Although actual temperature limitations on the coatings currently being used for CC oxidation protection will vary depending on specific operating conditions and component performance requirements, general guidelines can be established based on experimental obsevations and known thermochemical properties of the coating constituents. For example, unprotected B2O3 glass layers were shown to be effective in retarding the oxidation of CC substrates for extended periods at temperatures up to about 1000C, but volatilization of the glass is limiting at higher temperatures. When B2O3 was used as a crack sealant for hard outer coatings or as a binder in coatings composed of hard oxide particles, oxidation protection was extended to temperatures of more than 1200C.
The maximum temperature at which B2O3 can be used in contact with carbon at atmospheric pressure is approximately 1575C because of the disruption of the glass by equilibrium CO reaction product pressures greater than 10-1 Mpa (1 atm) at higher temperatures. The vapor pressure of B2O3 at this temperature also is projected to be about 10-3 Mpa under dry conditions and close to 10-1 Mpa in moist environments. These factors, combined with the tendency previously described for B2O3 to corrode the silicon-based outer coatings, suggest maximum temperatures for the borate glass internal and glass-sealed external coatings of 1500C to 1550C even for short periods of performance.
Certain applications do not require glass-sealed outer coatings because the component is reapidly heated to and cooled from temperatures high enough to close the coating coefficient of thermal expansion mismatch cracks. Relative to this situation, pure CVD SiC and Si3N4 coatings are viable in the 1700C to 1800C range under strongly oxidizing conditions. This temperature limit exists because at higher temperatures the SiO2 layers that normally protect these materials are disrupted by CO and N2 interfacial reaction product pressures that become greater than 10-1 Mpa, thus causing the coatings to erode by uncontrolled oxidation. Under less oxidizing conditions, the temperature limit can be reduced to the 1500C to 1600C range by active oxidation. Gaseous SiO is then formed instead of the protective SiO2, and coatings can fail by rapid loss of material.