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At least 757 records · Page 42

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NASA NTRS1988

Requirements and an approach for coating the Gravity Probe B gyroscope rotor

A process to coat a quartz gyroscope rotor uniformly with superconducting niobium as part of a NASA sponsored experiment to test general relativity is described. The requirements for uniformity, film adhesion, and superconducting properties of the niobium thin film rotor coating are discussed. A uniformity of 1.5 percent (peak to valley) (38 nm) with good adhesion, no pinholes larger than 0.25 mm, transition temperature of 9.8 K, and the ability to survive repeated thermal cycling to 4.2 K have been achieved. Problems concerning abrasive damage and electrical arc damage during earthbound testing have been observed. Many hours of ground-based gyro operation demonstrate the applicability of this approach to rotor coating.

Gill, D.↗

NASA NTRS1988

Adhesion, friction and micromechanical properties of ceramics

The adhesion, friction, and micromechanical properties of ceramics, both in monolithic and coating form, are reviewed. Ceramics are examined in contact with themselves, other harder materials, and metals. For the simplicity of discussion, the tribological properties of concern in the processes are separated into two parts. The first part discusses the pull-off force (adhesion) and the shear force required to break the interfacial junctions between contacting surfaces. The role of chemical bonding in adhesion and friction, and the effects of surface contaminant films and temperature on tribological response with respect to adhesion and friction are discussed. The second part deals with abrasion of ceramics. Elastic, plastic, and fracture behavior of ceramics in solid state contact is discussed. The scratch technique of determining the critical load needed to fracture interfacial adhesive bonds of ceramic deposited on substrates is also addressed.

Miyoshi, Kazuhisa↗

NASA NTRS1990

Centrifugal Barrel Finishing Of Turbine-Blade "Fir Trees"

Modified centrifugal barrel-finishing machine imparts desired residual compressive stresses to "fir trees" of turbine blades. Centrifugal forces generate compressive stresses, which are transmitted to turbine blades through abrasive slurries in which suspended. Eliminates need for shot peening, rounding of edges and burrs caused by shot peening and, consequently, need for mass finishing operations to remove burrs. Improves surface finish of "fir trees".

Mandel, Johnny L.↗

NASA NTRS1990

Grinding Away Microfissures

Treatment similar to dental polishing used to remove microfissures from metal parts without reworking adjacent surfaces. Any variety of abrasive tips attached to small motor used to grind spot treated. Configuration of grinding head must be compatible with configurations of motor and workpiece. Devised to eliminate spurious marks on welded parts.

Booth, Gary N.↗

NASA NTRS1990

Polishing Difficult-To-Reach Cavities

Springy abrasive tool used to finish surfaces of narrow cavities made by electrical-discharge machining. Robot arm moves vibrator around perimeters of cavities, polishing walls of cavities as it does so. Tool needed because such cavities inaccessible or at least difficult to reach with most surface-finishing tools.

Malinzak, R. Michael↗

NASA NTRS1989

Fundamental tribological properties of ion-beam-deposited boron nitride films

The adhesion, friction, and micromechanical properties of ion-beam-deposited boron nitride (BN) films are reviewed. The BN films are examined in contact with BN metals and other harder materials. For simplicity of discussion, the tribological properties of concern in the processes are separated into two parts. First, the pull-off force (adhesion) and the shear force required to break the interfacial junctions between contacting surfaces are discussed. The effects of surface films, hardness of metals, and temperature on tribological response with respect to adhesion and friction are considered. The second part deals with the abrasion of the BN films. Elastic, plastic, and fracture behavior of the BN films in solid-state contact are discussed. The scratch technique of determining the critical load needed to fracture interfacial adhesive bonds of BN films deposited on substrates is also addressed.

Miyoshi, Kazuhisa↗

NASA NTRS1989

Aeolian processes on Venus

This review assesses the potential aeolian regime on Venus as derived from spacecraft observations, laboratory simulations, and theoretical considerations. The two requirements for aeolian processes (a supply of small, loose particles and winds of sufficient strength to move them) appear to be met on Venus. Venera 9, 10, 13, and 14 images show particles considered to be sand and silt size on the surface. In addition, dust spurts (grains 5 to 50 microns in diameter) observed via lander images and inferred from the Pioneer-Venus nephalometer experiments suggest that the particles are loose and subject to movement. Although data on near surface winds are limited, measurements of 0.3 to 1.2 m/sec from the Venera lander and Pioneer-Venus probes appear to be well within the range required for sand and dust entrainment. Aeolian activity involves the interaction of the atmosphere, lithosphere, and loose particles. Thus, there is the potential for various physical and chemical weathering processes that can effect not only rates of erosion, but changes in the composition of all three components. The Venus Simulator is an apparatus used to simulate weathering under venusian conditions at full pressure (to 112 bars) and temperature (to 800 K). In one series of tests, the physical modifications of windblown particles and rock targets were assessed and it was shown that particles become abraded even when moved by gentle winds. However, little abrasion occurs on the target faces. Thus, compositional signatures for target rocks may be more indicative of the windblown particles than of the bedrock. From these and other considerations, aeolian modifications of the venusian surface may be expected to occur as weathering, erosion, transportation, and deposition of surficial materials. Depending upon global and local wind regimes, there may be distinctive sources and sinks of windblown materials. Radar imaging, especially as potentially supplied via the Magellan mission, may enable the identification of such areas on Venus.

Greeley, Ronald↗

NASA NTRS1990

Aeolian removal of dust from photovoltaic surfaces on Mars

It is well documented that Mars is totally engulfed in huge dust storms nearly each Martian year. Dust elevated in these global dust storms, or in any of the numerous local dust storms could settle on photovoltaic surfaces and seriously hamper photovoltaic power system performance. Using a recently developed technique to uniformly dust simulated photovoltaic surfaces, samples were subjected to Martian-like winds in an attempt to determine whether natural aeolian processes on Mars would sweep off the settled dust. The effects of wind velocity, angle of attack, height off the Martian surface, and surface coating material were investigated. Principles which can help to guide the design of photovoltaic arrays bound for the Martian surface were uncovered. Most importantly, arrays mounted with an angle of attack approaching 45 deg show the most efficient clearing. Although the angular dependence is not sharp, horizontally mounted arrays required significantly higher wind velocities to clear off the dust. From the perspective of dust-clearing it appears that the arrays may be erected quite near the ground, but previous studies have suggested that saltation effects can be expected to cause such arrays to be covered by sand if they are set up less than about a meter from the ground. Providing that the surface chemistry of Martian dusts is comparable to our test dust, the materials used for protective coating may be optimized for other considerations such as transparency, and chemical or abrasion resistance. The static threshold velocity is low enough that there are regions on Mars which experience winds strong enough to clear off a photovoltaic array if it is properly oriented. Turbulence fences proved to be an ineffective strategy to keep dust cleared from the photovoltaic surfaces.

Gaier, James R.↗

NASA NTRS1990

Plastic superconductor bearings any size, any shape, 77 k and up

Friction free bearings at 77 k or higher are possible using the high T(sub c) copper oxide ceramic superconductors. The conventional method for making such bearings is to use a sintered ceramic monolith. This puts great restraints on size, shape and postforming machining. The material is hard and abrasive. It's possible to grind up ceramic superconductors and suspend the granules in a suitable matrix. Mechanical properties improve and are largely dependent on the binder. The Meissner effect is confined to individual grains containing electron vortices. Tracks, rails, levitation areas and bearings can be made this way with conventional plastic molding and extruding machines or by painting. The parts are easily machined. The sacrifice is in bulk electrical conductivity. A percolating wick feel for LN2 can be used to cool remote superconductors and large areas quite effectively. A hollow spheroid or cylinder of superconductor material can be molded with the internal surfaces shielded by the Meissner effect. It might be thought of as the dc magnetic analogue of the Faraday cage and the inside can be called the Meissner space. It's selective. The ac fields are transmitted with minor attenuation. Particle size and distribution have a profound effect on final magnetic and electrical characteristics.

Reick, Franklin G.↗

NASA NTRS1990

Fundamental tribological properties of ion-beam-deposited boron nitride films

Miyoshi, K.↗

NASA NTRS1991

Superfiber For Strong, Light Fabrics

New ultrahigh-molecular-weight fiber offers major advantages as fabric for space suits, diving suits, sporting goods, sails, and ultralight aircraft. Polyolefin fiber has extremely high modulus of elasticity, low elongation, high specific strength, low specific gravity, resistance to chemicals, low moisture absorption, and high resistance to damage by flexure and abrasion. Consists of elongated, highly aligned, highly crystalline polyethylene molecules. Structure not only results in high strength-to-weight ratio but also gives high resistance to tearing and ripping. Fibers woven in many patterns. Offers combination of desirable properties not found in nylon, polyester, or aramid fabrics.

Dawn, Frederic S.↗

NASA NTRS1991

Deposition Of Diamondlike Films By ECR Microwave Plasma

Hard, amorphous hydrogenated carbon films of diamondlike quality deposited at room temperature on silicon, optical glass, and quartz through decomposition of CH4 in electron-cyclotron-resonance (ECR) microwave plasma of CH4 diluted with H2. Technique provides hard, abrasion-resistant coatings for lenses and other optical components. Films chemically inert and posses high electrical resistivity and breakdown fields, valuable properties in microelectronics applications.

Pool, Frederick S.↗

NASA NTRS1990

Revealing the microstructure of materials

The objectives are to demonstrate how the microstructure of materials may be revealed by abrasive polishing and chemical etching, and to illustrate how microstructural information is used to monitor manufacturing processes, provide in-depth inspection, and perform failure analysis. Microstructural analysis is the procedure used to reveal the internal microstructural details of a material or part by sectioning and polishing the cut surface so that it may be examined under a suitable microscope. A printed wiring board was selected as the test material because it contains both metals and nonmetals that have distinctive microstructures, and because this technique is used throughout the electronic industry as a key quality control tool. The three principle component materials in printed circuit boards are glass/epoxy laminates faced with copper foil; copper, deposited by both electroless and electrolytic plating; and tin/lead solder. Sample preparation, mounting, grinding, polishing, and examination and analysis are discussed.

Nelson, James A.↗

NASA NTRS1991

Processing of glass-ceramics from lunar resources

The goal is to fabricate useful ceramic materials from the by-products of lunar oxygen production processes. Specifically, the crystal nucleation and growth kinetics of ilmenite-extracted lunar regolith were studied in order to produce glass-ceramics with optimal mechanical, thermal, and abrasion resistant properties. In the initial year of the program, construction and calibration of a high temperature viscometer, used for determining the viscosity of simulated lunar glasses was finished. A series of lunar simulants were also prepared, and the viscosity of each was determined over a range of temperatures. It was found that an increase in the concentration of Fe2O3 decreases the viscosity of the glass. While this may be helpful in processing the glass, Fe2O3 concentrations greater than approximately 10 wt percent resulted in uncontrolled crystallization during viscosity measurements. Impurities (such as Na2O, MnO, and K2O) in the regolith appeared to decrease the viscosity of the parent glass. These effects, as well as those of TiO2 and SiO2 on the processability of the glass, however, remain to be quantified.

Fabes, B. D.↗

NASA NTRS1991

Sand and dust issues for the MESUR mission

The presence of particles in the Martian atmosphere increases the rate of erosion of the heat shield during the entry phase of the MESUR mission. Preliminary analysis has shown that under certain conditions particles will penetrate the bow shock of the entry vehicle, impact and erode the heat shield, above the anticipated rate from the ablation process. Knowledge of the distribution, sizes, and composition of particles suspended in the atmosphere will permit the estimation of the heat shield recession both for a nominal Mars atmosphere and in dust storm conditions. On the surface of Mars, sand and dust can effect the operations and design of the science instruments (i.e., imaging) and lander subsystems (i.e., solar arrays). Distribution and deposition of sand and dust on the planet's surface will dictate the feasibility of using solar arrays as a power source for the lander. Furthermore, the potential abrasive nature of blowing particles must be understood in order to properly select suitable materials for lander structure and any components exposed to the environment. The selection of lander material may also be dictated by corrosive and chemically active properties of particles at a potential landing site. Some key interests concerning sand and dust on the surface and in the atmosphere are summarized in a number of questions that are included.

Wercinski, P. F.↗

NASA NTRS1991

High-temperature, flexible, thermal barrier seal

This device seals the sliding interfaces between structural panels that are roughly perpendicular to each other or whose edges are butted against one another. The nonuniformity of the gap between the panels requires significant flexibility along the seal length. The seal is mounted in a rectangular groove in a movable structural panel. A plurality of particles or balls is densely packed in an outer sheathing. The balls are laterally preloaded to maintain sealing contact with the adjacent wall using a pressurized linear bellows. Distortions in the adjacent panel are accommodated by rearrangement of the particles within the outer sheathing. Leakage through the seal is minimized by densely compacting the internal particles and by maintaining positive preload along the back side of the seal. The braid architecture of the outer sheathing is selected to minimize leakage through the seal and to resist mechanical abrasion.

Sirocky, Paul J.↗

NASA NTRS1991

Experimental investigations of the effects of cutting angle on chattering of a flexible manipulator

When a machine tool is mounted at the tip of a robotic manipulator, the manipulator becomes more flexible (the natural frequencies are lowered). Moreover, for a given flexible manipulator, its compliance will be different depending on feedback gains, configurations, and direction of interest. Here, the compliance of a manipulator is derived analytically, and its magnitude is represented as a compliance ellipsoid. Then, using a two-link flexible manipulator with an abrasive cut off saw, the experimental investigation shows that the chattering varies with the saw cutting angle due to different compliance. The main work is devoted to finding a desirable cutting angle which reduces the chattering.

Lew, J.↗

NASA NTRS1990

An estimate of the amount of a material removed from the Martian fretted terrain

An estimate is made of the volume of plateau material removed from the area of Martian fretted terrain between 270 W and 360 W. Depending on the model chosen for the former extent of plateau, between 0.5 and 3.3 million cubic km have been removed. This corresponds to a global equivalent layer between 3 and 21 m or a lowland equivalent layer between 9 and 60 m of redistributed sediment. The preferred model is at the lower end of this range. The material could have been removed during a relatively short period of Martian history by aeolian abrasion following in situ weathering. The volume of material is an order of magnitude smaller than the total amount likely to be present in the northern lowlands.

Dimitriou, Andrew↗