Interface coatings weak oxides

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. 

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. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

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. 

All experimental data and discussions presented above seem to confirm the validity of the hypothesis that if reducible elements present in the surface state (including the ultrathin layer of plasma polymer) on which a cathodic E-coat is applied, those elements will be subjected to the cathodic reduction during the process of the cathodic E-coat deposition, and a weak boundary or defective spots would be created in the interface of the E-coat and the substrate (plasma polymer-coated steel). The validity of the hypothesis implies that the best result of a cathodic E-coat cannot be realized unless the adverse effect of the cathodic reduction can be minimized. In this context, the presence or absence of oxides is a very important factor in the use of a cathodic E-coat, besides the fact that the presence of oxides... 

Separation exclusively at the interface is improbable according to the weak boundary theory . Rather, the separation should always occur in the bulk regions of coating or substrate (cohesive failure) or in a finitely thick weak layer (interface layer) between the two substances. A failure within the weak interface layer is regarded as an adhesive failure. The weak layer can be a brittle oxide layer or an absorbed and occluded layer of gas and/or some other kind of contamination. 

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]. 

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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