Stress-strain curves by tensile testing of thin metallic films on thin polyimide foils: Al, AlCu, CuNi(Mn)

For better understanding the mechanical properties of thin films it is helpful to use the same experimental methods as for bulk material, like tensile tests, thereby being able to directly compare the results. However, tensile tests of free-standing metallic thin films are often difficult to perform for reasons of preparation, handling, and stresses in the films. By leaving the metallic film on an elastic substrate tensile tests were performed in a rather simple and precise manner, using a commercial tensile testing machine. Stress-strain curves were determined by separating the force working on the substrate from that working on the film-substrate compound. Those measurements were done at room temperature for Al, AlCu(0.5 wt %) and Cu0.57Ni0.42Mn0.01 thin (200-2000 nm) films prepared by magnetron sputtering on 8 mu m and 13 mu m thick polyimide (Kapton) foils. The film microstructure was characterized by scanning and transmission electron microscopy and X-ray diffraction. The tensile strength of the fine grained films was found to be up to one order of magnitude higher than for the corresponding coarse grained bulk material. Al and AlCu films showed little, CuNi(Mn) films showed no plastical behavior. Crack formation started between 0.3 % and 2 % strain depending on the material the thermal history, and the grain size.