Nanoidentation Study of the Mechanical Properties of Granular Metal Thin Films
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NANOINDENTATION STUDY OF THE MECHANICAL PROPERTIES OF GRANULAR METAL THIN FILMS M. R. SCANLON*, M. K. FERBER** AND R. C. CAMMARATA* * The Johns Hopkins University, Department of Materials Science and Engineering, Baltimore, MD 21218 ** High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-0117
ABSTRACT Nanoindenter techniques have been used to investigate the mechanical properties of Ag-A12 0 3 and Fe-SiO 2 granular metal films. A discontinuity in the rate of change of hardness as a function of metal volume fraction p was observed. The discontinuity occurs at the percolation threshold Pc of the metal, and appears to result from a change in the deformation mechanism at pc. A large peak in compliance (inverse modulus) as measured during indentation unloading was observed in the Ag-A120 3 films near Pc, but was not observed for the Fe-SiO2 films. The compliance peak displayed by Ag-A120 3 is believed to result from debonding at the metal-ceramic interface and subsequent interfacial sliding, and is not an intrinsic materials property.
INTRODUCTION Granular metals are two-phase composite materials that in their most common form consist of nanometer size metal granules embedded in a ceramic matrix [1]. These cermets exhibit unique properties due to the small crystallite size (below 10 nm) and large volume fraction of grain and interphase boundaries [1-5]. Although a wide variety of physical phenomena have been studied in several metal/insulator systems, not much work has been done to date on the mechanical properties of such systems. Results of the only reported investigation to date [2] indicated that dramatic changes in both the elastic and plastic properties as a function of metal volume fraction p occurred near the percolation threshold Pc
of the metal (that is, the value of the volume fraction where the metal changes from being in the form of noninteracting isolated granules to being an interconnected network). In this paper, the elastic and plastic properties of two granular metal systems, Ag-A1203 and Fe-
SiO 2 , as determined by nanoindentation, will be reported. EXPERIMENTAL PROCEDURE The granular metal films were produced by magnetron co-sputtering from a metal and an insulator target under an atmosphere of 10 mTorr of argon. The deposition rate of each of the materials was dependent on both the sputtering power and the target to substrate distance. Films with compositions ranging from p = 0 to p = I were produced by varying these parameters. The thickness of each film was about 3.0 jtm. Mechanical properties were measured using the nanoindenter at the High Temperature Materials Laboratory of the Oak Ridge National Laboratory. Five widely spaced indents were made in each sample using a three-sided diamond (Berkovitch) indenter. Force versus displacement curves were generated during both loading and unloading. Hardness values were. calculated as the nominal stress underneath the indenter. In addition, the effective elastic compliance, nominally equal to (1 - v 2 )/E, where
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