Mechanical Behavior of Nanocrystalline Platinum Thin Films

Open Access
Author:
Meirom, Roi Arie
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 02, 2011
Committee Members:
  • Christopher Muhlstein, Committee Chair
  • David Green, Committee Member
  • Suzanne Mohney, Committee Member
  • Mary Frecker, Committee Member
Keywords:
  • pt
  • platinum
  • thin films
  • materials science
  • mechanical behavior
  • nanocrystalline
Abstract:
Ever since the realization that an increase in grain boundary volume contributes to the strengthening of metals, there has been a drive to reduce the grain size of polycrystalline metals. The creation of materials with finer and finer grain sizes has led to the development of new fabrication techniques, and metals with grain size in the ‘nano’ (~1×10-9 m) regime are now commonplace. These days, nanocrystalline metals are used in a variety of nonstructural and structural applications. Insuring the durability and reliability of structural, nanograined, metallic films requires an in-depth understanding of their mechanical properties. This study focuses on establishing the mechanical properties of nanocrystalline platinum thin films, with a substantial emphasis on the cyclic fatigue behavior. Thin film mechanical testing specimens were fabricated of nanocrystalline platinum in two film thicknesses. Various analysis techniques established these films to be composed of fully dense (111) oriented columnar grains roughly 15-35 nm in size of at least 99.99% pure platinum.. An annealing process allowed for the modification of the grain morphology such that the columnar grains were reduced to a more polycrystalline morphology; grains were still vertically elongated, but multiple 14-42 nm sized grains were present through the thickness of the annealed films. Creep testing of all free-standing specimens determined that the mechanical properties of the films are not significantly time dependent, in agreement with nanoindentation studies of identical films adhered to a silicon substrate. Tension testing indicated that the films are mechanically robust, with as-received, columnar films having yield and ultimate tensile strengths higher than 1 GPa and a large strain to failure of ~0.03. Annealed films maintained their strength but lost their ductility, displaying a strain to failure of ~0.01, consistent with their altered grain morphology. Scanning and transmission electron transmission based fractography revealed that failure was the result of ductile void-rupture processes and that the grain morphology of these specimens was stable in spite of the high stresses involved. Fatigue tests were conducted on ~500 nm films with central notches and resulted in a high power law exponent of ~10.5, and a fracture toughness of about ~5 MPa √m. This behavior is more reminiscent of the fatigue behavior of brittle ceramic systems than that of ductile metallic such as the micrograined form of the platinum, which had a power law exponent of ~3.3, and approximately an order of magnitude greater fracture toughness. Fractography revealed that the fatigue crack path exhibited two different crack advance modes. At low crack growth rates, grain coarsening and intergranular crack advance dominated while at high crack growth rates, typical ductile transgranular in the absence of grain coarsening dominated, and at intermediate rates, both crack advance modes were witnessed. A large portion of the study was devoted to characterizing the reasons for and mechanisms behind these observations and dependencies on specimen thickness and grain morphology followed using ~1 μm specimens, both columnar and polycrystalline. Further testing of films revealed that the fatigue crack growth rate behavior is largely dictated by specimen thickness, as ~1 μm specimens displayed a much larger range of fatigue crack growth (~20 MPa √m) and a more typical power law exponent of 3.9-4.2. Although the fatigue behavior of the platinum films has a strong dependence on the thickness of the films, the fracture mode in the films did not show the same dependence with the ~1 μm columnar specimens displaying a similar range of the crack growth modes. The polycrystalline samples, however, displayed a shift in the transition from the intergranular mode to the transgranular mode, and thus a link between the crack advance modes and the grain morphology was formed. Finite element modeling of an elastic anisotropic single crystal of platinum was utilized to quantify stresses in the vicinity of crack tips. Shear stress magnitudes along slip directions indicated that through-thickness slip is preferred over in-plane slip under certain conditions, and is dependent on both vicinity to crack tip and grain orientation.