Nanotechnology in Lubricants: A Systematic Review of the Use of Nanoparticles to Reduce the Friction Coefficient

: The study of lubricating oil is paramount for the optimal functioning of modern engines, and it has generated intensive research in the automotive industry. The aim is to improve the tribological properties of lubricants by including nanomaterials as additives in base oils. This article presents an exhaustive bibliographic review of the experiments carried out to optimize the tribological properties of nano-lubricants in order to identify the nanoparticles and experimental processes used and analyze the results obtained. The methodology adopted combines inductive and deductive elements. It begins with the formulation of a general theory on the application of nanoparticles in lubricants, followed by the collection of specific data on the conceptualization and preparation of nano-lubricants. A total of 176 articles focused on the application of nanoparticles in lubricants, especially to reduce the coefficient of friction, are reviewed. These works, with impact levels Q1 and Q2, delve into the application and are analyzed to review the obtained results. Most researchers worked with a nanoparticle concentration range of 0% to 1% by volume.


Introduction
Lubricating oil is a fundamental component for the correct functioning of vehicle engines today; therefore, it is one of the areas with the greatest research and innovation within the automotive industry [1][2][3][4].This is mainly due to the functions that it performs, among which the following stand out: reduces friction [5], improves sealing [6], protects against wear and reduces corrosion [7], eliminates contaminants [8] and dissipates heat [9].It is estimated that 80% of mechanical failures are caused by a wear defect due to friction and lubrication [10].
Holmberg et al. in [11] state that 33% of the energy loss in an internal combustion engine is due to friction, attributing 11.5% to the moving parts of the engine, 5% to the transmission gears, 11.5% to rolling resistance and 5% to brake friction.In this way, due to the importance of lubrication, the need arises to improve the tribological properties of lubricants, which leads to a research approach focused on the inclusion of nanomaterials as additives in base oils [12][13][14][15][16].
These studies have obtained favorable results; for example, [27] shows an increase in the thermal conductivity ratio of a lubricant, from 1 to 1.15, using Copper Oxide (CuO) nanoparticles at concentrations of 0% to 4% of the volume with a size of 10 nm at temperatures of 20 to 50 • C.This finding makes nanofluids attractive as coolants for devices with a high energy density.
temperatures of 20 to 50 °C.This finding makes nanofluids attractive as coolants for devices with a high energy density.
Zawawi et al. [28] analyzed the behavior of the friction coefficient of nano-lubricants composed of Aluminum Oxide and Silicon Oxide (Al2O3-SiO2), and they concluded that, for a reduction in the friction coefficient of up to 4.78% and an improvement in the wear rate of 12.96%, the optimal volume concentration of nano-lubricants should be 0.02%.It is true that diverse and extensive nanomaterials are used as additives for lubricating oils; however, it is metal nanoparticles or their oxidants and sulfides that have received the most study and research [29,30] because metal nanoparticles enhance the lubricating oil's tribological characteristics through micro-bearing effects and various other mechanisms [31].
Considering the importance of the results presented in the previous paragraphs, this article presents a bibliographic review of the experiments conducted for the improvement of the tribological properties of nano-lubricants in order to identify the nanoparticles used for these experiments, identify the processes of experimentation and, finally, analyze the obtained results.

Materials and Methods
This article uses a mixed-approach methodology that combines inductive and deductive elements in the research.Initially, as shown in Figure 1, a general theory of the application of nanoparticles in lubricants is formulated.Subsequently, the data and specific characteristics of researchers' work are collected in the context of the conceptualization and preparation of nano-lubricants.The methods and selection of nanoparticles are mentioned, which, once applied to a lubricant, have an influence on the coefficient of friction.The methodology adopted follows the PRISMA checklist, allowing inductive and deductive research to feed into each other.A flow diagram is included in the Supplementary Materials.The inductive methodology is applied in the observation of specific cases, leading to the formulation of hypotheses on the confirmation of preparation, as well as the methods commonly applied to the production of nanofluids and their influence on the reduction in the friction coefficient.The researchers collected data on the one-step and two-step methods used to achieve lubricant stability and durability.These data were then focused on individual cases, thus deriving general principles applicable to the production of finished lubricants.However, the deductive methodology was used to develop generalizations where theoretical principles were analyzed to understand and explain specific phenomena of importance in the The researchers collected data on the one-step and two-step methods used to achieve lubricant stability and durability.These data were then focused on individual cases, thus deriving general principles applicable to the production of finished lubricants.However, the deductive methodology was used to develop generalizations where theoretical principles were analyzed to understand and explain specific phenomena of importance in the preparation of nanofluids.Once the methodology was defined, bibliographic materials were then applied, both academic articles and books intended for scientific dissemination.Information relevant to the characteristics of nano-lubricants was extracted from these materials.

Preparation of Nano-Lubricants
The base fluid to which nanoparticles are added in a stable suspension state is known as a nano-lubricant.Dispersion stability is an important challenge in the use of nanoparticles in lubricants, and this is because nanoparticles agglomerate easily due to their high surface tension, leading to the formation of non-dispersible aggregates [32].
To achieve a better dispersion of nanoparticles, methods are used to reduce their high surface energy, such as surface functionalization.This approach offers a solution to these challenges by altering the chemical composition of the nanoparticles to enhance their dispersibility and maintain their stability, using a variety of techniques, ranging from physical to chemical methods [33].
The physical method mainly focuses on the adsorption of surfactants on the surface of nanoparticles to modify their surface characteristics.Regarding chemical methods, a series of compounds have been created to functionalize the surface of nanoparticles of metals and metal oxides, giving them specific properties such as hydrophobicity, hydrophilicity and charge, which contribute to improving both their dispersion and their stability [34][35][36].
To understand the stability of the dispersion of nano-lubricants, colloidal theories can be used, which attribute the stability of the suspension to the interaction between the nanoparticles and the thermal agitation energy received by the base fluid [37].

Methods for the Preparation of Nano-Lubricants
In experimentation, the preparation of nano-lubricants is an important step to achieve stability and durability in mixtures; therefore, to produce nanofluids, two methods have been used, known as the one-step method and the two-step method [38][39][40].
Several techniques are used for this process.The first, the laser ablation technique, is schematized in Figure 2 in [47].It consists of focusing on the surface of a material (gas or liquid) using a laser beam, which causes its vaporization at the irradiated point.The impact between the evaporated part and the surrounding molecules results in the formation of a laser-induced plasma plume, which is subsequently confined in a specific region to disperse the nanoparticles so that the coagulation phenomenon is correctly controlled in the final stages of the process [48][49][50].
preparation of nanofluids.Once the methodology was defined, bibliographic materials were then applied, both academic articles and books intended for scientific dissemination.Information relevant to the characteristics of nano-lubricants was extracted from these materials.

Preparation of Nano-Lubricants
The base fluid to which nanoparticles are added in a stable suspension state is known as a nano-lubricant.Dispersion stability is an important challenge in the use of nanoparticles in lubricants, and this is because nanoparticles agglomerate easily due to their high surface tension, leading to the formation of non-dispersible aggregates [32].
To achieve a better dispersion of nanoparticles, methods are used to reduce their high surface energy, such as surface functionalization.This approach offers a solution to these challenges by altering the chemical composition of the nanoparticles to enhance their dispersibility and maintain their stability, using a variety of techniques, ranging from physical to chemical methods [33].
The physical method mainly focuses on the adsorption of surfactants on the surface of nanoparticles to modify their surface characteristics.Regarding chemical methods, a series of compounds have been created to functionalize the surface of nanoparticles of metals and metal oxides, giving them specific properties such as hydrophobicity, hydrophilicity and charge, which contribute to improving both their dispersion and their stability [34][35][36].
To understand the stability of the dispersion of nano-lubricants, colloidal theories can be used, which attribute the stability of the suspension to the interaction between the nanoparticles and the thermal agitation energy received by the base fluid [37].

Methods for the Preparation of Nano-Lubricants
In experimentation, the preparation of nano-lubricants is an important step to achieve stability and durability in mixtures; therefore, to produce nanofluids, two methods have been used, known as the one-step method and the two-step method [38][39][40].
Several techniques are used for this process.The first, the laser ablation technique, is schematized in Figure 2 in [47].It consists of focusing on the surface of a material (gas or liquid) using a laser beam, which causes its vaporization at the irradiated point.The impact between the evaporated part and the surrounding molecules results in the formation of a laser-induced plasma plume, which is subsequently confined in a specific region to disperse the nanoparticles so that the coagulation phenomenon is correctly controlled in the final stages of the process [48][49][50].The second technique is the submerged arc, used in [51][52][53].Its principle is based on submerging a pure copper rod in a dielectric liquid located in a vacuum chamber.At this point, the temperature increases to a range between approximately 6000 and 12,000 • C, producing an arc in the rod, which subsequently melts and vaporizes together with the dielectric liquid [54], as detailed in Figure 3.
The second technique is the submerged arc, used in [51][52][53].Its principle is based on submerging a pure copper rod in a dielectric liquid located in a vacuum chamber.At this point, the temperature increases to a range between approximately 6000 and 12,000 °C, producing an arc in the rod, which subsequently melts and vaporizes together with the dielectric liquid [54], as detailed in Figure 3. High-pressure steam removes vaporized metal, and, when applied in a vacuum chamber, the metal undergoes a condensation process, nucleation and growth, thus transforming into nanoparticles.
Finally, the vapor deposition technique, used in [55,56], is characterized by forming a thin layer of base fluid on the wall of a container under the action of the centrifugal force produced by a rotating disk (Figure 4).Subsequently, in a tank filled with inert gas at low pressure, the material is heated and evaporated.After this process, the nanofluid is ready when the vapors of the raw material condense due to the interaction with a thin film of swirling water and settle in the base fluid [57].

Two-Step Method
The two-step method consists of first preparing nanoparticles as dry powders through chemical or physical processes and then, through intense agitation, dispersing them in a base fluid.This method is the most used due to its low cost and ability to produce nanofluids on a large scale [58][59][60].High-pressure steam removes vaporized metal, and, when applied in a vacuum chamber, the metal undergoes a condensation process, nucleation and growth, thus transforming into nanoparticles.
Finally, the vapor deposition technique, used in [55,56], is characterized by forming a thin layer of base fluid on the wall of a container under the action of the centrifugal force produced by a rotating disk (Figure 4).Subsequently, in a tank filled with inert gas at low pressure, the material is heated and evaporated.After this process, the nanofluid is ready when the vapors of the raw material condense due to the interaction with a thin film of swirling water and settle in the base fluid [57].
The second technique is the submerged arc, used in [51][52][53].Its principle is based on submerging a pure copper rod in a dielectric liquid located in a vacuum chamber.At this point, the temperature increases to a range between approximately 6000 and 12,000 °C, producing an arc in the rod, which subsequently melts and vaporizes together with the dielectric liquid [54], as detailed in Figure 3. High-pressure steam removes vaporized metal, and, when applied in a vacuum chamber, the metal undergoes a condensation process, nucleation and growth, thus transforming into nanoparticles.
Finally, the vapor deposition technique, used in [55,56], is characterized by forming a thin layer of base fluid on the wall of a container under the action of the centrifugal force produced by a rotating disk (Figure 4).Subsequently, in a tank filled with inert gas at low pressure, the material is heated and evaporated.After this process, the nanofluid is ready when the vapors of the raw material condense due to the interaction with a thin film of swirling water and settle in the base fluid [57].

Two-Step Method
The two-step method consists of first preparing nanoparticles as dry powders through chemical or physical processes and then, through intense agitation, dispersing them in a base fluid.This method is the most used due to its low cost and ability to produce nanofluids on a large scale [58][59][60].

Two-Step Method
The two-step method consists of first preparing nanoparticles as dry powders through chemical or physical processes and then, through intense agitation, dispersing them in a base fluid.This method is the most used due to its low cost and ability to produce nanofluids on a large scale [58][59][60].
In the two-step method, the preparation of nanoparticles is carried out prior to mixing with the base fluid using the following techniques: The sol-gel technique is applied in [61-63] and specifically explained by Behnajady et al. [64] in the following steps: First, using the ultrasonic bath technique, a mixture of titanium and a solvent is sonicated.Deionized water is then added drop by drop to the mixture using magnetic agitation to carry out the hydrolysis process.In the end, the product from the previous steps is dried and calcined, producing a crystalline powder (see Figure 5).
The sol-gel technique is applied in [61-63] and specifically explained by Behnajady et al. [64] in the following steps: First, using the ultrasonic bath technique, a mixture of titanium and a solvent is sonicated.Deionized water is then added drop by drop to the mixture using magnetic agitation to carry out the hydrolysis process.In the end, the product from the previous steps is dried and calcined, producing a crystalline powder (see Figure 5).Another technique is Hydrothermal synthesis [65][66][67], which is applied in a high temperature range.In addition, low-or high-pressure conditions are established depending on the main composition of the reaction in order to control the morphology of the materials.The main advantage of hydrothermal synthesis is a negligible loss of materials [68].
Once the nanoparticles are obtained by the two-step method, a nanofluid is prepared, which involves the suspension and dispersion of the nanoparticles using the techniques described below.
Ultrasound technique: This technique consists of stirring the nanoparticles in a base fluid (Figure 6) with ultrasonic sound waves with a frequency greater than 20 kHz.In this way, the formation of nanometer-sized agglomerates is reduced by breaking intermolecular interactions.The agglomeration of the dispersed nanoparticles will lead to declined thermal performance, thermal conductivity and viscosity [57,[69][70][71][72][73][74].
Magnetic stirring technique [75][76][77][78]: This technique consists of the action of a rotating magnetic field created by stationary electromagnets or a set of rotating electromagnets that operate with stirring mechanisms.Another technique is Hydrothermal synthesis [65][66][67], which is applied in a high temperature range.In addition, low-or high-pressure conditions are established depending on the main composition of the reaction in order to control the morphology of the materials.The main advantage of hydrothermal synthesis is a negligible loss of materials [68].
Once the nanoparticles are obtained by the two-step method, a nanofluid is prepared, which involves the suspension and dispersion of the nanoparticles using the techniques described below.
Ultrasound technique: This technique consists of stirring the nanoparticles in a base fluid (Figure 6) with ultrasonic sound waves with a frequency greater than 20 kHz.In this way, the formation of nanometer-sized agglomerates is reduced by breaking intermolecular interactions.The agglomeration of the dispersed nanoparticles will lead to declined thermal performance, thermal conductivity and viscosity [57,[69][70][71][72][73][74].
Magnetic stirring technique [75][76][77][78]: This technique consists of the action of a rotating magnetic field created by stationary electromagnets or a set of rotating electromagnets that operate with stirring mechanisms.
PH adjustment: A different consideration establishes PH adjustment as a supplement to the previous techniques.The pH of the base fluid is adjusted to improve the suspension and dispersion of the nanoparticles.This technique raises the value of the zeta potential, which can be related to the stability of colloidal dispersions.The zeta potential decreases as the Ph value increases [79][80][81][82].
Wei et al. [83] and Shao et al. [84] apply a technique in which they combine all the processes described above in the preparation of a nanofluid to achieve better efficiency and stability.
Finally, Figure 6 summarizes the methods and techniques used for preparing nanolubricants through a diagram.
Wei et al. [83] and Shao et al. [84] apply a technique in which they combine all the processes described above in the preparation of a nanofluid to achieve better efficiency and stability.
Finally, Figure 6 summarizes the methods and techniques used for preparing nanolubricants through a diagram.

Nanofluid Stability
After preparing a nanofluid, it is necessary to verify its stability as a prior step to experimentation.The first technique is the UV-Vis spectrophotometer technique, which is applicable for all base fluids [85] and has achieved excellent results in several studies [86][87][88].This technique bases its operation on the action of taking advantage of the change in the intensity of light when it passes through a fluid.Using a wavelength of 200 to 900 nm, the instrument analyzes fluid dispersions and measures absorption by liquid [89].Jiang et al., in [90], apply the method as follows: First, by scanning, the aim is to find the maximum absorbance of the nanoparticles in order to prepare a standard that fits a linear relationship using at least three diluted concentrations (0.01-0.03%).Finally, the relative stability is measured, and the treated nanofluid is allowed to rest for a few days.
Another way to measure nanofluid stability is the zeta potential test, which uses the study of electrophoretic behavior to check the stability of nanofluids [91].In an experiment conducted by Wang [92], a 0.05% weight fraction of nanosuspension was needed to calculate the particle magnitude and zeta potential.The zeta potential and the stability of the suspension had particles with a similar charge; therefore, there was mutual repulsion causing these particles to not agglomerate.In [93], Lee et al. mention that a suspension with a zeta potential greater than 30 mV in R absolute value has correct stability.
The photo capture technique is the most important to observe the sedimentation of nanofluids.Photos are captured after preparation with the reservation of suspension quantities.When viewing the photos, sedimentation will be evident, as seen in [94-96].

Nanofluid Stability
After preparing a nanofluid, it is necessary to verify its stability as a prior step to experimentation.The first technique is the UV-Vis spectrophotometer technique, which is applicable for all base fluids [85] and has achieved excellent results in several studies [86][87][88].This technique bases its operation on the action of taking advantage of the change in the intensity of light when it passes through a fluid.Using a wavelength of 200 to 900 nm, the instrument analyzes fluid dispersions and measures absorption by liquid [89].Jiang et al., in [90], apply the method as follows: First, by scanning, the aim is to find the maximum absorbance of the nanoparticles in order to prepare a standard that fits a linear relationship using at least three diluted concentrations (0.01-0.03%).Finally, the relative stability is measured, and the treated nanofluid is allowed to rest for a few days.
Another way to measure nanofluid stability is the zeta potential test, which uses the study of electrophoretic behavior to check the stability of nanofluids [91].In an experiment conducted by Wang [92], a 0.05% weight fraction of nanosuspension was needed to calculate the particle magnitude and zeta potential.The zeta potential and the stability of the suspension had particles with a similar charge; therefore, there was mutual repulsion causing these particles to not agglomerate.In [93], Lee et al. mention that a suspension with a zeta potential greater than 30 mV in R absolute value has correct stability.
The photo capture technique is the most important to observe the sedimentation of nanofluids.Photos are captured after preparation with the reservation of suspension quantities.When viewing the photos, sedimentation will be evident, as seen in [94][95][96].
Likewise, the Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) tools are very useful to distinguish the shape, size and distribution of nanoparticles.However, with these tools, the real situation of nanoparticles in a base fluid cannot be visualized.These tools are only used on dry samples [92,94,97,98].
The light scattering method is also used to visualize the structure of colloidal particles in a suspension.The intensity of light scattered in a particle is related to its volume.Since the interaction of electromagnetic radiation with a small particle is weak, the scattering of light is mostly transmitted, and only a small amount is scattered [99,100].
Another method used to estimate the stability of nanoparticles is sedimentation balance, which consists of submerging the tray of a sedimentation balance in a fresh nanofluid.The weight of the sedimentation or its volume indicates the stability of the nanofluid.In general, nanofluids are considered stable if the concentration of the supernatant particles remains constant over time [101][102][103].
Finally, the colloidal stability of a nanofluid can also be determined by the three-omega method.It can be evaluated by detecting the increase in thermal conductivity caused by the sedimentation of nanoparticles in a wide range of volume fractions [104,105].

Influence of Nanoparticles on Lubricants
Nanoparticles play a vital role in friction zones, specifically in the limit lubrication regime, where the wear and friction margins reach their maximum value [106].
Typically, the lubrication mechanism of nano-additives can be described to have four main effects: micro-bearing, protective film, polishing and repair effects [107][108][109][110][111]. The main mechanism of nano-additive lubrication highlighted in the reviewed studies is the micro-bearing effect.
The diminutive size of nanoparticles allows them to penetrate the surface asperities in the contact zone [112], as shown in Figure 7. Also, when added to a fluid, their size is small enough to remain dispersed by Brownian motion.However, suspended particles can adhere to each other and form agglomerates, resulting in a loss of wear protection and friction reduction ability, hence the importance of the correct preparation of the nano-lubricant [89].
Rebaso et al. [113] concluded that friction significantly reduced as the stirring time increased in the preparation of their oil enriched with MoS 2 .Therefore, the stirring time in the preparation of a nano-lubricant plays an important role in the stability of the dispersion and consequently affects lubrication performance [114][115][116][117][118].
ance, which consists of submerging the tray of a sedimentation balance in a fresh nanofluid.The weight of the sedimentation or its volume indicates the stability of the nanofluid.In general, nanofluids are considered stable if the concentration of the supernatant particles remains constant over time [101][102][103].
Finally, the colloidal stability of a nanofluid can also be determined by the threeomega method.It can be evaluated by detecting the increase in thermal conductivity caused by the sedimentation of nanoparticles in a wide range of volume fractions [104,105].

Influence of Nanoparticles on Lubricants
Nanoparticles play a vital role in friction zones, specifically in the limit lubrication regime, where the wear and friction margins reach their maximum value [106].
Typically, the lubrication mechanism of nano-additives can be described to have four main effects: micro-bearing, protective film, polishing and repair effects [107][108][109][110][111]. The main mechanism of nano-additive lubrication highlighted in the reviewed studies is the micro-bearing effect.
The diminutive size of nanoparticles allows them to penetrate the surface asperities in the contact zone [112], as shown in Figure 7. Also, when added to a fluid, their size is small enough to remain dispersed by Brownian motion.However, suspended particles can adhere to each other and form agglomerates, resulting in a loss of wear protection and friction reduction ability, hence the importance of the correct preparation of the nanolubricant [89].
Rebaso et al. [113] concluded that friction significantly reduced as the stirring time increased in the preparation of their oil enriched with MoS2.Therefore, the stirring time in the preparation of a nano-lubricant plays an important role in the stability of the dispersion and consequently affects lubrication performance [114][115][116][117][118]. The proper concentration is another important factor affecting the lubrication characteristics of nano-lubricants [119][120][121][122].The addition of nanoparticles in an inadequate The proper concentration is another important factor affecting the lubrication characteristics of nano-lubricants [119][120][121][122].The addition of nanoparticles in an inadequate concentration, whether in excess or shortage, can cause negative effects in some cases, due to either increased friction or wear [123].
The intrinsic mechanical property of nanoparticles, such as hardness, is determined by their size, which, in turn, affects their tribological behavior.For materials in the size range of 100 nm or larger, hardness increases with particle size concentration [124].If the hardness of the nanoparticles is greater than that of the surface material, the result is notching and scratching, as indicated by Peña et al. [125] in their study.The high hardness (8-9 Mohs) of nano-Al 2 O 3 resulted in abrasive wear and the agglomeration of nanoparticles.
The shape of nanoparticles also plays an important role in this regard because the behavior of a nano-lubricant depends on the space between the nanoparticle and the lubricated surface at the time of loading [121].
Spherical shaped nanoparticles show a high loading capacity due to their ball-bearing effect.Figure 8 shows the linear contact that is associated with nanosheets and the planar contact that is associated with nanoplatelets.In most studies related to nano-lubricants, spherical nanoparticles have been used [18,22,[126][127][128][129][130].
behavior of a nano-lubricant depends on the space between the nanoparticle and the lubricated surface at the time of loading [121].
Spherical shaped nanoparticles show a high loading capacity due to their ball-bearing effect.Figure 8 shows the linear contact that is associated with nanosheets and the planar contact that is associated with nanoplatelets.In most studies related to nano-lubricants, spherical nanoparticles have been used [18,22,[126][127][128][129][130].

Preparation Methods
In the previous sections, the importance of preparing a nano-lubricant is defined, whereby, under this concept, the methods with the greatest application in the articles investigated are presented.A group of 39 works is considered for the analysis of the preparation methods used, considering that the publication date of these investigative works is greater than 2010 and with a significant impact (Q1 and Q2).
Of the analysis group, 23.07%used the one-step preparation method and 76.92% used the two-step method, as shown in Figure 9 and described in Table 1, which also details the type of dispersion used by the authors and its duration.La et al. [133] applied 12 h of ultrasonic stirring to their nano-lubricant, with this being the longest stirring time applied compared to the rest of the studies.Investigations of the tribological performance of nanoparticles as additives in lubricating oils are carried out with experiments using different tools such as four-ball tribometer, piston ring, ball-on-disc and pin-on-disc tools, using normal conditions of the ASTM, DIN and AISI standards [130][131][132].

Result Analysis 5.1. Preparation Methods
In the previous sections, the importance of preparing a nano-lubricant is defined, whereby, under this concept, the methods with the greatest application in the articles investigated are presented.A group of 39 works is considered for the analysis of the preparation methods used, considering that the publication date of these investigative works is greater than 2010 and with a significant impact (Q1 and Q2).
Of the analysis group, 23.07%used the one-step preparation method and 76.92% used the two-step method, as shown in Figure 9 and described in Table 1, which also details the type of dispersion used by the authors and its duration.La et al. [133] applied 12 h of ultrasonic stirring to their nano-lubricant, with this being the longest stirring time applied compared to the rest of the studies.bricated surface at the time of loading [121].
Spherical shaped nanoparticles show a high loading capacity due to their ball-bearing effect.Figure 8 shows the linear contact that is associated with nanosheets and the planar contact that is associated with nanoplatelets.In most studies related to nano-lubricants, spherical nanoparticles have been used [18,22,[126][127][128][129][130].Investigations of the tribological performance of nanoparticles as additives in lubricating oils are carried out with experiments using different tools such as four-ball tribometer, piston ring, ball-on-disc and pin-on-disc tools, using normal conditions of the ASTM, DIN and AISI standards [130][131][132].

Preparation Methods
In the previous sections, the importance of preparing a nano-lubricant is defined, whereby, under this concept, the methods with the greatest application in the articles investigated are presented.A group of 39 works is considered for the analysis of the preparation methods used, considering that the publication date of these investigative works is greater than 2010 and with a significant impact (Q1 and Q2).
Of the analysis group, 23.07%used the one-step preparation method and 76.92% used the two-step method, as shown in Figure 9 and described in Table 1, which also details the type of dispersion used by the authors and its duration.La et al. [133] applied 12 h of ultrasonic stirring to their nano-lubricant, with this being the longest stirring time applied compared to the rest of the studies.Conversely, Zheng et al. [158] applied 10 min of magnetic stirring and 15 min of ultrasonic stirring to their mixture; these are the values of the shortest stirring times.Thus, these two authors represent the extremes of agitation application time in nanofluids.In total, 56.41% of the experiments used the ultrasound dispersion method, 28.20% used magnetic stirring, and the remaining 15.38% mixed both methods (Figure 10a).Figure 10b shows the authors who achieved stable nano-lubricants (17.94%) and nonstable nano-lubricants (2.56%), as well as the authors who did not report stability (5.12%) and the authors who achieved stability for a certain time (74.35%).

Reduction in the Coefficient of Friction
To analyze the results of the reduction in the friction coefficient, 39 articles are considered.These are analyzed under the criteria of the maximum percentage reduction in the friction coefficient achieved, the concentrations of nanoparticles applied, the concentrations that produced the best results and the temperatures at which the experiments were carried out.
Figure 12 details the results obtained by the researchers in terms of reducing the friction coefficient.Singh et al. [136] achieved the highest percentage reduction in the friction coefficient, with a 91.6% reduction in the friction coefficient in their experiment.
This author experimented with graphite nanoparticles at room temperature, using an SAE-30 base oil and the pin-on-disc tool, with the application of variable forces between

Reduction in the Coefficient of Friction
To analyze the results of the reduction in the friction coefficient, 39 articles are considered.These are analyzed under the criteria of the maximum percentage reduction in the friction coefficient achieved, the concentrations of nanoparticles applied, the concentrations that produced the best results and the temperatures at which the experiments were carried out.
Figure 12 details the results obtained by the researchers in terms of reducing the friction coefficient.Singh et al. [136] achieved the highest percentage reduction in the friction coefficient, with a 91.6% reduction in the friction coefficient in their experiment.
This author experimented with graphite nanoparticles at room temperature, using an SAE-30 base oil and the pin-on-disc tool, with the application of variable forces between 20 and 50 N at 300 rpm.To understand Figure 12, it should be considered that some authors experimented with more than one type of nanoparticle.
Likewise, after the literature review, it was found that 64.10% used a concentration range in their experiments, with the remaining 35.89%only using a concentration value, as detailed in Table 2.
20 and 50 N at 300 rpm.To understand Figure 12, it should be considered that some authors experimented with more than one type of nanoparticle.
Likewise, after the literature review, it was found that 64.10% used a concentration range in their experiments, with the remaining 35.89%only using a concentration value, as detailed in Table 2. Table 2. Single concentration used by some authors.
Table 3 details the concentrations that were efficient for these researchers.The most efficient concentration values were less than 1; however, two authors [125,184] found the efficient concentration to be 2% and 3%.
Table 3. Effective concentration used by the authors who used a concentration percentage.
Table 3 details the concentrations that were efficient for these researchers.The most efficient concentration values were less than 1; however, two authors [125,184] found the efficient concentration to be 2% and 3%.

Base Lubricants
This study collected the results of 67 high-impact articles, in which the base lubricants most used for testing were as follows: SAE 5W-30 with 10 investigations, representing 16% of the total articles; SAE 10W-40 with 7 investigations, representing 11% of the total articles, and SAE 20W-50 with 6 investigations, representing 10% of the total articles.
The frequency with which these lubricants were used reflects the criteria of how affordable they are in the local market, as well as their ease of marketing.
Furthermore, their standard properties facilitate mixing with the nanoparticle for the subsequent study of this property.Table 4 shows the type of base oil, as well as how many and which authors use said oil, while Figure 14 graphically outlines this information.

Base Lubricant #Articles/Citations
It is important to consider sustainability in the nano-lubrication process.In this sense, a large number of nanoparticles are environmentally friendly, since they minimize the use of hazardous materials and additives, which is useful for environmental and economic sustainability.Nanoparticles can also facilitate a reduction in energy consumption during their production, reducing the carbon footprint, meeting the requirements of green tribology [36].
However, sustainability applies not only to nanoparticles but also to the base oil for comprehensive nano-lubricant production.Biomaterials based on vegetable oils offer a sustainable, biodegradable and low-cost alternative for various applications in engineering and biomedicine [191].Taha-Tijerina et al. found that halloysite nanotubular structures (HNSs) reinforcing natural ester lubricants significantly reduce the friction coefficient and wear scar diameter, offering environmentally friendly alternatives to mineral fluids for industrial applications [192].Gupta et al. focused their research on the development of ecological nano-lubricants, using mahua and linseed oils with h-BN nanoparticles as additives.
The addition of h-BN nanoparticles improved the rheological and tribological properties of the oils, making them more effective as lubricants.The study highlighted the potential of using these bio-based lubricants with nanoparticles for sustainable development and as a sustainable alternative to mineral oil [193].Finally, Hameed et al. stated that a mixture of 20% mahua biofuel with Al 2 O 3 and CeO 2 nanoparticles at 100 parts per million each resulted in a reduction in brake-specific fuel consumption (BSFC) of 3.25%; an increase in thermal efficiency brake performance (BTE) of 1.39%; and a reduction in hydrocarbon (HC) emissions, nitrogen oxides (NOX) and carbon monoxide (CO) of 30.73%, 1.27% and 44.13%, respectively [194].It is important to consider sustainability in the nano-lubrication process.In this sense, a large number of nanoparticles are environmentally friendly, since they minimize the use of hazardous materials and additives, which is useful for environmental and economic sustainability.Nanoparticles can also facilitate a reduction in energy consumption during their production, reducing the carbon footprint, meeting the requirements of green tribology [36].
However, sustainability applies not only to nanoparticles but also to the base oil for comprehensive nano-lubricant production.Biomaterials based on vegetable oils offer a sustainable, biodegradable and low-cost alternative for various applications in engineering and biomedicine [191].Taha-Tijerina et al. found that halloysite nanotubular structures (HNSs) reinforcing natural ester lubricants significantly reduce the friction coefficient and wear scar diameter, offering environmentally friendly alternatives to mineral fluids for industrial applications [192].Gupta et al. focused their research on the development of ecological nano-lubricants, using mahua and linseed oils with h-BN nanoparticles as additives.The addition of h-BN nanoparticles improved the rheological and tribological properties of the oils, making them more effective as lubricants.The study highlighted the potential of using these bio-based lubricants with nanoparticles for sustainable development and as a sustainable alternative to mineral oil [193].Finally, Hameed et al. stated that a mixture of 20% mahua biofuel with Al2O3 and CeO2 nanoparticles at 100 parts per million each resulted in a reduction in brake-specific fuel consumption (BSFC) of 3.25%; an increase in thermal efficiency brake performance (BTE) of 1.39%; and a reduction in hydrocarbon (HC) emissions, nitrogen oxides (NOX) and carbon monoxide (CO) of 30.73%, 1.27% and 44.13%, respectively [194].

Nano-Additives
For an analysis regarding nano-additives, a total of 63 impactful articles were taken into account.The most used nano-additives were as follows: MoS 2 (Molybdenum Disulfide) and ZnO (Zinc Oxide) with six investigations each, representing 10% of the total articles, and Gr (Graphene) and Al 2 O 3 /TiO 2 (Aluminum Oxide and Oxide of Titanium) with five investigations each, representing 8% of the total articles; this information is collected in Table 5 and schematized in Figure 15.The properties of nanoparticles are critically dependent on their dimensions, shape and morphological structure, which are characterized using different imaging methods: Field-Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) [40].The properties of nanoparticles are critically dependent on their dimensions, shape and morphological structure, which are characterized using different imaging methods: Field-Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) [40].Most of the studies presented characterized nanoparticles using FESEM.The nanostructure of Al 2 O 3 and TiO 2 nanoparticles is fairly spherical, which provides a very good rolling medium within engine oil [155,177,188].The characterization of MoS 2 nanoparticles clearly indicated its layered lamellar structure [120].In addition, ZnO NWs are vertically aligned, with a hexagonal shape and a relatively smooth surface [55].The characterization of SiO 2 nanoparticles indicated the uniform distribution of their sizes and their spherical shape [128].ZnO nanoparticles possess a polygonal shape with a wide range of sizes and an elongated morphology [139].
Other authors employed TEM to observe the morphology and particle size of TiO 2 [51,53].Finally, MWCNT nanoparticles are characterized using SEM and TEM.The tubular and filamentous morphology of nanotubes are shown to be very good when observed using these microscopes.Nanotubes, which are made, have an average diameter of about 10-20 nm and an average length of 10 µm [165,166].

Discussion
The application of nanoparticles in lubricants to reduce the friction coefficient has been the subject of numerous investigations, as demonstrated by this review of 194 articles, mainly with Q1 and Q2 impact levels.The examination of these studies reveals that the two-step method is the most common method for the formation of nano-lubricants since it is more economical than the one-step method.The dispersion of nanoparticles in a base fluid is mostly achieved by ultrasonic agitation, suggesting its effectiveness and wide application in the scientific community.
Crucially, although nanoparticles can reduce the coefficient of friction by penetrating the asperities of contact surfaces in boundary lubrication zones, their ability to increase wear by agglomerating is a major challenge.Controlling the stability of nano-lubricants is therefore essential to maximize their benefits.
Despite the efforts made, only a small percentage of researchers have managed to develop long-term stable nano-lubricants.A notable example is the work of Koshy et al., who managed to stabilize their MoS2-based nano-lubricant for 365 days.This highlights the need to continue researching and improving nano-lubricant stabilization techniques for practical application.
Regarding the concentration of nanoparticles, most researchers focused on a volume concentration range of 0% to 1%.This suggests that higher concentrations may not be necessary or may even be counterproductive in terms of lubricant stability and effectiveness.
Conversely, the use of advanced characterization techniques, such as spectrophotometry, the Z potential, SEM/TEM and light scattering, is crucial to understand the dispersion of nanoparticles in a base fluid and evaluate the stability of nano-lubricants.These tools allow researchers to visualize the distribution and agglomeration of nanoparticles, which is essential for optimizing the tribological properties of lubricants.
Additionally, it is important to consider the environmental impact of nano-lubricants and their production.Studies are needed to evaluate their biodegradability and possible harmful effects on the environment.
The potential benefits of using non-edible vegetable oils for bio-lubricant production include sustainability by utilizing crops grown on wasteland; avoiding interference with growing food on limited agricultural land; controlling soil erosion, land degradation and deforestation; and promoting a closed-loop life cycle with balanced carbon emissions.Furthermore, non-edible vegetable oils offer a high viscosity index, a high flash point, excellent lubricity and superior biodegradability, making them a suitable alternative raw material to mineral oil and capable of reducing environmental effects.
The addition of Al 2 O 3 and CeO 2 nanoparticles in biodiesel blends, specifically with mahua biofuel, has shown promising results in improving combustion efficiency; reducing the emissions of pollutants such as CO, HC, NOx and smoke; and improving overall engine performance.Studies emphasize the potential of nano-additives in biodiesel blends to achieve cleaner combustion and higher efficiency in diesel engines.

Conclusions
In conclusion, the application of nanoparticles in lubricants offers great potential to reduce the friction coefficient in boundary lubrication zones.However, more effort is required to develop long-term stable nano-lubricants and improve the understanding of their behavior under different operating conditions.
The two-step method and ultrasonic stirring are the most widely used for the preparation and dispersion of nanoparticles, respectively.These methods, along with the control of nanoparticle concentration, are key to maximizing the tribological benefits of nanostructured lubricants.
Further research is needed to optimize the tribological properties of commonly used base lubricants, such as SAE 5W-30, SAE 10W-40 and SAE 20W-50, to ensure optimal lubrication in automotive applications.
The most widely used nano-additives, such as ZnO, MoS 2 , Gr and Al 2 O 3 /TiO 2 , offer promising opportunities to improve the tribological properties of lubricants.However, a greater understanding of their long-term effects and those under different operating conditions is required for their practical implementation.

Future Perspectives
To advance in this field, it is necessary to explore new nanoparticles and preparation methods that can improve the stability and effectiveness of nano-lubricants.Furthermore, more studies should be conducted on the tribological behavior of nano-lubricants under real operating conditions, especially in automotive applications.
Optimizing the tribological properties of the most used base lubricants is also essential.Research must be conducted to find the right balance between nanoparticle concentration and lubricant stability, as well as to evaluate its long-term performance.
Additionally, it is important to consider the environmental impact of nano-lubricants and their production.Studies are needed to evaluate their biodegradability and possible harmful effects on the environment.
In summary, the use of nanoparticles in lubricants represents a promising area of research that has the potential to significantly improve the efficiency and durability of lubrication systems.However, more research and development are needed to bring these laboratory advances to practical and commercial applications.

Figure 1 .
Figure 1.Flowchart diagram of the methodology for the bibliographic review.

Figure 1 .
Figure 1.Flowchart diagram of the methodology for the bibliographic review.

Figure 2 .
Figure 2. Generation of particles with the laser ablation process.Figure 2. Generation of particles with the laser ablation process.

Figure 2 .
Figure 2. Generation of particles with the laser ablation process.Figure 2. Generation of particles with the laser ablation process.

Figure 3 .
Figure 3. Schematic diagram of a submerged arc nanoparticle synthesis system.

Figure 3 .
Figure 3. Schematic diagram of a submerged arc nanoparticle synthesis system.

Figure 3 .
Figure 3. Schematic diagram of a submerged arc nanoparticle synthesis system.

Figure 5 .
Figure 5. Scheme of the preparation of TiO2 nanoparticles via sol-gel.

Figure 5 .
Figure 5. Scheme of the preparation of TiO 2 nanoparticles via sol-gel.

Figure 6 .
Figure 6.Summary of the preparation methods of a nano-lubricant.

Figure 6 .
Figure 6.Summary of the preparation methods of a nano-lubricant.

Figure 8 .
Figure 8.Effect of nanoparticle shape on the contact pressures experienced upon loadingInvestigations of the tribological performance of nanoparticles as additives in lubricating oils are carried out with experiments using different tools such as four-ball tribometer, piston ring, ball-on-disc and pin-on-disc tools, using normal conditions of the ASTM, DIN and AISI standards[130][131][132].

Figure 9 .
Figure 9. Distribution of used methods.

Figure 8 .
Figure 8.Effect of nanoparticle shape on the contact pressures experienced upon loading.

Figure 8 .
Figure 8.Effect of nanoparticle shape on the contact pressures experienced upon loading

Figure 9 .
Figure 9. Distribution of used methods.

Figure 9 .
Figure 9. Distribution of used methods.
Figure10bshows the authors who achieved stable nano-lubricants (17.94%) and nonstable nano-lubricants (2.56%), as well as the authors who did not report stability (5.12%) and the authors who achieved stability for a certain time (74.35%).Figure details the stability times achieved by 74.35% of the studies examined in this analysis.

Figure 14 .
Figure 14.Statistics of base lubricants used.

Table 1 .
Methods, stirring process and stirring time applied in previous research.

Table 3 .
Effective concentration used by the authors who used a concentration percentage.

Table 4 .
Base lubricants used in investigations.

Table 5 .
Nano-additives used in research.