Sylvia M. Johnson

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.

Low-cost matrix development for an Oxide-Oxide composite

Continuous oxide fiber/oxide matrix composites are attractive for use as high temperature structural materials because they can combine composite properties with long-term oxidative stability. The development of a matrix for such a composite and prevention of matrix-fiber coating interaction is described here. The goal use temperature of this composite is 1100°C to 1200°C. The composite is being developed by the M C Consortium comprising 3M, Rockwell International, and SRI International. The composite consists of an alumina-based woven-tow fiber preform, coated with lanthanum phosphate (monazite) to promote fiber debonding and pullout, in an oxide matrix derived from a preceramic-polymer slurry filled with active and inert powders. This approach to the matrix enables conventional polymer matrix composite technology to be used in composite part fabrication. Only one infiltration of the matrix is required, a critical factor in keeping the cost low.

In situ observation of the orthorhombic‐tetragonal phase transformation in YBa2Cu3O7−δ

We have carried out in situ heat treatments of YBa2Cu3O7−δ at temperatures between 25 and 500 °C and oxygen pressure of 10 Torr. It was found that the original orthorhombic (OI phase) YBa2Cu3O7−δ transforms into the OII phase (still orthorhombic, but with a doubling of the a‐axis, and with a δ=0.5) at about 250 °C. As the temperature is increased further, the tetragonal phase of YBa2Cu3O7−δ forms. As the tetragonal phase is slowly cooled to room temperature, the orthorhombic phase OI reappears. Cycling the temperature repeatedly has indicated the regenerability of the OI phase.

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.

Fe Substituted, Laser Ablated YBa 2 Cu 3 O 7 films using Off-Stoichiometric Targets

We present data on thin films of YBa 2 Cu 3 O 7 (YBCO) and YBa 2 (Cu 1−x Fe x ) 3 O 7 prepared by laser ablation using a sequence of targets, including Cu-deficient YBa 2 Cu 2.4 O 7 , CuO, and (CuO) 1−y (FeO) y . This technique achieves mixing on an atomic scale. We find that stochiometric films, made using a combination of the Cudeficient target and CuO, have a sharp transition (width c . We have achieved substitution of Fe on the Cu sites, using a sequence of the three targets for a range of Fe concentrations. T c decreases nearly linearly with concentration and T c → 0 near 15% Fe. The T c suppression for the films is slightly less than the corresponding value obtained for bulk samples.

Continuous SiC Fiber/Glass Composites

Recently, research on ceramic-ceramic composites has significantly increased because of the potential of preparing materials with improved crack resistance. Most of the composites studied to date contain short fibers or whiskers as the toughening agent rather than continuous fibers, partly because various ceramic whiskers are widely available and it is relatively easy to process whisker composites. In contrast, only one type of continuous-tow SiC fiber is readily available, and methods of processing continuous fiber composites are more complex.