Oxide Fiber Coatings

Non-oxide fiber coatings research should be focused on ... 

Impact. Oxide fibers with improved creep resistance will allow higher temperature applications (e.g., combustors and heat exchangers) provided that suitable oxide fiber coatings are developed in parallel. 

The oxidation resistance of non-oxide fiber coatings must be improved, from the current calculated value of a few minutes (at 1,200°C [2,192°F]) for 1 pm (0.04 mils) BN coatings to the maximum possible calculated value of 100 hours for 1 pm (0.04 mils) siliea-forming coatings. 

Tests on tin oxide fiber coatings in model composite systems indicated some crack deflection at the coating-fiber interface (Siadati et al., 1991 Venkatesh and Chawla, 1992). However, tensile tests of tin oxide coated alumina fiber-reinforced alumina matrix composites demonstrated a decrease in the extent of fiber pullout as the density of the matrix phase was increased. This led to increasingly brittle fracture behavior in these composites (Goettler, 1993). Tin oxide also has thermal stability problems at elevated temperatures (Norkitis and Hellmann, 1991). For example, in the presence of air at temperatures above 1300°C (2,372°F), tin oxide (solid) decomposes into SnO (gas) and Oj (gas). This decomposition occurs at even lower temperatures when the partial pressure of oxygen in the test environment is reduced. 

FIGURE 6-18 Example of oxide fiber coatings deposited via the immiscible liquid coating technique (ErTa04-... 

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. 

Goettler, R.W. 1996. Oxide fiber coatings for Nextel 610 and Nextel 720 fibers. Presented at the 1996 Conference and Exposition on Composites, Advanced Ceramics, Materials and Structures (paper no. SI-13-96F), January 7-11, 1996, Cocoa Beach, Florida. 

The commonly used alternative to an engineered fiber-matrix interlayer is a relatively weak matrix [28 2]. It was observed that one type of weak interface is created by using a highly porous oxide fiber coating [43]. By extension, fabricating the composite such that the entire matrix is porous results in effective crack deflection and toughening [4]. 

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