Effect of surface modification on the stability and friction and wear properties of ZrO2 / lubricating oil nanofluids

Effect of surface modification on the stability and friction and wear properties of ZrO2 / lubricating oil nanofluids | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of surface modification on the stability and friction and wear properties of ZrO2 / lubricating oil nanofluids Tao Zhu, Shan Qing, Juan Duan, Zhihui Jia, Mingyue Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6241691/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 03 Jun, 2025 Read the published version in Journal of Nanoparticle Research → Version 1 posted 13 You are reading this latest preprint version Abstract In this paper, ZrO 2 / lubricating oil nanofluids modified by silane coupling agent γ-methacryloxypropyltrimethoxysilane ( KH570 ) were prepared by surface modification technology. Four surfactants and unmodified nanoparticles were added to the lubricating oil to prepare nanofluids with surfactants. Firstly, the characteristics of modified particles were studied. The results showed that the highest grafting rate of KH570 was found in the modified nanoparticles with 20ml KH570, and the grafting rate was 4.818%. At the same time, it was found that the modified nano-zirconia changed from hydrophilicity to hydrophobicity, and the lipophilicity and dispersion stability were also improved. The stability and friction and wear properties of the prepared nanofluids were studied. The results show that the nanofluids prepared by adding sodium dodecyl benzene sulfonate ( SDBS ) and unmodified nanoparticles in the surfactant have better stability. In contrast, the nanofluids prepared by KH570 modified nanoparticles showed more excellent stability. The modified nano- ZrO 2 has better tribological properties than the unmodified ZrO 2 . The best friction coefficient of the modified nano- ZrO 2 lubricating oil is 0.0753, and the L-TSA46 base oil is reduced by 40.71%. ZrO2 / lubricating oil nanofluids surfactant surface modification stability friction and wear Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 1. Introduction In today's era of accelerated industrialization, the efficient and stable operation of mechanical equipment has become a key goal pursued in various fields. Friction, as an inevitable phenomenon in mechanical systems [ 1 ] , not only leads to a large amount of energy loss, according to statistics, about one-third of the world's energy consumption in overcoming friction, but also causes serious wear of mechanical components, which greatly shortens the service life of equipment and may even lead to safety accidents [ 1 – 3 ] .Lubricating oil is the core medium to reduce friction and wear [ 4 ] .Its performance directly determines the lubrication effect of the friction pair, which in turn affects the working efficiency and life of the entire mechanical system. With the continuous improvement of mechanical performance requirements in modern industry, traditional lubricating oil has been difficult to meet the increasingly stringent working conditions. In this context, the rapid development of nanomaterial technology has brought new opportunities for the improvement of lubricating oil performance [ 5 ] . Nanomaterials are considered to be an environmentally friendly choice because of their low environmental impact and easy decomposition [ 6 ] , especially in applications as lubricant additives. Studies have shown that nanoparticles as additives for lubricating oil show more excellent extreme pressure bearing capacity, better wear resistance, and better lubrication effect than traditional lubricating additives [ 7 ] , which reveals its great development prospects as an advanced lubricating material. Zhao et al. [ 8 ] prepared Pb nanoparticles and revealed that Pb nanoparticles exhibited excellent wear resistance. Compared with the lubricating oil without Pb nanoparticles, the wear diameter was significantly reduced. The small size of Pb nanoparticles is the key factor to give it excellent lubrication characteristics. Asadauskas et al. [ 9 ] compared the tribological properties of Fe, Cu, and Zn nanoparticles in oil. Iron nanoparticles enhanced the anti-wear ability of antioxidant rapeseed oil. Zn nanoparticles reduce the wear rate of mineral oil and form smoother wear traces. Compared with soft metals such as tin ( Sn ), metal oxide nanoparticles have higher hardness, which allows them to provide more significant lubrication in environments with heavy loads and extreme pressures [ 10 ] . At the same time, the hard nature of metal oxides also gives them polishing effect [ 11 ] , which helps to reduce the surface roughness. Qian et al. [ 12 ] chemically modified the anatase TiO 2 nanoparticles prepared by them with stearic acid, and then investigated their tribological behavior. The results show that the modified TiO 2 nanoparticles can significantly improve the anti-friction and anti-wear properties and bearing capacity of paraffin oil.Ingole et al. [ 13 ] used nano-titanium dioxide and commercial titanium dioxide ( P25 ) as lubricant additives to study the tribological behavior. It was found that the addition of commercial titanium dioxide would lead to an increase in the friction coefficient. On the contrary, the addition of nano-titanium dioxide makes the friction coefficient more stable and decreases it. This phenomenon may be due to the formation of a uniform protective film on the sliding contact surface of nano-titanium dioxide. Battez et al. [ 14 ] discussed the anti-wear behavior of CuO, ZnO, and ZrO 2 nanoparticle suspensions in poly-α-olefin (PAO6). The results show that nano-ZnO, nano-ZrO 2 and nano-CuO used as lubricant additives can reduce friction and wear compared with base oil. The suspension with 0.5% ZnO and ZrO 2 has the best comprehensive tribological properties, showing the lowest friction coefficient and wear.Dejang et al. [ 15 ] prepared monoclinic ZrO 2 nanopowders and Al 2 O 3 / 3wt % ZrO 2 nanocomposite powders by the ball milling method. The results show that Al 2 O 3 / 3wt % ZrO 2 composite has the lowest friction coefficient and sliding wear rate, and its wear resistance is significantly better than that of single Al 2 O 3 . In addition, alumina ceramics containing zirconia additives show a lower friction coefficient and higher wear resistance [ 16 ] . These studies have shown that zirconium compounds have great potential as anti-wear and anti-friction additives, but there are few studies on single zirconium compounds. Compared with easily oxidized metal elements, metal oxides exhibit more stable properties [ 17 ] . However, due to their small size, high surface energy, and easy agglomeration [ 18 ] , the dispersion and stability of nanoparticles in lubricating oil have become one of the key challenges in the application of lubricating oil additives, which greatly limits the application of nanomaterials in many fields [ 19 – 23 ] .At present, the effective methods to improve the solubility of nanoparticles in base oil are surface modification [ 24 ] and the use of dispersants as surfactants [ 25 ] . However, there are few studies on the surface modification of nano-fluids. Ma et al. [ 26 ] studied the addition of sodium dodecyl sulfate (SDS), PVP, and cetyltrimethylammonium bromide (CTAB) to Al 2 O 3 / TiO 2 water and Al 2 O 3 / CuO-water nanofluids. It was found that the increase of surfactant concentration improved the stability and viscosity. Sarsam et al. [ 27 ] studied the effects of sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and gum arabic (GA) on the stability and thermophysical properties of aqueous graphene nanosheets (GNPs) nanofluids. The results show that the addition of SDBS makes the stability of the system the best, followed by the thermal conductivity. Zhang et al. [ 28 ] prepared γ-glycidoxypropyltrimethoxysilane (GPTMS)-modified TiO 2 /water nanofluids by surface modification technology and studied its stability. The results show that the nanofluids prepared by adding sodium dodecyl sulfate ( SDS ) and unmodified nanoparticles have moderate stability. The nanofluids prepared by GPTMS-modified nanoparticles showed excellent stability. Zhao et al. [ 29 ] modified the surface of commercial TiO 2 nanoparticles with 3-aminopropyltrimethoxysilane and 3-isocyanatopropyltrimethoxysilane by an aqueous phase process. They found that the dispersion stability of particles was positively affected by the increase of zeta potential of particles. It can be seen that the effects of different surfactants on ZrO 2 /lubricant nanofluids, as well as the detailed properties of surface modified ZrO 2 nano-lubricants and the effect of single zirconium compound as anti-wear and anti-friction additives, there are still some unknown problems in the research of this field. Therefore, in this paper, γ-methacryloxypropyltrimethoxysilane was used to modify the surface of ZrO 2 nanoparticles, and the modified nanoparticles were characterized. Then, the effects of four different surfactants and surface modification on the stability of ZrO 2 /lubricating oil nanofluids were investigated. The friction coefficient was tested by a four-ball tester. Finally, the tribological properties of pure nanoparticles and silane coupling agent-modified nanoparticles added to lubricating oil were investigated. The modified ZrO 2 nanoparticles have better anti-friction and anti-wear properties than pure ZrO 2 nanoparticles. 2. Experimental section 2.1 Materials In this study, nano-zirconia powder with a particle size less than 100 nm, isopropanol, silane coupling agent γ-methacryloxypropyltrimethoxysilane, sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), sodium dodecyl benzene sulfonate (SDBS) and sodium citrate were all from Shanghai Aladdin Biochemical Technology Co., Ltd. The lubricating oil for the steam turbine is Great Wall L-TSA46 (SINOPEC Lubricating Oil Company). All materials were utilized without further treatment. 2.2 Preparation and characterization of surface modified nanoparticles The surface-modified ZrO 2 nanoparticles were prepared by the following method: 1 g of dried nano-zirconia particles were weighed and added to 30 ml of distilled water and ultrasonically dispersed for 10 minutes (CP-3010GTS, 40 kHz) to obtain a ZrO 2 oxide suspension. The appropriate amount of isopropanol and γ-methacryloxypropyltrimethoxysilane (KH570) was weighed, and the two were mixed evenly and then slowly added to the nano-copper oxide suspension several times. After stirring, the mixed liquid was put into a high-speed desktop centrifuge and centrifuged three times at 9000 r/min for 5 min each time. At the end of each centrifugation, wash with deionized water and anhydrous ethanol to remove excess isopropanol and silane. Finally, it was placed in an electrothermal constant temperature drying oven at 70℃ for 16 h. After heating, it was taken out and ground, and the ground modified ZrO 2 powder was collected. The experimental flow of surface modification is shown in Fig. 1 . The surface composition of the modified ZrO 2 nanoparticles was analyzed by attenuated total reflection Fourier transform infrared spectroscopy (Thermo Fisher Scientific Nicolet iS20, USA), and the solid spectrum was obtained using KBr beads. The vibration band is reported as wave number (cm − 1 ). Band intensity was assigned as weak (w), medium (m), shoulder (sh), strong (s), and breadth (br). X-ray diffraction (Anton Paar XRDynamic500, Austria) was used to measure and analyze the sample powder, and the Cu Kα incident beam (k = 1.51418Å) was used to measure the diffraction pattern in the range of 10°—80° for 2 hours. The crystal structure changes of ZrO 2 nanoparticles before and after modification were analyzed. The surface characteristics of ZrO 2 nanoparticles before and after modification were compared by Chengde Dingsheng JY-82C video contact angle measuring instrument. With the test method of sessile drop method, the sample was placed in a 13 mm abrasive tool, and the sample was pressed with a pressure of 6 tons of infrared tableting machine. After that, a drop of 16 ul droplet was dropped on the sample with the instrument. The image was collected every 62 ms, and the contact angle was measured by selecting the picture when the droplet was just in contact with the sample. The grafting efficiency and thermal behavior of modified ZrO 2 nanoparticles were determined by thermogravimetric analyzer (Netzsch STA449F3, Germany) under nitrogen at a heating rate of 10℃/min from 20℃ to 800℃. The grafting ratio E is calculated by the ratio of the mass of the grafted silane (W 0 ) to the mass of the surface-modified ZrO 2 nanoparticles (W 1 ), as shown below: $$\:\text{E}\left(\text{%}\right)=\frac{{\text{W}}_{0}}{{\text{W}}_{1}}\times\:100$$ 1 In the formula, W 0 is the mass of the grafted functional group, and W 1 is the mass of the surface-modified nanoparticles. 2.3 Nanofluids preparation The unmodified or modified ZrO 2 nanofluids were prepared by directly dispersing nanoparticles into turbine lubricants with the assistance of 10-minute ultrasonic stirring (CP-3010 GTS, 40 kHz) and 90-minute magnetic stirring (JKI, 200–1500 rpm). The concentrations of the prepared nanofluids were 0.05 wt %, 0.1 wt %, 0.3 wt % and 0.5 wt %, respectively. In the preparation of nanofluids with surfactant (dispersant), the unmodified ZrO 2 nanoparticles were weighed, and an appropriate amount of turbine lubricating oil was added to make the mass fraction of ZrO 2 in the lubricating oil 0.05,0.1,0.3,0.5wt %, respectively. The mixed liquid was ultrasonically shaken for 10 min, and then magnetically stirred at 900 rpm for 90 min to obtain better stability of the prepared nano-lubricant. 2.4 Evaluation of nanofluids stability The morphology of aggregates in nanofluids was analyzed by scanning electron microscopy (ZEISS Sigma 300, Germany). The stability of nanofluids can be determined by observing the morphology of nanoparticles. Through the sedimentation experiment, the image of nano-lubricant oil was captured at a certain time interval to observe the change of nano-fluid with time. In unstable nanofluids, nanoparticles will attract each other to form larger aggregates and eventually precipitate to the bottom of the container under the action of gravity. Therefore, the stability of nanofluids can be intuitively evaluated by the sedimentation method. The zeta potential of the nanofluids in the initial state was measured by a zeta potential analyzer (Malvern Zetasizer Nano ZS90, UK) at a temperature of 25 ℃. The average size distribution of nanofluids is determined by dynamic light scattering (DLS, Malvern Zetasizer Nano ZS90, UK). Using the light scattering theory, the diffusion coefficient is obtained according to the fluctuation of scattered light intensity, and then the Stokes-Einstein equation is used to calculate the average particle size in the colloidal suspension based on the measured diffusion coefficient, as shown below: $$\:d\left(H\right)=\frac{kT}{3\pi\:\mu\:D}$$ 2 In the above formula, d (H) is the hydrodynamic particle size ; D is the translational diffusion coefficient ; k is Boltzmann constant, k = 1.381 * 10-23J / K ;µis liquid viscosity, mPa·s ; T is the absolute temperature. The ultraviolet absorbance (transmittance) of the nanofluid was measured by an ultraviolet-visible spectrophotometer (Yipu Instrument Manufacturing (Shanghai) Co., Ltd.). The sample was the upper liquid of the nanofluid, and then the agglomeration and precipitation of the particles in the nanofluid were monitored. 2.5 Friction and wear tests The anti-friction and anti-wear properties of nanofluids were investigated on an MRS-100 hydraulic four-ball friction and wear tester at room temperature, 1450 rpm and 392 N load. The friction coefficient and wear scar diameter were measured according to SH/T 0189–1992, and the test time was 20 min. The diameter of the ball is 12.7 mm, which is made of GCr15 bearing steel (AISI-52100) with an HRC of 64–66. At the end of each test, the wear scar diameter on three fixed balls was measured on a digital reading optical microscope with an accuracy of 0.01 mm, and the average wear scar diameter of the same three tests was calculated. The friction coefficient is automatically recorded by the strain gauge of the four-ball tester. 3. Results and discussion 3.1 Characterization of surface modified ZrO 2 nanoparticles TGA analysis can show the quality of nanoparticles before and after burning, and the grafting efficiency of the silane coupling agent with different volumes on the surface of ZrO 2 is obtained to determine the optimal amount of KH570. The prepared modified nanoparticles were calcined at a heating rate of 10℃/min using a thermogravimetric analyzer (TGA). The heating range is from 20℃ to 800℃. The silane grafted on the surface of the zirconia nanoparticles will be completely burned. The grafting rate of the silane coupling agent on the surface of the zirconia nanoparticles can be calculated by formula (1). Fig.2 shows the TGA curves of nanoparticles treated with different volumes of KH570 (1 ml, 5 ml, 15 ml and 20 ml). It can be seen that all nanoparticles have slight weight loss below 230℃, which can be attributed to the desorption of physically adsorbed water [30] . The weight loss of untreated zirconia (1) nanoparticles was the least, and other samples showed obvious weight loss before 650℃ due to the large-scale oxidative thermal decomposition of the KH570 organic chain grafted on the surface of the particles. The grafting rate of each sample can be calculated by subtracting the weight loss of the unmodified nanoparticles from the weight loss of the modified nanoparticles. Figure 3 shows the grafting rate of KH570 on the surface of zirconia nanoparticles under different volumes of silanes coupling agent. It can be seen that the grafting rate of 20 ml KH570 is the highest. At the same time, when the addition volume is less than 20ml, the grafting rate of KH570 in zirconia nanoparticles will increase with the increase of volume, and then when 25 ml and 30 ml KH570 are added, the grafting rate is lower than that when 20 ml KH570 is added. The highest KH570 grafting rate (4.818%) was found in the nanoparticles added with 20 ml KH570. Fourier transform infrared spectroscopy (FTIR) analysis can determine the functional groups on the surface of zirconia nanoparticles so as to identify whether the silane coupling agent is successfully grafted on the surface of zirconia nanoparticles. Fig. 4 shows the Fourier transform infrared spectroscopy (FTIR) spectra of silane coupling agent KH570, unmodified zirconia and KH570-modified zirconia. The broad absorption band near 3450 cm -1 and the peak at 1630 cm -1 in Fig. 3 are the stretching vibrations of adsorbed water and surface hydroxyl (-OH). The peaks at 2885 and 2932cm -1 are the asymmetric stretching vibration and symmetric stretching vibration of -CH 3 . In addition, the characteristic absorption peak of Si-O-C appears near 1049cm -1 . The 1719, 1460 and 1170cm -1 distributions represent the stretching vibration absorption peak of C=O, the bending vibration absorption peak of-CH 2 and the stretching vibration absorption peak of C-O-C. The above groups of vibration peaks once again confirmed the successful condensation reaction between the silane coupling agent and the hydroxyl group on the surface of zirconia. Due to the excessive (unreacted) and physically adsorbed silane that has been repeatedly washed away by ethanol and deionized water, the above peaks indicate that KH570 is successfully grafted on zirconia nanoparticles. The change of crystal structure of ZrO 2 nanoparticles before and after surface modification can be identified by X-ray diffraction (XRD) analysis. The XRD patterns of unmodified ZrO 2 and KH570 modified ZrO 2 are shown in Figure 5. There was no significant difference between the XRD images of unmodified ZrO 2 and KH570 modified ZrO 2 , and the characteristic peaks of (100), (-111), (111), (-112), (022) of ZrO2-PDF # 37-1484 can be corresponded. This indicates that the process of surface modification of ZrO 2 by silane coupling agent does not change the crystal structure of nanoparticles. The ability or propensity of a liquid to spread on a solid surface is known as its wettability. The wettability can be clearly determined by the contact angle. The wettability improves with decreasing contact angle. The wetting of water on solids depends on the relationship between interfacial tension (water/air, water/solid and solid/air). The ratio of these interfacial tensions determines the contact angle (θ) between the water droplets on a given surface and the surface. The effect of KH570 surface modification on the water contact angle of zirconia is shown in Figures 5a and 6b. For Fig. 6a, the water contact angle of unmodified ZrO 2 was observed to be 25.66°. Fig. 6b shows that the water contact angle of KH570-modified ZrO 2 is 104.69°. (The contact angle of 0°means complete wetting, while the contact angle of 180° corresponds to complete non-wetting. In general, the contact angle of 90°<θ<120°indicates that the surface is a hydrophobic surface with low wettability and low contact angle. In general, the larger the angle, the lower the surface energy. The above results show that the surface of nano-ZrO 2 modified by KH570 is hydrophobic and the surface energy is low. In addition, the contact angle is a crucial metric for assessing how lipophilic solid surfaces are. By measuring the contact angle of oil droplets on nano-ZrO 2 powder or film both before and after modification, the change in surface lipophilicity may be quantitatively examined. The effect of KH570 surface modification on the contact angle of zirconia oil is shown in Figures 6c and 6d. Fig. 6c shows that the water-oil contact angle of the zirconia is 53.13°. Fig. 6d shows that the oil contact angle of KH570-modified ZrO2 is 60.2°. The larger the contact angle, the stronger the lipophilicity of the surface. The greater the contact angle of the modified nano-ZrO 2 surface, indicating that the more successful the introduction of hydrophobic organic groups, the stronger the lipophilicity of the surface. Therefore, the lipophilicity of KH570 modified ZrO 2 is improved compared with unmodified ZrO 2 . The above results show that the modified nano-zirconia changes from hydrophilicity to hydrophobicity, and the lipophilicity is also improved. The improvement of lipophilicity is beneficial to the dispersion of nano-ZrO 2 in lubricating oil. The SEM image of figure 7 is the SEM image of nano-ZrO 2 generated at 100 nm. It can be seen from the figure that before modification, ZrO 2 nanoparticles have agglomerated particles, and the agglomeration is serious; the surface gap of the modified ZrO 2 nanoparticles increased significantly and was relatively uniform. Silane coupling agent modified nano-ZrO 2 effectively alleviated particle agglomeration and improved its dispersion. ZrO 2 nanoparticles were dispersed in ethanol and measured by laser particle size analyzer, as shown in Fig. 8. Dynamic laser scattering (DLS) measurements showed that the average particle size of the modified nano-zirconia particles decreased. When unmodified, the particle size distribution of zirconia is wide, and the average particle size is 505.4 nm. After modification, the particle size distribution is narrow and shifts to a smaller particle size value. At this time, the average particle size of nano-zirconia particles is 227.6 nm. The results show that the particle size of ZrO 2 nanoparticles modified by KH570 is smaller and more uniform than that of unmodified nanoparticles. Due to the formation of new chemical bonds between KH570 and ZrO 2 nanoparticles, the interaction between nanoparticles was broken and the agglomeration was effectively controlled. Zeta potential measurement is used to characterize the agglomeration between nanoparticles. An important factor affecting the stability of the solution is the surface potential of the particles. The larger the absolute value of zeta potential, the less likely the nanoparticles are to agglomerate. The absolute value of zeta potential of the nanoparticles was measured three times, and then the average value was taken to ensure the accuracy of the data. Table 1 lists the zeta potential of the nano-zirconia ethanol solution before and after modification. The average absolute potential of the unmodified zirconia zeta is 2.63 mV. At this time, the surface of the nanoparticles is less charged, the interaction between the nanoparticles is strong, and it is easy to flocculate in the suspension. The average absolute zeta potential of the modified zirconia was 18.1 mV, and the absolute value of the zeta potential of the modified ZrO 2 nanoparticles was greater than that of the unmodified ZrO 2 nanoparticles. The results show that the electrostatic repulsion of the modified nano-ZrO 2 particles is stronger than that of the unmodified nano- ZrO 2 particles, the absolute potential of the modified zirconia is increased, and the dispersion in the solution is improved, which means that the modified ZrO 2 nanofluid has better stability. Table1 The absolute value of ZrO 2 Zeta potential. Number 1 2 3 mean Raw ZrO 2 2.33 2.52 3.04 2.63 Modified ZrO 2 18.3 18.5 17.4 18.1 3.2 Stability of ZrO 2 /lubricant nanofluids One important element influencing the friction characteristics of nanofluids is colloid stability. In order to assess the kinds and amounts of dispersants used, sedimentation tests and UV-visible spectroscopy studies will be used to examine the suspension stability of a variety of nanofluid types based on the manufactured conventional and modified nanofluids. The effects of variables such nanoparticle surface modification on the suspension stability of nanofluids might eventually increase the stability of the nanofluids and serve as a guarantee and reference for further studies on the friction performance of nanofluids.. When the sedimentation experiment can not observe the sedimentation of the nanofluids, the UV-visible spectrophotometry experiment will be used to determine the sedimentation of the nanofluids by measuring the UV transmittance. Since the storage conditions of nanofluids are generally static storage at room temperature, all stability experiments are carried out at room temperature of 25℃. In this paper, four surfactants (dispersants) of sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), sodium dodecyl benzene sulfonate (SDBS) and sodium citrate were selected for experiments, and the stability of the samples was observed by sedimentation method, so as to explore the surfactants suitable for ZrO 2 nanofluids. After determining the surfactant with the best stability, the optimal addition ratio was selected by comparing the stability of different ratios added to the lubricating oil, and finally compared with the stability of the modified ZrO 2 nanofluid. Firstly, the concentration of unmodified ZrO 2 with the best stability was determined by measuring the ultraviolet transmittance. The greater the change of ultraviolet transmittance, the faster the precipitation rate of nanofluids and the higher the ultraviolet transmittance, indicating that the upper concentration of nanofluids is lower. Therefore, nano-ZrO 2 lubricants with mass fractions of 0.05 wt %, 0.1 wt %, 0.3 wt %, and 0.5 wt % were prepared. Fig.9 shows the UV transmittance of unmodified zirconia nanofluids. It can be seen from the diagram that the stability of the lubricating oil with 0.1wt % nano-ZrO 2 is the best when the ultraviolet transmittance of the lubricating oil changes little, 0.05wt % is slightly worse, and the stability of 0.3wt % and 0.5wt % is the worst. Therefore, 0.1wt % concentration of unmodified nano-ZrO 2 and dispersant are added to the lubricating oil. The sedimentation standing experiment is the simplest method to evaluate the stability of nanofluids, because the sedimentation behavior of nanofluids can be visually observed by photographing the images of nanofluids at different time intervals. The type of surfactant (dispersant) In the experiment, the proportion of nanoparticles, the concentration of nanoparticles and the amount of surfactant were controlled as invariants. Only the types of surfactants used in the preparation process were changed. The 0.1wt % ZrO 2 nanofluid was added with PVP, SDBS, SDS and sodium citrate according to S : N = 1 : 1, respectively. The samples obtained by ultrasonic dispersion for 10 min and magnetic stirring for 90 min were subjected to static experiments, as shown in Fig.10. It can be seen from the diagram that the ZrO 2 / lubricating oil nanocomposite liquid with sodium citrate and SDS added on the seventh day has obvious instability, and the nanoparticles have phase separation in a short time, and the sedimentation rate is fast, without further study. After 14 days, the nanofluids added with PVP also precipitated. In addition to the nano-lubricant added with SDBS, other sedimentation occurred. Therefore, sodium dodecyl benzene sulfonate (SDBS) was selected as the dispersant through experimental observation. In the experiment of exploring the stability of nano-lubricant, the concentration of nanoparticles and the type of dispersant (SDBS) were controlled as invariants, and only the proportion of surfactant (SDBS) and nano-copper oxide used in the preparation process was changed. Sodium dodecyl benzene sulfonate (SDBS) and nano-copper oxide were added to the lubricating oil in different proportions (1 : 2,1 : 1,2 : 1,3 : 1), respectively. The samples were obtained by ultrasonic dispersion for 10 min and magnetic stirring for 90 min at a speed of 900 rpm. The prepared nano-zirconia lubricating oil with dispersant was placed, and the UV transmittance of each was measured every 4 days, and its stability was evaluated. The UV transmittance of ZrO 2 nano-lubricant containing SDBS is shown in Fig.11. In the first 17 days, the UV transmittance of the four nano-lubricants showed the same trend. After 17 days, except for (SDBS : ZrO 2 = 1 : 2) lubricating oil, the other three nano-lubricants changed greatly, and the UV transmittance became higher. On the 25th day, the UV transmittance of (SDBS : ZrO 2 = 1 : 1) lubricating oil was 99.7 %, (SDBS : ZrO 2 = 2 : 1) lubricating oil was 91.5 %, (SDBS : ZrO 2 = 3 : 1) lubricating oil was 87.6 %. The (SDBS : ZrO 2 = 1 : 2) lubricating oil has a UV transmittance of 66.3 %, and its UV transmittance is relatively low and the stability is good. The nano-lubricant is the nano-lubricant with the best stability of unmodified zirconia nano-lubricant containing dispersant. In the previous section, it has been found that KH570-modified nanoparticles have a high grafting rate when 20 ml is added, so they are selected to prepare surface-modified nanofluids. The modified nano-ZrO 2 was added to the lubricating oil according to different mass fractions (0.05wt %, 0.1wt %, 0.3wt %, 0.5wt %), and the samples were obtained by ultrasonic dispersion for 10 min and magnetic stirring for 90 min. The prepared modified nano-ZrO 2 lubricating oil was placed, and the UV transmittance of each was measured every 3 days. As shown in Fig.12, the UV transmittance of 0.05wt % modified zirconia lubricating oil was 54.8 % on the first day and 96.2 % on the 13th day. The UV transmittance of 0.1wt % modified zirconia lubricating oil was 22 % on the first day and 85.6 % on the 13th day. The transmittance of the two changed greatly, and the UV transmittance was high, indicating that the modified nano-zirconia was basically precipitated in the lubricating oil on the 13th day, and no further research was needed. The modified zirconia lubricating oil with 0.3wt % and 0.5wt % has a small change in transmittance and a low UV transmittance, indicating that the modified nano-zirconia has a little precipitation in the lubricating oil on the 13th day and has good stability. Furthermore, the stability of 0.3wt % and 0.5wt % modified zirconia lubricating oil was compared with that of unmodified zirconia nano-lubricant containing dispersant. In order to facilitate the subsequent discussion, the corresponding code of the nanofluid is given. The sample codes S-1, M-1 and M-2 represent the added (SDBS : ZrO 2 = 1 : 2) nano-lubricants, 0.3wt % and 0.5wt % modified zirconia lubricants, respectively. The results of sedimentation standing test are shown in Fig.13. The three nanofluids can be stabilized for more than 7 days, and then after 14 days, S-1 appeared slight precipitation, M-1 and M-2 appeared phase separation, and after 35 days, S-1 appeared a large amount of precipitation. After 56 days, S-1 has completely precipitated, M-1 and M-2 have precipitated, but the supernatant still has more modified nanoparticle content. The above sedimentation behavior shows that the stability of M-1 and M-2 is better than that of S-1, that is, the stability of KH570 modified nano-zirconia lubricating oil is better than that of unmodified zirconia nano-lubricant containing dispersant SDBS, indicating that ZrO 2 nanoparticles modified by KH570 have stronger steric hindrance and electrostatic repulsion, which can more effectively prevent agglomeration between nanoparticles. 4. Friction coefficient and wear scar diameter of four-ball test and anti-wear and friction reduction mechanism mechanism A conventional four-ball test was conducted at room temperature, 1450 rpm, with a heavy load of 392 N, under extreme high-speed circumstances for 20 minutes in order to assess the coefficient of friction (COF). When the concentration percentage of modified ZrO 2 or pure ZrO 2 nanoparticles in lubricating oil varies over time, Fig. 14 illustrates how the average friction coefficient, wear scar diameter, and friction coefficient change. The computer automatically logs the friction coefficient per second, and the friction time is 1200 s. Figure 15 shows the wear morphology of the steel ball during the four-ball test run. It can be seen from the diagram that the AFC (Average coefficient of friction) of the lubricating oil without adding nanoparticles is 0.127, and the average wear scar diameter is 0.808 mm. Among them, the AFC of the nano-lubricant with unmodified ZrO 2 is 0.155,0.114,0.109 and 0.106 according to the mass fraction of unmodified ZrO 2 from small to large, and the average wear scar diameter is 0.973, 0.861, 0.733 and 0.758 mm. The AFC of 0.05 wt %, 0.1 wt %, 0.3 wt % and 0.5 wt % modified ZrO 2 were 0.0938, 0.0892, 0.0753 and 0.0764, respectively, and the average wear scar diameters were 0.818, 0.813, 0.581 and 0.731 mm, respectively. Compared with L-TSA46 base oil, the AFC of modified ZrO 2 decreased by 26.14%, 29.76%, 40.71% and 39.84%, respectively. The experimental results show that the KH570 modification has a certain improvement on the anti-friction effect of ZrO 2 lubrication. In Fig. 14 (b), the COF curves of L-TSA46 base oil and other unmodified ZrO 2 lubricants with different concentrations are very unstable with time, and the fluctuation is obvious. In Figure 14 (a), the COF curves of the lubricants with 0.05,0.1 and 0.5 wt % modified ZrO 2 have a certain degree of downward shift compared with the pure base oil, but there are still obvious fluctuations. The COF curve of the lubricant with 0.3 wt % modified ZrO 2 is relatively stable and the volatility is reduced. After about 700 s, there is almost no fluctuation and it maintains a slight downward trend. Compared with L-TSA46 base oil, the COF of unmodified ZrO 2 lubricating oil was reduced by about 16.54%. It was found that the addition of 0.05 wt % unmodified ZrO 2 could not reduce the friction coefficient of base oil under high load and high-speed conditions. Compared with pure base oil, the COF of ZrO 2 lubricating oil modified by KH570 is reduced by 40.71% at most, showing excellent anti-friction performance. After the four-ball test, the WSD image was observed with a metallographic microscope, as shown in Figure 15. When the concentration of modified ZrO 2 additive is 0.3wt %, the wear scar diameter is only 0.581 mm, showing a very smooth wear scar. Compared with the wear scar diameter of pure lubricating oil, it is reduced by 0.227 mm. Therefore, when the concentration of modified ZrO 2 additive is 0.3wt %, the anti-friction and anti-wear effect is the best. The above phenomenon also shows that the modified ZrO 2 in the base oil effectively lubricates the interface between the metal under heavy load and high friction speed conditions. This behavior indicates that the added modified ZrO 2 alleviates the interaction between the surfaces and acts as a 'small ball' or 'micro-bearing', changing the friction mode from sliding to rolling, thereby reducing the friction of the particles. The high hardness of ZrO 2 makes it have a polishing effect, which helps to reduce the surface roughness. In addition, the alkyl chain provides stability for the dispersion and acts as an additional non-sticky or protective layer, thereby also avoiding roughness and facilitating the synergistic lubrication mechanism. KH570 as an organic modification can improve the lipophilicity of ZrO 2 , reduce the surface chemical energy of nanomaterials, and help other substances to form an adsorption film to repair the defects of the friction surface. In general, compared with unmodified zirconia, the modified nanoparticles have better stability in addition to their own high hardness. 5. Conclusions This study used surface modification and surfactant addition to create ZrO 2 /lubricating oil nanofluids. The effects of KH570 treatment and the addition of different surfactants on the stability of nanofluids and the effects of surface modification on the friction and wear properties of nanofluids were investigated. The main conclusions are as follows: The grafting efficiency of KH570-modified ZrO 2 nanoparticles under different addition conditions was studied by thermogravimetric analysis. When the addition volume is less than 20 ml, the grafting rate of KH570 in zirconia nanoparticles will increase with the increase of volume. The highest grafting rate of KH570 was found in the nanoparticles with 20 ml KH570, and the grafting rate was 4.818%. By measuring the contact angle experiment, it was found that the modified nano-zirconia changed from hydrophilicity to hydrophobicity, and the lipophilicity was also improved. Through the sedimentation experiment and the UV transmittance test, it can be concluded that the stability of adding SDBS surfactant to pure ZrO 2 lubricating oil is better than that of adding the other three surfactants. The stability of modified ZrO 2 nano-lubricants is better than that of unmodified ZrO 2 nano-lubricants. The stability of ZrO 2 nano-lubricant with the concentration of 0.3 wt% and 0.5 wt % is better than that of unmodified zirconia nano-lubricant with dispersant, indicating that the chemical grafting of KH570 on the surface of nanoparticles can effectively improve the stability of nanofluids. At the same time, the stability of nanofluids before and after modification was evaluated by SEM images, zeta potential and average particle size measurement. The results also showed that ZrO 2 nanofluids modified by KH570 had better stability. Using the same preparation method, nanofluids with different concentrations before and after modification were prepared. The effects of surface modification and nanoparticle concentration on the friction and wear properties of nanofluids were studied. The four-ball test results show that the modified ZrO 2 nanomaterial as an additive can significantly reduce the friction coefficient and improve the anti-wear performance at the optimum addition amount of 0.3 wt %. Compared with pure ZrO 2 , the modified nano-ZrO 2 material has better anti-wear and anti-friction properties. During the friction process, the added modified ZrO 2 is deposited on the friction surface to form a protective film, which alleviates the interaction between the surfaces and changes the friction mode from sliding to rolling. The high hardness of ZrO 2 makes it have a polishing effect and reduces the surface roughness. Therefore, the friction coefficient and wear scar are significantly reduced. Declarations CRediT authorship contribution statement Tao Zhu: Writing – original draft, Funding acquisition, Data curation, Conceptualization. Shan Qing: Writing – review & editing, Resources, Formal analysis. Juan Duan: Supervision, Funding acquisition. Zhihui Jia : Resources, Project administration, Methodology. Mingyue Wang: Resources, Investigation, Formal analysis. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding Financial support from National Natural Science Foundation of China under Contract (NO. 51966005); Yunnan Fundamental Research Project (Grant No. 202201AU070114). Data availability No datasets were generated or analysed during the current study. References K Holmberg, P Andersson, A Erdemir. 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Cite Share Download PDF Status: Published Journal Publication published 03 Jun, 2025 Read the published version in Journal of Nanoparticle Research → Version 1 posted Editorial decision: Revision requested 22 Apr, 2025 Reviews received at journal 21 Apr, 2025 Reviews received at journal 11 Apr, 2025 Reviews received at journal 09 Apr, 2025 Reviews received at journal 27 Mar, 2025 Reviewers agreed at journal 25 Mar, 2025 Reviewers agreed at journal 25 Mar, 2025 Reviewers agreed at journal 25 Mar, 2025 Reviewers agreed at journal 25 Mar, 2025 Reviewers invited by journal 25 Mar, 2025 Editor assigned by journal 24 Mar, 2025 Submission checks completed at journal 23 Mar, 2025 First submitted to journal 17 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6241691","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":435585761,"identity":"e882e821-68a1-49e1-8ac0-ca40ac43d501","order_by":0,"name":"Tao Zhu","email":"","orcid":"","institution":"Kunming University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Tao","middleName":"","lastName":"Zhu","suffix":""},{"id":435585763,"identity":"70e6128b-ecff-40c8-9d1a-e7aa834c3278","order_by":1,"name":"Shan 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11","display":"","copyAsset":false,"role":"figure","size":2263022,"visible":true,"origin":"","legend":"\u003cp\u003eUV transmittance of nano lubricants with different (SDBS:ZrO\u003csub\u003e2\u003c/sub\u003e) mass ratio\u003c/p\u003e","description":"","filename":"image11.png","url":"https://assets-eu.researchsquare.com/files/rs-6241691/v1/8694fd7d23840adfc59249b8.png"},{"id":79769217,"identity":"85cabd20-1046-4aee-9f5d-9084bf2dd0c6","added_by":"auto","created_at":"2025-04-02 12:59:07","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":2583597,"visible":true,"origin":"","legend":"\u003cp\u003eUltraviolet transmittance of modified ZrO\u003csub\u003e2\u003c/sub\u003e/lubricants\u003csub\u003e \u003c/sub\u003ewith different mass fraction\u003c/p\u003e","description":"","filename":"image12.png","url":"https://assets-eu.researchsquare.com/files/rs-6241691/v1/88375840b71ed925dec2d0b8.png"},{"id":79768096,"identity":"43432bf5-287b-4735-b070-74de26dc7b41","added_by":"auto","created_at":"2025-04-02 12:43:07","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":1243797,"visible":true,"origin":"","legend":"\u003cp\u003eSedimentation images of raw ZrO\u003csub\u003e2\u003c/sub\u003e/lubricants nanofluids with SBDS and modified ZrO\u003csub\u003e2\u003c/sub\u003e/lubricants nanofluids taken at different time intervals.\u003c/p\u003e","description":"","filename":"image13.png","url":"https://assets-eu.researchsquare.com/files/rs-6241691/v1/6415f506bbed44672c44c4d5.png"},{"id":79768424,"identity":"ff2a317b-fefc-45d6-959c-f130e9ca34a7","added_by":"auto","created_at":"2025-04-02 12:51:07","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":370332,"visible":true,"origin":"","legend":"\u003cp\u003eMean coefficient of friction and diameter of abrasion marks and the friction coefficient variation with times for lubricating oil with different concentration of nanoparticles：(a) and (c) modified ZrO\u003csub\u003e2\u003c/sub\u003e；(b) and (d)unmodified ZrO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-6241691/v1/a21352cb3011b1d797c93b08.png"},{"id":79768429,"identity":"441e814e-e7ae-4364-8e56-20940440dc77","added_by":"auto","created_at":"2025-04-02 12:51:07","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":683732,"visible":true,"origin":"","legend":"\u003cp\u003eAbrasion morphology of steel balls in four-ball test running in (a) lubricant and (b) lubricant + unmodified zirconia nanoparticles 0.05% and (c) lubricant + modified zirconia nanoparticles 0.3%\u003c/p\u003e","description":"","filename":"image18.png","url":"https://assets-eu.researchsquare.com/files/rs-6241691/v1/c9d6e757d071a04dd3b85128.png"},{"id":84242567,"identity":"6a4c84ae-4241-411a-a206-89f60a43e223","added_by":"auto","created_at":"2025-06-09 16:09:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":20504755,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6241691/v1/494ceaec-4f53-4837-b2a4-92f1452bfed2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of surface modification on the stability and friction and wear properties of ZrO2 / lubricating oil nanofluids","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn today's era of accelerated industrialization, the efficient and stable operation of mechanical equipment has become a key goal pursued in various fields. Friction, as an inevitable phenomenon in mechanical systems\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e, not only leads to a large amount of energy loss, according to statistics, about one-third of the world's energy consumption in overcoming friction, but also causes serious wear of mechanical components, which greatly shortens the service life of equipment and may even lead to safety accidents\u003csup\u003e[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e.Lubricating oil is the core medium to reduce friction and wear\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e.Its performance directly determines the lubrication effect of the friction pair, which in turn affects the working efficiency and life of the entire mechanical system. With the continuous improvement of mechanical performance requirements in modern industry, traditional lubricating oil has been difficult to meet the increasingly stringent working conditions. In this context, the rapid development of nanomaterial technology has brought new opportunities for the improvement of lubricating oil performance\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eNanomaterials are considered to be an environmentally friendly choice because of their low environmental impact and easy decomposition\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e, especially in applications as lubricant additives. Studies have shown that nanoparticles as additives for lubricating oil show more excellent extreme pressure bearing capacity, better wear resistance, and better lubrication effect than traditional lubricating additives\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e, which reveals its great development prospects as an advanced lubricating material. Zhao et al. \u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e prepared Pb nanoparticles and revealed that Pb nanoparticles exhibited excellent wear resistance. Compared with the lubricating oil without Pb nanoparticles, the wear diameter was significantly reduced. The small size of Pb nanoparticles is the key factor to give it excellent lubrication characteristics. Asadauskas et al. \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e compared the tribological properties of Fe, Cu, and Zn nanoparticles in oil. Iron nanoparticles enhanced the anti-wear ability of antioxidant rapeseed oil. Zn nanoparticles reduce the wear rate of mineral oil and form smoother wear traces. Compared with soft metals such as tin ( Sn ), metal oxide nanoparticles have higher hardness, which allows them to provide more significant lubrication in environments with heavy loads and extreme pressures\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. At the same time, the hard nature of metal oxides also gives them polishing effect\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e, which helps to reduce the surface roughness. Qian et al. \u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e chemically modified the anatase TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles prepared by them with stearic acid, and then investigated their tribological behavior. The results show that the modified TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles can significantly improve the anti-friction and anti-wear properties and bearing capacity of paraffin oil.Ingole et al. \u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e used nano-titanium dioxide and commercial titanium dioxide ( P25 ) as lubricant additives to study the tribological behavior. It was found that the addition of commercial titanium dioxide would lead to an increase in the friction coefficient. On the contrary, the addition of nano-titanium dioxide makes the friction coefficient more stable and decreases it. This phenomenon may be due to the formation of a uniform protective film on the sliding contact surface of nano-titanium dioxide. Battez et al. \u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e discussed the anti-wear behavior of CuO, ZnO, and ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticle suspensions in poly-α-olefin (PAO6). The results show that nano-ZnO, nano-ZrO\u003csub\u003e2\u003c/sub\u003e and nano-CuO used as lubricant additives can reduce friction and wear compared with base oil. The suspension with 0.5% ZnO and ZrO\u003csub\u003e2\u003c/sub\u003e has the best comprehensive tribological properties, showing the lowest friction coefficient and wear.Dejang et al. \u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e prepared monoclinic ZrO\u003csub\u003e2\u003c/sub\u003e nanopowders and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e / 3wt % ZrO\u003csub\u003e2\u003c/sub\u003e nanocomposite powders by the ball milling method. The results show that Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e / 3wt % ZrO\u003csub\u003e2\u003c/sub\u003e composite has the lowest friction coefficient and sliding wear rate, and its wear resistance is significantly better than that of single Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. In addition, alumina ceramics containing zirconia additives show a lower friction coefficient and higher wear resistance\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. These studies have shown that zirconium compounds have great potential as anti-wear and anti-friction additives, but there are few studies on single zirconium compounds.\u003c/p\u003e \u003cp\u003eCompared with easily oxidized metal elements, metal oxides exhibit more stable properties\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. However, due to their small size, high surface energy, and easy agglomeration\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e, the dispersion and stability of nanoparticles in lubricating oil have become one of the key challenges in the application of lubricating oil additives, which greatly limits the application of nanomaterials in many fields\u003csup\u003e[\u003cspan additionalcitationids=\"CR20 CR21 CR22\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e.At present, the effective methods to improve the solubility of nanoparticles in base oil are surface modification\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003eand the use of dispersants as surfactants\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. However, there are few studies on the surface modification of nano-fluids. Ma et al. \u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e studied the addition of sodium dodecyl sulfate (SDS), PVP, and cetyltrimethylammonium bromide (CTAB) to Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e / TiO\u003csub\u003e2\u003c/sub\u003e water and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e / CuO-water nanofluids. It was found that the increase of surfactant concentration improved the stability and viscosity. Sarsam et al. \u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e studied the effects of sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and gum arabic (GA) on the stability and thermophysical properties of aqueous graphene nanosheets (GNPs) nanofluids. The results show that the addition of SDBS makes the stability of the system the best, followed by the thermal conductivity. Zhang et al. \u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e prepared γ-glycidoxypropyltrimethoxysilane (GPTMS)-modified TiO\u003csub\u003e2\u003c/sub\u003e/water nanofluids by surface modification technology and studied its stability. The results show that the nanofluids prepared by adding sodium dodecyl sulfate ( SDS ) and unmodified nanoparticles have moderate stability. The nanofluids prepared by GPTMS-modified nanoparticles showed excellent stability. Zhao et al. \u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e modified the surface of commercial TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles with 3-aminopropyltrimethoxysilane and 3-isocyanatopropyltrimethoxysilane by an aqueous phase process. They found that the dispersion stability of particles was positively affected by the increase of zeta potential of particles.\u003c/p\u003e \u003cp\u003eIt can be seen that the effects of different surfactants on ZrO\u003csub\u003e2\u003c/sub\u003e/lubricant nanofluids, as well as the detailed properties of surface modified ZrO\u003csub\u003e2\u003c/sub\u003e nano-lubricants and the effect of single zirconium compound as anti-wear and anti-friction additives, there are still some unknown problems in the research of this field. Therefore, in this paper, γ-methacryloxypropyltrimethoxysilane was used to modify the surface of ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles, and the modified nanoparticles were characterized. Then, the effects of four different surfactants and surface modification on the stability of ZrO\u003csub\u003e2\u003c/sub\u003e/lubricating oil nanofluids were investigated. The friction coefficient was tested by a four-ball tester. Finally, the tribological properties of pure nanoparticles and silane coupling agent-modified nanoparticles added to lubricating oil were investigated. The modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles have better anti-friction and anti-wear properties than pure ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1 Materials\u003c/h2\u003e\n \u003cp\u003eIn this study, nano-zirconia powder with a particle size less than 100 nm, isopropanol, silane coupling agent \u0026gamma;-methacryloxypropyltrimethoxysilane, sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), sodium dodecyl benzene sulfonate (SDBS) and sodium citrate were all from Shanghai Aladdin Biochemical Technology Co., Ltd. The lubricating oil for the steam turbine is Great Wall L-TSA46 (SINOPEC Lubricating Oil Company). All materials were utilized without further treatment.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2 Preparation and characterization of surface modified nanoparticles\u003c/h2\u003e\n \u003cp\u003eThe surface-modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles were prepared by the following method: 1 g of dried nano-zirconia particles were weighed and added to 30 ml of distilled water and ultrasonically dispersed for 10 minutes (CP-3010GTS, 40 kHz) to obtain a ZrO\u003csub\u003e2\u003c/sub\u003e oxide suspension. The appropriate amount of isopropanol and \u0026gamma;-methacryloxypropyltrimethoxysilane (KH570) was weighed, and the two were mixed evenly and then slowly added to the nano-copper oxide suspension several times. After stirring, the mixed liquid was put into a high-speed desktop centrifuge and centrifuged three times at 9000 r/min for 5 min each time. At the end of each centrifugation, wash with deionized water and anhydrous ethanol to remove excess isopropanol and silane. Finally, it was placed in an electrothermal constant temperature drying oven at 70℃ for 16 h. After heating, it was taken out and ground, and the ground modified ZrO\u003csub\u003e2\u003c/sub\u003e powder was collected. The experimental flow of surface modification is shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eThe surface composition of the modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles was analyzed by attenuated total reflection Fourier transform infrared spectroscopy (Thermo Fisher Scientific Nicolet iS20, USA), and the solid spectrum was obtained using KBr beads. The vibration band is reported as wave number (cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Band intensity was assigned as weak (w), medium (m), shoulder (sh), strong (s), and breadth (br).\u003c/p\u003e\n \u003cp\u003eX-ray diffraction (Anton Paar XRDynamic500, Austria) was used to measure and analyze the sample powder, and the Cu K\u0026alpha; incident beam (k\u0026thinsp;=\u0026thinsp;1.51418\u0026Aring;) was used to measure the diffraction pattern in the range of 10\u0026deg;\u0026mdash;80\u0026deg; for 2 hours. The crystal structure changes of ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles before and after modification were analyzed.\u003c/p\u003e\n \u003cp\u003eThe surface characteristics of ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles before and after modification were compared by Chengde Dingsheng JY-82C video contact angle measuring instrument. With the test method of sessile drop method, the sample was placed in a 13 mm abrasive tool, and the sample was pressed with a pressure of 6 tons of infrared tableting machine. After that, a drop of 16 ul droplet was dropped on the sample with the instrument. The image was collected every 62 ms, and the contact angle was measured by selecting the picture when the droplet was just in contact with the sample.\u003c/p\u003e\n \u003cp\u003eThe grafting efficiency and thermal behavior of modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles were determined by thermogravimetric analyzer (Netzsch STA449F3, Germany) under nitrogen at a heating rate of 10℃/min from 20℃ to 800℃. The grafting ratio E is calculated by the ratio of the mass of the grafted silane (W\u003csub\u003e0\u003c/sub\u003e) to the mass of the surface-modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles (W\u003csub\u003e1\u003c/sub\u003e), as shown below:\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\:\\text{E}\\left(\\text{%}\\right)=\\frac{{\\text{W}}_{0}}{{\\text{W}}_{1}}\\times\\:100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cp\u003eIn the formula, W\u003csub\u003e0\u003c/sub\u003e is the mass of the grafted functional group, and W\u003csub\u003e1\u003c/sub\u003e is the mass of the surface-modified nanoparticles.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Nanofluids preparation\u003c/h2\u003e\u003cp\u003eThe unmodified or modified ZrO\u003csub\u003e2\u003c/sub\u003e nanofluids were prepared by directly dispersing nanoparticles into turbine lubricants with the assistance of 10-minute ultrasonic stirring (CP-3010 GTS, 40 kHz) and 90-minute magnetic stirring (JKI, 200\u0026ndash;1500 rpm). The concentrations of the prepared nanofluids were 0.05 wt %, 0.1 wt %, 0.3 wt % and 0.5 wt %, respectively.\u003c/p\u003e\u003cp\u003eIn the preparation of nanofluids with surfactant (dispersant), the unmodified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles were weighed, and an appropriate amount of turbine lubricating oil was added to make the mass fraction of ZrO\u003csub\u003e2\u003c/sub\u003e in the lubricating oil 0.05,0.1,0.3,0.5wt %, respectively. The mixed liquid was ultrasonically shaken for 10 min, and then magnetically stirred at 900 rpm for 90 min to obtain better stability of the prepared nano-lubricant.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Evaluation of nanofluids stability\u003c/h2\u003e\u003cp\u003eThe morphology of aggregates in nanofluids was analyzed by scanning electron microscopy (ZEISS Sigma 300, Germany). The stability of nanofluids can be determined by observing the morphology of nanoparticles.\u003c/p\u003e\u003cp\u003eThrough the sedimentation experiment, the image of nano-lubricant oil was captured at a certain time interval to observe the change of nano-fluid with time. In unstable nanofluids, nanoparticles will attract each other to form larger aggregates and eventually precipitate to the bottom of the container under the action of gravity. Therefore, the stability of nanofluids can be intuitively evaluated by the sedimentation method.\u003c/p\u003e\u003cp\u003eThe zeta potential of the nanofluids in the initial state was measured by a zeta potential analyzer (Malvern Zetasizer Nano ZS90, UK) at a temperature of 25 ℃. The average size distribution of nanofluids is determined by dynamic light scattering (DLS, Malvern Zetasizer Nano ZS90, UK). Using the light scattering theory, the diffusion coefficient is obtained according to the fluctuation of scattered light intensity, and then the Stokes-Einstein equation is used to calculate the average particle size in the colloidal suspension based on the measured diffusion coefficient, as shown below:\u003c/p\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e$$\\:d\\left(H\\right)=\\frac{kT}{3\\pi\\:\\mu\\:D}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cp\u003eIn the above formula, d (H) is the hydrodynamic particle size ; D is the translational diffusion coefficient ; k is Boltzmann constant, k\u0026thinsp;=\u0026thinsp;1.381 * 10-23J / K ;\u0026micro;is liquid viscosity, mPa\u0026middot;s ; T is the absolute temperature.\u003c/p\u003e\u003cp\u003eThe ultraviolet absorbance (transmittance) of the nanofluid was measured by an ultraviolet-visible spectrophotometer (Yipu Instrument Manufacturing (Shanghai) Co., Ltd.). The sample was the upper liquid of the nanofluid, and then the agglomeration and precipitation of the particles in the nanofluid were monitored.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5 Friction and wear tests\u003c/h2\u003e\u003cp\u003eThe anti-friction and anti-wear properties of nanofluids were investigated on an MRS-100 hydraulic four-ball friction and wear tester at room temperature, 1450 rpm and 392 N load. The friction coefficient and wear scar diameter were measured according to SH/T 0189\u0026ndash;1992, and the test time was 20 min. The diameter of the ball is 12.7 mm, which is made of GCr15 bearing steel (AISI-52100) with an HRC of 64\u0026ndash;66. At the end of each test, the wear scar diameter on three fixed balls was measured on a digital reading optical microscope with an accuracy of 0.01 mm, and the average wear scar diameter of the same three tests was calculated. The friction coefficient is automatically recorded by the strain gauge of the four-ball tester.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003e\u003cstrong\u003e3.1 Characterization of surface modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTGA analysis can show the quality of nanoparticles before and after burning, and the grafting efficiency of the silane coupling agent with different volumes on the surface of ZrO\u003csub\u003e2\u003c/sub\u003e is obtained to determine the optimal amount of KH570. The prepared modified nanoparticles were calcined at a heating rate of 10℃/min using a thermogravimetric analyzer (TGA). The heating range is from 20℃ to 800℃. The silane grafted on the surface of the zirconia nanoparticles will be completely burned. The grafting rate of the silane coupling agent on the surface of the zirconia nanoparticles can be calculated by formula (1). Fig.2 shows the TGA curves of nanoparticles treated with different volumes of KH570 (1 ml, 5 ml, 15 ml and 20 ml). It can be seen that all nanoparticles have slight weight loss below 230℃, which can be attributed to the desorption of physically adsorbed water\u003csup\u003e[30]\u003c/sup\u003e. The weight loss of untreated zirconia (1) nanoparticles was the least, and other samples showed obvious weight loss before 650℃\u0026nbsp;due to the large-scale oxidative thermal decomposition of the KH570 organic chain grafted on the surface of the particles.\u003c/p\u003e\n\u003cp\u003eThe grafting rate of each sample can be calculated by subtracting the weight loss of the unmodified nanoparticles from the weight loss of the modified nanoparticles. Figure 3 shows the grafting rate of KH570 on the surface of zirconia nanoparticles under different volumes of silanes coupling agent. It can be seen that the grafting rate of 20 ml KH570 is the highest. At the same time, when the addition volume is less than 20ml, the grafting rate of KH570 in zirconia nanoparticles will increase with the increase of volume, and then when 25 ml and 30 ml KH570 are added, the grafting rate is lower than that when 20 ml KH570 is added. The highest KH570 grafting rate (4.818%) was found in the nanoparticles added with 20 ml KH570.\u003c/p\u003e\n\u003cp\u003eFourier transform infrared spectroscopy (FTIR) analysis can determine the functional groups on the surface of zirconia nanoparticles so as to identify whether the silane coupling agent is successfully grafted on the surface of zirconia nanoparticles. Fig. 4 shows the Fourier transform infrared spectroscopy (FTIR) spectra of silane coupling agent KH570, unmodified zirconia and KH570-modified zirconia. The broad absorption band near 3450 cm\u003csup\u003e-1\u003c/sup\u003e and the peak at 1630 cm\u003csup\u003e-1\u003c/sup\u003e in Fig. 3 are the stretching vibrations of adsorbed water and surface hydroxyl (-OH). The peaks at 2885 and 2932cm\u003csup\u003e-1\u003c/sup\u003e are the asymmetric stretching vibration and symmetric stretching vibration of -CH\u003csub\u003e3\u003c/sub\u003e. In addition, the characteristic absorption peak of Si-O-C appears near 1049cm\u003csup\u003e-1\u003c/sup\u003e. The 1719, 1460 and 1170cm\u003csup\u003e-1\u003c/sup\u003e distributions represent the stretching vibration absorption peak of C=O, the bending vibration absorption peak of-CH\u003csub\u003e2\u003c/sub\u003e and the stretching vibration absorption peak of C-O-C. The above groups of vibration peaks once again confirmed the successful condensation reaction between the silane coupling agent and the hydroxyl group on the surface of zirconia. Due to the excessive (unreacted) and physically adsorbed silane that has been repeatedly washed away by ethanol and deionized water, the above peaks indicate that KH570 is successfully grafted on zirconia nanoparticles.\u003c/p\u003e\n\u003cp\u003eThe change of crystal structure of ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles before and after surface modification can be identified by X-ray diffraction (XRD) analysis. The XRD patterns of unmodified ZrO\u003csub\u003e2\u003c/sub\u003e and KH570 modified ZrO\u003csub\u003e2\u003c/sub\u003e are shown in Figure 5. There was no significant difference between the XRD images of unmodified ZrO\u003csub\u003e2\u003c/sub\u003e and KH570 modified ZrO\u003csub\u003e2\u003c/sub\u003e, and the characteristic peaks of (100), (-111), (111), (-112), (022) of ZrO2-PDF # 37-1484 can be corresponded. This indicates that the process of surface modification of ZrO\u003csub\u003e2\u003c/sub\u003e by silane coupling agent does not change the crystal structure of nanoparticles.\u003c/p\u003e\n\u003cp\u003eThe ability or propensity of a liquid to spread on a solid surface is known as its wettability. The wettability can be clearly determined by the contact angle. The wettability improves with decreasing contact angle. The wetting of water on solids depends on the relationship between interfacial tension (water/air, water/solid and solid/air). The ratio of these interfacial tensions determines the contact angle (\u0026theta;)\u0026nbsp;between the water droplets on a given surface and the surface. The effect of KH570 surface modification on the water contact angle of zirconia is shown in Figures 5a and 6b. For Fig. 6a, the water contact angle of unmodified ZrO\u003csub\u003e2\u003c/sub\u003e was observed to be 25.66\u0026deg;. Fig. 6b shows that the water contact angle of KH570-modified ZrO\u003csub\u003e2\u003c/sub\u003e is 104.69\u0026deg;. (The contact angle of 0\u0026deg;means complete wetting, while the contact angle of 180\u0026deg; corresponds to complete non-wetting. In general, the contact angle of 90\u0026deg;\u0026lt;\u0026theta;\u0026lt;120\u0026deg;indicates that the surface is a hydrophobic surface with low wettability and low contact angle. In general, the larger the angle, the lower the surface energy. The above results show that the surface of nano-ZrO\u003csub\u003e2\u003c/sub\u003e modified by KH570 is hydrophobic and the surface energy is low.\u003c/p\u003e\n\u003cp\u003eIn addition, the contact angle is a crucial metric for assessing how lipophilic solid surfaces are. By measuring the contact angle of oil droplets on nano-ZrO\u003csub\u003e2\u003c/sub\u003e powder or film both before and after modification, the change in surface lipophilicity may be quantitatively examined. The effect of KH570 surface modification on the contact angle of zirconia oil is shown in Figures 6c and 6d. Fig. 6c shows that the water-oil contact angle of the zirconia is 53.13\u0026deg;. Fig. 6d shows that the oil contact angle of KH570-modified ZrO2 is 60.2\u0026deg;. The larger the contact angle, the stronger the lipophilicity of the surface. The greater the contact angle of the modified nano-ZrO\u003csub\u003e2\u003c/sub\u003e surface, indicating that the more successful the introduction of hydrophobic organic groups, the stronger the lipophilicity of the surface. Therefore, the lipophilicity of KH570 modified ZrO\u003csub\u003e2\u003c/sub\u003e is improved compared with unmodified ZrO\u003csub\u003e2\u003c/sub\u003e. The above results show that the modified nano-zirconia changes from hydrophilicity to hydrophobicity, and the lipophilicity is also improved. The improvement of lipophilicity is beneficial to the dispersion of nano-ZrO\u003csub\u003e2\u003c/sub\u003e in lubricating oil.\u003c/p\u003e\n\u003cp\u003eThe SEM image of figure 7 is the SEM image of nano-ZrO\u003csub\u003e2\u003c/sub\u003e generated at 100 nm. It can be seen from the figure that before modification, ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles have agglomerated particles, and the agglomeration is serious; the surface gap of the modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles increased significantly and was relatively uniform. Silane coupling agent modified nano-ZrO\u003csub\u003e2\u003c/sub\u003e effectively alleviated particle agglomeration and improved its dispersion.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles were dispersed in ethanol and measured by laser particle size analyzer, as shown in Fig. 8. Dynamic laser scattering (DLS) measurements showed that the average particle size of the modified nano-zirconia particles decreased. When unmodified, the particle size distribution of zirconia is wide, and the average particle size is 505.4 nm. After modification, the particle size distribution is narrow and shifts to a smaller particle size value. At this time, the average particle size of nano-zirconia particles is 227.6 nm. The results show that the particle size of ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles modified by KH570 is smaller and more uniform than that of unmodified nanoparticles. Due to the formation of new chemical bonds between KH570 and ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles, the interaction between nanoparticles was broken and the agglomeration was effectively controlled.\u003c/p\u003e\n\u003cp\u003eZeta potential measurement is used to characterize the agglomeration between nanoparticles. An important factor affecting the stability of the solution is the surface potential of the particles. The larger the absolute value of zeta potential, the less likely the nanoparticles are to agglomerate. The absolute value of zeta potential of the nanoparticles was measured three times, and then the average value was taken to ensure the accuracy of the data. Table 1 lists the zeta potential of the nano-zirconia ethanol solution before and after modification. The average absolute potential of the unmodified zirconia zeta is 2.63 mV. At this time, the surface of the nanoparticles is less charged, the interaction between the nanoparticles is strong, and it is easy to flocculate in the suspension. The average absolute zeta potential of the modified zirconia was 18.1 mV, and the absolute value of the zeta potential of the modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles was greater than that of the unmodified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles. The results show that the electrostatic repulsion of the modified nano-ZrO\u003csub\u003e2\u003c/sub\u003e particles is stronger than that of the unmodified nano- ZrO\u003csub\u003e2\u003c/sub\u003e particles, the absolute potential of the modified zirconia is increased, and the dispersion in the solution is improved, which means that the modified ZrO\u003csub\u003e2\u003c/sub\u003e nanofluid has better stability.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable1\u003c/strong\u003e The absolute value of ZrO\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eZeta potential.\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003eNumber\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003emean\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003eRaw ZrO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e2.33\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e2.52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e3.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e2.63\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003eModified ZrO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e18.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e18.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e17.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 114px;\"\u003e\n \u003cp\u003e18.1\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003cstrong\u003e3.2 Stability of ZrO\u003csub\u003e2\u003c/sub\u003e/lubricant nanofluids\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eOne important element influencing the friction characteristics of nanofluids is colloid stability. In order to assess the kinds and amounts of dispersants used, sedimentation tests and UV-visible spectroscopy studies will be used to examine the suspension stability of a variety of nanofluid types based on the manufactured conventional and modified nanofluids. The effects of variables such nanoparticle surface modification on the suspension stability of nanofluids might eventually increase the stability of the nanofluids and serve as a guarantee and reference for further studies on the friction performance of nanofluids.. When the sedimentation experiment can not observe the sedimentation of the nanofluids, the UV-visible spectrophotometry experiment will be used to determine the sedimentation of the nanofluids by measuring the UV transmittance. Since the storage conditions of nanofluids are generally static storage at room temperature, all stability experiments are carried out at room temperature of 25℃.\u003c/p\u003e\n\u003cp\u003eIn this paper, four surfactants (dispersants) of sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), sodium dodecyl benzene sulfonate (SDBS) and sodium citrate were selected for experiments, and the stability of the samples was observed by sedimentation method, so as to explore the surfactants suitable for ZrO\u003csub\u003e2\u003c/sub\u003e nanofluids. After determining the surfactant with the best stability, the optimal addition ratio was selected by comparing the stability of different ratios added to the lubricating oil, and finally compared with the stability of the modified ZrO\u003csub\u003e2\u003c/sub\u003e nanofluid.\u003c/p\u003e\n\u003cp\u003eFirstly, the concentration of unmodified ZrO\u003csub\u003e2\u003c/sub\u003e with the best stability was determined by measuring the ultraviolet transmittance. The greater the change of ultraviolet transmittance, the faster the precipitation rate of nanofluids and the higher the ultraviolet transmittance, indicating that the upper concentration of nanofluids is lower. Therefore, nano-ZrO\u003csub\u003e2\u003c/sub\u003e lubricants with mass fractions of 0.05 wt %, 0.1 wt %, 0.3 wt %, and 0.5 wt % were prepared. Fig.9 shows the UV transmittance of unmodified zirconia nanofluids. It can be seen from the diagram that the stability of the lubricating oil with 0.1wt % nano-ZrO\u003csub\u003e2\u003c/sub\u003e is the best when the ultraviolet transmittance of the lubricating oil changes little, 0.05wt % is slightly worse, and the stability of 0.3wt % and 0.5wt % is the worst. Therefore, 0.1wt % concentration of unmodified nano-ZrO\u003csub\u003e2\u003c/sub\u003e and dispersant are added to the lubricating oil.\u003c/p\u003e\n\u003cp\u003eThe sedimentation standing experiment is the simplest method to evaluate the stability of nanofluids, because the sedimentation behavior of nanofluids can be visually observed by photographing the images of nanofluids at different time intervals. The type of surfactant (dispersant) In the experiment, the proportion of nanoparticles, the concentration of nanoparticles and the amount of surfactant were controlled as invariants. Only the types of surfactants used in the preparation process were changed. The 0.1wt % ZrO\u003csub\u003e2\u003c/sub\u003e nanofluid was added with PVP, SDBS, SDS and sodium citrate according to S : N = 1 : 1, respectively. The samples obtained by ultrasonic dispersion for 10 min and magnetic stirring for 90 min were subjected to static experiments, as shown in Fig.10. It can be seen from the diagram that the ZrO\u003csub\u003e2\u0026nbsp;\u003c/sub\u003e/ lubricating oil nanocomposite liquid with sodium citrate and SDS added on the seventh day has obvious instability, and the nanoparticles have phase separation in a short time, and the sedimentation rate is fast, without further study. After 14 days, the nanofluids added with PVP also precipitated. In addition to the nano-lubricant added with SDBS, other sedimentation occurred. Therefore, sodium dodecyl benzene sulfonate (SDBS) was selected as the dispersant through experimental observation.\u003c/p\u003e\n\u003cp\u003eIn the experiment of exploring the stability of nano-lubricant, the concentration of nanoparticles and the type of dispersant (SDBS) were controlled as invariants, and only the proportion of surfactant (SDBS) and nano-copper oxide used in the preparation process was changed. Sodium dodecyl benzene sulfonate (SDBS) and nano-copper oxide were added to the lubricating oil in different proportions (1 : 2,1 : 1,2 : 1,3 : 1), respectively. The samples were obtained by ultrasonic dispersion for 10 min and magnetic stirring for 90 min at a speed of 900 rpm. The prepared nano-zirconia lubricating oil with dispersant was placed, and the UV transmittance of each was measured every 4 days, and its stability was evaluated. The UV transmittance of ZrO\u003csub\u003e2\u003c/sub\u003e nano-lubricant containing SDBS is shown in Fig.11. In the first 17 days, the UV transmittance of the four nano-lubricants showed the same trend. After 17 days, except for (SDBS : ZrO\u003csub\u003e2\u003c/sub\u003e = 1 : 2) lubricating oil, the other three nano-lubricants changed greatly, and the UV transmittance became higher. On the 25th day, the UV transmittance of (SDBS : ZrO\u003csub\u003e2\u003c/sub\u003e = 1 : 1) lubricating oil was 99.7 %, (SDBS : ZrO\u003csub\u003e2\u003c/sub\u003e = 2 : 1) lubricating oil was 91.5 %, (SDBS : ZrO\u003csub\u003e2\u003c/sub\u003e = 3 : 1) lubricating oil was 87.6 %. The (SDBS : ZrO\u003csub\u003e2\u003c/sub\u003e = 1 : 2) lubricating oil has a UV transmittance of 66.3 %, and its UV transmittance is relatively low and the stability is good. The nano-lubricant is the nano-lubricant with the best stability of unmodified zirconia nano-lubricant containing dispersant.\u003c/p\u003e\n\u003cp\u003eIn the previous section, it has been found that KH570-modified nanoparticles have a high grafting rate when 20 ml is added, so they are selected to prepare surface-modified nanofluids. The modified nano-ZrO\u003csub\u003e2\u003c/sub\u003e was added to the lubricating oil according to different mass fractions (0.05wt %, 0.1wt %, 0.3wt %, 0.5wt %), and the samples were obtained by ultrasonic dispersion for 10 min and magnetic stirring for 90 min. The prepared modified nano-ZrO\u003csub\u003e2\u003c/sub\u003e lubricating oil was placed, and the UV transmittance of each was measured every 3 days. As shown in Fig.12, the UV transmittance of 0.05wt % modified zirconia lubricating oil was 54.8 % on the first day and 96.2 % on the 13th day. The UV transmittance of 0.1wt % modified zirconia lubricating oil was 22 % on the first day and 85.6 % on the 13th day. The transmittance of the two changed greatly, and the UV transmittance was high, indicating that the modified nano-zirconia was basically precipitated in the lubricating oil on the 13th day, and no further research was needed. The modified zirconia lubricating oil with 0.3wt % and 0.5wt % has a small change in transmittance and a low UV transmittance, indicating that the modified nano-zirconia has a little precipitation in the lubricating oil on the 13th day and has good stability.\u003c/p\u003e\n\u003cp\u003eFurthermore, the stability of 0.3wt % and 0.5wt % modified zirconia lubricating oil was compared with that of unmodified zirconia nano-lubricant containing dispersant. In order to facilitate the subsequent discussion, the corresponding code of the nanofluid is given. The sample codes S-1, M-1 and M-2 represent the added (SDBS : ZrO\u003csub\u003e2\u003c/sub\u003e = 1 : 2) nano-lubricants, 0.3wt % and 0.5wt % modified zirconia lubricants, respectively. The results of sedimentation standing test are shown in Fig.13. The three nanofluids can be stabilized for more than 7 days, and then after 14 days, S-1 appeared slight precipitation, M-1 and M-2 appeared phase separation, and after 35 days, S-1 appeared a large amount of precipitation. After 56 days, S-1 has completely precipitated, M-1 and M-2 have precipitated, but the supernatant still has more modified nanoparticle content. The above sedimentation behavior shows that the stability of M-1 and M-2 is better than that of S-1, that is, the stability of KH570 modified nano-zirconia lubricating oil is better than that of unmodified zirconia nano-lubricant containing dispersant SDBS, indicating that ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles modified by KH570 have stronger steric hindrance and electrostatic repulsion, which can more effectively prevent agglomeration between nanoparticles.\u003c/p\u003e"},{"header":"4.\tFriction coefficient and wear scar diameter of four-ball test and anti-wear and friction reduction mechanism mechanism","content":"\u003cp\u003eA conventional four-ball test was conducted at room temperature, 1450 rpm, with a heavy load of 392 N, under extreme high-speed circumstances for 20 minutes in order to assess the coefficient of friction (COF). When the concentration percentage of modified ZrO\u003csub\u003e2\u003c/sub\u003e or pure ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles in lubricating oil varies over time, Fig. 14 illustrates how the average friction coefficient, wear scar diameter, and friction coefficient change. The computer automatically logs the friction coefficient per second, and the friction time is 1200 s. Figure 15 shows the wear morphology of the steel ball during the four-ball test run. It can be seen from the diagram that the AFC (Average coefficient of friction) of the lubricating oil without adding nanoparticles is 0.127, and the average wear scar diameter is 0.808 mm. Among them, the AFC of the nano-lubricant with unmodified ZrO\u003csub\u003e2\u003c/sub\u003e is 0.155,0.114,0.109 and 0.106 according to the mass fraction of unmodified ZrO\u003csub\u003e2\u003c/sub\u003e from small to large, and the average wear scar diameter is 0.973, 0.861, 0.733 and 0.758 mm. The AFC of 0.05 wt %, 0.1 wt %, 0.3 wt % and 0.5 wt % modified ZrO\u003csub\u003e2\u003c/sub\u003e were 0.0938, 0.0892, 0.0753 and 0.0764, respectively, and the average wear scar diameters were 0.818, 0.813, 0.581 and 0.731 mm, respectively. Compared with L-TSA46 base oil, the AFC of modified ZrO\u003csub\u003e2\u003c/sub\u003e decreased by 26.14%, 29.76%, 40.71% and 39.84%, respectively. The experimental results show that the KH570 modification has a certain improvement on the anti-friction effect of ZrO\u003csub\u003e2\u003c/sub\u003e lubrication. In Fig. 14 (b), the COF curves of L-TSA46 base oil and other unmodified ZrO\u003csub\u003e2\u003c/sub\u003e lubricants with different concentrations are very unstable with time, and the fluctuation is obvious. In Figure 14 (a), the COF curves of the lubricants with 0.05,0.1 and 0.5 wt % modified ZrO\u003csub\u003e2\u003c/sub\u003e have a certain degree of downward shift compared with the pure base oil, but there are still obvious fluctuations. The COF curve of the lubricant with 0.3 wt % modified ZrO\u003csub\u003e2\u003c/sub\u003e is relatively stable and the volatility is reduced. After about 700 s, there is almost no fluctuation and it maintains a slight downward trend. Compared with L-TSA46 base oil, the COF of unmodified ZrO\u003csub\u003e2\u003c/sub\u003e lubricating oil was reduced by about 16.54%. It was found that the addition of 0.05 wt % unmodified ZrO\u003csub\u003e2\u003c/sub\u003e could not reduce the friction coefficient of base oil under high load and high-speed conditions. Compared with pure base oil, the COF of ZrO\u003csub\u003e2\u003c/sub\u003e lubricating oil modified by KH570 is reduced by 40.71% at most, showing excellent anti-friction performance. After the four-ball test, the WSD image was observed with a metallographic microscope, as shown in Figure 15. When the concentration of modified ZrO\u003csub\u003e2\u003c/sub\u003e additive is 0.3wt %, the wear scar diameter is only 0.581 mm, showing a very smooth wear scar. Compared with the wear scar diameter of pure lubricating oil, it is reduced by 0.227 mm. Therefore, when the concentration of modified ZrO\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eadditive is 0.3wt %, the anti-friction and anti-wear effect is the best.\u003c/p\u003e\n\u003cp\u003eThe above phenomenon also shows that the modified ZrO\u003csub\u003e2\u003c/sub\u003e in the base oil effectively lubricates the interface between the metal under heavy load and high friction speed conditions. This behavior indicates that the added modified ZrO\u003csub\u003e2\u003c/sub\u003e alleviates the interaction between the surfaces and acts as a \u0026apos;small ball\u0026apos; or \u0026apos;micro-bearing\u0026apos;, changing the friction mode from sliding to rolling, thereby reducing the friction of the particles. The high hardness of ZrO\u003csub\u003e2\u003c/sub\u003e makes it have a polishing effect, which helps to reduce the surface roughness. In addition, the alkyl chain provides stability for the dispersion and acts as an additional non-sticky or protective layer, thereby also avoiding roughness and facilitating the synergistic lubrication mechanism. KH570 as an organic modification can improve the lipophilicity of ZrO\u003csub\u003e2\u003c/sub\u003e, reduce the surface chemical energy of nanomaterials, and help other substances to form an adsorption film to repair the defects of the friction surface. In general, compared with unmodified zirconia, the modified nanoparticles have better stability in addition to their own high hardness.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThis study used surface modification and surfactant addition to create ZrO\u003csub\u003e2\u003c/sub\u003e/lubricating oil nanofluids. The effects of KH570 treatment and the addition of different surfactants on the stability of nanofluids and the effects of surface modification on the friction and wear properties of nanofluids were investigated. The main conclusions are as follows:\u003c/p\u003e\n\u003cp\u003eThe grafting efficiency of KH570-modified ZrO\u003csub\u003e2\u003c/sub\u003e nanoparticles under different addition conditions was studied by thermogravimetric analysis. When the addition volume is less than 20 ml, the grafting rate of KH570 in zirconia nanoparticles will increase with the increase of volume. The highest grafting rate of KH570 was found in the nanoparticles with 20 ml KH570, and the grafting rate was 4.818%. By measuring the contact angle experiment, it was found that the modified nano-zirconia changed from hydrophilicity to hydrophobicity, and the lipophilicity was also improved.\u003c/p\u003e\n\u003cp\u003eThrough the sedimentation experiment and the UV transmittance test, it can be concluded that the stability of adding SDBS surfactant to pure ZrO\u003csub\u003e2\u003c/sub\u003e lubricating oil is better than that of adding the other three surfactants. The stability of modified ZrO\u003csub\u003e2\u003c/sub\u003e nano-lubricants is better than that of unmodified ZrO\u003csub\u003e2\u003c/sub\u003e nano-lubricants. The stability of ZrO\u003csub\u003e2\u003c/sub\u003e nano-lubricant with the concentration of 0.3 wt% and 0.5 wt % is better than that of unmodified zirconia nano-lubricant with dispersant, indicating that the chemical grafting of KH570 on the surface of nanoparticles can effectively improve the stability of nanofluids. At the same time, the stability of nanofluids before and after modification was evaluated by SEM images, zeta potential and average particle size measurement. The results also showed that ZrO\u003csub\u003e2\u003c/sub\u003e nanofluids modified by KH570 had better stability.\u003c/p\u003e\n\u003cp\u003eUsing the same preparation method, nanofluids with different concentrations before and after modification were prepared. The effects of surface modification and nanoparticle concentration on the friction and wear properties of nanofluids were studied. The four-ball test results show that the modified ZrO\u003csub\u003e2\u003c/sub\u003e nanomaterial as an additive can significantly reduce the friction coefficient and improve the anti-wear performance at the optimum addition amount of 0.3 wt %. Compared with pure ZrO\u003csub\u003e2\u003c/sub\u003e, the modified nano-ZrO\u003csub\u003e2\u003c/sub\u003e material has better anti-wear and anti-friction properties. During the friction process, the added modified ZrO\u003csub\u003e2\u003c/sub\u003e is deposited on the friction surface to form a protective film, which alleviates the interaction between the surfaces and changes the friction mode from sliding to rolling. The high hardness of ZrO\u003csub\u003e2\u003c/sub\u003e makes it have a polishing effect and reduces the surface roughness. Therefore, the friction coefficient and wear scar are significantly reduced.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTao Zhu:\u003c/strong\u003e Writing \u0026ndash; original draft, Funding acquisition, Data curation, Conceptualization. \u003cstrong\u003eShan Qing:\u003c/strong\u003e Writing \u0026ndash; review \u0026amp; editing, Resources, Formal analysis. \u003cstrong\u003eJuan Duan:\u003c/strong\u003e Supervision, Funding acquisition. \u003cstrong\u003eZhihui Jia\u003c/strong\u003e: Resources, Project administration, Methodology. \u003cstrong\u003eMingyue Wang:\u003c/strong\u003e Resources, Investigation, Formal analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFinancial support from National Natural Science Foundation of China under Contract (NO. 51966005); Yunnan Fundamental Research Project (Grant No. 202201AU070114).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo datasets were generated or analysed during the current study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eK Holmberg, P Andersson, A Erdemir. 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Polymers for Advanced Technologies, 2001, 12(5): 285-292.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-nanoparticle-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nano","sideBox":"Learn more about [Journal of Nanoparticle Research](http://link.springer.com/journal/11051)","snPcode":"11051","submissionUrl":"https://submission.nature.com/new-submission/11051/3","title":"Journal of Nanoparticle Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"ZrO2 / lubricating oil nanofluids, surfactant, surface modification, stability, friction and wear","lastPublishedDoi":"10.21203/rs.3.rs-6241691/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6241691/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this paper, ZrO\u003csub\u003e2\u003c/sub\u003e / lubricating oil nanofluids modified by silane coupling agent γ-methacryloxypropyltrimethoxysilane ( KH570 ) were prepared by surface modification technology. Four surfactants and unmodified nanoparticles were added to the lubricating oil to prepare nanofluids with surfactants. Firstly, the characteristics of modified particles were studied. The results showed that the highest grafting rate of KH570 was found in the modified nanoparticles with 20ml KH570, and the grafting rate was 4.818%. At the same time, it was found that the modified nano-zirconia changed from hydrophilicity to hydrophobicity, and the lipophilicity and dispersion stability were also improved. The stability and friction and wear properties of the prepared nanofluids were studied. The results show that the nanofluids prepared by adding sodium dodecyl benzene sulfonate ( SDBS ) and unmodified nanoparticles in the surfactant have better stability. In contrast, the nanofluids prepared by KH570 modified nanoparticles showed more excellent stability. The modified nano- ZrO\u003csub\u003e2\u003c/sub\u003e has better tribological properties than the unmodified ZrO\u003csub\u003e2\u003c/sub\u003e. The best friction coefficient of the modified nano- ZrO\u003csub\u003e2\u003c/sub\u003e lubricating oil is 0.0753, and the L-TSA46 base oil is reduced by 40.71%.\u003c/p\u003e","manuscriptTitle":"Effect of surface modification on the stability and friction and wear properties of ZrO2 / lubricating oil nanofluids","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-02 12:43:02","doi":"10.21203/rs.3.rs-6241691/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-04-22T12:38:33+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-21T15:18:02+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-11T07:53:51+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-09T07:13:28+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-03-28T00:08:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"18049813685562845846374396381782339334","date":"2025-03-26T03:43:50+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"53353709422889367494188036330545666651","date":"2025-03-25T15:57:43+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"180399423286817158503888982203663652191","date":"2025-03-25T14:09:09+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"168246308186292283783682668078687022789","date":"2025-03-25T08:36:22+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-03-25T07:10:46+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-03-24T20:35:44+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-03-24T02:55:47+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Nanoparticle Research","date":"2025-03-17T06:41:21+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-nanoparticle-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"nano","sideBox":"Learn more about [Journal of Nanoparticle Research](http://link.springer.com/journal/11051)","snPcode":"11051","submissionUrl":"https://submission.nature.com/new-submission/11051/3","title":"Journal of Nanoparticle Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"58a57833-ff37-4cc1-afcb-c7b7eab68c8d","owner":[],"postedDate":"April 2nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-06-09T16:00:55+00:00","versionOfRecord":{"articleIdentity":"rs-6241691","link":"https://doi.org/10.1007/s11051-025-06358-3","journal":{"identity":"journal-of-nanoparticle-research","isVorOnly":false,"title":"Journal of Nanoparticle Research"},"publishedOn":"2025-06-03 15:57:17","publishedOnDateReadable":"June 3rd, 2025"},"versionCreatedAt":"2025-04-02 12:43:02","video":"","vorDoi":"10.1007/s11051-025-06358-3","vorDoiUrl":"https://doi.org/10.1007/s11051-025-06358-3","workflowStages":[]},"version":"v1","identity":"rs-6241691","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6241691","identity":"rs-6241691","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}