Exploring the lubrication mechanism of CeO2 nanoparticles dispersed in engine oil by bis(2-ethylhexyl) phosphate as a novel antiwear additive
Graphical Abstract
Introduction
In automobile engines worldwide, the ability of lube oils to enhance fuel economy is important. Vehicles contribute approximately 19% of the global energy loss and 23% of the overall annual gaseous emissions, causing increasing concern regarding energy scarcity and environmental safety [1]. Friction is the principal reason for power dissipation in vehicle engines, with frictional losses expending 17–19% of the engine power generated [2]. Additionally, the new emission limitations imposed on automobiles are the principal driving force propelling the development of lube oils [3]. Thus, automotive engines require innovative lubricant formulations that are more fuel-efficient, eco-friendly, affordable, and sustainable. Consequently, researchers are focusing on decreasing the total frictional power loss and increasing the engine durability and fuel economy to improve the performance of vehicle engines. Advanced automotive engines require novel lube oils to operate under harsh mixed or boundary lubrication conditions, as well as run under heavy loads, high operating speeds, and high temperatures. Therefore, the development of engine lube oils to reduce tribo-induced energy dissipation, material losses, and CO2 emissions is pivotal.
From an energy-saving position, Skjoedt et al. [4] reported that the reduction of engine friction by 10% for all US automobiles (in 2007) would lead to a saving of 3.4 billion gallons in fuel consumption. In engines, lowering the viscosity of lubricating oils is a common strategy for improving fuel economy [5]. Recently, several researchers have suggested that enhanced lubricating efficiency and extended service durability can be achieved by supplementing engine oils with nanomaterials (nanolubricants). To obtain nanolubricants, nanoparticles with sizes below 100 nm are dispersed in lube oils [6]. Nanolubricants are emerging as a remarkably promising technology for enhancing antifriction/antiwear properties, performance under extreme pressure, heat transfer, and energy saving in automotive engines [7]. The thermal conductivity of nanolubricants is higher than those of base lube oils, resulting in improved heat transfer in the engine [8], [9]. Ali et al. [10] investigated the tribological characteristics of reciprocating sliding surfaces using Al2O3/TiO2 nanolubricants. The friction experiments revealed that the friction coefficient was diminished by 48–50% compared to that obtained with the fully formulated oil (5 W-30) during the boundary lubrication regime. To correlate the tribological results to the actual engine performance, Ali et al. [11] investigated the effect of TiO2/Al2O3 nanoadditives on the performance of a gasoline engine. Their findings illustrated that the fuel consumption with Al2O3/TiO2 nanolubricants decreased by 16–20%. The AVL engine dynamometer results by Ali et al. [12] illuminated that the fuel consumption and gaseous emissions (HC, CO2, and NOx) of an engine lubricated with graphene nanolubricants decreased by 17% and 2.7–5.4%, respectively. Eventually, nanolubricant application will become a viable and appealing lubricating modification strategy because it does not require any major hardware adjustments.
The motives for selecting the nanomaterial and surfactant type used in this study are stated below. CeO2 is a rare earth metal oxide with a cubic fluorite structure, and it presents a range of interesting chemical and physical properties [13]. CeO2 nanomaterials are known to be inexpensive or economical and possess low hardness [14]. CeO2 nanomaterials exhibit excellent antiwear properties and facilitate the formation of protective films on worn interfaces [7], [15], [16]. The dispersion of CeO2 nanomaterials could be improved using surface modification chemicals in the organic fluid phase [15]. Furthermore, CeO2 has been extensively employed as a nanoadditive in fuel [17]. CeO2 nanoparticles can function as an oxygen buffer, facilitating the simultaneous oxidation of hydrocarbons and a decrease in the concentration of nitrogen oxides, thereby decreasing emissions [18]. These events enhance the combustion process throughout the entire combustion stroke, caused by the chemical reactions between the tribolayer produced on the rubbing liner interface and the gaseous emissions. The proper selection of a surfactant is critical for the efficient dispersion of nanoparticles in lube oils. In that regard, bis(2-ethylhexyl) phosphate (HDEHP) is an oil-soluble ion extractant with a high surface activity, attributed to its phosphate head group and alkyl phosphate, which alters the nanoparticle surface [19]. The HDEHP molecule has a quaternary structure. Recently, HDEHP has been studied as a dispersant of nanomaterials in base oils [20], [21]; however, few studies on the interactions between HDEHP and the nanoparticle surfaces have been conducted. The dispersion stability of TiO2/Al2O3 hybrid nanoadditives dispersed by HDEHP in PAO6 oil was excellent after storage for 70 days without sedimentation [20]. Furthermore, phosphorus-containing composites are formulated to react chemically on worn interfaces, rapidly producing a tribolayer and, consequently, preventing harsh wear or seizure [22].
In respect of the tribological properties, Thottackkad et al. [14] added CeO2 nanoparticles (0.51 wt% and 30–150 nm) modified by Tween 20 into coconut oil under 1–2 MPa pressure applied. The key findings explicated that the wear rate and friction coefficient were lessened by 17% and 22%, respectively, compared with the base lube oil. Wu et al. [16] studied the effect of CeO2 nanoparticles dispersed by oleylamine in polyalphaolefin oil on the wear resistance. The results explained that the scar diameter decreased by 56.0% as compared with oil alone. Shen et al. [15] reported that adding 2.0 wt% CeO2 nanoadditives to a grease catalyzed the oxidation of Fe to generate a ferrite oxide-containing tribolayer, enhancing the antiwear capability of rubbing surfaces. Gupta et al. [23] illustrated that using CeO2 nanomaterials (0.1% w/v and 80 nm) modified by sodium dodecyl sulfate (SDS) improves anti-wear property by 16.7% compared to rapeseed oil without nanoadditives. Additionally, other studies have revealed improvement in tribological properties utilizing CeO2 nanoparticles in lube oils and composites [7], [24], [25], [26], [27], [28].
Here, we explore the synergistic effect of CeO2 nanoparticles and HDEHP in nanolubricant formulation and their rheological, thermal, and tribological performances. Furthermore, we address the most important question: what are the underlying key mechanisms responsible for the tribological events? To answer this question, we conducted a detailed systematic evaluation of the tribological performance of the liner–ring tribosystem lubricated by the 5 W-30 engine oil, HDEHP, and CeO2 nanolubricant. To the best of our knowledge, this is the first study to apply the HDEHP surfactant as an antiwear additive. To elucidate the mechanisms behind the tribological performance, the topographies of the rubbing interfaces were thoroughly studied using a field emission scanning electron microscope (FESEM), electron probe microanalyzer (EPMA), energy dispersive spectrometer (EDS), and 3D optical surface profilometer. Furthermore, we analyzed the composition of the tribofilm built on the frictional surfaces using X-ray photoelectron spectrometry (XPS).
Section snippets
Material characterization
A fully formulated synthetic engine oil (5 W-30) was used as a reference oil in the rheological, thermal, and tribological investigations to elucidate the synergistic effect of the CeO2 nanoadditives and the HDEHP surfactant on the properties of the nanolubricant formulation. The density of the 5 W-30 oil at 15 ℃ (ASTM D1298) is 0.84 g/cm3, and its dynamic viscosity (ASTM D78) is 7.91 and 45.41 mPa.s at 100 ℃ and 40 ℃, respectively. CeO2 nanoparticles with a size range of 20–25 nm were
Characterization of the CeO2 nanolubricants
In this section, the dispersion stability of the CeO2 nanolubricant was monitored via visual observation and UV analysis. Moreover, the rheological and thermal degradation performances of the CeO2 nanolubricant are discussed and compared with those of the reference oil (5 W-30). The dispersion stability of the CeO2 nanolubricant over time is essential for its potential commercialization. Fig. 4 shows the visual appearance and UV spectral intensity of the CeO2 nanolubricant after direct
Conclusions
This investigation studies the synergistic effect of CeO2 nanoparticles and HDEHP in nanolubricant formulation, as well as their rheological, thermal, and tribological performances. The key conclusions extracted from the experiments are as follows.
- 1.
The HDEHP surfactant stabilized the CeO2 nanomaterials in the fully formulated oil (5 W-30) for 30 days without sedimentation. Furthermore, the rheological evaluation revealed that the HDEHP surfactant can facilitate the successful application of
CRediT authorship contribution statement
Mohamed Kamal Ahmed Ali: Conceptualization, Methodology, Investigation, Formal analysis, Visualization, Data curation, Writing – original draft, Writing – review & editing. Hou Xianjun: Supervision, Resources, Data curation, Reviewing, Funding acquisition, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to express their sincere thankfulness to the National Natural Science Foundation of China (Project No. 51875423) for continuous support. M.K.A. Ali would like to acknowledge the assistance of Minia University and Wuhan University of Technology for their cooperation for the research work. Final appreciations are expressed to editors and reviewers for their helpful comments and precious time.
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