## 4 Conclusions  
We investigated the effects of MnO\({}_{2}\) doping on the microstructure, crystal structure, and tribological properties of CSZ under thermal stress. The addition of more than 2 mol% MnO\({}_{2}\) to 9 mol% CSZ stabilizes the ZrO\({}_{2}\) phase, resulting in a tetragonal phase fraction exceeding 88%. The cubic phase fraction of zirconia initially decreases with increasing MnO\({}_{2}\) content, but increases again a doping level of 4 mol% MnO\({}_{2}\). As the MnO\({}_{2}\) doping level increases, the concentrations of Mn\({}^{3+}\) and Mn\({}^{4+}\) ions in CSZ gradually decline, reaching 0% at 4 mol% MnO\({}_{2}\). This phenomenon is attributed to greater lattice distortion caused by the higher concentration of Mn\({}^{2+}\) ions, since Mn\({}^{2+}\) ions have a lower valence state than Zr\({}^{4+}\). MnO\({}_{2}\)-doped CSZ maintains a higher phase stability under thermal stress than 9 mol% CSZ; however, a rapid surge in the cubic phase is observed in MnO\({}_{2}\)-doped CSZ after thermal stress, which is also considered to result from enhanced lattice distortion due to Mn\({}^{2+}\).  
The tribological properties, as indicated by the Vickers hardness and specific wear amount, are superior in MnO\({}_{2}\)-doped CSZ compared with 9 mol% CSZ, primarily due to the enhanced sinterability imparted by MnO\({}_{2}\) doping. However, CSZ doped with 4 mol% MnO\({}_{2}\) exhibits inferior mechanical properties compared with the sample doped with 3 mol% MnO\({}_{2}\), both before and after thermal shock. This deterioration is attributed to the increased cubic phase fraction, which has inferior mechanical properties compared with the tetragonal phase. These results demonstrate that both the microstructural changes induced by MnO\({}_{2}\) doping and the phase composition of CSZ significantly influence its tribological properties. In particular, doping 3 mol% MnO\({}_{2}\) into 9 mol% CSZ results in the highest mechanical performance by fully stabilizing the ZrO\({}_{2}\) phase and maintaining a stable microstructure under thermal stress. These findings are expected to contribute to improving the performance and lifetime of CSZ in high-temperature applications requiring superior mechanical properties.  
Conceptualization, H.L. ([PERSON]); methodology, J.-j.K. and J.K.; software, H.L. ([PERSON]); validation, J.K., J.-j.K. and H.L. ([PERSON]); formal analysis, K.J.; investigation, K.J.; writing--original draft preparation, J.K.; writing--review and editing, H.L. ([PERSON]); visualization, [PERSON] ([PERSON]); project administration, [PERSON] ([PERSON]); funding acquisition, [PERSON] ([PERSON]). All authors have read and agreed to the published version of the manuscript. This research was funded by the National Research Foundation of Korea, grant number RS-2024-0045531462182065300101.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
Author [PERSON] was employed by the company Yeol Jeon Sa Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.