Non-Oxide Composites

Conversion of the preceramic paper preform into a ceramic pro-duct involves removal of the bio-organic pulp fibers and consolidation of the inorganic filler powder compact. Oxide ceramics are formed by annealing oxide filled preceramic paper preform in air to decompose and oxidise the fibers in the temperature range of 300 - 800 °C followed by sintering at elevated temperatures (1200 - 1600 °C) non-oxide composite ceramics involve formation of a highly porous biocarbon template preform into which a liquid or gas phase is infiltrated and final consolidation during... 

More surprising than these non-silica oxides, however, was the discovery of non-oxidic compositions Already relatively early it was shown that noble metals... 

The CMC market is divided into two classes, oxide and non-oxide materials. Oxide composites consist of oxide fibers (e.g., alumina [AI2O3]), interfacial coatings, and matrices. If any one of these three components consists of a non-oxide material (e.g., silicon carbide [SiC]), the composite is classified as a non-oxide composite. These classes have different properties, different levels of development, and different potential applications. 

This section centers on fiber coatings for non-oxide eomposites in which either the fiber or the matrix is a non-oxide ceramic. Although oxide fiber-reinforced composites have been studied, most of the research available in the literature has focused on SiC fiber-reinforced composites. For example, mullite (3Al203-2Si02) fiber-reinforced SiC matrix composites have been fabricated by CVI (chemical vapor infiltration). However, SiC fiber-reinforced SiC matrix (SiC/SiC) composites are superior for the following reasons (1) mullite fiber-reinforced composites do not improve resistance to oxidation, one of the major factors limiting the use of non-oxide composites and (2) SiC fibers have mechanical properties superior to those of mullite fibers. This section will be concerned primarily with SiC fiber-reinforced ceramic composites, which offer the best oxidation resistance of any non-oxide fiber at high temperatures (particularly above 1,100°C [2012°F]). 

The development of eeramic oxide composites has lagged behind the development of non-oxide composites because of the poor creep resistanee of oxide fibers (compared to SiC... 

The development of ceramic oxide composites has lagged behind the development of non-oxide composites because of the poor creep resistance of oxide fibers (compared to SiC fibers) and because of the lack of adequate oxide fiber coatings that promote fiber-matrix debonding. Recent advances in creep-resistant oxide fibers and progress on interface control has improved the potential for oxide ceramic composites in industrial and defense applications. However, an effective coating for oxide fibers that provides a weak fiber-matrix interface (and therefore tough composite behavior) remains to be demonstrated. As was discussed in Chapter 6, all oxide coating concepts discussed in the literature have been demonstrated with model systems rather than actual composite systems. 

Continuous-length polycrystalline ceramic fibers with non-oxide compositions are currently being developed and used for a variety of low and high-temperature structural applications. Recent literature reviews detail the process methods, properties, and applications for the many non-oxide fiber types that have been developed over the last 30 years [1- ]. 

It seems unlikely that polycrystalline oxide fibers will ever approach the use temperatures of SiC-based or C fibers (> 1500°C). Hence, it can be anticipated that even in long-term use, materials selection for refractory composites will balance the high temperature properties of non-oxide composites versus the superior enviro-thermal durability of oxide composites. 

A wider variety of all non-oxide composites has been made by hot pressing (Table 7) than by sintering. While some of these composites have... 

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