Compositions can be graded from porous substrates to impervious outer layers.
A family of tantalum-based ceramics has been invented as ingredients of high-temperature composite insulating tiles. These materials are suitable for coating and/or permeating the outer layers of rigid porous (foamlike or fibrous) ceramic substrates to (1) render the resulting composite ceramic tiles impervious to hot gases and (2) enable the tiles to survive high heat fluxes at temperatures that can exceed 3,000 °F (≈1,600 °C). Originally intended for use on the future space exploration vehicles, insulating tiles made with these materials may also be useful in terrestrial applications (e.g., some industrial processes) in which there are requirements to protect against flows of hot, oxidizing gases.
The starting ingredients of a ceramic of this invention are the following:
- Tantalum disilicide (TaSi2), which can act as either an emittance agent or a matrix material, depending upon the overall composition;
- Molybdenum disilicide (MoSi2), which, depending on the overall composition, acts as a secondary emittance agent or as an oxygen getter (to inhibit the oxidation of tantalum within the finished composite);
- A borosilicate glass (B2O3·SiO2), which acts as a source of boron and may, depending on the overall composition, act as an alternative matrix; and
- Silicon hexaboride (SiB6), which acts as a processing aid by facilitating sintering of the aforementioned ingredients.
These ingredients are milled together in ethanol, and the resulting slurry is sprayed or painted onto a porous ceramic substrate. The underlying layers (sublayers) of different compositions are used to integrate the outer layer (coating) with a wide variety of porous substrate materials. For this purpose, infiltration of these compositions into the substrate results in a functional gradient system that accounts for the difference in the coefficient of thermal expansion (CTE) between coating and substrate. The preferred composition for such underlying layers are in the approximate range of 20 to 60 weight percent of MoSi2, 0 to 60 weight percent of TaSi2, 40 to 80 weight percent of borosilicate glass, and 1 to 5 percent of SiB6. The ingredients are then sintered by heating the treated substrate at atmospheric pressure at either a temperature of 2,225 °F (≈1,218 °C) for 90 minutes or a temperature of 2,400 °F (≈1,316 °C) for 10 minutes. The milling, coating, and sintering process conditions are chosen to minimize the undesired oxidation of tantalum compounds.
For an outer coating layer, the preferred composition is between 20 and 45 weight percent of borosilicate glass, between 10 and 65 weight percent of TaSi2, between 5 and 30 weight percent of MoSi2, and between 1 and 5 weight percent of SiB6. An underlying layer or sublayer of different composition can be used to integrate the outer layer with the substrate and, for this purpose, may be allowed to infiltrate to some small depth below the surface of the substrate. The preferred composition for such an underlying layer lies in the approximate range of 20 to 60 weight percent of MoSi2, 40 to 80 weight percent of B2O3·SiO2, and 1 to 5 percent of SiB6. One or more intervening layer(s) of intermediate composition(s) could also be included (see figure). The precise composition of the sublayer should be chosen to match the CTE of the substrate, while the compositions of intermediate layers should be chosen to grade the transition from the CTE and porosity of the substrate to the CTE and full density of the outer coating layer.
This work was done by David A. Stewart, Daniel Leiser, Robert DiFiore, and Victor Katvala of Ames Research Center. This invention is owned by NASA, and a patent application has been filed. Inquiries concerning rights for the commercial use of this invention should be addressed to the Ames Technology Partnerships Division at (650) 604-2954. Refer to ARC-14743-1.