Sputtering synthesis and properties of molybdenum nanocrystals and Al/Mo layered nanocomposites
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T. E. Schlesinger and R. C. Cammarata Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218
M. E. Twigg and A. S. Edelstein Naval Research Laboratory, Washington, DC 20375 (Received 27 August 1990; accepted 27 December 1990) Molybdenum nanocrystals (4 nm < particle size < 12 nm) were synthesized in the vapor by sputtering in a thermal gradient at argon pressures between 0.2 and 0.6 Torr. The gradient was achieved by cooling the substrate table with liquid nitrogen in order to enhance the deposition rate of the nanocrystals. The size of these nanocrystals depended on the sputtering argon gas pressure. Using appropriate deposition parameters, these nanocrystals were incorporated, either as a dispersed phase in a major phase produced by normal sputtering or as a layer in a layered structure to produce nanocomposite films. Results of Auger electron spectroscopy (AES) study of the Al/Mo layered nanocomposite films are presented. The Knoop microhardness of these films was increased by as much as a factor of four compared with the hardness of homogeneous Al films. A correlation of the microhardness to the microstructures as revealed by transmission electron microscopy (TEM) is discussed.
I. INTRODUCTION
Nanocomposites are composites containing nanometer-size regions of the individual phases. Nanocomposites of immiscible phases can be fabricated as thin films and coatings by conventional sputterdeposition using cosputtering from a single composite target or from different targets.1'2 However, it is difficult to avoid undesirable compound or alloy formation when the phases are thermodynamically miscible. One way of making phase-separated composite films from miscible phases is to arrange the phases in a layered structure. This method has the disadvantage that one cannot control the particle size by post-deposition annealing without promoting undesirable alloying or compound formation. An alternative approach using sputter-deposition to fabricate phase-separated nanocomposite films has been recently reported.3 This new method circumvents the above limitations in making phase-separated nanocomposite films by forming the particles of the minor phase in the vapor by rapid condensation. The grain size and degree of aggregation of a)
Research Associate of the National Research Council, United States National Academy of Sciences. Current address: GeoCenters, Inc., Fort Washington, Maryland 20744; on-site at Code 6090, Center for Biomolecular Science and Engineering, Naval Research Laboratory, Washington, DC 20375. Address correspondence to this author. J. Mater. Res., Vol. 6, No. 4, Apr 1991
the particles can be controlled by varying the sputtering gas pressure without any post-deposition annealing. These particles are then incorporated into a major phase produced by normal sputtering. By controlling the thickness of the major phase and the deposition rate of the particles (thus the coverage or number of particles per unit area), either a dispersion or a layeredstructured nan
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