Thermal Stability of Self-Supported Metallic Multilayered Thin Films
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Thermal Stability of Self-Supported Metallic Multilayered Thin Films A. Misra, H. Kung and R.G. Hoagland, Materials Science and Technology Division, MS G755, Los Alamos National Laboratory, Los Alamos, NM 87545 ABSTRACT The morphological stability and strength retention following elevated temperature exposure or thermal cycling will be crucial in exploiting the extremely high strengths of nanolayered materials in advanced engineering applications. The effects of elevated temperature (≤ 800 °C) vacuum annealing on the morphological stability and mechanical properties of sputter deposited Cu-Nb multilayers with 75 nm bilayer period are reported here. Even after 800 °C/ 1 hour anneal, the continuity of the layered structure is maintained and the bilayer periods are unchanged. The in-plane grain sizes in both Cu and Nb coarsened but were anchored at grooved boundaries preventing further growth. For a constant bilayer period, the effect of increasing the in-plane grain size on the multilayer hardness is found to be insignificant. After annealing, the layers are observed to be offset by shear along a vertical plane at the triple point junctions that have equilibrium groove angles aligned in a zig-zag pattern. A new mechanism is proposed for the evolution of this “anchored” structure that is resistant to further morphological instability. INTRODUCTION Multilayered thin films on substrates are used in a variety of applications such as x-ray mirrors, magnetic recording media and heads, diffusion barrier coatings, wear resistant coatings, etc. Thermal stability of these films has been studied mostly for the instability mechanisms involving interdiffusion in miscible layers, chemical reaction to form a new phase/compound or phase transformation (e.g., amorphous to crystalline), etc. Recent studies have shown that metallic multilayers, composed of alternating layers of soft metals, typically possess unusually high strengths when the bilayer periods are on the order of a few to a few tens of nanometers [1,2]. In addition to coatings on substrates, these high-strength metallic multilayers may also have applications as self-supporting components. Most studies of the morphological stability of fiber or lamellar composites have been conducted for materials where the microstructural features are on the micron-scale. These studies show that although lamellar composites are relatively more stable than fiber composites, significant morphological instabilities may still occur in layered materials such as pearlitic steels, γ/α2 titanium aluminides, rolled Ni-W, etc [3,4]. Recently, Josell and Spaepen [5] have studied the stability of near-micron scale multilayered films, primarily through creep testing, and observed layer pinch-off due to grain boundary grooving as a major instability mechanism. These studies indicate that complete degradation of the multilayer structure is often possible during short anneals at elevated temperatures. Clearly, more detailed studies of the thermal stability of nano-scale multilayers are needed to elucidate
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