Defect-dependent elasticity: Nanoindentation as a probe of stress state
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J.D. Kielyb) and J.E. Houston Sandia National Laboratories, Albuquerque, New Mexico 87185-1415
P.E. Russell North Carolina State University, Raleigh, North Carolina 27695-7531 (Received 12 October 1999; accepted 1 May 2000)
Using an interfacial force microscope, the measured elastic response of 100-nm-thick Au films was found to be strongly correlated with the films’ stress state and thermal history. Large, reversible variations (2×) of indentation modulus were recorded as a function of applied stress. Low-temperature annealing caused permanent changes in the films’ measured elastic properties. The measured elastic response was also found to vary in close proximity to grain boundaries in thin films and near surface steps on single-crystal surfaces. These results demonstrate a complex interdependence of stress state, defect structure, and elastic properties in thin metallic films.
I. INTRODUCTION
While it has long been recognized that a material’s plastic properties are controlled by its defects, a material’s elastic properties are generally attributed to its bond character.1,2 However, a few reports have suggested a link between structure and elastic response for particular materials. Such elastic properties have been attributed to intercrystalline regions with differing interatomic spacings,3–8 potentials,8 or porosity.7,9 In most cases the link between elastic response and structure was found to be highly dependent upon processing conditions. A variety of thin films (Al, Cu, Au, Au–Ni, Ag–Ni) deposited under nonequilibrium conditions8,10–15 and nanocrystalline bulk materials (Cu, Pd, Au, Ni, Fe)3–7 have exhibited large variations of hardness, elasticity, and stress state as a function of processing. These reports suggest a link between measured elastic properties and defect structure. A previous interfacial force microscope (IFM) study revealed that the deformation response of 200-nm-thick Au films was dependent upon their defect structure.16 A survey of individual grains demonstrated that their initial plastic response was accommodated by intergranular deformations (grain boundary sliding). The authors speculated that grain to grain variations of the measured elastic
a)
Present address: Intel Corporation 2200 Mission College Boulevard, Santa Clara, California 95052-8119. b) Present address: Seagate Research, 2403 Sidney Street., Pittsburgh, Pennsylvania 15203-2116. J. Mater. Res., Vol. 15, No. 8, Aug 2000
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modulus and shear-stress at yield were attributed to local differences in the defect structure (subsurface free volume). More recently, the IFM was used to probe the elastic properties of Au films with smaller grain sizes (average grain diameter of 25–50 nm instead of 500 nm).17 For these samples, measurements of elastic moduli were consistent from point-to-point and sampleto-sample for samples made with similar deposition conditions. However, large sample-to-sample variations were observed as a function of film adhesion-layer combination and substr
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