WHIPOX CMCs matrix compositions

FIGURE 2. The system S1O2-AI2O3 (after Klug et al. [12]). Fiber and matrix compositions used for WHIPOX CMCs are indicated by arrows. 

TABLE 2. In-plane strength, interlaminate shear strength (ILSS) and Young s moduli for WHIPOX CMCs with different fibers and matrix compositions at room temperature... 

Various material properties (e. g. thermal diffiisivity, air permeability) have been determined for WHIPOX CMCs. Reliable data are important for potential applications such as thermal insulators, filters or burners. Thermal conductivity perpendicular to fiber orientation is about IW/mK. Closer inspection reveals lower conductivity if a mullite matrix is employed instead of alumina. Thermal conductivity in fiber direction, on the other hand, is about three times higher as perpendicular to the fiber direction, reflecting the non-isotropic structure of the composite (Figure 11). 

The air permeability of WHIPOX CMCs strongly depends on processing parameters, matrix composition, and on the composite architecture. According to Figure 12 flow rates range between 10 and 1001/cm busing overpressures of 250 mbar. It can be anticipated that... 

FIGURE9. WHIPOX CMC with mullite plus SI02 matrix a as-prepared, b 1600°C, I h left polished sections right fracture surfaces. Note that upon firing the matrix shows pore agglomeration, particle coarsening and reaction with the Nextel 720 fibers, thus leading to catastrophic embrittlement ofthe composite. 

At the DLR Institute of Materials Research an all-oxide CMC (WHIPOX = wound highly porous oxide) consisting of alumina fibres Nextel 610 or mullite based fibre (Nextel 720, both 3M) and an alumina or mullite matrix, respectively, has been developed in recent years [1,2]. Mullite-based CMCs typically offer higher creep stability than alumina-based composites but display lower thermal conductivity. Lower thermal stability of alumina-based materials, however, can be accepted, since service temperature is significantly lower for these materials as a result of the better cooling efficiency. Therefore material development was focused on alumina... 

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