# Automatic Transmission Fluids in Electrified Transmissions: Compatibility with Elastomers
## 3 Results and Discussion
### Changes in Hardness  
The abovementioned specifications were also used to analyse the compatibility of the materials regarding changes in hardness. Figure 2 shows the evolution in hardness of the different elastomers during the aging process expressed in Shore A scale.  
Acceptation limits values from manufacturers are not indicated in the figure as these limits are not absolute values but are also given related to initial hardness of the material [9]. While the FKM showed small changes in hardness in the first 168 h, the EPDM and silicone showed a huge reduction, but within the limits allowed by transmissions manufacturers in the case of the silicone. [PERSON] et al. [18] reported a similar drop in hardness for silicone, around 15 points (Shore A), after only 72 h of aging in engine oil, with a variety of additives and at 150 \({}^{\circ}\)C. There is no bibliographical reference to the behaviour of EPDM in oils, but its loss of hardness is by far the greatest of the three elastomers. [PERSON] et al. [19] reported a large reduction in hardness for EPDM seals after they were used in a water supply system  
Figure 1: Changes in volume (expressed as the ratio of the volume variation to the initial volume) of the FKM, silicone, and EPDM after aging in oils A, B, C, and D.  
for three years at temperatures between 20 and 45 \({}^{\circ}\)C. It seems to be the immersion in fluids that affects the hardness of the EPDM, as [PERSON] reported no hardness reduction after 360 h of thermo-oxidative aging at 120 \({}^{\circ}\)C [20]. The changes in hardness of each elastomer after aging in oils were not greatly affected by the oil used. Regarding the FKM, [PERSON] [17] reported negligible hardness changes after one week of aging at 150 \({}^{\circ}\)C in engine and wheel-bearing oils, and null at 250 \({}^{\circ}\)C without aging, confirming the results shown in Figure 2.  
From 168 h onwards, the silicone showed insignificant variation in hardness, but that was not the case for the EPDM and the FKM. The EPDM had a greater loss of hardness but there is no specification for this material regarding hardness change. While the EPDM showed a continuous reduction in hardness, that of FKM increased. Taking into consideration the specifications from manufacturers, the silicone fulfilled all the requirements respecting admissible changes in hardness, Ford's being the most restrictive (\(-\)30 ShA). This material showed a lower decrease in hardness when aged (672 h) at room temperature [21]. It was the high temperature, rather than the oil type, that led to a decrease in the hardness of the silicone.  
In the case of the FKM, its hardness variation exceeds the Ford, Allison Transmission, and Dexron VI specifications (+5 ShA) after 336 h for oils A, B, and C, and also after 504 h for oil D. The FKM also exceeds the GM specifications (+6 ShA) for oils A, B, and C after 504 h, and for oil D after 672 h. In addition, the FKM exceeds the Allison TES 439 specifications (+7 ShA) for oil B after 504 h and for oil C after 672 h. On the contrary, the FKM fulfilled the Ford Mercon V specifications (limit value: \(-\)8 ShA points) during the whole test and for all the oils tested. The results in [PERSON] et al. [22] confirm what is shown in Figure 2, as they obtained a slight hardening of FKM after prolonged aging in lubricants at 150 \({}^{\circ}\)C.  
Figure 2: Changes in hardness of the FKM, EPDM, and silicone after aging in oils A, B, C, and D.