ELASTIC BEHAVIOR OF NICKEL FILMS DURING MONOTONIC AND CYCLIC LOADING

Open Access
Author:
Collins, James Grayson
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 05, 2008
Committee Members:
  • Christopher Muhlstein, Thesis Advisor
Keywords:
  • nanocrystalline
  • nickel
  • thin films
  • electrodeposition
  • fatigue
Abstract:
Electrodeposited metallic thin films have exhibited mechanical behavior that differs dramatically from their bulk counterparts, including high strength and low ductility. Specifically, films with grain sizes less than 100 nm have exhibited low elastic modulus compared to the expected coarse grain material value, and this is often contributed to a large volume fraction of grain boundaries. The mechanical behavior of pulse electrodeposited (PED) nickel films with a micrograined microstructure was examined in this study and compared to the properties of nanycrystalline direct current deposited thin films and bulk material behavior. Tensile tests of the PED specimens were analyzed using digital image correlation (DIC). Engineering strain was determined using the DIC technique and, in conjunction with engineering stress, was used to calculate elastic modulus values between 81 and 112 GPa. The bulk coarse-grained value of elastic modulus is 200 GPa. DIC evaluation of the transverse strain ratio led to values as great as 3.4 for this ratio in the apparent elastic regime of the material, values greater than the elastic limit of 0.5 for uniaxial tension. These values corresponded to a volume change of -17.0% below the proportional limit of the material. The specimens had an average yield strength of 826 MPa and average tensile strength of 995 MPa. The use of incremental step fatigue tests conducted at a maximum stress of 876 MPa revealed that elastic modulus increased after initial loading and that this increase in elastic modulus was a stable and permanent effect. The average increase in elastic modulus was 51.7%. DIC evaluation indicated average modulus changes from 100 GPa to 160 GPa. Constant amplitude fatigue tests conducted at the nominal yield strength and at half of the yield strength confirmed the results of the incremental step tests with regards to elastic modulus. The specimens were heat treated and then incrementally step tested to observe the effect of thermal energy on the change in elastic modulus. Specimens were heat treated for 24 hours at 100°C and 25% RH. After heat treatment, the average change in elastic modulus was 14.0%, showing that thermal energy drove the mechanism responsible for elastic modulus evolution in the material. Additional specimens were creep tested at ~25°C for 24 hours in force control, and these specimens also showed increases in elastic modulus and also primary and secondary creep regimes. Both elastic strain energy and thermal energy appeared to drive the mechanism responsible for stabilization of the dilated material. Optical micrography on metallographically prepared specimens indicated the films were textured, and X-ray diffraction analysis confirmed a (111) orientation for the films. X-ray microanalysis indicated no grain reorientation or grain growth when comparing as-received and fatigued specimens. Analysis of the lattice parameter through X-ray diffraction showed that as-received specimens had a lattice parameter 0.1% greater than specimens that had been heat treated at 100°C and 25% RH for 30 minutes. Comparison of fatigued and heat-treated specimens to as-received specimens using ion contrast imaging revealed, qualitatively, recrystallization of grains below ~10 nm in size throughout the fatigued and heat-treated specimens. Density measurements supported the transverse strain results from tensile testing. The as-received specimens decreased 18.9% by volume after heat treatment at 100°C and 25% RH for 30 minutes. A possible mechanism to account for these changes in elastic modulus is lattice dilation. Current models of nanocrystalline materials are not applicable due to the low volume of grain boundaries, and there was no evidence of microcracking in the material, which is another mechanism that can reduce the apparent elastic modulus of the material. Through the use of thermodynamic models, the observed density volume changes would correspond to a reduction in elastic modulus of 48.9% with respect to the bulk material equilibrium value at 0 K, suggesting that this material system could be modeled using first principles thermodynamic calculations. The proposed mechanism for low elastic module in PED nickel thin films does not depend on grain boundary volume like the nanocrystalline material models, and the lattice dilation model correlated well with observed volume changes and elastic modulus changes in the material system.