Percolation Effects in the Mechanical Properties of Granular Ni-A1 2 O 3 thin films
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PERCOLATION EFFECTS IN THE MECHANICAL PROPERTIES OF GRANULAR Ni-A1 2 0 3 THIN FILMS T.E. SCHLESINGER*, A. GAVRIN**, R.C. CAMMARATA*, AND ý.-L. CHIEN* Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218 Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218 ABSTRACT The mechanical properties of sputtered Ni-AlO granular thin films were investigated by low load microharaness testing. It was found that the microhardness of these films displayed a percolation threshold at a nickel volume fraction of about 0.6, below which the hardness is greatly enhanced. This behavior is qualitatively similar to the electrical and magnetic properties of these types of films. A percolation threshold in hardness can be understood as due to a change in the mechanism for plastic deformation. INTRODUCTION Granular metal films are composed of nanometer size metal particles embedded in an immiscible (generally electrically insulating) matrix. This class of ultrafine microstructure materials has displayed unusual transport, magnetic, optical, and superconducting properties [1-4). Associated with many of these unusual properties is the percolation threshold, which is often near a metal volume fraction p of about 0.6 [5). Below the percolation threshold, isolated metal granules of nanometer sizes are imbedded in an amorphous insulating matrix. Above the percolation threshold, the metal granules are interconnected in an infinite network exhibiting metallic conductivity. The insulator is distributed throughout the film, filling the remaining volume. Because of their unique structure, granular metal films often exhibit synergistically enhanced properties compared to those of the individual components. For example, Ni-A12 03 granular films are harder and more corrosion resistant than films composed only of the metal, but still exhibit magnetic properties characteristic of Ni. In this paper, we present an investigation of the mechanical properties of granular metal films as measured by microhardness testing. As the volume fraction of metal is reduced below about 0.6, a dramatic increase in hardness is observed, similar to changes in the electrical and magnetic properties of this type of material. EXPERIMENTAL Thin granular Ni-A12 9 3 films were prepared using a highrate rf magnetron sputtering system with composite targets of Ni and A1l2 0.3 The films were examined by X-ray diffractometry, revealing that the Al 03 matrix was amorphous, and the nickel granules were crystalline, possessing the fcc Mat. Res. Soc. Symp. Proc. Vol. 195. 01990 Materials Research Society
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In films with smaller structure of bulk Ni (see Figure 1). 3 were about the Ni granules volume fractions of nickel, Above a volume fraction of 0.6 (near the nm in diameter. were granules the interconnected threshold), percolation All the samples were about 6 "m in somewhat larger (-6 nm). A series of thickness, deposited on sapphire substrates. films was produced with volume fraction p of Ni ranging from
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