The continuous-filament ceramic composite is becoming recognized as necessary for new, high-temperature structural applications. Yet because of the susceptibility of the filaments to damage from traditional methods for the preparation of ceramics, vapor-phase infiltration has become the fabrication method of choice. The chemical vapor infiltration methods for producing these composites are now being studied in earnest, with the complexity of filament weaves and deposition chemistry being merged with standard heat and mass-transport relationships. Two of the most influential effects on the mechanical properties of these materials are the adhesion and frictional force between the fibers and the matrix, which can be controlled by a tailored interface coating. A variety of materials are available for producing these composites including carbide, nitride, boride, and oxide filaments and matrices. Silicon carbide-based materials are by far the most advanced and are already being used in aerospace applications.

This feature article explores the concept of creating functionally graded metal‐ceramic composite microstructures for thermal barrier coatings used in gas‐turbine applications. From a thermomechanical perspective, this concept offers the possibility of significantly improving the life and reliability of thermal barrier coatings. However, prior research reveals that progress has been somewhat limited because of the oxidative instability exhibited by some metal‐ceramic composite microstructures. The present study addresses some of the materials criteria and research issues associated with preparing chemically stable, yet mechanically durable, graded metal‐ceramic microstructures for realistic application environments.

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Crystalline mullite was deposited by chemical vapor deposition (CVD) onto SiC/SiC composites overlaid with CVD SiC. Specimens were exposed to isothermal oxidation tests in high‐pressure air + H2O at 1200°C. Unprotected CVD SiC formed silica scales with a dense amorphous inner layer and a thick, porous, outer layer of cristobalite. Thin coatings (∼2 μm) of dense CVD mullite effectively suppressed the rapid oxidation of CVD SiC. No microstructural evidence of mullite volatility was observed under these temperature, pressure, and low‐flow‐rate conditions. Results of this preliminary study indicate that dense, crystalline, high‐purity CVD mullite is stable and protective in low‐velocity, high‐pressure, moisture‐containing environments.