# Tribological Analysis of Steels in Fuel Environments: Impact of Alloy Content and Hardness
## 3 Results and Discussion
### Characterization of the Wear Tracks  
Since the tribological tests were performed in the boundary lubrication regime, the interactions of the solid surfaces should be responsible for the observed differences in the tribological behaviors. Characterization of the wear tracks was performed to understand the tribochemical effects at the sliding interfaces (Figure 9). The Raman spectra indicated the presence of characteristic carbon D and G bands, at approximately 1350 cm\({}^{-1}\) and 1580 cm\({}^{-1}\), respectively. These Raman signatures have frequently been associated with the formation of carbon-based tribofilms [37; 38; 39; 40; 41; 42; 43; 44; 45]. Figure 9: (**a–e**) Raman 2D maps for G and D peaks (1 \(\upmu\)m lateral resolution) of hard 52100, soft 52100, soft CF2, soft D2 and hard D2 steel wear track in decane against alumina counterbody, (**f–j**) EDS maps of hard 52100, soft 52100, soft CF2, soft D2 and hard D2 steel wear track in decane against alumina counterbody, (**k–o**) Raman spectra inside the wear track of hard 52100, soft 52100, soft CF2, soft D2 and hard D2 steel wear track in decane against alumina counterbody. The acquired series of Raman maps, spectra, and energy-dispersive x-ray spectroscopy (EDS) maps demonstrated the formation of carbon films during the sliding process. The hard and soft variants of 52100 steel displayed significantly weaker D and G signals when compared with CF2 and D2 steels. The Raman spectra of both soft CF2 and both hard and soft D2 steels exhibited stronger D and G signals, indicating a more efficient formation of the carbon films in these materials. This observation suggests that the steel composition is responsible for the formation of the protective carbon film which plays a critical role in further wear reduction [46]. Prior studies demonstrated that the presence of catalytic elements, such as Cu, Ni, Mo, etc., activate tribocatalytic processes [10; 39; 40]. In this case, the Cu, Ni, Cr, Mo, and V may have contributed to the tribocatalytic formation of the observed carbon films. Importantly, more efficient formation of the tribofilms also correlates with the improved wear resistance of these steels and the alumina counterbody observed in Figure 7b,c. The low wear of the hard 52100 in decane with alumina may be due to the higher amount of oxide observed in the EDS map of Figure 9f The formation of oxides was identified as a contributing factor in the wear behavior of the steels. Raman spectra of soft variants of the CF2 and D2 steels displayed a higher intensity of the D band in comparison to that of the G band, indicating a higher level of disorder or structural imperfections within the formed carbon tribofilms [10; 39; 40; 41]. Among these steels, the CF2 steel shows more distinguished peaks, suggesting a more ordered and graphitic-like structure, underlining more complete dissociation of hydrocarbon molecules and prevalence of carbon-carbon bonds which is in line with prior reports on carbon film formation on the surface of Cu clusters [39]. Meanwhile, though D2 steel also shows characteristic carbon peaks, the presence of additional peaks between the D and G bands suggests incomplete dissociation of C-H and C-O bonds and a rather tribopolymer-like structure of the formed film [10; 40]. The current investigation suggests that chromium (Cr) and copper/nickel (Cu/Ni) are influential elements in D2 and CF2 steels, respectively. For instance, Cr\({}_{2}\)O\({}_{3}\), a well-known catalyst, is responsible for producing a significant portion of the world's polyethylene, and D2 steel, with its 11.5 wt.% Cr content, demonstrates higher catalytic potential compared to 52100 steel with only 1.45 wt.% Cr. Similarly, Cu/Ni acts as a catalyst for alkane dehydrogenation. Prior studies indicated that Cr\({}_{2}\)O\({}_{3}\) is able to catalyze the formation of oligomers/polymers from polyalphaolefin and dodecane [43]. Given that the tribotesting was conducted in the air, the formation of Cr\({}_{2}\)O\({}_{3}\) from Cr in D2 during testing aligns with these findings.