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The desire to make structural materials ever lighter and for use at higher and higher temperatures has prompted the development in the past 20 years of polyphase ceramic composites. The problem with ceramics, of course, is that they are brittle, that is, they have an exceedingly low fracture toughness because their lack of intrinsic ductility makes them very flaw sensitive. The composite approach addresses the flaw-sensitivity issue: elastic energy is converted to surface energy, not by the propagation of a single crack (which leads to catastrophic failure of the component), but rather by the delamination of the fiber reinforcement phase from the matrix phase and the retention of integrity in the part by the fibers. Further, frictional dissipation as fibers are pulled from the matrix absorbs substantial energy, too. One of the best examples of the approach was the use of polymer-derived, continuous silicon carbide fibers embedded in an aluminosilicate glass-ceramic: an oxidation reaction between the matrix and the fiber placed a (100-nm cylindrical-sheath interphase of nanocrystalline carbon between fiber and matrix: cracks would shred this interphase, the fibers would remain intact and the part could be as tough at 1300 deg C as aluminum alloys are at room temperature, provided that there was little oxygen in the environment. With oxygen reaching the interphase through matrix microcracks, the carbon interphase would rapidly convert to a silica interphase (strongly bonded to fiber and matrix) and the material would be as brittle as a monolithic ceramic. Our approach was to mimic the SiC/silicate composites with aluminum oxide fibers embedded in a silicate matrix, but create a "functional" interphase from a high-temperature, synthetic mica (e.g., fluorophlogopite: KMg3[Si3Al]O10F2). The idea(s) evolved from concepts of oxide/silicate equilibria as developed in the fields of metamorphic and igneous petrology; as such, we refer to the approach as petromimetics; i.e., mimicking rocks. The approach creates 1300 deg C+ materials that remain both mechanically tough and thermodynamically stable even in air; the toughness persists even in a moist-air environment despite the fact that water seeks to attack the F- in the mica (Figs. 4 and 5).
Figure 4: Engineered laminate of Al2O3 (alumina), MgAl2O4 (spinel) and KMg3[Si3Al]O10F2 (fluorophlogopite mica). The composite is a fracture tough, high-temperature ceramic. In this specimen, the width of the mica layer (phase with the bladed morphology) is approximately 80 mm.
Figure 5: Ambient-temperature mechanical response of alumina/spinel/fluorophlogopite/spinel/alumina laminates as a function of processing temperature (which affects mica morphology). In these flexure experiments, the tough laminates experience stable crack growth confined to the mica interphase. As a consequence, unloading and reloading the specimen allows monitoring the evolving compliance with delamination.
Our composite work now concentrates on reinforcement of nm-scale oxide grains with nm-scale ceramic whiskers. The work, part of the Materials Research Science and Engineering Center for Nanostructured Materials and Interfaces, is collaborative with Profs. Robert Carpick, Walter Drugan and Donald Stone. The idea is only similar in part to that described above. Carpick's research in atomic force microscopy (AFM) suggests that friction behavior between solids differs from the standard, macroscopic Coulomb description when the sub-micrometer/nanometer scale is approached. As such, one can envision a composite physics distinctly different if a nm-grain-size matrix is reinforced with high-aspect-ratio, ~1-nm diameter single-crystal whiskers. First-order effects can be imagined, then, based on chemical segregation of dopants either to or away from the whisker-matrix phase boundary. Our research will probe this and other possibilities of nm-scale chemical design of structural composites.
Sample Publications:
King, T.T. and R.F. Cooper (1994). Ambient-temperature mechanical response of alumina-fluoromica laminates. J. Am. Ceram. Soc., 77, 1699-1705.Download pdf
King, T.T., W. Grayeski and R.F. Cooper (2000). Thermochemical reactions and equilibria between fluoromicas and silicate matrices: A petromimetic perspective on structural ceramic composites. J. Am. Ceram. Soc., 83, 2287-2296.Download pdf
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