WHIPOX

Figure 1. Schematic view of a design concept for CMC combustion chambers, based on joined segments (left and center). WHIPOX combustion chamber shingle with laser drilled cooling channels for rig testing (right).

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... 

Mechanical tests using bending, tensile and compressive load conditions including the determination of elastic constants of the orthotropic material were carried out under room and high temperature conditions. For creep tests in tension four testing devices were established and creep tests longer than 6,000 h were carried out with different CMC qualities. The WHIPOX CMCs show much better creep resistance compared to state-of-the-art metallic combustor materials. 

Calculations and tests in a high pressure cooling rig shortly will demonstrate the reduction of cooling air using the all-oxide CMC WHIPOX as thermal protection system in combustion chambers. 

Another relatively mature porous-matrix composite, WHIPOX (i ound Highly orous Oxide Ceramic Composite) has been developed this material is the subject of a preceding chapter in this Handbook and will therefore not be addressed in this text. These materials utilize either Nextel 720 or 610 fibers filament wound into a highly porous mullite or alumina... 

M. Schmucker, A. Grafinuller, and H. Schneider, Mesostructure of WHIPOX all oxide CMCs, Comp. Part A, 34 613-622 (2003). 

H. Schneider, J. Goring, B. Kanka, and M. Schmucker, WHIPOX a new oxide fibre/oxide matrix composite for hi -temperature applications, Keram. Zeit., 53 [9] 788-791 (2001). 

In contrast to the fabric infiltration techniques used by various manufacturers WHIPOX is produced by a continuous winding process [7]. The matrix is derived from a commercial pseudoboehmite/amorphous silica precursor with overall compositions... 

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

FIGURE 4. Examples of WHIPOX components fabricated versus the DLR winding process. 

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... 

FIGURE 8. Short beam shear test to determine the yield shear strength (Tei of WHIPOX CMC (a) and cyclic fatigue test showing degradation if the toad exceeds (Tei (b). 

C. This suggests damage-tolerant fracture behavior of WHIPOX CMCs with alumina-rich matrix, even if they were heat-treated at 1600°C (Figure 10). 

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. 

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. 

FIGURE 11. Thermal conductivity of WHIPOX CMCs parallel and perpendicular to the fiber texture. 

FIGURE 12. Air permeability of different WHIPOX CMCs consisting of Nextel 610 fibers and Al203/mullite... 

M. Schmiicker, A. Grafmiiller, H. Schneider, Mesostmcture of Whipox all oxide CMCs, Composites A 34... 

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