## 3 Results
### Effect of Lubricants on the Lubrication Regime, Friction, and Wear
#### 3.2.3 Effect of Lubricants on Microhardness  
In boundary lubricated systems (which were achieved with lubricants L1 to L5), mechanical effects can occur, and these affects can be reflected by a change in the hardness of the materials on the worn zone [3]. This phenomenon can affect the performance of the machinery because, if hardness changes, both friction and wear can increase. In this work, as the systems operated in boundary and mixed lubrication conditions, the microhardness on the worn zone of the steel was affected according to the lubricant employed, as can be observed in Figure 8. It can be seen in this figure that the microhardness of the samples lubricated with water-based lubricants (L1, L2, and L3) and the synthetic oil (L4) increased in relation to the initial substrate hardness. Specifically, the hardness of the sample lubricated with L4 increased up to 22.5%. The opposite phenomenon occurred with the samples lubricated with mineral oil (L5) and bio-based oils L6 and L7, which showed a slight decrease in microhardness. It is important to note that the neat natural oil (L8) did not produce alterations in its substrate hardness, instead keeping its initial micro-hardness substrate value. In order to deepen the change of hardness of the steel samples as an effect of the lubrication process, three representative samples were cut transversely, polished, and analyzed. Then, new measurements were made on the transverse plane, as illustrated in Figure 9. The first sample was the lubricated one with synthetic lubricant (L4), with the highest increase in hardness; the second used the natural-based oil with synthetic esters (L7), whose hardness decreased; and the third used the natural-based lubricant with mineral oil (L6), whose hardness was not affected. The hardness on the transverse planes, from the surfaces to the substrates, was graphed and can be observed in Figure 10. It can be seen in this figure that the change in the microhardness of the steel did not only occur near the worn surface but also affected a considerable distance of the bulk substrate. These distances indicate the intensity of damage (boundary layers) that the lubrication process produced on the material and are also an indication of the effectiveness of the lubricants. In this sense, it can be seen that the bio-based lubricant with synthetic esters (L7) affected up to 175 \(\upmu\)m of the material, the synthetic lubricant (L4) affected 350 \(\upmu\)m, and the bio-based lubricant with mineral oil (L6) only affected the surface of the material. Figure 8: Influence of lubricants on the microhardness within the worn surfaces of the 4140 steel. The increase in hardness exhibited on samples lubricated with water-based lubricants (L1, L2, and L3) and synthetic lubricant (L4) could be due to the severe plastic deformation that took place during the sliding process. This plastic deformation can be corroborated on one side by the subsurface layer on the sample lubricated with L4 (Figure 9b) formed by plastic deformation via friction. On the other hand, the decrease in hardness could be related to a microstructural change, similar to that observed in the same steel under severe wear conditions [6]. Considering the previous results, it can be observed that the best friction, wear, and hardness response of the studied system was observed under lubrication with all the bio-based lubricants (L6, L7, and L8), with the neat Castor oil (L8) being the best lubricant in all the experiments. An explanation for these behaviors could be the chemical nature of the base lubricant--the type of fatty acids, the carbon chain length, and the polarity of the natural oil [2]. Such characteristics define their chemical, physical, and lubricating properties. In this way, natural oils are mostly composed by triglycerides, which are tri-esters of fatty acids and glycerol. Such esters are, in general, polar long-chain molecules and exhibit a high degree of linearity and flexibility [2; 4; 27].