Workers at NASA Ames Research center are endeavoring to develop durable, oxidation- resistant, foam thermal protection systems (TPSs) that would be suitable for covering large exterior spacecraft surfaces, would have low to moderate densities, and would have temperature capabilities comparable to those of carbon- based TPSs [reusable at 3,000 °F (≈1,650 °C)] with application of suitable coatings. These foams may also be useful for repairing TPSs while in orbit. Moreover, on Earth as well as in outer space, these foams might be useful as catalyst supports and filters.
Preceramic polymers are obvious candidates for use in making the foams in question. The use of these polymers offers advantages over processing routes followed in making conventional ceramics. Among the advantages are the ability to plastically form parts, the ability to form pyrolized ceramic materials at lower temperatures, and the ability to form high-purity microstructures having properties that can be tailored to satisfy requirements.
Heretofore, preceramic polymers have been used mostly in the production of such low-dimensional products as fibers because the loss of volatiles during pyrolysis of the polymers leads to porosity and large shrinkage (in excess of 30 percent). In addition, efforts to form bulk structures from preceramic polymers have resulted in severe cracking during pyrolysis. However, because the foams in question would consist of networks of thin struts (in contradistinction to nonporous dense solids), these foams are ideal candidates for processing along a preceramic-polymer route.
The present research explores the feasibility of forming ceramic foams using sacrificial blowing agents and/or sacrificial fillers in combination with preceramic polymers. The possibility of using reactive fillers in combination with the aforementioned ingredients is also investigated. The use of such reactive fillers as Ti or Si reduces the large shrinkage observed in pyrolysis of polymers. The fillers also react with excess carbon that, in the absence of such reaction, would be present in the foam pyrolysis products. A reactive filler becomes converted to a ceramic material with an expansion that reduces overall shrinkage in the pyrolized part. The expansion of the reactive filler thus compensates for the shrinkage of the polymer if the appropriate volume fraction of filler is present in a reactive atmosphere (e.g., N2 or NH3).
Previously, this reactive-filler approach yielded limited success in efforts to make fully dense structural composite materials (in contradistinction to foams). However, in the present research, this reactive-filler approach has been modified to enable processing of foams with minimal shrinkage.
Figure 1 shows representative foam microstructures. In all cases, the foams are isotropic, open-celled structures. Foams processed by use of a polyurethane blowing agent have large cell sizes (50 to 500 μm), whereas foams processed by incorporating sacrificial fillers (e.g., polymer microspheres) generally have much smaller cell sizes (as low as 3 μm, depending on the diameter of the starting sacrificial filler particles). In all three micrographs of Figure 1, it is evident that the original unpyrolized structure is retained after pyrolysis, without loss of spherical cell shape.