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NASA NTRS2008

Evaluation of Silicon Nitride for Brayton Turbine Wheel Application

Silicon nitride (Si3N4) is being evaluated as a risk-reduction alternative for a Jupiter Icy Moons Orbiter Brayton turbine wheel in the event that the Prometheus program design requirements exceed the creep strength of the baseline metallic superalloys. Five Si3N4 ceramics, each processed by a different method, were screened based on the Weibull distribution of bend strength at 1700 F (927 C). Three of the Si3N4 ceramics, Honeywell AS800, Kyocera SN282, and Saint-Gobain NT154, had bend strengths in excess of 87 ksi (600 MPa) at 1700 F (927 C). These were chosen for further assessment and consideration for future subcomponent and component fabrication and testing.

Freedman, Marc R.↗

NASA NTRS2005

Permeability of Candidate Stirling Heater Head Materials Measured

Researchers at the NASA Glenn Research Center are evaluating high-temperature materials for Stirling heater heads for second- and third-generation Stirling radioisotope power systems that would help to increase the system efficiency to 30 to 35 percent and the system specific power to 8 to 10+ W/kg. Ceramic materials could make it possible for the convertor hot-end temperature to be increased to 1050 to 1200 C, in comparison to the current 650 C with an Inconel 718 heater head. A hermetically sealed Stirling heater head must retain a constant internal pressure of nearly 400-psi helium (He) throughout its useful life (120,000 hr) at the design operating temperature. Therefore, He permeability was measured for eight potential materials and compared with the permeability of the current heater head material, Inconel 718. The eight materials included silicon nitride (Si3N4), silicon dioxide (SiO2), both sintered and chemical vapor deposited (CVD) silicon carbide (SiC), alumina (Al2O3), two types of melt-infiltrated (MI) SiC/SiC composites, and a carbon/SiC composite (C/SiC). Glenn submitted samples of each material to Porous Materials, Inc., Ithaca, New York, for permeability analysis. At room temperature and 30-psi He, four materials--Si3N4, Al2O3, SiO2, and sintered SiC--demonstrated lower permeability than Inconel 718. The CVD SiC and all the composite materials were significantly more permeable to He than the baseline material.

Freedman, Marc R.↗

NASA NTRS2001

Remote, Noncontact Strain Sensing by Laser Diffraction Developed

A system was developed at the NASA Glenn Research Center for continually monitoring, in real time, the in-plane strain tensor in opaque solids during high-temperature, long-term mechanical testing. The simple, noncontacting, strain-sensing methodology should also be suitable for measurement in hostile environments. This procedure has obvious advantages over traditional, mechanical, contacting techniques, and it is easier to interpret than moir and speckle interferometric approaches. A two-dimensional metallic grid of micrometer dimensions is applied to a metallographically prepared gauge section on the surface of a tensile test specimen by a standard photolithographic process. The grid on the fixtured specimen is interrogated by an He-Ne laser, and the resulting diffraction pattern is projected backwards onto a translucent screen. A charge-coupled device (CCD) camera is used to image the first-order diffraction peaks from the translucent screen. A schematic representation of the system is shown in the figure.

Freedman, Marc R.↗

NASA NTRS1991

Reliability analysis of a structural ceramic combustion chamber

The Weibull modulus, fracture toughness and thermal properties of a silicon nitride material used to make a gas turbine combustor were experimentally measured. The location and nature of failure origins resulting from bend tests were determined with fractographic analysis. The measured Weibull parameters were used along with thermal and stress analysis to determine failure probabilities of the combustor with the CARES design code. The effect of data censoring, FEM mesh refinement, and fracture criterion were considered in the analysis.

Salem, Jonathan A.↗

NASA NTRS1990

Reliability analysis of a structural ceramic combustion chamber

The Weibull modulus, fracture toughness and thermal properties of a silicon nitride material used to make a gas turbine combustor were experimentally measured. The location and nature of failure origins resulting from bend tests were determined with fractographic analysis. The measured Weibull parameters were used along with thermal and stress analysis to determine failure probabilities of the combustor with the CARES design code. The effect of data censoring, FEM mesh refinement, and fracture criterion were considered in the analysis.

Salem, Jonathan A.↗

NASA NTRS1990

Mechanical behavior and failure phenomenon of an in situ-toughened silicon nitride

The Weibull modulus, fracture toughness and crack growth resistance of an in-situ toughened, silicon nitride material used to manufacture a turbine combustor were determined from room temperature to 1371 C. The material exhibited an elongated grain structure that resulted in improved fracture toughness, nonlinear crack growth resistance, and good elevated temperature strength. However, low temperature strength was limited by grains of excessive length (30 to 100 microns). These excessively long grains were surrounded by regions rich in sintering additives.

Salem, Jonathan A.↗

NASA NTRS1989

Slurry-pressing consolidation of silicon nitride

A baseline slurry-pressing method for a silicon nitride material is developed. The Si3N4 composition contained 5.8 wt percent SiO2 and 6.4 wt percent Y2O3. Slurry-pressing variables included volume percent solids, application of ultrasonic energy, and pH. Twenty vol percent slurry-pressed material was approximately 11 percent stronger than both 30 vol percent slurry-pressed and dry-pressed materials. The Student's t-test showed the difference to be significant at the 99 percent confidence level. Twenty volume percent (300 h) slurry-pressed test bars exhibited strengths as high as 980 MPa. Large, columnar beta-Si3N4 grains caused failure in the highest strength specimens. The improved strength correlated with better structural uniformity as determined by radiography, optical microscopy, and image analysis.

Sanders, William A.↗

NASA NTRS1989

Grinding Si3N4 Powder In Si3N4 Equipment

Three methods of grinding compared. Report based on study of grinding silicon nitride powder in preparation for sintering into solid ceramic material. Attrition, vibratory, and ball mills lined with reaction-bonded silicon nitride tested. Rates of reduction of particle sizes and changes in chemical compositions of powders measured so grinding efficiences and increases in impurity contents from wear of mills and media evaluated for each technique.

Herbell, Thomas P.↗

NASA NTRS1988

Improved Consolidation Of Silicon Carbide

Colloidal techniques used to improve dispersion and suspension of SiC in water. In combination with slurry pressing, resulted in increase in strength of sintered SiC by 87 percent over SiC sintered after dry pressing. Improvement due to reduced porosity and reflected in improved densities of "green" and sintered specimens. Smaller sizes of pores, and reduced sizes of critical flaws contributed to increase in strength. Useful for making parts from particulate-, transformation-, and whisker-toughened ceramics and monolithic ceramics. Provides avenue for study of variables contributing to reliability of current structural ceramics.

Freedman, Marc R.↗

NASA NTRS1988

Slurry-pressing consolidation of silicon nitride

A baseline slurry-pressing method for a silicon nitride material is developed. The Si3N4 composition contained 5.8 wt percent SiO2 and 6.4 wt percent Y2O3. Slurry-pressing variables included volume percent solids, application of ultrasonic energy, and pH. Twenty vol percent slurry-pressed material was approximately 11 percent stronger than both 30 vol percent slurry-pressed and dry-pressed materials. The Student's t-test showed the difference to be significant at the 99 percent confidence level. Twenty volume percent (300 h) slurry-pressed test bars exhibited strengths as high as 980 MPa. Large, columnar beta-Si3N4 grains caused failure in the highest strength specimens. The improved strength correlated with better structural uniformity as determined by radiography, optical microscopy, and image analysis.

Sanders, William A.↗

NASA NTRS1988

Improved "Green" Forming Of Silicon Nitride

Advanced processing techniques reduce incidence of critical flaws. Critical flaws reduced by processing of powders to avoid organic and metallic contamination and combination of colloidal techniques with innovative slurry-pressing technique avoiding agglomeration. Silicon nitride considered for many applications ranging from components of turbine engines to industrial heat exchangers.

Freedman, Marc R.↗

NASA NTRS1988

A sintering model for SiC(sub)w/Si3N4 composites

Presented is a model which suggests that it should be possible to pressureless sinter a SiC(sub w)/ Si3N4 composite to theoretical density. Prior failure to achieve complete densification by sintering is attributed to the use of compositions which result in a glass deficit. There is one basic premise for this model. The ratio of glass amount to surface area of nonglass constituents must be the same for both composite and sinterable monolithic Si3N4. This model suggests that whisker and grain sizes and whisker loading influence the glass amount necessary for successful sintering of composites. According to the model, a large glass amount will be necessary for successful sintering of these composites. However, grain boundary thicknesses in the composite will be less than those in the analogous monolithic materials. This suggests that good high temperature strength may still be attained. A recent report supports the predictions of the model.

Freedman, Marc R.↗