Mechanical size effects in thin copper foils: An experimental study [thesis]

Gerd Simons, Jürg Dual, Jürgen Villain
The goal of this work is the investigation of the effect of several size parameters on the mechanical behavior of thin copper foils in tensile testing, in particular the question is whether a smaller sample has a different mechanical behavior than a larger one. Attention is paid to the most relevant size parameter, the thickness, and the influence of the microstructure of the foils, a factor which has not been accounted for systematically in literature up to now. Copper foils with 10, 20 and 34
more » ... µm thickness are tensile tested in-house, thicker foils (50, 100 and 250 µm) by a project partner (Laboratory of Materials for Mechatronics and Electrical Engineering, University of Applied Sciences Augsburg, Prof. Villain). For the tensile tests, a new setup was built which extends a previous setup developed by [Mazza, 1997] and allows for an automatic testing of the samples at a controlled strain rate. The standard strain rate applied is ˙ = 10 −4 1/s. Samples with a dogbone shape are tested, their geometry is scaled according to the thickness. The samples are produced by wet etching of rolled and electrodeposited copper foils (standard sample type, "as-received samples"). Some of the samples are heat treated after etching ("heat treated samples"). As the microstructure of a crystalline solid has a significant influence on its mechanical behavior it is characterized in detail. Different techniques such as metallography, X-ray diffraction and electron backscatter diffraction are applied for this task. The rolled samples (10 and 20 µm thick) have a strong cube texture with elongated grains with an oblate cross-section (typical length 100 µm, small diameter 5 µm, long diameter 30 µm). The electrodeposited samples have a columnar grain structure with a weak fibre texture. Heat treatment changes the microstructure of the rolled foils considerably. The grains are equi-axed with an average diameter of 15 µm. Thus, the 10 and 20 µm heat treated foils have only 1-2 grains per thickness. Rolling texture components with 111 parallel to the rolling direction form the preferred orientations, some grains are still in cube orientation. The most important result of the tensile tests is that the thickness of the foils has an influence on the mechanical behavior in the size regime studied. When the thickness is reduced from 250 to 10 µm the fracture strain decreases for the as-received foils from approximately 20% to 0.2% and for the samples with heat treatment from 35% to 15%. The tensile strength increases with smaller thickness for the as-received samples if the surface roughness is taken into account for the stress calculations (the surface roughness of the thinner foils is a considerable fraction of the total thickness). The 10 µm as-received foils have the highest tensile strength which is 400 MPa. The heat treated samples do not show a pronounced size dependence of the tensile strength. xi
doi:10.3929/ethz-a-004843017 fatcat:urucegiszna6pfbhxmpe2gfdqy