3D Printed silicon carbide scalable to meter-class segments for Origins Space Telescope

Abstract

During a NASA Phase II SBIR project the Goodman Technologies (GT) team developed and competed two advanced processes for producing large silicon carbide (SiC) mirror substrates and structures, both processes which could ultimately be performed in the microgravity environment of space. The process of scale-up from Phase I mirror substrates was anything but easy; over 250 unsuccessful trials were performed before achieving a printable substrate. A new Z-process allowed for conversion of moldable (but not printable) nanopastes into Pathfinder mirror components. The team also overcame delays associated with shutdowns due to COVID-19. The work did ultimately result in the demonstration of the world’s large 3D/AM SiC mirror substrates (25-cm scale). The first process, the Robocasting or Direct Ink Writing (DIW) printing process, was employed to 3D print engineered nanopastes consisting of SiC particulates with sizes ranging from the nanometer to micron scales mixed and suspended homogeneously in a preceramic polymer and liquid solvent solution. A large computer numerically controlled (CNC) platform with a 1.2-meter by 1.2-meter build bed was modified to become a large prototype “robocaster”. Modifications included incorporation of a large build plate (an optical bench top), a fluid supply system and syringes, a 750 W infrared (IR) heater, and programming. More than 200 experiments were conducted on the prototype robocaster, unsuccessfully, over a span of 16-months. Every print job would crack, and/or warp, and/or delaminate either during printing, or during low temperature curing, or during low-temperature polymer infiltration pyrolysis (PIP), or during high-temperature (PIP). Through massive effort, we finally overcame the engineering issues. Using a production robocaster, the ability to 3D print, join and then cure individual off-axis parabolic (OAP) mirror substrate segments (4 of them) to make a 25-cm monolithic Pathfinder mirror substrate for subsequent densification and pyrolysis was ultimately demonstrated. Two 25- cm monolithic substrates were printed and cured successfully. All of the robocast parts ultimately warped or bowed and/or cracked during low-temperature pyrolysis or high-temperature PIP steps. Through internally funded efforts performed by GT it was confirmed that as cured robocast material can be silicon melt infiltrated to form a very low silicon content of reaction bonded silicon carbide. GT also found that robocast material densified via polymer infiltration pyrolysis results in a polishable material. The second process GT developed and employed with UHM is the Z-process. The Z-process starts with a moldable (but not printable) nanopaste consisting of SiC particulates with sizes ranging from the nanometer to micron scales mixed and suspended homogeneously in a preceramic polymer and liquid solvent solution. The moldable nanopaste is compacted in a custom designed metallic and graphitic tooling set which also contains a precision mandrel. Compaction serves to squeeze out “extra” liquid phase material from the nanopaste while conforming the nanopaste to the shape of the mandrel prior to curing. Once cured the part is almost theoretical density and requires only a few steps of PIP to fully densify the part. PIP is accomplished at temperatures much lower than silicon melt infiltration (<1600 °C), chemical vapor deposition (<1450 °C), or conventional sintering (<2200 °C), providing enormous energy savings. The Z-process tooling can be used multiple times providing economy of numbers. The Z-process was successfully used to join 4-OAP segments and a backside lattice supporting structure. This paper shall discuss the results of progress made towards the manufacturing of Large SiC Space Optics traceable to meter-class segments for a far-infrared surveyor (the Origins Space Telescope, OST). The technology also is intended to fill priority technology gaps for the LUVOIR Surveyor, and Habitable Exoplanet Observatory (HabEx).