Hybrid Additives of 1,3-Diketone Fluid and Nanocopper Particles Applied in Marine Engine Oil

Abstract

The lubrication performance of the cylinder liner–piston ring (CLPR) is crucial for the energy efficiency and operating reliability of marine diesel engines. To enhance the boundary lubrication of marine engine oil, a 1,3-diketone fluid HPTD (1-(4-hexylphenyl) tridecane-1,3-dione, HPTD) was introduced as an ash-free friction modifier. Besides that, octadecylamine-functionalized nanocopper particles (ODA-Cu) were also added to the marine oil to improve its anti-wear behavior. Through cylinder-on-disk friction tests, the appropriate contents of HPTD and ODA-Cu were determined, which then formed hybrid additives and modified the engine oil. The tribological performance of the modified oil was analyzed under various normal loads, reciprocating frequencies, and testing temperatures. Based on the synergy of the tribochemical reaction of HPTD and the mending effect of ODA-Cu on the sliding surface, the modified oil not only had lower sulfated ash content but also exhibited superior lubrication performance (i.e., reduced coefficient of friction by 15%, smaller wear track by 43%, and higher maximum non-seizure load by 11%) than the pristine engine oil. The results of this study would be helpful for the design of novel hybrid eco-friendly additives for marine engine oil.

1. Introduction

Concerning the increasing attention to environmental protection, more and more new energy technologies, such as solar panels, wind generators, rigid and soft sails, batteries, and fuel cells, have been applied as the power source of marine vessels [1,2]. However, diesel engines will still play an important role in water transportation for decades due to their good reliability and wide applicability [3]. To further enhance their fuel economy and lower CO₂ emissions, the lubricating oil of internal combustion engines is evolving toward lower viscosity due to its better fluidity and cooling efficiency [4]. This low-viscosity tendency is more prominent in vehicle engines than in marine engines, because the latter are operated under harsher conditions [5]. For instance, the cylinder liner–piston ring (CLPR), one of the key frictional components in marine diesel engines, is in the boundary lubrication regime and suffers high temperatures, high pressures, and variable loads [6]. Therefore, novel highly efficient lubricating additives are expected to be explored to enhance the boundary lubrication of CLPR, especially toward the trend of low-viscosity engine oils [7,8].

Superlubricity, characterized by an ultra-low friction coefficient below 0.01, is recognized as a promising approach for enhancing the energy efficiency and material durability of mechanical systems [9,10]. A new synthetic 1,3-diketone lubricant could achieve superlubricity on steel surfaces through a combination of the tribochemical reaction and the formation of molecular layers [11]. The detailed mechanism of 1,3-diketone is schematically illustrated in the supporting information (Figure S1). Significantly, 1,3-diketone, composed solely of carbon, hydrogen, and oxygen, exhibits superior environmental compatibility compared to conventional additives (e.g., MoDTC and ZDDP) containing sulfur or phosphorus. Notably, 1,3-diketone also demonstrates outstanding anti-spreading behavior, facilitating the establishment of stable and durable lubrication [12]. Furthermore, our previous study has demonstrated the efficacy of oily 1,3-diketone as an additive in fully formulated precision instrument oil and air compressor oil in terms of friction reduction and extreme pressure capacity enhancement [13]. It should be noted that during the running-in stage, the tribochemical reaction between diketone lubricants and steel surfaces inevitably induces a certain degree of wear [13]. Therefore, when considering its application as a friction modifier in marine lubricants, it is essential to incorporate anti-wear additives to mitigate this initial wear and ensure long-term protective performance.

Complementary to 1,3-diketone additives, nano-additives exhibit remarkable anti-wear properties through mechanical processes such as the rolling effect [14], the mending effect [15], the polishing effect [16], and protective films [17]. Additionally, nano-additives improve the extreme pressure capacity of lubricant and facilitate the absorption of heat generated by friction, thereby reducing thermal wear [18]. Among various nano-additives, Cu is widely recognized as a representative soft metal due to its exceptional ductility [19,20]. Importantly, nanocopper demonstrates superior tribological performance in engine oils, particularly for moving parts subjected to high temperatures and alternating loads [21]. The uniform dispersion of nanoparticles within the lubricant plays a significant role in achieving optimal tribological performance [22]. To prevent the adverse effects of nanoparticle agglomeration, nanoparticles must be functionalized via various methods. For instance, Cu modified with diisooctyl dithiophosphoric acid was synthesized and utilized as nano-additives, showing excellent dispersion stability in polyalphaolefin oils [23].

This study proposes the use of 1,3-diketone fluid and nanocopper particles as hybrid additives for marine engine oil. A commercial marine engine oil 3015 was selected due to its relatively low viscosity (SAE 30 grade), in which the function of the lubricating additives would be more significant. Viscosity, sulfated ash content, and friction tests were conducted to determine the appropriate composition of the hybrid additives in 3015. Then the tribological performance of the modified 3015 was analyzed under various normal loads, reciprocating frequencies, and testing temperatures. Combined with the extreme pressure test and surface analysis on the worn surfaces, the lubrication mechanism of the hybrid additives in marine engine oil was discussed as well.

2. Materials and Methods

2.1. 1,3-Diketone Fluid

The 1,3-diketone fluid HPTD (1-(4-hexylphenyl) tridecane-1,3-dione, HPTD) was synthesized via a Claisen condensation reaction (Figure 1). Briefly, 2.65 g sodium ethoxide (Sigma-Aldrich, St. Louis, MO, USA) was dissolved in 33 mL ethyl undecanoate (Sigma-Aldrich) at −5 °C. Upon the formation of a yellow solution, 7 mL 4′-N-Hexylacetophenone (Sigma-Aldrich) was added dropwise. The reaction mixture was maintained at 0 °C for 12 h with continuous stirring, during which the color transitioned to brown. Subsequently, 40 mL of deionized water was added. The organic phase was separated via a separatory funnel, while the aqueous layer was extracted three times with 40 mL of ethyl acetate (Sigma-Aldrich). The collected organic layers were combined with the original organic phase and dried over sodium sulfate (Aladdin, London, UK), followed by solvent removal under rotary evaporation. After the purification via chromatography, employing n-hexane:ethyl acetate = 9:1 as an eluent, 4.96 g of HPTD was isolated.

To confirm the successful synthesis of HPTD, Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, Thermo Fisher, Waltham, MA, USA) and nuclear magnetic resonance spectrometry (NMR, Bruker AVANCE III, Billerica, MA, USA) were conducted on the obtained product after the reaction. The rheological properties of HPTD were characterized using a rotational rheometer (MCR102, Anton Paar, Graz, Austria) equipped with parallel-plate geometry (diameter = 50 mm, gap = 1 mm).

2.2. Functionalized Nanocopper Particles

To improve the dispersion stability of nanocopper particles in oil, octadecylamine-functionalized nanocopper particles (ODA-Cu) were synthesized using the Brust-Schiffrin method (Figure 2) [24]. Concisely, 375 mg copper (II) sulfate (Sinopharm Chemical Reagent, Shanghai, China) and 750 mg sodium octanoate (Sinopharm Chemical Reagent) were dissolved separately in 5 mL of distilled water and mixed to form a blue suspension, which was maintained at 30 °C. Then, 10 mL of dichloromethane (Sinopharm Chemical Reagent) was introduced and the mixture was stirred for 10 min before removing the aqueous phase to yield the Cu precursor solution. Subsequently, 20 mL of dichloromethane containing 1.9 g of octadecylamine (Sinopharm Chemical Reagent) and 8 mL of distilled water containing 0.18 g of NaBH₄ (Aladdin) were added to a three-necked flask. The mixture was stirred at 10 °C for 30 min. The Cu precursor solution was then slowly dripped into the three-necked flask and the reaction proceeded at 10 °C for 1 h with vigorous stirring. After completion, the aqueous phase was removed and ethanol was added to form a precipitate. Finally, the brownish-black ODA-Cu was collected by centrifugation (7000 rpm, 5 min), washed with ethanol (Aladdin), and vacuum-dried at 60 °C overnight.

To confirm the surface modification of nanocopper particles, FTIR was performed on both pure ODA and ODA-Cu samples. The influences of surface modification on the particle thermal stability were studied using a thermal gravimetric analyzer (STA449F3, NETZSCH, Selb, Germany). Additionally, transmission electron microscopy (TEM, JEM-1400Plus, JEOL, Tokyo, Japan) was utilized to analyze particle surface morphology and the particle size distribution was quantified using dedicated image analysis software.

2.3. Lubricating Oils

A commercial marine engine oil (3015, PetroChina Lubricating Oil Company, Beijing, China, SAE 30 grade) was selected as the lubricating oil to be modified. To explore the influence of HPTD and ODA-Cu additives on 3015, a series of 3015-based composite oils containing either HPTD, ODA-Cu, or both was synthesized.

Owing to the excellent oil solubility of HPTD, a homogeneous 3015+HPTD composite oil was successfully prepared through 30 min of ultrasonic treatment. Subsequently, 3015+HPTD containing varying HPTD mass fractions ranging from 1% to 9% (w/w) were systematically prepared for experimental characterization. Dynamic viscosity–temperature curves were determined under a constant shear rate of 10 s⁻¹ across a temperature range of 40–180 °C. Additionally, to ensure that the incorporation of HPTD maintains the viscosity grade of the pristine 3015 oil, kinematic viscosity tests were performed in accordance with ASTM D445 [25].

While the evaluation of HPTD’s influence on the viscosity of 3015 was a critical aspect of this study, the assessment of ODA-Cu additives requires a yet equally important consideration. Since Cu is a heavy metal, its use as an additive in engine oil must be evaluated for its impact on the sulfated ash content, which affects both engine performance and pollutant emissions. Therefore, ODA-Cu was first dispersed in dichloromethane using a 10 min ultrasonic dispersion process, followed by incorporation into 3015 during 30 min of continuous ultrasonication. After removing dichloromethane through vacuum drying, composite oils of 3015+ODA-Cu with mass concentrations ranging from 0.1% to 0.5% were obtained. The sulfated ash content of these blends was determined following the ASTM D874 standard procedure [26].

Following these individual assessments of viscosity and sulfated ash content, the combined formulation (3015 composite oil with HPTD and ODA-Cu) was developed and evaluated for its viscosity–temperature behavior, sulfated ash content, and dispersion stability.

2.4. Tribological Tests

Friction tests were conducted using a reciprocating cylinder-on-disk configuration (SRV-III, Optimal Instruments, Chorley, UK) integrated with real-time friction coefficient (COF) monitoring (Figure 3a). The temperature was precisely controlled by heating the test assembly. The tribo-pair specimens, fabricated from 100Cr6 bearing steel, included a cylinder (15 mm diameter × 22 mm length, Sa = 0.071 µm) and a disk (24 mm diameter × 7.9 mm height, Sa = 0.074 µm). In the experimental setup, the cylinder is moved to the sliding direction by around 10°. Before testing, all specimens were washed with ethanol and thoroughly dried. A controlled volume of 30 μL of lubricant was applied to the cylinder surface. In the present work, 150 N normal load, 30 Hz reciprocating frequency, 2 mm stroke length, and 150 °C testing temperature were established as typical operating conditions to evaluate the lubricating performance of 3015 oil with different HPTD and ODA-Cu contents. A parametric study was conducted by systematically varying one parameter at a time within its designated range while maintaining other parameters constant at baseline values to isolate individual effects on lubrication performance.

The extreme pressure (EP) behavior was performed in a four-ball tribometer (MFT-4BM, Rtec-Instruments, San Jose, CA, USA, Figure 3b). The ball (12.7 mm diameter, Sa = 0.03 µm) was made of 100Cr6 bearing steel and was always immersed in the tested oil. According to the GB/T 12583 standard [27] (similar to ASTM D2783 [28]), at room temperature (~25 °C), a series of tests were conducted at 1760 rpm for 10 s under progressively increasing loads, such as 834 N, 883 N, 932 N, 981 N, and so on. When the wear scar diameter under a specific load (e.g., 883 N) exceeded 5% of the designated value for that load, it signified the onset of seizure, and the former load (e.g., 834 N) was identified as the maximum non-seizure load (P_B). Each oil sample underwent five repeated tests. Although minor variations in wear scar diameters were observed, all experimental trials consistently identified the same P_B level as specified in the GB/T 12583 standard.

2.5. Wear Analysis

After the friction test, the wear track width on the cylinders was measured using a laser scanning microscope (LSCM, VK-9700, Keyence, Itasca, IL, USA). The adsorbed surface layer of HPTD was examined using X-ray photoelectron spectroscopy (XPS, Thermo, ESCALAB 250Xi). After extreme pressure testing, the wear scar diameter on the balls was measured using LSCM. The worn morphology and elemental composition of the balls were analyzed by scanning electron microscopy (SEM, JSM-7500 F, JEOL) and energy-dispersive spectrometer (EDS, X-ACT, OXFORD, Abingdon, UK).

3. Results and Discussion

3.1. Determination of HPTD Content

Based on the NMR analysis, the prepared HPTD had a purity of around 90% (Figure S2). In the FTIR spectra (Figure 4a), the peak at 1457 cm⁻¹ is ascribed to C=C stretching vibration [29] and the peak centered at 1607 cm⁻¹ is assigned to C=O stretching vibration [30]. These results confirm the successful preparation of HPTD. Figure 4b shows that HPTD exhibits a dynamic viscosity of 4 mPa·s at 100 °C. As a result, incorporating HPTD as a friction modifier into 3015 will dilute the viscosity of the oil. Therefore, precise control of HPTD concentration is essential during incorporation to avoid altering the viscosity characteristics of 3015.

To investigate the effect of HPTD concentration on the viscosity of 3015, Figure 5a illustrates the viscosity variations of 3015 blended with HPTD at five different concentration levels. While the inherently low viscosity of HPTD induces a reduction in the viscosity of 3015, this effect is notably minimized at elevated temperatures, with the 3015+HPTD composite oils showing negligible deviation from the pristine 3015 under high-temperature conditions (≥100 °C). Testing conducted according to ASTM D445 further confirms that HPTD has a negligible effect on the kinematic viscosity of 3015. The 3015+HPTD composite oils fully maintain the original SAE 30 viscosity grade of 3015 when HPTD content is controlled within 1–9% (Figure 5b). Therefore, the maximum allowable HPTD content added in 3015 is 9%. HPTD effectively preserves the high-temperature viscosity characteristics of 3015, which is particularly crucial for cylinder liner–piston ring interfaces under high-temperature conditions.

To thoroughly investigate the effect of HPTD content on the tribological properties of 3015, friction tests were conducted under typical operating conditions (150 N, 20 Hz, 3 mm, and 150 °C) using four oils: pristine 3015 and three HPTD-doped formulations (3%, 6%, and 9% HPTD). In general, all tested oils underwent a COF reduction during the running-in process (Figure 6). Another obvious tendency is that the COF of oils decreased with increasing HPTD content, where the 3015+9%HPTD exhibited an approximately 10% lower COF compared to the 3015, demonstrating the effective anti-friction capability of HPTD (the COF value in the histogram was the average of three resultant COF values at the end of each repeated test). LSCM analysis of the wear track width (WTW) on the cylinders further confirmed that 3015 containing HPTD produced significantly smaller wear than 3015. These improvements in both friction reduction and wear protection establish 9% HPTD as the optimal doping content for 3015.

3.2. Determination of ODA-Cu Content

FTIR measurements were performed to confirm the modification of Cu by ODA. As shown in Figure 7a, both ODA and ODA-Cu exhibited characteristic symmetric and asymmetric stretching vibrations of C-H bonds at 2920 cm⁻¹ and 2850 cm⁻¹, respectively [31]. Notably, the C-H bending vibration peak of ODA at 1469 cm⁻¹ exhibited a slight red shift to 1463 cm⁻¹ in ODA-Cu, which could be indicative of the interaction between ODA and Cu [32]. More significantly, the characteristic amine-related absorption peaks of ODA, including the N-H stretching vibration at 3331 cm⁻¹ and the amino wagging vibration at 977 cm⁻¹, were completely absent in the ODA-Cu spectrum [33]. The disappearance of these amine-specific absorption peaks provides clear evidence that ODA is covalently bonded to Cu through Cu-N bonds [34,35]. TGA was used to evaluate the thermal stability of ODA-Cu. As shown in Figure 7b, the distinct mass loss at 440 °C is attributed to the degradation of ODA [36]. The TGA curve at 800 °C reveals that ODA-Cu contains 40 wt% ODA, suggesting that the long alkyl chain and high organic content of octadecylamine benefit facilitating superior oil dispersibility of the nanocopper particles. Figure 7c shows the surface morphology of ODA-Cu nanoparticles as observed through TEM. The corresponding particle size distribution analysis (Figure 7d) demonstrates an average diameter of 2.7 nm, as quantified by the particle size analyzer.

Figure 8 presents the effect of the concentration of ODA-Cu in 3015 composite oils on the sulfated ash content. The data indicate that the sulfated ash content increase of 3015 was smaller than 2% when the ODA-Cu content was less than 0.3%. However, exceeding this threshold results in a significant increase in sulfated ash content (e.g., 0.4% ODA-Cu caused a 7% increment in the sulfated ash content). Therefore, it is recommended to limit the ODA-Cu content in 3015 composite oils to ≤0.3%.

Based on the established optimal 9% HPTD in 3015 and the predetermined 0.3% ODA-Cu threshold for acceptable sulfated ash content, composite oils including 3015 + 9% HPTD + 0.1% ODA-Cu, 3015 + 9% HPTD + 0.2% ODA-Cu, and 3015 + 9% HPTD + 0.3% ODA-Cu were tested under the typical operating conditions (Figure 9). Compared to the case of HPTD, the incorporation of ODA-Cu had less impact on the COF of the 3015. However, the corresponding wear scar analysis indicated that the added ODA-Cu led to an obvious reduction in WTW, suggesting that it exerts a mending effect at the contact interface. It is noted that the 0.3% ODA-Cu sample displayed a greater wear scar width compared to the 0.2% ODA-Cu. This phenomenon is potentially attributed to the elevated concentration of nanoparticles, which aggregate into larger particles, thereby inducing an interception effect within the inlet region of the lubricant film. Consequently, the optimum ODA-Cu content in 3015 was determined as 0.2%. At this stage, the optimal hybrid additives for 3015 were determined as 9% HPTD and 0.2% ODA-Cu, with the modified 3015 labeled as m3015.

As shown in Figure 10a, it was found that no precipitation behavior occurred in m3015 after one month, proving that the HPTD and ODA-Cu nanoparticles were well dispersed in 3015. m3015 maintained a similar viscosity to 3015 at 100 °C, while exhibiting a 12% reduction in sulfated ash content (Figure 10b). The unexpected decrease in sulfated ash content may be attributed to the dilution effect of HPTD on conventional ash-forming additives present in the original oil, which outweighed the ash content increase induced by ODA-Cu.

3.3. Friction Tests of m3015 Under Various Operating Conditions

3.3.1. Effect of Normal Load

The load-bearing capability of lubricating oil serves as a critical consideration when determining suitable lubrication systems for industrial applications. Thus, friction tests maintained typical operating conditions (30 Hz, 2 mm, and 150 °C) while applying three discrete normal loads (120 N, 150 N, and 180 N) to specifically characterize the load-dependent friction behavior. By analyzing the COF curves (Figure S3), it was found that the COF of m3015 decreases gradually with the increasing load. At 180 N, the COF was reduced by 13% compared to 3015 (Figure 11a). This improvement can be attributed to increased asperity contact on the friction surface under higher loads, which activates a tribochemical reaction between HPTD and the steel surface, thereby resulting in a more pronounced friction-reducing effect. Wear analysis (Figure S4 and Figure 11b) shows that the WTW of both 3015 and m3015 increased with load, yet m3015 exhibited a significantly smaller increase, demonstrating the enhanced anti-wear performance achieved through HPTD+ODA-Cu modification of 3015.

3.3.2. Effect of Reciprocating Frequency

Frequency as a fundamental parameter of reciprocating motion significantly influences the friction and wear characteristics of lubricating oil. In experiments conducted under constant normal load (150 N) and temperature (150 °C) conditions, various reciprocating frequencies, including 20 Hz, 30 Hz, and 40 Hz, were set up. To ensure consistent sliding distance (432 m), the stroke lengths were adjusted accordingly to 3 mm, 2 mm, and 1.5 mm. According to the COF curves (Figure S5) and the wear surface (Figure S6), the resultant COF and the WTW are summarized in Figure 12. It can be seen that the reciprocating frequency had minimal effect on the COF of both 3015 and m3015. However, as reciprocating frequency increases with reduced stroke, friction occurs in a more concentrated area, leading to increased WTW [37]. Under these conditions, m3015 exhibited a significantly narrower wear track width compared to 3015, highlighting the superior anti-wear properties of hybrid additives of HPTD and ODA-Cu.

3.3.3. Effect of Testing Temperature

Temperature is a critical factor affecting the tribological performance of engine oil because it involves the tribochemical reaction of additives. As demonstrated in Figures S7 and S8, the influence of different temperatures (120 °C, 150 °C, and 180 °C) on the tribological characteristics of 3015 and m3015 was investigated under constant operating parameters (150 N, 30 Hz, and 2 mm). The resulting COF and WTW are summarized in Figure 13. Both oils exhibited progressive COF reduction with rising temperature. This trend can mainly be ascribed to the enhanced tribochemical reaction between traditional lubrication additives and modified materials (HPTD and ODA-Cu) in 3015 at higher temperatures, which improved lubrication performance. Interestingly, the COF of oils increased to varying degrees in the later stages of the friction test at 180 °C (Figure S6), a phenomenon directly correlated with the quantity of supplied oils (30 μL). At higher temperatures, thermal migration of the oil, potentially induced by the Marangoni effect, leads to oil starvation in the contact zone [38]. The extent of the COF increase for m3015 was significantly less than that observed in 3015, owing to the strong chemical adsorption of HPTD on the steel surface, which effectively mitigates oil dispersion. By the end of the experiment, the COF of m3015 was 15% lower than that of 3015. Under oil-starved conditions, HPTD and ODA-Cu composite materials greatly enhance the tribological performance of 3015 engine oil. This effect is particularly evident during piston ring operation near the cylinder liner’s end. The analysis of WTW reveals that increasing temperature leads to more severe wear for both 3015 and m3015 (Figure 13b). This is primarily due to the decrease in oil viscosity at elevated temperatures, which diminishes both the load-bearing capacity and its ability to form a protective film. However, the m3015 demonstrated superior wear resistance across the entire temperature range compared to the 3015.

3.3.4. XPS Analysis on the Wear Track

Figure 14 illustrates the XPS spectral analysis of the cylinder’s wear track after friction testing with lubricating oils under typical operating conditions. The high-resolution XPS spectra of the C 1s reveal peaks located at 284.8 eV and 286.3 eV, which correspond to the C-C and C-O bonds, respectively [39]. Furthermore, the peak observed at 288.3 eV is indicative of the C=O bond, signifying the occurrence of a tribochemical reaction between 1,3-diketone and the steel surface [11]. Notably, the Cu element was not detected in the XPS spectrum of m3015, which may be attributed to the low content of ODA-Cu in m3015.

3.4. Extreme Pressure Test of m3015

The oil 3015 is a commercial lubricating oil employed in marine diesel engines for cylinder and crankcase lubrication, in which extreme pressure capacity serves as a critical performance parameter. In accordance with the ASTM D2783, the maximum non-seizure load (P_B) values for 3015 and m3015 lubricants were investigated using the four-ball method. As depicted in Figure 15, the m3015 exhibited an improvement in P_B value, increasing from 834 N for the pristine 3015 to 932 N. This significant improvement is attributed to the strong surface adsorption of HPTD and the surface deposition of ODA-Cu, which effectively enhances the resilience of the pristine oil.

Considering that XPS analysis failed to directly detect the ODA-Cu presence on cylinders under normal load conditions (150 N), and building on the extreme pressure tests showing that the PB value of 3015 was 832 N, the extreme pressure test was thereby conducted under a 755 N condition, which allowed us to obtain clear wear tracks and facilitated searching for the presence of ODA-Cu and analyzing its mechanism of action. SEM and EDS analyses were performed on the ball after the extreme pressure test. As depicted in Figure 16, the worn surface morphology of the ball utilizing 3015 shows surface damage with deep furrows, signifying serious damage via abrasive wear [40]. In contrast, the worn surface on the ball lubricated by m3015 was flatter, and the corresponding EDS analysis confirmed that the content of Cu was 4.71%. Furthermore, upon conducting EDS analysis on the different worn regions (Figure 17), it was determined that the copper elements predominantly originate from the darker areas of the wear surface (region 1), suggesting that the friction film was formed by the deposition of ODA-Cu. These results suggest that under conditions of extreme pressure, the absorbing layer produced by HPTD struggles to sustain such a significant pressure, with the ODA-Cu providing the primary load-bearing capacity of the oil.

4. Conclusions

The present work investigated the hybrid additives of 1,3-diketone HPTD and surface-functionalized nanocopper particles applied in the marine engine oil 3015. The influences of the hybrid additives on oil viscosity, sulfated ash content, friction/wear behavior, and extreme pressure capacity were tested. The main conclusions drawn from the results are as follows:

(1)

Following a comprehensive evaluation of the effects of hybrid additives on the viscosity, sulfated ash content, and friction/wear properties of 3015, the optimal composition of hybrid additives for 3015 was determined to be 9% HPTD and 0.2% ODA-Cu. This results in a modified 3015 (m3015) oil with a sulfated ash content decreased by 12%.

(2)

When operating conditions were concerned, higher loads and temperatures promoted the tribochemical reactions of HPTD, leading to lower friction for m3015. On the contrary, the influence of reciprocating frequencies was very little. Across all tested conditions, m3015 demonstrated superior lubrication performance than that of 3015, in which the maximum COF and WTW reduction were 15% and 43%, respectively.

(3)

In extreme pressure tests, the hybrid additives of HPTD and ODA-Cu increased the P_B of 3015 from 834 N to 932 N. Under such high loads, the load-bearing capacity of the oil was primarily provided by the mending effect of ODA-Cu.

(4)

As a commercial engine oil, 3015 itself contains regular anti-friction and anti-wear additives. On one hand, the results of this work suggest that the existing additives in 3015 showed no obvious antagonism with the incorporated HPTD and ODA-Cu; on the other hand, their exact coupling effect with the conventional additives, such as MoDTC or ZDDP, should be investigated in the future to further optimize the oil formulation.