Work is currently being performed at NASA Ames Research Center on development of a durable, oxidation-resistant reusable acreage foam TPS, with low-to-moderate density and temperature capability comparable to that of the carbon systems (reusable at 3000°F) with application of a suitable coating. This foam may also be used as an on-orbit repair material. Other potential applications for this material include catalyst supports and filters.

Preceramic polymers are obvious candidates for the processing of foams. Use of these polymers offers many advantages over conventional ceramic processing routes. Advantages include being able to plastically form the part form a pyrolized ceramic material at lower temperatures, and form high purity microstructures that are tailorable depending on property requirements. To date, preceramic polymers are mostly used in the production of low dimensional products such as fibers since loss of volatiles during pyrolysis leads to porosity and large shrinkage (in excess of 30%). In addition, formation of bulk structures has resulted in severe cracking during pyrolysis. However, the morphology of the network structure in the foam (thin struts) makes it an ideal candidate for processing from a preceramic polymer route.

The present research explores the feasibility of formation of ceramic foams using sacrificial blowing agents and/or sacrificial fillers in combination with pre-ceramic polymers. The possibility of using reactive fillers in combination with the aforementioned approach is also investigated. The use of reactive fillers (e.g., Ti or Si) reduces the large shrinkage observed in polymer pyrolysis. The fillers also react with excess carbon that would be present in the unfilled foam pyrolysis product. The reactive filler converts to a ceramic material with a volume expansion that reduces overall shrinkage in the pyrolized part. The expansion of the reactive filler thus compensates for the polymer shrinkage if the appropriate volume fraction of filler is present in a reactive atmosphere (e.g., N₂ or NH₂). This approach has resulted in structural composites with limited success. However, we have modified this filled pre-ceramic polymer approach to process foams with minimal shrinkage.

Representative foam morphologies are shown in Figure 1. In all cases the foams are isotropic open-celled structures. Foams processed using a polyurethane blowing agent approach have large cell sizes (50 to 500 µm), whereas foams processed incorporating sacrificial fillers (e.g., polymer microspheres) generally result in much smaller cell sizes (as low as 3 µm depending on the diameter of the starting sacrificial filler). In all three micrographs, it is evident that the original unpyrolized structure is retained after pyrolysis without loss of cell spherical shape.

Preliminary arc-jet testing was conducted on SiC foams processed from preceramic polymers. Arc-jet facilities are used to simulate the aerothermal heating environments experienced by space vehicles during atmospheric entry. The tests discussed herein were conducted in the NASA Ames 60-MW Interaction Heating Facility (IHF). In the IHF, flowing air is heated by an electric discharge. Small diameter disks (1.5 inch) were tested with 90 second exposure to 1650°C. Samples showed minimal degradation under these conditions. The two test samples were beveled 0.5-inch thick, 1.5-inch diameter disks. These samples were placed in a silicon-carbide coated graphite model holder and exposed to the arc-jet for 90 seconds (Figure 2).

In summary, the use of pre-ceramic polymers with the addition of sacrificial blowing agents and sacrificial/reactive fillers offers a feasible approach to form open-cell foam for insulation and other potential applications including filters and catalyst supports. Current work demonstrates that this is a viable method to form refractory ceramic foams at relatively low processing temperatures (1200°C) with encouraging properties. This approach allows for tailoring of foam properties (such as composition, pore size, strength, etc) by varying the processing conditions. The potential of these materials for use as an on orbit repair approach for leading edge damage to support NASA's Shuttle Return to Flight efforts is currently being explored.