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High-Temperature Coatings on Carbon Fibers and Carbon-Carbon Composites
James E. Sheehan
San Diego, California
High-Temperature Coatings on Carbon Fibers 226
Chemical Vapor Deposition 230
Physical Vapor Deposition 231
Liquid Precursor Methods 235
Liquid Metal Transfer 236
Space Shuttle Orbiter Thermal Protection 241
Structural Applications 242
Performance Issues 245 Internal Coatings 249
Liquid Precursor Method 250
Matrix Chemical Modifications 250
Chemical Vapor Deposition 251
Advantages and Limitations 252 Temperature Limitations 253
Current Coatings 253 Ultrahigh-Temperature Coatings 255 References 256
The use of coatings to protect carbon fibers and carbon fiber-carbon matrix (CC) composites from chemical attack at elevated temperatures is important for the development of lightweight structural materials for use in advanced aircraft and aerospace applications. Carbon fibers are the optimum reinforcement for many composite systems and are the constituent of choice for hot structures because of their unique retention of mechanical properties at extreme temperatures.
Methods that have proved to be particularly effective for producing thin, adherent inorganic coatings on micron-sized carbon fibers within yams are chemical vapor deposition (CVD). physical vapor deposition (PVD) processes, electroplating, and liquid precursor methods. Ceramic coatings made by CVD have been shown to substantially retard the oxidation of carbon fibers; coatings made by several techniques also have been effective in preventing fiber attack during the fabrication of metal-matrix composites. Although the mechanical properties of high-performance carbon fibers can be significantly reduced by coating, submicron coatings deposited under appropriate conditions produce minimal detrimental effects.
The primary reason that coatings are applied to CC composites is to provide oxidation or erosion protection. Except for limited-life rapid heating and cooling applications, external coatings on CC composites are usually applied as multilayer systems. The most successful external coating systems use a hard ceramic as the primary oxygen barrier with a glaze or glass that can flow to accommodate thermal mismatch strains and can seal defects in the hard ceramic coating. A coating system of this type prevents oxidation of the CC that is used to provide reusable thermal protection for the Space Shuttle orbiter vehicles. This general approach is also being employed to develop structural oxidation-protected CC composites for airbreathing engine and hypersonic vehicle airframe applications.
The coatings and coating methods now being used for external protection of structural CC composites are SiC and S'\^i outer coatings made by CVD. inner glass-former coatings of boron compounds made by slurry coating. CVD. and carbide conversion of the CC surface, and bond layers formed by the conversion of the CC surface to SiC. Performance issues associated with the current multilayer external coating systems are coating spallation due to thermal expansion mismatch with the CC. corrosion of the outer coating by the borate glass sealants, moisture sensitivity of borate glasses, and high oxygen permeability of borate glasses. These problems are being addressed by chemically modifying the inner coatings and developing coating arrangements that limit glass formation. Although the high oxygen permeability of borate glasses is a fundamental limitation, optimization of the current external coating approach and the use of coatings on pore and fiber surfaces within the CC should allow hundreds of hours of component performance.
Internal coatings have been made by impregnation with liquid precursors, chemical modifications of the carbon matrix, and chemical vapor infiltration (CVI). Adding borate glass-forming powders to chemically modify matrices is currently the most widely used internal coating method because of the simplicity of the process, the effectiveness of borate glasses, and the achievement of high concentrations of active materials. The principal drawbacks of the powder method are nonuniform glass-former distribution, reductions in composite mechanical properties associated with increased ply spacings in fabric laminates, and fiber damage during composite consolidation. These problems are being addressed by the use of fiber coatings, by chemical modifications at the molecular level, and by the optimization of powder particle sizes.
The use of borate glasses limits, even for very short times, the present oxidation-protected CC materials to temperatures below 1550°C. Certain shortterm applications do not require the use of borate glasses; here the limits are in the 1700°C to 1800°C range under optimum conditions and are set by the oxidative instability of the SiC and Si3N4 outer coatings. The most attractive oxygen barrier coating materials for temperatures above this range are iridium and SiC»2. Iridium has the advantages of ultralow oxygen and carbon permeabilities, and an excellent chemical compatibility with carbon. On the other hand, the high coefficient of thermal expansion (CTE) of iridium is a serious drawback for using the material as an external coating on CC. In contrast, vitreous SiO2 has a very low CTE and can flow to accommodate mismatched strains; however, SiO2 must be isolated from carbon by the use of a system of oxide and carbide layers of questionable thermochemical stability. Ultrahigh-temperature coatings that employ iridium and S1O2 as essential constituents are currently under investigation.
The purpose of this chapter is to review work that has been published in the open literature concerning coatings on carbon fibers and carbon fibercarbon matrix (CC) composites. The chapter focuses on inorganic coatings that are appropriate for providing protection from chemical attack at elevated temperatures. This subject is important because a key factor in improving the capabilities of high-performance aircraft and aerospace vehicles is the development of lightweight structural materials that can operate at very high temperatures in reactive environments.
Carbon fibers are among the strongest and stiffest known fibers, surpassing all others in strength retention and creep resistance at elevated temperatures. Superior structural properties combined with a very low density make carbon fibers the constituent of choice for many current composite structures. Carbon fibers have the greatest potential for reinforcing new composites with vastly improved hightemperature capabilities.
Although not attacked by many chemicals that are corrosive to other materials, carbon is particularly susceptible to oxidation at elevated temperatures. This is because the oxides of carbon are gases and, therefore, are not protective. Because the most important flight-related applications for high-temperature materials are as air-breathing engine components or structures that must withstand aerodynamic heating, effective oxidation protection is essential to utilize the full potential of carbon fiber composites in advanced aircraft and aerospace vehicles.
The application of discrete reaction-barrier coatings to carbon fibers and carbon fiber composites is the most effective method for protecting the materials from chemical degradation. In general, a coating must be both physically and chemically compatible with the underlying material and must resist degradation and permeation by the corrosive species. Furthermore, the coating must adhere strongly to the substrate, but its presence must not detract significantly from the important properties of the fibers or composite. Coatings that meet these requirements enable the development of advanced composites and composite structures for a broad range of high-temperature applications.
High-Temperature Coatings on Carbon Fibers
At ambient pressures, carbon crystallizes as closely packed atoms in hexagonal arrays. The covalent bonds that exist between carbon atoms in the arrays are the strongest bonds known (ref. 1). When continuity within arrays and alignment of the arrays are achieved, the resulting oriented layered structures can have extremely high strengths as well as elastic moduli in directions parallel to the alignment. Structures of this type are also highly anisotropic because the bonding is weak between atoms in adjacent layers.
The strong covalent bonds that are responsible for the strength and stiffness of carbon also make it unique in terms of property retention at high temperatures. The strong bonds inhibit thermally activated atom movements to such a degree that mechanical properties are retained at temperatures up to 2200°C. Other consequences of the strong bonding include very low thermal expansion parallel to the atom layers, and vapor pressures that become appreciable only at temperatures over 2750°C.
These mechanical and thermal attributes, along with densities of less than 2.2 g/cc, make carbon materials ideal for high-temperature, flight-related applications. The utility of carbon for rocket propulsion and atmospheric reentry thermal protection was recognized early in the late 1950's and early 1960's in the development of long-range strategic missiles (refs. 2 and 3) where synthetic graphites were used for thermal shock and thermal erosion resistance. These molded and pyrolytic graphite components could not be used as structures, however, because of strength and elastic modulus limitations. Pyrolytic graphites have a highly aligned layered structure, but growth defects limit tensile strengths and moduli to approximately 140 MPa and 28 GPa in the preferred directions (ref. 3).
Carbon is a brittle material, so even comparatively small flaws that are unavoidable in bulk processing prevent realization of the very high strength and modulus potential of aligned carbon structures. The extreme anisotropy of such structures greatly enhances the detrimental effects of crystal boundaries and regions of abrupt misalignment because local thermal expansion differences produce microcracks and residual stresses on heating and cooling due to constraints of the surrounding material (refs. 4 and 5). These difficulties are very pronounced in bulk material but can be minimized when highly oriented carbon is made in the form of flexible fine-diameter fibers.
The development of modern-day carbon fibers started more than 30 years ago. Methods for large-scale production of highly oriented micron-sized fibers with minimal defects were identified in the mid-1960's. Although the defect size that can influence strength is limited by the fiber diameter, eliminating pores, inclusions, surface defects, kinks, and abrupt changes in orientation was found to be essential for producing carbon fibers with consistently high mechanical properties.
All of the commercial high-performance carbon fibers currently available are made by the pyrolysis of either petroleum pitch or polyacrylonitrile (PAN) precursors. Highly oriented fibers made from reconstituted cellulose (rayon) were once available but are no longer a commercial product. Inexpensive, low-modulus carbon fibers used for a variety of filler, insulation, and nonstructural applications are now made from rayon and pitch precursors.
Commercial carbon fibers are produced by spinning the precursor fibers, carbonizing the precursor fibers by heating in an inert environment to at least 800°C, and heat-treating the carbonized fibers in inert gas at temperatures between 1200°C and 2700°C. Pitch and PAN are thermoplastics that require low-temperature oxidative cross-linking before carbonization.
The fabrication of high-performance carbon fibers is achieved by the alignment of the planar arrays of carbon atoms along the axis of each fiber. This alignment is accomplished in different ways, depending on the carbon precursor. The previously mentioned high-performance rayon precursor fibers were aligned by stretching the carbonized fibers at high temperatures (ref. 6). Currently, a strong preferred orientation is achieved in pitch- and PAN-derived fibers by aligning the precursors.