## 4 Conclusions  
Four conventional ATFs were tested for their compatibility with three polymeric materials that can be used in electrified transmissions. After testing volume, hardness, tensile strength and elongation at break, and measuring variation in these materials when aged in the ATFs, and additionally after some complementary tests to justify the results, we can conclude that:  
* PEEK and PTFE showed full compatibility in the short and long-term regarding volume, hardness, and tensile strength properties with all the ATFs used. PEEK showed good compatibility (lower variations) with all the ATFs for elongation at break also, while PTFE showed worse results than PEEK on this measure.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**ATFs**} & \multicolumn{3}{c}{**Polymers**} \\ \cline{2-4}  & **PEEK** & **PTFE** & **PA66** \\ \hline Oil A & *** & *** & ** \\ Oil B & *** & *** & ** \\ Oil C & *** & *** & ** \\ Oil D & *** & *** & ** \\ \hline \hline \end{tabular}  
* Medium compatibility; *** High compatibility.  
\end{table}
Table 8: Oil compatibility with the polymers studied.  
PA66 had worse compatibility with the oils than the other materials. This result was related to an increase in the grade of crystallinity of the PA66 that caused changes in the mechanical properties of the aged samples.
* According to the FTIR results, the variations in the mechanical properties of the materials are, in general, related more to the ageing time than to the oil type, with the exception of the PTFE, which does not seem to be much affected by the ageing time.
* The ATFs showed good compatibility with the studied polymers and only PA66 showed worsening of its elongation at break performance after ageing.
* The compatibility of these ATFs with the tested materials is a necessary condition, but is not in itself sufficient, to allow their use in electrified transmissions. Additional oil compatibility tests (electrical, magnetic, thermal, corrosion, aeration, and foaming at high speeds) must be performed to justify their use in electrified transmissions. In addition, the results of these acceptance tests should not be used for predicting the lifetime of a component in a real application.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12073608/s1](https://www.mdpi.com/article/10.3390/app12073608/s1)]([https://www.mdpi.com/article/10.3390/app12073608/s1](https://www.mdpi.com/article/10.3390/app12073608/s1)), Figures S1-S3: Tensile strength and elongation and break of the PEEK, PTFE and PA 66 (fresh and after 168, 336, 504 and 672 h of ageing in the ATFs); Figures S4-S6: FT-IR spectra of the PEEK, PTFE and PA 66 (fresh and after 168 and 672 h of ageing in the ATFs); Figure S7: DSC results for the different material/oil combinations (Exo-up); Tables S1-S3: [PERSON]'s statistic for every peak ratio for PEEK, PTFE and PA 66.  
Conceptualization, A.G. and J.L.V.; Methodology, A.G., A.G.-T. and A.F.-G.; Investigation, A.G., G.D.V. and A.F.-G.; Writing--original draft, A.G., A.G.-T. and A.F.-G.; Supervision, J.L.V. and A.H.B.; Project administration, J.L.V. and A.H.B.; Writing--review & editing, J.L.V. and A.H.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Ministry of Science, Innovation and Universities (Spain), grant number PID2019-109367 RB-100. Data Availability Statement: Data is contained within the article or Supplementary Material.  
The authors would like to acknowledge the Ministry of Science, Innovation and Universities (Spain) for supporting this work under the project PID2019-109367 RB-100 and the technical support provided by Scientific and Technical Services of the University of Oviedo.  
The authors declare no conflict of interest.