Nanobiolubricant grinding: a comprehensive review

Fatty acids are important factors in the performance of vegetable oils and have been demonstrated in friction and wear tests [53]. The polar groups of fatty acids are chemically active. Depending on the attraction between the atoms or molecules, they can be firmly adsorbed onto the surface of a workpiece or tool to form a layered and oriented molecular grid. Consequently, the friction interface is separated by the adsorbed film, and sliding manifests as external friction between the adsorbed films. This reduces the direct contact between the tool and workpiece with good lubrication properties [25].

Fatty acids are classified into saturated and unsaturated fatty acids, and most vegetable oils contain high levels of unsaturated fatty acids. Unsaturated fatty acids have the right viscosity and surface tension; therefore, they provide a good balance between cooling and lubrication. The C=C bond contained in unsaturated fatty acids is easily oxidized, resulting in poor thermal stability of vegetable oil, making the oil film unstable and the lubricity properties correspondingly reduced [54]. Oils high in saturated fatty acids have received increasing attention. These oils tend to have higher viscosity, and therefore, better lubricating properties. More importantly, as shown in Fig. 4, compared to unsaturated fatty acids, saturated fatty acid molecules can be arranged in thin straight chains because of the absence of C=C bonds. The strong stacking effect improves the intermolecular interactions, resulting in a strong filling effect that contributes to the lubricant film strength. Padmini et al. [55] compared the performance of machining AISI 1040 steel with MoS₂ nanofluid based on three different vegetable oils (coconut, sesame, and canola oils). The coconut oil-based nanofluid exhibited the best processing performance, with 31.58% lower tool wear than that of canola oil. This is mainly due to its saturated fatty acid content of up to 90% with excellent lubrication properties. Zhang et al. [56] compared the machining performances of 45 steel grinding using different vegetable oils (soybean, palm, and rapeseed oils). The best lubrication performance was obtained with palm oil-based nanofluids. Yin et al. [57] systematically revealed the lubrication mechanisms of different vegetable oils (cottonseed, palm, castor, soybeans, and peanuts) with varying physicochemical properties. Palm oil exhibited the best lubricating properties. This is mainly due to the high saturated fatty acid content and viscosity of palm oil, whose main fatty acids are palmitic and stearic acids. The strongest and most diffuse lubricant film was formed at the grinding wheel-workpiece interface.

Various fatty acids have different carbon chain lengths owing to their different types. Fatty acids with different carbon chain lengths exhibit different oil film strengths. Because the cohesion between molecules is proportional to the number of carbon atoms, the strength and lubricating properties of lubricant films formed using fatty acids with longer carbon chains are stronger than those formed using fatty acids with shorter carbon chains [58]. Erhan et al. [59] found that canola oil exhibited better lubricating properties than soybean oil. This is mainly because canola oil (C22) has a longer carbon chain compared to soybean oil (C18). In general, the longer the carbon chain, the higher the viscosity and better the lubricating properties. However, for the saturated fatty acids, the frictional and wear resistances of the lubricant film peaked and remained constant when the carbon number was greater than 16. In this case, the lubricating effect did not change as the number of carbon atoms in saturated fatty acids increased. For unsaturated fatty acids, the presence of polar unsaturated C=C groups reduces the lubricating properties of vegetable oils by reducing the oxidative and thermal stabilities [60].

As shown in Fig. 5a, the C=C content in unsaturated fat is the activity center of many reactions, such as oxidation, and the oxidative stability of oils is a key factor affecting the quality of processing. The main components of most vegetable oils are fatty acid glycerides, and the presence of unsaturated double bonds in the fatty acid carbon chains leads to poor oxidative stability. Regarding the relationship between different saturations and lubricating properties, on the one hand, the presence of C=C makes the unsaturated fatty acids susceptible to oxidation, which leads to the degradation of the vegetable oils, resulting in the failure of the physically adsorbable oil film formed. On the other hand, the higher the percentage of C=C. Quinchia et al. [61] found that canola oil had a lower degree of double bond unsaturation compared to soybean oil. As a result, canola oil has strong antioxidant properties. The addition of antioxidants (e.g., vitamin E) is undoubtedly the most direct way to improve the oxidative stability of vegetable oils. Chemical modification is another method of improving the antioxidant properties of vegetable oils. This method focuses on the chemical reaction between the carboxyl groups and carbon chains of unsaturated fatty acids to change the degree of unsaturation, carbon chain length, and branching of the fatty acids in vegetable oils, thus improving the thermal oxidative stability of vegetable oils. Common modification methods include hydrogenation, esterification, vulcanization, epoxidation, and isomerization.

Both the oxidative stability and available viscosity range are important factors that limit their application as industrial lubricants [62]. During the grinding process, the viscosity of vegetable oil mainly affects the lubrication and heat transfer properties. Vegetable oils have different viscosities owing to the molecular structures of different fatty acids. Vegetable oils with low viscosity exhibit good cooling performance, whereas those with high viscosity exhibit good lubricating performance. Several common types of vegetable oils and viscosity are shown in Figs. 5b, c.

The high viscosity of the oil molecules results in a strong absorption force between them, thereby generating a high-strength oil film with good lubrication properties. For natural vegetable oils, higher viscosities are usually caused by polar groups (such as –OH and –COOH). The presence of polar groups facilitated the formation of oil films and enhanced the lubricating properties. Zhang et al. [63] discussed the effect of the viscosity of the base oil on friction and found that vegetable oils with high viscosity had good lubrication properties. The main reason for this phenomenon is that a higher viscosity impedes the flow of the nanofluid, and the grinding fluid stays in the grinding zone for a longer period of time, thus improving lubrication at the wheel and workpiece interface and reducing friction and wear. However, excessive molecular absorption leads to an inactive Brownian motion and reduced cooling performance. Temperature can be considered as a composite result of cooling and lubricating properties. If high viscosity oil is used, the lubricating properties are enhanced, and therefore less heat is generated; however, the cooling properties are diminished. Therefore, there should be an optimum viscosity value to achieve a balance between cooling and lubricating properties.

To avoid degradation of the heat transfer performance due to high viscosity, some researchers have proposed mixing high-viscosity oils with low-viscosity oils. Sajeeb and Rajendrakumar [64] mixed saturated coconut oil with highly unsaturated mustard oil for tribological testing. A 1:1 mixture yielded the best performance. Compared to coconut oil, the blend had an 18.18% higher viscosity and a 159.7% lower pour point. Jia et al. [65] mixed castor oil with six other vegetable oils (palm oil, soybean oil, peanut oil, rapeseed oil, corn oil, and sunflower oil) for grinding nickel-based alloy processing experiments. Soybean oil and castor oil had better lubricity properties. This is mainly because the addition of soybean oil to castor effectively reduces the viscosity of castor oil and improves its flow, atomization, heat exchange and wettability. In addition, according to the heat transfer theory, solid materials have a greater heat transfer capacity than liquid materials, whereas liquid materials have a greater heat transfer capacity than gaseous materials. Therefore, the addition of nanoparticles is an effective method to ensure the cooling performance of highly viscous vegetable oils.

In addition, vegetable oils do not satisfy the requirements for friction resistance and heat transfer in the high-temperature and high-pressure working areas of grinding. When the friction load increases to a certain level, the continuous fluid lubricant film breaks. At this point, measures such as increasing the viscosity of the lubricant alone do not work, and polar additives must be added to the lubricant [44]. Ozcelik et al. [66] conducted comparative experiments using different proportions of extreme-pressure additives in rapeseed oil. Rapeseed oil and 8% extreme pressure (EP) additives had the lowest surface roughness and cutting force. Sani et al. [67] conducted experiments on vegetable oil cutting fluids containing phosphorus and ammonia particles. The results showed that the new green cutting fluid had better tribological properties than mineral oil.

However, polar additives can cause ecological damage because they include sulfur, chlorine, and phosphorus. Nanoparticles have become a good alternative to traditional polar additives because of their inherently good heat transfer, anti-friction, and wear reduction properties, as well as their green nature. A comparison of the performances of mineral oil, biolubricant, and nanobiolubricant is shown in Fig. 6.

Oxide nanoparticles are often added to lubricating base fluids to improve their anti-friction and anti-wear properties.

Aluminum oxide (Al₂O₃) nanoparticles are hard and sphere-like in shape, which can enable them to act as a class of bearings in the lubrication process, with excellent friction-reducing and anti-wear properties. They are often used as additives to enhance the anti-wear and anti-wear properties of base fluids because of their ease of enhancing the load-bearing capacity and anti-wear properties of oil films. Titanium oxide (TiO₂) exhibits good anti-wear and load-bearing capacities. They have been widely used because of the easy availability of raw materials, good thermal and chemical stability, and non-toxicity. Ferriferrous oxide (Fe₃O₄) is a common natural compound that has received considerable attention owing to its superparamagnetic properties. The film-forming lubricating ability of Fe₃O₄ nanofluids becomes stronger under the influence of magnetic fields. Cupric oxide (CuO) can form a friction film on a metal surface to reduce friction and wear at the interface. However, the melting temperature of CuO nanoparticles is relatively low compared with that of other materials. Applications under high thermal loads must be discussed in detail. Zinc oxide (ZnO) has attracted considerable attention because of its large surface area, high surface energy, strong adsorption, and easy sintering properties [70]. As a common metal oxide, ZnO nanoparticles are easily prepared. Therefore, their addition to base oils not only improves the tribological properties of lubricants but also significantly reduces costs. However, owing to the low solubility of ZnO in oil, dispersion in base oil may be a challenge [71]. Silicon dioxide (SiO₂) is a widely used ceramic material with good wear resistance, electrical insulation, and high thermal stability. Under the action of stress, SiO₂ nanoparticles can be tightly adsorbed on the grinding wheel and workpiece interface through hydroxyl groups and initially form a physical adsorption film. Thus, friction damage at the interface was reduced to achieve better workpiece surface quality [72]. Zirconium oxide (ZrO₂) nanoparticles exhibit excellent optical and antioxidant properties, wear resistance, high melting points, and thermal stability. Notably, ZrO₂ nanoparticles have a small particle size and large specific surface area, which makes them easier to disperse in a basic solution [73]. Thus, ZrO₂ nanoparticles have a wide range of applications in optics, electronics, and processing.

Carbon nanomaterials have the advantages of high thermal conductivity, stability and wear resistance. Composed of elemental carbon, they do not contain harmful substances and have a low environmental impact compared with other heat transfer materials, which is in line with the requirements of sustainable development. Therefore it has become a popular research topic.

Carbon nanotubes (CNTs) are elongated tubular structures with diameter of 1–2 nm. Owing to their high thermal conductivity, strength, and hardness, they do not grind into hard films under high loads. Its special tubular structure can effectively reduce the sliding friction in the grinding area and improve the lubrication effect of nanofluids [74]. CNTs are considered as promising working nanoparticles owing to their great potential for improving thermal conductivity. Graphene (GR) has a high mechanical strength, excellent thermal conductivity, good tribological properties, and chemical inertness. Monolayer graphene nanoparticles have an ultrathin laminar structure and large specific surface area; therefore, graphene nanoparticles can easily enter the workpiece-abrasive interface. During friction, graphene nanoparticles continuously precipitate and adhere to the surfaces of workpieces and tools, forming an adsorbed friction-reducing layer, which makes it a very promising material for high-load-bearing nanolubrication [75]. Nanodiamonds (NDs) are important carbon nanomaterials. Owing to their high hardness and stable structure, NDs are less prone to deformation and chemical reactions during processing. In addition, some ND nanoparticles may fill the interfacial gaps in the grinding zone, thereby acting as abrasive particles and providing a synergistic effect for material removal.

Metal nanoparticles are prepared purely from metal precursors. Because of their localized surface plasmon resonance, these nanoparticles exhibit unique optoelectronic properties. For example, metal nanoparticles such as gold (Au) and silver (Ag) are important in numerous fields such as semiconductors, magnetism, and catalysts because of their unique properties such as optics and electricity, as well as their high thermal conductivity. In recent years, several other properties of metal nanoparticles have attracted the interest of researchers. For example, in addition to their tribological effects, Cu nanoparticles exhibit excellent self-healing properties and are environmentally friendly [58, 76]. However, the introduction of these particles into nanofluids in the manufacturing sector is not cost-competitive, making their use difficult to scale up.

Molybdenum disulfide (MoS₂) is the most common nanofluidic component in grinding and is favored by researchers owing to its excellent heat transfer and lubrication properties. Owing to the strong Mo–S bond and the weaker bonding of sulfur atoms between the molecular layers, a plane of lower shear was created. This plane fractured along the molecular layer under intermolecular shear, creating a sliding plane. This structure provides the MoS₂ nanoparticles with a certain degree of friability, flexibility, and ductility. When cutting forces are present, the MoS₂ nanoparticles expand into a thin physical film in the cutting zone to reduce the wear and friction coefficients.

In addition, researchers have a positive attitude towards the composite use of different types of nanoparticles. Based on the mechanisms of action of different nanoparticles during processing, comprehensive nanofluids with multifunctional requirements can be achieved. Previous research has shown that the cooling and lubrication mechanisms change significantly after mixed use, and the cooling and lubrication performance can be maintained at a certain level simultaneously [77]. The improvement in the processing performance of the hybrid nanobiolubricants was significantly greater than expected.

The one-step approach involved dispersing the nanoparticles directly in the base solution during preparation. This method avoids the problems of collecting, storing, and transporting nanoparticles, minimizes the aggregation of nanoparticles, and effectively avoids the oxidation of nanoparticles in air.

The vapor-phase deposition method proposed by Souza et al. [78] has been the most widely used one-step method in recent years. As shown in Fig. 7, the vacuumed vessel contains a liquid of low-pressure vapor, and the liquid is rotated so that it forms a very thin film on the walls [79]. When the vapor of the raw material condenses and settles in the base fluid by interacting with the film, a nanobiolubricant is obtained, and the cooling system is used to prevent the liquid from heating up and increasing the pressure inside the vessel. As the entire process is performed under vacuum, oxidation reactions can be avoided and high-purity metal nanoparticles can be obtained. The Argonne laboratory in the United States prepared Cu nanobiolubricants using this method. The nanoparticles exhibited good dispersion and high suspension stability in basic solutions.

The chemical liquid-phase method involves the preparation of nanobiolubricants by chemical reactions from the molecular and atomic points of view. Sandhya and Nityananda [80] prepared Cu-based nanobiolubricants using this method. They also found that the particle diameter decreased as glucose concentration increased. This is mainly because, as the concentration of the reducing agent increased, the rate of reduction accelerated, and the number of precipitated metal ions increased dramatically, forming more nuclei and thus smaller particles during nucleation. The liquid phase also participates in the chemical reactions and cannot be directly replaced by the base solution. The direct preparation of nanofluids using the liquid-phase method is not as common as that using the gas-phase method.

The one-step method combines the preparation and dispersion of nanoparticles; the prepared nanoparticles have a small particle size and good dispersion stability. This is because of the avoidance of additional dispersion steps and the consequent reduction in the agglomeration of nanoparticles. However, this method is only suitable for the preparation of nanofluids in fluids with low vapor pressure, and is not suitable for high-volume industrial production owing to its high equipment requirements, harsh environmental requirements (e.g., vacuum), and high production costs.

As shown in Fig. 8, the two-step method involves the preparation and dispersion of nanoparticles in two steps [81]. Nanoparticles are prepared by certain physical or chemical methods and then dispersed in the base fluid by ultrasonic dispersion, high-pressure homogenization, mechanical stirring, and the addition of dispersants.

The one-step method is mainly suitable for the preparation of nanobiolubricants of metal nanoparticles, as it avoids direct contact with air. However, the two-step method is more suitable for the preparation of nanobiolubricants containing oxide nanoparticles; the two-step method works better [82]. The main disadvantage of the two-step method compared with the one-step method is the large amount of particle aggregation that accompanies the process. However, the one-step method is costly and difficult to operate, whereas the two-step method has the advantages of simplicity, low cost, applicability to almost all types of nanofluid preparations, suitability for practical high-volume production, and high commercialization potential. Therefore, a two-step method is commonly used when preparing nanobiolubricants.

In the process of preparing nanobiolubricants, particles are attracted to each other and tend to aggregate and settle owing to the size effect, especially during the drying, storage, and transport of nanoparticles. Clusters of nanoparticles not only cause settling and blocking of microchannels but also reduce thermal conductivity [83]. In addition, it is difficult to uniformly and stably suspend nanoparticles in a basic solution because of the strong attractive van der Waals forces between the nanoparticles, which lead to collision and aggregation effects. Therefore, when using a two-step process to prepare nanobiolubricants, one of the key requirements is to maintain long-term dispersion homogeneity of the nanobiolubricants suspension and prevent the formation of particle clusters.

Sedimentation is one of the simplest methods for evaluating the stability of nanobiolubricants. A high-resolution camera was used to capture images of the nanofluids to detect precipitation. As shown in Fig. 9, under the influence of gravity, the nanoparticles began to settle in the lower part of the container, and after a certain period, the particles began to cluster. Nanobiolubricants are considered stable when the nanoparticles in a nanofluid are uniformly dispersed and do not precipitate over time. According to the Stokes’ theorem, the smaller the number of nanoparticles, the smaller the deposition rate. As a result, smaller nanoparticles settle more slowly than larger ones. However, the size of the nanoparticles must not be small because when the size of the particles is small, the surface energy of the particles increases, resulting in aggregation of the particles [86]. However, this method is not suitable for assessing the stability dark-colored nanobiolubricants. Zeta potential is a measure of the strength of the mutual repulsion or attraction between particles and is an important indicator for characterizing the stability of a colloidal dispersion. The higher the absolute value of the zeta potential, the greater the repulsive force between the particles, which prevents particle aggregation, and the more stable the system. Conversely, a lower zeta potential indicates that the attractive forces between the particles exceed the repulsive forces; the dispersion is more easily disrupted; and agglomeration occurs. In general, precipitate formation is observed within a short period of time when the absolute value of the zeta potential is between 15 mV and 30 mV. Nanobiolubricants with zeta potentials greater than 30 mV are relatively stable; those greater than 40 mV have good stability; and those greater than 60 mV are considered to have excellent stability. Kim et al. [87] prepared gold nanobiolubricants using single-pulse laser beams without any dispersant, which still exhibited good stability after one month because of the large negative zeta potential of gold nanoparticles in water. Although transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are primarily used to the characterization of nanoparticles, they also play important roles in measuring nanofluid stability [88]. When using electron microscopy, the prepared nanobiolubricants are usually dropped onto a copper grid coated with carbon to observe the distribution of nanoparticles on the copper grid when the nanobiolubricants have completely evaporated. Because the evaporation of nanobiolubricants is always accompanied by the aggregation of nanoparticles, TEM is only applied when the concentration of nanoparticles is low [89]. Li et al. [90] studied the stability of 0.1% copper nanobiolubricants with different dispersant types and concentrations at pH 9.5 using TEM technology. The best dispersion was obtained at a concentrations of 0.43% for the dispersants. Xu et al. [8] evaluated the effect of different pH values on the stability of TiO₂ nanofluids with the same nanoparticle concentration using TEM and found the best stability at pH 12. This is mainly because when the pH is above the isoelectric point, the surface charge of the nanoparticles increases as the pH increases, leading to an increase in inter-particle repulsion and therefore an increase in the absolute value of the zeta potential [91].

Ultrasonic dispersion is an effective method to reduce nanoparticle agglomeration. By placing the nanobiolubricants in an ultrasonic field, the nanoparticles are vibrated by ultrasonic waves of a suitable frequency to overcome the gravitational force between the nanoparticles. This destroys the original agglomeration of the nanoparticles. And it destroys the equilibrium between the nanoparticles and the molecules of the base fluid. Thereby, it improves the stability of the nanofluid.

Sharif et al. [92] treated TiO₂ nanobiolubricants in a magnetic stirrer for 30 min and then in an ultrasonic bath for 2 h and found that the nanobiolubricants were stable for over seven months by stability analysis using TEM. Kalita et al. [93] prepared Fe₃O₄ nanobiolubricants that were stabilized for more than 30 d after 2 h sonication by an ultrasound probe. Sadeghi et al. [94] studied the effect of sonication on the stability of Al₂O₃ nanobiolubricants and found that the zeta potential increased with increasing sonication mixing time, reaching 50 mV with good stability after 150 min.

However, longer ultrasound treatments may change the internal structure and properties of the particles; therefore, a sustained increase in ultrasound time does not necessarily reduce the particle size [95]. Yang et al. [96] prepared gold nanobiolubricants using ultrasound-assisted preparation and found that the number of agglomerated particles decreased with increasing sonication time. However, after 45 min, no change in the particle size was observed. Kole and Khandekar [97] prepared ZnO nanobiolubricants and found that the particle size decreased from 459 nm to 91 nm with increasing sonication time. However, as the ultrasound treatment time increased to 100 h, the particle size increased to 220 nm. This can be attributed to the ability of the ultrasound to provide energy to the molecules. According to the law of energy conservation, the kinetic energy of the nanoparticles increases as the ultrasound time increases. Song et al. [98] found that when ultrasound was used to prepare nanobiolubricants, the temperature of the liquid gradually increased over time. This is mainly because when the ultrasonic energy is sufficiently high, ultrasonic cavitation is generated, creating a large number of microbubbles in the liquid medium. The formation and disappearance of microbubbles in a short period generates a large, localized high temperature and pressure in the liquid, which increases the temperature of the base liquid. This phenomenon leads to an intensification of the motion of the nanoparticles and an increase in the probability of particle collisions, which in turn leads to clusters.

Although the agglomeration size decreased and the stability increased with increasing sonication time, this was inaccurate for longer treatment times. Therefore, further exploration of sonication times is necessary. Surfactants, also known as dispersants, are a simple and economical way to improve the stability of nanobiolubricants. Surfactants can be adsorbed onto the surface of the particles, thus altering the surface properties of the nanoparticles, creating strong particle-to-particle repulsion, and allowing for proper dispersion [99]. When used, it is often combined with a dispersion method, such as ultrasound, to achieve better results.

As shown in Fig. 10, common surfactants can be classified as anionic, cationic, nonionic, or amphoteric. The choice of dispersant type must be based on the nature of the base fluid and the selection of the dispersant to cover the nanoparticles, which must be compatible with the nature of the base fluid to prevent cluster formation.

To achieve long-term stability, there should be good compatibility between the nanoparticles and base fluid. Hydrophilic nanoparticles are compatible with polar solvents, while hydrophobic nanoparticles are easily dispersed in non-polar solvents. However, if hydrophobic nanoparticles are dispersed in polar base fluids and hydrophilic nanoparticles are dispersed in non-polar base fluids, surfactants need to be added [97].

Different types of surfactants have different hydrophilicities and hydrophobicities; therefore, their compatibility is also different. Anionic surfactants, such as sodium dodecyl sulfate (SDS), are typically suitable for stabilizing hydrophilic nanoparticles. They have hydrophilic head groups and hydrophobic alkyl chains, which can form micellar structures in water and wrap around the nanoparticles. Cationic surfactants such as hexadecyltrimethylammonium bromide (CTAB) are suitable for stabilizing hydrophobic nanoparticles. They contain hydrophilic head groups and hydrophobic alkyl chains that can interact with the hydrophobic nanoparticles to form micellar structures.

Mao et al. [100] compared the dispersion stability of Al₂O₃ nanobiolubricants under the same ultrasonic conditions with and without the addition of SDS. The results showed that Al₂O₃ was more uniformly dispersed in the dispersion system with the application of the dispersant. Gao et al. [101] studied the effects of surfactants with different hydrocarbon chain lengths on the dispersion stability using palm oil as the base oil and CNTs as nanoparticles. The nanobiolubricants containing alkylphenol polyoxyethylene ether 10 (APE-10) exhibited optimal dispersion stability. This is because the length of the hydrocarbon chain of an ionic surfactant affects its hydrophilicity and lipophilicity, the longer the hydrocarbon chain, the more lipophilic it is. Zhou et al. [102] found that when the carbon number of the straight-chain alkane chain of the lipophilic group increased to 16, the long chain curled, thereby increasing the single-molecule cross-sectional area of the surfactant. As a result, the saturated adsorption of surfactants decreases and the adsorption of surfactants decreases, making the dispersion less stable. Table 2 summarizes the results of different studies on the effect of surfactants on dispersion stability based on zeta potential and dispersion time [90, 103‐107].

Although the addition of surfactants is an effective way to improve dispersion stability, excessive amounts of surfactants can affect the thermophysical properties of nanofluids by reducing their thermal conductivity [108]. Amrita et al. [109] studied the thermal conductivity of graphene nanobiolubricants using different dispersant types and found that the thermal conductivity tended to increase and then decrease with increasing concentrations of gum arabic (GA) and Triton X100 (TX 100). Wang et al. [110] studied the thermal conductivity of graphite nanobiolubricants at different dispersant concentrations and observed that the maximum value was achieved when the weight ratio of the dispersant to graphite was approximately 3:1. The main reason for this phenomenon is that when the surfactant concentration reaches a critical micelle concentration (CMC), it aggregates to form micelles. The formation of micelles can cause sudden changes in the surfactant properties, resulting in local clustering. As shown in Fig. 11, the number of micelles increased with increasing surfactant concentration, which affected the homogeneity of the nano-added phase composition and reduced dispersion stability. In addition, the micelles formed had an insulating layer that prevented heat transfer. Therefore, the dispersant can be effectively adsorbed on the particle surface only when the correct amount of surfactant is added. Considering that the volume fraction of the nano-added phase generally does not exceed 2%, the surfactant concentration should not be too high to avoid affecting physical properties.

Although the addition of surfactants is an effective method for improving the stability of nanoparticle dispersions, the use of surfactant may pollute the heat transfer medium and produce foam at high temperature, thus improving the thermal resistance [111]. In addition, the biodegradability of commonly used surfactants such as GA, SDBS, and SDS has not been reported [112]. Therefore, it is necessary to develop novel and efficient dispersants.

The most critical parameter affecting the thermophysical properties of nanobiolubricants is the thermal conductivity. Nanoparticles with higher thermal conductivity are generally considered to exhibit better heat transfer performance. Table 3 lists the thermal conductivity values of several common nanoparticles and the thermal conductivity of water at room temperature as a control.

Sen et al. [113] measured the thermal conductivity of Al₂O₃. They concluded that the thermal conductivity of the nanoparticles was highly dependent on their shape. Jeong et al. [114] studied the effect of different shapes of nanoparticles on thermal conductivity and found that the thermal conductivity of nanobiolubricants with rectangular nanoparticles increased by 5.9% compared to that of nanobiolubricants with spherical nanoparticles. This is mainly because with the increase of the length-diameter ratio of nanoparticles, the contact area increased and the collision heat transfer rate increased [115]. Hamid et al. [116] measured the thermal conductivities of TiO₂ nanobiolubricants with different shapes. The cylindrical nanoparticles had greater thermal conductivity than the spherical nanoparticles. This is mainly because the shape factor of cylindrical particles is larger than that of spherical particles. According to the Hamilton-Crosser model, nanoparticles with a large shape factor have a higher thermal conductivity.

The particle size is an important parameter that affects the thermal conductivity. Numerous studies have demonstrated that the thermal conductivity of nanobiolubricants increases with decreasing particle size. This was mainly due to the high surface energy of the nanoparticles, which increased as the size of the nanoparticles decreased for the same volume fraction. The coordination of unsaturated atoms produces a large number of vacant and unsaturated bonds, resulting in an increased activity and Brownian motion of the nanoparticles. This results in a faster separation of the nanobiolubricants from the workpiece and increased perturbation of the workpiece boundary, leading to enhanced heat transfer [117].

Yang et al. [118] studied the dynamic heat flow density during the microgrinding of Al₂O₃ nanoparticles of different sizes. The use of nanoparticles with sizes of 30 nm, 50 nm, 70 nm, and 90 nm reduced the temperature by 21.4%, 17.6%, 16.1%, and 8.3%, respectively. Maheshwary et al. [119] studied the thermal conductivity of particles of different sizes at a concentration of 2%. Compared with the pure base fluid, the thermal conductivity increased by 61% when the particle size was 101 nm and by 96% when the particle size was 31 nm. Sun et al. [120] measured the effective thermal conductivities of SiO₂ nanobiolubricants with particle sizes of 10 nm and 60 nm using a rotating cooter. The thermal conductivity was improved by 13% and 11% at particle sizes of 10 nm and 60 nm, respectively, compared with that of the pure base fluid.

The concentration of the nanobiolubricants had a significant effect on the thermal conductivity. Alawi et al. [32] studied the effect of the volume fraction of nanoparticles on the thermal conductivity of nanobiolubricants. The heat transfer coefficient increased with the volume fraction of Al₂O₃ nanoparticles. Duangthongsuk and Wongwises [121] found that the thermal conductivity increased by 13.2% when the volume concentration of TiO₂ in water increased from 0.2% to 2%.

High concentrations can lead to the agglomeration of nanoparticles and a reduction in thermal conductivity. Makhesana et al. [122] studied the thermal conductivity of MoS₂ nanoparticles added to rapeseed oil at different concentrations (0.5%, 1%, and 1.5%) and found that the thermal conductivity of the nanobiolubricants increased between 0.5% and 1% nanoparticle concentration and decreased slightly after 1%–1.5% concentration. Sahooli et al. [123] prepared CuO nanobiolubricants in the concentration range of 0.01%–0.1%. They found that thermal conductivity increased as the number of nanoparticles added to the base fluid increased. As shown in Fig. 12, Li et al. [124] conducted an experimental study on the grinding of Ni-based alloys with different NMQL concentrations. The lowest grinding temperature of 108.9 °C and the lowest energy scaling factor of 42.7% were obtained with a 2% nanobiolubricants concentration. However, as the concentration increases further, the thermal conductivity begins to decrease. This is because of the formation of clusters of nanoparticles in nanobiolubricants, which makes them unstable.

However, the optimum concentrations obtained by different scholars vary, with the optimum concentration distribution at most processing parameters ranging from 0.5% (volume fraction) to 2.5% (volume fraction). Therefore, to achieve high heat transfer, academics in the field must work together to establish a database that provides a table of optimum concentration recommendations for engineering applications.

Temperature variations in nanobiolubricants have a significant impact on their thermal conductivity by affecting the Brownian motion of the nanoparticles. Sezer et al. [125] found that the thermal conductivity of CNTs nanobiolubricants increased by 69% when the temperature increased from 20 °C to 30 °C at the same concentration. Sharma et al. [126] measured the thermal conductivity of CNTs nanobiolubricants samples at five temperatures (25 °C, 35 °C, 40 °C, 45 °C, and 50 °C) by a transient thermometer. The thermal conductivity of all the nanobiolubricants increased with increasing temperature. In addition, the relative rate of change of the base fluid to the nanofluid determines the thermal conductivity of nanobiolubricants. Therefore, an increase in temperature does not necessarily contribute to the enhancement of thermal nanobiolubricants, and sometimes, the result may be the opposite [127].

Ultrasonic vibration-assisted grinding superimposes high-frequency microamplitude ultrasonic vibrations onto conventional grinding. From a processing perspective, ultrasonic vibration and the superposition of the trajectory of the abrasive grain, such that the relative motion between the abrasive grain and the workpiece shows short contact and long separation, reduce the actual grinding time of the abrasive grain. In addition, the “contact-separation-contact-separation” cycle of intermittent grinding between the abrasive grits and the workpiece allows the coolant to effectively enter the grinding area, improve the heat transfer efficiency of the coolant and reduce the grinding temperature. It has the advantages of high cooling and heat transfer efficiencies, low grinding temperature, and high surface quality of the workpiece obtained. For NMQL, the penetration of the grinding fluid is hindered by the narrow gap at the tool-chip interface. Ultrasonic vibrations allow the grinding wheel to be separated from the grinding chips, providing a microspace for the lubricating medium to enter the grinding area. The grinding fluid was drawn into the space where the grinding chips were separated because of the pumping action of the transient vacuum, giving full play to its lubricating effect [102]. The ultrasonic vibration-assisted grinding device is shown in Fig. 26a, where (1) indicates the piezo actuator and transducer, (2) the horn and booster, (3) the work fixture (main plate) with the flexible joint, (4) the workpiece, (5) the MQL nozzle, (6) the grinding wheel, and (7) the force.

Elcioglu and Murshed [203] found that ZnO nanobiolubricant droplets in the presence of ultrasonic vibrations could explode into a microspray of smaller droplets and form a stable capillary wave, which enhanced the infiltration properties of the droplets. Yan et al. [204] found that under the radial ultrasonic vibration MQL process, the lubricating medium could enter the chip-tool contact interface more effectively and affect the friction under tool vibration. Yan et al. [204] applied ultrasonically empowered nanofluidic microlubrication for the microgrinding of biological bone materials. The grinding temperature was reduced by 33.5% and 10.0% compared to ultrasonic vibration alone and NMQL, respectively. Rabiei et al. [205] studied the cooling effect of an Al₂O₃ NMQL in tangential ultrasonic vibration-assisted grinding. A temperature reduction of up to 48% in the grinding zone compared to dry grinding, a glossy surface was obtained, and thermal damage was effectively avoided. Molaie et al. [206] compared the surface quality of the MoS₂ nanobiolubricant MQL grinding before and after the application of horizontal ultrasonic vibrations. The results are shown in Fig. 26b, which shows that the use of ultrasound together with the nanobiolubricant MQL significantly improved the surface roughness. This is partly because the nanoparticles penetrate the grinding zone more easily when vibration is superimposed and partly because the low frictional grain movement in horizontal vibration creates a higher chance of cutting surface burrs, resulting in an increase in surface integrity. Duan et al. [84] studied the machined surface of a Ti-6Al-4V titanium alloy with ultrasonic vibration-assisted nanobiolubricant grinding. The axial motion generated by the ultrasonic vibration created sinusoidal grinding marks on the surface of the workpiece, which reduced the adhesion phenomenon in the grinding of titanium alloys. Thus, slip and plowing during grinding were reduced, and the grinding temperature was reduced by 15.7%, thereby improving the quality of the machined surface.

The technological advantages of ultrasound-enabled MQL are even more significant for the machining of hard and brittle materials. Gao et al. [207] studied the material removal mechanism of ultrasonic vibration-assisted CNTs nanobiolubricant grinding of a carbon fiber-reinforced polymer (CFRP). The ultrasonic vibration-assisted treatment reduced the brittle fracture of the material and achieved the lowest grinding force and optimal surface quality compared with the pure nanobiolubricant. Wdowik et al. [208] examined the surface quality of ZrO₂ ceramics after ultrasonic-assisted and ordinary grinding. It was found that ultrasonic vibrations could significantly promote plastic material removal and form a special weave on the surface of the workpiece. Xu et al. [177] found that ultrasonic vibration-assisted grinding of CFRP composites effectively suppressed the formation of damage and obtained lower grinding forces and better surface morphology compared with conventional grinding. Liang et al. [209] found that ultrasonic vibration was effective in reducing the generation of scratches when grinding CFRP composites. The main reason for this is because axial ultrasonic vibration effectively increases the length of the grinding arc, and the superimposition between the trajectories of the grinding grains plays a reciprocal role in ironing the surface of the workpiece. In addition, based on ultrasonic cavitation, the rapid expansion and closure of the bubble generates a large pressure around the solid-liquid interface to produce repeated impacts and break down the surface residue of the workpiece and flake it off, which can have an ultrasonic cleaning effect on the surface of the workpiece to improve the quality of the processed surface [210].

In summary, the combination of ultrasonic assistance and NMQL changed the material removal mechanism and significantly improved the grinding performance. Simultaneously, by enhancing droplet wettability, ultrasonic-assisted grinding improved the cooling heat transfer performance and effectively reduced the grinding temperature.

Magnetic field empowerment involves the modulation of the processing properties using magnetic fields in conjunction with magnetic nanofluids. Nanoparticles exhibit a magnetic response to an applied magnetic field and can change their morphology and movement in response to changes in the magnetic field, such as alignment, aggregation, and dispersion. This enables the adsorption, separation, and manipulation of other substances using nanoparticles. Consequently, magnetic nanoparticles are more likely to adsorb onto the friction interface under the influence of a magnetic field, filling and repairing them and significantly improving their tribological properties [211]. As shown in Fig. 27, the magnetic field exerts an additional adsorption force on the lubricant compared to the conventional NMQL, which is much higher than the viscous force and gravity. The nanofluid can be stored and transported in the pores of the wheel, significantly improving its permeability [212]. In addition, by influencing the surface energy of the magnetic droplets, the magnetic field can reduce the contact angle of the droplets and improve heat transfer. Consequently, magnetic nanofluids under the influence of a magnetic field exhibit better friction and heat transfer capabilities, which reduce the surface roughness of the machined workpiece and improve the grinding performance.

Wang et al. [213] found that as the strength of the magnetic field increased; particles in the heat transfer channel aggregated; the degree of fluid movement increased, thereby improving the heat transfer performance. The heat transfer performance of the Fe₃O₄ nanofluid improved by 17.62% compared to that of pure water. Cui et al. [212] compared the grinding performances of graphene and Fe₃O₄ for grinding a Ti-6Al-4V alloy. They found that the permeability of Fe₃O₄ was enhanced by magnetic field adsorption and traction. Compared with graphene nanoparticles with high thermal conductivity, the grinding temperature was reduced by 13.8%. Lv et al. [214] found that the contact angle of a droplet decreased under the influence of a magnetic field. This not only improves the permeability of the Fe₃O₄ nanofluid but also increases the diffusion area of the droplet once it reaches the heat transfer surface, resulting in an increase in the evaporative heat transfer efficiency of the droplet and enhancing its lubrication and cooling efficiency. The magnetic-field-assisted Fe₃O₄ nanobiolubricant MQL had a 35.5% reduction in cutting temperature compared with the conventional vegetable oil nanofluid MQL. Gao et al. [207] found that the addition of an external magnetic field could guide Fe₃O₄ nanoparticles to penetrate the tool-chip interface, resulting in better friction reduction and improved machining performance. The surface roughness was reduced by 25.6% compared with that of pure NMQL.

In addition, scholars have studied the coupling between magnetic nanofluids and textured tools. The directional transport of magnetic nanofluids in the presence of a magnetic field allows magnetic nanoparticles to be concentrated in the grooves of textured tools, facilitating the formation of a lubrication layer between the workpiece and tool. The texture of the tool surface, on the other hand, holds the cutting fluid and further facilitates the penetration of the lubricating fluid, while also storing some of the abrasive chips and preventing them from scratching the substrate surface. Thus, coupling magnetic nanofluids and textured tools under the influence of a magnetic field can effectively improve tribological properties and surface quality. Guo et al. [215] revealed the coupling mechanism between textured tools and magnetic nanofluids in the presence of magnetic fields. The best performance was exhibited when the magnetic field direction was parallel to the groove direction, with a 19.4% reduction in the surface roughness compared with no magnetic field. Zhang et al. [216] studied the coupling of magnetic nanofluids and textured tools in a magnetic field. The surface roughness of the workpiece under the new process conditions was reduced by 49.1% compared with that under conventional lubrication.

In summary, magnetic field empowerment improves the frictional and heat transfer properties of nanofluids, and the enhancement of the processing performance has been widely demonstrated. However, the current selection of magnetic nanoparticle types is homogeneous. Therefore, different types of magnetic nanoparticles must be compared.

Their lower coefficient of friction, high antiwear properties, and high heat transfer properties make nanobiolubricants promising candidates for a wide range of grinding applications. The high temperature and pressure generated during the grinding process lead to friction and wear, thereby reducing machining quality and tool life. Nanobiolubricants reduce friction and wear by forming a nano-lubrication film on the grinding interface, which can be automatically repaired during the grinding process, thereby extending tool life and improving machining quality. Based on the physical and chemical properties of different nanoparticles, nanoparticles with an excellent heat exchange medium and high hardness are recommended under the condition of intense friction during the grinding of cemented carbide. In the grinding of soft medium-carbon steel, nanoparticles with multi-layer structures are recommended to effectively transfer the friction state of friction interface parts. Facing the boundary conditions of high temperature and high pressure during the grinding of difficult-to-machine materials, hybrid nano-biolubricants and multi-field empowerment modulation are effective methods to solve the problems of mechanical and thermal damage to workpieces.