Mechanical properties and microstructures of metal/ceramic microlaminates: Part I. Nb/MoSi 2 systems
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T. Y. Tsui and G. M. Pharr Department of Materials Science, Rice University, P.O. Box 1892, Houston, Texas 77251
W. C. Oliver Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6116 (Received 24 February 1992; accepted 23 June 1992)
Artificial multilayers, or microlaminates, composed of alternating layers of Nb and MoSi2 of equal thickness were synthesized by d.c, magnetron sputtering. Four different modulation wavelengths, A, were studied: 7, 11, 20, and 100 nm. The compositions, periodicities, and microstructures of the microlaminates were characterized by Auger electron spectroscopy and transmission electron microscopy. Structural characterization revealed that the as-deposited Nb layers are polycrystalline, while the MoSi2 layers are amorphous. The hardnesses and elastic moduli of the films were measured using nanoindentation techniques. Neither a supermodulus nor a superhardness effect could be identified in the range of wavelengths investigated; for each of the microlaminates, both the hardness and modulus were found to fall between the bounds set by the properties of the monolithic Nb and MoSi2 films. Nevertheless, a modest but a measurable increase in both hardness and modulus with decreasing wavelength was observed, thus indicating that behavior cannot be entirely described by a simple rule-of-mixtures. The hardness was found to vary linearly with A~1/2 in a manner similar to the Hall-Petch relationship. Annealing the microlaminates at 800 °C for 90 min produces significant increases in hardness and modulus due to chemical interaction of the layers. I. INTRODUCTION Synthetic multilayer structures consisting of alternating layers of crystalline metals and/or compounds have been reported to exhibit unique mechanical, 12 magnetic,3'4 electronic,4 and x-ray optical properties5-6 and may have a wide range of applications. One of the well-known phenomena resulting from periodic, thin-film multilayers is the "supermodulus effect", in which a 2- to 3-fold increase of the biaxial and flexural moduli has been reported for Cu-Ni, 7 ' 8 Au-Ni, 9 and Cu-Nb 10 ' 11 superlattices with modulation wavelengths, A, around 2 nm, although elastic softening has been observed for Mo/Ni12>13 superlattices as modulation wavelengths approach 2 nm. Similarly, the hardnesses of multilayers have also been reported to exhibit unusual behavior at small bilayer thicknesses. For example, metallic multilayers such as Mo/V and Nb/Ta and nonmetallic multilayers like TiN/VN have been reported to exhibit "superhardnesses" at bilayer thicknesses around 5 nm. 14~16 Although a number of models have been postulated to explain the observed elastic and plastic anomalies, they can, in general, be grouped into two categories: one attributed to structural effects and the other to electronic effects. In the first category, two theories have been J. Mater. Res., Vol. 7, No. 10, Oct 1992
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postulated, one related to coherent interfaces and the other related to in
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