Oxidation protection for carbon fiber composites

Carbon fiber/carbon matrix (C-C) composites are being considered for high-temperature structural applications. The most important of these applications require the C-C components to operate in oxidizing environments. Concepts currently being pursued to provide oxidation protection for structural C-C composites are based on earlier work conducted with graphite and the successfully employed oxidation-protected C-C material developed for the shuttle orbiter vehicles. Present C-C applications can be classified either as limited-life or extended-life depending on the component duty cycle. Limited-life applications generally require minutes to hours of operation at temperatures greater than 1650°C with minimal thermal cycling. The principal concerns for limited-life oxidation protection are coating erosion, coating spallation, and the oxygen permeability of intact coatings. Dense coatings of Si3N4 or SiC have been shown to perform well at temperatures in the 1700° to 1800°C range. At higher temperatures outer coatings that present significant thermal expansion problems must be considered inconjunction with either SiO2 or iridium as oxygen barriers. Extended-life applications involve numerous thermal cycles at temperatures below 1650°C and hundreds of hours of operation. Here C-C materials modified with boron and coated with dense Si3N4 or SiC outer coatings and boron-rich inner layers are currently the topic of extensive development. Experimental results and model calculations have shown that these materials have the potential for useful performance in important extended-life applications. Current issues include coating quality and reproducibility, coating spallation, and chemical effects associated with the borate glasses that are needed to seal cracks in the outer coatings.