Effects of Laser Surface Texturing and Lubrication on the Vibrational and Tribological Performance of Sliding Contact

: Various textures are fabricated by a picosecond laser machine on the surfaces of circular stainless steel specimens. Vibrational and tribological effects of laser surface textures are investigated by means of a tribometer and a data acquisition and signal processing (DASP) system. Experimental results show that surface textures can reduce the coefﬁcients of friction (COFs), enhance the wear resistance, and improve the dynamical performance of frictional surfaces. In this study, the surface with micro circular dimples in diameter of 150 µ m or textured area density of 25% has the best tribological and dynamical performance. Compared with the non-textured surface, the surface with circular dimples in diameter of 150 µ m and 15% textured area density has 27% reduction of COFs, 95% reduction of frictional vibrations, and 66% reduction of frictional noise. The frictional vibrations and noise in the sliding contacts can be effectively reduced by adding graphene to the lubrication oil, and the surface textures enhance the frictional noise reduction performance of lubrication.


Introduction
The phenomenon of friction and wear exists everywhere in the world, where there are surfaces in relative motion [1]. Although smoother surfaces can provide lower friction, recent studies show that rough surfaces with certain microstructures have better friction reduction, wear resistance, and lubrication performance [2][3][4][5][6]. In these studies, various techniques have been used to manufacture the surface micro structures, such as abrasive jet machining (AJM) [4], micro-electrical etching (MEE) [5], and laser surface texturing (LST) [6]. Since LST has the advantages of fast manufacturing and processing speed, no pollution to the environment, excellent shape and size control ability, this technology has attracted increasing attention in academic research and engineering applications.
Etsion et al. [6,7] are early representatives of the application research on LST. The authors used the laser micro modeling technology to process micro-pit arrays on the surface of piston rings. In addition, they studied the effect of depth, diameter, and area occupancy of the micro-structures on the tribological properties of functional surfaces. In the following decade, many studies used similar methods to experimentally study the effects of geometry, size, and density of surface texture and the contact mode of friction pair on the friction, wear, and lubrication properties of functional surfaces [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Various functional materials were used in these studies, including steels [8][9][10][11][12][13][14][15][16], cast iron [17][18][19], alloys [20][21][22], and compound materials [23,24], etc. The results of these experimental studies showed that LST had a positive effect on the reduction of friction and wear. However, some unsuccessful cases of using textures under high contact pressures of non-conformal contacts have been reviewed by Gachot et al. [25]. For instance, the reciprocating sliding tests of lubricated steel surfaces for a line contact under low contact pressures did not show positive effects of surface texturing on friction [26], and the pin-on-disk tests (unidirectional sliding) under full film lubrication [27], although surface texturing slightly increased the film thickness [26] and expanded the operation range of loads and speeds for hydrodynamic lubrication [27].
More recently, bionic principles have been employed to the design of surface textures. Greiner et al. [28] used the laser processing technique to process the snake and lizard skinlike surface textures. The research results showed that LST reduced the sliding frictional forces by more than 40% under the condition of dry friction. Zheng et al. [29] used the method of laser heat treatment to carry out the bionic coupling of hardness gradient and hexagonal structure on the pre-engraved steel sample with an hexagonal surface structure. The results showed that the hexagonal texture processed by laser (HT-L) not only exhibited lower friction coefficient and less friction fluctuation than the similar surface hardness generated by conventional heat treatment but without hardness gradient (HT-H) and smooth surface treated by the same laser heat treatment as the HT-L (SS-L surfaces). In addition, the results demonstrated a substantial reduction of wear loss compared to the HT-H and SS-L surfaces.
In the last decade, investigation of the vibration and noise reduction performance of surface textures has become an interesting research area. Sudeep et al. [30] reported the friction and vibration behaviors of lubricated point contacts of reciprocating rolling motion on the surface with textures. In a combination of higher load and rolling speed, the surface textured contacts yielded significant reduction in rolling friction and vibration amplitude at resonant frequencies in comparison with the smooth contacts. The study of Meng et al. [31] showed that appropriately decreasing the eccentricity ratio or rotational speed can reduce the noise of the compound textured bearing. In addition, there is a critical length/diameter ratio of the bearing for suppressing the noise of the compound textured bearing. Wang et al. [32] studied the effects of diameter, depth, and area density of circular dimples on the friction, wear, and frictional noise of sliding contacts by 16 orthogonal designed experiments. Experimental results showed that laser surface textures can significantly reduce the friction and wear under rich oil lubrication, and reduce the frictional noise generated in steel-steel dry sliding contacts.
In the present study, three groups of parametric studies are carried out to experimentally investigate the effects of texture diameter, textured area density, and lubrication on the tribological performance of the textured surfaces. In addition, the effects of LST and lubrication on the reduction of frictional vibrations and noise are studied under oil-rich lubrication conditions. Moreover, the effects of lubricant with graphene particles on the tribological and dynamical performance of stainless steel surfaces without and with textures are investigated, and the softer upper specimens (aluminum alloy balls) are used.

Test Specimen Preparation
The commercial MDP31 picosecond laser processing system was used for the laser texturing, as shown in Figure 1a. The standard laser texturing process was followed in this study [33]. Figure 1b demonstrates the overview of the textures on the surface of the lower specimen, while Figure 1c-e illustrates the geometrical parameters of a single circular dimple. The specific laser processing parameters are shown in Table 1.  The tribo-pair consists of a lower specimen and an upper specimen. The lower specimen was a circular stainless-steel plate in diameter of 70 mm, and the upper specimen was a bearing-steel or aluminum alloy ball in diameter of 5 mm. The material properties of the lower and upper specimens are shown in Table 2. Seven lower specimens with circular dimples on their surfaces were manufactured by means of the laser machine shown in Figure 1a. The depths of all the circular dimples are the same (25 μm). However, the diameters and area densities of surface textures on these specimens are different, and are divided into three groups for the convenience of comparison. Table 3 shows the experimental arrangement and the dimensional parameters of the laser machined specimens in this study.   The tribo-pair consists of a lower specimen and an upper specimen. The lower specimen was a circular stainless-steel plate in diameter of 70 mm, and the upper specimen was a bearing-steel or aluminum alloy ball in diameter of 5 mm. The material properties of the lower and upper specimens are shown in Table 2. Seven lower specimens with circular dimples on their surfaces were manufactured by means of the laser machine shown in Figure 1a. The depths of all the circular dimples are the same (25 µm). However, the diameters and area densities of surface textures on these specimens are different, and are divided into three groups for the convenience of comparison. Table 3 shows the experimental arrangement and the dimensional parameters of the laser machined specimens in this study.

Test Rig
In this experimental study, the UMT-TriboLab Mechanical Tester and Tribometer (manufactured by Bruker in the United States) was employed to investigate the tribological performance of the stainless-steel plate with surface textures. The loading force of the tribometer is in the range of 5~500 mN or 10~1000 N, the spindle speed is in the range of 0.1~5000 rpm, and the torque is up to 5 Nm. The accuracies of both the force and torque sensors are 0.02% of full-scale values. Figure 2a,b illustrates the experimental setup and the contact of the friction pair, respectively. The frictional vibrations were measured by a three-direction accelerometer (Kistler 8766A50M5) with a sampling rate of 51 kHz and processed by the data acquisition and signal processing (DASP) system developed by Beijing Oriental Institute of Vibrations and Noise (BOIVN), while the frictional noise was measured by means of a microphone (sound sensor INV3062W developed by BOIVN) with a sampling rate of 51 kHz, as shown in Figure 2a.

Test Rig
In this experimental study, the UMT-TriboLab Mechanical Tester and Tribometer (manufactured by Bruker in the United States) was employed to investigate the tribological performance of the stainless-steel plate with surface textures. The loading force of the tribometer is in the range of 5~500 mN or 10~1000 N, the spindle speed is in the range of 0.1~5000 rpm, and the torque is up to 5 Nm. The accuracies of both the force and torque sensors are 0.02% of full-scale values. Figure 2a,b illustrates the experimental setup and the contact of the friction pair, respectively. The frictional vibrations were measured by a three-direction accelerometer (Kistler 8766A50M5) with a sampling rate of 51 kHz and processed by the data acquisition and signal processing (DASP) system developed by Beijing Oriental Institute of Vibrations and Noise (BOIVN), while the frictional noise was measured by means of a microphone (sound sensor INV3062W developed by BOIVN) with a sampling rate of 51 kHz, as shown in Figure 2a.

Experimental Methodology
In the tribological tests with the UMT-TriboLab, the upper specimen was a stationary bearing steel or aluminum alloy ball, and the lower specimen was a rotating stainless steel plate without or with surface textures, as shown in Figure 2b. Each experiment was repeated three times on the same lower specimen with the same normal contact force and relative sliding speed at different locations on the surface of the lower specimen. In addition, a new identical bearing steel ball or aluminum alloy ball (the upper specimen) was used for each repeated test. To investigate the effect of lubrication on the tribological and vibrational performance of surface textures, two different lubricants were used (A: Castrol Lubricants 2022, 10, 10 5 of 14 5W-30 engine oil and B: Castrol 5W-30 engine oil with 0.1% graphene added) and the identical aluminum alloy balls were used as upper specimens in these tests. The properties of lubricants A and B are shown in Table 4. The lubricants A and B and the graphene used in this study was provided by the Center of Green Lubrication, Chinese Academy of Sciences Shanghai Advanced Research Institute. The lateral size of the graphene sheet is about tens of microns, and the graphene sheet thickness is between 4 and 5 nm. The theoretical thickness of a single layer of graphene is about 0.34 nm, thus the number of graphene layers is about 10, and the average size of graphene particles is about 16.2 µm in diameter. More details regarding the characteristics of the graphene used in this study can be found in [34]. Above the upper specimen, a force sensor and a torque sensor were fixed on the tribometer, which were used to measure the normal contact force and frictional force of the sliding contact during tests, respectively. The coefficients of friction (COFs) were obtained via the built-in data acquisition system of the tribometer. During the tests, the lower specimen (stainless steel plate) was fully covered by the lubrication oil. The tests were carried out at room temperature, and each experiment lasted for the same contact distance of 100 m under the same normal load of 10 N. After the tests, the specimens were ultrasonically cleaned and the wear losses (wear depths and volumes) of both the upper and lower specimens were measured using a white light interference profilometer, the Bruker ContourGT-K 3D Optical Microscope. Figure 3a demonstrates the effect of texture diameter on the measured COFs of bearing steel ball sliding, which is contacted with a stainless steel plate with 15% textured area density at a sliding speed of 1 m/s. In Figure 3a, it can be seen that the COFs are significantly reduced by the surface textures in all of the tests, although they are not so significant in the first 5 s of the tests. This is due to the fact that in the first few seconds the sliding contacts are in the transit states. Moreover, Figure 3a shows that the circular dimples in diameter of 150 µm have the best performance in friction reduction, 27% less than the specimens without a surface dimple, while the specimen with circular dimples in diameter of 130 µm has the similar performance in friction reduction with the specimen with circular dimples in diameter of 190 µm, about 10% less than the specimen without a surface texture. cant in the first 5 s of the tests. This is due to the fact that in the first few seconds the sliding contacts are in the transit states. Moreover, Figure 3a shows that the circular dimples in diameter of 150 μm have the best performance in friction reduction, 27% less than the specimens without a surface dimple, while the specimen with circular dimples in diameter of 130 μm has the similar performance in friction reduction with the specimen with circular dimples in diameter of 190 μm, about 10% less than the specimen without a surface texture.  Figure 3b shows the effect of textured area density on the COFs of bearing steel ball sliding which is contacted on a stainless steel plate with surface textures in diameter of 140 μm at sliding speed of 0.5 m/s. In Figure 3b, it can be seen that the specimen surface with 25% textured area density has a 20% reduction on the COFs, while an 8% reduction of the COFs is obtained when the textured area density is 35%. The effect of 15% textured area density on the friction reduction is in the middle of the three studied textured area densities. Figure 3c presents the friction coefficients of textured and non-textured surfaces under two different lubricants, lubricant A and lubricant B. In these studies, aluminum alloy balls were used as upper specimens, and circular stainless steel plates with surface textures in diameter of 140 μm and 25% textured area density were used as lower specimens, at the sliding speed of 1 m/s and normal load of 10 N.

Lubricant
In Figure 3c, it can be seen that under the same lubricant the COFs of the textured surface are significantly less than the non-textured surface. In addition, the COFs from the sliding contacts of non-textured surface and aluminum alloy balls are decreasing in the whole period of testing. However, on the textured surfaces, the COFs are decreasing only in the first 5 s and then become stable.
Moreover, Figure 3c shows that the COFs of both the textured and smooth surfaces are reduced by adding graphene to the lubrication oil. This is due to the fact that graphene  Figure 3b shows the effect of textured area density on the COFs of bearing steel ball sliding which is contacted on a stainless steel plate with surface textures in diameter of 140 µm at sliding speed of 0.5 m/s. In Figure 3b, it can be seen that the specimen surface with 25% textured area density has a 20% reduction on the COFs, while an 8% reduction of the COFs is obtained when the textured area density is 35%. The effect of 15% textured area density on the friction reduction is in the middle of the three studied textured area densities. Figure 3c presents the friction coefficients of textured and non-textured surfaces under two different lubricants, lubricant A and lubricant B. In these studies, aluminum alloy balls were used as upper specimens, and circular stainless steel plates with surface textures in diameter of 140 µm and 25% textured area density were used as lower specimens, at the sliding speed of 1 m/s and normal load of 10 N.

Lubricant
In Figure 3c, it can be seen that under the same lubricant the COFs of the textured surface are significantly less than the non-textured surface. In addition, the COFs from the sliding contacts of non-textured surface and aluminum alloy balls are decreasing in the whole period of testing. However, on the textured surfaces, the COFs are decreasing only in the first 5 s and then become stable.
Moreover, Figure 3c shows that the COFs of both the textured and smooth surfaces are reduced by adding graphene to the lubrication oil. This is due to the fact that graphene has an ultra-thin layered structure, which is easy to adhere to the frictional contact surfaces and reduce the direct contact between the friction counterparts [35][36][37].

Texture Diameter
In this study, a white light interference profilometer, the Bruker ContourGT-K 3D optical microscope was employed to measure the wear losses (volume) of the tested specimens. Figure 4a,b demonstrates the surface topographies of the upper and lower specimens without and with surface textures in area density of 15% and diameter of 130 µm, after sliding friction tests under the same operation conditions of normal load 10 N and sliding speed 1 m/s. Figure 4a,b shows that the wear volume of the textured surface is considerably smaller than the non-textured surface. The wear scar of the non-textured surface is about 10.43 µm in depth and 1200 µm in width, while the wear scar on the surface with textures in diameter of 130 µm is only about 3.2 µm in depth and 70 µm in width. In Figure 4a,b, it can be seen that the wear volume of the steel ball which is contacted with the textured surface is considerably smaller than the steel ball which is contacted with the untextured surface. sliding speed 1 m/s. Figure 4a,b shows that the wear volume of the textured surface is considerably smaller than the non-textured surface. The wear scar of the non-textured surface is about 10.43 μm in depth and 1200 μm in width, while the wear scar on the surface with textures in diameter of 130 μm is only about 3.2 μm in depth and 70 μm in width. In Figure 4a,b, it can be seen that the wear volume of the steel ball which is contacted with the textured surface is considerably smaller than the steel ball which is contacted with the untextured surface.   Figure 4c shows the wear volume of a specimen without a surface texture and three textured specimens with circular dimples in diameters of 130,   Figure 4c shows the wear volume of a specimen without a surface texture and three textured specimens with circular dimples in diameters of 130, 150, and 190 µm at the same sliding speed of 1 m/s and normal load of 10 N. In Figure 4c, it can be seen that the wear volume of the specimen without a surface texture is almost 10 times the specimen with a surface texture. The wear volume of the textured specimen with surface circular dimples in diameter of 150 µm is less than the specimen with 130 and 190 µm in diameter of circular dimples. Figure 4d shows the wear volume of the upper specimen (bearing steel balls) against the specimens without a texture and with circular dimples in diameters of 130, 150, and 190 µm at the same sliding speed of 1 m/s and normal load of 10 N. In Figure 4d, it can be seen that the wear volume of the steel ball against the specimen without a texture is about twice the amount of steel balls against the textured specimens. The wear volume of the steel ball against the textured specimens with surface circular dimples in diameter of 150 µm is less than the two specimens with circular dimples in diameters of 130 and 190 µm.

Area Density
Similarly, Figure 4e,f shows the surface topographies of the upper and lower specimens without a surface texture and with circular dimples in diameter of 140 µm and 25% textured Lubricants 2022, 10, 10 8 of 14 area density, respectively, after lubricated non-conformal sliding contacts. Figure 4e shows that the wear scar on the non-textured specimen surface is about 3.93 µm in depth and 900 µm in width, while in Figure 4f, the wear scar on the specimen with 25% textured area density is only about 2.24 µm in depth and 75 µm in width. Figure 4e,f also shows the topographies of the upper specimen after sliding which is contacted with the lower specimen without and with surface textures, respectively. In Figure 4e, the wear of the upper specimen (steel ball) is significantly larger than the wear shown in Figure 4f. Figure 4g demonstrates the effect of textured area density on the wear of the lower specimen of stainless steel plate. In Figure 4g, it can be seen that the surface textured area density has a significant effect on the wear of the contact surface. The surface with a textured area density of 25% has the smallest wear off compared with the surfaces with textured area densities of 15% and 35%. Figure 4h demonstrates the effect of textured area density of the lower specimen on the wear of the upper specimen (bearing steel ball). In Figure 4h, it can be seen that the surface textured area density of the lower specimen has a significant effect on the wear of the upper specimen. When the lower specimen has a 25% textured area density, the upper specimen (bearing steel ball) has the smallest wear compared with the area density against the lower specimens with textured area densities of 15% and 35%. Figure 5 shows the wear of the upper specimen (Al ball) in contact with the textured and non-textured surfaces lubricated with lubricant A and lubricant B. In Figure 5, it can be seen that when the normal contact force is the same and under the same lubricant, the wear of the upper specimen (Al ball) in contact with the textured specimen is significantly less than when it is in contact with the non-textured surface. Moreover, the wear of the upper specimen (Al ball) is significantly reduced by adding 0.1% of graphene to the lubricant for both the textured and non-textured surfaces.

The Mechanism of Friction and Wear Reduction by LST
Based on the tribological studies in the present and previous studies, the mechanism of the positive effect of LST on the friction and wear reduction can be summarized in general. First, the existence of surface textures changes the lubrication condition on the frictional surfaces. In the starved lubrication condition, textures store lubricants and improve the surface lubrication performance. In the oil-rich lubrication condition, textures have an hydrodynamic lubrication (HL) or Elasto-hydrodynamic lubrication (EHL) effect and improve the bearing performance of the lubricated surface. Second, surface textures reduce the friction contact area, store the abrasive particles, and prevent the wear caused by the third body (abrasive particles) between the frictional contact surfaces. Third, laser surface texturing (LST) increases the surface hardness and decreases the tensile residual stress [38] or increases the compressive residual stress of the laser textured surface [39]. Therefore, improving the wear resistance of the textured surface.
The effects of circular dimple diameter, textured area density or lubricants on the friction and wear performance of the textured surfaces can be summarized based on the

The Mechanism of Friction and Wear Reduction by LST
Based on the tribological studies in the present and previous studies, the mechanism of the positive effect of LST on the friction and wear reduction can be summarized in general. First, the existence of surface textures changes the lubrication condition on the frictional surfaces. In the starved lubrication condition, textures store lubricants and improve the surface lubrication performance. In the oil-rich lubrication condition, textures have an hydrodynamic lubrication (HL) or Elasto-hydrodynamic lubrication (EHL) effect and improve the bearing performance of the lubricated surface. Second, surface textures reduce the friction contact area, store the abrasive particles, and prevent the wear caused by the third body (abrasive particles) between the frictional contact surfaces. Third, laser surface texturing (LST) increases the surface hardness and decreases the tensile residual stress [38] or increases the compressive residual stress of the laser textured surface [39]. Therefore, improving the wear resistance of the textured surface.
The effects of circular dimple diameter, textured area density or lubricants on the friction and wear performance of the textured surfaces can be summarized based on the previous studies. Mohmad et al. [40] pointed out that textures can only function well at a certain range of diameter size. Smaller diameters have difficulty in generating the hydrodynamic loading capacity, while larger diameters may increase the roughness of contact surfaces and increase the coefficients of friction.
Li et al. [23] reported that the dimples on the surface with a lower textured area density were not sufficient for storing lubricants and had difficulty in generating the hydrodynamic loading capacity. In contrast, with an increase of the dimple density, the number of dimple increases and the surface roughness increases significantly, which reduces the wear life of oil film. A texture with moderate dimple density provides sufficient lubricant storage, and its relatively lower roughness is beneficial to the wear life of oil film.
In the present study, graphene was added to lubricant-B. in addition, the research of Sarno et al. [36] showed that the graphene platelets in oil easily form a protective film to prevent the direct contact between two contact surfaces, Therefore, this improves the frictional behavior of the lubricant. In Figure 6a-d, it can be seen that the surface textures have a significant effect on the reduction of frictional vibrations. In addition, the circular dimples in diameter of 150 µm have the best performance in the reduction of frictional vibrations. Moreover, Figure 6b,d shows that the frictional vibrations significantly reduced after 40 s of sliding contacts. However, for the smooth surface, the frictional vibrations increased after 40 s of sliding contacts, which is possibly due to the more serious wear of the surface without laser texturing, as shown in Figure 6a. contacts, which is possibly due to the more serious wear of the surface without laser texturing, as shown in Figure 6a. Table 5 lists the RMSs of measured vibrations in time domain, the vibration reduction rate compared with the non-textured specimen under the same working conditions, the main resonant frequency of vibrations, and the peak resonant vibration amplitudes. In Table 5, it can be seen that under the normal load of 10 N at sliding speed of 1 m/s with textured area density of 15%, the measured frictional vibrations from the surface with circular dimples in diameter of 150 μm are reduced to about 5% of the vibrations induced by the surface without a surface texture in time domain, which has a better performance in vibration reduction than the specimens with surface circular dimples in diameters of 130 and 190 μm.    Table 5 lists the RMSs of measured vibrations in time domain, the vibration reduction rate compared with the non-textured specimen under the same working conditions, the main resonant frequency of vibrations, and the peak resonant vibration amplitudes. In Table 5, it can be seen that under the normal load of 10 N at sliding speed of 1 m/s with textured area density of 15%, the measured frictional vibrations from the surface with circular dimples in diameter of 150 µm are reduced to about 5% of the vibrations induced by the surface without a surface texture in time domain, which has a better performance in vibration reduction than the specimens with surface circular dimples in diameters of 130 and 190 µm.

Area Density
Similarly, it can be seen from Figure 6e that the surface with textures in diameter of 140 µm and 25% textured area density has a better performance in vibration reduction than the 15% and 35% textured area densities. In Figure 6e, it can be seen that the peak resonance frequency of the frictional vibrations generated from the non-textured surface is 700 Hz. However, the peak resonant frequency of the frictional vibrations generated from the textured surfaces is changed to 550 and 600 Hz. The decrease of the peak resonant frequency may be due to the decrease in the contact stiffness caused by surface texturing. In addition, the reduction of frictional vibrations in amplitude at resonance may be attributed to the increased damping due to the surface dimple squeezing film effect [41].
Moreover, in Table 5, it can be seen that under the same sliding speed and texture diameter, the measured RMS vibration amplitudes from the specimen with 25% textured area density are reduced more than 87% in time domain, which have the best performance in vibration reduction among the three specimens with different textured area densities. Figure 6f presents the measured frictional vibrations in frequency domain from the Al alloy ball sliding contacts with the textured and un-textured lower specimen under two different lubrications. In these experiments, two different lubricants, A and B, were used under the experimental conditions of sliding speed of 1 m/s, normal load of 10 N, and the textured area density of 25%. In Figure 6f, it can be seen that both the surface texture and lubrication have a significant effect on the frictional vibrations. Moreover, Figure 6f shows that the peak vibration amplitude generated from the non-textured surface with lubricant B is 44% lower than the amplitude generated from the non-textured surface with lubricant A, while the peak vibration amplitude from the textured surface with lubricant B is 14% lower than the amplitude generated with lubricant A. Graphene as an additive of lubricants can effectively reduce the frictional vibrations on both textured and non-textured surfaces. However, the effect of lubrication on the reduction of frictional vibrations is reduced by surface textures.

Texture Diameter
The frictional noise measured during the tribological tests with a sound pressure sensor is shown in Figure 7. Figure 7a-d shows that the diameters of surface textures have a significant effect on the reduction of frictional noise, and the 150 µm in diameter circular dimples have better performance in the reduction of frictional noise than the textures in diameters of 130 and 190 µm.   Figure 7e shows that the surface with 25% textured area density has better performance in noise reduction than the 15% and 35% textured area densities, as the circular dimple diameter is 140 μm. In Table 6, it can be seen that the surface texture can significantly reduce the amplitude of frictional noise, and the surface with the textured area density of 25% has the most significant effect on the frictional noise reduction. Compared with the non-textured surface, 73% of the RMS of frictional noise in time domain generated by the textured surface (25% textured area density) is reduced. In addition, it is observed that the surface texture changes the peak resonant frequency of frictional noise.  Table 6 lists the RMSs of measured frictional noise in time domain, the noise reduction rate in time domain, the main resonant frequency of frictional noise, and the noise amplitude at peak resonant frequency. In Table 6, it can be seen that the frictional noise from the textured specimen is considerably lower than the non-textured specimen under the same experimental conditions. In addition, the RMS of frictional noise from the textured surface with 150 µm in diameter dimples has 66% reduction in time domain compared with the RMS from the non-texture specimen under the same working conditions.  Figure 7e shows that the surface with 25% textured area density has better performance in noise reduction than the 15% and 35% textured area densities, as the circular dimple diameter is 140 µm. In Table 6, it can be seen that the surface texture can significantly reduce the amplitude of frictional noise, and the surface with the textured area density of 25% has the most significant effect on the frictional noise reduction. Compared with the non-textured surface, 73% of the RMS of frictional noise in time domain generated by the textured surface (25% textured area density) is reduced. In addition, it is observed that the surface texture changes the peak resonant frequency of frictional noise. Figure 7f presents the measured frictional noise in frequency domain from the Al alloy ball sliding contacts with the textured and untextured lower specimen under two different lubrications. In these experiments, two different lubricants were used under the experimental conditions of sliding speed of 1 m/s, normal load of 10 N, and the textured area density of 25%.

Lubricant
In Figure 7f, it can be seen that both the surface texture and lubrication have a significant effect on the frictional noise. Figure 7f shows that the peak sound pressure amplitude generated from the non-textured surface with lubricant B is 5% lower than the non-textured surface with lubricant A, while the peak sound pressure amplitude from the textured surface with lubricant B is 8% lower than lubricant A. Therefore, graphene as an additive of lubricants can effectively reduce the frictional noise on both the textured and non-textured surfaces. Figure 7f demonstrates that surface textures (circular dimples) have a more significant effect on the frictional noise reduction than the lubricants, and surface textures enhance the noise reduction performance of lubrication.

The Mechanism of Frictional Vibration and Noise Reduction by LST
From the dynamic tests in this study, it can be seen that LST has a significant effect on the reduction of frictional vibrations and noise. The mechanism of frictional vibration and noise reduction by LST can be summarized. Since the frictional noise and vibrations are excited by frictional forces that are proportional to the COFs of the frictional system, the frictional vibrations and noise can be reduced by surface textures due to the reduction of COFs. In addition, the LST reduces the wear on the contact surfaces, which has a significant effect on the reduction of frictional vibrations and noise. Moreover, LST reduces the tensile residual stress of the textured surface [38], which has a positive effect on the reduction of frictional vibrations and noise. Furthermore, laser surface texturing facilitated better lubrication on the contact surfaces, resulting in the improved contact stiffness and damping [41], which is beneficial to the reduction of frictional noise and vibrations.

Conclusions
Experimental studies have been carried out to investigate the effects of LST and lubrication with graphene on the tribological and dynamical performance of non-conformal sliding contacts. Significant friction/wear and frictional vibrations and noise reductions are observed in this study, and the following conclusions can be drawn: (1) Under the operation conditions in this study, the circular dimples in diameter of 150 µm have a better tribological performance than those of 130 and 190 µm, while a textured area density of 25% has a better tribological performance than those with 15% and 35% textured area densities. (2) Laser surface textures have a significant effect on the reduction of frictional vibrations and noise generated from steel-steel lubricated non-conformal sliding contacts in both the time and frequency domains. In this study, the maximum RMS reduction of frictional vibrations and noise is more than 95% and 70%, respectively, in the time and frequency domains.
(3) Adding graphene to the Castrol 5W-30 fully synthetic engine oil can improve the tribological and dynamical performance of lubrication. Surface textures (circular dimples) have a more significant effect on the frictional noise and vibration reduction than the lubricants, and surface textures enhance the frictional noise reduction performance of lubrication. (4) When the upper specimen is an aluminum alloy ball, the effect of texture on the reduction of friction, vibration, and noise is more significant than the bearing steel ball upper specimen, which may be due to the more significant hydrodynamic effect generated by the surface textures sliding contact with the aluminum alloy balls. However, the mechanisms are required for further investigation.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest:
The authors declare no conflict of interest.