WHIPOX CMCs mullite matrix

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... 

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). 

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. 

FIGURE 10. Load/deflection curves of WHIPOX CMCs with alumina plus mullite matrix. Damage-tolerant fracture behavior upon firing at 1500°C and 1600°C is indicated. 

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