Volume diffusion of atomic oxygen in α-SiO₂ protective coating

The minimum thickness for an amorphous silicon dioxide (α-SiO₂) protective coating required to prevent volume diffusion of atomic oxygen in a low Earth orbit (LEO) was evaluated by measuring the oxide thickness formed on Si(001) wafers in a hyperthermal atomic oxygen beam. The thickness of oxide film was measured by x-ray photoelectron spectroscopy. The diffusion length of atomic oxygen in α-SiO₂ at temperatures between 297 K and 493 K, where exterior surfaces of a spacecraft may be heated in LEO, shows temperature and flux dependences, i.e. the diffusion length of atomic oxygen increases with increasing temperature and beam flux. It was also demonstrated that the atomic oxygen fluence is not a primary factor of the diffusion length since the oxide growth obeys a parabolic law. The ground-based testing condition to evaluate performances of protective coatings are also discussed, based on the experimental data obtained in the experiments.