Michael Gusman

Recent Developments in Ultra High Temperature Ceramics at NASA Ames

NASA Ames is pursuing a variety of approaches to modify and control the microstructure of UHTCs with the goal of improving fracture toughness, oxidation resistance and controlling thermal conductivity. The overall goal is to produce materials that can perform reliably as sharp leading edges or nose tips in hypersonic reentry vehicles. Processing approaches include the use of preceramic polymers as the SiC source (as opposed to powder techniques), the addition of third phases to control grain growth and oxidation, and the use of processing techniques to produce high purity materials. Both hot pressing and field assisted sintering have been used to make UHTCs. Characterization of the mechanical and thermal properties of these materials is ongoing, as is arcjet testing to evaluate performance under simulated reentry conditions. The preceramic polymer approach has generated a microstructure in which elongated SiC grains grow in the form of an in-situ composite. This microstructure has the advantage of improving fracture toughness while potentially improving oxidation resistance by reducing the amount and interconnectivity of SiC in the material. Addition of third phases, such as Ir, results in a very fine-grained microstructure, even in hot-pressed samples. The results of processing and compositional changes on microstructure and properties are reported, along with selected arcjet results.

Interactions between crystalline Si{sub 3}N{sub 4} and preceramic polymers at high temperature

Polymeric precursors to Si{sub 3}N{sub 4} were mixed with Si{sub 3}N{sub 4} powder, in various combinations, to form coatings in Si{sub 3}N{sub 4} and to control microstructure of Si{sub 3}N{sub 4}. The microstructure of polymer-derived ceramic and preceramic materials was examined by transmission electron microscopy (TEM) after heat treatment at 900 C and 1,200 C and sintering at 1,650 C. The observed microstructure suggests that the polymer-derived-material properties will approach those of conventionally formed Si{sub 3}N{sub 4}. With proper viscosity control, gaps between {alpha}-Si{sub 3}N{sub 4} powder particles as narrow as 5 to 10 nm are filled by the polymer ensuring full wetting of the {alpha}-Si{sub 3}N{sub 4}. Voids as large as 0.3 {mu}m between {alpha}-Si{sub 3}N{sub 4} particles are filled by the ceramic precursor, reducing the size of pores formed during subsequent sintering processes. Intimate bonding between the amorphous, polymer-derived-material and the crystalline Si{sub 3}N{sub 4} grains is observed after heat treatments or sintering, a necessary condition for achieving high-quality properties similar to conventional material. Acicular grains of {beta}-Si{sub 3}N{sub 4} formed from the equiaxed {alpha}-Si{sub 3}N{sub 4} powder/PCMS mixture upon sintering are also observed.