## 4 Discussion  
As seen from the phase composition analysis and microstructure data, the chosen conditions of the powder bed synthesis, i.e., three-layer graphite/SiC/graphite bed and exposure at 1550 \({}^{\circ}\)C for 3 h, favor the formation of a YSZ cubic solid solution in the  
Figure 10: Microhardness test results for ceramics Z0–Z2. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**Activation Energy of Conductivity E\({}_{\alpha}\) eV**} \\  & **Grain Component** & **Grain Boundary Component** \\ \hline Z0 & 1.10 & 1.31 \\ Z0.25 & 1.06 & 1.21 \\ Z1 & 1.23 & 1.35 \\ Z2 & 1.10 & 1.16 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Activation energy of conductivity of grain and grain boundary components of ceramics Z0–Z2. samples. High relative density values of 94-97%, crystallinity values higher than 93% for all compositions and large crystallites (112-114 nm, see Table 2) prove this assumption. At the same time, the obtained Raman spectra are quite different from the typical ones for YSZ/rGO composites. Bands referring to rGO have an intensity close to the baseline, which indicates the elimination of the carbon phase after sintering (see Figure 4). The spectrum of pure Z0 ceramics with no initial rGO added shows the presence of negligible carbon bands. That points to possible carbon diffusion into a specimen from the powder bed during sintering. Intensive bands at 1-1000 cm\({}^{-1}\) for all compositions can be hardly attributed to YSZ. Instead, they could be referred to SiC, which also could be the source of carbon. According to [32], all crystalline silicon carbide polytypes give Raman scattering from a longitudinal optic phonon (LO) at 973 cm\({}^{-1}\), which is presented in spectra, and a transverse optic (TO) phonon at approximately 790 cm\({}^{-1}\), which is slightly seen. Another intensive broad band at \(\sim\)480 cm\({}^{-1}\) in spectra can be referred to as amorphous SiC [38] or amorphous Si [39]. Thus, two processes taking place during sintering are expected: (i) diffusion of SiC into the specimen bulk and its decomposition with the formation of free carbon (ii) gradual rGO agglomeration at high temperatures like it was observed for SPS-sintered composites [8]. Silicon carbide powder is rather coarse, and its steady diffusion into the ceramic matrix bulk is limited (mean particle size of 36.22 \(\upmu\)m, see Figure S5, Supplementary Materials). The comprehensive consideration of the data on microstructure, conductivity and microhardness (see Figures 6 and 8-10) allows us to conclude that they are competing with each other depending on rGO concentration. The microstructure of the Z2 sample significantly differs from the other composites. According to SEM images (Figure 6), the structure of the near-surface layer is similar to the structure of the bulk, which points to the negligible diffusion of silicon carbide from the powder bed during sintering. At the same time, only this composition possesses an extensive network of pores with some large ones in the bulk, which, according to previous studies, corresponds to rGO agglomeration sites. Based on the EDX spectra data (see Figure S3 and Table S2 in Supplementary Materials), the large pores contain \(\sim\)70 at.% of carbon, and this fact proves the above assumption. The increased values of ionic conductivity and the decrease of microhardness observed for the Z2 sample prove the assumption on rGO agglomeration. Therefore, there is less dispersed rGO observed in the intergranular space. Both the microhardness value and electrochemical behavior of the Z2 sample are close to those for pure Z0 ceramics. According to the EDX spectra and elemental distribution maps, even the bulk of Z0 ceramics contains some carbon in pores (see Figures S1, S2 and S4 and Table S1). In addition, the rather loose structure of the bulk (Figure 7a) and increased activation energy of the grain boundary conductivity obtained for the sample indicate SiC diffusion into the sample took place rather intensively.