The use of reliable, high-temperature, lightweight materials in the manufacture of aircraft engines is expected to result in lower fossil and biofuel consumption, thereby leading to cost savings and lower carbon emissions due to air travel. Although nickel-based superalloy blades and vanes have been successfully used in aircraft engines for several decades, there has been an increased effort to develop high-temperature, lightweight, creep-resistant substitute materials under various NASA programs over the last two decades. As a result, there has been a great deal of interest in developing SiC/SiC ceramic matrix composites (CMCs) due to their higher damage tolerance compared to monolithic ceramics. Current-generation SiC/SiC ceramic matrix composites rely almost entirely on the SiC fibers to carry the load, owing to the premature cracking of the matrix during loading. Thus, the high-temperature usefulness of these CMCs falls well below their theoretical capabilities.
The objective of this work is to develop a new class of high-temperature, lightweight, self-healing, SiC fiber-reinforced, engineered matrix ceramic composites. Several engineered matrices were designed to be thermally compatible with SiC. Different tests were conducted on these matrices, which helped to down-select suitable compositions. Engineered matrix composites (EMCs) designed to match the coefficient of thermal expansion (CTE) of the SiC fiber are being fabricated by slurry casting and melt infiltration techniques. The matrix composition was designed to convert any ingressed oxygen into lowviscosity oxides so they can flow into the cracks due to capillary action and seal them, thereby activating its self-healing properties.
The concept uses the fundamental principles of physics and materials science to develop a new class of self-healing ceramic composites (SHCCs). Unlike current SiC/SiC CMC technology, the present concept develops SiC fiber-reinforced SiC-Si3N4-intermetallic matrix composites with a composition formulated to match the CTE of the fibers, and with an ability to absorb ingressed oxygen and self-heal cracks by filling them with low-viscosity oxides.
The concept provides considerable flexibility in designing the composite matrix for a wide variety of high-temperature applications. Depending on the composition, the intermetallic phases deform plastically at high temperatures, unlike SiC and Si3N4. Thus, the matrix is likely to be plastically compliant to the applied loading conditions at high temperatures rather than develop cracks. This important feature allows the matrix to carry some load before transferring to the reinforcing SiC fibers, thereby potentially extending the life of the composite. The ability of these matrices to selfheal fine cracks is also expected to increase composite life. For some matrices, the expected amount of free silicon after melt infiltration is expected to be low, which would allow composites made with this engineered matrix to be used in applications at or above 1,755 K.
This work was done by Sai V. Raj of Glenn Research Center, and Mrityunjay Singh and Ramkrishna Bhatt of the Ohio Aerospace Institute.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. LEW-18964-1