Microwave Sintering of Zirconia-Toughened Alumina Composites
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MICROWAVE SINTERING OF ZIRCONIA-TOUGHENED ALUMINA COMPOSITES*
H. D. Kimrey, J. 0. Kiggans, M. A. Janney, and R. L. Beatty Oak Ridge National Laboratory Oak Ridge, Tennessee
INTRODUCTION
Microwave sintering possesses unique attributes and has the potential to be developed as a new technique for controlling 1 microstructure to improve the properties of advanced ceramics. -6 Because microwave radiation penetrates most ceramics, uniform volumetric heating is possible. Thermal gradients, which are produced during conventional sintering because of conductive and radiative heat transfer to and within the part, can be minimized. By eliminating temperature gradients, it is possible to reduce internal stresses, which contribute to cracking of parts during sintering, and to create a more uniform microstructure, which may lead to improved mechanical properties and reliability. With uniform, volumetric temperatures, the generation of nonuniform particle/grain growth due to temperature gradients and associated sintering gradients can be regulated. Recent investigations have identified additional benefits to microwave sintering. Using 28-GHz radiation, it was demonstrated that alumina could be densified at 300-400°C below temperatures used in conventional processing and that a uniform, fine-grained microstructure could be obtained. 1 -3 With the advent of lowertemperature processing, it is possible to reduce grain growth, vaporization, and interactions between phases, which are often significant in the fabrication of advanced ceramics.
*Research sponsored by the U.S. Department of Energy Assistant Secretary of Conservation and Renewable Energy, Office of Industrial Technologies, Advanced Industrial Materials Program under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
Mat. Res. Soc. Symp. Proc. Vol. 189. ©1991 Materials Research Society
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BACKGROUND
One class of ceramics, in which microstructural control in conjunction with microwave processing is expected to yield improved properties, is transformation-toughened ceramics. The retention of the tetragonal zirconia phase (t-Zr02) has previously been shown to control the mechanical properties of composites that contain dispersed Zr02 particles. 7 - 9 The amount of t-Zr02 retained in the body and the microstructure of the composite significantly affect mechanical properties. Limitations of currently produced zirconia-toughened alumina are related to (1) variations in the composition of the stabilizing additive and (2) the location, grain size, and morphology of the dispersed tetragonal zirconia second phase. For the greatest toughening effects, the tetragonal Zr02 phase particles must be transformed at a relatively low applied tensile stress, and all particles must transform for similar conditions. This requires that the solute content in the body be uniform and optimized, that the Zr02 particles have a uniform desired size, and that the shape of the particles be similar (i.e., polyhedral particles transform more easily than do spherical ones). Improv
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