Strain Rate Sensitivity of the Nanoindentation Creep of Ag, Cu, and Ni Thin Films

Abstract

Ag, Cu, and Ni films each of 150 nm, 300 nm, 600 nm and 1000 nm thickness were deposited on Si using E-beam evaporation. The structural and surface properties were explored using FE-SEM and AFM. The crystal structure and orientation of the films were examined using x-ray diffraction. Nanocrystalline grain structure formation was observed to dominate the crystal film growth. The nanomechanical and creep properties were measured using nano-indentation. The hardness results as calculated based on tip calibration and contact stiffness for Ag differ due to excessive pile-up. Since Cu and Ni endure less pile-up, the hardness results exhibit little or no difference. For Ag films, the hardness based on tip calibration increases with the normalized depth of indentation as the film thickness decreases and remains nearly flat (no size effects). On the other hand, the hardness based on contact stiffness remains the same with the normalized depth of indentation regardless of the film thickness. For creep experiments, the strain rate sensitivity, m, of the hardness for the Ag, Cu, and Ni films is reported as 0.033 ± 0.001, 0.03 ± 0.003, and 0.04 ± 0.005 respectively. The normalized activation volume (V*/b3), when plotted versus the hardness (H), decreases with increasing hardness. V*/b3 measured at the surface of the bulk materials, i.e., shallow indents, is similar to V*/b3 measured for thin films when tested at 10–20% of the film thickness to circumvent substrate effects. V*/b3 coalesces with the literature data from conventional uniaxial testing and nano-indentation data for bulk Ag, Cu, and Ni samples. Although m is different from bulk to thin films due to the differences in grain sizes, the activation volume results for thin films extrapolate substantially to the bulk material except for Ni as the results experience a slight deviation.