Deformation Behavior And Length-Scale-Controlled Effect Of Cu-Au Multilayers And Cu Films

Thin film materials are widely used in micro/nano-systems, such as large-scale integrated circuits and micro-electro-mechanical systems. Mechanical properties of thin films are different from those of bulk materials. Thin film materials exhibit pronounced size effects during gradient dominated deformation, compared with bulk materials. Therefore, it is necessary to study mechanical properties of thin film materials under different loading conditions. In this paper, mechanical properties and deformation behavior of the Cu-Au multilayers with individual layer thickness (h) of 25nm, 50nm, 100nm, and 250nm were investigated using a nanoindentor. In addition, free-standing polycrystalline and single crystal Cu films with thicknesses of 25μm and 50μm were also investigated through microbending test system self-designed here. The characteristic length scale of polycrystalline and single crystal Cu were attained and the physical nature concerning the characteristic length scale of materials was understood.Nanoindentation tests on the Cu-Au multilayers show that the hardness of the Cu-Au multilayers increases with decreasing h. In the case of h≥50nm, the hardness increases linearlywith 1/h^1/2. When h is less than 50nm, the increase in hardness with increasing 1/h^1/2 deviates from the linear relation and shows a weak increase tendency. It has also been found that the elastic modulus of the Cu-Au multilayers exhibits a slight decrease with h. The elastic modulus of the Cu-Au multilayers is higher than that of the Cu and Au component films. In addition, the materials pile-up and shear bands around the indent on the Cu-Au multiplayers are also observed. Plastic deformation of the Cu-Au multilayers in the indent exhibits shear banding behavior. The width of the shear band decreases with decreasing h.Microbending tests on the free-standing polycrystalline and single crystal Cu films show that pronounced size effect is observed with gradient dominated deformation of polycrystalline and single crystal Cu, but the strengthening effect of polycrystalline Cu is stronger than that of single crystal cu. It can be determined that the characteristic length scale of the polycrystallineCu is 4.1±0.5|xm, while the characteristic length scale of the single crystal Cu with a single slip orientation is 3.8±0.8|jm. Further analysis shows that the material characteristic length scale is dependent on the density of geometrically necessary dislocation inside the materials. It is also suggested that the size effect dominated by strain gradient can be quantified by the material characteristic length scale.