**4. Discussion**

As already mentioned above, the penetration of oxidants into the EBC system and the following formation of a TGO cannot be prevented completely. However, with purposeful design of the EBC layer system, the transport processes and the following oxidation processes can be influenced. The first possibility should be the reduction of the transport processes for O2 and H2O. Coatings with a lower oxygen di ffusion coe fficient and water vapor solubility are reasonable trends for material development.

In the present study, the following oxidation and corrosion processes were modified. With the introduction of oxidable particles into an intermediate layer, it was possible to shift the oxidation processes from the interface between the Si bond coat and the Yb-silicate top coat to a volume in the intermediate layer. The processes occurring during oxidation and corrosion are schematically described in Figure 8.

**Figure 8.** Schematic diagram of the oxidation and corrosion mechanism of SiCF/SiC(N) composite with three-layer EBC system.

Independent of the thermal treatment, oxidation, or hot gas corrosion, the transport of oxidants and the following oxidation processes in the EBC system were found to be the first processes in the system. The penetrated O2 and H2O reacted with the SiC, resulting in the formation of a SiO2-based shell surrounding the SiC particles. In this way, the SiC particles served as a getter for the further transport of oxidants into deeper regions of the whole system. As the oxidants could not reach the Si bond coat, the formation of the TGO was prevented. In a hot gas atmosphere with increased water vapor pressure and high gas velocity, corrosion processes were observed. The water vapor penetrated this region and reacted with the SiO2 at the surface of the SiC particles and formed Si(OH)4. A Si(OH)4 gradient developed as a consequence of the high hot gas velocity outside, which was a driving force of the outward transport and evaporation of the Si(OH)4. Small pores were found to be the result of the corrosion process.

The beneficial gettering function of the SiC particulate will be a temporary e ffect only. After longer oxidation time or at higher temperatures, the SiC particles will be consumed and oxygen will reach the silicon bond coat to form the TGO. However, with the incorporation of the SiC particles, to

be oxidized during operation in hot gas atmosphere, the EBC system could be temporary stabilized. With the delayed formation of the TGO, their resulting damage mechanisms, cracking as a result of stresses by crystallization and phase transition processes and the gap formation caused by corrosion material loss, started at later application times. A very beneficial effect in terms of long-term stability and lifetime can only be achieved by simultaneous improvement of the oxygen and H2O permeation behavior of the whole EBC layer system. The lower the transport of the oxidants into the material, the longer the gettering function of the SiC particles can be used. Further studies have to be performed to optimize the EBC system regarding composition and microstructure with special focus on the transport mechanisms during service in hot gas environments.

**Author Contributions:** H.K. conceptualized the idea; H.K., K.S. and W.K. developed the coating; K.S. and W.K. performed the experiments and analyzed data; H.K. supervised the project and acquired funding; H.K. prepared the manuscript; K.S. and W.K. contributed in editing and submission. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the German Federal Ministry for Education and Research, gran<sup>t</sup> number 03EK3544C and Fraunhofer Funding MAVO CMC engine.

**Acknowledgments:** The authors gratefully thank B. Gronde and F.L. Toma for support in plasma spraying technology.

**Conflicts of Interest:** The authors declare no conflict of interest.
