Anisotropic Microstructure and Chemo-Optical Properties of Fluorocarbon Films Deposited by Ablative Pulsed Plasma...

Ablative pulsed plasma thruster (APPT)-induced fluorocarbon contamination severely degrades microsatellite optics and solar panels, but the spatial heterogeneity of its chemo-optical properties and underlying plasma-film mechanisms remain poorly understood. In this work, X-ray photoelectron spectroscopy (XPS), scanning probe microscopy (SPM), scanning electron microscopy (SEM), and ultraviolet-visible (UV-Vis) spectrophotometry were used to characterize the chemical structure, morphology, thickness, and chemo-optical properties of PTFE-derived fluorocarbon films, while high-speed imaging and triple Langmuir probe diagnostics were used to map plume conditions governing film formation. The film F/C ratio (0.6-1.0) is far lower than that of pristine PTFE (2.0), controlled by competition between energetic ion sputtering and delayed neutral C₂F₄ deposition. In the electrode-perpendicular plane, the F/C ratio reaches a minimum ~0.6 at 30°, corresponding to a transmittance of <20% at 550 nm; anode-side films exhibit lower F/C ratios and transmittance yet higher reflectance than cathode-side counterparts. Films in the electrode-parallel plane show quasi-symmetric bond distributions and higher F/C ratios and transmittance, with F/C minima at ±15°. By correlating plasma anisotropy (cathode-biased electron density and anode-biased electron temperature) with film chemistry and optics, we identify this ion-neutral competition as the origin of angle-dependent film properties. This work provides design guidelines: optics within 30° of the thruster axis in the perpendicular plane require active protection, enabling quantitative anisotropic contamination risk assessment for microsatellites.