# Evaluation of Anti-Wear Properties of Metalworking Fluids Enhanced with Halloysite Nanotubes
## 3 Results and Discussion
### Surface Analysis  
Micrographs of worn surfaces of tested blocks with MO + HNT and SF + HNT suspensions are shown in Figures 7 and 8, respectively. The worn block in Figure 7a with MO shows pitting throughout the sample. Fewer regions with pitting are found for Figure 7b with MO + 0.01 wt. % HNTs. Pitting wear is further reduced with higher nanoparticle content on the worn surfaces observed in Figure 7c,d with 0.05 wt. % and 0.10 wt. % HNTs. Figure 8 with SF nanofluids presents overall smoother surfaces compared to MO nanofluids (Figure 7). Figure 8a (unfilled SF) shows very few regions with pitting and shallow furrows. Lower groove density with increasing HNT concentration is evident in Figure 8b-d, with a very smooth surface found with 0.10 wt. % HNTs. Surface roughness profiles of worn blocks shown in Figures 7 and 8 taken perpendicularly to the sliding direction of the test are depicted in Figures 9 and 10, respectively. Table 2 presents surface roughness parameters of \(Ra\), \(Rq\), and \(Rz\) characterized by an optical measurement system. For both MO and SF, the nanofluids roughness parameter values were very similar, and decreased with increasing HNT concentration. Reductions in \(R_{a}\) by 36% and 34% were obtained for MO and SF nanofluids, respectively, at 0.10 wt. % HNTs. Lower \(R_{a}\) values achieved with HNTs were due to a higher amount of removed material during the wear testing that provided a polishing effect, as observed in Figures 7 and 8. Similar results have been found by [PERSON] et al. [20] with nanoparticles effectively reducing the \(R_{a}\) of surfaces through metal removal. Figure 6: Coefficient of friction values for MO and SF nanofluids with varying HNT nanoparticle concentration. Figure 8: _Cont._  
Figure 7: Scanning electron micrographs of blocks after tribological testing of: (**a**) MO, (**b**) MO + 0.01 wt. % HNTs, (**c**) MO + 0.05 wt. % HNTs, (**d**) MO + 0.10 wt. % HNTs. Arrows indicate the direction of sliding. Elemental analysis performed by EDS on the worn blocks tested with MO + 0.10 wt. % HNTs and SF + 0.10 wt. % HNTs can be observed in Figure 11a,b, respectively. No Al and/or Si elements were detected on the surface, suggesting that at the contact pressure of this test of 0.5 GPa there was no HNT deposition and formation of a protective film. Furthermore, nanoparticles were too large to fill valleys or grooves of surfaces and provide a mending tribological mechanism. This is in contrast to previously reported results by our group [33], where at 3.5-3.9 GPa (in a four-ball extreme pressure test) Al and Si groups were found on the surface, likely due to HNT exfoliation due to the compression forces. As explained by [PERSON] et al. [34], HNTs are hard aluminosilicates with interlayer water molecules where weak Van der Waals forces and H-bonds are present, and shearing occurs due to the slip motion between these layers, particularly for larger nanoparticle diameters (>120 nm). A similar behavior has also been reported experimentally by [PERSON] et al. [35] for inorganic fullerene-like (IF)-MoS\({}_{2}\) nanoparticles. At contact pressures of 1 GPa exfoliation of outer fullerene sheets was found to be the predominant mechanism, and at lower pressures (0.1 GPa) a rolling/sliding mechanism was observed. It is explained that under compression forces the IF nanoparticles gradually deform and start to exfoliate, which is enhanced by the large number of defects in the layers. It is then proposed that the enhancement in tribological properties provided by HNTs can be attributed to a third body mechanism, where nanoparticles separate surfaces reducing the real area of contact and reducing wear, as proposed by [PERSON] et al. [36], and their possible sliding between surfaces. The schematic of the anti-wear mechanism provided by HNTs is shown in Figure 12. As observed in Figures 4 and 5, increasing HNTs concentration resulted in lower wear mass loss due to more nanoparticles on the wear zone that were able to reduce the area of contact and decreased friction and wear.