Study on friction and wear behavior of gray cast iron with different carbon content at different temperatures

In this paper, we study the friction and wear properties of gray cast iron with different carbon contents at various ambient temperatures. We also examine the failure forms of gray cast iron friction and wear. The research concludes that under low-temperature wear conditions, the graphite in the gray cast iron structure can enter into the interface between the friction pair, have a lubricating effect on the wear surface, and reduce the friction coefficient and wear loss of the gray cast iron material. As the graphite content in the gray cast iron structure increases, its lubrication and protection effects enhance. Consequently, the primary cause of wear failure in gray cast iron is fatigue peeling induced by plastic deformation. Under high-temperature wear conditions, an oxide layer gradually forms on the wear surface. As the experimental temperature increases, the thickness of the oxide layer on the wear surface also increases. When the oxide layer formed on the wear surface reaches a certain level of thickness, the internal expansion stress of the oxide layer increases considerably, causing the oxide layer to peel off and increase the roughness of the wear surface, friction coefficient, and wear loss. Additionally, an increase in the graphite content in the gray cast iron structure makes the surface more prone to oxidation. This leads to increased friction coefficient and wear loss, with the wear failure of gray cast iron primarily caused by the peeling of the oxidation layer.


Introduction
In the rapid development of modern industry, gray cast iron is widely used in engineering machinery, heavy industry, and transportation due to its good mechanical properties, excellent strength and hardness, and lower cost.Especially in the manufacturing of machine tool beds, automobile engine blocks, brake discs, pipes, and various brackets, gray cast iron, with its excellent wear resistance and good casting performance, has become one of the indispensable materials.
Although gray cast iron performs well in many aspects, with the increasingly stringent conditions of industrial applications, the current performance of gray cast iron has shown limitations and cannot meet the current needs, even posing serious challenges to industrial production efficiency and safety.To overcome these limitations, scientists and engineers have been exploring ways to improve the performance of gray cast iron to adapt to more demanding industrial environments.
To adapt to the development of the industry, continuously improving the various properties of gray cast iron is a consensus in the field of material research.Early researchers conducted extensive studies on the mechanical properties of gray cast iron, but the research on its wear resistance is still insufficient, especially regarding the impact of carbon content on the friction and wear of gray cast iron [1][2][3].Increasing the carbon content of gray cast iron can lead to improvements in thermal conductivity, damping performance, castability, and machinability; however, it can also result in a reduction in the material's mechanical properties.As research into enhancing the mechanical properties of gray cast iron progresses, the adverse effects of increased carbon content on its mechanical properties can be mitigated through other means, leading to the successful application of highcarbon gray cast iron.
On the other hand, in the research on friction and wear of gray cast iron by A. Vadiraj and colleagues, the authors added lubricants such as MoS2, boric acid, graphite, and TiO2 to the friction pairs.The study showed that the addition of graphite and MoS2 significantly improved lubrication [4].However, graphite is the main constituent phase of gray cast iron, and during the friction and wear process of gray cast iron, due to its lower mechanical properties, graphite can fracture and enter into the friction pair.Additionally, due to the unique hexagonal layered structure of graphite, it acts as a lubricant in the friction pair, leading to a self-lubricating phenomenon in the gray cast iron material during the friction and wear process [5][6][7][8][9].Nonetheless, the presence of graphite also brings certain negative impacts, such as potentially reducing the overall mechanical performance of the material.Graphite exists in the matrix in a form akin to defects, acting to cleave the continuity of the matrix [10,11].Specifically, cracks tend to initiate at the tips of graphite and then extend into the matrix, leading to material failure [12][13][14].
Research on the friction and wear of gray cast iron materials at different temperatures is also somewhat limited.Studies have shown that the oxide film formed on the wear surface and the temperature of the wear surface can have a significant impact on the friction and wear of gray cast iron [15,16], but there is little mention of oxidative wear at higher temperatures and the friction and wear of high carbon content gray cast iron at different temperatures.
Therefore, in-depth research on the friction and wear performance of gray cast iron, especially under different carbon content and temperature conditions, has become an important research direction.By systematically exploring the wear behavior under different conditions, not only can we better understand the mechanism of graphite in friction and wear, but we can also reveal how carbon content and temperature affect the friction and wear performance of gray cast iron, providing theoretical basis and practical guidance for designing and developing higher performance gray cast iron materials.
This study, through friction and wear experiments between gray cast iron and GCr15 steel, aims to comprehensively analyze the wear behavior of gray cast iron with different carbon contents under different temperature conditions, exploring its friction and wear performance and failure mechanisms.The research results will not only help to understand the impact of carbon content and temperature on the friction and wear performance of gray cast iron but also provide a significant scientific basis and technical support for improving the performance of gray cast iron materials, extending their service life, and expanding their application range.Through these efforts, it is expected to promote the wider application of gray cast iron in modern industry, especially in those fields where higher material performance is required.

Experimental materials
For this experiment, gray cast iron materials were prepared using a 10 kg induction furnace, equipped with a thermocouple for accurate temperature measurement.The temperature of the molten iron when tapped was about 1530 °C.Inoculation treatment was conducted inside a Si-Fe alloy ladle, with the addition of inoculant (mass fraction) being 0.7 wt%.After slag removal, the molten iron was cast into resin sand molds.Circular specimens were cut using electrical discharge cutting equipment from symmetrical positions to ensure consistent microstructures.The specimens were then polished and cleaned to have identical surface conditions before conducting friction and wear experiments.Details of the friction and wear test are illustrated in figure 1(a).
To study their effects, friction and wear experiments were conducted on high-carbon gray cast iron specimens with various graphite contents at different temperatures.The chemical compositions of the experimental materials are shown in table 1.
Some of the cut specimens were prepared for metallographic examination.The specimens were polished and then etched for 6-9 s using a 4% nitric acid alcohol solution.The graphite morphology was observed under an optical microscope, and the graphite structure was statistically analyzed using image analysis software.The matrix microstructures of the experimental gray cast iron were also observed under an optical microscope.

Experimental methods
The wear experiments were conducted on an MS-HT1000 high-temperature friction and wear testing machine, which is produced by Lanzhou Huahui Instrument Technology Co., Ltd The counterpart used in the wear experiments is a 6 mm diameter GCr15 steel ball.The chemical composition of GCr15 material includes Carbon (C) 0.95%-1.05%,Silicon (Si) 0.15%-0.35%,Manganese (Mn) 0.25%-0.45%,Phosphorus (P) 0.025%, Sulfur (S) 0.025%, Chromium (Cr) 1.40%-1.65%;tensile strength: 861Mpa, hardness: 62HRC.The experimental parameters were set as shown in table 2, and the test temperature was set at 20 °C, 180 °C, 340 °C, and 500 °C.The friction pair is placed inside a furnace for the experiment.The furnace is a sealed space capable of maintaining a constant temperature, with the experimental temperature being the temperature inside the furnace.Each friction and wear condition was tested twice to ensure the experiment's repeatability and consistency.The average friction coefficient and wear loss were calculated for each condition, and the standard deviation of these values was determined to establish the error band.
The sampling position and the wear process of the wear specimens are shown in figure 1.For the friction and wear experiments, the specimen was fixed to the motor and rotated, while the counterface remained stationary.A force sensor, positioned on the counterface, measured the frictional force acting on the specimen.The friction coefficient μ was calculated according to equation (1).The experimental setup is illustrated in figure 1(c).In equation (1), Ff is the frictional force acting on the specimen, which is numerically equal to the measured reaction force of the sensor, and N is the applied load on the counterface.
For the worn samples, different positions were cut for SEM scanning, as shown in figure 1(b), to observe the wear morphology and analyze the wear mechanism.

Experimental results and discussion
3.1.Microstructure characteristics of the experimental gray cast iron Figures 2 and 3 illustrate the graphite and matrix microstructures of the experimental gray cast iron, respectively.As shown in figure 2, the graphite structure in all three experimental gray cast iron samples is type A, characterized by uniform distribution and lack of directionality.The Graphite Length Grade consistently remains at Level 4, with similar graphite lengths across the samples.Table 3 presenting the statistical analysis of the microstructure characteristics, indicates that an increase in the carbon content of the experimental gray cast iron correlates with a higher graphite content in its microstructure.Figure 3 shows that the matrix microstructure of the experimental gray cast iron is pearlite, with approximately uniform interlamellar spacing.Therefore, the three types of experimental gray cast iron possess similar graphite types, graphite length grades, pearlite content, and pearlite interlamellar spacing, differing only in graphite content.the friction coefficients differently.Under room temperature and 180 °C conditions, the friction coefficients of experimental gray cast iron #1 and #2 exhibit some fluctuation in the initial wear stage before gradually stabilizing.In contrast, the friction coefficient of experimental gray cast iron #3 shows minimal fluctuation.At 340 °C, the friction coefficients of all three experimental gray cast iron types initially increase and then stabilize, showing minimal fluctuation.At 500 °C, the friction coefficients of all three types of experimental gray cast iron exhibit substantial fluctuations.This is particularly notable for experimental gray cast iron #3, where the friction coefficient significantly increases in the later stages of the wear experiment.Figure 5(a) presents the average values of the friction coefficients recorded during the experiment.As the experimental temperature rises, the friction coefficients of gray cast iron with varying carbon contents exhibit a similar trend: initially increasing, then decreasing, and finally increasing again.Under room temperature and 180 °C conditions, the friction coefficient of gray cast iron decreases with increasing carbon content.With the condition of 340 °C, the friction coefficients of the three experimental gray cast iron are very close.However, there is a trend that the friction coefficient increases with the increase in carbon content.At 500 °C, the influence of carbon content on the friction coefficient of gray cast iron becomes more pronounced.That is, the friction coefficient increases with the increase in carbon content.

Friction coefficient and wear loss analysis
Figure 5(b) illustrates the wear loss of gray cast iron with varying carbon contents across different temperatures.As the experimental temperature increases from room temperature to 180 °C, the wear loss decreases, and the wear loss is the lowest with the condition of 180 °C.The wear loss increases slightly as the wear experiment temperature increases to 340 °C.With the condition of 500 °C, the wear loss increases sharply.The wear loss data indicate that under room temperature and 180 °C, the wear loss of gray cast iron decreases with increasing carbon content.With the conditions of 340 °C and 500 °C, the wear loss increases as the carbon content of the experimental gray cast iron increases.
Figure 6 displays the wear surfaces of gray cast iron with varying carbon contents under different experimental temperatures, with arrows indicating the motion direction of the grinding pair.Under identical experimental temperature conditions, the wear surface characteristics of gray cast iron remain consistent.However, the differences in wear surface characteristics are significant as the wear experiment temperature changes.Image-Pro Plus software was employed for the statistical analysis of wear morphology under various conditions.
At room temperature, as depicted in figure 6(a), the wear surface exhibits a few holes and a wavy morphology, with a white powder-like substance present in localized areas.The presence of 'holes' suggests local material peeling.In contrast, the 'wavy morphology' suggests that the material has undergone plastic deformation and will gradually fall off as wear debris under the subsequent action of friction force [17].The 'white powder-like substance' indicates that the material has been oxidized and part of it gradually falls off in the later stage.Comparing the three types of experimental gray cast iron, it is observed that the amount of 'white powder-like substance' on the wear surface decreases as the carbon content increases.
With the condition of 180 °C, as shown in figure 6(b), the wear surface characteristics are similar to those under the room temperature wear condition, i.e. a few holes, wavy morphology, and a white powder-like substance.The wear loss mechanisms under this condition continue to be fatigue wear and surface oxidation wear.Compared to room temperature conditions, the wear surface at 180 °C shows increased plastic deformation and more white powder-like substance, while the number of holes decreases.And there is an increased degree of plastic deformation on the wear surface.From the comparison between the three types of experimental gray cast iron, as the carbon content in gray cast iron increases, the width of the wear marks decreases.
With the condition of 340 °C, as shown in figure 6(c), the wear surface exhibits distinct smooth and rough areas, with the smooth area being particularly dense.Oxygen element analysis on the wear surfaces, shown in figure 7, reveals heavy oxidation in the smooth areas, indicating the formation of an oxide layer.In contrast, the rough area is caused by the shedding of the oxide layer in the smooth area [16,18,19].It can be seen that the wear mechanisms of the materials are fatigue wear and surface oxidation wear.However, compared with the surface morphologies under 180 °C, the oxidized area on the wear surface increases significantly.Comparing the three types of experimental gray cast iron, an increase in carbon content corresponds to an enlargement of the smooth area.
With the condition of 500 °C, as shown in figure 6(d), there is an increased proportion of smooth areas, and the edges of these areas show fracturing.This is particularly noticeable in the high-carbon content #3 experimental gray cast iron, which exhibits apparent surface cracks [20].Under these conditions, the material undergoes more severe surface oxidation, with the shedding of the oxide layer becoming the predominant wear mechanism.
At low temperatures, the main types of wear are adhesive wear, fatigue wear, and slight abrasive wear.As the temperature increases, the types of wear gradually transition to oxidative wear and fatigue wear.At 500 °C, all samples exhibit clear signs of oxidation wear, especially in those with higher carbon content.Integrating these observations, it can be concluded that as the temperature increases, the nature of wear shifts from mechanical friction to thermal friction and oxidative wear.
For gray cast iron with different carbon contents, the difference in carbon content leads to different graphite contents in the gray cast iron, which results in different wear characteristics during the wear process and affects its wear resistance performance.Different temperatures also have a certain impact on the wear surface.The presence of oxide film alters the type of friction and wear of gray cast iron.

Analysis and discussion
4.1.Influence of wear test temperature on the constituent phases on the wear surfaces Figure 6 demonstrates that varying degrees of oxides form on the wear surface at different friction and wear temperatures.This section delves into how these experimental temperatures influence the friction and wear process in gray cast iron.Considering the graphite structure plays a crucial role in gray cast iron, this section focuses on the analysis of the #3 sample with the highest carbon content in order to magnify the effect of the graphite structure.thickness.In contrast, no significant accumulation of oxygen (O) elements is observed.At 180 °C, the worn surface shows no accumulation of carbon elements, but a slight accumulation of oxygen elements is noticeable.At 340 °C and 500 °C, the worn surface lacks carbon accumulation but exhibits a clear accumulation of oxygen elements.This suggests the formation of an oxide layer of considerable thickness, which significantly increases with rising wear test temperatures.At 340 °C and 500 °C, the worn surface lacks carbon accumulation but exhibits a clear accumulation of oxygen elements.This suggests the formation of an oxide layer of considerable thickness, which significantly increases with rising wear test temperatures.
Figure 9 shows the XRD analysis spectra of the worn surfaces of the experimental gray cast iron #3 specimen at different temperatures.At 20 °C and 180 °C, the XRD spectra reveal only the matrix Fe on the worn surface, indicating that the oxide layer, as shown in figure 8(b), is too thin to be detected in these spectra.At 340 °C, Fe 2 O 3 and Fe 3 O 4 oxides can be detected on the worn surface.At 500 °C, the peak values of Fe 2 O 3 and Fe 3 O 4 oxides increase, indicating the increase of oxides.When compared to the 340 °C condition, there is a relative increase in the amount of Fe 2 O 3 on the worn surface at 500 °C.
Gray cast iron is characterized by a significant amount of flake graphite structure.During the wear process, graphite can be pressed into the friction pair and spread across the worn surface.This leads to the formation of a carbon layer, which provides lubrication [21].However, as the wear test temperature increases, the carbon (C) in the graphite gradually oxidizes, generating gas and shedding from the friction pair.Meanwhile, the gray cast iron matrix will gradually oxidize, forming an oxide layer.The higher the wear test temperature, the thicker the oxide layer formed.
The presence of both carbon and oxygen accumulation on the worn surfaces of gray cast iron influences the friction coefficient and the wear failure mechanism.Under the room temperature wear condition, a certain amount of graphite is on the worn surface, which has a lubricating effect and can reduce the friction coefficient during the wear process.As the wear test temperature rises, the graphite oxidizes and loses its lubricating effect.Therefore, the friction coefficient of gray cast iron with the condition of 180 °C is higher than that under room temperature.However, as the wear test temperature continues to increase, the worn surface gradually forms an oxide layer, and the dense oxide layer reduces the friction coefficient during the wear process, leading to a decrease in the friction coefficient at 340 °C [18].At 500 °C, despite the thicker oxide layer on the worn surface, the continuous oxidation increases the surface roughness and, consequently, the friction resistance [5,22,23].This, in turn, leads to an increased friction coefficient.
Under room temperature conditions, a carbon accumulation layer forms on the worn surface.The primary wear mechanism involves plastic deformation-induced failure at the edges of indentations.As the wear test temperature increases, the carbon accumulation layer on the worn surface undergoes oxidation, leading to the gradual formation of an oxide layer.When the oxide layer on the worn surface is relatively thin, its expanding stress is insufficient to cause shedding.In this state, the oxide layer provides a certain level of protection to the worn surface [16,[24][25][26].However, during the friction wear process, the oxide layer can be damaged, leading to a loss of its protective effect [27,28].
While the wear of the material is still primarily due to plastic deformation-induced fatigue, at very high wear test temperatures, the oxide layer formed on the worn surface becomes significantly thicker.The expanding stress inside the oxide layer exceeds the bonding force between the oxide layer and the substrate.As a result, the oxide layer gradually delaminates and then flakes off.Oxidation wear gradually becomes the dominant wear mechanism.Therefore, with the condition of 500 °C, the material wear loss increases sharply.

Effect of carbon content on oxidation layer of high carbon gray cast iron wear surface
An increase in the carbon content of gray cast iron leads to a higher graphite content.Consequently, more graphite accumulates on the worn surface during the wear process, reducing friction resistance [7,15], and, in turn, lowering the friction coefficient.Figure 10 shows that no obvious carbon film is present on the surfaces of samples #1 and #2, whereas a carbon film is evident on the surface of sample #3.This suggests that as the carbon content increases, graphite is more evenly distributed on the wear surface during the friction and wear process, thus offering lubrication and protection.Consequently, under low-temperature wear conditions, the friction coefficient of gray cast iron decreases with increasing carbon content.As the temperature rises, oxide formation on the friction and wear surfaces, along with the oxidation of the graphite structure, causes the surface carbon film to gradually disappear.This leads to a progressive weakening of graphite's lubricating effect.
Figure 4(d) shows that at 500 °C, the real-time friction coefficient experiences significant fluctuations.To further elucidate the friction and wear behavior at this temperature, a laser confocal test was performed on the wear sample.The cross-sectional profile obtained from this test is presented in figure 11.The figure reveals that at 500 °C, the wear depth escalates with increasing carbon content, correlating with the wear weight loss observed at this temperature.
This study employs the maximum height method to measure surface roughness.This method calculates roughness as the sum of the maximum peak height and the maximum valley depth in the profile.Figure 11 displays the measured surface roughness.The results indicate that at 500 °C, surface roughness intensifies with an increase in carbon content.The surface roughness is shown in table 4.
Figure 12 illustrates the microstructure changes due to the oxidation process on the worn surface of sample #3 at 340 °C and 500 °C.The arrows indicate the direction of motion of the grinding pair.In figure 12(a), we observe that the exposed graphite structure on the worn surface undergoes gradual oxidation.Oxygen from the air penetrates the surface of the gray cast iron via channels formed by the oxidation of graphite, leading to internal oxidation of the matrix [29].With an increase in wear test temperature, figure 12(b) shows that the surface's exposed graphite oxidizes more intensely.This results in increased matrix oxidation and a gradual increase in the oxide layer's thickness.Once the oxide layer on the worn surface reaches a certain thickness, the internal expanding stress causes it to shed, thereby increasing the surface roughness [30,31].As the carbon content of gray cast iron increases, the interconnectedness of the graphite structure becomes more significant, thereby facilitating the entry of oxygen into the matrix structure under the guidance of graphite.
Consequently, graphite plays a critical role in guiding and accelerating the high-temperature oxidation process of gray cast iron.As the carbon content in gray cast iron rises, so does the graphite content.This leads to more intense surface oxidation, increased surface roughness, and a higher friction coefficient.
Under low-temperature conditions, an increased carbon content in gray cast iron results in greater graphite accumulation on the worn surface.This accumulation not only provides lubrication but also protects the matrix of the gray cast iron [32].Consequently, as the carbon content in gray cast iron increases, the wear loss decreases.However, with rising wear test temperatures, the surface of high-carbon gray cast iron becomes more susceptible to oxide layer formation.The shedding of this oxide layer contributes to material wear, thereby increasing the wear loss.Thus, under high-temperature conditions, the wear loss in gray cast iron escalates with increasing carbon content.

Conclusion
(1) Wear failure mechanisms of high carbon gray cast iron: At low temperatures: The wear mechanisms of high carbon ash cast iron include fatigue spalling, adhesive wear, abrasive wear, and a small amount of oxidative wear.The primary cause is the peeling of the surface layer due to plastic deformation.
At high temperatures: The wear mechanisms shift to oxidative wear, fatigue wear, abrasive wear, and adhesive wear.The predominant factors are oxidative wear facilitated by the formation of an oxidation layer at elevated temperatures and fatigue wear due to the peeling of this oxidation layer.
(2) Influence of carbon content on friction and wear properties of gray cast iron: At low temperatures: With an increase in carbon content, the wear surface of high carbon gray cast iron becomes enriched with more graphite, resulting in a reduction in friction coefficient and a decrease in wear loss.
At high temperatures: As carbon content increases, the oxidation degree of the wear surface intensifies, leading to an increase in friction coefficient and an augmentation of wear loss.
(3) Protective role and failure of oxide layer on the matrix: To a certain thickness, the oxide layer effectively protects the matrix material.However, with rising temperature and increased carbon content, the oxide layer thickens and deepens, introducing internal stresses.These internal stresses cause microcracks within the oxide layer.Under the influence of frictional stress, cracks form between the oxide layer and the matrix, ultimately resulting in the flaking of the oxide layer in a sheet-like manner and causing substantial wear loss.

Figure 1 .
Figure 1.Sampling position, sample size, and experimental principle (a): sample location and the surface of the test piece; (b): Size of wear sample and SEM scanning position; (c): Schematic diagram of experimental principle).

Figure 4
displays the real-time friction coefficients of gray cast iron with various carbon contents at different temperatures.The real-time friction coefficient curves reveal that changes in experimental temperature affect

Figure 5 .
Figure 5. (a) The average friction coefficient of gray cast iron with different carbon content at different temperatures (b) The wear loss of gray cast iron with different carbon content at different temperatures.

Figure 7
Figure 7 shows that the degree of surface oxidation in the #3 experimental gray cast iron sample progressively increases with rising test temperatures.The arrows in the figure indicate the motion direction of the grinding pair.Figure 8 illustrates the distribution of carbon and oxygen elements on the worn surfaces of the experimental gray cast iron #3 specimen at different wear temperatures.The arrow in the figure indicates that the grinding pair moves inward along the plane of the paper.easurements of C and O elements on the worn surface reveal that under room temperature conditions, there is an accumulation of carbon (C) elements with a noticeable

Figure 8
Figure 7 shows that the degree of surface oxidation in the #3 experimental gray cast iron sample progressively increases with rising test temperatures.The arrows in the figure indicate the motion direction of the grinding pair.Figure 8 illustrates the distribution of carbon and oxygen elements on the worn surfaces of the experimental gray cast iron #3 specimen at different wear temperatures.The arrow in the figure indicates that the grinding pair moves inward along the plane of the paper.easurements of C and O elements on the worn surface reveal that under room temperature conditions, there is an accumulation of carbon (C) elements with a noticeable

Figure 11 .
Figure 11.Profile of wear section at 500 °C.

Table 2 .
Friction and wear experiment parameters.

Table 3 .
Microstructure content and size of gray cast iron used in the experiment.