Additive Manufacturing of Silicon Carbide Mirrors

Due to its high elastic modulus, low density, low thermal expansion, and high thermal conductivity, silicon carbide is an ideal material for many demanding space applications. Examples include: lightweight freeform mirrors and structural components for ground/space-based imaging systems. The ability to print near-net shapes in silicon carbide will increase design freedom and reduce production cost/time. This research will therefore have significant impact for space exploration.This development effort will generate new knowledge about the additive manufacturing of silicon carbide and enable the production of silicon carbide mirrors. Unlike existing processes, such as hot pressing, slip casting, or polymer binding, the new approach to be studied does not rely on: 1) high temperatures/pressures within a mold; or 2) a polymer binder that serves as the preliminary support structure until it is removed by sintering. Rather, sodium hydroxide will be applied to silicon carbide powder in a powder bed, liquid binder-jetting process. The sodium hydroxide will oxidize the silicon carbide to form an amorphous silica layer. When heat is added, this amorphous layer will crystallize to form a network of connecting rods and plates between the adjacent silicon carbide grains through secondary crystal growth. This crystalline silica network will serve to reduce the porosity while increasing the density and strength. This represents a transformative step forward in additive manufacturing that is applicable not only to silicon carbide, but can also be extended to other material systems. The supporting oxidation and crystal growth modeling efforts will enable a deterministic approach to process parameter selection.