NASA/TM—2008-214803 Evaluation of Silicon Nitride for Brayton Turbine Wheel Application Marc R. Freedman Glenn Research Center, Cleveland, Ohio January 2008 NASA STI Program . . . in Profile Since its founding, NASA has been dedicated to the • CONFERENCE PUBLICATION. Collected advancement of aeronautics and space science. The papers from scientific and technical NASA Scientific and Technical Information (STI) conferences, symposia, seminars, or other program plays a key part in helping NASA maintain meetings sponsored or cosponsored by NASA. this important role. • SPECIAL PUBLICATION. Scientific, The NASA STI Program operates under the auspices technical, or historical information from of the Agency Chief Information Officer. It collects, NASA programs, projects, and missions, often organizes, provides for archiving, and disseminates concerned with subjects having substantial NASA’s STI. The NASA STI program provides access public interest. to the NASA Aeronautics and Space Database and its public interface, the NASA Technical Reports Server, • TECHNICAL TRANSLATION. English- thus providing one of the largest collections of language translations of foreign scientific and aeronautical and space science STI in the world. technical material pertinent to NASA’s mission. Results are published in both non-NASA channels and by NASA in the NASA STI Report Series, which Specialized services also include creating custom includes the following report types: thesauri, building customized databases, organizing and publishing research results. • TECHNICAL PUBLICATION. Reports of completed research or a major significant phase For more information about the NASA STI of research that present the results of NASA program, see the following: programs and include extensive data or theoretical analysis. Includes compilations of significant • Access the NASA STI program home page at scientific and technical data and information http://www.sti.nasa.gov deemed to be of continuing reference value. NASA counterpart of peer-reviewed formal • E-mail your question via the Internet to professional papers but has less stringent [email protected] limitations on manuscript length and extent of graphic presentations. • Fax your question to the NASA STI Help Desk at 301–621–0134 • TECHNICAL MEMORANDUM. Scientific and technical findings that are preliminary or • Telephone the NASA STI Help Desk at of specialized interest, e.g., quick release 301–621–0390 reports, working papers, and bibliographies that contain minimal annotation. Does not contain • Write to: extensive analysis. NASA Center for AeroSpace Information (CASI) 7115 Standard Drive • CONTRACTOR REPORT. Scientific and Hanover, MD 21076–1320 technical findings by NASA-sponsored contractors and grantees. NASA/TM—2008-214803 Evaluation of Silicon Nitride for Brayton Turbine Wheel Application Marc R. Freedman Glenn Research Center, Cleveland, Ohio National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio 44135 January 2008 Trade names and trademarks are used in this report for identification only. Their usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. Level of Review: This material has been technically reviewed by technical management. Available from NASA Center for Aerospace Information National Technical Information Service 7115 Standard Drive 5285 Port Royal Road Hanover, MD 21076–1320 Springfield, VA 22161 Available electronically at http://gltrs.grc.nasa.gov Evaluation of Silicon Nitride for Brayton Turbine Wheel Application Marc R. Freedman National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio 44135 Summary Silicon nitride (Si N ) is being evaluated as a risk-reduction alternative for a Jupiter Icy Moons 3 4 Orbiter Brayton turbine wheel in the event that the Prometheus program design requirements exceed the creep strength of the baseline metallic superalloys. Five Si N ceramics, each processed by a different 3 4 method, were screened based on the Weibull distribution of bend strength at 1700 °F (927 °C). Three of the Si N ceramics, Honeywell AS800, Kyocera SN282, and Saint-Gobain NT154, had bend strengths in 3 4 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. Introduction Recently, a coupled fluid-structural analysis of the Brayton rotating unit (BRU) turbine wheel concluded that because of the low probability of failure predicted for a silicon nitride (Si N ) wheel at the 3 4 highest fluid-induced stress location, a properly designed Si N rotor could be successful (ref. 1). Thus, 3 4 Si N was chosen as a back-up material for the turbine wheel of the Jupiter Icy Moons Orbiter (JIMO) 3 4 Brayton cycle nuclear engine. In this application, the component will operate at a maximum temperature of 1700 °F (927 °C) and a maximum stress of 40 ksi (276 MPa), in He-Xe (trace O ) for >100 000 hr. The 2 temperature and stress conditions are easily achieved with this material, but there are no reports in the literature about the long-term behavior, especially creep (slow crack growth), under these conditions. Reference 1 also emphasizes that a ceramic rotor cannot be directly substituted for a metal part mostly because of ceramic manufacturing limitations, particularly blade thickness. With manufacturing limitations in mind, the evaluation of five state-of-the-art Si N ceramics has 3 4 begun. Each material has been processed differently by its manufacturer, and thus each represents a different way the turbine wheel can be made. The materials are briefly described in table I. To narrow the choice of materials for long-term testing, the strength of each material was determined by a four-point bend test at 1700 °F (927 °C). There is no existing bend strength data in the literature for any of these materials at 1700 °F (927 °C). A statistical population of each material was tested so that a Weibull modulus could be reliably calculated. The Weibull distribution function, F = 1 – exp[–(σ/σ )m], where F f 0 is the cumulative probability of fracture, σ is the characteristic strength, m is the Weibull modulus, and σ 0 f is the fracture strength, is used to describe a population density distribution. A larger m indicates a lower scatter or dispersion in the Weibull distribution and a greater material reliability. Thus, desirable materials would have both high strength and high Weibull modulus. The purpose of this work is twofold. The first is to screen a larger number of candidate materials using a simple, inexpensive test and to choose the best material(s) for the more expensive, long-term sustained tensile-load testing. The second is to determine the baseline properties of the materials at 1700 °F (927 °C). NASA/TM—2008-214803 1 TABLE I.—CANDIDATE SILICON NITRIDE MATERIALS Vendor Material Designation Process Honeywell Ceramic Components SiN AS800 Gas-pressure sintered (GPS) 3 4 Ceradyne, Inc. Si N 147–31E Sintered, reaction bonded (SRBSN) 3 4 Boride Products SiAlON TK4 Sintered Kyocera Industrial Ceramics Si N SN282 GPS 3 4 Saint Gobain Ceramics & Plastics, Inc. SiN NT154 Hot isostatically pressed (HIPed) 3 4 Procedures All high-temperature flexural strength testing adhered to both the ASTM C 1211 and MIL–STD– 1492(MR) protocols (ref. 2). Thirty specimens of each of the materials listed in table I were longitudinally ground with a 400-grit diamond wheel to 3 mm high by 4 mm wide by 45 mm long. All long edges had a 0.12-mm bevel. The specimens were heat treated in air at 2100 °F (1150 °C) to relieve residual stress and heal machining damage. The densities of each specimen were determined by dividing the measured weight by the calculated volume. Four-point bend strength was determined at 1700 °F (927 °C) at a crosshead rate of 0.5 mm/min. The inner and outer spans of the flexure fixture were 20 and 40 mm, respectively. Selected failed specimens of each material were mounted with the two tensile surfaces glued together, and fractography was performed in a scanning electron microscope (SEM). Representative virgin specimens of each material were vacuum-mounted in epoxy; some were polished only and some were polished followed by plasma etching to evaluate their respective microstructure. Both the polished and the polished and etched specimens were examined by both SEM and a field emission scanning electron microscope (FESEM). Selected specimens were examined by energy dispersive analytical x-ray (EDAX) while in the FESEM to discern localized variation in chemical composition. Results and Discussion Weibull parameters were determined by the linear regression method from lnln(1/(1 – F)) = mlnσ – f mlnσ as shown in figure 1, where the Weibull modulus is obtained directly from the slope and the 0 characteristic strength is calculated from the intercept. Based upon this initial analysis, the Kyocera SN282 (Kyocera Industrial Ceramics Corp., Vancouver, WA), Honeywell AS800 (Honeywell Ceramic Components, Torrance, CA), and Saint-Gobain NT154 (Saint-Gobain Ceramics & Plastics, Inc., Northborough, MA) had significantly greater average strengths and higher Weibull moduli than the materials from Boride Products (Boride Products, Traverse City, MI) and Ceradyne (Ceradyne, Inc., Costa Mesa, CA). Clearly, the Boride Products material had an average strength below the required 276 MPa. The Weibull modulus of the Ceradyne material (15.89) was a little more than half that of the AlliedSignal material (28.12) and less than half that of the current ductile metal competitors (~40). Honeywell AS800 AS800 is an in-situ-reinforced silicon nitride produced by gas-pressure liquid-phase sintering with La O , Y O , and SrO additives followed by heat treatment (ref. 3). The measured average density of this 2 3 2 3 material was 3.63 g/cm3 (standard deviation = 0.004). As shown in figure 2(a), the material had a relatively uniform distribution of fine (<5 μm) porosity throughout. At higher magnification, figure 2(b), the typical acicular, interlocking grain structure of an in-situ-reinforced Si N is apparent. This structure is 3 4 due to the α- to β-Si N dissolution-reprecipitation sintering mechanism and results from subsequent 3 4 anisotropic growth of the resultant hexagonal β-Si N grains to maximize the low-energy (100) prismatic 3 4 planes (ref. 3). Lin et al. have described the grain structure to consist of ~80 percent equiaxed, ~0.5-μm NASA/TM—2008-214803 2 grains, and ~20 percent acicular, 1.5- to 2-μm grains with aspect ratios from 5 to 12 (ref. 4). One benefit of this microstructure compared to fine-grained Si N is relatively high fracture toughness, ~8 MPa⋅m1/2 3 4 (ref. 5). The EDAX examination of the surface reveals a glassy grain boundary phase containing La, Y, Si, O, and N (fig. 3). Others have identified two crystalline phases, La Si O N and either Y Si O N (ref. 5 3 12 5 3 12 3) or Y Si O N (ref. 4), within the grain boundary. This material typically failed from either a surface 10 7 23 pore or an exaggerated grain at the surface. A typical fracture origin, a ~100- by 11-μm Si N grain, is 3 4 shown in figure 4(a) and (b). Room-temperature flexural strengths have been reported to be ~800 MPa with a Weibull modulus of 21 (refs. 5 and 6). Reference 6 indicates that the flexure strength at 1700 °F (927 °C) obtained in this study is in agreement with the extrapolated values obtained for AS800 made in 1995. Stepped stress rupture results at 982 °C (essentially no strength degradation at stresses up to 450 MPa for up to 1000 hr) in that same reference provided encouraging justification for continued creep characterization of this material for this application. NASA/TM—2008-214803 3 Kyocera SN282 SN282 is another in-situ-reinforced silicon nitride. It is produced by gas-pressure liquid-phase sintering with Lu O additive followed by heat treatment (ref. 7). The average density of the material in 2 3 this study was 3.38 g/cm3 (standard deviation = 0.005). As shown in figure 5, the material had a relatively uniform distribution of fine (<5 μm) porosity throughout, similar to AS800. Also apparent in figure 5 NASA/TM—2008-214803 4 is the typical acicular, interlocking grain structure of this in-situ-reinforced Si N . Compared to AS800, 3 4 SN282 has significantly fewer and larger acicular grains, resulting in a much finer overall microstructure and finer distribution of intergranular glass phase. The EDAX examination of the surface reveals a glassy grain boundary phase containing Lu, Si, O, and N (fig. 6). This is consistent with two crystalline phases, Lu Si O and Lu SiO (ref. 7), within the grain boundary. This material typically failed from either a 2 2 7 2 5 surface pore or an exaggerated grain (or both) at the surface. One such failure origin, both a grain and a pore, is shown in figure 7. Room temperature flexural strengths have been reported to be 595 MPa with a NASA/TM—2008-214803 5 Weibull modulus of 11 (ref. 5). Although this is not the highest strength material in the present study, SN282 remains interesting because of its reported improved creep and oxidation resistance at higher temperatures (refs. 8 and 9). It is not known whether these benefits will also be evident at 927 °C. Significantly, the strength does not decrease at 927 °C, which indicates a more refractory grain boundary phase in this material relative to the other materials in this study. Saint Gobain NT154 NT154 is produced by hot isostatic pressing (HIP) of a glass-encapsulated powder followed by heat treatment. In this case the sintering additive is Y O , and the resulting grain boundary phase is Y Si O 2 3 2 2 7 (refs. 10 and 11). The average density of the material in this study was 3.22 g/cm3 (standard deviation = 0.005). As evident in figure 8, this material departs from the conventionally sintered materials, AS800 and SN282, with a finer microstructure devoid of highly elongated grains. Figure 9 also clearly shows small and dispersed pores, pore clusters, and extremely fine equiaxed grains that fill the space between relatively small acicular grains. There also appears to be substantially less glassy phase, as would be expected from a HIPed material. In contrast to the sintered materials, this microstructure contributes to significantly lower fracture toughness, 5.5 to 6.0 MPa⋅m1/2 (2005, Saint-Gobain Ceramics & Plastics, Inc., Northborough, MA, product literature). Failure in this material can be attributed to pore clusters (fig. 10(a)) and surface machining defects (fig. 10(b)). Room temperature flexural strength is reported to be 900 to 1000 MPa with a Weibull modulus of 10 to 21 (refs. 11 and Saint-Gobain product literature). At 982 °C, the strength drops to 740 MPa with a Weibull modulus of 13 (Saint-Gobain product literature). This reported strength is in agreement with the strength measured in this study, but the modulus determined in this study is significantly higher. The manufacturer also reports a tensile creep rate at 1260 °C of 1.9×10–8 s–1, which is very encouraging for our lower temperature application. NASA/TM—2008-214803 6