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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Thermal barrier coatings for the space shuttle main engine turbine blades

The Space Shuttle Main Engine (SSME) turbopump turbine blades experience extremely severe thermal shocks during start-up and shut-down. For instance, the high pressure fuel turbopump turbine which burns liquid hydrogen operates at approximately 1500 F, but is shut down fuel rich with turbine blades quenced in liquid hydrogen. This thermal shock is a major contributor to blade cracking. The same thermal shock cause the protective ZrO2 thermal barrier coatings to spall or flake off, leaving only the NiCrAlY bond coating which provides only a minimum thermal protection. The turbine blades are therefore life limited to about 3000 sec for want of a good thermal barrier. A suitable thermal barrier coating (TBC) is being developed for the SSME turbine blades. Various TBCs developed for the gas turbine engines were tested in a specially built turbine blade tester. This tester subjects the coated blades to thermal and pressure cycles similar to those during actual operation of the turbine. The coatings were applied using a plasma spraying techniques both under atmospheric conditions and in vacuum. Results are presented. In general vacuum plasma sprayed coatings performed much better than those sprayed under atmospheric conditions. A 50 to 50 blend of Cr2O3 and NiCrAlY, vacuum plasma sprayed on SSME turbopump turbine blades appear to provide significant improvements in coating durability and thermal protection.

Bhat, B. N.↗
NASA NTRS1989

Ballistic-electron-emission microscopy investigation of Schottky barrier interface formation

Ballistic-electron-emission microscopy (BEEM) has been used to investigate the origin of defects at the Au/GaAs(100) Schottky barrier interface. In addition, molecular beam epitaxy (MBE) and in situ fabrication methods have been employed to control Schottky barrier interface properties. BEEM characterization combined with MBE methods has enabled the development of a near-ideal Schottky barrier interface with drastically reduced defect density.

Hecht, M. H.↗
NASA NTRS1988

Vacuum application of thermal barrier plasma coatings

Coatings are presently applied to Space Shuttle Main Engine (SSME) turbine blades for protection against the harsh environment realized in the engine during lift off-to-orbit. High performance nickel, chromium, aluminum, and yttrium (NiCrAlY) alloy coatings, which are applied by atmospheric plasma spraying, crack and spall off because of the severe thermal shock experienced during start-up and shut-down of the engine. Ceramic coatings of yttria stabilized zirconia (ZrO2-Y2O3) were applied initially as a thermal barrier over coating to the NiCrAlY but were removed because of even greater spalling. Utilizing a vacuum plasma spraying process, bond coatings of NiCrAlY were applied in a low pressure atmosphere of argon/helium, producing significantly improved coating-to-blade bonding. The improved coatings showed no spalling after 40 MSFC burner rig thermal shock cycles, cycling between 1700 and -423 F. The current atmospheric plasma NiCrAlY coatings spalled during 25 test cycles. Subsequently, a process was developed for applying a durable thermal barrier coating of ZrO2-Y2O3 to the turbine blades of first stage high-pressure fuel turbopumps utilizing the vacuum plasma process. The improved thermal barrier coating has successfully passed 40 burner rig thermal shock cycles without spalling. Hot firing in an SSME turbine engine is scheduled for the blades. Tooling was installed in preparation for vacuum plasma spray coating other SSME hardware, e.g., the titanium main fuel valve housing (MFVH) and the fuel turbopump nozzle/stator.

Holmes, R. R.↗
NASA NTRS1990

Metallographic techniques for evaluation of thermal barrier coatings

The performance of ceramic thermal barrier coatings is strongly dependent on the amount and shape of the porosity in the coating. Current metallographic techniques do not provide polished surfaces that are adequate for a repeatable interpretation of the coating structures. A technique recently developed at NASA-Lewis for preparation of thermal barrier coating sections combines epoxy impregnation, careful sectioning and polishing, and interference layering to provide previously unobtainable information on processing-induced porosity. In fact, increased contrast and less ambiguous structure developed by the method make automatic quantitative metallography a viable option for characterizing thermal barrier coating structures.

Brindley, William J.↗
NASA NTRS1992

Thermal Barrier For Vented O-Ring Seal

Barrier allows gases to seat seal without damaging it. Ring of tungsten-wire mesh forms protective barrier between hot, pressurized combustion gases and O-rings. Mesh cools and depressurizes gases so they safely push on and thereby help to seat primary O-ring or secondary O-ring if primary O-ring fails to form seals. Barrier devised for use in rocket motor. Potential terrestrial applications includes aircraft engines, furnaces, and ducts carrying hot gases.

Schick, H.↗
NASA NTRS1991

Chemical nature of the barrier in Pb/YBa2Cu3O(7-x) tunneling structures

Several reports of reproducible tunneling measurements on YBa2Cu3O(7-x) thin films or single crystals with a Pb counterelectrode have recently appeared. The nature of the tunnel barrier, formed by air exposure, in these structures has been unknown. In the present work, the chemical nature of the tunnel barrier is studied with X-ray photoelectron spectroscopy (XPS). Laser-ablated films grown on LaAlO3 which have been chemically etched and heated in air are found to form nonsuperconducting surface Ba species, evident in an increase of the high binding energy Ba 3d and O 1s signals. A deposited Pb film about 10 A thick is found to be oxidized, and Cu(+2) is partially reduced to Cu(+1). The tunneling barrier thus appears to consist of species resulting from a combination of the air exposure and a reaction between the superconductor and the deposited Pb counterelectrode.

Vasquez, R. P.↗
NASA NTRS1993

Computed barrier heights for H + CH2O yields CH3O yields CH2OH

The barrier heights (including zero-point effects) for H + CH2O yields CH3O and CH3O yields CH2OH have been computed using complete active space self consistent field (CASSCF)/gradient calculations to define the stationary point geometries and harmonic frequencies and internally contracted configuration-interaction (CCI) to refine the energetics. The computed barrier heights are 5.6 kcal/mol and 30.1 kcal/mol, respectively. The former barrier height compares favorably to an experimental activation energy of 5.2 kcal/mol.

Walch, Stephen P.↗
NASA NTRS1992

Thermionic emission current in a single barrier varactor

From I-V measurements on Single Barrier Varactors (SBV) at different temperatures we concluded that thermionic emission across the barrier of the actual device is mainly due to transport through the X band. The same structure was also modeled with a one-dimensional drift-diffusion model, including a 'boundary condition' for thermionic emission across the heterojunction interface. By including thermionic field emission through the top of the triangular barrier of a biased diode and the effect of a non-abrupt interface at the heterojunction, we obtained good agreement between the modeled and measured I-V characteristics.

Hjelmgren, Hans↗
NASA NTRS1992

New approach to the design of Schottky barrier diodes for THz mixers

Near-ideal GaAs Schottky barrier diodes especially designed for mixing applications in the THz frequency range are presented. A diode fabrication process for submicron diodes with near-ideal electrical and noise characteristics is described. This process is based on the electrolytic pulse etching of GaAs in combination with an in-situ platinum plating for the formation of the Schottky contacts. Schottky barrier diodes with a diameter of 1 micron fabricated by the process have already shown excellent results in a 650 GHz waveguide mixer at room temperature. A conversion loss of 7.5 dB and a mixer noise temperature of less than 2000 K have been obtained at an intermediate frequency of 4 GHz. The optimization of the diode structure and the technology was possible due to the development of a generalized Schottky barrier diode model which is valid also at high current densities. The common diode design and optimization is discussed on the basis of the classical theory. However, the conventional fomulas are valid only in a limited forward bias range corresponding to currents much smaller than the operating currents under submillimeter mixing conditions. The generalized new model takes into account not only the phenomena occurring at the junction such as current dependent recombination and drift/diffusion velocities, but also mobility and electron temperature variations in the undepleted epi-layer. Calculated diode I/V and noise characteristics are in excellent agreement with the measured values. Thus, the model offers the possibility of optimizing the diode structure and predicting the diode performance under mixing conditions at THz frequencies.

Jelenski, A.↗
NASA NTRS1993

Computed barrier heights for H + CH2O yields CH3O yields CH2OH

The barrier heights (including zero-point effects) for H + CH2O yields CH3O and CH3O yields CH2OH have been computed using complete active space self consistent field (CASSCF)/gradient calculations to define the stationary point geometries and harmonic frequencies and internally contracted configuration-interaction (CCI) to refine the energetics. The computed barrier heights are 5.6 kcal/mol and 30.1 kcal/mol, respectively. The former barrier height compares favorably to an experimental activation energy of 5.2 kcal/mol.

Walch, Stephen P.↗
NASA NTRS1990

Newton modified barrier method in constrained optimization

In this paper, we develop and investigate the Newton method for solving constrained (non-smooth) optimization problems. This approach is based on the modified barrier functions (MBF) theory and on the global converging step-size version of the Newton method for smooth unconstrained optimization. Due to the excellent properties of the MBF near primal-dual solution, the Newton modified barrier method (NMBM) has a better rate of convergence, better complexity bound, and is much more stable in the final stage of the computational process than the methods which are based on the classical barrier functions (CBF).

Polyak, R.↗
NASA NTRS1991

Thick thermal barrier coatings for diesel components

An engineered thick thermal barrier coating consisting of multiple layers of zirconia and CoCrAlY with a zirconia top layer and having a system thermal conductance less than 410 w/m(exp 2)K exceeded the 100 hour engine durability goals set forth in this program. The thermal barrier coatings were intact at the test conclusion. Back to back single cylinder research engine tests were conducted with watercooled, metal hardware and oil-cooled, thermal barrier coating insulated hardware to determine apparent heat release and fuel economy. Apparent heat release data revealed that the insulated engine had a shorter ignition delay and a longer combustion duration than the metal engine. The insulated engine fuel economy was approximately two percent worse on average for this series of tests. There was no attempt to optimize engine efficiency of the insulated engine by modifying the engine timing, coating, or other techniques.

Yonushonis, T. M.↗
NASA NTRS1995

Thick thermal barrier coatings for diesel engines

Caterpillar's approach to applying Thick Thermal Barrier Coatings (TTBC's) to diesel engine combustion chambers has been to use advanced modeling techniques to predict engine conditions and combine this information with fundamental property evaluation of TTBC systems to predict engine performance and TTBC stress states. Engine testing has been used to verify the predicted performance of the TTBC systems and provide information on failure mechanisms. The objective of Caterpillar's subcontract with ORNL is to advance the fundamental understanding of thick thermal barrier coating systems. Previous reviews of thermal barrier coating technology concluded that the current level of understanding of coating system behavior is inadequate and the lack of fundamental understanding may impede the application of TTBC's to diesel engines. Areas of TTBC technology being examined in this program include powder characteristics and chemistry; bond coat composition; coating design, microstructure, and thickness as they affect properties, durability, and reliability; and TTBC 'aging' effects (microstructural and property changes) under diesel engine operating conditions. Methods to evaluate the reliability and durability of TTBC's have been developed that attempt to understand the fundamental strength of TTBC's for particular stress states.

Beardsley, M. B.↗
NASA NTRS1995

Some aspects of the hot corrosion of thermal barrier coatings

This paper provides a pro tem review of the hot corrosion of zirconia-based thermal barrier coatings for engine applications. Emphasis is placed on trying to understand the chemical reactions, and such other mechanisms as can be identified, that cause corrosive degradation of the thermal barrier coating. The various approaches taken in attempts to improve the hot corrosion resistance of thermal barrier coatings are also briefly described and critiqued.

Jones, R. L.↗
NASA NTRS1995

High temperature barrier coatings for refractory metals

Improvements in high temperature oxidation resistant metal coating technology will allow NASA and commercial entities to develop competitive civil space transport and communication systems. The success of investigations completed in this program will have a positive impact on broadening the technology base for high temperature materials. The work reported herein describes processes and procedures for successfully depositing coherent oxidation barrier coatings on refractory metals to prevent degradation under very severe operating environments. Application of the new technology developed is now being utilized in numerous Phase 3 applications through several prominent aerospace firms. Major achievements have included: (1) development of means to deposit thick platinum and rhodium coatings with lower stress and fewer microcracks than could be previously achieved; (2) development of processes to deposit thick adherent coatings of platinum group metals on refractory substrates that remain bonded through high temperature excursions and without need for intermediate coatings (bonding processes unique to specific refractory metals and alloys have been defined; (3) demonstration that useful alloys of refractory and platinum coatings can be made through thermal diffusion means; (4) demonstration that selected barrier coatings on refractory substrates can withstand severe oxidizing environments in the range of 1260 deg and 1760 deg C for long time periods essential to the life requirements of the hardware; and (5) successful application of the processes and procedures to prototype hardware. The results of these studies have been instrumental in improved thermal oxidation barrier coatings for the NASP propulsion system. Other Phase 3 applications currently being exploited include small uncooled thrusters for spacecraft and microsatellite maneuvering systems.

Malone, G. A.↗
NASA NTRS1994

Barrier potential design criteria in multiple-quantum-well-based solar-cell structures

The barrier potential design criteria in multiple-quantum-well (MQW)-based solar-cell structures is reported for the purpose of achieving maximum efficiency. The time-dependent short-circuit current density at the collector side of various MQW solar-cell structures under resonant condition was numerically calculated using the time-dependent Schroedinger equation. The energy efficiency of solar cells based on the InAs/Ga(y)In(1-y)As and GaAs/Al(x)Ga(1-x)As MQW structues were compared when carriers are excited at a particular solar-energy band. Using InAs/Ga(y)In(1-y)As MQW structures it is found that a maximum energy efficiency can be achieved if the structure is designed with barrier potential of about 450 meV. The efficiency is found to decline linearly as the barrier potential increases for GaAs/Al(x)Ga(1-x)As MQW-structure-based solar cells.

Mohaidat, Jihad M.↗
NASA NTRS1995

Thick thermal barrier coatings for diesel engines

Caterpillar's approach to applying thick thermal barrier coatings (TTBC's) to diesel engine combustion chambers has been to use advanced modeling techniques to predict engine conditions and combine this information with fundamental property evaluation of TTBC systems to predict engine performance and TTBC stress states. Engine testing has been used to verify the predicted performance of the TTBC systems and provide information on failure mechanisms. The objective Caterpillar's program to date has been to advance the fundamental understanding of thick thermal barrier coating systems. Previous reviews of thermal barrier coating technology concluded that the current level of understanding of coating system behavior is inadequate and the lack of fundamental understanding may impeded the application of TTBC's to diesel engines. Areas of TTBC technology being examined in this program include powder characteristics and chemistry; bond coat composition; coating design, microstructure, and thickness as they affect properties, durability, and reliability; and TTBC 'aging' effects (microstructural and property changes) under diesel engine operating conditions. Methods to evaluate the reliability and durability of TTBC's have been developed that attempt to understand the fundamental strength of TTBC's for particular stress states.

Beardsley, M. Brad↗