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Article

Analysis of Tribological Properties of Expanded Graphite—Alloy Steel Pair Under High Loads in Dry and Humid Conditions

by
Aleksandra Rewolińska
1,*,
Kamil Leksycki
2,
Łukasz Wojciechowski
1,* and
Karolina Perz
1
1
Faculty of Civil and Transport Engineering, Poznan University of Technology, Piotrowo Street, 60-965 Poznan, Poland
2
Institute of Mechanical Engineering, University of Zielona Gora, 4 Prof. Z. Szafrana Street, 65-516 Zielona Gora, Poland
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(7), 4022; https://doi.org/10.3390/app15074022
Submission received: 13 March 2025 / Revised: 1 April 2025 / Accepted: 3 April 2025 / Published: 5 April 2025
(This article belongs to the Special Issue Surface Metrology in Advanced and Precision Manufacturing)
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Abstract

:
Expanded graphite significantly improves the tribological properties of materials in friction pairs, but there is a lack of research in the literature on its cooperation with metals and the effect of water on friction and wear mechanisms. It is particularly important to understand the phenomenon of graphite layer formation on the steel surface and its effect on tribological properties. The aim of this study is to evaluate the tribological properties of an expanded graphite–alloy steel combination operating under selected loads in both dry and humid conditions. The tests were carried out on a block-on-ring tribological tester (where the blocks were made of expanded graphite and the rings were made of AISI 4130 steel) at a rotational speed of 150 rpm, with loads of 200 and 650 N. The frictional behavior was analyzed on the basis of the measured values of the friction torque and the coefficient of friction (COF) calculated from it (and the applied load). In dry conditions, the friction torque was stable, while in humid conditions it showed cyclical changes. An increase in load from 200 to 650 N caused an increase in the average friction torque by 235% in dry conditions and by 209% in humid conditions. The presence of water reduced the friction pair temperature by 12% at 200 N and by 18% at 650 N; however, it simultaneously increased graphite consumption–by 1179% at 200 N and by 100% at 650 N. The amount of graphite deposited on the steel surface depended on the load–in humid conditions, it increased by 114% at 200 N, while it decreased by 250% at 650 N. The conducted research expanded the understanding of the influence of operating conditions on the tribological properties of the expanded graphite–alloy steel pair. It also provided new data on the friction and wear mechanisms of this material combination in humid conditions, which may have significant engineering applications.

1. Introduction

Modern engineering faces many challenges related to energy efficiency and material consumption in mechanical processes. Approximately 23% of the world’s total energy consumption is related to tribological processes. Of this, 20% is for overcoming frictional resistance, while 3% is for the energy required to produce a new part [1,2,3]. Through the use of new materials and advanced lubrication technologies, it is possible to reduce energy losses due to friction and wear by 18% in the short term (8 years) and by 40% in the long term (15 years) [2].
One promising solution for reducing the costs associated with tribological processes is the use of modern carbon materials, such as carbon nanotubes, fullerenes, graphene, nanodiamonds, and expanded graphite. The use of these materials can significantly reduce friction and wear in polymers [4,5,6], metals [7,8,9,10], and ceramic materials [11,12,13]. Of these materials, expanded graphite is particularly noteworthy, as it exhibits favorable tribological properties due to its layered crystalline structure [14,15,16,17,18,19]. This structure provides strong bonds within the layers, weak bonds between the layers, and good susceptibility to delamination. Due to these properties, expanded graphite is widely used in engineering applications as a component of lubricants and composite materials. In engineering components such as bearings, brakes, seals, and brush motors, it significantly improves performance and extends service life.
The researchers analyzed the potential to improve the tribological properties of selected engineering components, such as brakes and bearings, by adding expanded graphite. Manoharan et al. [20] conducted a comparative study of brake pads made from composites containing carbon materials in the form of expanded graphite, natural graphite flakes, and synthetic graphite flakes. It was shown that pads with the addition of expanded graphite had the highest thermal stability. Furthermore, the use of expanded graphite in the composite contributed to a decrease in wear and an increase in fade resistance. Jin et al. [17] introduced expanded graphite into resin-based friction materials, resulting in a 22.4% decrease in the wear rate compared to natural graphite. The stability of the friction coefficient was also improved, especially at high operating temperatures. According to Tang et al. [21], the addition of 2% expanded graphite to friction materials increased the coefficient of friction by 24.1% at 350 °C and enhanced resistance to fading by 19%. However, for a graphite content of 4%, the wear rate decreased by 37.3%. It was also indicated that during braking, the presence of expanded graphite in brake pads reduces the development of cracks and delamination of the material.
Expanded graphite is also used in sliding materials, which, unlike friction materials, have low coefficients of friction. Sun et al. [18] investigated the effect of adding expanded graphite to a polyetherimide (PEI) composite. The addition of graphite was shown to reduce the friction coefficient and wear index by 35% and 87.2%, respectively, compared to the composite without the additive. Xue et al. [22] indicated that temperature played a key role in reducing the friction coefficient and wear index values of the composites tested with the expanded graphite additive. It was found that operating at elevated temperatures (200–250 °C) promotes the formation of a homogeneous graphite sliding layer between the mating surfaces. This layer provides a reduction in the coefficient of friction and wear of the mating surfaces. It has also been reported that at high temperatures, surface wear increases with the rising load. Jia et al. [23] investigated that the tribological properties of polyimide-based (PI) composites containing nanoscale expanded graphite (nano-EG) are highly dependent on the nano-EG content under dry sliding conditions. Compared to pure PI, the wear resistance of PI/nano-EG composites has increased 200-fold. Optimal tribological properties were achieved when the nano-EG content was 15 wt%. The low wear rate was attributed to the smooth and continuous sliding layer formed on the mating surface of the self-lubricating PI/nano-EG composite. The positive effect of the formed sliding layer on tribological phenomena was confirmed by Struchkova et al. [24] and Wang et al. [25]. It should be noted that the use of expanded graphite in sliding materials does not always imply a decrease in the coefficient of friction and wear of the cooperating materials. Somberg et al. [26] showed that the addition of expanded graphite to polyethylene composites under dry conditions increased the coefficient of friction by up to 38% compared to the pure polymer. Changing the operating conditions by introducing water as a lubricant resulted in an increase in friction for the pure polymer, which was attributed to the formation of a polymeric sliding layer. The introduction of expanded graphite hindered the formation of the polymeric sliding layer under water lubrication conditions, resulting in less friction and wear compared to the pure polymer. As reported by Japić et al. [27], the tribological properties of composites containing expanded graphite are also influenced by graphite concentration and particle size. A low friction coefficient of the composite was obtained by adding 10 wt% of expanded graphite. However, this amount of graphite led to an increase in the wear of the composite.
A new application of expanded graphite in sliding materials is its use as a component of self-lubricating microcapsules, which allows a slow and uniform release of lubricant between mating surfaces. Li et al. [28] improved the tribological properties of the polyurethane composite by enriching it with microcapsules containing expanded graphite combined with paraffin. They found that the coefficient of friction reached 0.18 and 0.08 under dry and water lubrication conditions, respectively, which were 77% and 62% lower compared to the pure polyurethane composite. Feng et al. [29] developed a three-dimensional expanded graphite in which they uniformly dispersed SP80/PW microcapsules. The resulting capsules were placed in an epoxy resin. It was found that the friction coefficient and wear rate of the mating materials decreased by 79.7% and 99.5%, respectively, compared to the material without graphite capsules. Similar results were obtained by Feng et al. [30] by introducing expanded graphite microcapsules into a PA6 composite. They showed a significant reduction in friction coefficient and wear intensity compared to the pure PA6 composite. The addition of graphite also reduced the temperature at the friction node.
Expanded graphite is also a material widely used in sealing technology, where it is employed in both resting joints and rotary motion, such as in sealing pump glands, as well as in reciprocating motion, such as in industrial fittings [31,32]. These seals often operate in aqueous environments, such as in the marine industry [33] and the nuclear power industry [34], where they are required to have a long service life. A proposal for the use of seals made of a steel liner coated with a layer of expanded graphite in nuclear reactors was presented by Staf et al. [35]. Long-term tests showed that these seals provide high seal integrity and durability with respect to seals without expanded graphite.
In summary, the use of expanded graphite significantly improves the tribological properties of cooperating materials at friction pairs. Its presence in friction and sliding materials contributes to increased durability and efficiency of technical components operating in demanding conditions. However, there is essentially no research in the literature on the interaction of expanded graphite with metallic materials. Furthermore, the mechanism and the role of the formed sliding layer at the interface of the mating surfaces are not fully explained. There is a lack of research on the behavior of expanded graphite when operating in the presence of water. Meanwhile, the literature indicates that water plays an important role in the friction processes of a pair in which graphite occurs. The lubricating properties of graphite result, among other factors, from its ability to form strong chemical bonds with water. Water vapor from the environment is adsorbed on the edges of the crystallographic lattice, which causes weakening of the bonds between carbon planes and, consequently, ease of shearing and transfer of crystal planes to the surface of cooperating friction pairs [36]. It is known that graphite contributes to the formation of a sliding layer on cooperating surfaces. However, the formation of the layer is limited under certain conditions. For example, Jia et al. [37] and Hirani and Goilkar [38] reported that water hinders the formation of a sliding layer on the cooperating surface. However, Restuccia et al. [39] indicated that the presence of water affects the COF decrease in the tribological pair containing graphite additive. The role of water in a tribological pair containing graphite is not explained. An industrial example of expanded graphite and steel working together is the sealing of valve glands, which typically operate in an aqueous environment. The presence of water can significantly affect the mechanisms of friction and surface wear, altering the properties of the sliding layer and, consequently, the effectiveness and durability of the seal. These issues represent a clear research gap that needs to be addressed and clarified.
The aim of this study is to evaluate the tribological properties of an expanded graphite–alloy steel combination operating under selected loads in both dry and humid conditions.

2. Materials and Methods

Tribological tests were conducted using a block-on-ring Amsler model A135 tester (Figure 1). The friction machine enables continuous and static load regulation, changes in the counter-sample speed, and temperature measurement using a thermocouple. In this system, the sample consisted of expanded graphite with an industrial purity of 98%, and dimensions of 10 × 10 × 15 mm (H × W × D). The samples were made at the seal manufacturer’s plant. The expanded graphite samples were made from natural flake graphite, which was chemically treated and then rapidly heated, causing it to expand and form a light, porous material. In the next step, the expanded graphite is pressed into sheets of a specified thickness and density, creating a graphite foil. The foil is then layered and compressed. The counter-sample was AISI 4130 alloy steel, from which rings with an external diameter of 45 mm and a width of 12 mm were made. The counter-samples were ground to a surface roughness parameter Ra ~0.5 μm, resulting in an anisotropic surface morphology of the alloy steel. The chemical composition, as well as the physical and mechanical properties of the tested materials, are presented in Table 1 and Table 2, respectively.
In order to identify the tribological phenomena occurring in the expanded graphite–alloy steel combination, tests were carried out under dry and humid operating conditions. The humid operating conditions were achieved by the dropwise application of 0.3 mL of water at 3 min intervals. The choice of test conditions was dictated by the actual operating conditions of the combination, which often alternates between humid and dry regimes. The humidity in the test room was 41%. The following kinetic conditions were applied: a ring speed of 150 rpm, loads of 200 and 650 N, respectively, and an operating time of 30 min (Table 3).
Tribological behavior was analyzed on the basis of the measured values of the friction torque and the coefficient of friction (COF) calculated from it, as well as the operating temperature of the friction pair. The data were collected cyclically, which means that measurements were taken every 10 s. For statistical purposes, every test was repeated three times.
In order to assess the mass wear of the graphite samples, they were weighed before and after the friction test. The amount of wear was determined from Equation (1):
w e a r w t % = m 1 − m 2 m 1 × 100 %
where m1—mass of the sample before the friction test, m2—mass of the sample after the friction test.
Similar measurements were carried out for the rings, and the aim was to assess the presence of the resulting expanded graphite sliding layer on the steel surface. The percentage of the layer mass was calculated from Equation (2):
l a y e r   m a s s w t % = m 2 − m 1 m 1 × 100 %
where m1—mass of the steel ring before the friction test, m2—mass of the steel ring after the friction test.
After tribological testing, the graphite-coated alloy steel surfaces were subjected to SEM and elemental composition analyses by EDS.

3. Results

3.1. Analysis of the Friction and Temperature Indices of Expanded Graphite–Alloy Steel Pairs

The recorded changes in friction torque over time for an expanded graphite–alloy steel pair, operating under dry and humid conditions as a function of load, are shown in Figure 2. With a load of 200 N under dry conditions, the friction torque remains between 0.54 and 0.6 Nm throughout the test. In humid conditions, the value varies between 0.4 and 0.8 Nm. Characteristic cycles of frictional torque were observed during the 30 min test under humid conditions. Immediately after water is applied, a sharp decrease in friction torque occurs, followed by a rapid increase. At a load of 650 N in dry conditions, the friction torque remained at approximately 1.9 Nm, although three minutes before the end of operation, it increased to around 2.35 Nm. The application of water to the tribological contact at a load of 650 N caused a sharp decrease in friction torque to approximately 0.7 Nm, followed by stabilization and a return to around 2 Nm. This phenomenon occurred cyclically throughout the 30 min friction process.
The recorded changes in the COF of the tested pair operating under dry and humid conditions, depending on the load, are shown in Figure 3. Similar trends in COF variations to those of friction torque were observed. Under a load of 200 N in dry conditions, the COF remained at approximately 0.13. In humid conditions, a sharp drop in COF to around 0.09 was observed immediately after water application, followed by a rapid increase to approximately 0.17. At a load of 650 N in dry conditions, the COF remained stable at around 0.13 until the 27th minute, after which a sudden increase to approximately 0.16 was recorded. In humid conditions, the direct application of water to the friction pair caused a sharp decrease in COF to about 0.05, followed by an increase to approximately 0.14.
The average variations in friction torque and COF depending on operating conditions and load are presented in Figure 4. Under dry conditions, an increase in load from 200 to 650 N results in a friction torque increase of approximately 235%, while under humid conditions, the increase is around 209%. No significant average changes in COF were observed for the tested operating conditions and loads.
The variation in COF and temperature over time as a function of load under dry operating conditions is presented in Figure 5. Up to the 27th minute, no influence of load on COF was observed. After this point, at a load of 650 N, a significant increase in COF occurred, preceded by a sudden rise in temperature. This behavior can be explained by the fact that at higher loads, elevated temperature may promote water vapor desorption and surface oxidation of graphite, which may affect the friction pair’s operating characteristics [40,41]. At a load of 200 N, the operating temperature remains stable at around 40 °C, whereas at 650 N, it gradually increases, eventually reaching approximately 120 °C.
The time course of COF and temperature change as a function of load for humid operating conditions is shown in Figure 6. For both 200 and 650 N loads, there is a cyclic loss of COF stability. Characteristic decreases in COF occur when water is applied to the contact zone, which may be related to the presence of an adsorbed water layer on the graphite surface. The presence of water reduces friction by decreasing adhesion forces and facilitates sliding between graphite layers, which freely move due to their lamellar structure and weak interlayer bonds [42]. At a load of 200 N, the operating temperature remains stable at around 40 °C, while at a load of 650 N, it gradually increases and eventually reaches about 70 °C.
The average percentage changes in temperature as a function of operating conditions and load are presented in Figure 7. At a load of 200 N, humid conditions result in a 12% reduction in temperature compared to dry operating conditions, while at a load of 650 N, the reduction is 18%.

3.2. Evaluation of Graphite Mass Loss and Its Transfer to the Steel Surface After the Friction Cycle

The percentage changes in the mass of expanded graphite after the friction cycle, depending on operating conditions and load, are presented in Figure 8. Dry operating conditions significantly reduce graphite surface wear compared to humid conditions. Compared to humid operating conditions, dry operation at a load of 200 N results in a reduction in graphite surface wear by approximately 1179%, while at a load of 650 N, the reduction is around 100%.
The mass values of layers transferred to steel surfaces, depending on working conditions and load, are shown in Figure 9. At a load of 200 N, compared to dry working conditions, humid conditions ensured an increase in the mass of the graphite layer transferred to the steel surface by approx. 114%, while for a load of 650 N the amount of mass was lower by approx. 250%.

3.3. Scanning Evaluation of Alloy Steel Surface After Friction Cycle

SEM images of the alloy steel surface after a friction cycle, depending on working conditions and load, are shown in Figure 10. The EDS results are presented in Table 4.
After a dry friction cycle at a load of 200 N, an unevenly applied graphite layer (approx. 10% of the mass composition) is observed on the steel surface, particularly concentrated in the areas of the peaks of surface micro-irregularities, as well as traces of its delamination. On the uncoated surface, individual wear products are observed. On the other hand, at a load of 650 N, unevenly applied graphite layers (approx. 19% of the mass composition) and their delamination are visible, similarly to the lower load. Large amounts of wear products are observed on the uncoated and graphite-coated surfaces.
After a friction cycle in humid conditions at a load of 200 N, large areas of graphite (approx. 84% of the mass) are observed on the steel surface, filling the grooves and covering the peaks of micro-irregularities. Graphite takes on a characteristic layered structure, and local secondary plateaus and areas of microcracks in the graphite layers can also be seen [20]. In turn, at a load of 650 N, few and small graphite areas (approx. 27% of the mass composition) are observed on the steel surface, which fill the grooves and cover the peaks of micro-irregularities.

4. Discussion

The conducted studies clearly indicate that the working environment has a significant impact on the tribological processes occurring in the expanded graphite–alloy steel combination. One of the key phenomena associated with friction in this tribological pair is material transfer. This mechanism involves the transfer of graphite particles to the steel surface, which can consequently lead to a change in the conditions of cooperation of the system from the initial graphite–steel contact to graphite–graphite contact.
The transfer of material takes place under the influence of mechanical and adhesive interactions resulting from the interaction of micro-uniformities of friction surfaces. The characteristics of the graphite layer formation process depend on the operating conditions and can affect the friction and wear parameters of the combination elements to varying degrees. Analysis of the chemical composition showed that in combinations operating in a humid environment, the concentration of carbon on the steel surface was higher, with its highest content recorded under low load conditions. In contrast, an inverse relationship was observed in dry conditions, with a higher amount of carbon accumulating on surfaces subjected to higher loads. In addition, EDS analysis showed that in humid conditions, the amount of oxygen on the tested steel surface increased three times compared to dry conditions. Researchers Restuccia et al. [39] confirm that the presence of water in the tribological pair significantly increases the amount of oxides in the surface layer. The amount of water introduced into the graphite–steel pair is crucial for their tribological properties. Studies conducted by Tahir et al. [43] have shown that a coating containing graphite most effectively reduces friction and wear at relative humidity levels of 55% and 95%. Water molecules adsorbed on graphite act as a lubricant, allowing its layers to slide freely. However, at 95% humidity, graphite agglomeration occurs, causing excessive adhesive wear, shortening the durability of the layer compared to conditions at 55% humidity. In this case, water molecules can act as a binder, causing the graphite particles to bond together. The performed research confirms that the presence of water affects the decrease in COF (Figure 3) while simultaneously leading to an increase in wear (Figure 8).
In the conducted studies, in humid conditions, cyclical changes in COF are observed. At a load of 200 N, after an initial decrease in friction, a temporary increase in its value is observed, followed by stabilization at the initial level. This mechanism can be explained by the change in humidity in the tribological pair, resulting from the friction process. Researchers Bhowmick et al. [44] and Li et al. [45] state that the amorphous carbon structure features a large number of edges and dangling bonds, which can easily interact with water molecules. Repeated destruction and formation of this amorphous carbon structure lead to fluctuations in the friction coefficient. Moreover, under high relative humidity, water molecules have more opportunities to passivate dangling bonds, thereby lowering the friction coefficient of the material. In contrast, no sharp increase in friction was observed at a load of 650 N, which may be due to the effect of temperature (~70 °C) on the lubricating properties of expanded graphite. As the temperature increases, the adhesion of graphite to the steel surface decreases, and the shearing of layers is facilitated, which contributes to faster smoothing of the graphite layer. Homogeneously distributed graphite layers improve the lubricating properties of the system, reducing wear on tribological components. This phenomenon is described by researchers Chen et al. [46], according to which, in temperatures from 25 to 300, the main wear mechanism is abrasive wear, and oxidation is minimal. Graphite can easily form a stable lubricating film consisting of graphite and oxides, which improves tribological properties.
The results of the tests are mostly consistent with previous literature reports on tribological processes in combinations containing carbon materials. As described by Chen et al. [47], the mechanism of material transfer is strongly correlated with operating conditions. Similar relationships regarding the effect of moisture on the tribological properties of metallic materials were described by Bregliozzi et al. [48] and De Bates et al. [49], where it was shown that under humid conditions the chemical bonding processes of carbon to the metal surface are intensified, leading to a reduction in the coefficient of friction. However, Jia et al. [37] showed that for a bronze–graphite composite, the presence of water increases the coefficient of friction with respect to operation under dry conditions. On the other hand, the wear rate of the composite in humid conditions decreased. This was attributed to the hindered transfer of the composite material to the steel surface during friction in the presence of water. It follows that the tribological phenomena associated with the presence of water in combination with carbon materials have not yet been fully clarified.
In addition, as described by Hirani and Goilkar [38], operating conditions affect the material transfer process in seals used in rotary joints in the paper industry. Higher loads under dry conditions can lead to intense wear of the mating surfaces. Material transfer is also influenced by surface texture. Ma and Lu [50] described how higher loading leads to intensive surface wear, which increases the transfer of graphite to the steel surface. This, in turn, affects the morphology and continuity of the transferred layer.

5. Conclusion

This paper evaluates the tribological properties of an expanded graphite–alloy steel pair operating at high loads (200 and 650 N) under dry and humid conditions. It was found that:
  • For the tested loads and operating conditions in dry conditions, the friction torque remained stable, whereas in humid conditions, cyclic variations were observed due to the application of water to the friction zone. Similar trends of change and behavior after the application of water are observed for COF. Under dry conditions, an increase in load from 200 to 650 N results in an increase in average friction torque of about 235%, and under humid conditions, by about 209%. In contrast, no clear average COF changes were observed for the operating and load conditions tested.
  • Compared to dry running conditions, humid conditions contribute to a decrease in friction node temperature of about 12% for a 200 N load and about 18% for a 650 N load.
  • Regardless of the load, compared to dry operating conditions, humid conditions contribute to significantly higher wear on the graphite surface. Water acts as a lubricant, reducing direct contact between surfaces while weakening the cohesive bonds of the graphite. For a load of 200 N, an increase in wear of about 1179% was recorded, and for 650 N, an increase of about 100%.
  • Compared to dry conditions, under humid conditions at a 200 N load, the mass applied to the steel surface increased by about 114%, while at 650 N it decreased by about 250%, indicating a dependence on the amount of deposited graphite on load and operating conditions.
  • After a dry friction cycle with loads of 200 and 650 N, an unevenly deposited graphite layer, traces of its delamination, and isolated wear products were observed on the steel surface. After a humid friction cycle at a load of 200 N, large areas of graphite are observed on the steel surface. The graphite takes on a characteristic layered structure, and local secondary plateaus and areas of microcracks in the graphite layers can also be seen. At a load of 650 N, only sparse and small areas of graphite are observed on the steel surface.
  • The research results obtained may be helpful for engineers looking for possibilities to increase the durability and efficiency of technical seals operating in demanding conditions.

Author Contributions

Conceptualization, A.R.; methodology, A.R. and K.L.; software, A.R., K.L., and Ł.W. validation, A.R., K.L., Ł.W., and K.P.; formal analysis, A.R., K.L., and K.P.; investigation, A.R., K.P., and Ł.W.; resources, A.R.; data curation, A.R. and K.L.; writing—original draft, A.R., K.L., and K.P.; writing—review and editing, A.R. and K.L.; visualization, A.R. and K.P.; supervision, A.R., Ł.W., and K.L.; project administration, A.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
COFCoefficient of Friction

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Figure 1. Diagram of the friction node under analysis.
Figure 1. Diagram of the friction node under analysis.
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Figure 2. Variation in friction torque over time under dry and humid conditions for a load of: (a) 200 N, (b) 650 N.
Figure 2. Variation in friction torque over time under dry and humid conditions for a load of: (a) 200 N, (b) 650 N.
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Figure 3. Variation in COF over time, under dry and humid conditions for load: (a) 200 N, (b) 650 N.
Figure 3. Variation in COF over time, under dry and humid conditions for load: (a) 200 N, (b) 650 N.
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Figure 4. Average variations in (a) friction torque and (b) COF depending on operating conditions and load.
Figure 4. Average variations in (a) friction torque and (b) COF depending on operating conditions and load.
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Figure 5. Variation in COF and temperature over time, depending on load under dry operating conditions.
Figure 5. Variation in COF and temperature over time, depending on load under dry operating conditions.
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Figure 6. Changes in COF and temperature over time depending on the load under humid operating conditions.
Figure 6. Changes in COF and temperature over time depending on the load under humid operating conditions.
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Figure 7. The average percentage changes in temperature as a function of operating conditions and load.
Figure 7. The average percentage changes in temperature as a function of operating conditions and load.
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Figure 8. Mass consumption of expanded graphite depending on operating conditions and load.
Figure 8. Mass consumption of expanded graphite depending on operating conditions and load.
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Figure 9. The mass value of layers transferred to steel surfaces depending on working conditions and load.
Figure 9. The mass value of layers transferred to steel surfaces depending on working conditions and load.
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Figure 10. SEM images of the alloy steel surface after the friction cycle as a function of operating conditions and load.
Figure 10. SEM images of the alloy steel surface after the friction cycle as a function of operating conditions and load.
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Table 1. Chemical composition of cooperating materials.
Table 1. Chemical composition of cooperating materials.
MaterialExpanded Graphite Alloy Steel (AISI 4130)
Chemical composition [%]C-98, O-0.5-1, S-0.01-0.1, Si,Al,Fe,Ca,Mg-0.5-2Fe-96, C-0.28-0.33, Cr-0.8-1.1, Mn-0.4-0.6, P-0.035, Si-0.15-0.35, S-0.035
Table 2. Physical and mechanical properties of the expanded graphite and AISI steel.
Table 2. Physical and mechanical properties of the expanded graphite and AISI steel.
SampleCounter-Sample
Properties expanded graphite alloy steel (AISI 4130)
Density [g/cm3]1.67.85
Roughness Ra [μm]-0.5
Hardness [HB]-217
Expansion [mL/h]200 ÷ 300-
Tensile strength [MPa]1–10560
Yield strength [MPa]-460
Thermal conductivity (W/mK) 25–47042.7
Table 3. Operating conditions of the friction pair during testing.
Table 3. Operating conditions of the friction pair during testing.
Conditions dry humid
Humidity in the test room [%]41
Test duration [min]30
Rotational speed [rpm]150
Load [N]200650200650
Table 4. EDS results of the alloy steel surface after friction cycle, depending on working conditions and load.
Table 4. EDS results of the alloy steel surface after friction cycle, depending on working conditions and load.
Load [N]Conditions No.Element At. [%]
CFeOSiSP
200dry19.8884.274.310.540.190.08
humid283.974.1911.270.150.090.16
650dry318.5775.484.290.50.230.17
humid427.6358.2611.00.880.231.02
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Rewolińska, A.; Leksycki, K.; Wojciechowski, Ł.; Perz, K. Analysis of Tribological Properties of Expanded Graphite—Alloy Steel Pair Under High Loads in Dry and Humid Conditions. Appl. Sci. 2025, 15, 4022. https://doi.org/10.3390/app15074022

AMA Style

Rewolińska A, Leksycki K, Wojciechowski Ł, Perz K. Analysis of Tribological Properties of Expanded Graphite—Alloy Steel Pair Under High Loads in Dry and Humid Conditions. Applied Sciences. 2025; 15(7):4022. https://doi.org/10.3390/app15074022

Chicago/Turabian Style

Rewolińska, Aleksandra, Kamil Leksycki, Łukasz Wojciechowski, and Karolina Perz. 2025. "Analysis of Tribological Properties of Expanded Graphite—Alloy Steel Pair Under High Loads in Dry and Humid Conditions" Applied Sciences 15, no. 7: 4022. https://doi.org/10.3390/app15074022

APA Style

Rewolińska, A., Leksycki, K., Wojciechowski, Ł., & Perz, K. (2025). Analysis of Tribological Properties of Expanded Graphite—Alloy Steel Pair Under High Loads in Dry and Humid Conditions. Applied Sciences, 15(7), 4022. https://doi.org/10.3390/app15074022

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