Nanocrystalline (n-) Cu films with average grain size of about 40 nm are fabricated by thermal evaporation. Film thickness is between 100 ~ 500 nm and these free-standing films are sitting on Si wafer with a rectangular window. Mechanical properties of those n-Cu films are characterized by plane-strain bulge test. Yield stress and Young’s modulus are obtained.

• Nanocrystalline Cu shows much higher strength than the coarse-grained counter part (Hall-Petch relationship)
• Characteristic deformation mechanisms for nanocrystalline materials are observed on these films
• More interestingly, these nanocrystalline Cu films show special behavior of “Plastic Strain Recovery” which means after deformed, the plastic deformation on the specimen disappears after a period of time!!

A Trick

There is a trick in plane-strain bulge test: As shown at the right-lower corner of the above picture, the shear stress is highly concentrated at the four corners of the rectangular film. Thus, under a large deformation, failure might occurs at those corners even though in the middle of the film, material is still in a safe regime. Crack will nuclear at any of those corners and propagate into the center quickly and cause a complete failure of the specimen.

• So, although the plane-strain bulge does compare materials’ strength, the experimental curves will not show the ultimate strength of the material: The real strength might be much higher than the maximum value in the curve, especially for less ductile materials.