With the enhancement of the environmental protection, the significance of green lubricating oil additives is growing, to meet the harsh restrictions on energy-saving and emission-reduction under the strict requirements of anti-friction and anti-wear ^[1][2][3][1,2,3]. Nano-scale additives applied to lubricating oil have great advantages in the field of precision manufacturing equipment because of the character of their surface–interface effect and high specific surface area [4]. In the process of operation of mechanical equipment, even if it is under the desired liquid lubrication condition, the interface of a friction pair can be in the boundary lubrication state under the influence of impact load, heavy load, low viscosity and repeated start-and-stop factors [5]. Although the traditional lubricating oil additives can improve the lubrication performance of tribo-pairs, their high reactive activation energy results in tribochemical corrosion wear, and their application is greatly limited in the friction parts of high-precision equipment [6]. Compared with traditional lubricating oil additives, nanoparticles have the unbeatable advantage of low film forming energy barrier, no film forming induction time, quick-forming of boundary protective film and micro-bearing effect on the friction surface–interface, further reducing equipment maintenance costs and improving the anti-friction and anti-wear performance of sliding components [7]. The research on nano-copper as lubricant additive has greatly progressed in the field of tribology, but nano-copper easily loses the characteristics of nanoparticles because the surface activity is relatively high, the primary cause being that a large number of unsaturated bonds exist on the surface of nanoparticles. In addition, the agglomeration of nano-copper, which is caused by VDW (Van der Waals’ force), electrostatic action and hydrogen bonding between particles, increases the size of secondary forming particles and loses the characteristics of nanoparticles. The above situation has a negative effect in the stability of nano-lubricating oil systems and easily causes oil circuit blockages in industrial applications. Therefore, it is important to prevent agglomeration of nano-copper particles, involving suspension time improving, dispersion stability and the tribological properties of nanoparticles in lubricating oil for nano-lubrication technology [8].

2. Lubrication Mechanism of Nano-Copper as Additives

2.1. Forming Mechanism of Lubrication Film

After the nanoparticles arrive to the contact area in the sliding process, a deposition film or strengthening layer is formed by chemical and physical action, and the friction film can be formed on the sliding track, thus reducing the shear strength of the friction interface and the direct contact of the friction interface. In addition, it has a certain filling and self-repairing effect on the damaged parts of the surface (Figure 15). Liu Weimin modified copper nanoparticles (Cu-DDP, 6 nm) on the surface of dialkyldithiophosphoric acid (DDP) and tested their lubrication properties as lubricating oil additives. Due to the deposited copper nanoparticles having a small particle size, low melting point and good ductility, the atomic radii of Cu and Fe are very closed, and the covalent radii are the same, so a stable deposition film can be formed on the worn track since the low melting point and ductility under high temperature and high stress in the sliding process as shown in Figure 26. Good anti-wear, anti-friction and extreme pressure properties were obtained ^[9][50]. Based on the study of DLC solid–liquid composite lubrication system, Zhang et al. ^[10][51] pointed out that the high activity of nano-copper particles in NPCuDDP formed a lubricating film through tribological chemical reaction by comparing the tribological mechanism of nano-copper additive (NPCuDDP) and zinc dithiophosphate (ZDDP), which reduced the wear rate of all DLC coatings by 2–3 orders of magnitude compared with that of ZDDP or without additives. Yi D Z et al. ^[11][52] claimed that the improvement in the anti-wear ability and anti-fatigue performance of the steel–steel pair could be closely related to the special worn-surface repairing effect of the nano-Cu additive and the tribochemical reaction. Borda et al. ^[12][53] showed that the introduction of nano-additives can form an effective lubricating film on the sliding track and realize the transfer of contact stress, thus improving the anti-friction and anti-wear effect of lubricating oil.

Figure 15. Schematic of lubrication enhancement of proposed lubrication mechanisms.

Figure 26. Mechanism of lubrication film forming.

However, there is also a different point of view that nano-copper softens to form a high viscosity fluid under the induction of friction heat in friction, which exerts the pressure-bearing lubrication effect on the friction interface to a certain extent ^[13][54]. Jiang et al. ^[14][55] prepared homopolymer polypropylene (PP-H) composite films with different nano-copper concentrations, presenting that the addition of nano-copper improves the thermal and mechanical properties of the composite film. Shi et al. ^[15][56] discussed the tribological mechanism of graphene/copper hybrid particles and hydroxypropyl methylcellulose composite solid lubricating coating, showing that the addition of graphene/Cu hybrid nanoparticles improves the lubrication performance.

2.2. Self-Repair Mechanism

Nano-copper particles with high activation characteristics are easy to adhere to the sliding surface or to bury in the micro-pit and micro-damage wear surface because of their large specific surface area and high surface energy, thus playing an effective protective role. Zhang et al. ^[16][57] think that the self-repairing effect is mainly accomplished by the coordination of physical, chemical and electrochemical action, further forming a better anti-wear effect. Yang et al. ^[17][58] observed a filling and self-repairing effect under the action of friction pair extrusion after adding 0.4% nano-copper to the lubricating oil, leading to the furrows and abrasions being significantly reduced on the sliding track; the deposition of copper nanoparticles plays a self-repairing role on the worn surface. Ye and Liu et al. ^[18][19][59,60] investigated the self-filling and self-repairing effects of the nano-copper precipitated from the surface defects of the coating since of copper nanoparticles and inferred that the friction heat will cause the nano-copper particles to absorb heat and produce an agglomeration effect, resulting in small aggregates of nanoparticles aggregating to form a lubricating boundary film during the sliding process ^[20][61]. Furthermore, the nano-copper particles can reduce the shear stress of the friction interface and weaken the adhesive and abrasive wear under high load, playing the role of lubrication protection on the friction surface ^[21][62].

The friction pair wear interface can constantly generate new metal surface during the sliding process, which is beneficial to forming the boundary film with self-healing effect. The surface metal activation caused by the tribo-chemical action is beneficial for the strengthening of self-healing effect in the application of nano-copper additives ^[22][63]. Shi et al. ^[23][64] verified that the friction chemistry is beneficial to the self-healing behavior of worn surfaces under the addition of modified nano-copper as self-healing materials. It has also been documented that nanoparticles can penetrate into the interior of the material lattice under frictional forces and play a self-healing role on the worn surface as shown in Figure 37, but this viewpoint lacks direct support and still needs to be refined and verified ^[24][65]. Wang et al. ^[25][66] conducted the tribological performance of nano-copper with self-healing additive in different lubricating oils, which showed that the repair film formed on the wear scar with the copper element. The self-healing effect of nano-copper is mainly related to the motion load and the content of nano-copper. If the load is too small, it is difficult to obtain the friction heat required for self-healing, but if the load is too large, the self-healing aggregates scarcely fall, resulting in the exacerbation of the lubrication state.

Figure 37. Self-healing mechanism.

2.3. Micro-Rolling Bearing Effect

Another opinion proposes that the thickness of the oil film under the boundary lubrication be increased after the nanoparticles enter the contact zone of sliding components and that the relative motion between the friction pairs be changed from the sliding friction state to the rolling friction state so as to avoid the direct contact between the friction pairs. Wang et al. ^[26][67] showed that the nano-copper particles can make the friction pair from the sliding state to the rolling state in the lubricating oil according to the experimental analysis, exhibiting good anti-wear and friction-reducing effects.

There are also certain differences in the effects of different scales of nano-copper on the performance of the lubricating fluid. In addition, excessive nano-copper can cause excessive wear because of the excessive size of agglomerate as shown in Figure 48. Because the spherical nano-copper is nearly spherical and the grains have dislocation distortion, the lattice will slip when a shear force arise, revealing that the nanoparticle resembles the ball bearing on the contact surface.

 

 

Figure 4. Lubrication mechanism of nano-copper lubricant additives with different sizes. Reprinted with permission from ref. ^[26]. Copyright 2020, China Petroleum Processing & Petrochemical Technology Press.

The spherical nanoparticles play a rolling role between the friction pairs, which can reduce the friction resistance, as shown in Figure 59. However, the application scope of the micro-ball bearing effect is effective under the low load condition, yet it can still maintain good rigidity under a heavy load and high temperature on the premise of well dispersed and spherical or similar to spherical particles. Although it is reported that the microsphere bearing effect can reduce friction for nano-copper, it still lacks direct experimental evidence for nano-copper as a soft metal ^[27][68].

Figure 59. Schematic diagram of micro-ball bearing effect.

In addition, the load-carrying capacity of the nanoparticles can be affected by the shape or size. Chen et al. ^[28][69] considered that the key factors of particle lubrication are the ability of particles to enter the contact zone and whether the particles can stay in the contact zone. Wu et al. ^[29][70] indicated that flaky nanoparticles such as MoS₂ nano sheets are easily enter the contact zone of micro-convexity surface due to their small thickness, while spherical or linear three-dimensional particles are easily pushed away by micro-convexity because of their large size. Therefore, it is difficult to enter the sliding interface. The thickness of the boundary lubrication film is in the order of nanometers, which is much smaller than the comprehensive roughness of the common surface. If the diameter of the nanoparticles is larger than about 10 nm, it is more difficult to enter the gap (oil film thickness) between the tribo- pairs under the boundary lubrication. Therefore, the key to producing the lubrication effect is whether the nanoparticles can enter the contact zone ^[30][71].

At present, the anti-friction mechanism of “ball bearing” of nano-copper particles has been reported to some extent, but there is still a lack of direct experimental evidence. Because of its high viscosity, the effect of oil solubility of the lubricating oil can be ignored. Therefore, most soft metal particles can be added to grease as additives ^[31][72]. In the grease, the addition of nano-copper particles can improve the anti-wear performance and the service quality, which can bring fully into play the self-repairing of soft metals in the grease ^[32][33][73,74].

2.4. Lubrication Theory and Simulation of Nanoparticle Lubricating Oil Additives

At present, the lubrication theory, simulation methods and theoretical modeling of nanoparticles as lubricating oil additives are varied, mainly including molecular dynamics, CFD fluid simulation, mechanical modeling and ANSYS simulation. Particularly, molecular dynamics theory plays a guiding role in nanoparticles modelling, friction heat, nano-adsorption and boundary film formation. For the simulation of copper particles, fewer reports are related to tribology, and the main research focuses on molecular dynamics simulations of the aggregation of monocrystal and polycrystal nanoparticles ^[34][75], besides the simulation of common lubricating oil additives, such as nano-MoS2 ^[35][76].

Leng et al. ^[36][77] established the molecular dynamics model of nanoparticles by Lammps software and studied the effect of different sizes of the thermal conductivity of castor oil dispersion, which has a certain guiding effect on the mechanical properties and anti-friction mechanism of nano fluids. Through CFD simulation and physical modeling, Wu et al. ^[37][38][78,79] found that the dispersant polyisobutyleneamine succinimide (PIBS) plays a decisive role in the dispersion state and lubrication performance of the lubrication system. In the case of low PIBS percentage and no PIBS, nanoparticle aggregates can easily enter the friction contacted zone and form a uniformly distributed boundary lubrication film, resulting in reduced friction coefficient and wear. When a high percentage of PIBS is used to improve the dispersion state, there is a “flow around” effect of the contact point, resulting in poor lubrication performance. Zhou et al. ^[39][80] found that the introduction to additives forms an effective lubrication protective film on the friction surface; through ANSYS simulation and analysis of the worn surface, the local wear or micro-damaged surface can be self-repaired adaptively, thus improving the tribological performance.