Interface coatings oxidation resistant

Several types of reactions involving solids with gases or liquids occur at the interface between the two phases. The most important reaction of this type is corrosion. Efforts to control or eliminate corrosion involve research that spans the spectrum from the coatings industry to the synthesis and production of corrosion-resistant materials. The economic ramifications of corrosion are enormous. Although there are numerous types of reactions that can be represented as taking place at an interface, the oxidation of a metal will be described. Figure 8.6 represents the oxidation of a metal. 

Yttrium is also used in other areas of metallurgy notably as a component of certain nickel-base and cobalt-base superalloys of the NiCrAlY and CoCrAlY type.(3) These alloys possess excellent corrosion and oxidation resistance, properties that have attracted the attention of the aero-engine industry where they are used as protective coatings on turbine blades. The alloys, when applied by vapour deposition, form an oxide coating that exhibits remarkable adhesion, a property attributed largely to the yttrium component acting to prevent the formation of voids at the oxide/substrate interface.(4)... 

Interface engineering to develop low-cost, oxidation-resistant fiber coatings and/or surface modifications that impart the desired interface properties. 

Implantations of yttrium and cerium in 15 % Cr/4% A1 steel and aluminized coatings on nickel-based alloys did not improve the high-temperature oxidation resistance even though conventional yttrium alloy addition had an effect. The differences for the various substrates are attributed to different mechanisms of oxidation of the materials. The austenitic steel forms a protective oxide film and the oxidation proceeds by cation diffusion. Thus, the yttrium is able to remain in a position at the oxide/metal interface. The other materials exhibit oxides based on aluminum. In their growth anion diffusion is involved which means an oxide formation directly at the oxide/metal interface. The implanted metals may, therefore, be incorporated into the oxide and lost by oxide spalling. 

Continuous-length ceramic fibers used to reinforce CMCs must have optimal mechanical, physical, and chemical properties (described in Chapter 2). This chapter reviews the characteristics of fibers that are commercially available and fibers that are at an advanced stage of development. The performance characteristics of interest include stiffness (i.e.. Young s modulus), strength, thermal and electrical conductivity, creep and rupture resistance, oxidation resistance, all as a function of temperature, and strength and stiffness retention as a fimetion of serviee history. The critical issue of chemical compatibility with prospective interface coatings and the eeramie matrix is addressed in Chapter 4 and Chapter 6. 

Many layered oxide coatings show promise as crack deflecting interfaces for oxide-oxide composites. Although the jl-aluminas have been shown to be unstable with Nextel 720 fibers, they are still candidate fiber coatings (especially the less volatile RbAlnOiy) for ceramic composites if other creep-resistant oxide fibers become available. Research into an improved creep-resistant Nextel 610 fiber that incorporates additions of rare-earth oxides or garnet-based fibers, may make the (5-... 

The main advantage of CMCs over monolithic ceramics is their superior toughness, tolerance to the presence of cracks and defects, and non-catastrophic mode of failure. It is widely accepted that in order to avoid brittle fiacture behavior in CMCs and improve the damage toloance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subsequent fiber pullout . Historically, following the development of SiC fibers, fiber coatings such as C or BN have been employed to promote the desired composite behavior. However, the non-oxide fiber/non-oxide matrix composites generally show poor oxidation resistance , particularly at intermediate... 

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