Weak oxide coatings

Monazites and xenotimes are two classes of weak oxide coatings that satisfy the criteria for crack deflection for the currently available fibers [84, 85]. Of these, monazite (LaP04) has been the most widely studied. 

Carbon is a commonly used and successful weak interfacial coating. For high temperature appHcations, however, carbon is not the best solution, because it oxidizes, leaving a physical gap between the reinforcement and the matrix or allowing interfacial reactions that result in a strong interface bond. Much research has been conducted to develop alternative high temperature debond coatings, with tittle success to date. 

Although stable at room temperatures, hohnium will corrode at higher temperatures and humidity. Its oxide coating is a yellowish film that reacts slowly with water and dissolves in weak acids. Holmium has one of the highest magnetic properties of any substance, but it has little commercial use. 

In natural anoxic environments, the major alternative oxidants are iron(III) and manga-nese(IV) oxides and hydroxides. Both are common in natural systems, as crystalline or amorphous particles or coatings on other particles. In the absence of photocatalysis, however, iron and manganese oxides are weak oxidants. As a result, they appear to react at significant rates only with phenols and anilines (45, 59-64). 

Sometimes, other support materials besides oxides, e.g., carbon, are attractive and advantageous for various reasons. In fine chemicals production a lot of experience exists with carbon-supported catalysts. Often they show good performance, and a high intrinsic activity is observed. Carbon support is pronounced for a weak interaction with the active phase and has a high surface area. An often-cited disadvantage of conventional carbon support is its mechanical weakness. Carbon-coated monolith can overcome this disadvantage due to the strength of the monolithic substrate. 

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. 

Recommendation 2b. Coating approaches that promise to provide damage tolerant oxide composites should be evaluated to prove or disprove their viability. Based on the preliminary results discussed in Chapter 6, the committee has concluded that research should be focused on the following areas weakly bonded, thermally stable oxide coatings (e.g., rare-earth phosphates of the general formula Me P04) and the development of oxide composites that do not require fiber... 

TABLE 3. Oxide-Oxide Composites with Weak Oxide (ABO4) Interface Coatings... 

Although limited, research into composites containing weak oxide interfaces is on-going and will likely continue until a commercial product is developed and the full advantages of these coatings are realized. 

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