High temperature tribological properties of Fe-Mo-Ni-Cu-Graphite self-lubricating guide sliding plates

High-temperature self-lubricating Fe-Mo-Ni-Cu-graphite materials with varying copper contents were prepared by powder metallurgy technology. The microstructure and wear surface of the sintered alloy were observed and analyzed using optical microscopy and scanning electron microscopy. The focus was on discussion of the influence of copper content on the tribological properties of the iron-based material. The results indicate that the friction coefficient and wear rate of the sintered material against a 40Cr steel disc show a decreasing trend after friction with an increase in copper content. Particularly, at a copper content of 15%, the friction coefficient is lowest at both room temperature and 500 °C, exhibiting the best wear resistance. The wear rate is in the order of 10–7 cm3/N•m, indicating mild wear. The predominant wear mechanism for both the material and the counterpart disc is adhesive wear. During friction, the formation of a black-brown lubricating composite film composed of Fe2O3, graphite, Fe2O3 • Fe3O4, CuO, and Fe3O4 on the material’s surface plays a crucial role in providing excellent high-temperature anti-friction properties.


Introduction
Iron-based powder metallurgy materials are rapidly developing engineering materials with significant application potential [1,2].Mechanical components manufactured using iron-based powder metallurgy methods exhibit excellent mechanical strength and wear resistance, and have a wide range of applications in the fields of automotive, aerospace, and mechanical engineering [3,4].Iron-based lubrication components produced through powder metallurgy methods benefit from a wide range of low-cost raw material sources [5,6].They have been increasingly adopted and are gradually replacing some traditional forged and cast materials.These components are used in the manufacturing of complex geometric parts with intricate machining or high processing costs, such as automotive gears, toothed components (such as pulleys, sprockets, and hubs) and camshafts [7].The addition of elements to iron-based materials such as nickel and molybdenum can strengthen the matrix, enhancing the material's mechanical and tribological properties [8].The addition of a small amount of copper (Cu) element in low-alloy steel can enhance the alloy's strength, ductility, and corrosion resistance [8,9].Currently, there is abundant research on the tribological behavior of oil-containing self-lubricating ironbased materials at room temperature [10].However, research of iron-based materials in high-temperature conditions is limited, and there is a lack of systematic investigation in this area.This limitation directly hinders the development of iron-based self-lubricating materials for high-temperature solid lubrication applications [11].
In general, iron-based materials have higher hardness and mechanical strength compared to copper-based and nickel-based materials, but they exhibit poorer wear compatibility sliding with shafts, and are susceptible to corrosion.This study based on prior research [12,13], involves the addition of alloying elements such as molybdenum, nickel, and copper to iron-based materials, focus on investigating the influence of varying copper content on the overall performance of iron-based materials.In particular, there has been insufficient in-depth investigation into the impact of varying copper content on the comprehensive performance of iron-based materials, and the requirements for practical operational conditions have not been clearly defined.Copper, as a significant alloying element in iron-based powder metallurgy, has been studied by numerous researchers both domestically and internationally [14,15].However, research methods and perspectives have varied, and there has been insufficient in-depth exploration of the impact of different copper content on the comprehensive performance of iron-based materials.Additionally, the requirements for practical operating conditions have not been clearly defined.Limited research has been conducted on high-temperature friction reduction performance, and the understanding of the mechanisms involved remains incomplete.
In this study, powder metallurgy technology was employed to incorporate varying amounts of copper into Fe-10Mo-5Ni-graphite materials.The research aimed to analyze the microstructure, mechanical properties, and tribological behavior of materials with different copper contents.The investigation focused on understanding the relationship between copper content and material properties, particularly in the context of hightemperature tribology.

Raw materials
The raw materials selected for this study include: Höganas NC100.24reduced iron powder with a particle size of −100 mesh and carbon content < 0.01%.Molybdenum powder with a particle size of −200 mesh.Nickel powder with a particle size of −200 mesh.Copper powder with a particle size of −200 mesh.High-purity graphite with a particle size of −325 mesh.It's important to note that the purity of all the above-mentioned powder materials is 99.5%.

Preparation of Fe-based composite
Iron (Fe), molybdenum (Mo), nickel (Ni), and copper (Cu) powders were used as the main raw materials.The specifications of these raw materials are listed in table 1. Various iron-based self-lubricating composite materials with different copper contents were prepared according to the powder ratios as presented in table 1.As the copper content increases, hardness and compressive strength exhibit two peak values at copper contents of 5% and 15%.The consistency increases with increasing copper content.Following the specified ratios, each powder raw material was carefully measured and mixed thoroughly in a three-dimensional mixer (Self-made equipment).The mixture was then pressed under a pressure of 400 to 450 MPa, held under pressure for 1 min, and subsequently demolded.The samples were sintered in a tubular resistance furnace with a hydrogen gas protective atmosphere.Sintering temperatures ranged from 1050 °C to 1100 °C with a holding time of 40 to 60 min.After sintering, the samples were naturally cooled within the furnace.

Characterization
The following characterization techniques were employed in this study.Density of the sample can be calculated by formula of ρ = m/V, and the quality (m) of the sample was determined using an electronic balance with accuracy with precision of 0.1 mg (FA1204, LICHEN Instrument Equipment Co., LTD, Shanghai, China), the volume was obtained by calculation since it was a regular disk.The brinell hardness (HBS) of the samples was tested by a digital brinell hardness tester with a test load of 62.5 kg according to ASTM E10, a dwell time of 30 s, and a 2.5 mm diameter indenter as shown in figure 1, the calculation of brinell hardness is based on the equation (1). ´--¼ Friction and wear properties were testing using a Pin-on-Disk tribometer (HT1000, Lanzhou Zhongke Kaihua technology development Co. Ltd, Lanzhou, China) as illustrated in figure 2, which employed asprepared samples for its static ball against rotating disc of 40Cr steel specimen (diameter Φ45×5 mm, surface hardness ∼30 HRC), surface roughness of 40Cr steel specimens were polished to around 0.8 μm, which is consistent with the actual surface roughness value of the workpiece.The friction test conditions were under a load of 10 N and a sliding speed of 0.5 m/s for 30 min at both room temperature and 500 °C.
The morphology of the friction layer was observed using a JSM-5600 scanning electron microscope.The composition of the friction layer was analyzed using EDS spectroscopy system.The microstructure of the materials was observed and analyzed using a high-temperature metallographic microscope (Zesis Imager A2m).X-ray diffraction analysis was conducted on the friction layer material and its composition.These characterization techniques were employed to comprehensively study the properties and behaviors of the ironbased self-lubricating composite materials under various conditions.Raman spectrum was obtained at 532 nm by a confocal Raman microscopic system (LabRAM HR Evolution, HORIBA Jobin Yvon Co. LTD, France), the spectral range was 1000-3500 cm −1 .Raman spectroscopy was used to measure the degree of graphitization of samples.100×, 500× and 1000× magnifications (figures 3(d), (e) and (f)).All images are photographs taken after corrosion using a metallographic etchant.The composition of the etchant consists of 4% HNO 3 and 96% anhydrous ethanol.

Results and discussion
In the microstructure of Fe-Mo-Ni-5Cu-graphite material, as observed from the figures 3(a)-(c), the dark gray areas represent fine martensitic laths (typical needle shaped ) and a small amount of red copper and pearlite are intermixed within the matrix [16].The black regions depict pearlite dispersed within the matrix.Due to the substantial addition of copper, the precipitation of free copper during the cooling process is more evident in the microstructure of Fe-Mo-Ni-15Cu-graphite sample as shown in the figures (3(d)-(f)), which is attributed to the fact that the solubility of copper in iron is 8%, and its solubility decreases with decreasing temperature.During the cooling stage after the completion of sintering, copper dissolved in the iron matrix tends to precipitate.
As the copper content increases, the loose martensitic structure forms a continuous phase, leading to a lower quantity of pearlite compared to Fe-Mo-Ni-5Cu-graphite sample, which suggests that the addition of copper restrains pearlite formation [12].The main reason is associated with grain refinement and solid solution strengthening of Cu addition, hardness also increases with Cu content, as the stacking fault energy decreased, leading to increased dislocation densities and twinning [17,18].Cu element has the effect of promoting graphitization, which means that addition of Cu can weaken the binding force between iron and carbon atoms and promote the production of free graphite [19].In addition, Cu also contributed to improved hardenability, as has been validated in Fe steel [20].Pearlite is a mixture of ferrite and cementite.As the copper content increases, the amount of pearlite in the black area decreases and more graphite is released, which is consistent with literature reports.Copper is a non-carbon element that can suppress the formation of cementite.During the heating process, the solubility of copper in iron is relatively high, leading to mutual dissolution of iron and copper.This reduces the diffusion capacity of carbon in iron, and during the cooling process, the abundant precipitation of copper increases the interfaces between iron and carbon, inhibiting the formation of cementite [21].

Friction and wear properties
From the figure 5, it is evident that for iron-based materials, both at room temperature and 500 °C, the friction coefficient decreases with increasing copper (Cu) content.The iron-based self-lubricating material with 15% copper content demonstrates favorable tribological performance at both room temperature and the elevated temperature of 500 °C, particularly exhibiting superior tribological performance at 500 °C.
Figure 6 depicts the influence of varying copper (Cu) content on the friction coefficient and wear rate of the material at different temperatures.The experiments were conducted under dry friction conditions in an atmospheric environment.The paired counterpart was a 40Cr steel disk with a rotational speed of 0.5 m s −1 and a friction radius of 30 mm.During dry friction, the overall trend of the friction coefficient decreases as the copper content increases.At a copper content of 15% and a friction temperature of 500 °C, the average friction coefficient is 0.28, which exhibits the best tribological properties, with the wear rate being in the order of 10 -7 cm −3 /N•m, indicating mild wear.At room temperature, the friction coefficient decreases but the wear rate increases as the copper content increases, indicating an increase in the lubricating phase (graphite), but a decrease (metal carbides) that act as wear-resistant phases, which is consistent with previous conclusions, that addition of Cu can weaken the binding force between iron and carbon atoms, metal carbides are the key to improving hardness and wear resistance, leading to an increase in wear at room temperature [19].The underlying reason is due to that more Cu precipitates were located on the martensitic lath boundary indicates that the existence of Cu-rich clusters weakens the binding force of the martensite lath boundary, in addition, Cu atoms weaken the bonding force of Fe-Fe bonds on the lath boundary [22,23].Under high-temperature conditions, metal oxides act as high-temperature lubricating phases, the wear rate of the material decreases with increasing copper content, due to that Cu has grain refinement and solid solution strengthening effect and improved matrix hardness [24].As the content of precipitated free copper increases, the amount of copper oxide formed also increases at high temperature.This thicker lubricating film further prevents direct contact between the matrix and counterpart, optimizing the friction and wear performance.The D peak represents the degree of defects and disorder in carbon materials, while the G peak is associated with lattice vibration and order.A lower G peak value indicates the higher degree of graphitization and disorder in carbon materials, suggesting that the structure of carbon materials is relatively complete and orderly.I D /I G value is used to describe the intensity relationship between the D and G peaks, so I D /I G value indicates the lattice vibration and order degree of carbon materials.A lower I D /I G value indicates a more compact and stable structure of carbon materials.As shown in figure 7, the I D /I G value decreases as the copper content increases, indicating an increase in the degree of graphitization of carbon elements.The elemental carbon content increases with the increase of copper elements.In room temperature friction testing, the disorder of carbon elements on the surface increased after friction testing, indicating a decrease in graphite content on the worn surface, which is consistent with the results in the figure 6.

Analysis of wear surface morphology and composition of friction disk
Figure 8 displays the friction morphologies of iron-based self-lubricating materials with varying copper (Cu) content against 40Cr steel disks.For materials with 5% and 10% copper content, adhesive and delamination phenomena are observed on the worn surfaces of the 40Cr steel disk.Some wear tracks are torn into fragments (figure 8(a)).At this point, the anti-wear performance is poorer, resulting in a higher wear rate, as confirmed by wear measurements.The friction mechanism of the material with 13% copper content involves oxidation wear and abrasive wear on the friction surface.Little evidence of adhesive wear is seen, and the oxide layer within the wear tracks is denser compared to figure 6(a).Slight plowing phenomena are observed at the edges of the friction region (figure 8(b)).The friction mechanism on the friction surface of the material with 13% copper content involves abrasive wear and oxidation wear (figure 8(c)).As the copper content reaches 15%, significant plastic deformation is observed on the friction surface of the iron-based material.Some friction surfaces exhibit adhesive wear, resulting in narrower wear tracks and reduced wear rates (figure 8(d)).Figure 9 presents the evidence that iron-based material with varying copper content exhibit transfer during the sliding contact with 40Cr steel disks.As the copper content increases, the amount transferred to the counterpart also increases.Interestingly, despite the increased transfer, the wear rate decreases.This indicates that oxidation reactions occur during the transfer process.The increase in copper content is the primary reason for reduction in the friction coefficient and wear rate of the material.The addition of copper improves the hightemperature tribological performance of the material.
Figure 10 illustrates friction testing of Fe-Mo-Ni-15Cu-graphite material at 500 °C, where through frictional transfer; a black-brown lubrication oxide layer is formed on the 40Cr steel disk.The primary components of this  To further investigate the friction and wear mechanisms of composite materials under high-temperature testing, x-ray Photoelectron Spectroscopy (XPS) analysis was conducted on the wear tracks.Figure 11 displays the full XPS spectrum of the wear track of Fe-Mo-Ni-Cu-graphite self-lubricating material.In addition, figure 12 shows the corresponding fitted spectra for Fe2p, Mo3d, Ni2p, Cu2p, C1s, and O1s, providing insight into the chemical states of the reaction products on the worn surface during the 500 °C friction experiment.The results are as follows: The peaks in the Fe2p spectrum are mainly attributed to metallic Fe, FeO, and Fe 2 O 3 [25].
In the fitted Mo3d spectra, the peaks correspond to metallic Mo, MoO 2 , and MoO 3 [26].The Ni2p fitted spectrum features peaks, which are associated with metallic Ni, NiO, and Ni(OH) 2 .Due to the relatively low amount of Ni added, the spectral curve for Ni is not as pronounced.In the Cu2p fitted spectrum, the strong peaks mainly attributed to metallic Cu and Cu 2 O and accompanied by satellite peaks of Cu 2+ , suggesting the presence of Cu in an oxidized form in the wear debris [27].The C1s binding energy peaks are indicative of graphite.In the O1s fitted spectrum, peaks in the range of binding energies between 529 eV and 530 eV are attributed to metal oxides, corresponding to oxidation phenomena observed during high-temperature testing.

Figure 1 .
Figure 1.The measurement process of Brinell hardness.

Figure 2 .
Figure 2. Section view of high temperature tribometer.

Figure 5 .
Figure 5. Friction coefficient as a function of time for iron-based materials with different copper contents at room temperature and 500 °C (a):20 °C, (b): 500 °C).

Figure 6 .
Figure 6.Tribological properties of as-prepared samples at different temperatures for iron-based materials with different copper contents ((a): Average friction coefficient, (b): Wear rate).

Figure 9 .
Figure 9. EDS analysis of the wear track on the 40Cr counterparts at 500 °C.

Figure 11 .
Figure 11.spectrum of XPS on the wear track of Fe-Mo-Ni-15Cu-graphite composite at 500 °C.