Graphene composite lubricating additive as well as preparation method and application thereof

Abstract

The invention discloses a graphene composite lubricating additive as well as a preparation method and application thereof, and belongs to the technical field of lubricating additive preparation. Modified graphene, nano molybdenum disulfide, a dispersing agent and thiophosphate amine salt are mixed and react to obtain the graphene composite lubricating additive. According to the invention, a triple synergistic stabilization mechanism of chemical anchoring, physical blocking and steric hindrance is utilized, so that the compatibility of graphene and base oil is improved, and the dispersion stability of the graphene composite lubricating additive is improved. Meanwhile, the prepared graphene composite lubricating additive can reduce the temperature of a friction pair, can form a friction reaction film containing C, Mo, P and O elements on the surface of the friction pair, reduces friction wear, shows good friction reduction and wear resistance, provides a theoretical basis and technical support for development of high-performance lubricating oil additives and application of high-temperature working conditions, and has wide application prospects. And the high-temperature lubrication problem of the marine diesel engine is solved, and remarkable engineering value and application prospects are achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lubricating additive preparation, and particularly relates to a graphene composite lubricating additive and a preparation method and application thereof. BACKGROUND

[0002] A ship engine, as a key power source of a ship, generates high temperature inside after long-time high-power operation. The high temperature increases the friction between engine components, which not only wastes energy and reduces efficiency, but also accelerates the aging of parts. Therefore, there is an urgent need for high-performance lubricating additives. Traditional lubricating additives often perform poorly at high temperatures, are easily decomposed, oxidized or volatilized, lose lubricating effect, increase friction and wear, and may also pollute the environment.

[0003] In the research of high-temperature lubricating additives for ship engines at home and abroad, on the one hand, the development of new lubricating additives is focused on, including graphene, borate and its composite materials, and the preparation method and performance improvement approach thereof are studied; on the other hand, the tribological properties and action mechanism of these additives under actual working conditions such as high temperature and heavy load are concerned, and the influence of the additives on the friction coefficient, wear amount and surface morphology of the friction pair is analyzed in depth through various testing methods, which provides a rich theoretical basis and scientific basis for the research and development of lubricating oil additives for ship engines. However, the dispersion stability of the additives still needs to be further improved to adapt to more complex working conditions, and future research needs to be continuously deepened in terms of material innovation, preparation process optimization and performance improvement. SUMMARY

[0004] The technical problem to be solved by the application is to provide a graphene composite lubricating additive and a preparation method and application thereof, so as to solve the technical problem of poor dispersion stability and friction-reducing and wear-resistant performance of existing lubricating additives at high temperatures.

[0005] To achieve the above-mentioned purpose, the technical scheme adopted by the application is to provide a preparation method of a graphene composite lubricating additive, comprising the following steps: S1, dispersing graphene in water, then adding a modifier, adding acid to make the pH value of the mixed solution 5-6, and finally placing the mixed solution in water at 160-200 DEG C for 5-10 h of hydrothermal reaction, washing after the reaction, drying, and obtaining modified graphene; the ratio of graphene and modifier is 2-5 g: 5-10 mL; the modifier is ethylenediamine; S2, the modified graphene, nanometer molybdenum disulfide and dispersant are mixed in water, the nanometer molybdenum disulfide and the graphene form a 'layered + flaky' composite structure, fill the friction surface micro-pit, enhance the carrying capacity; then ball milling 10-15h, homogenization, then ultrasonic treatment at 50-70 DEG C for 1-3h, finally adding thiophosphate amine salt stirring 30-60min, drying, the graphene composite lubricating additive is obtained; the mass ratio of the modified graphene, nanometer molybdenum disulfide, dispersant and thiophosphate amine salt is 0.5-1:1-2:2-5:1-3; the dispersant is polyisobutylene succinimide, the dispersant prevents the graphene from agglomerating through steric hindrance effect, improves long-term dispersion stability.

[0006] On the basis of the above technical scheme, the application can also be improved as follows: Further, the acid solution is hydrochloric acid.

[0007] Further, the temperature of the hydrothermal reaction is 180 DEG C, and the time is 8h.

[0008] Further, the washing liquid used for washing is anhydrous ethanol.

[0009] Further, the temperature of S1 and S2 drying is 70-90 DEG C, and the time is 3-5h.

[0010] Further, the thiophosphate amine salt is zinc dipropyl thiophosphate or T307, and the thiophosphate amine salt forms a chemical protective film in the use process, further reducing wear.

[0011] Further, the rotating speed of ball milling is 1800-2200r / min.

[0012] Further, the pressure of homogenization is 100-200MPa.

[0013] The application further discloses the graphene composite lubricating additive prepared by the preparation method.

[0014] The application further discloses the application of the graphene composite lubricating additive in preparing a lubricant.

[0015] The application has the following beneficial effects: 1、The triple synergistic stabilization mechanism of "chemical anchoring + physical barrier + steric hindrance", the nano MoS2 can be inserted between the graphene layers, increase the interlayer spacing, weaken the attraction between the layers, avoid the formation of large area stacking agglomerates, the polar groups (-P=O, -S-) in the thiophosphate amine salt molecules can form hydrogen bonds, coordination bonds with the surface defects of graphene (such as hydroxyl) or the amino groups of amino-modified graphene (rGO-NH2), and are firmly adsorbed on the surface of the graphene layers, avoiding the direct adsorption and agglomeration between the layers due to van der Waals force, improving the compatibility of graphene and base oil, and improving the dispersion stability of the graphene composite lubricating additive.

[0016] 2、The graphene composite lubricating additive prepared by the application can reduce the temperature of the friction pair, can form a friction reaction film containing C, Mo, P and O elements on the surface of the friction pair, and can reduce friction and wear. When 0.05wt% of the graphene composite lubricating additive (sample A) is added in the base oil 500N under high temperature working condition (200℃), the friction coefficient and the wear volume are reduced by 48.9% and 48% respectively, which shows good friction reduction and wear resistance, provides a theoretical basis and technical support for the development of high-performance lubricating oil additives and high-temperature working condition application, is beneficial to solve the high-temperature lubrication problem of the ship diesel engine, and has remarkable engineering value and application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is the gravity sedimentation test result of the base oil after adding the graphene composite lubricating additive; wherein figure (a) is the actual state when the graphene composite lubricating additive and the base oil are just mixed, and figure (b) is the actual state when the graphene composite lubricating additive and the base oil are mixed and stand for 72h; Figure 2 It is the friction coefficient change curve; Figure 3 It is the friction force change graph; Figure 4 It is the surface morphology graph of the base oil friction pair without adding the graphene composite lubricating additive; Figure 5 It is the surface morphology graph of the friction pair after adding sample D; Figure 6 It is the surface morphology graph of the friction pair after adding sample C; Figure 7 It is the surface morphology graph of the friction pair after adding sample B; Figure 8 It is the surface morphology graph of the friction pair after adding sample A; Figure 9 It is the surface wear amount comparison graph of the friction pair; Figure 10Fig. 1 is an infrared thermal image of different lubricating oils at 200℃; wherein Fig. (a) is an infrared thermal image of the base oil before the test, Fig. (a1) is an infrared thermal image of the base oil after the test, Fig. (b) is an infrared thermal image of the sample A compound oil before the test, and Fig. (b1) is an infrared thermal image of the sample A compound oil after the test; Figure 11 Fig. 2 is an infrared thermal image of different lubricating oils at room temperature; wherein Fig. (a) is an infrared thermal image of the base oil before the test, Fig. (a1) is an infrared thermal image of the base oil after the test, Fig. (b) is an infrared thermal image of the sample A compound oil before the test, and Fig. (b1) is an infrared thermal image of the sample A compound oil after the test; Figure 12 Fig. 3 is a scanning electron microscope morphology diagram of the friction pair after being rubbed in the base oil at 200℃; Figure 13 Fig. 4 is an energy spectrum diagram of the friction pair after being rubbed in the base oil at 200℃; Figure 14 Fig. 5 is a scanning electron microscope morphology diagram of the friction pair after being rubbed in the sample A compound oil at 200℃. DETAILED DESCRIPTION

[0018] The specific embodiments of the present application are described below to facilitate the understanding of the present application by those skilled in the art, and the specific conditions are not specified in the examples, which are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, which are all conventional products that can be purchased on the market. However, it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, any changes that are obvious within the spirit and scope of the present application as defined and determined by the appended claims are all included in the protection of the present application.

[0019] Example 1 A preparation method of a graphene composite lubricating additive, comprising the following steps: S1, dispersing 3g of graphene in 100mL of water, then adding 8mL of ethylenediamine, adding 10mL of dilute hydrochloric acid to make the pH value of the mixed solution 5.5, and finally placing the mixed solution in water at 180℃ for 8h of hydrothermal reaction, washing with anhydrous ethanol after the reaction, and drying at 80℃ for 4h to obtain modified graphene; S2, mixing 0.8g of modified graphene, 1.5g of nanometer molybdenum disulfide and 4g of polyisobutylene succinimide in 50mL of water, then ball milling under argon atmosphere at 2000r / min for 12h, homogenizing 3 times at 150MPa, then ultrasonic treatment at 60℃ for 2h, finally adding 2g of T307 and stirring for 50min, and drying at 80℃ for 4h to obtain a graphene composite lubricating additive (sample A).

[0020] Example 2 A preparation method of a graphene composite lubricating additive, comprising the following steps: S1, dispersing 2 g of graphene in 60 mL of water, then adding 5 mL of ethylenediamine, adding 7 mL of dilute hydrochloric acid to make the pH value of the mixed solution 6, and finally placing the mixed solution in a hydrothermal reaction at 160°C for 10 h, washing with anhydrous ethanol after the reaction is completed, and drying at 70°C for 5 h to obtain modified graphene; S2, mixing 1 g of modified graphene, 2 g of nanometer molybdenum disulfide, and 2 g of polyisobutylene succinimide in 60 mL of water, then ball milling under an argon atmosphere at 1800 r / min for 15 h, homogenizing 3 times under 100 MPa, then ultrasonic treatment at 70°C for 1 h, finally adding 3 g of zinc dipropyl thiophosphate and stirring for 30 min, and drying at 70°C for 5 h to obtain a graphene composite lubricating additive.

[0021] Example 3 A preparation method of a graphene composite lubricating additive, comprising the following steps: S1, dispersing 5 g of graphene in 110 mL of water, then adding 10 mL of ethylenediamine, adding 15 mL of dilute hydrochloric acid to make the pH value of the mixed solution 5, and finally placing the mixed solution in a hydrothermal reaction at 200°C for 5 h, washing with anhydrous ethanol after the reaction is completed, and drying at 90°C for 3 h to obtain modified graphene; S2, mixing 0.5 g of modified graphene, 1 g of nanometer molybdenum disulfide, and 5 g of polyisobutylene succinimide in 50 mL of water, then ball milling under an argon atmosphere at 2200 r / min for 10 h, homogenizing 3 times under 200 MPa, then ultrasonic treatment at 50°C for 3 h, finally adding 1 g of zinc dipropyl thiophosphate and stirring for 60 min, and drying at 90°C for 3 h to obtain a graphene composite lubricating additive.

[0022] Comparative Example 1 The difference between this comparative example and Example 1 is that step S2 of T307 is omitted, and the rest of the implementation conditions are the same as those of Example 1, to obtain a graphene lubricating additive (sample B).

[0023] Comparative Example 2 The difference between this comparative example and Example 1 is that the polyisobutylene succinimide in step S2 is omitted, and the rest of the implementation conditions are the same as those of Example 1, to obtain a graphene lubricating additive (sample C).

[0024] Comparative Example 3 The difference between this comparative example and Example 1 is that the nanometer molybdenum disulfide and T307 in step S2 are omitted, and the rest of the implementation conditions are the same as those of Example 1, to obtain a graphene lubricating additive (sample D).

[0025] The graphene lubricating additive samples A-D prepared in Example 1 and Comparative Examples 1-3 were used as samples to perform the following experiments.

[0026] Experimental Example 1: Gravity Settling Experiment Samples A-D were added to 500N base oil at an additive amount of 0.05wt% and uniformly dispersed, and after ultrasonic oscillation for 45min at room temperature, the dispersion stability was observed after standing for 72h. The results are shown in Table 1, wherein sample A had the best dispersion in the oil sample and no obvious stratification, because the combination of "graphene + MoS2 + sulfonamidate + polyisobutylene succinimide" contained in the graphene composite lubricating additive formed a triple synergistic stabilizing mechanism of "chemical anchoring + physical barrier + steric hindrance". Figure 1

[0027] The polar groups (-P=O, -S-) in the sulfonamidate molecule can form hydrogen bonds and coordination bonds with the surface defects (such as hydroxyl groups) of graphene or the amino groups of amino-modified graphene (rGO-NH2), and are firmly adsorbed on the surface of the graphene sheet layer, avoiding direct adsorption and agglomeration between sheet layers due to van der Waals forces. The non-polar alkyl long chain (such as C12-C18) at the other end of the molecule can fully fuse with the hydrocarbon chains of the base oil, improving the compatibility of graphene with the base oil, allowing the graphene adsorbed with the amine salt to smoothly integrate into the oil phase, rather than being isolated and suspended to cause settling.

[0028] The size difference between nano-MoS2 (particle size 50-100nm) and graphene sheet layer (0.5-2μm) is large, and MoS2 can penetrate between graphene sheet layers, increase the distance between sheet layers, and weaken the attraction between sheet layers, avoiding the formation of large-area stacked agglomerates. In subsequent ball milling, homogenization and other processes, MoS2 and graphene are simultaneously exfoliated and dispersed, and MoS2 particles can reduce the collision probability of graphene sheet layers and reduce the risk of "secondary agglomeration" during the dispersion process, thereby improving the dispersion efficiency.

[0029] As a high molecular dispersant, polyisobutylene succinimide forms a "three-dimensional protective network" on the surface of graphene, which prevents agglomeration for a long time. The long chain (molecular weight tens of thousands) of the high polymer can wrap around the surface of the graphene sheet layer (including graphene that has adsorbed amine salt), forming a thick three-dimensional protective layer. When two graphene sheet layers approach each other, the long chains will repel each other, creating steric hindrance and physically preventing the sheet layers from contacting and agglomerating. The motion resistance of the wrapped graphene particles in the base oil is increased, and the settling speed is significantly reduced. Even after long-term standing, stratification and precipitation caused by gravity can be reduced, and the storage stability can be improved.

[0030] Experimental Example 2: Wear Test at Normal Operating Temperature ​The samples A-D were added to 500N base oil at an addition amount of 0.05wt% and uniformly dispersed, and after ultrasonic oscillation for 45min, composite oil sample liquid was obtained, and an equal volume of 500N base oil (Base Oil) was used as a control group sample liquid, and the test temperature was uniformly controlled at 40°C (simulating the conventional working condition oil temperature).

[0031] 1. Four-ball machine test: test the wear scar diameter and PB value (GB / T 3142-2019).

[0032] Clean GCr15 bearing steel (diameter 12.7mm), and after drying, install it on the MQ-800 type four-ball friction and wear tester, and add 5mL of sample to be tested.

[0033] Measure the wear scar diameter: set the load to 392N, the speed to 1450r / min, and the time to 60min, and after the test, remove the steel ball, and measure the wear scar diameter of the three balls under the microscope (take the average value).

[0034] Measure the PB value (extreme pressure load): start from low load (294N), increase by 98N each time, run for 10s under each load, until the steel ball is obviously stuck or the wear scar diameter changes suddenly, and record the load at this time as the PB value.

[0035] 2. Reciprocating friction and wear test: test the average friction coefficient (GB / T 12583-2008).

[0036] Use UMT-3 type reciprocating friction and wear tester for testing, and uniformly apply the sample to be tested on the surface of the counterpart (upper sample: GCr15 steel ball, diameter 6mm; lower sample: 45# steel sheet, size 20mm×20mm×5mm).

[0037] Set parameters: load 50N, reciprocating frequency 50Hz, stroke 5mm, time 30min, and record the friction coefficient curve in real time.

[0038] Take the average value of the friction coefficient 20min after the test (exclude unstable data in the initial running-in stage).

[0039] The above experimental results are shown in Table 1.

[0040] Table 1 Wear data of oil temperature in each group of composite oil

[0041] As can be seen from Table 1, the wear scar diameter (0.35 mm) of sample A (Example 1) is lower than the qualified standard line (0.4 mm), and is more than 36.36%-51.39% smaller than samples B-D (Comparative Examples 1-3), and is more than 60% smaller than the base oil (control group), indicating that the synergistic anti-wear effect of "graphene + MoS2+ thiophosphate amine salt + polyisobutylene succinimide" is significant.

[0042] Only the PB value (950 N) of sample A (Example 1) meets the standard (qualified standard: ≥800 N), which is increased by 24.21% compared with Comparative Example 1 without thiophosphate amine salt (720 N), verifying the effect of the chemical anti-wear agent thiophosphate amine salt on improving the extreme pressure performance.

[0043] The average friction coefficient (0.065) of sample A (Example 1) meets the standard (qualified standard: ≤0.08), which is reduced by 40.87% compared with the control group (0.115), and is better than samples B-D, indicating that the compounded components can effectively reduce friction.

[0044] Experimental Example 3 High-temperature (200℃) tribology experiment Samples A-D were added to 500N base oil at an addition amount of 0.05wt% and uniformly dispersed, and after ultrasonic oscillation for 45 min, composite oil sample liquid was obtained, and an equal volume of 500N base oil (Base Oil) was used as a control group sample liquid. A CFT-I type material surface performance comprehensive tester was used for friction and wear experiment, a disc-pin reciprocating motion was used to simulate the reciprocating motion of the cylinder sleeve-piston ring of the ship engine, the friction pair material was gray cast iron, the disc diameter was 29.5 mm, the pin diameter was 4 mm, the experimental temperature was 200℃, the load was 100N, the rotation speed was 300t / m, and the duration was 60 min.

[0045] 1. Friction coefficient The friction coefficient was accurately recorded by the friction and wear machine software at an interval of 0.2s, and the results are shown in Figure 2 and Figure 3 The average friction coefficient of the control group during the friction period was 0.176, and the average friction force was 17.6N. The friction coefficient of the composite oil added with graphene lubricating additive was significantly lower than that of the base oil, specifically: the friction coefficient of sample D composite oil tended to be stable around 0.131, which was reduced by 25.6% compared with the base oil, and the average friction force was 13.3N, which was reduced by 24.4%. The friction coefficient of sample C composite oil was about 0.121 when it was stable, which was reduced by 31.2%, and the average friction force was 12.5N, which was reduced by 28.9%. The friction coefficient of sample B composite oil was around 0.10 when it was stable, which was reduced by 43.3%, and the average friction force was 11.2N, which was reduced by 36.4%. The friction coefficient of sample A composite oil was stable at about 0.09, which was reduced by 48.9%, and the average friction force was about 10N, which was reduced by 43.2%.

[0046] During the whole experiment, the friction coefficient of base oil gradually increased with time, because the oil film was easy to be thinned or even broken due to oxidation and evaporation under high temperature environment, resulting in the increase of the friction coefficient. The friction coefficient of the composite oil only had slight fluctuation, because the graphene composite lubricating additive prepared by the application could improve the repair efficiency and strength of the oil film, prevent the agglomeration of graphene and the oxidation of MoS2, and prevent the deterioration of the base oil, so that the performance of the lubricating additive would not decrease under high temperature working conditions.

[0047] When the oil film appeared micro-cracks or local thinning under high temperature, the sheet structure of graphene could be quickly adsorbed on the friction surface to fill the gap of the oil film; the layered particles of nano-MoS2 could be embedded in the micro-holes between the graphene layers to form a “sheet-layer interwoven” physical support structure to prevent the oil film from being broken due to the expansion of defects.

[0048] The polar groups of the thiophosphate amine salt adsorbed on the surface of graphene could enhance the compatibility of graphene and the base oil, and reduce the sedimentation of graphene due to the decrease of the viscosity of the oil film under high temperature. At the same time, under high temperature and high pressure, the thiophosphate amine salt would decompose active elements (P, S) and react with the metal friction surface to form a dense phosphate and sulfide chemical protective film (thickness about 5-10 nm). This film could not only replace the damaged physical oil film to play a lubricating role, but also could enhance the high temperature resistance of the oil film to avoid the failure of the oil film due to high temperature oxidation.

[0049] The long chain of polyisobutylene succinimide could still maintain good flexibility under high temperature, and could still wrap graphene to form steric hindrance to prevent the collision and agglomeration of the sheet layers due to the increase of thermal motion. At the same time, polyisobutylene succinimide had certain anti-oxygen dispersion effect, could adsorb the free radicals generated by the oxidation of the base oil, reduce the generation of oil sludge and carbon deposition under high temperature, and avoid the wrapping of graphene and MoS2 by the oxidation products to cause the loss of lubrication and repair ability.

[0050] 2. Surface morphology of friction pair The surface morphology of the friction pair was observed by VK-X250 confocal microscope, and the wear volume was measured, and the results are shown in Tables 2, 3 and Figures 4-9

[0051] Table 2 Influence of different composite oils on the friction pair

[0052] Table 3 Wear volume and anti-wear performance of the friction pair in each group of composite oil

[0053] ​As can be seen from Table 2 and Table 3, the average wear volume of the composite oil added with samples A-D is 703456 μm 3 , 643466 μm 3 , 546685 μm 3 and 493583 μm 3 respectively, all of which are less than the wear volume of the base oil (954496 μm 3 ), and the wear volume of the friction pair in sample A composite oil is the smallest, which is reduced by 48% compared with the base oil, which shows that the graphene composite lubricating additive in the composite oil has good anti-wear performance at a high temperature environment of 200℃, effectively reducing the wear amount of the friction pair surface.

[0054] Figures 4-8 The surface morphology contrast chart of the pin-on-disk test friction pair is shown in the figure, from which it can be seen that when the base oil is used as the lubricating medium, the abrasive wear degree of the friction pair surface is serious, there are many pits on the friction pair surface, and there are many gullies with high depth, in which the maximum depth of the gully is 14.5 μm; when the composite oil is used as the lubricating medium, the abrasive wear phenomenon of the friction pair surface is obviously improved, the width of the gully on the friction pair surface is about 25 μm, and the maximum gully depth is reduced to 4.8 μm. This shows that the graphene composite lubricating additive in the composite oil has good anti-wear performance at a high temperature environment of 200℃.

[0055] Figure 9 The wear amount of the friction pair surface in the pin-on-disk test is shown, it can be seen that when the base oil is used as the lubricating medium, the wear degree of the friction pair surface is serious; when the composite oil is used as the lubricating medium, the wear phenomenon of the friction pair surface is obviously improved, which shows that the graphene composite lubricating additive in the composite oil has good anti-wear performance at a high temperature environment of 200℃.

[0056] 3. Surface temperature field analysis The FLUKE Ti25 infrared thermal imager was used to detect the temperature field distribution of sample A composite oil and 500N base oil before and after the pin-on-disk reciprocating friction test at room temperature and 200℃ high temperature, and the temperature change was recorded, and the results are shown in Figure 10 and Figure 11 .

[0057] Before the test, the temperature of the base oil was 189.4℃, and the temperature of the sample A composite oil was 190.7℃; after 60 min of friction test, the temperature of the base oil rose to 195.3℃, increased by 3.1%, and the temperature of the sample A composite oil increased to 193.2℃, increased by only 1.3%, and the temperature increase of the composite oil was significantly lower than that of the base oil.

[0058] At room temperature, the frictional heat increase on the friction pair surface was 2.2% in the base oil and 1.7% in the composite oil. However, at 200℃, the frictional heat increase was greater in the base oil and less in the composite oil. This indicates that sample A has excellent tribological properties at high temperatures, effectively transferring some of the heat during friction, reducing the possibility of lubricant oxidation between friction pairs, and improving the thermal stability of the composite oil.

[0059] 4. Oil fluid testing and analysis The changes in elemental content in sample A composite lubricating oil before and after the friction test were detected using an MOA II oil analysis spectrometer. The results are shown in Table 4.

[0060] Table 4. Changes in Fe and B element content (ppm) before and after the composite oil test at 200℃

[0061] As shown in Table 4, during the friction process, wear occurs on the surfaces of the friction pairs due to their interaction, leading to the generation of iron filings that mix into the oil system, resulting in a significant increase in the Fe content of the oil. Simultaneously, the graphene composite lubricant additive in the composite oil gradually migrates and adheres to the surface of the friction pairs in the high-temperature friction environment, filling the pits and grooves and achieving a repair effect. During this process, the P content in the composite oil decreases due to participation in related reactions or physical adhesion to the friction surface. This phenomenon indicates that the thiophosphate amine salt in the composite oil participates in the repair and protection of the friction pair surface during the friction process, highlighting the positive role and important value of this graphene composite lubricant additive under high-temperature friction conditions. It provides strong evidence and important reference for further exploring the mechanism of action of lubricants in the field of high-temperature tribology.

[0062] 5. Surface analysis of the friction pair By studying the microstructure and elemental composition of the friction surfaces lubricated by 500N base oil and sample A composite oil, the tribological principles were analyzed. The results are as follows: Figures 12-14 As shown.

[0063] from Figure 12 It can be seen that the friction pair surface lubricated by the base oil exhibits severe abrasive wear characteristics, with numerous and deep grooves, extremely uneven surface, and a large number of abrasive particles. EDS analysis ( Figure 13It can be concluded that the wear region is mainly composed of Fe and C elements, with a small amount of O and Si elements. In a high-temperature environment of 200℃, the stability of the base oil film is significantly reduced, and it is extremely easy to break, which in turn induces serious adhesive wear and abrasive wear phenomena. In addition, the iron filings generated by friction are rapidly oxidized in a high-temperature environment, which further exacerbates the degree of wear and causes more serious damage to the surface of the friction pair, greatly affecting its performance and service life in high-temperature working conditions.

[0064] Compared with the base oil, the surface condition is significantly improved when lubricated with sample A composite oil. As shown in Figure 14 the surface of the friction pair tends to be flat, the depth of the gully is relatively shallow, and a large number of particulate substances are adsorbed on the surface. During the friction process, such particles produce a rolling friction effect on the surface of the friction pair, which is the core mechanism for reducing the phenomenon of scratches, effectively reducing the damage degree of the friction pair. It is particularly important to detect a large amount of O element, which reveals that Mo and P elements in the composite oil react with O element during the friction process, thereby generating an oxidation film with high elastic properties. Due to its special high-elastic property, the oxidation film can highly adapt to the harsh working environment of high temperature and heavy load, becoming a key factor for significantly improving the tribological properties of the composite oil, and providing a strong theoretical basis and practical support for the application of the composite oil in high-temperature and heavy-load friction scenarios.

Claims

1. A method of preparing a graphene composite lubricating additive, characterized by, The method comprises the following steps: S1, dispersing graphene in water, then adding a modifier, adding acid to make the pH value of the mixed solution 5-6, and finally placing the mixed solution in hydrothermal reaction at 160-200℃ for 5-10h, washing and drying after the reaction to obtain modified graphene; the ratio of graphene and modifier is 2-5g:5-10mL; the modifier is ethylenediamine; S2, mixing modified graphene, nano-molybdenum disulfide and dispersant in water, then ball milling for 10-15h, homogenizing, then ultrasonic treatment at 50-70℃ for 1-3h, finally adding thiophosphate amine salt and stirring for 30-60min, and drying to obtain graphene composite lubricating additive; the mass ratio of modified graphene, nano-molybdenum disulfide, dispersant and thiophosphate amine salt is 0.5-1:1-2:2-5:1-3; the dispersant is polyisobutylene succinimide.

2. The method of claim 1, wherein the graphene composite lubricating additive is prepared by the steps of: The acid is hydrochloric acid.

3. The method of claim 1, wherein the graphene composite lubricating additive is prepared by the steps of: a) mixing graphene oxide and a metal salt to form a mixture; b) heating the mixture to form a graphene composite; and c) reducing the graphene composite to form the graphene composite lubricating additive. The temperature of the hydrothermal reaction is 180℃, and the time is 8h.

4. The method of claim 1, wherein the graphene composite lubricating additive is prepared by the steps of: a) mixing graphene oxide and a metal salt to form a mixture; b) heating the mixture to form a graphene composite; and c) reducing the graphene composite to form the graphene composite lubricating additive. The washing liquid used for washing is anhydrous ethanol.

5. The method of claim 1, wherein the graphene composite lubricating additive is prepared by the steps of: a) mixing graphene oxide and a metal salt to form a mixture; b) heating the mixture to form a graphene composite; and c) reducing the graphene composite to form the graphene composite lubricating additive. The temperature of drying in S1 and S2 is 70-90℃, and the time is 3-5h.

6. The method of claim 1, wherein the graphene composite lubricating additive is prepared by the steps of: a) mixing graphene oxide and a metal salt to form a mixture; b) heating the mixture to form a graphene composite; and c) reducing the graphene composite to form the graphene composite lubricating additive. The thiophosphate amine salt is zinc dipropyl thiophosphate or T307.

7. The method of claim 1, wherein the graphene composite lubricating additive is prepared by the steps of: a) mixing graphene oxide and a metal salt to form a mixture; b) heating the mixture to form a graphene composite; and c) reducing the graphene composite to form the graphene composite lubricating additive. The rotating speed during ball milling is 1800-2200r / min.

8. The method of claim 1, wherein the graphene composite lubricating additive is prepared by the steps of: a) mixing graphene oxide and a metal salt to form a mixture; b) heating the mixture to form a graphene composite; and c) reducing the graphene composite to form the graphene composite lubricating additive. The pressure during homogenization is 100-200MPa.

9. A graphene composite lubricating additive characterized in that, The method is prepared by any one of claims 1-8.

10. The use of the graphene composite lubricating additive of claim 9 in the preparation of a lubricant.

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