Research on maintenance-free design and low friction characteristics of industrial robot sliding components

With the continuous progress of industrial automation, the application of industrial robots in various fields has become increasingly common, and ensuring their efficient and stable operation and reducing maintenance costs is crucial. This study used powder metallurgy technology, combined with multiple sintering-rolling processes and oil-immersed vacuum assistance, to successfully prepare Fe-Cu-Ni-Sn-graphite oil-immersed self-lubricating composite materials to meet this demand. In-depth microstructure and wear surface studies revealed that this porous oil-immersed self-lubricating composite material exhibits long-term low friction and high wear resistance and reduces the friction coefficient by 40% compared to untreated samples after a specific composite process. In addition, the material exhibits excellent friction performance in the high-temperature pin-on-disc friction and wear test machine. Even after continuous sliding for 24 hours, its friction coefficient remains low and stable. The study also found that its lubrication mechanism may be attributed to solid-liquid synergistic lubrication, thanks to the appearance of oil-graphite mixtures around the wear track. This design ensures high rigidity and reduced frictional loss, providing a strong reference for the design and optimization of sliding components of industrial robots, and is highly suitable for the widespread application of industrial robots.


Introduction
With the increasing depth of industrial automation, the application of industrial robots is becoming increasingly widespread [1][2][3] .The low friction and long lifespan characteristics of sliding components are crucial to ensure the stable operation of industrial robots and reduce maintenance costs.Traditional lubrication methods may be difficult to meet the long-term and efficient operation requirements, particularly in high-frequency and high-load working environments.The demand for low-friction materials to enhance the long-term serviceability of industrial robot joint components is becoming increasingly strong [4,5] .Self-lubricating materials can maintain low friction characteristics without external lubrication or the need for lubricants, making them highly applicable in this field.
Iron-based self-lubricating materials prepared by powder metallurgy technology exhibit good selflubricating properties, with the lubricating phase uniformly distributed in the material, demonstrating excellent self-lubricating performance [6][7][8] .In addition, these materials also possess excellent antifriction and anti-wear properties while ensuring mechanical strength due to the uniformly distributed lubricating phase in their structure, such as graphite and molybdenum disulfide, providing continuous ICAMIM-2023 Journal of Physics: Conference Series 2720 (2024) 012027 IOP Publishing doi:10.1088/1742-6596/2720/1/012027 2 lubrication.The porous characteristics of powder metallurgy products provide conditions for storing lubricants, allowing the formation of microscopic lubricant reservoirs on the surface of sliding components, thus achieving a self-lubricating effect [9] .When lubricant consumption or wear occurs, the internal lubricant can replenish the surface, achieving lasting lubrication.Graphite, as a common solid lubricant, has a low friction coefficient, which enables porous materials combined with it to have outstanding anti-friction and anti-wear properties [10] .However, it remains a great challenge to maintain the unity of self-lubricating performance and mechanical strength.The rolling and secondary sintering methods are used to ensure both mechanical strength and long-term serviceability.
Traditional sliding components of industrial robots often require regular lubrication, leading to higher maintenance costs.In our early exploration, we sought to create these components using powder metallurgy technology.However, we encountered a challenge: the low density of porous P/M materials resulted in unsatisfactory strength and hardness under high load conditions [1][2][3][4][5][6][7][8][9][10][11][12][13] .To address this issue, inspired by applying porous materials in self-lubricating sliding components, we utilized the unique properties of oil-containing porous powder metallurgy and employed secondary sintering and rolling to enhance the frictional characteristics of the components.Furthermore, we added graphite during the material mixing process to optimize the lubrication effect to achieve synergistic lubrication between solid and liquid phases [14,15] .This design not only ensures the strength of the components but also achieves lower frictional losses, providing a reference for the application of sliding components such as joints in industrial robots.

2.1
Raw materials Table 1.Specifications and participation ratio.The raw materials used in this experiment are high-purity copper powder, iron powder, tin powder, nickel powder, and graphite powder, with their main performance indicators and addition ratios shown in Table 1.According to the proportions in Table 1, the five powders are mixed in a drum-type blender for 3 hours, adding 1% machine oil (N460) as an auxiliary material before mixing to aid in pore formation.The mixed powders are then pressed in a steel mold at a pressure of 350~400 MPa, held for 1 minute, demolded, and sintered in a hydrogen-protected tube resistance furnace at 800°C for 30~50 minutes.After sintering, the sample is cooled in the furnace to obtain Sample 1. Sample 2 is obtained by rolling the sintered sample at a temperature of 400°C, rolling speed of 0.08 m/s, and rolling precision of ±5 mm.Sample 3 is obtained by sintering-rolling-secondary sintering at a temperature of 650°C, with the same rolling process as Sample 2. After preparation, each sample undergoes vacuum oil impregnation at 120°C, using oil with a room temperature viscosity of 375 cps.Finally, the samples are precision machined to the target dimensions.Table 2 lists the parameters of the sintering and pressing processes.

2.2
Experimental method The surface morphology of the friction layer was observed using a scanning electron microscope (SEM, model JSM-6610), and XRD and EDS analyzed its composition.The surface of the samples was observed using an optical metallographic microscope, the porosity of the samples was estimated through binary processing, and the average pore size was calculated.The friction and wear performance of the material was studied on a high-temperature reciprocating friction and wear test machine, where the test piece was made of a developed material, the pin size was Ø5 mm, the test plate was made of 40Cr steel with a size of Ø40 mm, and a hardness greater than 25 HRc.The experiment was conducted at room temperature and in an atmospheric environment for 45 minutes, as shown in Figure 1, with a load of 3 MPa, a friction radius of 25 mm, a sliding time of 24 hours, and a speed of 0.8 m s -1 .Different loads and speeds were considered in the experiment, and the mass before and after wear was measured using an electronic balance, and the mass loss and wear rate were calculated.Hardness testing of sintered samples was performed on a Brinell hardness tester, using a 10 mm diameter hardened steel ball or tungsten carbide ball as the indenter, applying an experimental force of 62.5 kgf (approximately 613.3 Newtons) and pressing it into the surface of the tested material.An electronic balance was used to determine the density of the sample, and its mechanical properties at room temperature were evaluated using a universal materials testing machine.

The microstructure and phase composition of composite materials
As shown in Figure 2, the metallographic morphology of the Cu-Fe-Ni-Sn-Graphite sample varies with different preparation processes.For the sintered sample (a), the phase composition appears relatively dispersed, with various non-continuous phases.However, after rolling, the phase composition of sample b becomes more continuous.Most notably, sample c, after sintering-rollingsintering, exhibits the most uniform and continuous phase distribution with significantly reduced porosity.With the increase in process steps, the microstructure of the sample gradually becomes more uniform and dense, and the distribution of each component becomes more continuous, which may be related to the compression during the rolling process and the solid-phase diffusion during the sintering process.Rolling is a process of deformation of materials by applying pressure.During this process, the material is compressed, leading to enhanced contact and bonding between powder particles, reducing porosity and voids.Additionally, sintering is a heat treatment process under high-temperature conditions that binds powder particles through solid-phase diffusion mechanisms, promoting atomic migration between particles and material redistribution, further enhancing the density of the sintered layer.Therefore, the combined effects of compression during rolling and solid-phase diffusion during sintering result in a more uniform and dense sample.
The XRD results shown in Figure 3 indicate that only the sintered Sample 1 and the sintered-rolled Sample 2 have a similar composition, existing mainly as a solid solution of Cu, Sn, Fe, and Ni, suggesting that rolling did not alter the composition of the samples.For Sample 3, after rolling and secondary sintering, there is almost no detection of the diffraction peak for Fe/(Fe, Ni, Cu) in the matrix.At the same time, the surface composition is mainly a Cu-Sn-Fe-Ni solid solution, indicating that the secondary sintering process increased the degree of alloying in the sample.These results show that the sintering-rolling-sintering composite process can effectively increase the alloying degree of the matrix and improve the material's hardness.This is mainly due to the significant grain refinement of the matrix after rolling deformation, which can effectively improve the strength of the composite material.The deeper reason is that rolling increases the dislocation density in the matrix, causing work hardening and increasing the strength and hardness of the matrix, which is consistent with the hardness trend in Table 2 [16][17][18] .Figure 5 shows the porosity of powder metallurgy samples prepared under three different process conditions: sintering, sintering-rolling, and sintering-rolling-sintering.The results indicate that the porosity decreases as the process steps increase.Compared to single sintering, the sintering-rolling and re-sintering process can effectively reduce the porosity of the material and increase its density, making it the preferred process for obtaining denser powder metallurgy components.This provides references for further optimization of powder metallurgy processes.4, friction coefficient testing of Cu-Fe-Ni-Sn-Graphite powder metallurgy samples prepared by three different processes revealed that the average friction coefficient of the sinteringrolling-secondary sintering process samples decreased from 0.18 to 0.13 compared to samples sintered only, indicating a 27% improvement in friction performance.After the running-in period, the friction coefficient of the sintering-rolling-secondary sintering process samples remained at 0.12, representing a 40% increase in friction performance compared to samples sintered only.This indicates that the sintering-rolling-secondary sintering process can significantly improve the anti-friction performance of the material.The reason is that the sintering-rolling-secondary sintering process improves the material's density and hardness, refines the grain size [19] , promotes carbide generation, and enhances the wear resistance of the material [20] .This provides a reference for further optimizing the anti-friction performance of copper-iron-based self-lubricating composite materials.At the same time, rolling can promote more graphite particles to precipitate from the matrix surface, which is beneficial to enhance the lubrication performance of the samples, consistent with the EDS results in Figure 4 showing an increase in carbon content with secondary sintering and rolling [21] .

Abrasion mechanism
To analyze the influence of different processes on the wear performance of materials, we observed the wear surface of the prepared materials through an optical metallographic microscope.As shown in Figure 7(a), the wear surface of the metal prepared by the sintering process only presents obvious furrows, with significant pores and a large amount of worn abrasive particles.As shown in Figure 7(b), the wear surface of the samples prepared by rolling and rolling-secondary sintering processes is smoother, with improved wear and less worn abrasive particles.Additionally, the samples of the rolling-secondary sintering process exhibit a more obvious metallic luster, which is associated with the increased hardness of the samples.The wear surface of the samples prepared by the sintering-rollingsintering process is the smoothest, with no apparent worn abrasive particles, indicating a significant increase in material hardness and the best wear resistance.In general, with the improvement of the preparation processes, from sintering to sintering-rolling and then to sintering-rolling-sintering, the wear resistance of the metallic materials is significantly enhanced.This is mainly due to the denser material structure and finer grains achieved through rolling and secondary sintering, which substantially increases hardness and significantly improves wear resistance.This result is consistent with the friction performance results in Figure 6.After rolling, the porosity of the samples decreases, allowing better oil retention after immersion, preventing oil loss, and promoting solid-liquid synergistic lubrication.Additionally, after rolling, some fine graphite particles are extruded at the corresponding wear marks, forming a lubricating film and improving the lubrication performance of the samples.The metal carbides generated by secondary sintering also contribute to the improvement of wear resistance [20] .

Conclusions
This study provides important references for the optimization design of sliding components in industrial robots.The developed materials, which are not only high in mechanical strength and low in friction loss but also highly applicable in the field of sliding components such as industrial robot joints, offer an effective approach to extending the service life of industrial robots and reducing maintenance costs.The main conclusions are as follows:  This study utilized powder metallurgy technology to prepare Fe-Cu-Ni-Sn-Graphite porous self-lubricating composite materials with ultra-low friction properties capable of effectively storing lubricating oil.
 Through the sintering-rolling-secondary sintering process, the material's density and hardness were significantly improved, with an 18% increase in density and a 43% increase in hardness, providing a reference for optimizing the process of self-lubricating composite materials.
 Materials processed by the sintering-rolling-secondary sintering technique showed a 40% reduction in the average friction coefficient after the running-in period.They maintained a low and stable friction coefficient of 0.12 after continuous sliding for 24 hours, significantly better than samples processed only by sintering.This reveals the wear mechanism of the sliding interface of materials processed by sintering-rolling-secondary sintering, achieving solid-liquid synergistic lubrication with long-lasting lubricating properties.

Figure 1 .
Figure 1.Schematic diagram of pin-on-disk tribometer and sample testing process.

Figure 4 .
Figure 4. EDS results of a sintered layer of Cu-Fe-Ni-Sn-Graphite samples prepared by different processes.(a) Sintering; (b) Sintering-Rolling (c) Sintering-Rolling-Sintering.Figure5shows the porosity of powder metallurgy samples prepared under three different process conditions: sintering, sintering-rolling, and sintering-rolling-sintering.The results indicate that the porosity decreases as the process steps increase.Compared to single sintering, the sintering-rolling and re-sintering process can effectively reduce the porosity of the material and increase its density, making it the preferred process for obtaining denser powder metallurgy components.This provides references for further optimization of powder metallurgy processes.

Table 2 .
Parameters of sintering and pressing processes.