# Tribological Analysis of Steels in Fuel Environments: Impact of Alloy Content and Hardness
## 4 Conclusions  
This study investigated the friction and wear behaviors of several steels when exposed to different lubrication environments and counterbody materials. Two types of fuel, ethanol and decane, two distinct counterbody materials, alumina and 52100 steel, and two hardness values were used to investigate how they influenced the tribological performance of the steels. The elastohydrodynamic analysis evaluated the changes in the film thickness during the initial test duration, revealing a modest dependence of film thickness on the counterbody material and the fuel chemistry, and confirmed that all tests were conducted in the boundary lubrication regime. The results revealed that the tribological behavior trends were similar for alumina and 52100 steel counterbodies across different environments. Even though soft 52100 steel demonstrated low friction, its wear was the largest for both tested environments and counterface materials. Among all the tested materials, hard D2 steel experienced the lowest wear. 52100 and D2 steels showed opposite friction behavior when comparing hard and soft steels, with lower friction observed for softer 52100 and harder D2 steels. Meanwhile,the wear of both flat and ball counterfaces was lower for harder candidates than for softer ones independent of the environment and counterbody. Results from the Raman characterization suggested that alloying elements played a crucial role in determining the wear characteristics of the steels under investigation. Raman analysis revealed the presence of characteristic D and G bands in the Raman spectra collected from the wear tracks, suggesting the formation of carbon-based tribofilms contributing to additional protection of surfaces during sliding. Interestingly, softer variants of CF2 and D2 steels exhibited a higher D band intensity compared to that of the G band, indicating increased disorder or structural imperfections. Contrarily, the harder D2 steel showcased a stronger G band, indicative of a more ordered structure of carbon-carbon bonds. Furthermore, the role of alloying elements in the wear process was noteworthy. These elements appeared to impact the material's resistance to wear, suggesting that the specific composition of the alloys played a key role in determining the tribological performance. The results of the study highlight the significance of alloy composition as the primary factor influencing wear resistance. In support of this, literature studies indicate that specific elements play a crucial role in catalyzing the transformation of hydrocarbon molecules into polymers during rubbing. These findings underscore the intricate relationship among steel alloy composition, structural properties (hardness), oxide formation, and the efficiency of carbon film formation during tribocatalytic processes, reflecting the wear performance of these steels. Through comprehensive evaluations of friction, wear, and surface topography, we highlight the importance of material compositions, counterbody materials, and environmental factors in defining the tribological performance of fuel-lubricated materials. Conceptualization, D.B., A.A.V., S.M.A. and S.P.B. ; methodology, D.B., A.A.V., S.M.A. ; experimental validation, A.Z.M., V.I.M., E.C. and M.E. ; computational analysis, S.L. and Q.J.W. ; resources, D.B., A.A.V., S.M.A., S.P.B., Y.-W.C. and Q.J.W. ; data curation, A.Z.M., V.L.M., E.C., M.E., S.L., Q.J.W., S.P.B., S.M.A., A.A.V. and D.B. ; writing--original draft preparation, A.Z.M., S.P.B., S.L. and D.B. ; writing--review and editing, Y.-W.C., Q.J.W., S.M.A. and A.A.V. ; supervision, D.B., S.M.A., A.A.V. and S.P.B. All authors have read and agreed to the published version of the manuscript. This research was sponsored by the DEVCOM Army Research Laboratory and was accomplished under Cooperative Agreement Numbers W911 NF-20-2-0198 and W911 NF-21-2-0199.