Application of nanocrystalline materials as matrices of composites: processing and performance advantages

Nanocrystalline materials, due to their small crystallite sizes in the range of 1 to 10 nm, exhibit high diffusivity, a capability for creep deformation at relatively low temperatures and the potential for alloying in systems that are ordinarily immiscible in solid state. A combination of these properties allows nanocrystalline materials to be densified at temperatures significantly lower than what their conventional crysalline counterparts require. As a result, nanocrystalline materials may be used as matrices of composites which can be processed at lower temperatures. This would reduce or eliminate thermal and chemical damage to the structure, which are common in conventional high-temperature composite processing. The benefits are demonstrated for composite made with single crystal sapphire fiber as reinforcement and zirconia, both in nanocrystalline as well as polycrystalline form as the matrices. The conventional zirconia composite with matrix powder particle sizes in the range of 30–40 μm was fabricated by hot pressing at 1400 °C (1673 K) and 16 ksi (112 MPa) while the composite with the nanocrystalline matrix having particle size 4–5 nm was hot pressed at 1040 °C (1313 K), keeping the pressure the same. Comparative analyses of the microstructures indicated that the nanocrystalline-based composite was denser and, as a result of a significant lowering of consolidation temperature, there was no indication of thermal damage to the fiber and no reaction between the fiber and the matrix.