The anti-wear and friction-reducing effect of functional gradient materials on the surface of cylinder liners was studied to improve the tribological properties of cylinder liner-piston ring pairing. A groove with a regular texture arrangement at a 30° angle was prepared on the surface of the cylinder liner by a laser marking machine. Iron-based powders with different nickel contents were clad in the grooves by coaxial powder feeding. The metallographic structure of the cladding alloy layer was analyzed, and the tribological properties of the functional gradient material cylinder liner under a certain load pressure were studied by a reciprocating friction and wear tester. The metallographic structure of the cladding layer showed a gradient arrangement, and the molecular atoms of the cladding layer and the matrix material diffused mutually to form a good metallurgical bond. The functional gradient material prepared by powder with a grade of 350, a nickel mass fraction of 4.2%, and an iron mass fraction of 80% had the most obvious friction reduction effect. Compared with the untreated control group, the average friction coefficient decreased by 25.4%, and the average contact resistance of this group was the largest in the test, which was increased by 63.45% compared with the control group. The surface support index of the cylinder liner prepared by powder with grade 330, 9% nickel mass fraction and 70% iron mass fraction is the highest at 0.78, which is 35% higher than that of the untreated control group, and the wear of the piston ring in the test group after friction is less than that of the control group. Conclusion The functional gradient material with a 30° angle distribution was prepared, which can make the lubricating oil film thickness of the friction pair of the cylinder liner-piston ring tribological system thicker during operation and more stable during operation, so as to reduce friction and reduce friction factor. At the same time, its support capacity and ability to store lubricating oil are enhanced, so that the friction factor of the prepared cylinder liner is significantly reduced. The material with grade 350 has the best effect in improving tribological properties.
The internal combustion engine is the core device of the ship power system. Reducing the friction loss of the internal combustion engine is of great significance for saving energy consumption. Studies have shown that when an internal combustion engine is working, the friction loss of the friction pair accounts for about 18% of the total energy loss, of which the loss of the cylinder liner-piston ring friction pair accounts for 70% to 80% of the friction loss of the piston assembly [1-2]. Therefore, improving the friction and wear performance of the cylinder liner-piston ring has become a focus of energy conservation, as well as an important part of developing a green economy and energy transformation and upgrading [3].
In recent years, many scholars have studied the role of surface texture in reducing friction and anti-wear of the piston ring-cylinder liner friction pair [4]. In the 1980s, surface texture technology was applied to the piston ring-cylinder liner friction pair. The research on cylinder liner surface texture has developed from the study of the role of surface micro-convexities and micro-dimples. A large number of studies have shown that micro-dimples can significantly improve the tribological properties of the piston ring-cylinder liner friction pair [5-6]. Profito et al. [7] studied the micro-groove texture processed by laser processing. By real-time monitoring of the change of friction coefficient, the influence of micro-groove texture on the friction performance of the friction pair under different lubrication conditions was analyzed. Lv Yonggang et al. [8] verified that a bionic texture can effectively reduce the surface roughness after wear, reduce abrasive wear and improve the lubrication effect. Xu Changkun et al. [9] processed circular textures with different texture densities on piston rings, and screened out the texture density with the best improvement effect after experimental verification. Rao Xiang et al. [10] conducted experiments under different working conditions using cylinder liners with different types of surface textures. The experiment screened out a groove texture structure with a 30° angle arrangement suitable for a variety of operating conditions. This structure has better tribological properties, and when its width is 2 mm, the groove structure can significantly improve the friction and wear performance of the low-speed friction pair. Tong Dehui et al. [11] divided the texture into different zones and studied the differences in their improvement of tribological properties. Yin Bifeng et al. [12] established a theoretical model for mixed lubrication of cylinder liner-piston ring friction pairs and simulated and analyzed the influence of groove angle.
At the same time, since functional gradient materials have the characteristic that material properties change with the change of material composition or structure, they are also increasingly used to improve the tribological properties of mechanical parts [13-14]. Wang Zhihao et al. clad an iron-cobalt gradient coating on the surface of a copper alloy, which greatly improved the wear resistance of the cladding surface. In the experiment, the adhesive wear of the functional gradient material was greatly reduced and the abrasive wear was weakened. Du Baoshuai et al. [16] used titanium iron and graphite as pre-set powders to prepare an iron-based wear-resistant composite coating on a carbon steel matrix. The element ratios in the coating were different, and the coating phase structure evolved accordingly. The obtained TiC was uniformly distributed in the coating in the form of dendrites and petals, which significantly improved the hardness of the cladding layer. Hao Yunbo et al. [17] used direct laser deposition technology to prepare SS316/Ni20/Fe90 gradient materials on impeller machinery. The hardness and wear resistance of the top of the gradient material were significantly improved compared with the bottom. Among the methods for preparing functional gradient materials, laser cladding is a digital additive manufacturing technology based on synchronous powder feeding. Thanks to the free movement of the laser and the flexible component control, directed energy deposition has become the most suitable additive manufacturing process for preparing metal functional gradient materials. Directed energy deposition controls the powder feeding rate of different powders and then regulates the component gradient of the material. Wang Bingtao et al. [18] also used a combination of numerical simulation and experiment to verify that nickel-based alloy powder can prepare a gradient layer with good metallurgical bonding on the surface of stainless steel. At present, some scholars have studied the preparation of gradient coatings by laser cladding from the perspective of the selection of cladding substrates [19-21], and some studies have also started from the composition of cladding powders [22-23]. The above studies show that the functional gradient materials produced by laser cladding additives can effectively improve the tribological properties of workpieces.
At present, many scholars have conducted a lot of research on the improvement of friction properties of functional gradient materials and the influence of surface texture on the cylinder liner-piston ring tribological system [24], but there are few studies on the influence of functional gradient materials on the cylinder liner-piston ring tribological system, and there are also few studies on the coupling effect between the two. To further explore its influence and mechanism, based on the optimized texture 30° groove texture structure selected in reference [10], this paper laser clads iron-based powders with different nickel contents to prepare functional gradient materials on the inner surface of cylinder liner slices, and compares them with the untreated original cylinder liner, and analyzes their tribological behavior by a reciprocating friction and wear tester.
1 Experiment
1.1 Materials and preparation
The experiment uses a ZS1115 diesel engine cylinder liner, and obtains 80 mm×120 mm cylinder liner slices by wire cutting. The material is wear-resistant alloy cast iron. The piston ring surface is not coated. The piston ring slice length is 60 mm and the material is ductile iron. The cylinder liner slices are first cleaned with anhydrous ethanol, and the texture is processed using a YLP-D series laser marking machine from Han’s Laser. The working parameters of the marking machine are shown in Table 1. The 30° groove texture in the literature [10] was selected, and 8 identical and parallel texture grooves were processed on the inner wall of each cylinder liner slice. Three cylinder liners were selected for the above processing, and one unprocessed cylinder liner was selected as the control group.
Laser cladding was performed on the pits formed on the inner surface of the cylinder liner after laser marking. The cladding equipment used was a Raycus fiber laser. Three cladding powders with different nickel contents were used. The powder grades and main components are shown in Table 2.
The processing adopted a coaxial powder feeding method. The three processed cylinder liners in the test group were laser clad. The same powder and processing parameters were used for the 8 grooves of each cylinder liner slice. The laser processing process is shown in Table 3. The three grades of powder were all from the outline of Wuhan Iron and Steel Huagong Laser Company. The specific powder process parameters were selected based on the company’s actual engineering experience.
The inner surface of the cylinder liner after laser cladding was ground with a white marble grinding wheel to make its inner wall smooth. At the same time, another cylinder liner was selected and processed with the same three process parameters. Wire cutting was performed from the middle section of the cladding path to obtain the cross section of the cladding area of the cylinder liner. The cross section was polished and etched with 4% nitric acid alcohol, and its metallographic structure was observed under a metallographic microscope.
As can be seen from Figure 1, the cross section after laser cladding treatment can be roughly divided into three parts: cladding layer, heat-affected zone and matrix. During the laser treatment process, the alloy layer formed a fine dendritic and cellular structure due to cooling, and a metallurgical bond was formed between the laser cladding layer and the matrix material. The two materials formed a bond through the mutual diffusion between atoms or molecules, and an obvious bonding band can be seen on the cross section. After magnifying the bonding area, it can be seen that the phosphorus eutectic in the cast iron material, that is, the matrix, diffuses into the bonding layer. Further magnification can be observed to observe the smaller flake pearlite in the cast iron, and the pearlite interlamellar spacing is small, which proves that the material in this area has high strength and hardness. At the same time, the flake graphite in the bonding area is also clearly visible. The composition and structure of the material show a gradient change, and the metallurgical bonding between the cladding layer and the matrix material forms a gradient material.
1.2 Test device
The test was carried out on the MWF-10 reciprocating friction and wear tester, and the overall structure is shown in Figure 2. It includes a drive device, a transmission device, a fixture, and a signal acquisition device. The motor drives the fixture to reciprocate on the guide rail through the crank-connecting rod mechanism, and the load is achieved by the mechanical pressure device to achieve the preset contact pressure. With the reciprocating motion of the slide plate, the cylinder liner and the piston ring are reciprocated. The stroke of the slide plate is 100 mm. The pressure and friction are measured by the force sensor set at the corresponding position. The pressure and friction during the test are measured by the sensor. By adjusting the motor speed and the pressure bearing device, the friction environment of the diesel engine under different working loads can be simulated. The lubrication method is drip lubrication. During the test, a certain amount of lubricating oil is evenly dripped into the friction pair. The pressure, friction and contact resistance are collected during the test using the USB6009 acquisition card.
1.3 Test method
To simulate the running environment of a low-speed diesel engine, the motor speed is set to 100 r/min and the load is set to 100 N. The influence of different cladding gradient layers on the friction performance of the cylinder liner-piston ring friction pair is analyzed at the same speed and the same load. Three groups of three grades of cladding powder and an untreated fourth group are set. The sample is a cylinder liner slice made by wire cutting of the cylinder liner of a ZS1115 single-cylinder diesel engine. The material is wear-resistant alloy cast iron. The piston ring slice is of the same size as the cylinder liner slice and is made of ductile iron, as shown in Figure 3.
1.4 Data acquisition
The data to be collected in this test include the friction between the cylinder liner and the piston ring during the test, the voltage signal between the cylinder liner and the piston (converted into contact resistance), and the surface morphology of the cylinder liner after the test. During the test, the friction force and contact resistance are collected 10 data per second by the acquisition card connected to the sensor. After the test contact, the contact surface profiler was used to collect the wear surface morphology characteristics of the cylinder liner sample after friction. Four sample points were collected for each cylinder liner to assist in analyzing the wear characteristics of the cylinder liner-piston ring friction pair [25]. The selected measurement area size was a square area of 0.8 mm × 0.8 mm. The collected data generated a three-dimensional morphology map of the surface of the tested sample and the corresponding characteristic parameters on the PC side.
2 Results and discussion
2.1 Analysis of friction force and friction factor
The trend of friction force changes during the four groups of tests is shown in Figure 4, and the average friction factor of the test is shown in Figure 5. It reflects the overall level of friction performance of the four groups of cylinder liners and their change trend over time. As can be seen from Figure 4, under room temperature conditions with a certain load pressure, the friction force of group 350 is the smallest within the selected 1.5 h (5400 s), while the friction force change trend of group 330 is the most stable during this period. During the 1.5 h experiment, the friction force fluctuated around a fixed value. Although the D30 group fluctuated 15 min before the test, the friction force value remained at 10-20 N afterwards. The control group had a large fluctuation after 30 min, and the friction force value changed more and the mean value increased accordingly.
As can be seen from Figure 5, the average friction coefficient of the three groups of experiments for processing functional gradient materials was significantly lower than that of the control group under the operating conditions of 100 N load and 100 r/min speed. The friction reduction effect of the functional gradient material prepared by 350 powder was the most significant, with a friction coefficient of 0.230 3, which was 25.4% lower than that of the control group. The other two groups also decreased by about 20% compared with the control group.
The friction in the control group increased because the wear debris generated by friction adhered to the surface of the friction pair, aggravating wear and increasing friction. The friction force and average friction coefficient of the first three groups of tests were better than those of the control group, which may be due to the self-lubricating film formed by the functional gradient material on its surface, which reduced friction and wear. Functional gradient materials have layers of different hardness to achieve higher wear resistance and lower friction coefficient. The three cladding powders selected for the experiment all have a certain nickel content, and nickel can show self-lubricating properties in friction and sliding contact. This self-lubricating property is mainly due to the oxide or sulfide film generated on the nickel surface. The lubricating film mainly includes nickel oxide, nickel sulfide and other compounds generated under specific conditions. These compounds form stable chemical bonds on the nickel surface and form a dense film layer on the surface, which reduces the direct contact between the metal surfaces, thereby reducing friction and wear. In addition, the self-lubricating film also has good corrosion resistance and oxidation resistance. At the same time, during the experiment, black adhesives were observed on the surface of the friction pair, which was due to the increase in temperature and pressure and the slight carbonization of the base lubricant. Although carbonization of lubricating oil and wear debris will intensify wear, the sulfur in the base oil can also form a nickel sulfide film with the nickel in the functional gradient material, and the oxide film can play a role in reducing friction and resisting wear. In comparison, the functional gradient material prepared by powder cladding of grade 350 has the best friction characteristics, while 330 shows the stability of the friction value during the friction process. The tribological properties of the cylinder liners prepared with functional gradient materials are significantly better than those of the untreated control group.
2.2 Wear analysis
Before the test, the piston ring was cleaned with anhydrous ethanol and then weighed using an electronic balance. After 2 h of friction test, it was removed for cleaning, weighed again and recorded, and the material mass loss caused by wear before and after the test was calculated and recorded as mass wear. The wear of the 4 groups of experimental piston rings is shown in Figure 6.
As can be seen from Figure 6, the wear of the 330 group and the 350 group is the smallest, which is 0.001 5 and 0.001 533 g. The wear of these two groups is lower than that of the control group (0.007 63 g), which is about 1/5 of the wear of the control group, and the wear of the D30 group is only 1/2 of the control group. The wear of D30 is larger. From the value of the contact resistance change with time in Figure 7, it can be inferred that the lubrication is seriously insufficient during the operation period of 30~60 min. From the overall data, it can be seen that the prepared functional gradient material has a wear-reducing effect on the friction pair, and this result is consistent with the result of the friction coefficient of friction force.
2.3 Contact resistance analysis
Due to the large difference in resistivity between the lubricating oil film and metal conductors such as cylinder liner and piston ring, the resistance value of the system composed of cylinder liner-oil film-piston ring can be measured by applying a measurement circuit between the cylinder liner-piston ring friction pair. Since the resistance of the metal conductor is small, the main influence of the contact resistance comes from the oil film thickness. Therefore, the oil film thickness can be judged by the size of the contact resistance, so as to understand the oil lubrication during the friction process. Take the measured value of the contact resistance within 100 minutes of the running time, take an average of every 100 measured values, and its change over time is shown in Figure 7. Using the formula obtained from the contact resistance working principle circuit diagram, the actual contact resistance of the friction pair during the test can be calculated by the measured value, and its average value is shown in Figure 8.
In Figure 8, the contact resistance of the 350 group and the D30 group is significantly greater than that of the control group. When the contact resistance method is used to measure the thickness of the lubricating film, the magnitude of the resistance is positively correlated with the thickness of the lubricating film, that is, the oil film thickness of the 350 group and the D30 group is greater than that of the control group. Among them, the average contact resistance value of the cylinder liner clad with 350 powder is the largest, which is 1.488 15 Ω, which is 63.45% higher than that of the original control group; followed by the cylinder liner clad with D30 powder, which is 44.52% higher, which is consistent with the results of the friction factor and friction force, indicating that this group has formed an effective lubricating oil film during the experimental friction process; and the contact resistance of the 330 group is slightly lower than that of the control group. From the change of the contact resistance measurement value over time in Figure 4, it can be found that the oil film thickness of 330 is relatively small, which may be due to the short-term eccentric wear after 30 minutes of the test, which leads to its average contact resistance being lower than that of the control group. It can also be intuitively seen from Figure 7 that after the eccentric wear condition ends and a relatively stable lubricating oil film is formed, its oil film thickness is greater than that of the untreated cylinder liner in the control group. This phenomenon occurs because as the surface of the cylinder liner wears, the surface roughness may decrease. Due to the mutual filling between materials and the change of the surface micro-geometry, the oil film formed by the lubricating oil between the surfaces is finally improved, making it easier to form a thicker oil film. The oil film of the 350 group also reached a stable state after running for a period of time.
After removing some data with abnormal contact resistance values in the 330 group, it is not difficult to see that the lubricating oil film of the cylinder liner prepared with functional gradient materials is generally better than that of the untreated control group cylinder liner. This is because laser etching and laser cladding are used in the processing of functional gradient materials, and the arrangement of the functional material on the surface of the cylinder liner has a certain texture. This makes the stress of the 30° arranged functional gradient material different from that of the ordinary surface of the cylinder liner when the friction pair surface is subjected to load. The oil stored at the junction of the arranged material and the ordinary material will be released to relieve friction during the friction process, and the texture of this arrangement can enhance the ability to form a dynamic pressure oil film, forming a lubricating oil film with appropriate thickness and relatively stable, which can improve the friction performance of the friction pair.
2.4 Surface morphology analysis
The surface of the cylinder liner after the test was measured using a laser interferometer displacement surface profiler, and the size of the measurement evaluation area was 0.8 mm × 0.8 mm. The worn surface was evaluated by three characteristic parameters: surface root mean square deviation Sq, valley liquid retention index Svi, and surface support index Sbi. In order to reduce errors and avoid the randomness of the measurement results, four random points were taken on each cylinder liner surface for measurement, and the measured values were averaged. The three characteristic parameters of the cylinder liner wear surface and their comparison are shown in Figure 9, and the more representative cylinder liner wear surfaces after four groups of tests are shown in Figure 10.
Sbi can be used to indicate the support performance of the surface. The larger the value, the better the support performance of the surface. As can be seen from Figure 9, the support index of the cylinder liner surface processed by functional gradient processing is improved compared with the unprocessed control group. The Sbi of the cladding 330 powder is the highest, which is 0.774 25, and the increase is 35% compared with the untreated control group. The above phenomenon is mainly due to the fact that the functional gradient material cladding on the cylinder liner surface has improved its surface hardness through design and adjustment of its composition.
Sq reflects the degree of deviation of the surface profile from the reference plane and is an important parameter to measure whether the measured surface is flat. According to the information in Figure 9, the wear of the D30 powder cladding layer is more serious, and the flatness of the other three groups is not much different. During the wear process, as the wear debris increases, the gap between the friction pairs increases, which is consistent with the results of Figure 10.
Svi reflects the ability of the cylinder liner surface valley area to store lubricating oil. The larger the Svi, the better the retention performance of the surface valley area liquid. As can be seen from Figure 9, the Sq of the first three groups are all greater than the control group, indicating that the cylinder liner surface has better oil storage capacity after processing, and the cylinder liner-piston ring friction pair has better lubrication characteristics and better lubrication. This result is consistent with the trend of contact resistance. In addition to the fact that this functional gradient material with optimized texture promotes the formation of a stable lubricating oil film, the Sq results also show that part of the oil storage capacity may come from the gap caused by wear between the friction pairs.
The above results show that the preparation of functional gradient materials on the surface of the cylinder liner significantly improves the surface quality and surface hardness, and the synergistic effect of the texture and the material can effectively improve the generation of a stable lubricating oil film when the friction pair is working, and improve the friction performance, which is also the same as the result of the friction coefficient of friction force.
3 Conclusions
This paper studies the effect of functional gradient materials with different components on the surface of the cylinder liner on the friction performance of the cylinder liner-piston ring slice friction pair by a reciprocating friction and wear tester. The conclusions are as follows:
1) The surface of the cylinder liner is processed by laser marking and laser cladding of iron-based alloy powders with different nickel contents, and the laser cladding layer forms a good metallurgical bond with the cylinder liner substrate. From the cross-sectional morphology, there are obvious bonding bands, alloy zones and heat-affected zones. Subsequent experiments also verified that the functional gradient material has good tribological properties.
2) In the test, the friction coefficient of the functional gradient material prepared with a nickel mass fraction of 4.2% powder was the lowest. The appropriate content of nickel formed a dense film layer in the friction pair, reducing the direct contact between the metal surfaces and showing the best tribological performance.
3) The functional gradient material prepared at a 30° angle on the cylinder liner surface can enable the cylinder liner-piston ring tribological system to better produce a stable lubricating oil film during operation, thereby reducing friction and friction coefficient, and enhancing its support capacity and ability to store lubricating oil. At the same time, a stable lubricating oil film can also reduce the wear of the friction pair.
