The effect of film thickness on the failure strain of polymer-supported metal films

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Information

Wiki entry by : Dongwoo Lee, AP225 Fall 2010.

Paper in this Wiki : Nanshu Lu, Zhigang Suo, Joost Vlassak, The effect of film thickness on the failure strain of polymer-supported metal films. Acta Materialia 58, 1679-1687 (2010)

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Summary

The paper illustrates the in-depth failure mechanism of polymer-supported metal thin films. The authors shows the effects of thickness, grain size, and crystallographic texture on the failure mode of the substrate by using both experiments and finite element analysis. Figure 1 shows the grain size difference(fig1. a), texture difference(fig1.b), and yield strength difference(fig1. c) as the film thickness changes. It was found that the grain size decreases, (111) texture dominates, and yield strength increases as film thickness decreases. Those characteristics of thin film lead to following results : a) As shown in the fig. 2, resistance increase according to strain is steeper when thickness decreases in the range of t < 500nm. This is because the crack density increases markedly and the cracks are interconnected as t decreases. b) As shown in the fig. 3, the failure mode becomes brittle when thickness is below 100nm. This is because of dimensional constraints on dislocation activity. c) Thin films of fcc metals tend to have a strong (111) texture, while thicker films also have a (100) texture. Generally, (110) texture has lower values of Young's modulus and Poisson's ratio.

Discussion

The failure mechanism of thin film on a polymer substrate was considered complicated. This paper gives clear solutions to that and reveals that there are three main mechanisms of the size effect: Very thin films fail as a result of intergranular fracture. As thickness increases, the failure mode switches to transgranular fracture. Also, as the yield strength decreases as thickness increases, debonding between metal and polymer becomes difficult. Those results will be very helpful for creating multi-functional materials for a variety of applications including soft robotics and flexible electronincs - good adhesion, uniform crystallographic texuture, and a larger grain size may lead to larger strain for the double layered material. Further research can be conducted to investigate the strain rate of the tensile test to observe the effect of viscoelasticity.