Thermal Decomposition of Ablating Gap Filler Material for Atmospheric Entry Heatshields

Atmospheric entry heatshields conventionally follow a tiled configuration, where numerous tiles of ablative material are bonded by a gap-filler. Atmospheric entry missions with large capsules starting with the Mars Science Laboratories to the most recent Mars 2020 mission use Room Temperature Vulcanized Silicone (RTV) as the gap-filler. This work discusses the response of RTV to high-temperatures that include mass-loss and microstructure change due to pyrolysis. As RTV pyrolysis poses unique problems such as foaming, intumescence and formation of a glassy SiO₂ layer, understanding these complex phenomena is crucial for developing robust and accurate material response models. In this paper, thermogravimetric analysis was conducted to quantify the mass-loss of RTV as it is heated to 1000^◦ C, in inert atmosphere. The thermally degraded samples were imaged using environmental scanning electron microscopy elucidating the microstructural foaming in RTV. Additionally, in-situ scanning electron microscopy was carried out on spin-coated RTV samples to observe in-situ the microstructure change as a function of temperature. TGA results revealed large mass loss, and SEM showed that the initially non-porous virgin RTV undergoes a marked change in microstructure, ending as a highly porous char.