VELOCITY DEPENDENCE OF FATIGUE CRACK GROWTH MECHANISMS IN NANOCRYSTALLINE PLATINUM

Open Access
Author:
Meirom, Roi Arie
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 19, 2008
Committee Members:
  • Christopher Muhlstein, Thesis Advisor
  • Dr Christopher Muhlstein, Thesis Advisor
Keywords:
  • Nanomaterials
  • Fatigue
  • MEMS
  • Platinum
  • Thin Films
Abstract:
Thin films are present in virtually every corner of industry, from holding our banking information on a thin magnetic strip, to lining the insides of our soda bottles as invisible diffusion barriers, to acting as switches and actuators in microelectromechanical systems (MEMS) barely visible to the naked eye. Material selection is of prime importance to the end-function of a thin film. Noble metals are a set of metals which have excellent resistance to oxidation, and their high electrical conductivity makes them ideal candidates for use in several MEMS applications. These applications often require the films to be mechanically loaded time and time again, and it is expected that they function reliably throughout their lifetime. The material’s response to fatigue must therefore be evaluated in order to ensure that these thin films behave as expected. Thin film mechanical testing specimens were fabricated as free-standing, 430nm platinum films having primarily (111) oriented columnar grains roughly 25-40 nm in diameter on a 20 nm titanium adhesion layer. These specimens had dog-bone geometry and a notch at the center of the gauge length, allowing for the localization of plastic deformation. The films were established to be fully dense (i.e. no voids or cracks) and no interphase regions between the grains were found. Further analysis confirmed purity of the platinum structural film within the detectability limits of the techniques. A tensile test conducted on one specimen established an upper bound for the ultimate tensile strength of 3.4 GPa, consistent with yield strength values found by nanoindentation and tensile testing studies conducted by others. Four constant stress amplitude (load ratio &#963;min/&#963;max= R = 0.1, sinusoidal waveforms at 1 Hz) fatigue tests were also conducted. An extremely limited range of stable fatigue crack growth was observed (< 3 MPa &#8730;m) as well as an order of magnitude reduction in Kmax (4.88 MPa &#8730;m) when compared to microcrystalline platinum. The power law exponent , m, associated with the crack growth rates was ~10.5, significantly larger than the microcrystalline platinum at ~3. This study also shows that crack closure does not affect crack advance, and that at low crack growth rates, fatigue damage localizes at grain boundaries, creating intergranular failure surfaces, but transitions to a transgranular crack path at higher velocities. The limited span of stable fatigue crack growth of the nanocrystalline platinum, in addition to the large m value suggests that the crack growth rate behavior of the nanocrystalline film is more similar to a ceramic or ordered intermetallic than that of a pure bulk metal. This research shows that the atomistic simulations and experimental observations of crack advance in nanograined metals may not be effectively generalized to other pure metal systems such as platinum.