Synergistic Effects of CuO and ZnO Nanoadditives on Friction and Wear in Automotive Base Oil ^†

Abstract

Efficient lubrication lowers friction, wear, and energy losses in automotive drivetrain components. Advanced lubricants are key to sustainable transportation performance, durability, and efficiency. This study analyzes the tribological performance of Group III base oil with CuO and ZnO nanoadditive mixtures. These additives enhance the performance of Group III base oils, making them highly relevant for automotive lubricant applications. An Optimol SRV5 tribometer performed ball-on-disk sliding contact tests with 100Cr6 steel specimens subjected to a 50 N force and a temperature of 100 °C. The test settings are designed to mimic the boundary and mixed lubrication regimes commonly seen in the automobile industry. During the tests, the effect of nanoparticles on friction was measured. Microscopic wear analysis was performed on the worn specimens. The results demonstrate that adding 0.3 wt% CuO nanoparticles to Group III base oil achieves a 19% reduction in dynamic friction and a 47% decrease in disk wear volume compared to additive-free oil. Notably, a 2:1 CuO-to-ZnO mixture produced synergy, delivering up to a 27% friction reduction and a 54% decrease in disk wear. The results show the synergistic effect of CuO and ZnO in reducing friction and wear on specimens. This study highlights the potential of nanoparticles for lubricant development and automotive applications.

1. Introduction

Automotive component friction and wear reduction is a major tribology challenge that affects energy efficiency, mechanical dependability, and sustainability. Wear causes 80% of moving mechanical systems to fail, accounting for 1–2% of industrialized nations’ GDP [1]. Using nanoadditives in lubricants has emerged as a promising strategy to improve tribological performance. Among these, metal oxide nanoparticles such as zinc oxide (ZnO) and Copper(II) oxide (CuO) have shown considerable potential through mechanisms like tribofilm formation, ball-bearing effects, surface polishing, and the mending effect [2,3]. CuO nanoparticles can enhance tribological properties in lubricants if dispersion is stable and concentration is optimal. ZnO shows best friction reduction at 0.5 wt% in PAO 6, while CuO performs best at 1 wt% [4]. However, ZnO’s high surface energy often results in agglomeration, reducing its effectiveness [5], whereas CuO tends to form stable tribofilms and can repair worn surfaces, extending component lifespan [3].

Surface-activated CuO nanoparticles have significantly reduced friction and wear in Group III base oils, even after thermal aging, particularly when introduced prior to aging—a property highlighting their anti-aging potential [6]. However, the primary challenge with copper-based nanoparticles lies in their propensity for agglomeration within lubricant matrices, which can lead to unstable dispersions, increased abrasive wear, and potential lubrication system obstruction. Despite their efficacy in generating protective tribofilms, copper-based nanoparticles exhibit susceptibility to agglomeration within lubricant matrices, resulting in unstable dispersion, heightened abrasive wear, and potential obstruction of lubrication systems [7]. Abdel-Rehim et al. [8] reported a 42.9% friction reduction and substantial wear improvement by dispersing 0.5 wt% of cylindrical CuO nanoparticles (35 ± 5 nm) in SAE 20W-50 mineral oil with oleic acid used as a surfactant. This influential study inspired further research into nano-ball bearings and tribosintering effects while providing measurement standards for oxide nanoparticle effectiveness in lubricants. Moreover, advancements in manufacturing, such as sonochemical methods, have further improved nanoparticle synthesis. Azam and Park [9] demonstrated that incorporating 0.1 wt% CuO nanoplatelets into 5W-40 synthetic oil reduced friction by 32%, with zeta potential results (92 mV) indicating stable dispersions.

Yeoh Jun Jie Jason et al. [10] provided a comprehensive review of nanolubricant technologies involving metals, oxides, sulfides, carbons, and alloys, highlighting the individual strengths of CuO and ZnO in lubrication—such as tribofilm formation, nano-ball bearing effects, repair, and surface polishing. The findings indicate that CuO-ZnO hybrid additives can outperform single-component systems due to synergistic tribofilm and mending effects, but these hybrids require further experimental studies to address long-term stability and real-world performance. Later studies have added to this overall framework. For example, Waqas et al. found that CuO at 0.8–1.6 wt% lowers the coefficient of friction by 13–28% and ZnO at 0.6 wt% lowers it by 22%. These oxides roll, fix, polish, and form tribofilms [11].

Environmentally friendly ZnO nanoparticle synthesis is also advancing. Gautier di Confiengo et al. [12] showed that extract type, calcination temperature, and precursors affect nanoparticle shape and dispersion. When 0.5 wt% spherical ZnO (50–200 nm) from acetate was blended with vegetable oils, the optimum wear resistance and lowest friction coefficients were observed, underlining the crucial influence of synthesis conditions on tribological performance and the sustainability of lubricant formulations. Research into nanoparticle synthesis techniques, such as electric discharge, has shown that key particle features directly impact tribological properties. Parnian et al. [13] found that smaller, oxidized CuO particles (23 nm, 54% oxide content) could reduce calcium-based grease friction from 0.028 to 0.0016, attributing this to enhanced tribofilm formation and greater surface adherence. These fundamental insights into the relationship between nanoparticle size, shape, and performance inform improvements in CuO-ZnO hybrid systems.

Recent work by Veerendra and Kumar [14] demonstrated CuO-ZnO mixed metal oxide nanoparticles (18–30 nm) in SAE 20W-50 lubricant at 0.5%, 1%, and 2% loadings. At 1%, the coefficient of friction and wear scar size were minimized, confirming synergy in tribofilm formation, surface repair, and nano-bearing effects. The addition of nanoparticles resulted in a significant reduction in friction and wear. The friction coefficient decreased from 0.119 for the base oil to 0.085 at a 2% nanoparticle concentration. Similarly, the wear scar diameter was markedly reduced from 1.744 mm (base oil) to 0.875 mm at the same level of nanoparticle loading. Notably, proper concentration optimization is key to suppressing agglomeration and maximizing performance. This study showed that concentration optimization is important for balancing performance improvement with agglomeration prevention. It also provided the first direct sign of the possibility of CuO-ZnO synergy.

New studies on the features of metal oxide nanoparticles have helped to understand how they work in various conditions. Yehia et al. found that carbon nanotubes significantly increased viscosity in water-based fracturing solutions [15]. Nano-zinc oxide (1 g/L) and nano-copper oxide (10 g/L), on the other hand, made the fluids much more stable and thicker than blank fluids. They found that controlling the shape and dispersion of CuO and ZnO is important for making them work better as lubricants and ensuring their rheological and stability qualities are at their best in harsh fluid conditions. These findings support the idea that CuO-ZnO mixtures may be helpful for more than one thing, like making things better at rubbing together, staying stable at high temperatures, and having a thicker consistency.

Discoveries in characterizing hybrid oxide materials have helped researchers understand how they can be used for multiple applications and how they can be mixed with other materials. Bakkardouch et al. [16] reported that mixed materials made from modified cellulose and ZnO and CuO nanoparticles were better at insulating and had higher thermal stability than those with CuO only. Understanding how oxides and matrices work is crucial for making stable, high-performance nanolubricant formulations.

To prove the idea of additive synergy, researchers have shown that nanoparticles, organic friction enhancers, and conventional additives can all work together. Guan et al. [17] studied how nano-biochar interacted with popular oil additives like MoDTC (molybdenum dithiocarbamate), ZDDP (zinc dialkyldithiophosphate), detergent, and dispersant. They found that nano-biochar cut down on friction by 9.4% when mixed with antiwear additives and by 4.5% when mixed with friction-lowering additives. Thampi et al. [18] noted that 0.1–0.5 wt% nanoparticle additions (CuO or ZnO) improved tribological properties for all oil types studied, via rolling, polishing, mending, and coating production, while excessive CuO (2%) elevated friction.

On the other hand, ZnO at 0.5 wt% had the opposite effect. Because they may be so helpful, composite and hybrid nanoparticles should be further investigated. This in-depth study adds to the scientific case for studying CuO-ZnO hybrids to maximize the benefits of both oxide types working together. Adding Zn and ZnO nanoparticles as extreme pressure (EP) additives in synthetic lubricants has substantially enhanced tribological performance under high-load conditions. When dispersed at optimal concentrations, especially 0.1 wt% ZnO, these nanoparticles increased the lubricant’s load-carrying capacity by up to 250% compared to the base fluid, primarily due to their higher hardness and the formation of a protective tribofilm on rubbing surfaces. Surface analyses confirmed that both Zn and ZnO nanoparticles were tribosintered onto steel surfaces, reducing surface roughness and preventing direct metal-to-metal contact, which led to lower friction and wear rates and improved the overall efficiency of mechanical components [19].

Researchers who have compared different oxide mixtures have found that hybrid metal oxide systems have much potential. In their research on CuO-TiO₂ hybrid nanofluids in gear oil, Kumar found that a mix of 0.3% CuO and 0.3% TiO₂ lowered the coefficient of friction by 19–23%. They also found that a mix of 0.1% CuO and 0.3% TiO₂ lowered wear by 23% and increased the load-carrying capacity by 89% compared to base oil. Their mechanical study showed that TiO₂ acts as nano-bearings to roll things, and CuO makes protective tribofilms. This shows how important it is to carefully optimize the ratios and distribution to get the most out of the synergistic interactions. With careful tweaking of concentration ratios and dispersion methods, these data support the idea that CuO-ZnO blends could achieve similar or even better performance through complementary mechanisms [20].

The examination of existing evidence reveals both advantages and drawbacks. Single-component nanoadditives such as CuO and ZnO yield quantifiable enhancements but have ongoing challenges: The pronounced propensity of ZnO for agglomeration reduces its efficacy, whereas the possibility for CuO to agglomerate may exacerbate abrasive wear and elevate obstruction risk. Hybrid or composite nanoparticles, combined with new synthesis techniques (e.g., sonochemical and green synthesis), mitigate specific challenges; yet, their synergistic mechanisms and long-term practical effectiveness necessitate further validation. Additional research has demonstrated that altering particle morphology and distribution enhances performance, including lubricant stability, viscosity, and high-temperature durability. Moreover, hybrid nanomaterial systems frequently surpass their single-oxide equivalents for insulation and thermal stability. The interaction between nanoparticles and prevalent lubricant additives (e.g., MoDTC, ZDDP, detergents, dispersants) substantiates the argument for synergistic approaches; however, attaining the appropriate equilibrium is precarious: for instance, an excess of CuO may elevate friction instead of diminishing it.

Although the advantages of CuO and ZnO as separate nanoadditives are established, comprehensive research on the ideal ratios, underlying synergistic mechanisms, and long-term dispersion stability of CuO-ZnO mixtures in automotive base oils is limited. Research on large-scale synthesis, environmental implications, and integration with conventional lubricant formulations under actual operating circumstances is relatively restricted. This study systematically evaluates the tribological performance, mechanistic interactions, and stability of CuO-ZnO mixtures through controlled synthesis, advanced characterization, and rigorous performance testing to establish the scientific foundation for synergistic enhancement in automotive lubrication.

This experimental investigation addresses these gaps by carefully assessing the tribological performance, mechanistic relationships, and stability of CuO-ZnO mixtures using controlled synthesis, improved characterization, and rigorous performance testing. The method utilizes sophisticated surface alterations and an extensive analysis of the mechanisms that dictate synergy. The primary objective is to elucidate the scientific concepts underlying CuO-ZnO synergy, facilitating the development of advanced high-performance automotive lubricants and enhancing sustainability, efficiency, and durability in mechanical systems.

2. Materials and Methods

For this investigation, a Group III base oil with a kinematic viscosity of 6 cSt at 100 °C (supplied by MOL-LUB Ltd., Almásfüzitő, Hungary) was selected as the base lubricant due to its widespread use in automotive engine oils. Spherical cupric oxide (CuO) and zinc oxide (ZnO) nanoparticles were employed as additives to enhance the tribological performance of the base oil. The CuO nanoparticles had an average particle size ranging from 30 to 50 nm and a purity of 99%, while the ZnO nanoparticles had an average particle size of approximately 50 nm and a purity of 97%. The bulk Vickers hardness of CuO is approximately 130–200 HV, while ZnO exhibits a significantly higher hardness of about 1000–1200 HV. It should be noted that at the nanoscale, these values can increase due to grain refinement and surface effects. Both types of nanoparticles were procured from a certified supplier.

CuO and ZnO nanoparticles were surface-modified using oleic acid as a surfactant to improve their dispersion stability in hydrocarbon-based lubricants. The surface modification procedure with oleic acid was previously validated on SiO₂ nanoparticles in a similar tribological system, as described in [21], and the same method was applied here for CuO and ZnO nanoparticles.

The preparation of the tested lubricant began by measuring the right amount of pre-surface-modified nanopowders into a beaker. A total of 6 mL of toluene was added to the dry nanopowder, and the mixture was stirred using a magnetic mixer for 1 min (200 rpm) at room temperature to reduce the size of larger bulks in the powder. Toluene helps to separate the individual nanoparticles from each other. Then, a proper amount of Group III 6 cSt base oil was added to the beaker to reach the desired 0.3 wt% nanoparticle concentration. This mixture was then mechanically stirred (1000 rpm) using a magnetic stirrer for 20 h under ambient conditions to allow complete evaporation of toluene, facilitating surface interaction between the nanoparticles and the surfactant. Final homogenization to ensure the proper initial dispersion for the tribotests was performed using an ultrasonic homogenizer at 50 °C for 15 min to eliminate residual agglomerates and achieve a stable and homogeneous nanolubricant suitable for tribological evaluation. Five lubricant formulations were prepared for comparative testing:

• Pure Group III base oil (6 cSt) as the reference lubricant.
• Base oil with 0.3 wt% CuO nanoparticles.
• Base oil with 0.2 wt% CuO and 0.1 wt% ZnO.
• Base oil with 0.1 wt% CuO and 0.2 wt% ZnO.
• Base oil with 0.3 wt% ZnO nanoparticles.

The lubricant formulation steps are shown in

Table 1. Steps of the lubrication formation.

Step	Task	Duration	Temperature [°C]
1	Add nanoparticles	-	21
2	Add toluene	-	21
3	Mechanical mixing (200 rpm)	1 min	21
4	Add base oil	-	21
5	Mechanical mixing (1000 rpm)	20 h	21
6	Utrasonication	15 min	50

.

Each formulation was evaluated using a tribological testing protocol on an Optimol SRV^®5 tribometer (Optimol Instruments GmbH, Munich, Germany), operated in the Tribological Laboratory of the Department of Propulsion Technology, Széchenyi István University, Győr, Hungary. The tribometer was configured in ball-on-disk mode in compliance with ISO 19291:2016 standards [22]. The ball specimens, made of 100Cr6 bearing steel (1.3505), were 10 mm in diameter, polished to an Ra value of 0.025 ± 0.005 µm, and exhibited a hardness of 60 ± 2 HRC. The disk specimens (24 mm diameter × 6.9 mm thickness) underwent vacuum arc remelting, spheroidizing annealing, and subsequent grinding and lapping to reach a surface roughness of Ra = 0.035–0.05 µm and hardness of 62 ± 1 HRC.

Before testing, all specimens were ultrasonically cleaned at 50 °C for 15 min using brake disk cleaner to eliminate surface contaminants and ensure identical initial conditions. The tribological test cycle consisted of three sequential stages:

• preheating at 100 °C under a 50 N preload,
• a 30 s run-in phase at 50 N to promote tribofilm formation, and
• a 2 h steady-state friction test at 150 N.

Sinusoidal oscillation motion was applied (1 mm stroke at 50 Hz), and the temperature was maintained at 100 °C throughout. An external oil circuit, preheated and regulated via a peristaltic pump at 225 mL/h, was integrated to provide consistent lubricant flow and prevent localized overheating at the contact interface. The tribometer recorded the average coefficient of friction in real time with a 1 s data logging interval.

Post-experiment surface characterization of the wear scars was performed using a confocal microscope (Leica DCM 3D, Leica Camera AG, Wetzlar, Germany), a scanning electron microscope (Hitachi S-3400N, Tokyo, Japan), and its energy dispersive X-ray spectroscope (EDX). All tribological and surface analytical tests were performed with four repetitions. All recorded data were processed in Microsoft Excel using the AVERAGE() and STDEV() functions to determine mean values and standard deviations. The graphical representation of the results was also generated using Excel.

2.1. Test Setup

The capacity of this setup to simulate the lubrication regimes and contact mechanics encountered by actual machine parts is its main selling point. A 10 mm diameter, a hardness grade of about 60 HRC, and an ultra-smooth surface roughness of Ra = 0.035 µm characterize the test balls made of 100Cr6 bearing steel. Disks made of 100Cr6 steel also underwent the same vacuum arc remelting process, leaving them with a final hardness of 62 HRC and relatively low surface roughness. The balls and disks were thoroughly cleaned and soaked in naphtha using an ultrasonic cleaner set to 50 °C for 15 min before testing. By removing all potential sources of contamination, this crucial step rendered the test results completely reliable. The tribological data produced are reproducible and accurately represent the harsh operating conditions encountered in contemporary industrial and automotive gear because of these meticulously controlled processes.

The process (as shown in Figure 1) of conducting a tribological test consists of three stages. As a first stage, preheating ensures that the specimens and lubricant will be in thermal equilibrium by heating them to 100 °C with a 50 N preload. Following that, a short period of consistent temperature and load is known as running-in. This step involves using a low-amplitude oscillating motion to create a stable tribofilm on the contact surfaces. Keeping the temperature at 100 °C, delivering a greater normal load (often 100 or 150 N), and running the test at 50 Hz with a 1 mm stroke (as shown in Figure 2) constitute the meat of the testing. The external oil circulation system prevents overheating in certain regions while evenly distributing preheated lubricant to the contact at a controlled flow rate of 225 mL/h, mimicking real-world continuous lubrication.

2.2. Friction Evaluation

Laboratory friction testing must accurately record both the instantaneous fluctuations and cumulative frictional behavior in lubricated contacts to resemble real-world operating conditions. Optimol SRV^®5 tribometers constantly measure the static friction (COF) during each test using a 25 kHz high-frequency data collecting system. The extremely high sampling rate allows for the simultaneous recording of steady-state frictional properties and the exact detection of transient phenomena, such as micro-slip events, boundary layer transitions, and asperity interactions. The friction absolute integral (FAI) is an additional analytical metric used in the lab in addition to the standard COF metrics. By considering the amount and duration of friction, the FAI measures the total energy dissipation across the sliding distance, offering a more realistic assessment of tribological performance. This dual-metric approach (COF + FAI) bridges the gap between controlled laboratory conditions and the complex transient regimes encountered in automotive transmissions, bearing assemblies, and other critical tribosystems, delivering unprecedented resolution in characterizing dynamic friction behavior (see Figure 3).

For each oscillation cycle, this technique averages the friction during the static and dynamic stages by combining the effects of the boundary, mixed, and hydrodynamic lubrication regimes into a single number. The FAI is excellent for comparing the frictional performance of different lubricant formulae and discovering minor improvements caused by additions, since it records the entire energy dissipated by friction during the test [23]. To exclude unstable, non-steady states from the evaluation, the average of the comparative COF and FAI values was taken throughout the last 1000 s of the test. All analyses were conducted consistently in this way. When used together, COF and FAI provide a comprehensive view of peak and average frictional reactions, essential for conducting reliable tests on advanced lubricants.

2.3. Wear Evaluation

Test specimens have been measured for their wear volumes using a Leica DCM 3D confocal microscope. By scanning both the worn and unworn sections, this device accurately calculates the volume loss by comparing the average height of the unworn surface to the depth of the wear track. This volumetric method provides a more reliable depiction of wear than linear measurements. Both the ball and the disk specimens were fully digitized using this microscope. The wear volume measured on the ball is referred to as the ball wear volume (BWV), while that on the disk is denoted as the disk wear volume (DWV). Figure 4 shows a height map of one of the recorded wear marks from the disk. The sum of the two yields the total wear volume (TWV), providing a comprehensive assessment of the material loss within the tribosystem. Due to the design of the tribometer, the ball material is slightly softer than the disk to prevent potential damage to the system; as such, the specimen pairing may offer limited insight into the effect of material hardness on wear behavior. In particular, the BWV indicates the ball’s wear resistance; due to the ball’s spherical geometry, a higher BWV corresponds to a larger wear scar diameter. This, in turn, reflects the contact pressure conditions and the pressure resistance of the tribosystem, where a larger BWV implies reduced load-carrying capacity.

2.4. Scanning Electron Microscopy and Elemental Surface Analysis

To gain insight into the tribological mechanisms at play, worn surface morphologies were examined using both optical microscopy and scanning electron microscopy (SEM). SEM imaging enabled high-resolution observation of surface features, including signs of abrasion, adhesion, and potential tribofilm development. Additionally, SEM analysis allowed the detection of residual nanoadditives on the wear scars, offering evidence of additive interaction with the surface.

This study conducted SEM investigations on the disk specimens, focusing on the worn tracks. Imaging was performed at 1000× magnification, using a 10 kV accelerating voltage, a working distance of approximately 10 mm, and in secondary electron (SE) mode to highlight topographical features. For each wear scar, three characteristic locations were selected for detailed analysis, based on the tribological relevance of contact pressure and relative sliding velocity (see Figure 5).

• The first point was located at the dead center (DC) of the wear track, i.e., along the central axis in the direction of motion where the ball reverses direction. This point corresponds to the minimum relative velocity and maximum static contact duration.
• The second point (WTHP) was at the geometrical center of the wear scar, along the central line of motion. The contact pressure and sliding velocity are at their highest, making this location critical for understanding dynamic wear processes.
• The third point (WTLP) was selected at the outer edge of the wear scar, where relative sliding velocity is maximal, but contact pressure is minimal.

These three regions were selected because they allowed the combined assessment of the two most relevant factors influencing tribofilm formation—contact pressure and relative sliding speed—under otherwise constant conditions, thus providing insight into the wear mechanisms and how these parameters affect the role and effects of the nanoparticles in the tribological system. At each selected location, energy-dispersive X-ray spectroscopy (EDX) was used to perform two types of analyses:

• Elemental mapping, to visualize the spatial distribution of elements associated with tribofilms or transferred material;
• Quantitative elemental composition, providing the atomic percentage of all detected elements, particularly those related to the additive nanoparticles (O, Cu, and Zn) and boundary layers (Fe, Cr, Si, O, and C).

This comprehensive microstructural and compositional characterization allowed for the correlation of observed wear features with lubricant composition and further validated the additive behavior under boundary lubrication conditions.

3. Results

The tribological performance of the Group III base oil containing various concentrations of CuO and ZnO nanoparticles was evaluated by analyzing frictional behavior and wear characteristics. The assessment involved measuring the dynamic and static coefficients of friction and the wear volumes of both tribological counterparts. Specifically, ball wear volume (BWV), disk wear volume (DWV), and total wear volume (TWV) were determined using 3D confocal microscopy.

The experimental results demonstrate apparent friction reduction and material loss trends, with notable synergistic interactions observed between CuO and ZnO nanoadditives. These interactions affected not only the overall wear resistance of the tribosystem but also the distribution of wear between the contact surfaces. The combination of these two oxides showed a more pronounced effect than either additive alone, suggesting beneficial mechanisms such as complementary tribofilm formation, particle rolling, or protective layer stabilization. Detailed quantitative results are presented in the following sections, accompanied by comparative analysis and graphical representations of the measured parameters.

3.1. Friction Results

Frictional behavior was first evaluated using the friction absolute integral (FAI), representing the cumulative dynamic friction over the entire test duration (see results in Figure 6). The results indicate that adding CuO nanoparticles significantly reduced friction compared to the additive-free reference. Even the lowest tested concentration of CuO led to a marked reduction, with a 0.3 wt% CuO addition decreasing the FAI by approximately 19%. This effect was further enhanced by partially substituting CuO with ZnO nanoparticles. A 2:1 CuO-ZnO mixture resulted in the highest observed reduction of 27%, while a 1:2 CuO-ZnO ratio achieved a 24% decrease. In contrast, ZnO nanoparticles alone had minimal effect, reducing the FAI by only 1%, indicating their limited friction-reducing potential in this configuration.

A similar curved trend was observed in static friction measurements. The pure CuO nanoadditive reduced static friction by 15%, while ZnO caused a slight 1% increase. However, the combined application of both oxide nanoparticles also proved beneficial. The 2:1 CuO-ZnO mixture showed a synergy, achieving the most favorable result, yielding a 17% reduction in static friction relative to the reference.

These findings confirm that CuO nanoparticles consistently exhibit the expected friction-reducing effect in this tribological system, whereas ZnO nanoparticles alone do not significantly influence frictional behavior. Importantly, a partial substitution of CuO with ZnO—specifically, a one-third replacement—improved performance in static and dynamic friction scenarios. This suggests a synergistic interaction between the two oxides, enabling optimized tribological characteristics through careful compositional tuning.

3.2. Wear Volume Results

Wear volume analysis revealed clear and distinct trends in how the nanoadditives influenced material loss on the disk (slightly harder) and ball (slightly softer) specimens. The disk wear volume (DWV) followed a similar trend to the friction results. Using 0.3 wt% CuO nanoparticles alone reduced the DWV by 47% compared to the additive-free reference, demonstrating adequate surface protection. In contrast, adding ZnO nanoparticles alone increased the DWV by 20%, indicating a detrimental effect under the given test conditions. Notably, the 2:1 CuO-ZnO mixture outperformed the pure CuO containing lubricant, reducing the DWV by 54%, marking the lowest disk wear observed in the study (see Figure 7).

However, the ball wear volume (BWV) results showed a markedly different trend. While CuO nanoparticles slightly reduced BWV by 9%, ZnO had a pronounced adverse effect. Due to its high hardness relative to the steel ball, the presence of ZnO nanoparticles, especially at higher concentrations, resulted in substantial abrasive wear on the ball surface. Both mixtures (2:1 and 1:2 CuO-ZnO) increased BWV by 48–49%, while pure ZnO led to a drastic 180% increase. These findings suggest that ZnO, despite its potential for surface reactivity, exhibits significant abrasiveness, even when interacting with similarly hard counterfaces, without adequate film-forming components.

When aggregating wear data through the total wear volume (TWV)—calculated as the sum of BWV and DWV—a comprehensive picture of system-level wear performance emerges. The pure CuO formulation showed the most substantial wear reduction, achieving a 33% decrease in TWV relative to the reference. The 2:1 and 1:2 CuO-ZnO mixtures also offered measurable benefits, with 16% and 12% reductions, respectively. However, no apparent synergistic effect on wear behavior could be identified in these combinations. Although frictional improvements were observed when ZnO was partially introduced, the overall wear performance remained superior with CuO alone.

These findings underscore that CuO nanoparticles are highly effective in reducing wear across both components, while ZnO requires careful control due to its abrasive potential, particularly in systems lacking sufficient film-forming additives. The absence of wear synergy in the nanoparticle mixtures highlights the complexity of multi-nanoparticle systems and the importance of material pairing and surface compatibility in achieving tribological performance improvements.

3.3. Scanning Electron Microscopy Results

SEM analysis of the worn disk surface lubricated with the Group III base oil revealed typical abrasive wear features across all regions (see Figure 8). Fine and parallel grooves dominated the surface at the outer edge (WTLP—high sliding speed, low contact pressure), indicating mild abrasion with no evidence of tribofilm formation. The dead center region (low speed, high pressure) showed localized material transfer and slight adhesion-related features, likely due to prolonged static contact. The central zone, characterized by high contact pressure and high speed, exhibited the most severe wear, including deep grooves, micro-cracks, and signs of plastic deformation, indicating a combination of abrasive and adhesive wear mechanisms under boundary lubrication conditions.

The reference sample’s EDX spectrum (Figure 9) reveals the presence of Fe, Cr, Si, C, and O elements, which correspond to the base elements and alloying components of the 100Cr6 bearing steel. The detected carbon and oxygen signals may also partially originate from the base oil residue or tribo-induced surface oxidation. No foreign elements related to nanoadditives were observed, confirming the sample’s additive-free nature.

SEM and EDX analysis were performed at the wear scars’ dead center (DC) to evaluate CuO and ZnO nanoparticles’ distribution and tribological role in various lubricant formulations (see Figure 10). The images revealed a clear transition in wear severity and surface topography as the ratio of CuO to ZnO decreased.

In the case of the pure CuO formulation (0.3 wt%), the wear surface exhibited shallow, parallel grooves typical of mild abrasive wear. EDX mapping showed a dense and homogeneous distribution of Cu, suggesting the formation or presence of a tribofilm composed of elemental copper due to triboreduction [24], likely contributing to the observed wear protection. Wear marks with the 2:1 CuO-ZnO mixture became slightly rougher, with more localized material displacement. Cu remained the dominant element, although Zn presence increased, appearing more scattered. This suggests partial incorporation of ZnO without entirely disrupting the tribological benefits provided by CuO, indicating limited synergy. For the 1:2 CuO-ZnO formulation, surface degradation was more pronounced, with deeper grooves and signs of adhesive interactions. Zn became the dominant species, while Cu signals weakened significantly. The Zn distribution was more uniform, but the absence of an effective tribofilm structure corresponded with worsened wear performance. Finally, the pure ZnO sample (0.3 wt%) exhibited the most severe wear, characterized by fractured surfaces, irregular debris accumulation, and deep abrasive scars. While Zn was highly concentrated across the wear track, it did not appear in cohesive film-like structures. Instead, it likely acted as an abrasive due to particle agglomeration or poor dispersion stability.

The results confirm that CuO nanoparticles form effective protective tribofilms, especially under static high-pressure contact conditions. In contrast, ZnO lacks film-forming ability under these conditions, and its presence, particularly in higher proportions, correlates with increased surface damage. The transition from copper-dominant to zinc-dominant systems reflects a shift from protective to abrasive wear behavior.

To assess tribofilm behavior under the most severe operating conditions, SEM and EDX mapping were conducted at the wear scar’s central region, where contact pressure and sliding speed are at their maximum (WTHP). Figure 11 shows that the topographical and elemental trends confirm that additive composition plays a decisive role in surface protection at this location.

In the case of 0.3 wt% CuO, the surface exhibited regular abrasive grooves with moderate wear severity. EDX analysis revealed a homogeneous and widespread distribution of copper, particularly along the edges of wear grooves and material flow lines. This suggests the formation of a tribochemically anchored Cu-based film that mitigates wear under high stress. In the 2:1 CuO-ZnO formulation, the WTHP region showed a notably high Cu signal, indicating the presence of a Cu-rich tribofilm. Flattened, smoother areas were visible, suggesting regions covered by Cu-induced tribofilms, which are known to promote lower friction (FAI). While primary abrasion grooves were still observable, particularly in the upper part of the image, signs of self-repairing effects could also be identified, where the tribofilm partially overlaid the abrasion grooves. The distribution of the tribofilm was less uniform across the width of the wear track but showed more pronounced extension along the sliding direction.

In contrast, in the 1:2 CuO-ZnO mixture, more intense abrasive features and some crater-like fatigue damage began to appear. Importantly, when interpreting the behavior of CuO-ZnO mixtures, it must be considered that tribofilm quantity does not necessarily equal quality; even though a higher Cu signal may be present, the resulting tribofilm may be thinner, patchier, and less coherent. Additionally, ZnO particles can increase local abrasive stress, damaging the surface despite higher Cu incorporation. Cu may accumulate in localized clusters at higher ZnO ratios rather than forming a uniform protective layer. Overall, the interplay between oxides is complex: chemical film formation and mechanical abrasion occur simultaneously, and their combined effect is not necessarily additive.

The 0.3 wt% ZnO formulation displayed the most severe wear morphology, characterized by deep cracks, delamination zones, and massive surface damage. While Zn was highly concentrated across the surface, it was present in a fragmented, non-film-forming pattern. These features suggest abrasive behavior caused by particle agglomeration or unstable surface interactions.

In summary, CuO-containing systems could form effective tribofilms under extreme contact conditions. As the ZnO ratio increased, the surface protection deteriorated, and wear severity increased. The findings underline that CuO is essential for maintaining tribological stability, while ZnO alone or in excess disrupts film integrity and increases abrasive wear.

SEM and EDX investigations are shown in Figure 12 from the outer edge of the wear scar (WTLP), where sliding velocity is at its maximum and normal load is minimal. This region is typically characterized by lower mechanical stress but higher shear rates, making it sensitive to film stability and additive dispersion.

In the case of 0.3 wt% CuO, the surface showed fine, regular abrasion marks without signs of severe damage. Copper was uniformly distributed across the scanned area, indicating good dispersion of the CuO nanoparticles and the likely presence of a thin tribofilm even under low-pressure conditions. This supports the stability of the Cu-based tribofilm across the entire wear track. The surface roughness of the 2:1 CuO-ZnO formulation increased slightly, and some localized disruptions became visible. While copper remained dominant, the zinc signal began appearing primarily as isolated clusters rather than continuous layers. The partial overlap between Cu and Zn may indicate competition for surface adsorption sites, with Cu still providing a degree of surface protection. When testing the 1:2 CuO-ZnO sample, the wear track displayed more pronounced micro-abrasive features, with shallower but less uniform grooves. Copper was sparsely distributed, while zinc was dominant across the surface. However, the Zn signal again lacked coherence, showing no signs of organized film formation, suggesting ZnO particles remained dispersed or aggregated, contributing little to surface protection. The pure ZnO (0.3 wt%) sample revealed a rough and irregular surface, with evident abrasion and micropitting. EDX mapping showed dense but chaotic Zn distribution, likely due to particle accumulation and incomplete dispersion. As with previous regions, no tribofilm-like morphology was observed, and the surface appeared heavily affected by unprotected shear contact.

In summary, CuO additives ensured consistent surface coverage and film stability, even in low-pressure, high-speed regions. As the proportion of ZnO increased, the protective effect diminished, and the nanoparticles tended to agglomerate rather than integrate into the boundary layer. ZnO alone proved ineffective in forming a stable, shear-resistant layer under these conditions.

Figure 13 compares EDX spectra in the Cu (Lα = 0.93 keV) and Zn (Lα = 1.01 keV) energy regions for five lubricant formulations tested at different points along the wear scar. The spectra are color-coded:

• black—reference (additive-free),
• red—0.3 wt% CuO,
• orange—0.2 wt% CuO + 0.1 wt% ZnO,
• light green—1:2 CuO-ZnO,
• dark green—0.3 wt% ZnO.

The spectral data correlate well with the spatial element maps shown earlier. As expected, no Cu or Zn peaks are observed in the reference sample, validating the additive-free condition. The pure CuO formulation (red) shows a distinct Cu peak with no Zn presence, confirming selective incorporation of copper species. Interestingly, in the case of nanoparticle mixtures, the Cu signal intensity increases compared to pure CuO, suggesting that the presence of ZnO enhances CuO incorporation into the tribological interface. The 2:1 and 1:2 CuO-ZnO mixtures exhibit comparable peak intensities for Cu and Zn, supporting the similarity in their observed tribological performance (friction and wear volume). However, the increase in Cu content in the presence of Zn appears to be a key factor in improved film formation and surface protection. This points to a synergistic effect where Zn facilitates the deposition or retention of Cu at high-pressure regions, without excessively promoting Zn incorporation itself. In contrast, the pure ZnO formulation (dark green) exhibits a strong Zn peak and the absence of Cu, consistent with high Zn incorporation but lacking any balancing effect from CuO. This supports earlier findings of severe abrasive wear, as excessive Zn-containing accumulations lead to unstable and damaging particle-surface interactions without a Cu-based film.

These results reinforce that tribofilm composition and structure are strongly influenced by additive interactions. ZnO alone behaves abrasively, but when co-applied with CuO, it contributes to controlled CuO deposition and limits ZnO overaccumulation. The observed synergy extends beyond friction reduction, manifesting in wear mechanism modulation and nanomaterial integration into the tribofilm.

Figure 14 shows the atomic percentages of Cu and Zn detected at three characteristic regions (DC, WTHP, WTLP) for each lubricant formulation. The reference sample showed negligible Cu and Zn, as expected.

In the 0.3 wt% CuO formulation, Cu levels ranged from 1.26% to 1.41%, with no Zn detected. This confirms consistent Cu integration and effective tribofilm formation. The 2:1 CuO-ZnO mixture exhibited Cu (0.50–1.84%) and Zn (0.82–1.38%), with some Cu values exceeding those in pure CuO. This suggests that ZnO nanoparticles promote Cu incorporation, while CuO presence controls Zn levels in the tribofilm, indicating synergy. In the 1:2 CuO-ZnO formulation, Cu content was lower and more variable, while Zn remained moderate. The reduced Cu limits tribofilm effectiveness, yet Zn is stabilized, reflecting weaker synergy. Cu was absent in the 0.3 wt% ZnO sample, and Zn levels were highest (up to 4.02%), correlating with the most severe abrasive wear and lack of protective film. These results confirm that ZnO enhances Cu deposition but requires Cu to moderate its incorporation. The best tribological performance aligns with the 2:1 CuO-ZnO ratio, where both elements are balanced within the boundary layer.

4. Discussion and Conclusions

This study demonstrates that combining CuO and ZnO nanoparticles in lubricating oil formulations significantly enhances the tribological performance of Group III base oils. The results confirm a clear synergy between the two metal oxides: CuO predominantly contributes to wear reduction through stable tribofilm formation, while ZnO reduces friction and improves dispersion stability through nano-rolling and tribosintering effects. Among the tested formulations, the 2:1 CuO-ZnO ratio offered the most balanced tribological performance, reducing average FAI by up to 27% and static friction by 17% relative to the reference oil, while achieving a 54% wear reduction on the disk surface (DWV).

Elemental analysis of the tribofilm composition by EDX supports these functional roles: the Cu content within the wear scar region correlated positively with reduced friction and improved wear protection, while Zn presence was associated with smoother friction profiles but often accompanied by increased localized wear. This suggests that partial substitution of CuO by ZnO is feasible, but with trade-offs. Specifically, ZnO incorporation reduces the protective Cu-rich tribofilm density, increasing wear on sacrificial components. Therefore, this strategy may be ideal for systems where one component must be preserved at all costs, and the other can tolerate increased wear or be more easily replaced. Compared to the literature, these findings reinforce and refine several established trends:

• The tribofilm-forming role of CuO is again confirmed, aligning with other research works [9], where CuO lowered wear by over 40% [8].
• The observed friction reduction is similar in scale to previous ZnO-based findings, where up to 22–23% reductions were achieved [11,19], though here they were maximized only when ZnO was combined with CuO.
• The synergistic behavior noted in Veerendra and Kumar’s study [14] and hypothesized in major reviews [10,20] is validated and quantified with a practical formulation.

Importantly, this study addresses gaps in the literature regarding the need for systematic studies on CuO-ZnO mixtures, including ratio optimization and real-surface analysis. Unlike many earlier works focused on friction alone, this investigation incorporates dynamic friction metrics and direct wear volume measurement on dual counterfaces, providing a more complete tribological picture. The results also show that ZnO reduces friction and may enhance CuO deposition; however, the exact mechanism behind this potential cross-effect remains hypothetical. Based on the observed microstructural features and supporting literature [15,16], it can be speculated that ZnO, through its nano-rolling and heat dissipation effects, modifies local contact conditions in a way that facilitates CuO triboreduction and the incorporation of Cu into the tribofilm. Further studies will be needed to confirm this mechanism. To capitalize on these findings, future research should

• investigate the surface modification of CuO and ZnO to prevent agglomeration and enhance dispersion stability;
• study the tribochemical pathways that govern synergistic tribofilm formation, especially in Zn-rich environments;
• evaluate the long-term performance and oxidative stability of these mixtures under thermomechanical stress;
• explore integration with existing additive packages, as compatibility and cumulative effects are not yet well understood [17,19]; and
• extend testing to realistic engine and aerospace scenarios, where fluctuating loads and temperatures require advanced lubricating behavior.

In conclusion, the optimized 2:1 CuO-ZnO formulation demonstrates a compelling balance between friction reduction and wear protection. Its tunable synergy allows strategic prioritization of component preservation versus overall efficiency. This makes CuO-ZnO mixed nanoadditives promising candidates for next-generation lubricants that meet the rising demands of energy-efficient, high-durability, and thermally stable systems in transportation and machinery.