The effect of film thickness on the failure strain of polymer-supported metal films
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)
This paper describes a new technique to study fracture mechanics of systems that has complex spatially and temporally varying mechanical properties; "traction force microscopy" was used to image the stress near the tip of an advancing interface crack. In the paper, the authors tried to draw the 3D displacement and stress distributions of a system that consists of colloidal coating. As the coating dries, a crack was created, and image analysis was performed for entire history of the crack propagation. Fig. 1 A shows the schematic diagram of the system in the paper, and fig. 1 B and C shows the interface crack in a colloidal coating for side view and top view, respectively. Using the image data in Fig. 1, it is possible to draw three dimensional displacement and stress distribution as shown in the Fig. 2. As expected, the stress shoots up rapidly just ahead of cracking. This technique allows scientists to see the spatially and temporally heterogeneous mechanical properties of materials.
The paper illustrates the in-depth failure mechanism of polymer-supported metal thin films. The authors shows the effect 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.
Image analysis is a good tool to figure out the mechanical property of a material when the resolution is sufficient. The authors used the technique to a system as colloid in it dries and succeed to draw 3D stress distribution. This is helpful for studying the fracture mechanics. Once the resolution of this technique is improved, it would be possible to probe the mechanical properties of homogeneous materials because the lattice parameters of a system would vary when it deforms. This may give remarkable knowledge expansion in material science.