## 3 Results and Discussion
The flexural strength before and after thermal shock for 9-CSZ and MnO\({}_{2}\)-doped CSZ is shown in Figure 8. The flexural strength of 9-CSZ before thermal shock was 100.2 MPa, \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Length of** & **Length of b** & **Angle** & **Estimated** \\  & **a (Å)** & **(Å)** & **Between a and b(\({}^{\circ}\))** & **Phase** \\ \hline
9-CSZ & 2.91 & 3.81 & 87.58 & Monoclinic \\ \hline
2 MnCSZ & 3.01 & 3.16 & 69.96 & Tetragonal \\ \hline
3 MnCSZ & 2.99 & 3.13 & 71.32 & Tetragonal \\ \hline
4 MnCSZ & 2.98 & 2.96 & 59.93 & Cubic \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of interplanar spacings, angles, and estimated phases obtained from HRTEM images (Figure 5). Figure 6: Vickers hardness of 9-CSZ and MnO\({}_{2}\)-doped CSZ before (red scattered line) and after (black scattered line) post heat treatment. whereas those of 2-4 mol% MnO\({}_{2}\)-doped CSZ were 387.8 MPa, 411.8 MPa, and 382.4 MPa, respectively. Thus, the flexural strength of MnO\({}_{2}\)-doped CSZ increased sharply by more than 386% compared with 9-CSZ. After thermal shock, the flexural strength of all samples declined; the value for 9-CSZ dropped to 88.18 MPa, while those for 2-4 mol% MnO\({}_{2}\)-doped CSZ were 195.2 MPa, 270.7 MPa, and 261.5 MPa, respectively, showing the same trend as observed before thermal shock. The flexural strength of 4 mol% MnO\({}_{2}\)-doped CSZ was slightly lower than that of 3 mol% MnO\({}_{2}\)-doped CSZ. This pattern was similar to the trends found in the Vickers hardness and specific wear amount. Figure 8: Flexural strength of MnO\({}_{2}\)-doped CSZ before (red scattered point) and after (black scattered point) post heat treatment. Figure 7: Specific wear amount of MnO\({}_{2}\)-doped CSZ before (red scattered point) and after (black scattered point) post heat treatment. MnO\({}_{2}\) doping enhances the sinterability of CSZ, as demonstrated by the microstructural results. This improvement likely contributes to the superior mechanical properties of MnO\({}_{2}\)-doped CSZ compared with 9-CSZ. However, the fact that the mechanical properties did not improve proportionally with increasing MnO\({}_{2}\) doping levels implies that factors beyond sinterability, particularly the phase fraction of CSZ, also play a crucial role. Consequently, doping 9 mol% CSZ with MnO\({}_{2}\) enhanced the phase stability of CSZ and promoted sintering, resulting in excellent mechanical properties. Specifically, doping with 3 mol% MnO\({}_{2}\) fully stabilized the CSZ phase. However, the addition of more than 3 mol% MnO\({}_{2}\) led to an increase in the cubic phase fraction of CSZ, which in turn caused a deterioration in the mechanical properties. [PERSON] et al. investigated the mechanical properties of yttrium-stabilized zirconia (YSZ) according to its phases and found that the cubic phase possesses a lower elastic modulus, greater ductility, and superior plasticity compared with the monoclinic and tetragonal phases [PERSON] et al. (2018). Materials with a high elastic modulus and low plastic deformation typically exhibit high hardness. Therefore, it can be inferred that the hardness and wear resistance of the cubic phase are lower than those of the tetragonal phase. The microstructure images of each specimen after thermal stress are shown in Figure 9. MnO\({}_{2}\)-doped CSZ displayed relatively uniform and larger grain sizes compared with 9-CSZ, consistent with the observations before thermal shock. These results indicate that the decline in the properties after thermal stress is not related to changes in the microstructure. Figure 9: FE-SEM images after thermal shock of **(a)** 9-CSZ, **(b)** 2 MnO\({}_{2}\)-doped 9-CSZ, **(c)** 3 MnO\({}_{2}\)-doped 9-CSZ, **(d)** 4 MnO\({}_{2}\)-doped 9-CSZ.