Metal-Matrix/Hollow-Ceramic-Sphere Composites

These materials are relatively inexpensive, lightweight, stiff, tailorable, and machinable.

A family of metal/ceramic composite materials has been developed that are relatively inexpensive, lightweight alternatives to structural materials that are typified by beryllium, aluminum, and graphite/epoxy composites. These metal/ceramic composites were originally intended to replace beryllium (which is toxic and expensive) as a structural material for lightweight mirrors for aerospace applications. These materials also have potential utility in automotive and many other terrestrial applications in which there are requirements for lightweight materials that have high strengths and other tailorable properties as described below.

The ceramic component of a material in this family consists of hollow ceramic spheres that have been formulated to be lightweight (0.5 g/cm3) and have high crush strength [40–80 ksi (≈276–552 MPa)]. The hollow spheres are coated with a metal to enhance a specific performance — such as shielding against radiation (cosmic rays or x rays) or against electromagnetic interference at radio and lower frequencies, or a material to reduce the coefficient of thermal expansion (CTE) of the final composite material, and/or materials to mitigate any mismatch between the spheres and the matrix metal. Because of the high crush strength of the spheres, the initial composite workpiece can be forged or extruded into a high-strength part. The total time taken in processing from the raw ingredients to a finished part is typically 10 to 14 days depending on machining required.

For purposes of further processing, the material behaves like a metal: It can be processed by conventional machining (including formation of threads) or electrical-discharge machining, and pieces of the material can be joined by techniques commonly used to join metal pieces. The material is also receptive to coating materials and exhibits highly variable thermal conductivity from metal to ceramic depending on loading.

Typical mechanical properties of such a material include a density less than that of beryllium (ranging from 1.2–17 g/cm3 while the density of beryllium is 1.85 g/cm3) and modulus as high as 25 Msi (≈170 GPa). In contrast, the modulus of aluminum is generally 14 Msi (≈97 GPa). The CTE, the thermal conductivity, and the specific heat can be tailored, through the formulation of the ceramic and metal matrix ingredients of the composite.

This work was done by Dean M. Baker of Advanced Powder Solutions, Inc. for Goddard Space Flight Center. GSC-15348-1

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