Materials

Advanced High-Temperature Engine Materials Technology Makes More Progress
Enabling Propulsion Materials Program Restructured
Titanium-Matrix Composite Fatigue Studied in Realistic Engine Cycle
Creep-Rupture Goals Set for Ceramic Fiber Reinforcement
New Tensile Test Determines CMC Interfacial Properties
Oxidation-Resistant Coating Stable to 1400 deg.C Identified for SiC/RBSN Composites
Strong, Tough Sapphire-Fiber-Reinforced Alumina Developed
Lewis Distinguished Paper and Lewis Materials Division Paper of the Year for 1993, Interfacial Chemistry of Perfluoroalkylether Lubricant Studied
NASA Solid Lubricant Composite Technology Receives Technical Award
Screen-Cage Ion Plating Developed
Rigid-Rod Polyimide Fibers Studied
Long-Term Aging Effects Measured for PMR-15 Composites
Nontoxic Low-Cost Resin Replaces PMR-15
Optimization Study of Electrically Conductive Polymers
New Refractory Oxide Coating Protects Silicon-Based Ceramics
Oxidation-Resistant TiAlCr Alloys Developed
Twin-Knudsen-Cell Mass Spectrometer Studies Alloy Thermodynamics
Structural Composite Made Strong, Tough, and "Pest" Resistant

The objective of the Advanced High-Temperature Engine Materials Technology (HITEMP) Program is to generate technology for advanced materials and structural analysis that will lead to increased fuel economy, improved reliability, extended life, and reduced operating costs for 21st century civil aviation propulsion systems. The primary focus is on fan and compressor materials (polymer-matrix composites, PMC's), compressor and turbine materials (superalloys; metal- and intermetallic-matrix composites, MMC's and IMC's), and turbine materials (ceramic-matrix composites, CMC's). These advanced materials are being developed by in-house researchers and on grants and contracts.

NASA considers this program to be a focused materials and structures research effort that builds upon our base research programs and supports component development projects, such as NASA's new initiative to develop the technology for advanced subsonic transport engines. HITEMP is closely coordinated with the Advanced Subsonic Technology (AST) Program and the Department of Defense/NASA Integrated High-Performance Turbine Engine Technology Program. Advanced materials from HITEMP may be used in these future applications.

Use of advanced materials in engines

A Lewis-developed polymer resin with greatly improved processability has been transferred to the AST program to be further developed and scaled up for engine components. An oxidative life prediction model has been developed and verified for nickel aluminum (NiAl) and delivered to General Electric Co. and Pratt & Whitney. An x-ray tomographic system has been developed and is being applied for the nondestructive evaluation of composite materials. Protective coatings on silicon carbide ceramic composites have demonstrated a threefold improvement in thermal mechanical fatigue life. Thin-film palladium-chromium strain gages have been developed and demonstrated at temperatures as high as 1900 deg.F. A new single-fiber microcomposite test technique has been developed to determine the interface properties of ceramic-matrix composites. Coated silicon carbide ceramic composite vanes have been successfully engine tested to 2000 deg.F in a cooperative project with the Army and Williams International.

The seventh annual review of the HITEMP program was held October 25 and 26, 1994. Details of research accomplishments are published in a conference report, NASA CP-10146.

Lewis contacts: Dr. Hugh R. Gray, (216) 433-3230;
Carol A. Ginty, (216) 433-3335
Headquarters program office: OA

The EPM program focuses on developing advanced materials technologies for HSCT propulsion system components that have the greatest impact on environmental and economic barriers. A high-temperature, long-life combustor liner is required to allow the use of advanced low-emissions combustion concepts. High-temperature, lightweight, durable materials are required for subcomponents in noise-suppressing exhaust nozzles. The economics of an HSCT can be severely impacted if advanced materials technologies are not developed for fan containment, turbine airfoil alloys, and compressor/turbine disks. Novel design concepts and possibly new materials are required for a lightweight, compact fan containment system that must be effective at elevated temperatures. Advances in turbine airfoil alloys, processes, and coatings are required to achieve acceptable lifetimes. New alloys as well as validation technologies are required for compressor and turbine disks that will be large and must operate at elevated temperatures for extended times during supersonic cruise.

One key feature of the EPM program is that the two major U.S. aircraft engine manufacturers, GE Aircraft Engines and Pratt & Whitney, have teamed up to conduct the contractual effort. Other U.S. engine companies, material suppliers, and component manufacturers are subcontractors to the GEAE/P&W HSCT team. NASA Lewis is an integral partner of this team, contributing to the overall critical-path effort. EPM is dedicated to maintaining U.S. leadership in the aircraft industry and is accomplishing this by restricting key data to the U.S. HSCT community. Conversely, strategic technology transfer is required to ensure sufficient markets for continuation of advanced technologies developed under EPM.

Another key aspect of EPM is the use of integrated technology development (ITD) teams. The ITD process implies that all disciplines required to achieve the end product, in this case materials technologies, are united into teams that work concurrently. This process is ideally suited for the EPM program because of the complex teaming arrangements among GEAE, P&W, NASA, and other U.S. industrial companies and because of the challenging technical and schedule milestones. An ITD team has been established for each critical component, as well as for business management (finance, contracts, procurement, etc.) and product assurance (change control, quality of product, process, etc.).

A ceramic-matrix composite combustor liner is deemed necessary for low-emissions combustors, such as a rich-burn, quick-quench, lean-burn or a lean, premixed, prevaporized concept. Silicon carbide (SiC) fiber-reinforced SiC composites have been selected for this application. Significant accomplishments in fiber strength, matrix processing, test methods, and design approaches have been achieved during the past year. As a backup, efforts using high-temperature superalloy liners for low-emissions combustor concepts have begun.

A restructuring of exhaust nozzle program goals due to HSR engine-cycle selections has resulted in lower temperature requirements for nozzle structural materials. State-of-the-art material systems can likely be utilized for an HSCT exhaust nozzle, but advances in scaleup capability, designs, and life prediction method-ologies are still required. The effort in developing these materials technologies for HSCT exhaust nozzles is well under way, with some relatively large-scale hardware already being fabricated.

The fan containment effort has focused on establishing system requirements and developing novel design concepts. Preliminary advanced superalloy compositions have been identified, fabricated, and tested for HSCT turbine airfoil alloy and compressor/turbine disk applications. Additional iterations of alloy development and testing will be required before downselecting candidates for each application.

Likewise, thermal barrier coating systems for turbine airfoils have been identified and initial testing has been completed.

EPM and the entire HSR program has undergone a major rebaselining effort completed at the end of government fiscal year 1994. The timing of this extensive programmatic restructuring is consistent with many of the technical changes occurring in the EPM program: revised exhaust nozzle requirements; initiation of technical efforts for fan containment, turbine airfoil systems, and compressor/turbine disks; and establishment of a feasible approach toward combustor backup materials. The outlook for fiscal 1995 is very positive as our efforts are now streamlined and focused toward achieving our goals.

Lewis contact: Joseph Doychak, (216) 433-8560
Headquarters program office: OA

A current-generation TMC, SCS-6/Ti-6-4 (carbon-coated silicon carbide fibers in a titanium alloy matrix containing 6% aluminum and 4% vanadium), was employed in this program. It was fabricated as eight-ply, unidirectional panels with a modest fiber content of 35%. Dogbone specimens cut from these panels were tested using a load-controlled, uniaxial test system to simulate the fiber-direction loads in a ring-reinforced impeller of an advanced turboshaft engine under development at Textron Lycoming.

The mission test, a complex nonisothermal fatigue test, produces a peak temperature and stress of 800 deg.F and 160 ksi and has a 14-min period. Each cycle simulates the stress and temperature waveforms expected under actual operating conditions of a turboshaft engine in which engine speed changes in a prescribed fashion.

Initial mission testing lasted 9528 cycles. In comparison, the isothermal fatigue life at comparable stress levels was about 22,000 cycles. As the time at elevated temperatures and stresses is much longer for the mission test, the shorter cyclic life was not unexpected. However, the fracture mode in the mission test was similar to that in isothermal fatigue tests. Initiation sites at cut edge fibers and other surface anomalies were followed by large regions of flat fatigue crack propagation, transitioning to a more ductile, tensile-overload region.

Temperature and stress waveforms used in mission testing of
titanium-matrix composites.

Although initial mission testing did not achieve the desired 15,000-cycle life, a common compressor life goal for human-rated turbine engines, the measured life of 9528 cycles suggests that the design goal is achievable by increasing fiber content and/or slightly decreasing peak stress or temperature. Alternatively, more advanced TMC's with stronger fibers and more creep-resistant Ti-alloy matrices are under development. These show considerable promise to exceed the 15,000-cycle life goal without cutting back on peak temperature or stress. Future work will attempt to answer these questions.

Bibliography

• Gayda, J.; Aksoy, S.Z.; and Gabb, T.P.: Fatigue Behavior of [0]8 SCS-6/Ti-6Al-4V Composite Subjected to High Temperature Turboshaft Design Cycles. Thermo-Mechanical Fatigue Behavior of Materials: 2nd Volume, ASTM STP 1263, M.J. Verrilli and M.G. Castelli, eds., American Society for Testing and Materials, 1995.

Fiber minimum creep and rupture strength goals based on
composite performance equivalent to superalloys.

References

1. DiCarlo, J.A.: Property Goals and Test Methods for High Temperature Ceramic Fiber Reinforcement. Eighth CIMTEC Proceedings, 1994.
   
2. International Nickel Inc.: High Temperature, High Strength, Nickel Base Alloys. Company brochure, 1984.
   
3. Yun, H.M.; Goldsby, J.C.; and DiCarlo, J.A.: Tensile Creep and Stress Rupture Behavior of Polymer Derived SiC Fibers. NASA TM-106692, 1994.
   
4. Goldsby, J.C.; DiCarlo, J.A.; Yun, H.M.; and Morscher, G.N.: Thermomechanical Properties of Advanced Polycrystalline Oxide Fibers. HITEMP Review 1993: Advanced High-Temperature Materials Technology Program, NASA CP-19117, 1993, p. 85.
   
5. Sayir, A.; Farmer, S.C.; and Dickerson, P.O.: Status of Single Crystal and Directionally Solidified Oxide Fibers. ibid, p. 83.

Large quantities of coated fiber are required to make CMC test specimens. But developmental fiber coating processes can only reliably coat small quantities of fiber--often insufficient for making even a single CMC test specimen. To address this need, NASA Lewis developed a single-fiber microcomposite test that requires only a small amount of fiber (~15 cm per test specimen), so that critical tests can be performed to evaluate the interface.

Microcomposite test setup and load displacement behavior.

The single-fiber microcomposites tested in this work were made of a chemically vapor-deposited silicon carbide fiber (~0.143 mm in diameter) produced by Textron Specialty Materials (Lowell, Mass.), an interfacial coating, and a chemically vapor-deposited silicon carbide sheath (~0.15 mm thick) produced by B.F. Goodrich (Cleveland, Ohio). Conventional carbon and boron nitride interfaces were chosen because their known interfacial properties enable the accuracy of this test to be determined. The actual microcomposite tensile test is performed with a universal testing machine. Acoustic emission is monitored to "listen" for crack events, the microcomposites are monitored in situ with a traveling optical microscope, displacement is measured with a laser device, and a furnace can be inserted to perform tensile tests at high temperatures in oxidizing environments.

We used a variety of techniques to determine the interfacial properties from the tensile tests at room temperature. Three techniques worked considerably well: a matrix crack saturation approach, a load/displacement cyclic technique (hysteresis loop), and a crack opening displacement technique. The techniques were used to determine the interfacial shear strength indirectly. The interfacial shear strength was then measured directly with conventional push-out and pull-out techniques. Very good agreement was found between the indirect and direct methods. However, the microcomposite test better represents actual composite behavior and can be performed at the test conditions of interest. Therefore, we are now working to determine the interfacial properties at high temperatures in corrosive environments on developmental interfaces.

Bibliography

• Morscher, G.N.; Martinez-Fernandez, J.; and Purdy, M.J.: Determination of Interfacial Properties Using a Single Fiber Microcomposite Test. HITEMP Review 1994: Advanced High-Temperature Engine Materials Technology Program, NASA CP-10146, 1994.

Lewis contact: Gregory N. Morscher, (216) 433-8675
Headquarters program office: OA

Under fast-fracture conditions the SCS-6/RBSN composites display a metal-like, stress-strain behavior and graceful failure beyond matrix fracture at temperatures to 1500 deg.C in air. Even after a 100-hr exposure test in an oxidizing environment from 1200 to 1400 deg.C, these composites show excellent strength retention (>60%) at room temperature. However, prolonged exposure between 600 and 1000 deg.C causes significant strength loss (~50%). This loss was attributed to oxygen diffusion through the porous RBSN matrix and oxidation of the carbon interface coating on SCS-6 fibers, as indicated by large oxidative weight loss between 600 and 1000 deg.C. To avoid the intermediate-temperature oxidation problems in SiC/RBSN composites, NASA Lewis is investigating a variety of techniques. Surface coating applied by a CVD method is one of them.

A commercially available CVD external coating called RT-42 (an SiC-based coating from Chromalloy) was applied to SCS-6/RBSN composite test coupons. Oxidation and burner rig tests were performed from 600 to 1400 deg.C for up to 100 hr to determine oxidation behavior and cyclic stability of the coated composites. The oxidation tests were performed at NASA Lewis and the burner rig tests at Williams International under a cooperative agreement. Coated composites showed less oxidation attack than the uncoated composites. Also coated composites showed no evidence of internal or coating damage after five thermal cycles from 25 to 1400 deg.C in burner rig tests.

Oxidation data and photomicrographs of SiC/RBSN composites before (top) and after
(bottom) 10-hr exposure at 1400 °ree;C for five thermal cycles

We concluded that CVD surface coating can significantly improve the oxidative stability of SiC/RBSN composites and that the coated composites can be used for thermally loaded components in high-temperature applications.

Bibliography

• Bhatt, R.T.: Oxidation Effects on the Mechanical Properties of an SiC-Fiber-Reinforced Reaction-Bonded Si3N4 Matrix Composite. J. Am. Cer. Soc., vol. 75, no. 2, 1992, pp. 406-412.
  
• Fohey, W.; Bhatt. R.T.; and Baaklini, G.Y.: Burner Rig and Engine Test Results of SiC/RBSN Composites. Vol. III, NASA CP-19117, 1993, pp. 68-1 to 68-11.
  
• Fox, D.S.: Oxidation Kinetics of Coated SiC/RBSN Composites. Vol. III, NASA CP-19117, 1993, pp. 69-1 to 69-10.

To optimize composite mechanical behavior, various composite processing conditions have been studied in-house at NASA Lewis. The effectiveness of the interfacial coatings was evaluated by fabricating and testing composites with 30 vol% of either coated or uncoated fibers. The uniform fiber coatings were porous, fine-grained, unstabilized zirconia, approximately 1 to 2 um thick. Additional composites were fabricated for further heat treatment at 1400 deg.C for 8 or 24 hr in an oxidizing environment. Then the as-fabricated and heat-treated composites were tensile tested at room temperature.

Alumina-matrix composite tensile strength before and after
heat treating

For the processing conditions studied, average tensile strengths to 421 MPa were measured for composites containing zirconia-coated fibers. In comparison, similar processing of composites containing uncoated fibers yielded samples with much lower tensile strengths of 130 MPa. The coated-fiber composites also displayed considerable strength retention after heat treatment. After treatment at 1400 deg.C for 8 or 24 hr in air the composites retained as much as 95 or 87%, respectively, of their as-fabricated tensile strength.

These results indicate the potential usefulness of oxide composites in oxidizing atmospheres at high temperatures. To fully explore the possibilities of these composites and the effectiveness of the zirconia interfacial coating, a series of high-temperature tests comparing the performance of these composites and nonoxide composites in oxidizing environments will be necessary.

Lewis contact: Martha H. Jaskowiak, (216) 433-5515
Headquarters program office: OA

Recent progress in electronics has resulted in longer life, lower weight satellites. Lubrication, and hence mechanism, failure is now becoming the life-limiting factor on spacecraft. New or improved (e.g., with soluble additives) lubricants are needed for long-term operation in space, requiring a deep understanding of the chemistry and interactions that take place at the metal/lubricant interface.

The interfacial chemistry of Fomblin Z25, a commercial perfluoropolyether used as a lubricant for space applications, with different metallic surfaces--440C steel, gold, and aluminum--was studied. Thin layers of Fomblin Z25 were evaporated onto the vacuum-cleaned, oxide-free substrates, and the interfacial chemistry was studied by x-ray photoelectron spectroscopy (XPS) and temperature desorption spectroscopy (TDS). The reactions were induced by heating the substrate and rubbing it with a steel ball. These experiments were done in ultrahigh vacuum to preserve the cleanliness of the surfaces.

Gold was found to be completely unreactive toward Fomblin at any temperature. Reaction at room temperature was observed only for the aluminum substrate, the most reactive toward Fomblin Z25 of the substrates studied. It was necessary to heat the 440C steel substrate to 190 deg.C to induce fluid decomposition. The degradation of the fluid was indicated by a debris layer at the interface. This debris layer, composed of inorganic and organic reaction products, when completely formed, passivated the surface from further attacking the Fomblin on top. The tribologically induced reactions on 440C steel formed a debris layer chemically like the thermally induced layer. In all cases the degradation reaction resulted in preferential consumption of the difluoroformyl carbon (-OCF2O-).

Presently, the interfacial chemistry of Krytox, another PFPE widely used as a lubricant for space applications, with the same surfaces is being studied. These results will provide insight into the interfacial chemistry of the PFPE, knowledge necessary for developing inhibitors or additives that will improve the performance and extend the life of these fluids.

High-resolution XPS of worn area after sliding a 440C ball against
Fomblin Z25 film 50 Å thick on 440C steel disk.

Bibliography

• Herrera-Fierro, P.; Jones, W.R., Jr.; and Pepper, S.V.: Interfacial Chemistry of a Perfluoropolyether Lubricant Studied by X-Ray Photoelectron Spectroscopy and Temperature Desorption Spectroscopy. J. Vac. Sci. Technol. A, vol.11, no. 2, 1993, pp. 354-367.

• Herrera-Fierro, P.; Pepper, S.V.; and Jones, W.R., Jr.: X-Ray Photoelectron Spectroscopy Study on the Stability of Fomblin Z25 on the Native Oxide of Aluminum. J. Vac. Sci. Technol. A, vol. 10, no. 4, 1992, pp. 2746-2751.

Lewis contact: Dr. Pilar Herrera-Fierro, (216) 433-6053
Headquarters program offices: OA and OSAT

Maximum use temperatures of PM212 and conventional
bearing materials.

Bibliography

• DellaCorte, C.; Sliney, H.E.; and Bogdanski, M.S.: Tribological and Mechanical Comparison of Sintered and HIPped PM212: High Temperature Self-Lubricating Composites. Lubr. Eng., vol. 48, Nov. 1993, pp. 877-885. (Also NASA TM-105379.)

The primary objective of this development was to apply silver lubricating films to high-temperature ceramic components of advanced combustion engines so as to reduce friction and wear. Other potential uses for SCIP include coating substrates with metal for protection against corrosion, depositing electrical conductors on dielectric substrates, making optically reflective surface layers, and applying decorative metal coats to ceramic trophies or sculptures.

The SCIP system basically consists of a dc-diode configuration. The ceramic (Al2O3) substrate is mounted and surrounded with a screen cage to which a negative potential (-3000 V, 80 mA) is applied in an argon pressure of 20 mtorr. The effectiveness of SCIP is attributed to its ability to provide a high-energy flux of ions and energetic neutral atoms that contribute to the excellent adherence and desirable microstructure of the deposited film. An important additional advantage of this technique is known in the industry as "throwing power"--the ability to coat even non-line-of-sight surfaces to produce three-dimensional coverage of the substrate.

Screen-cage ion plating (SCIP) for ceramics.

The deposited silver lubricating films reduce the coefficient of friction by 50% during sliding contact and thereby also reduce the surface tensile stresses that contribute to undesirable subsurface cracking and subsequently to severe wear. In this research the effect of friction on the critical stress for crack initiation was accurately predicted by a mathematical model.

The process is further extended to utilize reactive (oxygen-argon) glow discharge to deposit silver and gold films on ceramic (Al2O3) surfaces with high adherence. The oxygen presence in the glow discharge dramatically increases metallic film adherence. SCIP offers a simple, economic coating process to ion plate silver, gold, and other metals on ceramic surfaces with excellent adherence and three-dimensional coverage.

Bibliography

• Sliney, H.E.; and Spalvins, T.: The Effect of Ion-Plated Silver and Sliding Friction on Tensile Stress-Induced Cracking in Aluminum Oxide. STLE J., vol. 49, no. 2, pp. 153-159. (This paper received STLE Al Sonntag Award (best solid lubricant paper written) at the 1994 STLE Annual Meeting in Pittsburgh.)
  
• Spalvins, T.; and Sliney H.E.: Frictional Behavior and Adhesion of Ag and Au Films Applied to Aluminum Oxide by Oxygen-Ion-Assisted Screen Cage Ion Plating (SCIP).

• NASA TM-106522, 1994.
  
• Tech Brief: LEW-15858, June 1994.

Use temperature of polyimide fibers.

NASA Lewis began a research program to synthesize additional rigid-rod polyimides and to evaluate them along with PIF and PIM against Kevlar as fabric wraps for soft-wall fan containment designs in engine applications. A novel polyimide, tetramethyl benzene (TMBZ), was prepared at NASA Lewis by substituting four bulky methyl groups for the hydrogens on the noncoplanar structure units along the polyimide backbone. The objective was to raise the glass transition temperature T g and thus increase the use temperature. In collaboration with researchers at the University of Akron, an investigation is currently in progress under the Enabling Propulsion Materials (EPM) Program to spin the TMBZ fiber and to evaluate the effect of isothermal aging at 200 to 250 deg.C on the properties of TMBZ, PIF, and PIM polyimide fibers along with Kevlar and PBO.

Lewis contact: Dr. Chun-Hua Chuang, (216) 433-3227
Headquarters program office: OA

Results of aging PMR-15 composites at 500 degC in air.

Although the study is still ongoing, aging times of nearly 15,000 hr have been logged on composite specimens at 400 and 500 deg.F, revealing a pronounced effect of machined surface area on composite weight loss. Specimens with high machined surface areas had significantly higher weight losses at 500 deg.F than samples with low machined surface areas. Photomicrographs of aged specimens show that oxidative attack is more aggressive at machined surfaces because of the exposed fibers (ref. 2). Similar trends were observed for compressive strength loss.

References

1. Nelson, J.B.: Thermal Aging of Graphite/Polyimide Composites. Long-Term Behavior of Composites , ASTM STP813, T.K. O'Brien, ed., 1982, pp. 206-221.
   
2. Bowles, K.J.; and Kamvouris, J.E.: Penetration of Carbon-Fabric Reinforced Composites by Edge Cracks During Thermal Aging. NASA TM-106530, 1994. (Also to appear in the Journal of Advanced Materials.)

Lewis contact: Dr. Kenneth J. Bowles, (216) 433-3197
Headquarters program office: OA

A recent NASA Lewis study resulted in the development of a nontoxic version of PMR called AMB-21. This MDA-free PMR resin formulation should eliminate most of the rigid handling regulations and reduce costs to suppliers and users of PMR materials. AMB-21 compares favorably with PMR-15 in terms of material cost, ease of processing, mechanical properties, and thermo-oxidative stability (see graph).

Weight loss of graphite-reinforced AMB-21 and PMR-15
composites after exposure to air at 260 degC.

AMB-21 composite laminates can be fabricated by using either low-pressure autoclave or high-pressure compression molding process parameters identical to those used for PMR-15 composite materials. Because of the excellent flow characteristics of AMB-21 resin, resin transfer molding and braided "towpreg" fabrication processes are being investigated by Fiber Innovations Inc. under a NASA/General Electric contractual program.

MDA-free AMB-21 is available from several commercial suppliers in prepreg, powder, and solution forms and is being evaluated as a PMR-15 replacement by a number of major engine and aerospace companies.

Lewis contact: Raymond D. Vannucci, (216) 433-3202
Headquarters program office: OA

The highest conductivities reported (.4-W[-1]cm[-1]) were achieved with polythiophene in a polystyrene host polymer (ref. 9). The best films using a polyimide as base polymer (refs. 6 and 7) were four orders of magnitude less conductive than the polystyrene films. The authors suggested that this was because polyimides were unable to swell sufficiently for infiltration of monomer as in the polystyrene. It was not clear, however, if the different conductivities obtained were merely the result of differing oxidation conditions. Oxidation time, temperature, and oxidant concentration varied widely among the studies.

Many aerospace applications require a combination of properties. Thus, hybrid films made from polyimides or other engineering resins are of primary interest, but only if conductivities like those with a polystyrene base can be obtained. Hence, a series of experiments was performed to optimize the conductivity of polyimide-based composite films. The polyimide base chosen for this study was Kapton. 3-Methylthiophene (3MT) was used for the conductive phase.

Three processing variables were identified for producing these composite films, time, temperature, and oxidant concentration for the in-situ oxidation. Statistically designed experiments were used to examine the effects of these variables and synergistic/interactive effects among variables on the electrical conductivity and mechanical strength of the films. Temperatures were varied from -10 to 30 deg.C, times from 2 to 8 hr, and oxidant concentrations from 0.4 to 1.2 M. The experiments were carried out in a randomized order. Kapton films were soaked for 24 hr at room temperature with stirring in 3 MT. The monomer-treated films were then oxidized under the selected design conditions. Conductivity was measured with a standard four-point test using direct current. Maximum mechanical stress at maximum load was also measured with a tensile test. The measured values obtained under different oxidation conditions were analyzed by multiple linear regression. The conductivities were log transformed before analysis.

Predicted response surface for maximum mechanical stress in
terms of oxidation conditions of conductive polymer films.

Multiple linear regression analysis of the tensile data revealed that temperature and time have the greatest effect on maximum stress. The response surface of maximum stress versus temperature and time (for oxidant concentration at 1.2 M) is shown. At the oxidation conditions predicted to give maximum conductivity for p-3MT/Kapton, the maximum stress is predicted to be 27,000 psi. If better mechanical properties are needed, compromise conditions can be chosen from the response surface models that give slightly lower conductivities. For example, to obtain predicted maximum stress values closer to untreated Kapton (~34,000 psi) for p-3MT/Kapton, oxidation time can be limited to 2 hr or temperature can be lowered to 10 deg.C.

Conductivity of the composite films, measured for over 150 days in air at ambient temperature, dropped only fivefold in that time. Films aged under vacuum at ambient temperature diminished slightly in conductivity in the first day but did not change thereafter. This suggests that if the films are protected from air and perhaps moisture, the conductivity will persist indefinitely.

References

1. Meador, M.A.B.; Gaier, J.R.; Good, B.S.; Sharp, G.R.; and Meador, M.A.: A Review of Properties and Potential Aerospace Applications of Electrically Conducting Polymers. SAMPE Q., vol. 22, Oct. 1990, pp. 23-31.
   
2. Li, C.; and Song, Z.: Diffusion-Oxidative Polymerization of Transparent and Conducting Polypyrrole-Poly(Ethylene-Terephthalate) Composites. Synth. Met., vol. 40, 1991, pp. 40, 23.
   
3. Stanke, D.; Hallensleben, M.L.; and Toppare, L.: Electrically Conductive Poly(Methyl Methacrylate-G-Pyrrole) via Chemical Oxidation. Synth. Met., vols. 55-57, 1993, pp. 1108-1113.
   
4. Morita, M.; Hashida, I.; and Nishimura, M.: Conducting Polypyrrole Composite Thin Films Chemically Prepared by Spreading on Surface of Aqueous Solution Containing Oxidizing Agent. J. Appl. Polym. Sci., vol. 36, 1988, pp. 1639-1650.
   
5. Van Duk, H.; Aagaard, O.; and Schellekens, R.: Precursor Monomer Route: A Novel Concept for Producing Highly Conductive Polypyrrole Films. Synth. Met., vols. 55-57, 1993, pp. 1085-1090.
   
6. Tieke, B.; and Gabriel, W.: Conducting Polypyrrole-Polyimide Composite Films. Polymer, vol. 31, 1990, pp. 20-23.
   
7. Dao, L.H.; Zhong, X.F.; Menikh, A.; Paynter, R.; and Martim, F.: Proceedings of Annual Technical Conference of Society of Plastic Engineers, vol. 49, 1991, p. 783.
   
8. Ruckenstein, E.; and Park, J.S.: The Electromagnetic Interference Shielding of Polypyrrole Impregnated Conducting Polymer Composites. Polym. Compos., vol. 12, no. 4, Aug. 1991, pp. 289-292.
   
9. Ruckenstein, E.; and Park, J.S.: Polythiophene and Polythiophene-Based Conducting Composites. Synth. Met., vol. 44, 1991, pp. 293-306.
   

A new fully crystalline plasma-sprayed mullite coating was developed at NASA Lewis. It exhibits dramatically better resistance to thermal shock and molten salt corrosion than conventionally plasma-sprayed mullite coatings (ref. 1). Although mostly free of microcracks and debonding, the new coating still tends to develop through-thickness cracks that can propagate into the substrate under repeated thermal cycling (ref. 2). In environments containing water vapor and/or a reducing atmosphere, the mullite coating offers only limited protection against silica vaporization because of the fairly high (~0.4) silica activity (ref. 2).

In an effort to overcome these limitations various overlay coatings were studied. A cordierite overlay coating on mullite-coated silicon carbide (SiC) and SiC/SiC effectively inhibited the propagation of cracks in the mullite base coating under thermal cycling between room temperature and 1200 deg.C. This system also demonstrated excellent thermal shock resistance. Yttria-stabilized zirconia overlay coating reduced silica vaporization in water vapor and/or a reducing atmosphere.

The large mismatch in coefficient of thermal expansion between mullite (5.4310[-6 ] deg.C[-1]) and yttria-stabilized zirconia (10310[-6] deg.C[-1]) is of concern. However, SiC and SiC/SiC composites coated with mullite/yttria-stabilized zirconia dual-layer coating exhibited excellent thermal shock resistance under thermal cycling between room temperature and 1300 deg.C. This system also exhibited excellent chemical compatibility at the mullite/zirconia interface and appears to be effective even in inhibiting the propagation of through-thickness cracks in the mullite base coating by deflecting the cracks at the mullite/zirconia interface. Study of this dual-layer system will continue, to optimize its effectiveness as a high-temperature protective coating for silicon-based ceramic materials.

Bibliography

• Lee, K.N.; Miller, R.A.; and Jacobson, N.S.: A New Generation of Plasma-Sprayed Mullite Coatings on Silicon-Based Ceramics. Accepted in J. Am. Ceram. Soc., 1994.

• Lee, K.N.; Jacobson, N.S.; and Miller, R.A.: Refractory Oxide Coatings on SiC Ceramics. MRS Bulletin, Oct. 1994. (Also NASA TM-106677.)

We adopted an experimental approach to elucidating the relationships between microstructure, oxidation, and ductility in the Ti-Al-Cr system. The strategy involved

• Studying phase equilibria of alumina formers in the TiAlCr system
  
• Identifying key constituent phases
  
• Characterizing alloy oxidation resistance and Vickers microhardness

Isothermal 100-hr weight gain at 800 and 1000 degC in air for
Ti-5l.25Al-12.25Cr.

The relevant phases were identified as t(Al67Ti25Cr8), g(TiAl), r-TiAl2, TiCrAl (laves), and Cr2Al. Alumina formation was associated with t, Al-rich TiCrAl, and fine g + TiCrAl mixtures. Brittleness was associated with TiCrAl, Cr2Al + TiCrAl mixtures, and t decomposition to r-TiAl2 and Cr2Al. Two-phase g + TiCrAl alloys were identified that offered the potential for protective alumina formation up to 1000 deg.C in air and limited room-temperature ductility (~0.5 to 1% based on literature data for single-phase g alloys). A g + TiCrAl alloy (Ti-51.25Al-12.25Cr at.%) was produced in which the g phase was continuous, in order to increase ductility by interrupting the continuity of the brittle TiCrAl phase, and where protective alumina formation was observed. Future work will concentrate on optimizing ductility through reduced Al content and by additions of refractory elements.

References

1. Meier, G.H.; Perkins, R.A.; Schaeffer, J.C.; and McCarron, R.L.: GE Aircraft Engines Interim Report No.1. Naval Air Development Center Contract N62269-90-C-0287, Mar. 1991.
   
2. Becker, S.; Rahmel, A.; Schorr, M.; and Schutze, M.: Mechanism of Isothermal Oxidation of the Intermetallic TiAl and of TiAl Alloys. Oxidation of Metals, vol. 38, nos. 5/6, 1992, pp. 425-464.

One of the most versatile and direct ways to measure thermodynamic activity is by vapor pressure. The vapor pressure of an alloy component is measured and divided by the vapor pressure of the pure component at a particular temperature. This gives the activity from a first-principles definition. One way to measure a particular component's vapor pressure is with a Knudsen-cell mass spectrometer. A Knudsen cell is a small enclosure that allows equilibrium between a solid and vapor phase to be attained. A well-defined orifice allows this vapor to be sampled mass spectrometrically by measuring an ion intensity. This ion intensity is directly related to vapor pressure with an appropriate calibration constant. However, on most instruments the calibration constant changes from one analysis to another.

These changes in calibration constant can be avoided by using an internal standard. Instead of using of single Knudsen cell, two cells are used in such a way that a measurement may be taken on the pure component and then on the alloy without breaking the vacuum. There are several experimental problems with this approach. The cells are in a vacuum system and typically heated to 1000 deg.C or greater. They must be kept at exactly the same temperature and translated to and from the sampling region from outside the vacuum system. The most formidable problem is that the molecular beams emerging from each cell must not be allowed to mix.

Only a few research groups have built systems that completely address these problems. Some groups have separated the cells by a large distance to avoid beam mixing; others have used a system of apertures so that the ionizer effectively sees only inside the cell being sampled. Our approach involves a system of shutters.

Schematic of twin-Knudsen-cell flange.

The entire furnace and cell assembly is translated from outside the vacuum system by stepper motors controlled by a "joystick." Linear potentiometers are mounted on the translator to provide a digital position indication. The entire cell assembly is mounted on an 8.25-in. flange attached to a commercial mass spectrometer. When the desired cell is in place, the shutter is opened and the signal intensity measured. The net signal is the difference between the shutter opened and the shutter closed.

Even with the shutters a limited amount of beam mixing was unavoidable owing to the mobility of molecules at these temperatures. For measuring higher activities (>0.05) this was not a problem, and the alloy component was compared with the pure component, as described. For measuring very low activities a second calibration material, such as gold, was used and the necessary corrections were made for different mass spectrometric detection characteristics.

The system has been tested on silver-copper and iron-aluminum alloys, for which extensive literature data exists. Our results show very good agreement with these literature values. Studies continue on the technically important nickel-aluminum and titanium-aluminum systems, for which few data are available.

Lewis contact: Dr. Nathan Jacobson, (216) 433-5498
Headquarters program office: OA

The pesting phenomenon in MoSi2 is most pronounced at approximately 930 deg.F and has been attributed to the accelerated formation of voluminous molybdenum trioxide (MoO3). Both monolithic MoSi2 and its composites suffer total disintegration within 100 hr at this temperature. NASA Lewis has attempted to identify a new MoSi2-based composition with excellent "pest" resistance, lower CTE, and good mechanical properties as a matrix suitable for SiC fiber reinforcement.

We investigated the addition of thermodynamically stable silicon nitride (Si3N4) as a low-CTE phase. A mixture containing commercial-purity MoSi2 powder and 30 to 50 vol% of fine Si3N4 powder was mechanically blended in an attritor. The MoSi2-Si3N4 powder and the SCS-6-fiber-reinforced MoSi2-Si3N4 composite (fabricated by the powder cloth technique) were consolidated by vacuum hot pressing followed by hot isostatic pressing to produce fully dense panels. Mechanical and oxidation tests were conducted on both monolithic and composite materials.

Adding Si3N4 particulates to MoSi2 lowered the low-temperature accelerated oxidation rate (by more than an order of magnitude) by forming an Si2ON2 protective scale and thereby eliminated the catastrophic "pest" failure. Adding Si3N4 also improved the high-temperature oxidation resistance and strength, doubled the room-temperature toughness, and more importantly, significantly lowered the CTE of the MoSi2 (from 9310[-6] to 6310[-6 ] K[-1]) and eliminated matrix cracking in SCS-6-reinforced composites even after thermal cycling. The composite exhibited excellent "pest" oxidation resistance to 930 deg.F. The SCS-6 fiber reinforcement provided attractive tensile strength and improved toughness by nearly an order of magnitude.

Scanning electron micrograph of fully dense, crack-free
SCS-6/MoSi2-30Si3N4 composite fabricated by powder
cloth technique.

It is envisioned that small-diameter SiC tow reinforcement of the MoSi2-Si3N4 matrix will most economically produce complex-shaped composite structures with a better combination of strength, environmental resistance, and reliability than any other MoSi2-base material. This hybrid composite will compete with metal-, intermetallic-, and ceramic-matrix composites. Advanced processing techniques and mechanical tests to validate this vision are in progress.

Bibliography

• Hebsur, M.G.: In Intermetallic Matrix Composites III. J.A. Groves, R.R. Bowman, and J.J. Lewandowski, eds., Materials Research Society, Pittsburgh, PA, 1994, pp. 177-182.

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