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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Creep of Refractory Fibers and Modeling of Metal and Ceramic Matrix Composite Creep Behavior

Our concentration during this research was on the following subprograms. (1) Ultra high vacuum creep tests on 218, ST300 and WHfC tungsten and MoHfC molybdenum alloy wires, temperature range from 1100 K to 1500 K, creep time of 1 to 500 hours. (2) High temperature vacuum tensile tests on 218, ST300 and WHfC tungsten and MoHfC molybdenum alloy wires. (3) Air and vacuum tensile creep tests on polycrystalline and single crystal alumina fibers, such as alumina-mullite Nextel fiber, yttrium aluminum ganet (YAG) and Saphikon, temperature range from 1150 K to 1470 K, creep time of 2 to 200 hours. (4) Microstructural evaluation of crept fibers, TEM study on the crept metal wires, SEM study on the fracture surface of ceramic fibers. (5) Metal Matrix Composite creep models, based on the fiber creep properties and fiber-matrix interface zone formation.

Tewari, S.N.↗
NASA NTRS1999

Sensor Lead Wires Positioned on SiC-based Monolithic Ceramic and Fiber- reinforced Ceramic Matrix Composite Subcomponents with Flat and Curved Surfaces

There is strong interest in the development of silicon carbide-based monolithic ceramic and composite materials and components for demanding, high-temperature applications. Thorough characterization of material properties, including high-temperature testing under simulated or actual operating conditions, is a high priority for programs involved in developing these silicon carbide- (SiC) based materials and components. Members of the Sensors and Electronics Technology Branch at the NASA Lewis Research Center are developing minimally intrusive methods of measuring the properties (such as the surface temperature, strain, and heat flux characteristics) of components and subelements that are being tested or operated in hostile, high-temperature environments. Their primary goal is to instrument the test article or operating component with durable sensors that have a minimal effect on test conditions such as the gas flow across the surface of the item and the material response (including the through-thickness conduction of heat). Therefore, the main thrust of their work has been the development of thin-film sensors (e.g., thermocouples or strain gauges) for use on various advanced material test articles, including SiC/SiC composite components. There was a need for a better method of securing sensor lead wires on SiC-based components and subelements that would be tested at temperatures to 1000 C (or higher), to enhance the durability of the overall minimally intrusive sensor system. To address this need, Lewis researchers devised an alternative approach for positioning the sensor lead wires (which are connected to the thin-film sensors) on SiC or SiC/SiC components. A reaction-forming method of joining was used to strongly bond hoop-shaped monolithic SiC and SiC/SiC composite attachments of various sizes to both flat and curved surfaces of SiC/SiC composite subelements (see the photos). This approach is based on an affordable, robust ceramic joining technology, named ARCJoinT, which was developed at Lewis for the joining of SiC-based ceramics and fiber-reinforced composites.

Kiser, James D.↗
NASA NTRS2003

Probabilistic Evaluation of Advanced Ceramic Matrix Composite Structures

The objective of this report is to summarize the deterministic and probabilistic structural evaluation results of two structures made with advanced ceramic composites (CMC): internally pressurized tube and uniformly loaded flange. The deterministic structural evaluation includes stress, displacement, and buckling analyses. It is carried out using the finite element code MHOST, developed for the 3-D inelastic analysis of structures that are made with advanced materials. The probabilistic evaluation is performed using the integrated probabilistic assessment of composite structures computer code IPACS. The affects of uncertainties in primitive variables related to the material, fabrication process, and loadings on the material property and structural response behavior are quantified. The primitive variables considered are: thermo-mechanical properties of fiber and matrix, fiber and void volume ratios, use temperature, and pressure. The probabilistic structural analysis and probabilistic strength results are used by IPACS to perform reliability and risk evaluation of the two structures. The results will show that the sensitivity information obtained for the two composite structures from the computational simulation can be used to alter the design process to meet desired service requirements. In addition to detailed probabilistic analysis of the two structures, the following were performed specifically on the CMC tube: (1) predicted the failure load and the buckling load, (2) performed coupled non-deterministic multi-disciplinary structural analysis, and (3) demonstrated that probabilistic sensitivities can be used to select a reduced set of design variables for optimization.

Abumeri, Galib H.↗
NASA NTRS2007

Improving Turbine Performance with Ceramic Matrix Composites

Under the new NASA Fundamental Aeronautics Program, efforts are on-going within the Supersonics Project aimed at the implementation of advanced SiC/SiC ceramic composites into hot section components of future gas turbine engines. Due to recent NASA advancements in SiC-based fibers and matrices, these composites are lighter and capable of much higher service temperatures than current metallic superalloys, which in turn will allow the engines to operate at higher efficiencies and reduced emissions. This presentation briefly reviews studies within Task 6.3.3 that are primarily aimed at developing physics-based concepts, tools, and process/property models for micro- and macro-structural design, fabrication, and lifing of SiC/SiC turbine components in general and airfoils in particular. Particular emphasis is currently being placed on understanding and modeling (1) creep effects on residual stress development within the component, (2) fiber architecture effects on key composite properties such as design strength, and (3) preform formation processes so that the optimum architectures can be implemented into complex-shaped components, such as turbine vanes and blades.

DiCarlo, James A.↗
NASA NTRS2000

Silicon-Based Ceramic-Matrix Composites for Advanced Turbine Engines: Some Degradation Issues

SiC/BN/SiC composites are designed to take advantage of the high specific strengths and moduli of non-oxide ceramics, and their excellent resistance to creep, chemical attack, and oxidation, while circumventing the brittleness inherent in ceramics. Hence, these composites have the potential to take turbine engines of the future to higher operating temperatures than is achievable with metal alloys. However, these composites remain developmental and more work needs to be done to optimize processing techniques. This paper highlights the lingering issue of pest degradation in these materials and shows that it results from vestiges of processing steps and can thus be minimized or eliminated.

Thomas-Ogbuji, Linus U. J.↗
NASA NTRS1990

Intermetallic and ceramic matrix composites for 815 to 1370 C (1500 to 2500 F) gas turbine engine applications

Revolutionary improvements in gas turbine engine specific fuel consumption and specific thrust are expected to be gained through incorporation of CMCs and of MMCs (whose intermetallic matrices are reinforced by highly refractory ceramic fibers). A status development evaluation is presented for NASA's Advanced High Temperature Engine Materials Technology Program, with a view to projections of early-21st century transport aircraft performance levels obtainable through the use of MMCs and CMCs in ultrahigh bypass turbofan engines.

Stephens, Joseph R.↗
NASA NTRS1988

A test for interfacial effects and stress transfer in ceramic matrix composites

The efforts to fabricate single embedded filament specimens of carbon and SiC fibers were unsuccessful largely due to the thermal stresses resulting from differences in thermal coefficient of expansion. Other factors appear to have been involved including embrittlement of the metal substrate by the H2 gas in the chemical vapor deposition flow stream and reaction layers formed between the silicon carbide and the metal substrate. The carbon fiber may have been attacked by the CVD reactant. It is concluded that these differential stresses are so large as to make the embedded fiber test impractical for the study of interphase effects and stress transfer in fiber ceramic matrix systems.

Bascom, Willard D.↗
NASA NTRS1993

Ceramic fiber reinforced glass-ceramic matrix composite

A slurry of BSAS glass powders is cast into tapes which are cut to predetermined sizes. Mats of continuous chemical vapor deposition (CVD)-SiC fibers are alternately stacked with these matrix tapes. This tape-mat stack is warm-pressed to produce a 'green' composite which is heated to burn out organic constituents. The remaining interim material is then hot-pressed to form a BSAS glass-ceramic fiber-reinforced composite.

Bansal, Narottam P.↗
NASA NTRS1986

Polymer precursors for ceramic matrix composites

The synthesis and characterization of a polycyclohexasilane is reported. Because of its cyclic structure, it is anticipated that this polymer might serve as a precursor to SIC having a high char yield with little rearrangement to form small, volatile cyclic silanes, and, as such, would be of interest as a precursor to SiC composite matrices and fibers, or as a binder in ceramic processing. Several approaches to the synthesis of a bifunctional cyclic monomer were attempted; the most successful of these was metal coupling of PhMeSiCl2 and Me2SiCl2. The procedure gives six-membered ring compounds with all degrees of phenyl substitution, from none to hexaphenyl. The compounds with from 0-2 groups were isolated and characterized. The fraction with degree of phenyl substitution equal to 2, a mixture of cis and trans 1,2-; 1,3-; and 1,4 isomers, was isolated in 32 percent yield. Pure 1,4 diphenyldecamethylcyclohexasilane was isolated from the mixed diphenyl compounds and characterized. Diphenyldecamethylcyclohexasilanes were dephenylated to dichlorodecamethylcyclohexasilanes by treating with H2SO4.NH4Cl in benzene. The latter were purified and polymerized by reacting with sodium in toluene. The polymers were characterized by HPGPC, elemental analysis, proton NMR, and IR. Thermogravimetric analyses were carried out on the polymers. As the yield of residual SiC was low, polymers were heat treated to increase the residual char yield. As high as 51.52 percent residual char yield was obtained in one case.

Litt, M. H.↗
NASA NTRS2003

Nondestructive Evaluation of Ceramic Matrix Composite Combustor Components

Combustor liners fabricated from a SiC/SiC composite were nondestructively interrogated before and after combustion rig testing by X-ray, ultrasonic and thermographic techniques. In addition, mechanical test results were obtained from witness coupons, representing the as-manufactured liners, and from coupons machined from the components after combustion exposure. Thermography indications were found to correlate with reduced material properties obtained after rig testing. The thermography indications in the SiC/SiC liners were delaminations and damaged fiber tows, as determined through microstructural examinations. [copyright] 2003 American Institute of Physics

X-ray analysis↗
NASA NTRS1985

A ceramic matrix composite based on polymerization and pyrolysis of ethynylated aromatics

A number of ethynylated aromatic monomers recently have been synthesized which thermally homopolymerize and copolymerize to produce rigid, highly cross-linked polymers with high thermal stability (Tg of about 450 C). On pyrolysis, these polymers lose few volatiles (more than 85 percent char yield) to yield carbon bodies of relatively low porosity. These properties render the ethynylated aromatics of significant interest as matrices for high temperature composites. Incorporation of a SiC particle filler in the matrix improves the rheology of the system and minimizes shrinkage during pyrolysis. Several unidirectional composites have been fabricated combining a graphite or boria-alumina-silica continuous reinforcement with an ethynylated aromatic polymer matrix and SiC filler. Thermogravimetric analysis of composite pyrolysis behavior was used to determine reaction kinetics and to establish a composite fabrication cycle. Composites retained 95 percent of their green weight on pyrolysis. Microstructure of the green and pyrolyzed composites is characterized for materials pyrolyzed at 600 C in vacuum and argon as well as for laminates heated at 1200 C in argon following pyrolysis.

Hurwitz, F. I.↗
NASA NTRS1988

Auger analysis of a fiber/matrix interface in a ceramic matrix composite

Auger electron spectroscopy (AES) depth profiling was used to characterize the fiber/matrix interface of an SiC fiber, reaction bonded Si3N4 matrix composite. Depth profiles of the as received double coated fiber revealed concentration oscillations which disappeared after annealing the fiber in the environment used to fabricate the composite. After the composite was fractured, the Auger depth profiles showed that failure occurred in neither the Beta-SiC fiber body nor in the Si3N4 matrix but, concurrently, at the fiber coating/matrix interface and within the fiber coating itself.

Honecy, Frank S.↗
NASA NTRS1988

Auger analysis of a fiber/matrix interface in a ceramic matrix composite

Auger electron spectroscopy (AES) depth profiling was used to characterize the fiber/matrix interface of an SiC fiber, reaction bonded Si3N4 matrix composite. Depth profiles of the as received double coated fiber revealed concentration oscillations which disappeared after annealing the fiber in the environment used to fabricate the composite. After the composite was fractured, the Auger depth profiles showed that failure occurred in neither the Beta-SiC fiber body nor in the Si3N4 matrix but, concurrently, at the fiber coating/matrix interface and within the fiber coating itself.

Honecy, Frank S.↗
NASA NTRS1989

An investigation of the thermal cycling damage of 25 vol. pct SiCw/alumina ceramic matrix composite

An investigation was made of the thermal cycling damage of a 25 vol pct SiC whisker/alumina (SiCw/Al2O3) composite. Thermal cycling tests were conducted by subjecting a composite specimen to two different fluidized beds. After thermal cycling the composite specimens were subjected to elastic modulus and effective fracture toughness measurements. The thermal cycled specimens were investigated with SEM and TEM studies. It was found that this composite has a relatively high resistance to thermal cycling.

Armstrong, William D.↗
NASA NTRS1989

Effects of loading rate and temperature on dynamic fracture of ceramic matrix composites

A hybrid experimental-numerical procedure was used to determine the dynamic fracture initiation toughnesses and the dynamic stress intensity factors of alumina, TiB2-particulate/SiC-matrix, and SiC(w)/Al2O3-matrix composites at room and elevated temperatures under static and impact loadings. The dynamic fracture initiation toughnesses at room and elevated temperatures were greater than the corresponding static fracture toughness. The crack velocity versus the dynamic stress intensity factor relations showed minor differences due to temperature changes but significant differences due to the change in loading rates.

Yang, K. H.↗
NASA NTRS1991

Thermal shock of fiber reinforced ceramic matrix composites

Monolithic silicon carbide and silicon nitride and a Nicalon fiber reinforced silicon carbide composite were subjected to severe thermal shock conditions via impingement of a hydrogen-oxygen flame. Surface heating rates of 1000 C/sec to 2500 C/sec were generated. The performance of the monolithic reference materials are compared and contrasted to the significantly greater thermal shock resistance of the composite. Ultrasonic and radiographic NDE techniques were used to evaluate the integrity of the composite subsequent to thermal shock. Tensile tests were performed to determine the residual tensile strength and modulus. Physical property changes are discussed as a function of number and severity of thermal shock cycles.

Eckel, Andrew J.↗
NASA NTRS1991

Mechanical behavior of fiber reinforced SiC/RBSN ceramic matrix composites: Theory and experiment

The mechanical behavior of continuous fiber reinforced SiC/RBSN (Reaction Bonded Silicon Nitride) composites with various fiber contents is evaluated. Both catastrophic and noncatastrophic failures are observed in tensile specimens. Damage and failure mechanisms are identified via in-situ monitoring using NDE (nondestructive evaluation) techniques through the loading history. Effects of fiber/matrix interface debonding (splitting) parallel to fibers are discussed. Statistical failure behavior of fibers is also observed, especially when the interface is weak. Micromechanical models incorporating residual stresses to calculate the critical matrix cracking strength, ultimate strength, and work of pull-out are reviewed and used to predict composite response. For selected test problems, experimental measurements are compared to analytical predictions.

Chulya, Abhisak↗