Frictional performance of surface-textured phenolic resin-impregnated graphite materials under water lubrication

The frictional performance between impregnated graphite materials and YG-8 cemented carbide is investigated on a self-developed rotating-type tribometer. The test configuration is a cemented carbide plate rotating against the graphite surface. Three types of concave-type textures, including linear grooves, circular dimples and isosceles triangular dimples, are fabricated by laser process on the graphite surfaces to improve the tribological properties. The test environments include dry condition and water lubrication. The corresponding friction coefficients, surface micrographs of graphite samples and cemented carbide plates are analyzed. To reveal the underlying mechanism of the frictional modification effect under water lubrication, the formation of graphite transfer film on cemented carbide plates, the water contact angles of graphite surface and the hydrodynamic effect are further discussed. It is verified that the introduction of textures on graphite surfaces can significantly improve the tribological performance of water-lubricated graphite materials.


Introduction
The friction and wear characteristics of critical mechanical components, represented by bearings and seals, are among the most influential factors in determining the energy consumption, stability, and reliability of rotating machinery [1,2].In recent years, low-viscosity water-based lubricants have demonstrated fascinating application prospects due to low cost, small internal friction, environmental friendliness, and notable cooling effect [3].To further address the challenges posed by the low viscosity characteristics of water-based lubricants and their adverse corrosion effects on metal surfaces, new frictional materials and functional lubricant additives have been introduced into water lubrication sequentially [4,5].Among them, graphite materials have been proven to possess effective self-lubricating properties, stemming from their typical layer-lattice structure [6].Regarding the environment of water lubrication, graphite materials have been utilized as bearings in nuclear canned coolant pumps [7,8] and as mechanical face seals for rotary joints [9].Inside the graphite, the carbon atoms in each layer are strongly bonded through covalent bonds while two neighboring layers are weakly bonded by the interplanar van der Waals force [10].Moreover, the clearance between two neighboring layers is as large as 3.4 Å.As a result, graphite materials produce low resistance when they are shearing parallel to the layers because of the slipperiness of the lamellar structure.In water or moist environments, graphite materials can form adsorption films and decrease the friction coefficients with the cooperation of water lubrication [11].
However, it has already been verified that the tribological behaviors of graphite materials show an obvious scale effect and considerable friction force still exists for engineering applications [12].To further improve the tribological performance of water-lubricated graphite materials, several technical approaches have been proposed by the researchers, mainly concentrating on mechanical strengthening and surface treatment [13,14].Compared with pure graphite, impregnated graphite materials that incorporate metal or polymer are more competitive due to their increased hardness and enhanced anti-wear properties [15].Zhang et al [16] tested the friction behaviors of resin-impregnated and non-impregnated graphite materials sliding against a cemented carbide in water, and found that the impregnated one exhibited much better friction properties and could remain a stable friction regime under heavy load.Similarly, the friction and wear characteristics of graphite materials can also benefit from the reinforcement of the counterpart, such as the usage of cemented carbide [17,18].Besides, laser texturing is well known as a promising approach to modifying the interaction surfaces for better lubrication, lower friction coefficients and improved wear resistance [19,20], which can also be employed in graphite materials.By producing artificial microstructures with determined size, shape and distribution on the surface, the concave textures on the lubricated surfaces can enhance the hydrodynamic effect, store more lubricant, and trap wear debris during sliding friction [21].Generally, in a frictional pair containing a hard sample and a soft sample, surface textures are usually introduced on the surface of the hard sample.However, Wang et al [22] pointed out that the textures should be arranged on the soft one to prevent the convexity of the soft material from being embedded into the texture of the hard material, which can reduce the wear degree of the soft material.
In this study, concave surface textures are produced on the surface of phenolic resin-impregnated graphite samples by laser process.Frictional performance between graphite and cemented carbide is investigated in the environments of dry condition and water lubrication, and the underlying mechanism of the frictional modification effect of surface textures is analyzed.

Experimental procedures
The frictional configuration is a YG-8 cemented carbide plate rotating against disc-shaped graphite samples and thereby the contacting is in the form of face-to-face.The diameter and thickness of the YG-8 plate are 66 mm and 2 mm, respectively.YG-8 materials are mainly composed of 92% tungsten carbide and 8% cobalt.The mechanical parameters are listed in table 1.Compared with conventional steel, YG-8 cemented carbide has larger hardness and elastic modulus, which are beneficial for decreasing the friction coefficient [23].
Four disc-shape phenolic resin-impregnated graphite samples are used for frictional tests.The diameter and thickness are 55 mm and 15 mm, respectively.These samples are made from pure graphite materials by impregnation process and machining post-treatment.Pure graphite specimens are immersed in phenolic resin for 2 h and the immersion temperature is kept around 333 K.Then, the specimens are solidified in the air for 48 h.After impregnation, the phenolic resin could infiltrate into the graphite matrix through the pores and channels.The existing investigation [16] demonstrates that the water-lubricated friction coefficients of impregnated graphite are much smaller than that of pure graphite due to the decrease in the open pores and the increase in hydrodynamic effect.Three of the impregnated graphite samples are treated by laser process to produce surface textures on the surfaces and the left untextured sample is used for reference.Before texturing, the surfaces of the three samples are polished by a high-precision lapping machine UL-810.Three types of surface textures are fabricated by a YLP-F10 nanosecond fiber laser machine, including linear grooves, circular dimples and isosceles triangular dimples.
Figure 1 displays the structural drawings and practical photographs of three textured impregnated graphite samples, which are marked as S1, S2, and S3, respectively.The depth of all the textures is controlled at 50 μm.The diameter of circular dimples is 400 μm.The width of linear grooves is 300 μm.The base length and height of isosceles triangular dimples are 400 and 700 μm, respectively.Besides, the untextured sample for reference is marked S0.Sample S1 has six sectorial textured domains with a center angle of 25°.Each domain is uniformly distributed with 20 linear grooves in the radial direction.The inner 9 grooves have a tilting angle of 65°with the radial direction and the outer 11 grooves have a tilting angle of 85°.The clearance between the centers of two neighboring grooves is 1 mm.On the basis of sample S1, textured samples S2 and S3 are acquired by adding circular dimples and isosceles triangular dimples, respectively.The centers of the outermost circular dimples and the centers of the base of the outermost isosceles triangular dimples are on a circle with a radius of 26.5 mm.The centers of the innermost circular dimples and the centers of the base of the innermost isosceles triangular dimples are on a circle with a radius of 11.5 mm.The clearance between the centers of two neighboring circular dimples and the clearance between the centers of bases of two neighboring isosceles triangular dimples both are 1 mm.Usually, textures are created on the surface of the metal components since it will be difficult to destroy them during the friction between soft-hard pairs.However, the pronounced thermal effect of the laser texturing process can result in roughness around the edges of textures, leading to significant wear on the soft parts, such as the graphite materials employed in this study.Producing textures on the graphite materials can avoid such undesirable excess wear.The structural details of the original graphite surface and three types of textures are illustrated in figure 2. After impregnation, there are still a lot of pores on the surface and some pores are filled with solidified phenolic resin.Generally, the fabrication quality of the three types of surface textures is well.Each texture is fabricated by several times of radiation of laser beam, which can suppress the heat effect as much as possible and therefore the edges of surface textures are quite smooth.Before frictional tests, the textured samples are ultrasonic-cleaned for 15 min to clean the possible residual debris in the concave textures.
The frictional properties are tested on a self-developed rotating-type tribometer for face-to-face pairs.As shown in figure 3, the tribometer mainly consists of a rotor, a three-jaw chuck, a container of lubricant, a spherical bearing, a force sensor and a torque sensor.The YG-8 cemented carbide plate is clamped by the threejaw chuck and conducts rotation.The graphite samples are fixed inside the container of lubricant using the spherical bearing, which can alleviate the degree of unbalanced loading through self-adaptive adjustments.The load is applied by the pneumatic loading system from the below.Force and torque sensors can acquire the realtime loading force and frictional torque, respectively.The measurement system can calculate the friction coefficients automatically by simple numerical calculations.
During the frictional tests, the environment temperature is around 26 °C and the rotating speed of the cemented carbide plate is kept at 300 r min −1 .The test conditions include dry and deionized water lubrication.Each graphite sample is tested under four different loading forces, including 100, 150, 200, and 250 N. The test time for each loading force is 10 min and the average friction coefficient is calculated accordingly.coefficient of the untextured sample S0 is around 0.246.For the three textured samples, the coefficients decrease to 0.190, 0.188, and 0.185, respectively.The decrease degrees are around 22.8%, 23.6%, and 24.80%, respectively.However, the influence of loading force on friction coefficients doesn't exhibit a regular pattern, which should be attributed to the structural complexities and the face-to-face test configuration.Although the test sample is placed on the spherical bearing, it is impossible to completely avoid unbalanced loading [24].

Results and discussion
The optical surface micrographs of graphite samples after frictional tests are displayed in figure 5.Under the action of rotating friction, the untextured area appears evident wear marks, which are mainly caused by the abrasive wear action.The interfacial wear debris easily scratches the graphite surface in a confined space.Regarding the textured area, the concave space facilitates the capture and trapping of wear debris, which can alleviate the degree of abrasive wear.The linear grooves and isosceles triangular dimples are almost filled with wear debris, while very little wear debris can be observed inside the circular dimples.Different from the other two types of textures, the edges of circular dimples are smooth without sharp angles.The applied loading force cannot generate obvious stress concentration phenomena, resulting in reduced wear debris generation.Additionally, the mass loss of the graphite sample can reflect the wear characteristics at the macro scale.The mass values are measured on a high-precision analytical balance.The graphite samples were ultrasonic cleaned for 15 min and dried completely at a temperature of 110 °C before measurements.The initial mass values of samples S0, S1, S2 and S3 before friction tests are 63.5087 g, 63.8224 g, 63.0740 g and 63.8431 g, respectively.After dry friction tests, they decrease to 62.9117 g, 63.3437 g, 62.6451 g and 63.3260 g, with corresponding wear rates of 0.94%, 0.75%, 0.68% and 0.81%, respectively.The introduction of surface textures can decrease the wear degree to some extent.
Similarly, the optical surface micrographs of the cemented carbide plates, used in the tests of four graphite samples, are given in figure 6.Whether the graphite surface is textured or not, oxidative wear can be observed on the surfaces of cemented carbide plates, with the most significant wear occurring in the case of the untextured sample S0.Moreover, the formation of graphite transfer film on the plate surface is confirmed.Generally, the coverage area of graphite transfer film is much larger for textured samples.The appearance of graphite transfer film can improve tribological behaviors due to the low shear resistance between graphite layers [25][26][27].
Furthermore, the tribological performance of graphite samples lubricated by deionized water is tested and the friction coefficients are illustrated in figure 7.After the tests of dry friction, the surfaces of graphite samples and cemented carbide plates are polished and new surface textures are fabricated again.The frictional modification effect of surface textures is more prominent under the participation of liquid lubricant.When the loading force is 150 N, the water-lubricated friction coefficient of untextured sample S0 is about 0.0562.For the three textured samples, the coefficients decrease to 0.0172, 0.0176, and 0.0183, respectively.The largest decrease degree in friction coefficient is about 69%, which demonstrates the prominent effect of surface textures under water lubrication.
Before water-lubricated tests, the initial mass values of samples S0, S1, S2 and S3 are 62.3493 g, 60.6525 g, 60.2899 g and 60.3713 g, respectively.After tests, they decrease to 62.0874 g, 60.5373 g, 60.2236 g and 60.2747 g, with corresponding wear rates of 0.42%, 0.19%, 0.11% and 0.16%, respectively.Sample S2 textured by circular dimples demonstrates the best wear resistance, which is attributed to the smooth edges and slight stress concentration.Figure 8 displays the optical surface micrographs of graphite samples after the frictional tests in the water.Compared with dry friction, the wear marks are much slighter and the number of wear debris trapped in concave textures is decreased significantly.Under the rotation of the cemented carbide plate, the water has an excellent washing effect and prevents the wear debris from stable residing inside the textures.Therefore, the frictional modification effect of surface textures possibly originates from graphite transfer film or the lubrication effect of water.However, the formation characteristics of graphite transfer film in the water environment are inferior to the dry condition.As shown in figure 9, although no obvious oxidative wear phenomenon can be found, the coverage area of graphite transfer film is much smaller, featuring some small domains of discontinuous transfer film.Compared with the dry condition, it is difficult to form large continuous transfer film under water lubrication.On one hand, the washing effect of water during rotation can take the graphite debris away.On the other hand, water molecules absorbed on the surface [28] prevent the adhesion of graphite debris on the surface of the cemented carbide.Therefore, it can be deduced that the frictional modification of surface textures in water mainly originates from the lubrication effect.
Before frictional tests, the water contact angles on the graphite surfaces are acquired by an optical contact angle and interface tension meter (SL 200KS), as shown in figure 10.The tests repeat five times to ensure reproducibility.For each test, a water drop with a volume of 0.5 μl and a diameter of around 1 mm is placed on  the graphite surface.After a wetting and spreading time of 30 seconds, the shape of the water drop is recorded and the contact angle is calculated.The water contact angle on the untextured surface is around 100°.For the surfaces textured by linear grooves, circular dimples, and isosceles triangular dimples, water contact angles are 56°, 50°, and 42°, respectively.As well known, the contact angle determines a given liquid whether can spread well or not on a solid surface [29].A smaller water contact angle typically indicates the water can well spread on the graphite surface and enhance the lubrication capability, which is beneficial for reducing the friction coefficient.Except for the well spreading of water on textured surfaces, the surface textures lubricated by water can generate a hydrodynamic effect and reduce the friction coefficient [30,31].Since a low speed of 300 r min −1 is used in this study, a fluid simulation is employed to evaluate whether a prominent hydrodynamic effect can be generated to improve the tribological performance of the water-lubricated graphite samples.The linear velocity at the outer radius of the graphite samples is around 0.80 m•s −1 .The lubricating water is regarded as an incompressible fluid, and the flow mode is turbulent flow.The environment temperature for simulation is maintained at 293 K.The mass density and dynamic viscosity of the used deionized water are 1000 kg•m −3 and 1.002 × 10 −3 Pa•s, respectively.A convergence film thickness of 10 μm was given at first to compare the hydrodynamic performance.The graphite surface including the contour faces of the textures is set as the wall.Then, the investigated fluid domain is meshed into tetrahedra-type and hexahedra-type cells through sweep mode, which can ensure that the flow direction is perpendicular to the grid surface.figure 11 displays the hydrodynamic pressure distributions of the three types of surface textures.The maximum hydrodynamic pressure can reach 4.30 × 10 5 Pa.Compared with flat surfaces without concave-type textures, the existence of concave textures can produce considerable hydrodynamic pressure, which is beneficial for reducing the friction coefficients [32].
In a word, the frictional modification effect of surface textures under water lubrication mainly originates from the enhancement in lubrication capability and the additional hydrodynamic field produced.

Conclusions
In this study, the frictional performance of impregnated graphite materials under dry condition and water lubrication is investigated by a face-to-face testing configuration.Three types of surface textures, including linear grooves, circular dimples and isosceles triangular dimples, are fabricated on graphite surfaces by laser process to improve frictional behaviors.The main conclusions are as follows: (1) Since the concave-type surface textures can capture and trap wear debris, the dry friction coefficient can be reduced to some extent and oxidative wear degree is suppressed.Obvious graphite transfer film can be observed on the surface of the cemented carbide plates.
(2) The introduction of surface textures can significantly decrease the friction coefficients of water-lubricated graphite samples, and the maximum reduction can reach up to 69%.
(3) The washing effect of water prevents the wear debris from stable residing inside the textures and the formation of graphite transfer film.
(4) Based on the measurements of water contact angles and simulation of computation fluid dynamics, the frictional modification effect of surface textures in water is ascribed to the well wettability of surface textured graphite and hydrodynamic effect of concave-type textures.

Figure 4
Figure 4 displays the friction coefficients of four graphite samples tested under dry condition.Generally, the introduction of surface textures can decrease the dry friction coefficient to some extent.The frictional modification effect of surface textures is the most evident under the loading force of 150 N. The dry friction

Figure 2 .
Figure 2. Structural details of graphite surface of (a) untextured area, (b) linear grooves textured area, (c) circular dimples textured area, and (d) isosceles triangular dimples textured area.

Figure 3 .
Figure 3. Structural diagram of the frictional test configuration.

Figure 4 .
Figure 4. Friction coefficients of untextured and three textured graphite samples under dry condition.

Figure 7 .
Figure 7. Friction coefficients of untextured and three textured graphite samples under water lubrication.

Figure 10 .
Figure 10.Water contact angles on the graphite surface of the (a) untextured area, (b) linear grooves textured area, (c) circular dimples textured area, and (d) isosceles triangular dimples textured area.