The use of coatings to protect carbon fibers and carbon-carbon | CC composite 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 miron-sized carbon fibers within yarns are chemical vapor deposition (CVD), physical vapor deposition (PVD) processes, electroplating, and liquid precursor methods. Ceramic coating made by CVD have been shown on substantially retard the oxidation of carbon fibers; coatings made by several techniques also have been effective in preventing fiber attach 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 system 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 coatings. 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 air-breathing engine and hypersonic vehicles airframe applications. cfccarbon.com
The coatings and coating methods now being used for external protection of structural CC composites are SiC and Si3N4 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 composite 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 coating 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 CC composite 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 particles sizes.
The use of borate glasses limits, even for very short times, the present oxidation-protected CC composite materials to temperatures below 1550C. Certain short-term applications do not require the use of borate glasses; here the limits are in the 1700C to 1800C 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 SiO2. 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 of iridium is serious drawback for using the material as an external coating on CC composite. In contrast, vitreous SiO2 has very low CET and can flow to accommodate mismatched strains; however, SiO2 must 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 SiO2 as essential constituents are currently under investigation.