# Tribological Analysis of Steels in Fuel Environments: Impact of Alloy Content and Hardness
## 3 Results and Discussion
### Friction and Wear Performance  
The friction and wear of the steels were tested under boundary lubrication conditions in ethanol and decane. One representative test result of friction variation and microscope  
Figure 4: Central and minimum film thickness values for tests using steel and alumina ball, reciprocating motion with a maximum speed of 0.628 m/s and a normal load of 10 N. The lubricant is decane. Hc means central film thickness and Hmin minimum film thickness. Figure 3: Central and minimum film thickness values for tests using steel and alumina balls, 10 mm in diameter, reciprocating motion with a maximum speed of 0.628 m/s and a normal load of 10 N. The lubricant is ethanol. Hc means central film thickness and Hmin minimum film thickness. image of surface wear are shown for each of the two different counterbodies, alumina and 52100, with ethanol lubrication in Figure 5. In Figure 5a, we compare the coefficient of friction of the soft and hard steels against the alumina counterbody with ethanol. The softer and harder steels had comparable friction behavior. In Figure 5c, when we used 52100 as the counterbody material, the coefficient of friction of the various materials varied over a greater range. Soft and hard 52100 had generally the lowest and highest values, while hard D2 dropped to a lower value towards the end of the test. Figure 5b,d show the worn surfaces of both the flat specimens and the counterbody balls. When we used alumina as the other material (Figure 5b), the width of the wear scar was less on both the ball and the flat surface. Representative coefficient of friction and optical microscope wear results for the hard and soft steels with the two counterbodies using decane as a lubricant are given in Figure 6. Figure 6a reveals the coefficient of friction in the decane environment with the alumina counterbody. Soft D2, hard D2, and soft 52100 had steady coefficients of friction which were also the lowest, while soft CF2 and hard 52100 had higher and more variable coefficients of friction throughout. Figure 6c illustrates a different result when 52100 was used as the counterbody material. As with ethanol lubrication, the 52100 counterbody resulted in a larger range of coefficients of friction. Hard D2 had the lowest coefficient of friction, with soft 52100 and soft CF2 slightly higher but close in value. Figure 6b,d show the wear track on both the flat and counterbody surfaces. The wear tracks caused by alumina as the counterbody material (Figure 6b) are visibly smaller on both the surfaces of the ball and the flat samples than those from the 52100 counterbody (Figure 6d). Figure 5: (**a,c**) Coefficient of friction behavior of soft and hard 52100, soft CF2, and soft and hard D2 steel against (**a**) Alumina counterbody (**c**) 52100 counterbody under high-frequency reciprocating at the frequency of 20 Hz under 10 N as a function of cycles in ethanol environment. (**b,d**) Surface morphology of wear tracks and counterface ball surfaces. The scale bar is 200 \(\upmu\)m.  
We analyzed the steels (52100, CF2, and D2) by examining the average coefficients of friction, wear rate of the ball and the flat samples from three tests conducted in two fuel environments, and the wear profile from the tribology tests in ethanol and decane environments using both alumina and 52100 counterbody materials. The results are plotted in Figures 7 and 8. Figure 6: (**a,c**) Coefficient of friction behavior showing the performance of soft and hard 52100, soft CF2, and soft and hard D2 steel against (**a**) Alumina counterbody (**c**) 52100 counterbody under high-frequency reciprocating at the frequency of 20 Hz under 10 N as a function of cycles in decane environment. (**b,d**) Surface morphology of sliding paths and counterface ball surfaces.