Corrosion of Composites Materials

299

In an effort to find an interphase or coating for alumina and mullite fibers, Cooper and Hall [7.26] developed a synthetic fluorophlogopite*, based upon their geochemical approach, that when reacted with alumina formed an intermediate spinel phase that was stable after heating to 1200°C in air for 150 hr. Thus by coating alumina fibers with spinel and then using the fluorophlogopite as an interphase, an alumina matrix composite proved successful. Above about 1280°C, the alumina reaction with fluorophlogopite produced forsterite, leucite, and spinel along with the volatile fluorides SiF4, AlF3, and KF [see Eq. (7.1) below], making the spinel-coated alumina fiber/ fluorophlogopite laminate unstable at those high temperatures. Reactions between mullite and fluorophlogopite formed cordierite in addition to the phases mentioned above. This was not successful as a mullite fiber composite since the cordierite allowed potassium diffusion from the fluorophlogopite continually deteriorating the mullite. In the alumina fiber case, the spinel coating acted as a barrier to potassium diffusion.

(7.1)

Cooper and Hall reported that reaction (7.1) occurred at temperatures above 1230°C in flowing dry argon, although thermodynamic calculations indicated that the reaction proceeded only after the temperature reached 1279°C. This was attributed to the partial pressures of the gaseous phases not summing to 1 atm during the experiment in flowing argon.

7.2.3 Particulates

For a discussion of the various mechanisms involved in toughening composites when particulates are used as the

* Cooper and Hall use the term fluorophlogopite interchangeably with the terms mica, fluoromica, and fluorophyllosilicate, which may cause some confusion unless the reader is well versed in mineralogy.

Copyright © 2004 by Marcel Dekker, Inc.