Investigation Into Silicone-Silicate Conversion Due to Atomic Oxygen in the Low Earth Orbit Environment

Author

Justin T. Self, California Polytechnic State University, San Luis ObispoFollow

Available at: https://digitalcommons.calpoly.edu/theses/3018

Date of Award

6-2025

Degree Name

MS in Aerospace Engineering

Department/Program

Aerospace Engineering

College

College of Engineering

Advisor

Kira Abercromby

Advisor Department

Aerospace Engineering

Advisor College

College of Engineering

Abstract

Silicones have been widely used over the years in spacecraft for a variety of functions, but atomic oxygen (AO) exposure in Low Earth Orbit causes a silicate (SiO₂) surface layer to rapidly develop on the silicone which results in a permanent change in its thermal and optical properties. Although this silicone to silicate conversion is known to occur, statistical predictions of layer formation as a function of time on orbit are not found in literature. This thesis utilized optical and scanning electron microscopy, Fourier-Transform Infrared Spectroscopy, diffuse reflectance spectroscopy, and nonlinear regression statistical techniques after RF plasma asher exposure tests to study this phenomenon. Samples were exposed to AO fluences ranging from 2.83 ± 0.214 ×10¹⁹ to 1.88 ± 0.0520 ×10²¹ atoms/cm². The estimated time at the International Space Station altitude and orbit for CV-2960 and RTV-S691 silicones to form a fully-developed silicate layer is predicted to be between four to nine days, depending on the solar activity. Percent of sample surface area occupied by cracks ranged from 0% to 52.8% and was found to follow a logarithmic model for both CV-2960 and RTV-S691 silicones (R² = 0.974 and 0.981, respectively) and was significantly linked to AO fluence (p < .0001). CV-2960 silicone was observed to develop a surface silicate layer more readily than the phenyl-containing RTV-S691. Ground-exposed DC 93-500 samples were compared to DC 93-500 MISSE-6 flight articles and were observed to correlate very well in diffuse reflectance features at similar fluence values. Phenyl-containing silicones may be more resistant to AO-induced silicate layer formation which leads to a longer time on orbit before this layer becomes saturated with crack structures.