**1. Introduction**

Most of the failures and energy losses of mechanical systems are due to friction and wear, so lubrication could be considered one of the most effective ways to reduce surface damage and energy dissipations. Lubricants decrease wear and friction in mutual contacts between coupled surfaces. Several types of additives in lubricants (e.g., Extreme pressure (EP), antiwear (AW), Frequency-modulation (FM)) yield specific requirements for good lubrication. The previous research proved that lubricants' tribological properties have been improved using nanoparticles as a new additive [1]. Nanoadditives enhance the lubricating characteristics due to the tiny particles' size and morphology that can fill the surface asperities and realize the lubrication mechanisms (i.e., rolling effect, mending effect, polishing effect, protective film formation [1,2]). Research has focused on several typologies of nanomaterials, such as chalcogenides, metal oxides, and carbon-based additives in oil or grease [3–9]. For instance, experimental research has proved that metal oxides added to lithium grease can significantly improve the lubricants' tribological behavior. As reported in [10], the lubricating properties of grease specimens with different concentrations of Al2O3 nanoparticles have been investigated using a pin-on disc apparatus. Results have shown a reduction of COF and wear scar width by approximately 57.9% and 47.5%, respectively. Moreover, research has been carried out on the effects of agglomeration of selected nanoparticles, such as zirconia, within a lithium grease [11]. Experimental analysis on friction properties has proved the addition of 1 wt% ZrO2 nanoparticles to pure lithium grease can decrease the friction coefficient to 50%. Nevertheless, the agglomeration of

**Citation:** Senatore, A.; Hong, H.; D'Urso, V.; Younes, H. Tribological Behavior of Novel CNTs-Based Lubricant Grease in Steady-State and Fretting Sliding Conditions. *Lubricants* **2021**, *9*, 107. https://doi.org/ 10.3390/lubricants9110107

Received: 15 September 2021 Accepted: 26 October 2021 Published: 29 October 2021

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ZrO2 nanoparticles in the lithium grease can increase the friction coefficient by two times compared to that for the pure grease. Applying nanomaterials and nanotechnology in lubrication has also become increasingly popular to achieve green manufacturing and its sustainable development. To this end, water-based nanolubricants are emerging as promising alternatives to the traditional oil-based lubricants [12].

As nanomaterials are of growing interest for lubricating systems, more and more attention has been given to carbon nanotubes (CNTs). Their tubular structure exhibits excellent mechanical characteristics (i.e., high tensile strength, high elastic modulus [13,14], high thermal and electrical conductivity [15–18]) and good lubrication properties, enhancing wear and coefficient Friction (CoF) reduction [19].

In the last few years, the use of carbon nanotubes as additives in lubricating oil/grease has increased, and many research studies have proved that CNTs, with the single-wall (SWCNT) or multi-wall (MWCNT) structure, can improve the tribological performances of the lubricant significantly, compared to a traditional one. For instance, the tribological behavior of Co-based single wall carbon nanotubes has been investigated as an antiwear and extreme pressure additive to SAE base oil. Friction and wear have been evaluated, and it has been found that the SWCNTs are more efficient than the commercial oil additives; tests proved the decreasing of the friction coefficient, using only 0.5 wt% carbon nanotubes to the base oil [20]. Different concentrations of multi-wall carbon nanotubes have been added as additives to two mineral oils; the samples were tested for their antiwear, load carrying capacity, and friction coefficients. The experimental results have shown that the addition of MWCNTs to base oils exhibited good friction reduction and antiwear properties: wear and friction have decreased by −68%/ −39% and −57%/ −49%, respectively, in the case of MWCNTs-based mineral oils compared with the two reference mineral oils [21].

A weighted percentage of 1 wt% of carbon nanotubes in a traditional lithium-based grease can improve EP properties, load-carrying capacity, AW, and friction performances [22]. Moreover, the CNTs concentration affects the lubricant's tribological characteristics, as shown in [23], whereas the different percentage of nanoparticles has been added to a common calcium-based grease. Carbon nanotubes have been added together with other nanoparticles in a calcium grease, and improvements have been observed not only in reduced wear and friction but also in other important characteristics of a lubricant grease, such as shear stress, apparent viscosity, drop point, and thermal conductivity [24–26].

CNTs have been used with different nanoparticles and materials to improve the tribological behavior of lubricants [27–29]. Wang et al. coated the surface of carbon nanotubes with a uniform copper nanoparticle. This successful coating for the nanocomposite was achieved through the modification of the CNTs surface with spontaneous polydopamine (PDA). The friction and wear were reduced by 33.5% and 23.7% when 0.2 wt% of the prepared nanocomposite was used. The improvement in the lubrication properties was attributed to uniform coating and the formation of transfer films on the rubbing surfaces [30]. Akbarpour et al. studied the wear and friction of a composite made of MWNTs, copper, and SiC. The result revealed that 2 vol% of SiC and 4 vol% CNT and Copper had the highest wear resistance and a lower friction coefficient [31]. Song et al. used the chemical vapor deposition technique to grow several layers of MoS2 on the surface of the carbon nanotubes. The CNTs were oxidized by acids to prepare strongly oxidized CNTs (SOCNTs). 0.02 wt% of SOCNTs with dibutyl phthalate (DBP) achieved a reduction in the friction coefficient and wear scar diameter by 57.93% and 19.08%, respectively [32].

According to the ASTM D288, the definition of lubricating grease is "a solid to a semi-fluid product of a thickening agen<sup>t</sup> in a liquid lubricant. Other ingredients imparting special properties may be included" [33]. The thickener agen<sup>t</sup> gives a solid to semi-fluid structure to the lubricant grease and affects many of its main properties. Recently, a new class of nanogrease has been defined using carbon nanotubes as a thickener phase in the lubricant. SWCNTs and MWCNTs have been added into the polyalphaolefin (PAO) oils to form stable and homogeneous CNT grease with potential heat transfer, conductive, and lubricative properties. The rheological investigation of these CNT greases has

provided information concerning the formation of a stable tridimensional structure and particle-particle interactions of CNT suspensions, explaining its excellent lubricating performances [34]. The characterizations of four selected CNT greases have shown high dropping temperatures, very low oil leaking percentages even at high temperatures, and a substantial increase in thermal and electrical conductivities [35]. Experimental tests have been carried out to investigate the enhancing tribological properties of CNT grease compared to the base oil grease. It has been showing that CNTs play a more significant part in lubrication, which greatly improves the lubricating effect on wear and friction [36–39].

In this research, CNTs, single-wall and multi-wall (SWCNTs and MWCNTs) have been added separately into polyalphaolefin oils (DURASYN\_166), with and without MoS2, to form stable and homogeneous CNT greases. The tribological behavior has been investigated in steady-state and fretting sliding conditions, and performance comparisons have been conducted with commercial calcium and lithium greases.
