Alumina-mullite matrix

Alternately, an alumina-mullite matrix was developed in an effort to improve long-term thermal stability of porous-matrix CMCs. This matrix utilizes a bimodal mixture of mullite ( 1 jLm) and alumina ( 0.2 p,m) powders [4], The mullite exhibits relatively slow sintering that inhibits shrinkage and maintains the overall porous structure, while the fine alumina sinters more readily, cementing the mullite particles and providing strength to the porous matrix structure [66, 67]. Zirconia has also been studied as the primary phase, with mullite used to stabilize the matrix [68]. 

Fused alumina- mullite matrix Tabular alumina- corundum matrix Alumina chrome Alumina chrome spall resistant... 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

As mentioned previously, the main body of research on whisker-reinforced composites was concerned with alumina, mullite, and silicon nitride matrix materials. None the less, selected work examined zirconia, cordierite, and spinel as matrix materials.16-18 The high temperature strength behavior reported for these composites is summarized in Table 2.5. As shown, the zirconia matrix composites exhibited decreases in room temperature strength with the addition of SiC whiskers. However, the retained strength at 1000°C, was significantly improved for the whisker composites over the monolithic. Claussen and co-workers attributed this behavior to loss of transformation toughening at elevated temperatures for the zirconia monolith, whereas the whisker-reinforcement contribution did not decrease at the higher temperature.17,18... 

A.T. Hunt, CCVD Processing of Lanthanum Phosphate and Beta-Alumina Fiber Coatings for Mullite Matrix Composites, Contract DM19561712, NSF SBIR Phase 1 Grant, 1996. 

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... 

The aforementioned fibers are combined with various matrix materials to produce oxide-oxide CMCs. Coatingless, porous-matrix CMCs based on fabric reinforcement are the most fully developed and examples of these materials are given in Table 2. General Electric s (GE) GEN-IV was one of the earliest porous matrix, all-oxide fiber composites, while COI Ceramics (COI ) is currently the most prominent commercial supplier of oxide-oxide composites. The matrix materials are primarily aluminosilicate (ex. manufacturers GE, COI), alumina-mullite (University of California, Santa Barbara [UCSB]), and/or alumina (COI). 

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). 

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. 

Zirconia-toughened ceramics (ZTCs). These are ceramic matrix composites. The matrix is alumina, mullite, or spinel. The reinforcements are tetragonal or monoclinic zirconia particles. 

Other Aluminosilicates, Transparent mullite glass-ceramics can be produced from modified binary Al C —Si02 glasses (21). In these materials, the bulk glass phase separates into tiny alumina-rich droplets in a siliceous matrix. Further heat treatment causes these droplets to crystallize to mullite spherulites less than 0.1 Jim in size. When doped with ions such as Cr3+, transparent mullite glass-ceramics can be made to absorb broadly in the visible while fluorescing in the near-ii (22,23), thereby making them potentially useful for luminescent solar collectors. 

Fig. 8.4 Phase-contrast TEM image of SiC whisker in alumina matrix that has undergone oxidation. The silicate glass (G) has begun to react with alumina to form a mullite nucleus (M), and graphitic carbon (C) surrounds the crystallite.14...

The combination of infiltration and reaction that characterizes DMO has been exploited to make a number of composites. As long ago as 1953, it was shown that silica containing refractories were reduced by molten aluminum to form alumina and silicon [4], Subsequently [27], the displacement reaction was extended to the formation of composites of alumina with residual Al-Si. More recently, the Al-Si02 displacement reaction has been used in the infiltration of dense preforms of silica [28] and mullite [29,30] by molten aluminum. Extension of the reactive infiltration process to porous silica-containing preforms [31,32] has resulted in the fabrication of metal-matrix composites in which the silica was replaced by a mixture of about 65% alumina and 35% metal, while the pores were infiltrated by molten alloy. In contrast to DMO, the displacement reaction appears to proceed at a critical temperature of 1100-1200°C and without the need for a volatile solute element or oxygen. Borosilicate glass has also been used as an initiator to enable the infiltration of Al-Si alloys into alumina preforms [33]. 

The fibers typically consist of carbon (C), silicon carbide (SiC), alumina (AI2O3), or mullite (Al203-Si02). For the matrix components, alumina, zirconium oxide, and silicon carbide are most commonly used. The terminology of CMC usually follows the principle t3q>e of fiber/t3q>e of matrix. C/SiC stands for a carbon-fiber-remforced silicon carbide. Today, the most important CMCs are C/C, C/C-SiC, C/SiC, and SiC/SiC. In some cases, the term is preceded with the abbreviation of the manufacturing process. 

FIGURE 4. Typical microstructure of porous matrix composite (Nextel 720/mullite-alumina, UCSB material)... 

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... 

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