Once the temperature limit for SiC and Si3N4 coatings is exceeded, identifying coatings with acceptably low oxygen permeabilities becomes the issue. For an external 100-um-thick coating, estimated coating permeabilities of approximately 10-8 and 10-10 g/cm.s or less would be required to provide acceptable short- and long-term oxidation protection of CC composites. At such high temperatures, materials that form refractory oxides such as ZrO2 and HfO2 oxidize very rapidly because the oxides are inherently permeable to oxygen. Based on previous work and the considerations discussed here, current ultrahigh-temperature coating development, for all but very short-term applications, is focused on concepts that use iridium and SiO2 as essential constituents.
More than 20 years ago, extensive work was conducted on iridium and iridium alloy coatings to be used to protect synthetic graphite bodies from oxidation at very high temperatures. Iridium is attractive because of its 2400C melting point, extremely low oxygen and carbon permeabilities, and inertness to carbon. Coatings were made by a variety of techniques including CVD, electroplating, and a combination of plasma spraying and hot-isostatic pressing. Although excellent protection was demonstrated for short times in the 2000C to 2100C range, deifficulties in fabricating high-quality coatings and the cost and availability of iridium were deterrents to further development. Iridium and rhodium are also susceptible to erosion by the formation of volatile oxides and have CTE values that are extremely high compared with those of CC composite materials. Coating erosion may or may not be an issue depending on oxygen partial pressure, gas flow conditions, and required time of performance. Oxide overcoatings have been proposed to inhibit erosion, but the CTE mismatch problem is profound and might very well prohibit the use of these materials as external coatings on CC composites. This same problem exists for Al2O3 and most other oxides.
As an external coating, the advantages of SiO2 are a very low CTE and an ability to flow at high temperatures to accommodate differential strains. The viscosity of SiO2 is about 107 Dpa-s at 1800C. Then, the glass will adhere on contact, begin to flow under its own weight, and can be considered to behave as a sealant. Because SiO2 is reduced by carbon and is unstable in contact with carbides at very high temperatures, SiO2 glass must be separated from the CC substrate by a hard oxide or some other compatible material. An outer coating must be used for most applications to prevent flow erosion and excessive vaporization. The vapor pressure of SiO2 under strongly oxidizing conditions is projected to be somewhat less than 10-5 Mpa at 2000C. Low oxygen pressures increase the volatility of SiO2 by promoting SiO formation. cfccarbon.com
Inner and outer coatings are required to envelop and protect the SiO2, which invariably leads to the consideration of materials that are unattractive from the CTE standpoint. It is interesting to note that several refractory oxides are known to exhibit very low CTE values. This phenomenon is a result of low-temperature microcracking, which produces extreme crystallographic anisotropy. The recombination of the microcracks produces significantly increased CTE values at higher temperatures. Furthermore, any appropriate oxide in contact with carbon at very high temperatures will form an interfacial carbide. This reaction raises the issue of continued conversion of the oxide to the carbide by transport of carbon through the carbide. The presence of the carbide also reintroduces the CTE problems. The most refractory of the low-CTE oxide is HfTiO4, which melts incongruently at about 1980C.
Materials selection for internal coatings to perform at very high temperatures is even more limited than the selection for external oxygen and carbon diffusion barriers. The coatings need to be chemically stable when in contact with carbon, and it is likely that this stability must result from thermodynamic compatibility rather than transport limitations because of the very short diffusion distances. Because the oxides are reduced by carbon at the temperatures in question and because rhodium forms a eutectic melt with carbon at about 1700C, the only material that clearly meets the above criteria is iridium. Totally replacing the carbon matrix with a ceramic would prevent matrix gasification, but experience has shown that this replacement would do little over extended periods of time to protect carbon fibers from oxidation. Fiber protection would require thin, adherent barrier coatings that are chemically compatible with the fibers and matrix. Again, iridium apprears to be the preferred material.