## 3 Results
### Microstructure and Density of Sintered Ceramics  
The experimental density data are presented in Table 2. The relative density of Z0 ceramics of 94.5 \(\pm\) 1.3% was obtained using the crystallographic data for cubic YSZ. The addition of 0.25 wt.% rGO to the ceramic matrix causes the density increase up to 96.3 \(\pm\) 1.3% with the same standard deviation. A further increase up to 1 wt.% rGO results in the highest density and minimal standard deviation of 97.1 \(\pm\) 0.4% in the entire series of ceramics. For the Z2 sample, with 2 wt.% rGO, a slight decrease of density to 95.6 \(\pm\) 0.7% takes place. Thus, a minimum of defects and the most uniform structure are expected for Z1 ceramics. SEM images of Z0-Z2 ceramics are presented in Figures 5-7. Structures of surfaces (Figures 5 and 6) can be divided into the near-surface layer and the bulk structures. All the near-surface layers contain an extensive microcrack network (Figure 5). Along with that, they contain a number of pores, and their amount increases with the increase of rGO content, which may be due to carbon additive agglomeration in the zirconia matrix during sintering.  
The microstructure of the surface layer differs significantly from the more uniform ceramic bulk (Figure 6). The surface layer of Z0 ceramics contains a number of prolongated cracks (Figure 6a), and their amount significantly decreases with rGO addition up to 1 wt.% (Figure 6b,c). According to [7], the fracture toughness of composites increases due to the addition of rGO, which stops the propagation of the cracks in the ceramics matrix. For the Z2 sample, the near-surface structure gradually turns into the bulk structure (Figure 6d). Clear borders between the edge area and the bulk are visible for the Z0 and Z0.25 samples and, especially, the Z1 sample (Figure 6c).  
Figure 5: SEM images of the near-edge surfaces of sintered samples: (**a**) ceramics Z0, (**b**) ceramics Z0.25, (**c**) ceramics Z1, (**d**) ceramics Z2.  
Cross sections of the specimens are characterized by different microstructures compared to surfaces (Figure 7). The image of Z0 has two structural zones. The near-edge layer is characterized by a large network of voids (Figure 7a). Furthermore, the area corresponding to the bulk of the Z0 sample has almost zero pores or defects. With the addition of 0.25 wt.% of rGO, the microstructure becomes almost monolithic and more uniform. The network of large voids vanishes, although relatively small pores are still present (Figure 7c). The boundaries of individual grains are still visible though less clearly (Figure 7d). With the increase of rGO content to 1 wt.%, the number of pores and their sizes decrease noticeably (Figure 7e-f). Along with that, grains become almost indistinguishable (Figure 7f). A further increase of rGO content to 2 wt.% leads to the formation of the extensive network of elongated pores with several large ones reaching the estimated sizes of 30 \(\times\) 20 \(\upmu\)m.  
The performed EDX analysis shows that carbon content in the pores is -75 at.% while it is absent on the surfaces for all studied compositions. Silicon is also absent both on the surface and inside the pores of ceramics (see Figures S2 and S4 and Tables S1 and S2 in Supplementary Materials).