A process that involves reaction bonding makes it possible to form strong joints, with tailorable thicknesses and compositions, between high-temperature-resistant structural parts made of SiC-based ceramic materials. These parts and materials are being developed for use as engine components, radiant-heater tubes, heat exchangers, components of fusion reactors, furnace linings, furnace bricks, and components for diffusion processing in the microelectronics industry. The process can be used to join simply shaped parts to make complexly shaped structures, and to repair such parts and structures.
The present process is a successor to the process reported in "Joining of SiC-Based Ceramic and Fiber-Reinforced Composite Parts" (LEW-16405), NASA Tech Briefs,Vol. 22, No. 5 (May 1998), page 54. The process begins with the application of a carbonaceous mixture (typically in paste form) to the joint regions between parts, immediately followed by clamping of the parts in a fixture. The thickness of the joint can be tailored by choice of the properties of the carbonaceous mixture and of the clamping force. The carbonaceous mixture is cured at a temperature between 110 and 120 °C for 10 to 20 minutes. The curing step fastens the parts together (albeit not yet at full strength), making it unnecessary to fabricate a special fixture to hold the parts together during subsequent high-temperature processing.
After curing of the carbonaceous layer, silicon or a silicon alloy in tape, paste, or slurry form is applied to the joint region. Then the parts are heated to a temperature between 1,250 and 1,425 °C for 5 to 10 minutes, the precise temperature and time depending on the applied material. The heating causes the silicon to melt, infiltrate the joint, and react with carbon. As a result, the finished joint contains silicon carbide with amounts of silicon and other phases that can be tailored by choice of the compositions of the reactants. Consequently, the process results in joints with tailorable microstructures and thus tailorable thermomechanical properties. The properties of the joints can thus be tailored to approximate closely those of the joined parts.
The figure depicts the microstructures of selected experimental reaction-formed joints of three different thicknesses between samples of a commercially available reaction-bonded silicon carbide. The thickest of these joints was found to consist mainly of Si with small amounts of SiC, and to be susceptible to brittle fracture. The thinnest of these joints were found to consist of SiC and Si phases. At both room temperature and temperatures up to 1,350 °C in air, the strength of the material in the thinnest joints was found to be at least equal to and, in some cases, greater than, that of the adjacent SiC sample materials.
This work was done by M. Singh of NYMA, Inc., for Lewis Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
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Refer to LEW-16661.