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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS2024

Assessing Mechanical Properties of Spacecraft Materials under Simulated Low Earth Orbit Atomic Oxygen Conditions

During a space mission, spacecraft surface materials are exposed to various damaging environmental factors including high-energy photons, electrons, atomic oxygen (AO) neutrals and ions, micrometeoroids and orbital debris, vacuum, and large temperature fluctuations. The resulting change in spacecraft material properties can significantly impact the performance and durability of spacecraft systems. Even though all aspects of the space environment can lead to the deterioration of spacecraft components, in low Earth orbit (LEO), the threat posed by AO is especially severe in terms of structural and optical damage, particularly to exterior spacecraft components that are susceptible to oxidation. A comprehensive understanding of material AO-induced weathering is essential for mission planning in the LEO environment. The presented work aims to evaluate the alterations in mechanical properties of selected innovative spacecraft materials and surface electronic system designs, such as Kapton® CR film coated front and back-side with polyimide coating containing AO-resistant filler, Kevlar EXO, and Bendable Electrodynamic Dust Shield (BEDS) architecture, under simulated AO exposure, utilizing the photoelasticity phenomenon in which birefringence is induced in a material when it is subjected to mechanical stress. Changes in stress patterns during the deformation of AO-exposed polymers were assessed using a large-field polariscope under varied deformation types. Patterns of colors were used for qualitative evaluations of residual stress. The stress patterns of the AO-exposed polymers were compared to those of the unexposed ones. Also, these patterns were correlated with data from Bidirectional Reflectance Distribution Function (BRDF) and surface morphology studies.

Yuliya Kuznetsova↗
NASA NTRS1986

The evolution and growth of Al2O3 scales on beta-NiAl

The formation and growth of Al2O3 scales on (beta)-NiAl were studied using electron microscopy and other analytical techniques to gain an understanding of the oxidation properties of (beta)-NiAl and of alumina-forming alloys, in general. The transient and mature stages of oxidation were studied as well as the transformation stage during which the oxide scale transforms from metastable Al2O3 phases to the thermodynamically stable alpha-Al2O3 phase. The transient oxidation stages were studied at 800 deg C and for short times at 1100C. At 800C, the scales consist predominantly of delta-Al2O3 which forms by cation vacancy ordering in the defective spinel lattice of gamma-Al2O3. At 1100C, a fast-growth morphology of theta-Al2O3 forms as a surface layer over delta-Al2O3. For both oxidation temperatures, the scales are often epitaxially oriented with respect to the metal. The transient scales grow by outward cation diffusion as evidenced by surface growth morphologies. The transformation to alpha-Al2O3 occurs within 1 hour at 1100C by a nucleation and radial growth process. The large volume decrease associated with the transformation results in a highly strained alpha-Al2O3 microstructure. A change in scale growth mechanism from outward cation to inward anion diffusion allows transient surface morphologies to be smoothed by surface diffusion. The mature stage of oxidation involves the growth of an alpha-Al2O3 scale having the lacey morphology formed as a result of the gamma yields alpha transformation. Growth of the scale occurs by counterdiffusion along grain boundaries resulting in ridges formed by impingement of alpha-Al2O3 nuclei during the transformation stage. Also scale growth occurs by inward oxygen diffusion through healed cracks; the cracks result from transformation stresses. The measured growth rates of scales having the lacey morphology are an order of magnitude less than fine-grained alpha-Al2O3 scales. Metal orientations were found to have a large effect on oxide morpholoies during all stages of oxidation.

Doychak, J. K.↗
NASA NTRS1996

Surface Areas and Morphology of Thin Ice Films

Thin ice films formed by deposition from the vapor phase in a fast flow-tube reactor have been used to simulate polar stratospheric cloud surfaces in order to obtain laboratory data on uptake and heterogeneous reaction rates. Surface areas are determined from BET (Brunauer, Emmett, and Teller) analysis of gas adsorption isotherms. The results for ices prepared at 196 K or 77 K are consistent with previous data on thicker ice films. Environmental scanning electron microscopy is used to obtain particle sizes and shapes, and to investigate the morphology of the ices on borosilicate or silicon windows. In addition, the uptake of HCI on ice films prepared at 196 K is investigated. The results suggest that the layer model we have previously developed for analysis of uptake and heterogeneous reaction rates on ice films is valid. Detailed information will be presented at the conference.

ice films↗
NASA NTRS1994

The Morphology of Low Surface Brightness Disk Galaxies

Images of a sample of low surface brightness (LSB) disk galaxies are presented. These galaxies are generally late types; however, they are not dwarfs, being intrinsically large and luminous. The morphology of LSB galaxies is discussed in terms of the physical interpretation of the Hubble sequence, the stages of which are found to be nonlinear in the sense that smaller physical differences separate mid to early type spirals than late types.

extragalactic astronomy↗
NASA NTRS2003

Classification of Mars Pathfinder Rock Surfaces Using Quantitative Morphologic Indices

We have examined the morphology of rocks in two regions of the MPF landing site in terms of location, size and dimensions, sphericity sphericity and elongation, and have correlated this information with spectral data extracted from associated rock surfaces, with the goal of improving the likelihood of discerning between rock and rock surface types. We use four highly diverse Imager for Mars Pathfinder (IMP) images centered on Mini-Matterhorn and the Rock Garden because they are suited to demonstrate the wide morphologic variation of rocks at the site. A sampling of rocks was chosen at these locations that represented a range of shapes, textures and spectral signatures. In this initial analysis we focused upon the largest rocks that are situated in such a way as to allow easy viewing of most of the faces.

Yingst, R. A.↗
NASA NTRS1992

Quantitative analysis of surface characteristics and morphology in Death Valley, California using AIRSAR data

The Jet Propulsion Laboratory Airborne Synthetic Aperture Radar (JPL-AIRSAR) is used to collect full polarimetric measurements at P-, L-, and C-bands. These data are analyzed using the radar analysis and visualization environment (RAVEN). The AIRSAR data are calibrated using in-scene corner reflectors to allow for quantitative analysis of the radar backscatter. RAVEN is used to extract surface characteristics. Inversion models are used to calculate quantitative surface roughness values and fractal dimensions. These values are used to generate synthetic surface plots that represent the small-scale surface structure of areas in Death Valley. These procedures are applied to a playa, smooth salt-pan, and alluvial fan surfaces in Death Valley. Field measurements of surface roughness are used to verify the accuracy.

Kierein-Young, K. S.↗
NASA NTRS1986

Vapor transport processes of Ge-chalcogenides under normal and reduced gravity conditions

Experimental and theoretical studies of the GeSe-GeI4 system revealed the multicomponent, multireaction nature of this system and the existence of a diffusion boundary layer; they also yielded equations for the prediction of diffusion limited mass transport rates. Vapor transport and crystal growth experiments of the GeSe-GeI4 and GeSe-Xe systems on earth and in a microgravity environment demonstrated the effects of convection and of microgravity on mass flux and crystal morphology. The surface and bulk morphology of GeSe crystals obtained for both systems in microgravity is considerably improved relative to ground-based specimens. Unexpected crystal growth phenomena were observed for the GeSe-Xenon system on recent Shuttle flights.

Wiedemeier, H.↗
NASA NTRS1991

Growth morphologies of crystal surfaces

A previously proposed Monte Carlo model (Xiao et al., 1988, 1990) is extended to three dimensions, and reevaporation after accommodation and growth on dislocation-induced steps are included. It is found again that, for a given set of growth parameters, the critical size beyond which a crystal cannot retain its macroscopically faceted shape scales linearly with the mean free path in the vapor. However, the three-dimensional systems show increased shape stability as compared with the corresponding two-dimensional cases. The experimental observation that the critical size of a growing crystal depends on the prevailing growth mechanism is confirmed, and detailed insight into the processes leading to the loss of face and facet stability is obtained.

Xiao, Rong-Fu↗
NASA NTRS1980

The surface of Io - Geologic units, morphology, and tectonics

The geologic units, morphology and tectonics evident on a preliminary geological map obtained from best-resolution Voyager 1 images and a preliminary pictorial map of the surface of Io are discussed. Nine volcanic units are identified on the surface which are composed of mountain materials, intervent, layered and eroded layered plains, and wall and floor, pit crater flow, shield crater flow, fissure flow and crater cone units associated with volcanic vents, along with seven types of structural features. The cumulative volcanic crater size distribution of 170 Ionian vents with craters greater than 14 km is similar to that of impact craters on other solar system bodies, with a surplus of small craters and a preponderance of volcanic vents in the Io equatorial zone. The observed surface morphologies provide evidence for a mixture of silicates and sulfur as the dominant surface material. Lineaments and grabens are also apparent on the surface, in a common planetary grid pattern.

Schaber, G. G.↗
NASA NTRS1977

Mercurian bright patches - Evidence for physio-chemical alteration of surface material

Morphologically and photometrically anomalous patches of highly-reflective material exist inside several large Mercurian craters. Calculations assuming that the patches exhibit an average lunar photometric function yield normal albedos of .39-.45, roughly 60% higher than Aristarchus, the brightest feature on the moon. Color-ratio images derived from Mariner 10 imaging data indicate that bright patches are bluer than typical Mercurian surface material, and are surrounded by material redder than typical surface material. Local physio-chemical alteration along impact-induced fractures may have been involved in production of these uniquely Mercurian features.

Dzurisin, D.↗
NASA NTRS1985

The geomorphology of Rhea - Implications for geologic history and surface processes

Morphological analyses of landforms on Rhea are used to define three physiographic provinces: cratered terrain 1 undifferentiated; cratered terrain 1 lineated; and cratered terrain 2. The important statigraphic relationships between the different provinces are examined with respect to major impact basins and tectonic features. It is shown that the formation of multiringed basins may have caused, or at least controlled the locations of major resurfacing and mantling events. The diameters of the central peaks relative to the impact crater diameters are found to be significantly larger than those within the craters of the moon or Mercury. Both cratered and noncrater lineaments have regional orientations which do not fit current global or regional stress models. On the basis of the morphological analysis, a chronological order is established for the origin of the three provinces: the cratered terrain 1 province was formed first; and cratered terrain 1 lineated and cratered terrain 2 were formed second, and last, respectively. It is shown that the chronological order is generally consistent with current theoretical models of the evolution of Rhea.

Moore, J. M.↗
NASA NTRS1981

Chemical and morphological characteristics of lithium electrode surfaces

Lithium electrode surfaces were analyzed for chemical and morphological characteristics, using electron spectroscopy chemical analysis (ESCA) and scanning electron microscopy (SEM). Samples included lithium metal and lithium electrodes which were cycled in a 1.5 M lithium arsenic hexafluoride/two-methyl tetrahydrofuran electrolyte. Results show that the surface of the as-received lithium metal was already covered by a film composed of LiO2 and an Li2O/CO2 adduct with a thickness of approximately 100-200 A. No evidence of Ni3 was found. Upon exposure of the lithium electrode to a 1.5 M LiAsF6/2-Me-THF electrochemical environment, a second film was observed to form on the surface, consisting primarily of As, Si, and F, possibly in the form of lithium arsenic oxyfluorides or lithium fluorosilicates. It is suggested that the film formation may be attributed to salt degradation.

Yen, S. P. S.↗
NASA NTRS1973

Chemical and morphological changes of reusable surface insulation coatings as a function of convectively heated cyclic testing

The effects of convective heating upon reusable surface insulation coatings were studied utilizing scanning electron microscopy, X-ray fluorescence, and X-ray diffraction. Samples of coated silica, mullite, and ceramic mullite fiber were cycled in an arc plasma stream up to 15 times for 15 minutes per cycle at surface temperatures simulating those on the space shuttle vehicle. The surfaces of ceramic mullite fiber and mullite coatings were roughened substantially by the convectively heated environment while the silica was significantly smoothed after testing. Scanning electron microscopy also showed surface cracking of varying degrees in all of the coatings. The surface chemistry of the coatings as examined by X-ray fluorescence revealed that significant changes in composition were occurring during cycling, particularly within the mullite coating.

Leiser, D. B.↗
NASA NTRS2002

Additive Effects on Si3n4 Oxidation/Volatilization in Water Vapor

Two commercially available additive-containing silicon nitride materials were exposed in four environments which range in severity from dry oxygen at 1 atm pressure, and low gas velocity to an actual turbine engine. Oxidation and volatilization kinetics were monitored at temperatures ranging from 1066 to 1400 C. The main purpose of this paper is to examine the surface oxide morphology resulting from the exposures. It was found that the material surface was enriched in rare earth silicate phases in combustion environments when compared to the oxides formed on materials exposed in dry oxygen. However, the in situ formation of rare earth disilicate phases offered little additional protection from the volatilization of silica observed in combustion environments. It was concluded that externally applied environmental barrier coatings are needed to protect additive-containing silicon nitride materials from volatilization reactions in combustion environments. Introduction Si3N4 is proposed for use as components, such as vanes, in turbine applications. Tens of thousands of hours of life are needed for both land-based turbines and aeropropulsion applications. Additive-containing SisN4 materials are

Opila, Elizabeth J.↗
NASA NTRS1973

Effects of sea salts on the physical characteristics of reusable surface insulation

Test results show that sea salt contamination and thermal cycles cause significant changes in the morphology of surface insulation coatings with increased melting. Sea salt contamination and thermal cycles do not have a significant effect on emittance of mullite and silica composite coatings at high temperatures (1300 K). At the lower temperatures there is apparently some effect on emittance but the evidence is inconclusive because of the absence of multiple tests.

Ransone, P. O.↗