**3. Results**

A summary of the weight changes of the two coating systems in comparison to the base CMC substrate without coating observed during oxidation and the burner rig test is summarized in Table 3. A weight gain was observed for all materials investigated after both tests.

**Table 3.** Weight change after oxidation and burner rig test, 1200 ◦C, 100 h. Comparison of EBC-coated systems with base ceramic matrix composites (CMC) material without EBC coating.


The values are the average of the three samples each. In connection with the interpretation of these results, some inaccuracies as a consequence of inhomogeneous EBC layers (thickness, pores, or cracks) should be considered. Furthermore, during the burner rig test, both processes, weight gain and weight loss, were observed. These results cannot be correlated directly with the TGO scale thickness obtained after the tests; however, some general tendencies can be followed as described in the microstructural discussion of the material investigated.

The main results were obtained by comparison of the microstructure of the CMC with di fferent EBCs after oxidation and hot gas corrosion tests both at 1200 ◦C and 100 h. After oxidation, a weight gain was observed. During the test, oxygen di ffused through the EBC layer and finally reacted at the first non-oxide surface in the system (base SiCF-SiC(N) composite or Si bond coat). Usually, a TGO layer was formed. In the case of the hot gas corrosion test, a second reaction has to be considered as well. Water vapor penetrated through the EBC layer, reacted with the SiO2 of the TGO layer, and formed volatile Si(OH)4. As a consequence of the high gas speed, the equilibrium of the corrosion reaction in Equation (1) was strongly shifted to the formation of Si(OH)4. In this way, the water vapor corrosion of the TGO became more dominant, resulting in material loss and a gap formation between the silica TGO and the EBC top layer.

This behavior is described in Figure 1 showing the comparison of polished cross-sections of the microstructure after oxidation and corrosion of a model EBC system consisting of a Si bond coat and an Yb2SiO5 top coat. While a TGO was formed (A) after oxidation, corrosion processes with the formation of gaps at the Yb2SiO5-Si interface (B) were observed.

**Figure 1.** Polished cross-section of TGO in Si-Y2SiO5 EBC system after (**A**) 100 h oxidation at 1200 ◦C in air and (**B**) 100 h hot gas corrosion at 1200 ◦C.

#### *3.1. CMC Substrate Material*

The results of the high-temperature tests of the base CMC without EBC were characterized by investigation of the microstructure of the surface region. After both tests, oxidation and the burner rig, a weight gain was observed. During oxidation, a protective layer of mainly SiO2 was formed at the surface of the material, limiting the diffusion of oxygen into the material (Figure 2A). This behavior was comparable to dense monolithic SiC. Both oxidation (weight gain) and corrosion (weight loss) were observed during the burner rig test. Caused by the water vapor corrosion of the protecting SiO2 surface layer, oxygen and water vapor were able to diffuse deeper into the material and oxidized the matrix and the fibers too (Figure 2B). As a consequence of these oxidation processes inside of the material, a higher weight gain was found after the burner rig test.

**Figure 2.** Microstructure of polished cross-sections of the base CMC after thermal treatment. (**A**) 100 h oxidation at 1200 ◦C; (**B**) 100 h burner rig test at 1200 ◦C.

#### *3.2. Al2O3–YAG EBC Coating System*

In the case of the material coated with Al2O3-YAG, a weight gain was observed after both tests. The values were found to be higher in comparison to the base CMC. Two effects are assumed to be the reason; first, a low protective ability caused by the inhomogeneity of the double layer with a high amount of cracks and porosity, and second, the TGO as the rate controlling factor for oxidation based on an alumosilicate glass with a significantly higher diffusion ability in comparison to the surface oxidation layer formed during oxidation of the base CMC [25].

The microstructure of the Al2O3-YAG EBC coating on the SiCF-SiC(N) composite in the coated condition is illustrated in Figure 3.

**Figure 3.** Microstructure of EBC consisting of Al2O3 bond coat and YAG top coat.

In principle, both processes, as described above, were observed after oxidation and the hot gas test. A comparison of polished cross-sections with the Al2O3/YAG coating is shown in Figure 4 after oxidation in air (A) and the burner rig test (B) at 1200 ◦C and 100 h.

**Figure 4.** Comparison of microstructure in polished cross-sections of SiCF/SiC(N) composite coated with Al2O3-YAG. ( **A**) TGO formation and Al2O3-SiO2 glass in and under the alumina layer after 100 h oxidation at 1200 ◦C. (**B**) Corrosion of TGO and grain boundary in the volume of Al2O3 layer after burner rig test 100 h and 1200 ◦C.

During the oxidation test, oxygen di ffused through the YAG/Al2O3 layer and oxidized the SiC fibers and SiC(N)-matrix to SiO2. Consequently, a TGO layer at the interface of the CMC base material and the Al2O3 bond coat was formed. Furthermore, a part of the oxidation product was found in the grain boundaries and triple junctions of the Al2O3 bond coat. The composition of the oxidation products in both the TGO and Al2O3layer was a glassy alumosilicate (Figure 4A).

As described above, corrosion processes were observed additionally after the burner rig test (Figure 4B). The alumosilicate glass in both the TGO and the grain boundaries and triple junctions were found to be corroded, forming small pores and voids in the Al2O3 bond coat and gaps between the top of the TGO and the bond coat. With increasing time, both corrosion processes will damage the EBC system. Especially, the corrosion of the TGO will form large defects, finally leading to failure of the EBC. The smaller pores and voids in the alumina bond coat, however, are much more stable from a mechanical point of view.

Notwithstanding other damage mechanisms, this behavior opens an idea to the enhance high-temperature stability of the EBC in principle. By the shifting of the oxidation processes from the interface of the TGO into the volume of the bond coat, it should be possible to decelerate the TGO formation. Furthermore, the damage mechanism in the TGO (crystallization processes and corrosion) could be avoided, and only smaller corrosion defects in the bond coat should be observed.

#### *3.3. Si–Yb2Si2O7*/*SiC–Yb2SiO5 EBC Coating System*

This mechanism described above was considered in the design of a three-layer coating system investigated next with a silicon bond coat, an intermediate layer consisting of Yb2Si2O7 with SiC particles, and Yb2SiO5 as the top coat featured by a superior hot gas stability. The microstructure of this EBC is demonstrated in Figure 5. Few microcracks were observed in the Yb2SiO5 top layer, probably as a consequence of the CTE mismatch between the top and intermediate layer (CTE Yb2Si2O7 4.2 × 10−<sup>6</sup> <sup>K</sup>−1; CTE Yb2SiO5 6.8 × 10−<sup>6</sup> <sup>K</sup>−1).

**Figure 5.** Polished cross-section of EBC layer system with Si bond coat, Yb2Si2O7/SiC, and Yb2SiO5 layer fabricated by atmospheric plasma spaying (Si) and suspension plasma spraying (Yb2Si2O7/SiC, Yb2SiO5).

In comparison to the other materials investigated, only a small weight gain was measured after both tests at elevated temperatures. This should be caused by a protecting function, especially of the Yb2Si2O7/SiC layer. Oxygen diffusion through the EBC layers and the oxidation reaction in the Yb2Si2O7/SiC layer were found to be the main processes observed after the oxidation test. The SiC particles in the intermediate layer were oxidized, consequently leading to the formation of a SiO2 scale on the SiC particles. The diffusion of oxygen through this SiO2-based layer controlled the oxidation process of the whole system during the testing time performed. A typical example for the mechanism is demonstrated in Figure 6A, presenting a polished cross-section at the interface of the Yb2SiO5 top coat and the Yb2Si2O7/SiC intermediate layer.

**Figure 6.** Polished cross-section of Si–Yb2Si2O7/SiC–Yb2SiO5 EBC coating system. SiC particle in Yb2Si2O7 layer at interfaces (**A**) Yb2SiO5–Yb2Si2O7/SiC and (**B**) Yb2Si2O7/SiC–Si after oxidation for 100 h and 1200 ◦C.

The SiC particles in this layer operated as a getter for the diffusion of oxygen, preventing further oxygen at diffusing deeper into the material. This effect is illustrated in Figure 6B with the

microstructure at the interface between the Si bond coat and the Yb2Si2O7/SiC intermediate layer. As the diffusion of oxygen reacted with SiC particles in the upper region of this layer, no oxidation processes were observed in this area. The SiC grains did not show an oxidation layer. Furthermore, the formation of a TGO was not observed. Similar results on the graded oxidation of SiC-particles have been reported in context of the mechanical self-healing ability of Yb2Si2O7/SiC composites [26,27].

In Figure 7, the microstructure of this EBC system is shown after the 100 h hot gas corrosion test at 1200 ◦C. Although some additional cracks formed, the EBC coating is still intact, protecting the CMC from the direct hot gas corrosion attack (Figure 7A).

**Figure 7.** Polished cross-section of SiCF/SiC(N) composite with three-layer EBC (Si–Yb2Si2O2/SiC–Yb2SiO5) after 100 h hot gas test (**A**) at 1200 ◦C, overview; (**B**) at the interface to the Yb2SiO5 top coat and the Yb2Si2O7/SiC interlayer, and (**C**) at the interface between the Yb2Si2O7/SiC interlayer and the Si bond coat; energy-dispersive X-ray spectroscopy (EDX) analysis of different phases in the microstructure, analysis locations demonstrated in (**D**).

In comparison to the oxidation test, stronger oxidation processes were observed in the Yb2Si2O7/SiC layer after the test in hot gas conditions. A higher amount of oxidants reached and oxidized the SiC particles in the intermediate layer of the EBC coating system as a consequence of additional water vapor permeation. In agreemen<sup>t</sup> to the literature [28,29], water vapor was found to be the dominant oxidant caused by the significantly higher solubility of H2O in SiO2. A second reasonable possibility is the formation of interconnected open splat boundaries in the Yb-silicate layer [26]. Further investigation has to be performed in this field. The SiC particles were found to be oxidized to SiO2 up to the bottom of the Yb2Si2O7/SiC layer. As a part of the oxidant reached the Si-bond coat the TGO layer started to grow (Figure 7C). Results of EDX analysis of microstructural details of this part are presented in Figure 7.

Additional processes were observed in the top region of the Yb2Si2O7/SiC layer. Some of the SiO2 areas formed by the oxidation of the SiC particles were found to be corroded by water vapor, finally leading to the formation of pores in the top region of the intermediate layer (Figure 7B).
