussion

The combination of metal films and soft substrate can give a tremendous chance to designing flexible electronics for various applications. In the paper, the authors investigated the mechanism of large elongation of Au film on a PDMS substrate for over 30% elongation. The soft matter under the metal film allows the film to deflect without large stress concentration. Given this fact, future works can be done for finding out the optimized Young's modulus of the soft matter for larger elongation of the film. Softer is better in terms of conformation to the metal film but rigid is better in terms of maintaining the shape of the structure. Also, for the real application of the metal film / polymer structure to flexible electronics, future works should be done for faster cyclic experiments and this will improve the reliability of the structure. One question that I have for this work is that how the thickness of the Au film effect on the elongation of the structure. Very thin ductile metal film can become brittle when its thickness is very small. In the paper, Au film was just 25nm. This can limit the movement of the dislocation and decrease the plasticity of the material. I expect that once the optimized thickness of the Au film is figured out, the structure would show much better performance.