Mechanical properties and microstructures of metal/ceramic microlaminates: Part II. A Mo/Al 2 O 3 system
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S. D. McAdams 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)
Synthetic multilayers consisting of periodic layers of the refractory metal Mo and the oxide ceramic AI2O3 have been produced by alternating d.c. and r.f. reactive sputter deposition. Microlaminates with four different modulation wavelengths—5, 20, 30, and 100 nm—were investigated in this study. The compositions, periodicities, and microstructures of the microlaminates were characterized by Auger electron spectroscopy, low-angle x-ray diffraction, and transmission electron microscopy, including high resolution lattice imaging and microdiffraction. Transmission electron microscopy from the microlaminates indicated that the as-deposited Mo layers are polycrystalline, while the as-deposited A12O3 layers are primarily amorphous. The Mo and A12O3 layers are thermally compatible at 800 °C for 6 h, showing no evidence of atomic interdiffusion between the layers. The mechanical properties of the microlaminates, as well as those of monolithic films of Mo and A12O3 (i.e., the baseline materials), were investigated using nanoindentation methods. A higher than expected modulus and hardness were observed for the microlaminate with the longest wavelength (100 nm); otherwise the mechanical properties are explainable by a rule-of-mixtures. The enhanced mechanical properties of the 100 nm microlaminate may be attributed to crystallization of the amorphous A12O3 layers and the evolution of a structural texture within this phase.
I. INTRODUCTION Artificial multilayer structures have been reported to exhibit a unique combination of structural,1'2 magnetic,3'4 electronic,4 and x-ray optical properties5'6 and may have a wide range of applications. One well-known phenomenon reported for artificial multilayers is the "supermodulus effect," in which a 2- to 5-fold increase of the biaxial and flexural moduli has been claimed for some multilayer structures with modulation wavelengths in the range of 2 nm. Since it was first reported for the Au-Ni system,7 the supermodulus effect has received considerable attention, both experimentally and theoretically, and has been studied in a variety of systems, e.g., Cu-Ni, 8 - 9 Cu-Nb, 10 ' 11 and Mo/Ni.12-13 In addition to anomalous elastic properties, anomalously high hardnesses have also been reported for certain multilayers including Nb/Ta, 14 Mo/V, 14 and TiN/VN. 15 It is noted, however, that many of the reported experimental results are inconsistent,16'17 and even in the extensively studied Cu/Ni system, the existence of a supermodulus effect has recently been questioned.9'18'19 2774
http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 10, Oct 1992
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One of the fundamental factors leading to the confusion surrounding the supermodulus effect is the validity of comparing elastic moduli determined b
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