This innovation is a hybrid metal-ceramic matrix composite (CMC) turbine blade in which a SiC/SiC CMC airfoil section is bonded to a single-crystal superalloy root section in order to mitigate risks associated with an all-CMC blade inserted in a superalloy disk. This will allow current blade attachment technology (SX blade with a dovetail attachment to a slotted Ni disk) to be used with a ceramic airfoil. The bond between the CMC and single crystal will be primarily mechanical in nature, and enhance with clamping arising from thermal expansion mismatch. Two single-crystal root sections will be bonded to each other using diffusion bonding at temperatures near 1,200 °C. The single crystals will form a clamshell around the CMC, with little or no gap between the metal and ceramic. Upon cooling, the metal will shrink around the CMC to firmly clamp it. It is envisioned that this will allow the blade root to operate at temperatures up to about 800 °C. Single crystals will resist stress relaxation at this temperature, thus maintaining clamping loads for long lives. The hybrid concept plus the method of manufacture is new technology.
The state of the art for ceramic blades involves inserting CMC blades directly into powder metallurgy superalloy disks. The hybrid blade concept is intended as a risk mitigation strategy should insurmountable issues arise with inserting the CMC blades directly into the superalloy disk. Given the brittle nature of CMC materials, this approach facilitates increased precision and ease of handling for CMC blades during assembly.
Because there is no true ductility in CMC materials, there is a potential issue of cracking due to stress concentrations that occur at the notches that are inherent in fir tree attachment designs used to insert blades directly into a disk. These stresses can be exacerbated by slight geometric perturbations in the mating surfaces between the blade root and disk. Fretting is an additional issue that can initiate fatigue failure in such an attachment. The proposed hybrid blade concept allows heritage fir tree geometries to attach the blade to the disk, where contact stresses and fretting issues have already been solved for metals. In the contact regions of the fir tree, the single-crystal alloy clamshell would be able to plastically flow, thus mitigating the high stresses, whereas a CMC counterpart could not.