Tribological Properties of Polymer Friction Improvers Combined with MoDTC/ZDDP at Different Temperatures

<jats:p>To expand the applications of polymer friction modifiers in low viscosity oils suitable for hybrid power systems, the tribological properties of five kinds of oil samples at different temperatures were studied, which were compounded with polymer friction modifiers Perfad XG 2500, molybdenum dialkyl dithiocarbamate (MoDTC), zinc dialkyl dithiophosphate (ZDDP). When Perfad XG 2500 is used alone, it forms a brush-like surface film at low temperatures, which can provide lower initial friction. With the rise in temperature, its antifriction effect decreases significantly, which is related to the desorption behavior of physical film. When Perfad XG 2500 is combined with MoDTC, it has the best synergistic antifriction effect, and the average friction coefficient is reduced by a maximum of 20.83% (90 °C). Its friction coefficients decrease significantly with the increase in temperature due to the dissociation process of MoDTC catalyzed by high temperature. However, there are many furrows on the worn surfaces, and the sharp features of the profiles are obvious. When Perfad XG 2500 is combined with ZDDP, it has the best synergistic anti-wear effect, and the total wear depth is reduced by 49.6%. Its worn surface is smooth and defect-free, with minimal residual strain. The friction coefficients are hardly affected by temperature, and the average friction coefficient is reduced by a maximum of 14.4% (30 °C). When Perfad XG 2500, MoDTC and ZDDP are used together, the average friction coefficient is reduced by up to 15.2% (30 °C). Its friction coefficient increases slightly with the rise in temperature. The reason for its moderate tribological performance may be related to the superficial competition of various additives.</jats:p>

section_content: With the continuous upgrading of energy conservation and environmental protection in various countries, engine oil technology is developing in the direction of lower viscosity and longer service life. Lower viscosity can result in less fluid friction, and friction modifiers can reduce the friction and wear of surface asperities. The combination of the two can improve the fuel economy of the engine while ensuring engine reliability [1] [2] [3] [4] [5] [6] [7] [8]. Polymer friction modifier (PFM) is environmentally friendly. It provides low initial friction and is not consumed during driving, effectively extending the oil change cycle. It is applicable to cutting edge hybrid powertrain designs with low temperature operating regimes and low viscosity engine oils. Molybdenum dialkyl dithiocarbamate (MoDTC) has excellent sliding friction reduction in boundary regime, and is widely used in low viscosity oils. As a multifunctional additive, zinc dialkyl dithiophosphate (ZDDP) has become an indispensable part of the engine oil additives due to its anti-wear properties, oxygen resistance, and corrosion resistance. It is of great significance to explore the tribological characteristics and antifriction mechanisms of the PFM combined with MoDTC/ZDDP. 

PFMs have multiple polar functional groups that can change the rheological properties of fluids and form a strong adsorption film on the tribological surface [9]. Aoki [10] studied the effects of speed and temperature on the friction properties of different polymers, demonstrating the antifriction mechanism of multipoint adsorbed polymers under boundary lubrication conditions. Gmür [11, 12] tested the antifriction property and film forming ability of polymers in hexadecane solution. Cyriac [13] studied the effects of organic and polymer friction improvers on the boundary lubrication of steel surfaces in the presence of ZDDP by using a ball-on-disk friction tester. The results showed that PFM had a better antifriction effect. Shen [14] studied the interaction between fatty acids and ZDDP in engine oil. Guegan [15] studied the friction measurement of engine oil containing organic friction improver, oil-soluble organic molybdenum, and functionalized polymers. The results showed that polymer was the most beneficial in sliding-rolling contacts. Kossoko [16] studied the conditions for obtaining low friction with PFMs under rolling-sliding and reciprocating pure sliding. Delamare [17] designed PFMs with lipophilic brush blocks and anchor blocks, and studied its adsorption and lubricity. Murdoch [18] revealed the relationship between adsorption and adhesion of functionalized olefin copolymers in boundary lubrication. Yamashita [19] studied the effect of temperature on the structure and tribological properties of PFM adsorption layer, and analyzed the low friction mechanism of adsorption layer at higher temperatures. Moody [20] studied the performance and mechanism of PFM in 0W20 engine oil. 

Although MoDTC has excellent antifriction performance, it has some disadvantages such as high price, short service life and easy-to-form sediments [21, 22]. ZDDP has good anti-wear performance, but it is not friendly to the environment [23, 24], so its dosage should be strictly controlled. Liskiewicz [25] revealed the potential synergistic mechanism between hydrogen and ZDDP or MoDTC, and analyzed the impact of additives on the friction behavior of nitrided and sulphonitrided surfaces. Roshan [26] established a friction prediction model under boundary lubrication, and analyzed the sensitivity of the model to MoDTC and ZDDP. Balarini [27] studied the effects of reciprocating and unidirectional rotational friction and wear tests on the tribological properties of MoDTC-containing oils. The results showed that MoDTC can activate reciprocating tests. Lu [28] monitored the process of friction films formation between MoDTC and ZDDP, and proved that MoDTC accelerated the formation of ZDDP friction films. Xu [29] used Raman microscope and atomic force microscope to study the relationship between the formation and removal of MoDTC/ZDDP friction films. Experiments showed that MoS2 friction films are easier to remove from friction contact than ZDDP anti-wear friction films. De Feo [30] analyzed the relationship between the chemical behavior of MoDTC additive in the process of thermal oxidative degradation and its antifriction ability. Espejo [31] conducted automotive tests on fully formulated oil containing MoDTC, demonstrating that it can reduce engine friction torque. 

According to the above-mentioned researches, most studies analyzed the adsorption and low friction mechanism of PFM under different lubrication conditions when used alone. In addition, the studies of MoDTC and ZDDP mostly focused on the friction mechanism when they were used alone and on the analysis of their synergistic effect. However, there were few studies on the tribological behavior of PFM combined with MoDTC, ZDDP. This research can fill this gap. This article used PFM, ZDDP and MoTDC to prepare five 0W20 engine oils. The tribological tests were conducted on the surface of CSS-42L steel parts. The variation rules of friction force, load and worn surface morphology at different temperatures were obtained. The tribological performance of oils at different temperatures was compared and analyzed, and the antifriction mechanism of the PFM combined with MoDTC/ZDDP was revealed. This research work provides theoretical and experimental support for the application of PFM in low viscosity lubricating oils.
section_name: Introduction

section_name: Materials and Methods

section_content: The technical formulations of the test oils are shown in Table 1. The baseline oil (BO) is 0W20 grade, which is composed of base oil and compound functional additive package. The specific composition is shown in Table 2. PFM adopts Perfad XG 2500, a new polymer friction modifier produced by Croda. BO + PFM (BP) investigates the friction performance of Perfad XG 2500 when used alone. BO + PFM + MoDTC (BPM) and BO + PFM + ZDDP (BPZ) explore the tribological performance of Perfad XG 2500 combined with MoDTC/ZDDP, respectively. BO + PFM + ZDDP + MoDTC (BPMZ) studies the tribological coupling effect of Perfad XG 2500 combined with MoDTC/ZDDP simultaneously. The oil samples are shown in Figure 1a.
section_name: Sample Preparation

section_content: The technical formulations of the test oils are shown in Table 1. The baseline oil (BO) is 0W20 grade, which is composed of base oil and compound functional additive package. The specific composition is shown in Table 2. PFM adopts Perfad XG 2500, a new polymer friction modifier produced by Croda. BO + PFM (BP) investigates the friction performance of Perfad XG 2500 when used alone. BO + PFM + MoDTC (BPM) and BO + PFM + ZDDP (BPZ) explore the tribological performance of Perfad XG 2500 combined with MoDTC/ZDDP, respectively. BO + PFM + ZDDP + MoDTC (BPMZ) studies the tribological coupling effect of Perfad XG 2500 combined with MoDTC/ZDDP simultaneously. The oil samples are shown in Figure 1a. The ball/disc reciprocating tribometer (Rtec Instruments, San Jose, CA, USA) was used. The counterpart ball (Si3N4, diameter 6. 3 mm) had higher hardness than the test plate. The plate is made of CSS-42L steel. The hardness and chemical composition of ball/disc is shown in Table 3. Before testing, the test surface of the plate was polished for 15 min on the polishing machine (MP1A) with 60 µm and 200 µm sandpaper, respectively, which ensures that the surface roughness of all the test plates is less than 0. 06 µm. Finally, an ultrasonic cleaning machine (KM-240ST, K&M Technologies LTD, London, UK) and alcohol were used to clean the test samples.
section_name: Sample Preparation

section_content: The tests were conducted on the MFT-5000Rtec tribometer (Rtec Instruments, San Jose, CA, USA), and the schematic and scene of the test system are shown in Figure 1b, c. The heating rod and the probe of the thermometer were immersed in the oil pool, and the temperature of the oils was measured in real time by thermometer UT320D (Uni-Trend Technology, Shanghai, China). The lubricating oils were heated to the specified temperature, and the tests were carried out after the oil temperature stabilized. The test conditions are shown in Table 4. A total of 100 friction coefficients per second were collected. The average value was calculated as the experimental results per second. The friction coefficients at 30 • C, 60 • C, 90 • C and 120 • C were measured at the same wear scar. The test lasted 10 min at each temperature stage. After the friction test, the plate samples were cleaned again by ultrasonic cleaning machine. The white light interference profiler (UP-3D, Rtec Instruments, San Jose, CA, USA) was used to measure the profile parameters of the worn surface. In addition, the micro-morphology of the wear scar on the test plate was observed by an electron probe microanalyzer (JXA-8230, JEOL, Showashima, Tokyo, Japan). The phase of the worn surface was analyzed by energy dispersive spectroscopy (Inca-X-ACT, Oxford, Abingdon, Oxfordshire, UK) and X-ray diffractometer (D-Max-RB, Rigaku, Showashima, Tokyo, Japan).
section_name: Experimental Procedures

section_name: Results and Discussions

section_name: Tribological Properties of Samples

section_content: As shown in Figure 2a, at 30 • C, the BO curve presents a zigzag shape and the friction coefficients fluctuate greatly. The average friction coefficient of the stable stage (150-600 s) is the highest of all oils, reaching 0. 125. At the initial stage (0 s to 50 s), the friction coefficient of BP is relatively low, and then gradually increases. At the stable stage, the friction coefficients of BP and BPM are close, which are 0. 113 and 0. 114, respectively. The curve fluctuations of BPZ and BPMZ are small, indicating that the friction surface is smooth. The average friction coefficients of the two are 0. 107 and 0. 106, respectively, which have an excellent antifriction effect. Compared to BO, BP/BPM/BPZ/BPMZ can reduce the average friction coefficient by 9. 60%, 8. 80%, 14. 40%, and 15. 20%, respectively. As shown in Figure 2b, at 60 °C, the friction coefficients of BO are larger in the 0-200 s stage, and decrease gradually and remain stable in the later stage. The friction coefficient curves of BO, BP and BPM are zigzag, indicating that the morphology of the friction surface is quite uneven. The average friction coefficient of BPM is the lowest and the antifriction performance is the best, indicating that the increase in temperature accelerates the tribochemical reaction of MoDTC on the surface. The curves of BPZ and BPMZ are relatively smooth, which may be due to the fact that ZDDP forms an anti-wear layer on the friction surface and reduces the surface damage. The friction coefficients of BPZ and BPMZ in the stable stage are relatively close. The average friction coefficients of the five oils are 0. 121 (BO), 0. 118 (BP), 0. 104 (BPM), 0. 107 (BPZ), and 0. 108 (BPMZ). Compared with BO, BP/BPM/BPZ/BPMZ can reduce the friction coefficients by 2. 48%, 14. 05%, 11. 57%, and 10. 74%, respectively. 

As shown in Figure 2c,d, the variation of the friction coefficient curves at 90 °C and 120 °C is almost the same. The friction coefficients of BO and BP in the initial stage (0-200 s) are higher, and decrease slightly in the later stage and tend to stabilize gradually. BPM exhibits excellent friction reduction capabilities at high temperatures, with the lowest average friction coefficient. The curves of BPZ and BPMZ are relatively smooth, benefiting from the anti-wear layer of ZDDP. At 90 °C, the average friction coefficients of the five oils are 0. 12, 0. 118, 0. 095, 0. 109, and 0. 108, respectively. Compared with BO, As shown in Figure 2b, at 60 • C, the friction coefficients of BO are larger in the 0-200 s stage, and decrease gradually and remain stable in the later stage. The friction coefficient curves of BO, BP and BPM are zigzag, indicating that the morphology of the friction surface is quite uneven. The average friction coefficient of BPM is the lowest and the antifriction performance is the best, indicating that the increase in temperature accelerates the tribochemical reaction of MoDTC on the surface. The curves of BPZ and BPMZ are relatively smooth, which may be due to the fact that ZDDP forms an anti-wear layer on the friction surface and reduces the surface damage. The friction coefficients of BPZ and BPMZ in the stable stage are relatively close. The average friction coefficients of the five oils are 0. 121 (BO), 0. 118 (BP), 0. 104 (BPM), 0. 107 (BPZ), and 0. 108 (BPMZ). Compared with BO, BP/BPM/BPZ/BPMZ can reduce the friction coefficients by 2. 48%, 14. 05%, 11. 57%, and 10. 74%, respectively. 

As shown in Figure 2c,d, the variation of the friction coefficient curves at 90 • C and 120 • C is almost the same. The friction coefficients of BO and BP in the initial stage (0-200 s) are higher, and decrease slightly in the later stage and tend to stabilize gradually. BPM exhibits excellent friction reduction capabilities at high temperatures, with the lowest average friction coefficient. The curves of BPZ and BPMZ are relatively smooth, benefiting from the anti-wear layer of ZDDP. At 90 • C, the average friction coefficients of the five oils are 0. 12, 0. 118, 0. 095, 0. 109, and 0. 108, respectively. Compared with BO, BP/BPM/BPZ/BPMZ can reduce the friction coefficients by 1. 67%, 20. 83%, 9. 17%, and 10. 00%, respectively. At 120 • C, the average friction coefficients of the five oils are 0. 121, 0. 119, 0. 096, 0. 108, and 0. 110, respectively. Compared with BO, BP, BPM, BPZ, and BPMZ can reduce the friction coefficients by 1. 65%, 20. 66%, 10. 74%, and 9. 09%, respectively. 

The friction coefficient of BO is the highest at 30 • C, and decreases slightly after the increase in temperature, which may be caused by the viscosity-temperature effect. The friction coefficients of BP increase with the increase in temperature, which may be due to the desorption of the physical adsorption film formed by PFM on the friction surface. The friction coefficients of BPM decrease with the increase in temperature, because the increase in temperature accelerates the tribochemical reaction of MoDTC on the surface. Temperature has almost no effect on the friction coefficients of BPZ. With the increase in temperature, the friction coefficients of BPMZ slightly increase.
section_name: Average Friction Coefficients

section_content: The ball reciprocates on the surface of the test disc. As it approaches the end of the stroke, the speed gradually decreases to zero, and then begins to move in the opposite direction. As shown by Figure 3, the ball approaches the first end of the stroke at 599. 23 s and begins to decelerate, and the friction coefficient increases gradually under the condition of boundary lubrication. At 599. 31 s, the ball reaches the end point. The velocity returns to zero and the friction coefficient suddenly changes to zero. Then, the ball moves in the opposite direction and reaches the other end of the stroke at 599. 82 s. The closer the test ball is to the end point, the smaller the relative velocity is. According to the Stribeck curve, under the condition of boundary lubrication, the smaller the speed is, the thinner the oil film is. The contact of the micro-convex body increases, which leads to the fluctuation of loads and frictional forces. The friction coefficient of BO is the highest at 30 °C, and decreases slightly after the increase in temperature, which may be caused by the viscosity-temperature effect. The friction coefficients of BP increase with the increase in temperature, which may be due to the desorption of the physical adsorption film formed by PFM on the friction surface. The friction coefficients of BPM decrease with the increase in temperature, because the increase in temperature accelerates the tribochemical reaction of MoDTC on the surface. Temperature has almost no effect on the friction coefficients of BPZ. With the increase in temperature, the friction coefficients of BPMZ slightly increase.
section_name: Analysis of Instantaneous Characteristics

section_content: The ball reciprocates on the surface of the test disc. As it approaches the end of the stroke, the speed gradually decreases to zero, and then begins to move in the opposite direction. As shown by Figure 3, the ball approaches the first end of the stroke at 599. 23 s and begins to decelerate, and the friction coefficient increases gradually under the condition of boundary lubrication. At 599. 31 s, the ball reaches the end point. The velocity returns to zero and the friction coefficient suddenly changes to zero. Then, the ball moves in the opposite direction and reaches the other end of the stroke at 599. 82 s. The closer the test ball is to the end point, the smaller the relative velocity is. According to the Stribeck curve, under the condition of boundary lubrication, the smaller the speed is, the thinner the oil film is. The contact of the micro-convex body increases, which leads to the fluctuation of loads and frictional forces. Because the transient curves of the same oil at different temperature stages are almost the same, only the test results at 120 • C are shown here. As shown in Figure 3a, the load fluctuates greatly near the end point. The fluctuation amplitude of BPM is the smallest and that of BPZ is the largest among the five oils. The fluctuation amplitude of the load is related to the morphology, hardness and strength of the friction surface. If the hardness and strength of surface with uneven profile are high, the fluctuation range will be larger. As shown in Figure 3b, c, the friction force and coefficient of BO and BP increase sharply as they approach the end of the stroke. At this moment, the lubricating oil film in some areas ruptures and the micro convex body contacts in large amounts. The friction coefficient of BPM/BPZ/BPMZ fluctuates slightly near the end point, possibly due to the formation of chemical reaction films on the surface under the action of high temperature and friction.
section_name: Analysis of Instantaneous Characteristics

section_name: Worn Surface Analysis

section_content: As shown in Table 5 and Figure 4, the surface and profile features of wear scars are compared horizontally (different temperatures) and longitudinally (different oil samples), respectively. (1) BO: There is a deep furrow on the friction surface. Under the action of load and friction, the materials in the middle of the wear scar are squeezed to both sides, and the bearing capacity of the oil film is poor. At 60 • C, the wear scar depth of BO increases significantly. When the temperature continues to rise to 120 • C, the depth of the wear scar almost does not change, but the width of the wear scar increases significantly. The profile of the wear scar is uneven and there are many spikes. (2) BP: There are still some extruded materials on both sides of the wear scar. There is no obvious furrow on the surface. The profile of the wear scar is smooth. The depth of wear scar increases almost uniformly at different temperature. The width of wear scar increases obviously at 120 • C. Temperature has little effect on the anti-wear properties of BP. (3) BPM: There are many furrows on the worn surface and some extruded materials on both sides. The profile has a large number of spikes. The width of wear scar increases uniformly in the four temperature stages. The depth of wear scar is smaller at 90 • C. (4) BPZ: The surface of the wear scar is smooth and the wear depth is shallow, indicating that the oil film has strong bearing capacity and anti-wear performance. There is a small amount of extruded material on both sides and at 1/4 of the wear scar. In the early stage of the experiment (30 • C), the wear amount obviously increased, but when the tests were carried out at the later stage (120 • C), the depth of the wear scar almost did not increase. The increase in temperature is conducive to the formation of an anti-wear layer for ZDDP. (5) BPMZ: The change rule of wear scar morphology and wear difference at different temperatures is similar to that of BPZ, but the wear amount is larger than that of BPZ. A small amount of extruded material is present at the quarter of the wear scar. The depth distribution of the wear scar is uneven. 

Comparing the five oils, it can be seen that BO has the largest wear amount. At the end of the test (40th min), the width and depth of the wear scar are 541. 2 µm and 5. 89 µm, respectively. There are deep grooves and spikes in the cross section of wear scars. Compared with BO, the width and depth of BP wear scar decreased by 15. 2% and 27. 9% respectively. There are still some peaks on the wear surface, but the peaks are not as steep as those of BO. The wear width of BPM is the smallest, but the quality of the worn surface is poor, and there are a large number of steep peaks and valleys in the profile. Compared to BO, the width and depth of the wear scar decreased by 25. 4% and 32. 4%, respectively. The depth of BPZ wear scar is the smallest. The wear profile is smooth and the anti-wear performance is excellent. The width and depth of the wear scar decreased by 11. 1% and 49. 6%, respectively. The wear profile of BPMZ is relatively smooth. The wear depth and width decreased by 33. 9% and 7. 3% compared to BO, respectively. Overall, the anti-wear properties of oil samples are in the following order: BPZ > BPMZ > BPM > BP > BO. both sides and at 1/4 of the wear scar. In the early stage of the experiment (30 °C), the wear amount obviously increased, but when the tests were carried out at the later stage (120 °C), the depth of the wear scar almost did not increase. The increase in temperature is conducive to the formation of an anti-wear layer for ZDDP. ( 5 ) BPMZ: The change rule of wear scar morphology and wear difference at different temperatures is similar to that of BPZ, but the wear amount is larger than that of BPZ. A small amount of extruded material is present at the quarter of the wear scar. The depth distribution of the wear scar is uneven.
section_name: Analysis of Wear Morphology

section_content: both sides and at 1/4 of the wear scar. In the early stage of the experiment (30 °C), the wear amount obviously increased, but when the tests were carried out at the later stage (120 °C), the depth of the wear scar almost did not increase. The increase in temperature is conducive to the formation of an anti-wear layer for ZDDP. ( 5 ) BPMZ: The change rule of wear scar morphology and wear difference at different temperatures is similar to that of BPZ, but the wear amount is larger than that of BPZ. A small amount of extruded material is present at the quarter of the wear scar. The depth distribution of the wear scar is uneven.
section_name: BO

section_content: both sides and at 1/4 of the wear scar. In the early stage of the experiment (30 °C), the wear amount obviously increased, but when the tests were carried out at the later stage (120 °C), the depth of the wear scar almost did not increase. The increase in temperature is conducive to the formation of an anti-wear layer for ZDDP. ( 5 ) BPMZ: The change rule of wear scar morphology and wear difference at different temperatures is similar to that of BPZ, but the wear amount is larger than that of BPZ. A small amount of extruded material is present at the quarter of the wear scar. The depth distribution of the wear scar is uneven. As shown in Figure 5a -j, after the friction tests, there are some deep furrows in the middle of the BO wear scar, some areas have peeling phenomena, and there are many wear debris on the surface. In the process of friction, the metal materials crack and separate from the surface of the matrix material to form wear debris, showing the surface morphology of delamination. There are many peeling pits in the middle of the BP wear scar. There are slight furrows along the edge of the wear scar, accompanied by debris particles. The repeated action of contact stress causes cracks in the surface layer, and the infiltration of lubricating oil accelerates the development of cracks to form spot spalling. The worn surface of BPM has a large number of deep furrows, accompanied by larger particles of wear debris. The worn surface of BPZ is smooth. There are slight furrows and cracks near the edge of the wear scar, and the surface damage is minimal. The middle part of the BPMZ wear scar is relatively smooth. There are obvious furrows and cracks near the edge of the wear scar, and there are large particles of wear debris on the surface. 

The roughness along the direction of the wear scar (yellow line) and the direction of the vertical wear scar (red line) are shown in Figure 5k-t. The roughness of BPM is the highest in both directions. There is a deep valley in the middle of the BO roughness curve, which indicates that there is a deep damage on the surface. The roughness of BPZ is the lowest in both directions, indicating that ZDDP forms a smooth anti-wear layer on the friction surface. 

As shown in Figure 6, the contour curves of BP and BPM fluctuate up and down along the movement direction, with more peaks and valleys. The profile of BO, BPZ, and BPMZ along the sliding direction assumes a parabolic shape, which leads to the fluctuation of the test load. The profile parabola of BO is the highest. Both BPZ and BPMZ have formed a hard antiwear layer on the friction surface. Therefore, the load fluctuations of the three are more obvious, which is consistent with the analysis results in Figure 3.
section_name: BO

section_content: As shown in Figure 7a, amplitude parameters (Ra, Rsk, Rku), spatial parameters (λa), hybrid parameters (∆a, Ir) and statistical curves (Peak Count, Bearing Ratio) are used to describe the surface profile characteristics of wear scars. The roughness average (Ra) of BPZ is the smallest, which is 0. 1 µm. Its surface is the smoothest. The Ra value of BPM (0. 208 µm) is the highest, and the worn surface is the most uneven. The skewness (Rsk) values of the five oils are all positive, indicating that the proportion of valleys in the profile is larger than that of peaks, and the higher the Rsk value, the greater the proportion of valleys. Valleys can cause surface damage. The Rsk values of BPZ and BPMZ are smaller, indicating that ZDDP can obviously reduce the surface damage. The kurtosises (Rku) of all oil samples are greater than three (when equal to three, the contour amplitude is the most uniform), and the larger the value is, the more obvious the sharp feature of the contour is. The sharp feature of BO is the most obvious. Overall, BPZ has the best wear resistance due to its smooth surface, small differences in peak to valley ratios, and small sharp contour features. 

the vertical wear scar (red line) are shown in Figure 5k-t. The roughness of BPM is the highest in both directions. There is a deep valley in the middle of the BO roughness curve, which indicates that there is a deep damage on the surface. The roughness of BPZ is the lowest in both directions, indicating that ZDDP forms a smooth anti-wear layer on the friction surface. 

As shown in Figure 6, the contour curves of BP and BPM fluctuate up and down along the movement direction, with more peaks and valleys. The profile of BO, BPZ, and BPMZ along the sliding direction assumes a parabolic shape, which leads to the fluctuation of the test load. The profile parabola of BO is the highest. Both BPZ and BPMZ have formed a hard antiwear layer on the friction surface. Therefore, the load fluctuations of the three are more obvious, which is consistent with the analysis results in Figure 3.
section_name: Surface Profile Characteristics

section_content: As shown in Figure 7a, amplitude parameters (Ra, Rsk, Rku), spatial parameters (λa), hybrid parameters (Δa, Ir) and statistical curves (Peak Count, Bearing Ratio) are used to describe the surface profile characteristics of wear scars. The roughness average (Ra) of BPZ is the smallest, which is 0. 1 μm. Its surface is the smoothest. The Ra value of BPM (0. 208 μm) is the highest, and the worn surface is the most uneven. The skewness (Rsk) values of the five oils are all positive, indicating that the proportion of valleys in the profile is larger than that of peaks, and the higher the Rsk value, the greater the proportion of valleys. Valleys can cause surface damage. The Rsk values of BPZ and BPMZ are smaller, indicating that ZDDP can obviously reduce the surface damage. The kurtosises (Rku) of all oil samples are greater than three (when equal to three, the contour amplitude is the most uniform), and the larger the value is, the more obvious the sharp feature of the contour is. The sharp feature of BO is the most obvious. Overall, BPZ has the best wear resistance due to its smooth surface, small differences in peak to valley ratios, and small sharp contour features. 

As shown in Figure 7b, the average absolute slope (Δa) indicates the steepness of the profile. The length ratio (Ir) indicates the wrinkle level of the profile. The Δa and Ir of BPM are the highest, the steepness and wrinkle level of the surface are the highest. The stress is quite concentrated. The Δa and Ir of BPZ are the smallest and the surface features are excellent. The average wavelength of the profile (λa) represents the distance between the local peaks and valleys. The λa of BPM is the smallest, and the peak and valley are denser. The λa of BPZ is the largest with sparse peaks and valleys. As shown in Figure 7c,d, the peak counting curves (PC) of the five oils show that the total peak number and peak number of BPZ and BPMZ are obviously less than those of other oils, indicating that they have excellent contour characteristics. The smaller the slope of bearing ratio curve (BRC), the greater the proportion of spikes and valleys in the profile. The slope of BPM is the smallest and the profile characteristic is the worst.
section_name: Surface Profile Characteristics

section_content: The elemental distribution analysis of the BPMZ worn surface was carried out by energy dispersive spectroscopy (EDS). As shown in Figure 8a-d, compared with before and after friction testing, the newly added elements on the worn surface include O/S/P/Zn. As shown in Figure 8e -h, more Mo/O/P/S elements are distributed on the worn surface As shown in Figure 7b, the average absolute slope (∆a) indicates the steepness of the profile. The length ratio (Ir) indicates the wrinkle level of the profile. The ∆a and Ir of BPM are the highest, the steepness and wrinkle level of the surface are the highest. The stress is quite concentrated. The ∆a and Ir of BPZ are the smallest and the surface features are excellent. The average wavelength of the profile (λa) represents the distance between the local peaks and valleys. The λa of BPM is the smallest, and the peak and valley are denser. The λa of BPZ is the largest with sparse peaks and valleys. 

As shown in Figure 7c,d, the peak counting curves (PC) of the five oils show that the total peak number and peak number of BPZ and BPMZ are obviously less than those of other oils, indicating that they have excellent contour characteristics. The smaller the slope of bearing ratio curve (BRC), the greater the proportion of spikes and valleys in the profile. The slope of BPM is the smallest and the profile characteristic is the worst.
section_name: Element Analysis of Worn Surface

section_content: The elemental distribution analysis of the BPMZ worn surface was carried out by energy dispersive spectroscopy (EDS). As shown in Figure 8a-d, compared with before and after friction testing, the newly added elements on the worn surface include O/S/P/Zn. As shown in Figure 8e -h, more Mo/O/P/S elements are distributed on the worn surface than in the non-worn area. The line scan is performed in the vertical direction of the wear scar (red line in Figure 8a ). As shown in Figure 8i -l, the content of Mo/O/P/S in the middle of the curve (wear area) increases obviously. This indicates that BPMZ undergoes frictional chemical reactions during the test, and generates some compounds containing these elements. This result provides guidance for peak analysis of XRD spectra.
section_name: Element Analysis of Worn Surface

section_content: As shown in Figure 9a, compared to the initial surface, some weak new crystal peaks appeared in the X-ray diffraction pattern of the wear scar, indicating that frictional chemical reactions occurred on the friction surface and the content of the products was low. During the test, under the action of high temperature and friction behavior, BO and BP reacted with the air dissolved in oil on the friction surface, resulting in a small amount of
section_name: XRD

section_content: As shown in Figure 9a, compared to the initial surface, some weak new crystal peaks appeared in the X-ray diffraction pattern of the wear scar, indicating that frictional chemical reactions occurred on the friction surface and the content of the products was low. During the test, under the action of high temperature and friction behavior, BO and BP reacted with the air dissolved in oil on the friction surface, resulting in a small amount of Fe 2 O 3 and Fe 3 O 4. The anti-wear layer is formed on the surface of BPZ, which is mainly composed of complex compounds formed by the interaction of S/P elements. The surface layer is mainly composed of FeS and ZnS, which has low shear strength and high melting point to protect the metal from further wear. Some studies showed that ZDDP can also produce FePO 4, FeSO 4, and ZnSO 4, forming the second anti-wear layer [32]. However, there is no clear crystal peak in the spectrum, perhaps because the content is too small or not in the surface layer. During the initial stage of the BPM test, low concentrations of iron oxides also form on the wear scars. However, with the increase in temperature, MoDTC decomposes on the surface to form MoS 2 and FeS friction films [33] [34] [35]. The friction film inhibits the further oxidation of iron on the worn surface, which may be the reason why there is no obvious crystal peak of iron oxide in BPM. The surface of BPZ/BPMZ is the same. As shown in Figure 9b, the surface crystallinity decreases after wear, which indicates that the friction behavior damages the crystal structure to varying degrees [36]. Among all the oils, BO has the lowest crystallinity and the highest residual strain. Compared with BO, BP/BPM/BPZ/BPMZ can reduce the residual strain by 13. 2% ((0. 121-0. 105)/0. 121), 24. 0% ((0. 121-0. 092)/0. 121), 66. 1% ((0. 121-0. 041)/0. 121), and 52. 9% ((0. 121-0. 057)/0. 121), respectively.
section_name: XRD

section_content: As shown in Figure 10, the test ball is immersed in lubricant and moves on the surface of the test disc at a relative velocity V under the action of load FN and friction FX. During ball sliding, the friction modifiers form a physical or chemical lubrication film on the worn surface to reduce friction and wear. The viscosity at low temperature is significantly higher than that at high temperature, and the oil film is thicker. The polymer molecules of PFM are adsorbed on the solid surface to form brush-like surface films having polymer concentrations. At high temperature, the viscosity of the lubricant decreases significantly and the oil film becomes thinner. At this time, there are a large number of micro-convex contacts, and some of the polymer molecules begin to desorb. The fluid entrainment causes a small number of polymers enter the friction zone and form a compressed film. MoDTC needs temperature catalysis to quickly form a chemical reaction film with low shear strength on the friction surface. ZDDP can also rapidly undergo frictional chemical reactions at low temperatures, forming an antifriction layer on the surface. The friction coefficient does not increase at high temperature, indicating that the chemical film is very
section_name: Interaction Mechanism of PFM with MoDTC/ZDDP

section_content: As shown in Figure 10, the test ball is immersed in lubricant and moves on the surface of the test disc at a relative velocity V under the action of load F N and friction F X. During ball sliding, the friction modifiers form a physical or chemical lubrication film on the worn surface to reduce friction and wear. The viscosity at low temperature is significantly higher than that at high temperature, and the oil film is thicker. The polymer molecules of PFM are adsorbed on the solid surface to form brush-like surface films having polymer concentrations. At high temperature, the viscosity of the lubricant decreases significantly and the oil film becomes thinner. At this time, there are a large number of micro-convex contacts, and some of the polymer molecules begin to desorb. The fluid entrainment causes a small number of polymers enter the friction zone and form a compressed film. MoDTC needs temperature catalysis to quickly form a chemical reaction film with low shear strength on the friction surface. ZDDP can also rapidly undergo frictional chemical reactions at low temperatures, forming an antifriction layer on the surface. The friction coefficient does not increase at high temperature, indicating that the chemical film is very stable. In the boundary lubrication state, PFM has synergistic effects with MoDTC and ZDDP, respectively. The compressed polymer film reduces the contact of micro convex bodies with the chemical film, and avoids the rupture of the chemical film caused by longterm wear. The friction is reduced and the service life of the chemical film is prolonged. The wear interface is composed of oil film, antifriction layer (MoS2/FeS/ZnS), anti-wear layer (sulfate, phosphate), oxide layer, damage layer and metal matrix. In the process of friction, cracks occur on the surface, and the entry of lubricant leads to the continuous propagation of cracks and the final fracture to form wear debris. The synergistic effect of Perfad XG 2500 and MoDTC can significantly improve the friction force and friction coefficient and obtain the best antifriction performance. Perfad XG 2500 and ZDDP have the best synergistic anti-wear effect with excellent surface characteristics and low residual strain. Perfad XG 2500, MoDTC, and ZDDP in BPMZ preempt friction surfaces, creating fierce surface competition. Its antifriction performance at high temperature is not as good as that of BPM, and its anti-wear performance is worse than that of BPZ. The optimal compounding ratio of three or more friction modifiers still needs further research.
section_name: Interaction Mechanism of PFM with MoDTC/ZDDP

section_content: In this paper, the tribological behavior of polymer friction modifiers and its blends with MoDTC and ZDDP at different temperatures were studied. The friction coefficient, friction force, and load of the five oils were measured by the ball/disc reciprocating tribometer. The three-dimensional profile, SEM-EDS, and XRD analysis were performed on the wear scars of the test disk. According to the above research, the following conclusions can be drawn:

(1) Perfad XG 2500 can provide lower initial friction at low temperatures (30 °C). Compared with BO, BP has the best antifriction effect at 30 °C (9. 6%). When the temperature increases, the antifriction performance becomes worse. At the same time, PFM can improve the wear performance of BO to some extent. 

(2) The compound use of PerfadXG2500 and MoDTC has synergistic effect and has
section_name: Conclusions

section_content: In this paper, the tribological behavior of polymer friction modifiers and its blends with MoDTC and ZDDP at different temperatures were studied. The friction coefficient, friction force, and load of the five oils were measured by the ball/disc reciprocating tribometer. The three-dimensional profile, SEM-EDS, and XRD analysis were performed on the wear scars of the test disk. According to the above research, the following conclusions can be drawn:

(1) Perfad XG 2500 can provide lower initial friction at low temperatures (30 • C). Compared with BO, BP has the best antifriction effect at 30 • C (9. 6%). When the temperature increases, the antifriction performance becomes worse. At the same time, PFM can improve the wear performance of BO to some extent. 

(2) The compound use of Perfad XG 2500 and MoDTC has synergistic effect and has the best antifriction performance. Compared with BO, the friction reduction effect of BPM at 90 • C is as high as 20. 83%, and the wear depth is reduced by 32. 4%. However, the quality of the worn surface is poor and the sharp features of the profile are obvious. 

(3) The compound use of PFM and ZDDP also has synergistic effect, and the anti-wear performance is the best. Compared with BO, BPZ can reduce friction by up to 14. 4% (30 • C) and wear depth by 49. 6%. The worn surface is smooth and free from defects, and the residual strain is the smallest. 

(4) When combined with PFM, MoDTC, and ZDDP, the average friction coefficient can be reduced by up to 15. 2% (30 • C) and the wear depth can be reduced by 33. 9% compared to BO. The simultaneous use of the three forms a fierce superficial competition.
section_name: Conclusions

section_name: Data Availability Statement:

section_content: The data presented in this study are available in this article.
section_name: Data Availability Statement:

section_content: The authors declare no conflict of interest.
section_name: Conflicts of Interest: