*2.3. Tribological Characterization*

The tribological test rig for exploring nano additives' effectiveness is the Wazau TRM 100 (Dr.-Ing. Georg Wazau Meß Prüfsysteme GmbH, Berlin, Germany) with the ball-onrotational disc setup as in the drawing in Figure 1. The selected mating materials are: (ball) steel X45Cr13, hardness: HRC 52-54, diameter: 8 mm, mean surface roughness:

0.2 μm; (disc) steel X155CrVMo12-1, hardness: HRC 60, diameter: 105 mm, mean surface roughness: 0.5 μm.

**Figure 1.** Ball on disk setup and rotational tribometer.

The tribological tests were carried out to investigate two different lubrication regimes and fretting operating conditions. A normal load level of 20 or 95 N was applied to attain an average hertzian contact pressure around 1.00 and 1.67 GPa, respectively. The average grease temperature was kept constant at room temperature in each test. The relative motion between the steel ball and the disc was pure sliding at two-speed levels: 5 and 500 mm/s. According to previous results for the same tribopair geometry, average contact pressure, temperature, the boundary lubrication regimes, and the mixed lubrication superposition of the boundary, and elastohydrodynamic lubrication (EHL) regimes were covered. The fretting test was designed by applying an alternate rotating motion to the disk to cover a 75◦ angle with 2 Hz as the frequency.

Both the ball and the disk were pre-cleaned, and the grease was then evenly pasted on the disk sliding pathway by forming a layer with 2 mm thickness. The test length was 60 min in steady-state condition and 120 min for fretting. Thus, the actual sliding distance was 18 and 1800 m for the speed of 5 and 500 mm/s, respectively. The sliding distance was 1131 m for the fretting portion/phase of testing.

The friction coefficient has been indirectly measured in real-time using a torque sensor located under the ball holder plate. After each tribotest, the wear damage circle on the top of the worn steel ball was offline measured through a 3D confocal microscopy.

#### **3. Results and Discussion**

## *3.1. Tribological Results*

The results of the frictional tests were performed under the operating conditions described above in Section 2.3. In both, the investigated lubrication regimes are summarized in Table 1. Tests were also performed to analyze the behavior of the lubricating grease samples for the wear damage of the steel ball surface. In particular, the worn surface of the steel ball was analyzed using a 3D confocal microscope to measure the wear scar diameter (WSD). According to the ISO/IEC Guide 98-3:2008, the expanded uncertainty of the frictional data and WSD measurements are 5.0 × 10−<sup>3</sup> and 1 μm, respectively, by assuming the coverage factor k = 2.

The high frequency acquired frictional data were processed through a rolling mean to filter out spike values and typical oscillations of such measurements.

The steady-state test at 5 mm/s of sliding speed and initial average hertzian pressure equal to 1.00 GPa provided the graphs collected in Figure 2. The graphs show CoF values in a narrow range of 0.12–0.14, with high similarities between Calcium soap, Lithium soap, and CNT-based grease (0.13 as average). The two remaining samples, i.e., Lithium soap with MoS2 and CNTs with MoS2 provided lightly higher CoF. This outcome confirms the effectiveness of MoS2 as an additive in oil and lubricant grease in extreme–pressure conditions, tougher than non-critical conditions as from this test.

**Figure 2.** Friction coefficient in boundary regime under 1.00 GPa average contact pressure.

In Figure 3, the increasing value of sliding speed (500 mm/s) results in a marked difference in the steady-state value of the friction coefficient. Except for the Calcium soap grease, showing high CoF variability in the first 25 min of the test, all the samples reach their asymptotic value quickly. The former result is not unexpected as the Calcium soap grease with NLGI grade 2 does not bear sliding condition with speed as high as 500 mm/s; the latter behavior underlines a good reduction achieved by the CNTs based samples with CoF in the range 0.11–0.12 with a reduction up to −35% compared to conventional greases over the second part of the test characterized by stationary behavior.

**Figure 3.** Friction coefficient in mixed regime under 1.00 GPa average contact pressure.

The measurements at a sliding speed of 5 mm/s and initial average hertzian pressure of 1.67 GPa are shown in Figure 4. In these conditions, the friction coefficient values are in the range 0.10–0.13, and for this test, the trend of the graphs is relatively constant. Again, the CoF values are comparable for some samples, such as Lithium soap, Lithium soap with MoS2 additive, and CNTs based grease. Interesting performances are observable for the Calcium soap grease and CNTs based grease with MoS2 additive, whose CoFs are around 0.10. This behavior is expected for these tested samples. As previously explained, the

Calcium based grease allows good tribological behavior in non-critical working conditions, e.g., at low temperatures and low speeds, whereas the properties of molybdenum bisulphide and carbon nanotubes, combined in the grease sample increase its tribological characteristics.

**Figure 4.** Friction coefficient in boundary regime under 1.67 GPa average contact pressure.

Furthermore, all samples exhibited decreased friction coefficient values while increasing normal load, demonstrating good lubricating capacity under high loads and low speeds in a boundary lubrication regime. As a matter of fact, the friction coefficient showed a tendency to decrease with increasing contact pressure. According to previous works [3,4], the shear stress increased less rapidly in proportion to the contact pressure. This leads to a reduction of the friction coefficient with increasing pressure at a given level of sliding speed.

Among the frictional tests discussed in this paper in a steady-state condition, the most demanding combination of sliding speed and load is given by the test in a mixed lubrication regime obtained at 500 mm/s and 1.67 GPa, seen in Figure 5. Once again, it is worth noting that the trend of the graphs in Figure 5 is nearly constant for all samples, except for Calcium soap grease with CoF peaks as high as 0.27, as is further confirmed in the discussion regarding the test at low speed and load, Figure 4.

**Figure 5.** Friction coefficient in mixed regime under 1.67 GPa average contact pressure.

The samples of Lithium soap, Lithium soap with MoS2, and CNTs based on the CoF curves are approximately superimposable, presenting CoF falling in the same range of the boundary test at 1.67 GPa, 0.12–0.13. In contrast, the CNTs based grease with MoS2 showed a noticeable reduction in CoF: −28% compared to the three previous samples, by attaining an outstanding reduction to 0.09.

The last set of graphs collected in Figure 6 includes measurements of frictional behavior exhibited by the five grease samples under the characteristic fretting condition by applying sliding alternate motion to the frictional conjunction with frequency 2 Hz. The test length was 120 min with a covered sliding distance of 1131 m. In fretting conditions, unlike the previous ones in which the sliding speed is constant, stability at high speed is not expected for each CoF graph since the oscillating conditions may require a longer running-in time for the tribopairs' steel surfaces as well as the grease thickener structure. Along with the already exhibited lack of stationarity of the Calcium soap grease, even the Lithium soap and Lithium soap with MoS2 additive show high variability of CoF over the whole 2-h test with average values attaining 0.15, 0.13, respectively.

**Figure 6.** Friction coefficient in the fretting test under 1.67 GPa average contact pressure.

The CNTs based greases show lower CoF values, with an average value under 0.09 and a remarkable reduction of −35% with respect to the best performing conventional grease. Even the amplitude of CoF oscillations in testing CNTs based grease is drastically reduced by showing enhanced structural stability of the thickener and overall industrial reliability of the lubricant.

The frictional and wear data also correlate well, resulting in a positive correlation (0.8 Pearson index), Tables 1 and 2. This outcome is not obvious in such test conditions. Table 3 underlines the outstanding results achieved by using CNTs based grease with wear parameter reduction close to −60% in comparison with the worst conventional grease and −22% in comparison with the best conventional grease.


**Table 1.** The average friction coefficient in boundary and mixed regime.

**Table 2.** Wear scar diameter (WSD) in boundary and mixed regime.


**Table 3.** The friction coefficient and wear scar diameter in the fretting test.


## *3.2. Surface Analysis Results*

The SEM images were acquired on the worn surfaces of steel ball specimens after the fretting test. By comparing SEM-XRD spectra collected on the worn surface of the steel ball specimen after tribological tests with conventional greases and CNTs based grease, a significant difference in carbon content was found on the steel surface. As shown in Figure 7, the carbon content on the steel ball's worn surface after test with Calcium soap grease is around 15%, and other SEM measurements confirm this on a different portion of the worn surface.

**Figure 7.** (**a**) SEM image of a worn surface ball of steel ball: Calcium soap grease; (**b**) EDX spectrum: Calcium soap grease.

The same analysis performed on the steel ball's worn surface post-tribological test with CNTs based grease shows lower carbon content (around 6–8%), Figure 8. In the same spectrum, it is worth noting the presence of Molybdenum.

**Figure 8.** (**a**) SEM image of a worn surface ball of steel ball: CNTs based grease with MoS2; (**b**) EDX spectrum: CNTs based grease with MoS2.

The well-known literature analysis of the friction reduction mechanism introduced by nanoparticles as lubricant additives finds in the following list the more convincing physical explanations: rolling-sliding "rigid" motions together with flexibility properties, nano additive exfoliation, and material transfer to the metal surface to form the so called "tribofilm" or "tribolayer", electronic effects in tribological interfaces, surface roughness improvement effect or "mending"; along with the more classical hypothesis of surface sliding on lower shear stress layers due to weak interatomic forces, valid also for microscale additives used for decades. In the case of the samples proposed in this paper, along with the transfer of nanoparticles from semifluid lubricant to steel mating surfaces, CNTs used as sole thickener seem to provide to the bulk structure of the grease a marked stability of the tribological response over the whole spectrum of the performed tests from a qualitative point of view. From a quantitative point of view, frictional reduction is always good, the CoF has a marked reduction in fretting conditions. On the other hand, wear reduction is substantial only in the case of the fretting test and further optimization may be needed to increase wear protection in a broader range of load/speed combinations. The outstanding behavior in fretting conditions could be attributed to a more consistent and

fatigue-resistant performance of CNTs thickener in comparison to conventional greases based on metallic soap.
