Thomas N. Wittberg

Space Environment Exposure of Polymer Films on the Materials International Space Station Experiment: Results from MISSE 1 and MISSE 2

A total of thirty-one samples were included in the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) Polymer Film Thermal Control (PFTC) and Gossamer Materials experiments, which were exposed to the low Earth orbit environment for nearly 4 years on the exterior of the International Space Station (ISS) as part of the Materials International Space Station Experiment (MISSE 1 and MISSE 2). This paper describes objectives, materials, and characterizations for the MISSE 1 and MISSE 2 GRC PFTC and Gossamer Materials samples. Samples included films of polyimides, fluorinated polyimides, and Teflon® fluorinated ethylene propylene (FEP) with and without second-surface metalized layers and/or surface coatings. Films of polyphenylene benzobisoxazole (PBO) and a polyarylene ether benzimidazole (TOR-LM TM) were also included. Polymer film samples were examined post-flight for changes in mechanical and optical properties. The environment in which the samples were located was characterized through analysis of sapphire contamination witness samples and samples dedicated to atomic oxygen (AO) erosion measurements. Results of the analyses of the PFTC and Gossamer Materials experiments are discussed.

The oxidation of an aluminum nitride powder studied by bremsstrahlung-excited Auger electron spectroscopy and x-ray photoelectron spectroscopy

Bremsstrahlung-excited Auger electron spectroscopy (AES) was used to study the oxidation kinetics of an aluminum nitride powder oxidized in air at 750, 800, 850, and 900 ‐ C. An equation was derived to calculate the average surface oxide film thickness from the aluminum AES spectra. The oxidation of this powder was found to follow a parabolic rate law within this temperature range. The measured activation energy was 230 6 17 kJymol (55 6 4 kcalymol). Analysis with x-ray photoelectron spectroscopy (XPS) showed that in addition to the nitride N 1s peak, there was a second N 1s peak. This peak has been observed in previous studies and can be attributed to N‐ O bonding either within the growing oxide film or at the Al 2O3yAlN interface.

Low‐temperature oxygen diffusion in alpha titanium characterized by Auger sputter profiling

Anodic TiO2 films were grown on titanium foils under galvanostatic conditions at 20 mA/cm2 in an aqueous saturated solution of ammonium tetraborate. The samples were then heat treated at temperatures between 450 and 550 °C and changes in the profile of oxygen concentration as a function of depth were monitored using Auger electron spectroscopy (AES). The oxygen diffusivities were calculated from the Boltzmann–Matano method and compared with those obtained using an analytical solution to Fick’s second law. The activation energy for oxygen diffusion in this temperature range was then calculated to be 28 kcal/mol.

XPS study of the dehydration of clay and kaolin powders

X-ray photoelectron spectroscopy (XPS) was used to analyze samples of a commercial clay powder and a kaolin standard. Samples were analyzed both in the as-received condition and after heating to temperatures of ≥750 °C. There was a loss of water during heating that corresponded to the transformation from a clay to an aluminosilicate. The XPS measurements showed that there was a decrease in the O/(Al + Si) ratio and a decrease of 0.2–0.3 eV in the aluminum Auger parameter value. The silicon Auger parameter value was unchanged after heating. Copyright

Solid lubrication of silicon nitride with cesium-based compounds : Part II : Surface analysis©

Energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) were used to characterize the wear surfaces of selected samples from Part 1 of the authors study. Results are presented for films generated on silicon nitride (Si3N4) originally coated with cesium oxytrithiotungstate (Cs2WOS3), cesium sulfate (Cs2SO4), and a hydrated cesium silicate (Cs2O·3SiO2·nH2O), all applied in a sodium silicate binder (Na2SiO3). Results show the presence of mostly Si, O, and Cs within the wear tracks of post-tested specimens. In some cases, W and S were not detected on samples that originally contained these elements, suggesting that decomposition had taken place. To simulate the reactions that might occur in a tribo-contact, mixtures of Si3N4 and Cs2WOS3 powders were heated in air to 700°C and analyzed using XPS and Bremsstrahlung-excited AES. It was found that Cs2WOS3 accelerates the formation ofSiO2 on Si3N4 under static conditions. These results support our hypothesis that high temperature chemical reactions between the cesium-containing compounds and the Si3N4 surface form a lubricious cesium silicate film. A mechanism is proposed based on the glass-modifying tendency of alkali metals and the hot-corrosion of Si3N4 Presented at the 55th Annual Meeting Nashville, Tennessee May 7–11, 2000

The nitridation of a silicon powder studied by XPS and X-ray-induced AES

XPS and x-ray-induced AES have been used to study the reaction layers formed on silicon powder samples heated in ultra-high purity nitrogen at temperatures between 1100 and 1200°C. An equation was derived to calculate the average surface reaction layer thicknesses from the silicon AES spectra. The results indicate that samples where the reaction layer is relatively thin may have some silicon oxynitride within this layer. For samples with calculated reaction layer thicknesses >1.5 nm, the N Is peak binding energy and Si KLL peak kinetic energy are characteristic of bulk Si 3 N 4 . The nitridation kinetics follow a linear rate low within this temperature range with a measured activation energy of 280 ± 60 kJ mol -1 .