氧气流量对等离子体增强化学气相沉积硅氧烷涂层的防原子氧性能的影响OA北大核心

Introduction Polyimide film is one of the most commonly used organic film materials for spacecrafts.In the low-Earth orbit(LEO)environment,the polyimide film applied to the surface of spacecraft will be eroded by atomic oxygen.Generally,performance failure will occur in about one year due to the erosion effect,affecting the service life of spacecrafts.The siloxane coating prepared by plasma-enhanced chemical vapor deposition(PECVD)method has good atomic oxygen resistance,which can reduce the atomic oxygen erosion rate of polyimide film to less than one-thirtieth.At the same time,the coating also has good light transmittance,flexibility and bendability.In recent years,there have been many studies on the effects of electrode spacing,power,pressure and other parameters on the properties of siloxane coatings prepared by PECVD at home and abroad,but the effect of oxygen flow rate on the atomic oxygen resistance of siloxane coatings has not been reported.This work focuses on the deposition rate,cross-sectional morphology,composition and structure of siloxane coatings prepared by PECVD under different oxygen flow rates.The atomic oxygen effect test was carried out on the coatings,and the mass loss data of different samples were successfully collected.The oxygen flow rate parameters of coatings with excellent atomic oxygen resistance has been obtained,which provides support for the research of atomic oxygen resistant coatings. Methods The coating samples were prepared by the PECVD method using a self-developed continuous roll-to-roll PECVD coating equipment.The substrate material for coating was 50 μm thick DuPont Kapton HN polyimide film.The reactant gases were Alfa Aesar hexamethyldisiloxane monomer(HMDSO)and oxygen(O2).With a fixed HMDSO flow rate of 25 mL/min,the O2 flow rate was adjusted to 4,8,12,16,20,and 24 mL/min respectively,while maintaining a deposition pressure of 8 Pa and a reaction power of 400 W.This setup was used to investigate the deposition rate,structure and composition of deposited coatings under different precursor ratios.The coatings were subjected to atomic oxygen irradiation tests using a microwave atomic oxygen generator.The atomic oxygen flux was calibrated using DuPont Kapton HN polyimide film.The composition of the coatings was analyzed using the INVENIO R Fourier Transform Infrared(FTIR)spectrometer from Bruker Optics,Germany.In this test,the siloxane coating was deposited on the surface of a nearly mid-infrared transparent potassium bromide(KBr)single crystal to reduce the influence of the substrate on the test results.The surface and cross-sectional morphology of the coatings were examined using a Zeiss Sigma 500 scanning electron microscope from Germany,and the thickness of the coatings was also determined. Results and discussion Under low oxygen flow rates,the deposition rate of the coating was significantly higher than that under high oxygen flow rates.This was attributed to the fact that at lower oxygen flow rates,most of the precursor reactants contribute to the formation of the coating,resulting in a higher deposition rate.Conversely,at higher oxygen flow rates,the probability of reactions between HMDSO and O2 increases,leading to an increase in the amount of gaseous products generated and a corresponding decrease in the deposition rate.All the siloxane coating samples exhibited a fairly dense cross-sectional morphology with no observable crystallization.When the oxygen flow rate was 8,12,or 16 mL/min,the coating cross-sections were smooth and free of texture.At 4 mL/min,however,local damage was observed in the coating cross-section,possibly related to an incomplete formation of the composite network structure.At 20 and 24 mL/min,the coating interfaces remained dense but exhibited a textured structure,indicating the formation of local atomic ordered arrangements within the coating and the generation of structural stress.The FTIR spectra of the coatings revealed that as the oxygen flow rate increased,the intensities of rocking and stretching vibration peaks(both symmetric and asymmetric)belonging to the Si-O bond increased,indicating an increase in the content of silicon oxides in the coating.Concurrently,the intensities of the methyl bending vibration peaks in Si—(CH3)2,Si—(CH3)3,and Si—CH3,as well as the C—H stretching vibration peak,decreased with increasing oxygen flow rate,suggesting a reduction in the methyl content of the coating.The stretching vibration of the O—H bond became more pronounced,indicating that higher oxygen concentrations favored the combination of H and O elements.Atomic oxygen exposure tests on siloxane coatings prepared under different oxygen flow rates showed that as the oxygen flow rate increased from 4 mL/min to 20 mL/min,the mass loss of the coatings decreased exponentially.However,when the oxygen flow rate reached 24 mL/min,the mass loss increased sharply.Coatings prepared at lower oxygen flow rates exhibited smooth surface morphologies without cracks,holes,or other defects.In contrast,the siloxane coating prepared at 24 mL/min exhibited a large number of cracks,with the coating tilting at the crack sites.These severe cracks provided channels for atomic oxygen erosion,leading to a higher atomic oxygen erosion rate for the sample. Conclusions During the deposition process of siloxane coatings,the deposition rate is highest when the oxygen flow rate is 8 mL/min,and then it decreases as the oxygen flow rate increases.The cross-section of the siloxane coatings grown under low oxygen flow conditions does not exhibit visible grains or grain boundaries.In contrast,the cross-section of siloxane coatings grown under higher oxygen flow conditions indicates the occurance of textures,although they haven't displayed typical grain morphologies,which may be related to the stress within the coatings.The increase in oxygen flow rate can facilitate the oxidation of CH3 in the gas phase during PECVD and the formation of siloxane coatings.As the oxygen flow rate increases,the mass loss of the coating under atomic oxygen exposure decreases at first and then increases.At lower oxygen flow rates,insufficient oxidation of HMDSO results in the presence of more methyl and hydroxyl groups in the coating.These methyl and hydroxyl groups are easily oxidized by atomic oxygen to produce volatile gases,leading to higher mass loss.However,at higher oxygen flow rates,cracks appear in the coating,causing an increase in mass loss.This study demonstrates that a suitable oxygen flow rate is crucial in optimizing the performance of siloxane coatings by promoting the reactions of HDMSO and forming a dense microstructure.