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NASA NTRS2002

Coated article and method of making

An article includes a silicon-containing substrate and a modified mullite coating. The modified mullite coating comprises mullite and a modifier component that reduces cracks in the modified mullite coating. The article can further comprise a thermal barrier coating applied to the modified mullite coating. The modified mullite coating functions as a bond coating between the external environmental/thermal barrier coating and the silicon-containing substrate. In a method of forming an article, a silicon-containing substrate is formed and a modified mullite coating is applied. The modified mullite coating comprises mullite and a modifier component that reduces cracks in the modified mullite coating.

Wang, Hongyu↗

NASA NTRS2003

Coated article and method of making

Wang, Hongyu↗

NASA NTRS1990

Modeling of the flexural behavior of ceramic-matrix composites

This paper examines the effects of matrix cracking and fiber breakage on the flexural behavior of ceramic composite beams. A model has been proposed to represent the damage evolution of the beam, of which the matrix fracture strain is smaller than that of the fibers. Close form solutions of the critical loads for the initiation of matrix cracking and fiber breakage in the tension side of the beam have been found. The effects of thermal residual stresses and fiber/matrix debonding have been taken into account. The initial deviation of the load-deflection curve from linearity is due to matrix cracking, while fiber breakages are responsible for the drop in the load carrying capacity of the beam. The proportional limit as well as the nonlinear behavior of the beam deflection have been identified. The growth of the damaged zone has also been predicted. A three-point bending case is given as a numerical example.

Kuo, Wen-Shyong↗

NASA NTRS1993

NASA-UVA Light Aerospace Alloy and Structures Technology Program (LA2ST)

The NASA-UVA Light Aerospace Alloy and Structures Technology (LA2ST) Program continues a high level of activity. Progress achieved between 1 Jan. and 30 Jun. 1993 is reported. The objective of the LA2ST Program is to conduct interdisciplinary graduate student research on the performance of next generation, light weight aerospace alloys, composites, and thermal gradient structures in collaboration with NASA-Langley researchers. The following projects are addressed: environmental fatigue of Al-Li-Cu alloys; mechanisms of localized corrosion and environmental fracture in Al-Cu-Li-Mg-Ag alloy X2095 and compositional variations; the effect of zinc additions on the precipitation and stress corrosion cracking behavior of alloy 8090; hydrogen interactions with Al-Li-Cu alloy 2090 and model alloys; metastable pitting of aluminum alloys; cryogenic fracture toughness of Al-Cu-Li + In alloys; the fracture toughness of Weldalite (TM); elevated temperature cracking of advanced I/M aluminum alloys; response of Ti-1100/SCS-6 composites to thermal exposure; superplastic forming of Weldalite (TM); research to incorporate environmental effects into fracture mechanics fatigue life prediction codes such as NASA FLAGRO; and thermoviscoplastic behavior.

Gangloff, Richard P.↗

NASA NTRS1995

Analytical investigation of the hygrothermal effects and parametric study of the Edge Crack Torsion (ECT) mode 3 test lay-ups

A shear deformation theory including residual thermal and moisture effects is developed for the analysis of either symmetric or unsymmetric laminates with mid-plane edge delamination under torsion loading. The theory is based on an assumed displacement field which includes shear deformation. The governing equations and boundary conditions are obtained from the principle of virtual work. The analysis of the (90/(+/- 45)(n)/(-/+ 45)(n)/90)(s) ECT mode 3 test lay-up indicates that there are no hygrothermal effects on the mode 3 strain energy release rate because the laminate, and both sublaminates above and below the delamination, are symmetric lay-ups. A further parametric study reveals that some other lay-ups can have negligible hygrothermal effects even when the sublaminates above and below the delamination are not symmetric about their own mid-planes. However, these lay-ups may suffer from distortion after the curing process. Another Interesting set of lay-ups investigated is a class of antisymmetric laminates with (+/-(theta/(theta -90)(2)/theta))(n) lay-ups. It is observed that when n takes on even numbers (2 and 4), both hygrothermal and mode 1 effects can be neglected. From this point of view, these lay-ups provide a way to determine the mode 3 toughness between two dissimilar layers. However, when n takes on odd numbers (1 and 3), both hygrothermal and mode 1 effects may be strong in these lay-ups. In particular, when theta equals 45 deg, the lay-ups are free from both hygrothermal and mode 1 effects irrespective of n.

Li, Jian↗

NASA NTRS1997

Thermal Characterization of Defects in Aircraft Structures Via Spatially Controlled Heat Application

Recent advances in thermal imaging technology have spawned a number of new thermal NDE techniques that provide quantitative information about flaws in aircraft structures. Thermography has a number of advantages as an inspection technique. It is a totally noncontacting, nondestructive, imaging technology capable of inspecting a large area in a matter of a few seconds. The development of fast, inexpensive image processors have aided in the attractiveness of thermography as an NDE technique. These image processors have increased the signal to noise ratio of thermography and facilitated significant advances in post-processing. The resulting digital images enable archival records for comparison with later inspections thus providing a means of monitoring the evolution of damage in a particular structure. The National Aeronautics and Space Administration's Langley Research Center has developed a thermal NDE technique designed to image a number of potential flaws in aircraft structures. The technique involves injecting a small, spatially controlled heat flux into the outer surface of an aircraft. Images of fatigue cracking, bond integrity and material loss due to corrosion are generated from measurements of the induced surface temperature variations. This paper will present a discussion of the development of the thermal imaging system as well as the techniques used to analyze the resulting thermal images. Spatial tailoring of the heat coupled with the analysis techniques represent a significant improvement in the delectability of flaws over conventional thermal imaging. Results of laboratory experiments on fabricated crack, disbond and material loss samples will be presented to demonstrate the capabilities of the technique. An integral part of the development of this technology is the use of analytic and computational modeling. The experimental results will be compared with these models to demonstrate the utility of such an approach.

Cramer, K. Elliott↗

NASA NTRS1990

Cyclic hot firing results of tungsten-wire-reinforced, copper-lined thrust chambers

An advanced thrust liner material for potential long life reusable rocket engines is described. This liner material was produced with the intent of improving the reusable life of high pressure thrust chambers by strengthening the chamber in the hoop direction, thus avoiding the longitudinal cracking due to low cycle fatigue that is observed in conventional homogeneous copper chambers, but yet not reducing the high thermal conductivity that is essential when operating with high heat fluxes. The liner material produced was a tungsten wire reinforced copper composite. Incorporating this composite into two hydrogen-oxygen test rocket chambers was done so that its performance as a reusable liner material could be evaluated. Testing results showed that both chambers failed prematurely, but the crack sites were perpendicular to the normal direction of cracking indicating a degree of success in containing the tremendous thermal strain associated with high temperature rocket engines. The failures, in all cases, were associated with drilled instrumentation ports and no other damages or deformations were found elsewhere in the composite liners.

Kazaroff, John M.↗

NASA NTRS1995

Low earth orbit durability of protected silicone for refractive photovoltaic concentrator arrays

Photovoltaic power systems with novel refractive silicone solar concentrators are being developed for use in low Earth orbit (LEO). Because of the vulnerability of silicones to atomic oxygen and ultraviolet radiation, these lenses are coated with a multilayer metal oxide protective coating. The objective of this work was to evaluate the effects of atomic oxygen and thermal exposures on multilayer coated silicone. Samples were exposed to high-fluence ground-laboratory and low-fluence in-space atomic oxygen. Ground testing resulted in decreases in both total and specular transmittance, while in-space exposure resulted in only small decreases in specular transmittance. A contamination film, attributed to exposed silicone at coating crack sites, was found to cause transmittance decreases during ground testing. Propagation of coating cracks was found to be the result of sample heating during exposure. The potential for silicone exposure, with the resulting degradation of optical properties from silicone contamination, indicates that this multilayer coated silicone is not durable for LEO space applications where thermal exposures will cause coating crack development and propagation.

McCollum, Timothy A.↗

NASA NTRS1972

Comparison of experimental and theoretical thermal fatigue lives for five nickel base alloys

The alloys Nimonic 90, IN 100, coated IN 100, B 1900, coated B 1900, MAR M200, and MAR M200DS (directionally solidified) were studied. Maximum temperatures ranged from 770 C to 1120 C (1420 F to 2050 F). Specimen geometries included tapered disks, double-edged wedges, and cambered airfoils. The disks and wedges were heated and cooled in fluidized beds. The airfoil specimens were heated by a Mach 1 natural gas burner and rapid-air-cooled, with and without spanwise loading. Life calculations included two distinct failure modes: conventional low cycle fatigue and cyclic creep. Required material properties were limited to conventional thermal, tensile, and creep rupture data. The complete life calculation system included the calculation of transient temperature distributions, thermal strains, stresses, creep damage, fatigue damage, and cycles to first crack. Calculated lives were within a factor of two for 76 of the 86 data points analyzed. Cyclic creep accounted for 81% of all the calculated damage.

Spera, D. A.↗

NASA NTRS1984

Literature survey on oxidations and fatigue lives at elevated temperatures

Nickel-base superalloys are the most complex and the most widely used for high temperature applications such as aircraft engine components. The desirable properties of nickel-base superalloys at high temperatures are tensile strength, thermomechanical fatigue resistance, low thermal expansion, as well as oxidation resistance. At elevated temperature, fatigue cracks are often initiated by grain boundary oxidation, and fatigue cracks often propagate along grain boundaries, where the oxidation rate is higher. Oxidation takes place at the interface between metal and gas. Properties of the metal substrate, the gaseous environment, as well as the oxides formed all interact to make the oxidation behavior of nickel-base superalloys extremely complicated. The important topics include general oxidation, selective oxidation, internal oxidation, grain boundary oxidation, multilayer oxide structure, accelerated oxidation under stress, stress-generation during oxidation, composition and substrate microstructural changes due to prolonged oxidation, fatigue crack initiation at oxidized grain boundaries and the oxidation accelerated fatigue crack propagation along grain boundaries.

Liu, H. W.↗

NASA NTRS1998

A Comparison of Space and Ground Based Facility Environmental Effects for FEP Teflon

Fluorinated Ethylene Propylene (FEP) Teflon is widely used as a thermal control material for spacecraft, however, it is susceptible to erosion, cracking, and subsequent mechanical failure in low Earth orbit. One of the difficulties in determining whether FEP Teflon will survive during a mission is the wide disparity of erosion rates observed for this material in space and in ground based facilities. Each environment contains different levels of atomic oxygen, ions, and vacuum ultraviolet (VUV) radiation in addition to parameters such as the energy of the arriving species and temperature. These variations make it difficult to determine what is causing the observed differences in erosion rates. This paper attempts to narrow down which factors affect the erosion rate of FEP Teflon through attempting to change only one environmental constituent at a time. This was attempted through the use of a single simulation facility (plasma asher) environment with a variety of Faraday cages and VUV transparent windows. Isolating one factor inside of a radio frequency (RF) plasma proved to be very difficult. Two observations could be made. First, it appears that the erosion yield of FEP Teflon with respect to that of polyimide Kapton is not greatly affected by the presence or lack of VUV radiation present in the RF plasma and the relative erosion yield for the FEP Teflon may decrease with increasing fluence. Second, shielding from charged particles appears to lower the relative erosion yield of the FEP to approximately that observed in space, however it is difficult to determine for sure whether ions, electrons, or some other components are causing the enhanced erosion.

Rutledge, Sharon K.↗

NASA NTRS2000

Evaluation of Thermo-Mechanical Stability of COTS Dual-Axis MEMS Accelerometers for Space Applications

Microelectromechanical systems in MEMS is one of the fastest growing technologies in microelectronics, and is of great interest for military and aerospace applications. Accelerometers are the earliest and most developed representatives of MEMS. First demonstrated in 1979, micromachined accelerometers were used in automobile industry for air bag crash- sensing applications since 1990. In 1999, N4EMS accelerometers were used in NASA-JPL Mars Microprobe. The most developed accelerometers for airbag crash- sensing are rated for a full range of +/- 50 G. The range of sensitivity for accelerometers required for military or aerospace applications is much larger, varying from 20,000 G (to measure acceleration during gun and ballistic munition launches), and to 10(exp -6) G, when used as guidance sensors (to measure attitude and position of a spacecraft). The presence of moving parts on the surface of chip is specific to MEMS, and particularly, to accelerometers. This characteristic brings new reliability issues to micromachined accelerometers, including cyclic fatigue cracking of polysilicon cantilevers and springs, mechanical stresses that are caused by packaging and contamination in the internal cavity of the package. Studies of fatigue cracks initiation and growth in polysilicon showed that the fatigue damage may influence MEMS device performance, and the presence of water vapor significantly enhances crack initiation and growth. Environmentally induced failures, particularly, failures due to thermal cycling and mechanical shock are considered as one of major reliability concerns in MEMS. These environmental conditions are also critical for space applications of the parts. For example, the Mars pathfinder mission had experienced 80 mechanical shock events during the pyrotechnic separation processes.

Sharma, Ashok K.↗

NASA NTRS2001

Degradation Of Environmental Barrier Coatings (EBC) Due To Chemical and Thermal Expansion Incompatibility

Current environmental barrier coatings (EBCs) consist of multiple layers, with each layer having unique properties to meet the various requirements for successful EBCs. As a result, chemical and thermal expansion compatibility between layers becomes an important issue to maintaining durability. Key constituents in current EBCs are mullite (3Al2O3-2SiO2), BSAS (BaO(1-x)-SrO(x)-Al2O3-2SiO2), and YSZ (ZrO2-8 wt.% Y2O3). The mullite-BSAS combination appears benign although significant diffusion occurs. Mullite-YSZ and BSAS-YSZ combinations do not react up to 1500 C. Thermally grown SiO2- BSAS and mullite-BSAS-YSZ combinations are most detrimental, forming low melting glasses. Thermal expansion mismatch between YSZ and mullite or BSAS causes severe cracking and delamination.

Lee, Kang N.↗

NASA NTRS2017

Tutorial on Atomic Oxygen Effects and Contamination

Atomic oxygen is the most predominant specie in low Earth orbit (LEO) and is contained in the upper atmosphere of many other planetary bodies. Formed by photo-dissociation of molecular oxygen, it is highly reactive and energetic enough to break chemical bonds on the surface of many materials and react with them to form either stable or volatile oxides. The extent of the damage for spacecraft depends a lot on how much atomic oxygen arrives at the surface, the energy of the atoms, and the reactivity of the material that is exposed to it. Oxide formation can result in shrinkage, cracking, or erosion which can also result in changes in optical, thermal, or mechanical properties of the materials exposed. The extent of the reaction can be affected by mechanical loading, temperature, and other environmental components such as ultraviolet radiation or charged particles. Atomic oxygen generally causes a surface reaction, but it can scatter under coatings and into crevices causing oxidation much farther into a spacecraft surface or structure than would be expected. Contamination can also affect system performance. Contamination is generally caused by arrival of volatile species that condense on spacecraft surfaces. The volatiles are typically a result of outgassing of materials that are on the spacecraft. Once the volatiles are condensed on a surface, they can then be fixed on the surface by ultraviolet radiation andor atomic oxygen reaction to form stable surface contaminants that can change optical and thermal properties of materials in power systems, thermal systems, and sensors. This tutorial discusses atomic oxygen erosion and contaminate formation, and the effect they have on typical spacecraft materials. Scattering of atomic oxygen, some effects of combined environments and examples of effects of atomic oxygen and contamination on spacecraft systems and components will also be presented.

Earth orbital environments↗

NASA NTRS2018

Atomic Oxygen Environments, Effects, and Mitigation

Oxygen atoms↗

NASA NTRS2020

Residual Stress Measurements of Ni-base Superalloys through Neutron Diffraction

Linear friction welding (LFW) is being considered for joining single crystal to polycrystalline nickel-base alloys for advanced high temperature gas turbine disks. Unfortunately, the transient thermal cycle during the welding generates undesirable residual stresses. Additionally, upon thermal treatment to relieve these residual stresses the weld joints often crack. To understand this issue, state-of-the-art neutron diffraction was performed near weld joints to quantify the residual stresses created by varying linear friction weld conditions. The residual stresses were measured along three orthogonal axes in both the polycrystalline LSHR and single crystal SC-180 alloys. Residual stress measurements using a contour method were used to compare the calculations obtained by neutron diffraction. The results suggest high residual stresses can be successfully reduced using both pre- and post-weld processing steps. Moreover, the effect of grain size and pre-heat temperatures on the LFW process were explored.

Superalloys↗

NASA NTRS2013

Sprayable Aerogel Bead Compositions With High Shear Flow Resistance and High Thermal Insulation Value

A sprayable aerogel insulation has been developed that has good mechanical integrity and lower thermal conductivity than incumbent polyurethane spray-on foam insulation, at similar or lower areal densities, to prevent insulation cracking and debonding in an effort to eliminate the generation of inflight debris. This new, lightweight aerogel under bead form can be used as insulation in various thermal management systems that require low mass and volume, such as cryogenic storage tanks, pipelines, space platforms, and launch vehicles.

Ou, Danny↗

NASA NTRS2010

Fabrication of Lanthanum Telluride 14-1-11 Zintl High-Temperature Thermoelectric Couple

The development of more efficient thermoelectric couple technology capable of operating with high-grade heat sources up to 1,275 K is key to improving the performance of radioisotope thermoelectric generators. Lanthanum telluride La3-xTe4 and 14-1-11 Zintls (Yb14MnSb11) have been identified as very promising materials. The fabrication of advanced high-temperature thermoelectric couples requires the joining of several dissimilar materials, typically including a number of diffusion bonding and brazing steps, to achieve a device capable of operating at elevated temperatures across a large temperature differential (up to 900 K). A thermoelectric couple typically comprises a heat collector/ exchanger, metallic interconnects on both hot and cold sides, n-type and ptype conductivity thermoelectric elements, and cold-side hardware to connect to the cold-side heat rejection and provide electrical connections. Differences in the physical, mechanical, and chemical properties of the materials that make up the thermoelectric couple, especially differences in the coefficients of thermal expansion (CTE), result in undesirable interfacial stresses that can lead to mechanical failure of the device. The problem is further complicated by the fact that the thermoelectric materials under consideration have large CTE values, are brittle, and cracks can propagate through them with minimal resistance. The inherent challenge of bonding brittle, high-thermal-expansion thermoelectric materials to a hot shoe material that is thick enough to carry the requisite electrical current was overcome. A critical advantage over prior art is that this device was constructed using all diffusion bonds and a minimum number of assembly steps. The fabrication process and the materials used are described in the following steps: (1) Applying a thin refractory metal foil to both sides of lanthanum telluride. To fabricate the n-type leg of the advanced thermoelectric couple, the pre-synthesized lanthanum telluride coupon was diffusion bonded to the metal foil using a thin adhesion layer. (2) Repeating a similar process for the 14-1-11 Zintl p-type leg of the advanced thermoelectric couple. (3) Bonding thick CTE-matched metal plates on the metallized lanthanum telluride and Yb14MnSb11 to form the hot and cold sides of the thermoelectric couple. The calculated conversion efficiency of such an advanced couple would be about 10.5 percent, about 35 percent better than heritage radioisotope thermoelectric technology that relies on Si-Ge alloys. In addition, unlike Si-Ge alloys, these materials can be combined with many other thermoelectric materials optimized for operation at lower temperatures to achieve conversion efficiency in excess of 15 percent (a factor of 2 increase over heritage technology).

Ravi, Vilupanur A.↗