Effects of groove-textured surfaces filled with Sn-Ag-Cu and MXene-Ti3C2 composite lubricants on tribological properties of CSS-42L bearing steel

To improve the tribological performance of CSS-42L bearing steel, smooth surfaces (SSs), groove-textured surfaces (GSs), GSs with Sn-Ag-Cu (GSs-SAC), and GSs with Sn-Ag-Cu-Ti3C2 (GSs-SACT) were prepared on CSS-42L. In addition, experimental studies were conducted on tribological properties. The obtained results indicated that GSs-SACT exhibited the best anti-friction and noise reduction performances. These remarkable tribological performances were attributed to the synergistic effects of grooves, Sn-Ag-Cu, and MXene-Ti3C2. The inconsistent rules of frictional forces were improved by the grooves and SACT, which inhibit the friction-induced noise. The micro-nano size-effects of MXene-Ti3C2 enhanced the repairing effect and anti-friction property of composite lubricants, which improved the profile characteristics of GSs-SACT.


Introduction
As the core component of equipment, bearings determine reliability and the performances of bearing steels are the key factors that ensure the durability of bearings. Owing to the rapid development of highend equipment, bearing steels are required to operate reliably for longer life-spans under higher speed, larger load conditions, and harsher environments. Presently, CSS-42L, a representative third generation aerospace bearing steel, is the most promising bearing material. Hence, investigations on the tribological properties of CSS-42L are of great engineering significance and crucial for improving the working performance of high-end equipment bearings [1][2][3].
With low shear strength, high bearing capacity, and large surface area ratio characteristics, two dimensional (2D) MXene exerts unique enhancement effects on nano-micro scale friction systems, which have been widely investigated in the tribology field [4][5][6][7][8][9]. The structural features and tribological properties of MXene/UHMWPE nanocomposites were studied by Zhang et al. [10]. The results obtained from this study demonstrated that MXene-Ti 3 C 2 suppressed the adhesion wear at the interface and improved the tribological properties, and creep resistance of composites. However, Lian et al. [11] evaluated the tribological properties of Ti 3 C 2 coatings on copper plate surfaces. The obtained results showed that during the sliding process, the graphitized Ti 3 C 2 coatings reduced the direct contact between the sliding surfaces, thereby inhibiting the production and adhesion of wear debris on the worn surfaces. Rosenkranz et al. [12] prepared MXene coatings on stainless steel (AISI 304) and performed the ball-disc friction experiments. The experimental results indicated that MXene coatings exhibited excellent tribological properties. The friction coefficient was reduced by approximately 300%, and the adhesive wear on the surface was effectively prevented. In another study, polymericbased composites reinforced with MXene and nano-Al 2 O 3 were prepared by Guo et al. [13]. They verified that hybrid lubrication films with ultra-low friction coefficients were formed at the interface, which could achieve super-lubricity on the surface.
The Sn-Ag-Cu alloy with a weak shear strength is a solid lubricant with outstanding lubrication abilities [14][15][16]. Liu et al. [17] filled Sn-Ag-Cu into microholes on M50 surfaces, and then studied the selfrepairing function of self-lubricating composites. It was observed that owing to the high fluidity of Sn-Ag-Cu, this alloy can form glaze/lubrication films, which exhibit a self-healing effect by covering the cracks and filling the pits on the worn surfaces. To improve the tribological properties of materials, lubricants, nano-reinforcers, surface textures, and synergistic lubrication technologies have been investigated by several researchers [18][19][20][21]. Zhou et al. [22] studied the possible lubrication and selfhealing effect of Sn-Ag-Cu and TiC by filling the micro-holes on a TC4 surface with the lubricant composites. A newly formed uniform lubrication/ protective layer was observed on the surfaces, which improved the tribological properties of TC4. Hua et al. [23] prepared Gr (graphite)-MoS 2 -PI (polyimide)-CNTs (carbon nanotubes) composite lubricants, filling into the surface textures of GCr15 bearing steel. It was reported that the synergistic effects of hybrid lubricants improved the wear resistance and service life of materials.
Friction-induced noise is a common problem in the solid lubrication field, which could reduce the friction system reliability and the durability of materials. Studies on the generation mechanisms and reduction technologies of friction-induced noise have been conducted by several scholars [24][25][26][27]. Friction-induced vibration and noise are the inherent properties of solid lubricated surfaces under dry friction. During the friction process, the tribological properties of materials induce friction-induced vibration at the friction interface, which is a self-excited vibration type. The vibration triggering the friction-induced noise could exacerbate the surface wear. The relationships between noise and friction could be established by analyzing the frictional force changes and surface profiles. This noise is related to the friction behavior, which can be reduced by improving the antifriction performance on the surfaces [28]. The changes in the contact conditions from elastic to plastic effectively suppress the noise [29]. In addition, noise can be reduced by the surface modification technology [30]. The groove-textured surface with a damping element can suppress the self-excited vibration of friction system [31]. Mo et al. [32] studied the characteristics of friction-induced vibration and noise on groove surfaces. They verified that the noise was related to friction fluctuation. In addition, the grooves on the surface could reduce the friction fluctuation, thereby suppressing the self-excited vibration of the friction system. It was observed that the synergistic effects of grooves and the Sn-Ag-Cu alloy improved the tribological properties of GCr15 and reduced the adhesive wear on the surfaces, thus eliminating the high-frequency noise and low-frequency chattering [33].
The lubricant composite of MXene and Sn-Ag-Cu can improve the lubrication performances, thus extending the application field of Sn-Ag-Cu. The MXene with weak boundary multilayer structures exhibits a significant application potential in the tribology field. Nonetheless, the experimental studies and theoretical analyses on the tribological properties of MXene are negligible. In this study, smooth surfaces (SSs), groove-textured surfaces (GSs), GSs with Sn-Ag-Cu (GSs-SAC), and GSs with Sn-Ag-Cu-Ti 3 C 2 (GSs-SACT) were prepared on CSS-42L. Furthermore, the anti-friction and wear resistance mechanisms of GSs-SACT were analyzed, and the synergistic effects of grooves and Sn-Ag-Cu-Ti 3 C 2 on the friction-induced vibration and noise were investigated. This study could provide the experimental basis for the application of MXene in CSS-42L bearings to achieve excellent antifriction, wear-resistance, and friction-induced noise reduction performances.

Sample preparation
First, Ti 3 C 2 nanosheets were obtained by etching Ti 3 AlC 2 powders in hydrogen fluoride (HF) solution. Ti 3 AlC 2 powders were added into 40% of the HF acid www.Springer.com/journal/40544 | Friction solution and stirred at room temperature (20-25 °C) with an electromagnetic stirrer for 8 h. Subsequently, the suspensions were centrifuged for 30 min and washed with deionized water. Ti 3 C 2 nanosheets were obtained by drying the sediments at 70 °C under vacuum conditions for 24 h. The microstructural morphology and elemental distributions of Ti 3 C 2 are illustrated in Figs. 1(a)-1(d). Next, the raw Sn-Ag-Cu powders were prepared according to the compositions presented in Table 1, and the powders were milled by a vacuum ball mill for 12 h. Sn-Ag-Cu spherical powders were prepared via vacuum atomization at a temperature of approximately 500-600 °C, and the spherical powder sizes were approximately 20-50 μm. The Sn-Ag-Cu spherical powders are illustrated in Fig. 1(e). According to the mixing ratios of Ti 3 C 2 (5 wt% ) and Sn-Ag-Cu (95 wt%), the mixed powders were stirred by the electric mixer at 1,000 rpm for 4 h. Finally, the lubricant composites of Sn-Ag-Cu and Ti 3 C 2 were obtained.
The grooves were prepared on the surfaces of CSS-42L bearing steel (Table 2) using the electrical discharge machining technology. The texture density was designed to highlight the tribological enhancement effects of grooves in the range of wear tracks during the tests, and the groove sizes are presented in Table 3. The lubricant composites of MXene-Ti 3 C 2 and Sn-Ag-Cu were melted at a temperature of 500-550 °C, into the grooves on CSS-42L surfaces, using the vacuum melting and infiltration technology for 50-60 min. After the infiltration, the temperature was reduced to 150-200 °C for 20-30 min. Subsequently, power was cut off, and the samples were cooled to room temperature (20-25 °C) inside a temperature-controlled furnace.  Furthermore, the samples were ground with 1500# sandpapers in the grinder for 20 min. The sample surfaces were polished by 4000# diamond grinding pastes in the grinder for 20 min. Finally, all samples were cleaned by the ultrasonic cleaner using alcohol, and the obtained samples are presented in Fig. 1(f).

Experimental
A test system for studying the tribological behavior and friction-induced noise was developed to synchronously collect friction coefficients, vibration acceleration, and noise in the test, as illustrated in Fig. 2. The 2D mechanical sensor with high tribometer sensitivity (MFT-5000 Rtec, USA) was used to collect the frictional force and load, and then the friction coefficients were automatically calculated. Moreover, the loading servo controller of the tribometer constantly modified the loading forces during the tests, which ensured that the test results were not affected by the load fluctuation. The tested samples were maintained in a reciprocating motion at dry conditions, and each test was repeated. The counterpart ball (Si 3 N 4 , diameter: 6.3 mm) has the highest hardness among the samples, which means that it could evaluate the tribological characteristics of samples more accurately.
To obtain stable tribological properties of selflubricating materials, the tribological standard test procedures were adopted. The test conditions were set based on the practical working conditions and tribological properties of the materials, as presented in Table 4. Meanwhile, the vibration acceleration signals during the tests were recorded by the acceleration sensor (PCB352C33, Sensitivity of 50 mV/g, USA), which was installed on the loading rod and kept in the horizontal direction to the friction. In addition, the distance from the friction interface was approximately 15-20 mm. The friction-induced noise was recorded by the microphone (PCB378B02, USA) with a sensitivity of 50 mV/Pa and the dynamic measurement range of 15-137 dB, with a horizontal distance from the friction interface at approximately 80-100 mm. The data acquisition system (Ipotest-08) was used to synchronously collect the vibration acceleration and noise signals. The sampling rate of the system was 10 ksps. It was verified that the sound pressures of measured signals in the tests were more than 10 dB larger than that of the environmental noise. The precision of measured signals was not influenced by the environment.
After the tests, the power spectral density (PSD), spectrum rules, and coherence of vibration acceleration and sound pressure signals were analyzed by the MATLAB software (USA). Meanwhile, the profile parameters on the worn out surfaces of the samples were measured by the white light interference profilometer (UP-3D Rtec, USA). Each parameter was measured ten times, and an average value was taken as the measurement result. In addition, the microstructural morphology and elemental distributions of Ti 3 C 2 were observed using a scanning electron microscope (SEM, JSM-IT300/EDS-X-MaxN20, Japan). Furthermore, the morphological characteristics on the worn surfaces were observed by a electronic probe microanalyser (EPMA, JAXA-8230, Japan), and the friction and wear mechanisms were analyzed. An field emission scanning electron microscope (FESEM, ULTRA PLUS, Germany) was utilized to observe the internal morphology of materials in the worn surface section, which analyzed the material damage characteristics. The phase analyses on the surfaces were performed using a rotation anode high power X-ray diffractormeter (XRD, D-MAX-RB, Japan) and an energy dispersive spectrdmeter (EDS, Inca-X-ACT, UK), and the crystallinity, residual stress, and grain diameter were calculated using the MDI Jade 9 software (USA).

Tribological behavior and friction-induced noise of samples
As illustrated in Fig. 3, the friction coefficient and equivalent sound pressure increase with the increase in applied load. As illustrated in Figs Under all load conditions, as the applied load increases, the friction coefficient takes a shorter time to reach the steady state. Moreover, the friction coefficients and friction-induced noise sound pressures increase; however, the increase in sound pressures is more significant. The friction coefficients of GSs and GSs-SAC gradually approach those of the SSs, and the noise sound pressures of GSs and GSs-SAC exhibited the same trend. However, the friction coefficients and noise sound pressures of GSs-SACT could be maintained at the lowest levels. Therefore, loads exhibit the least effect on the anti-friction and noise reduction performances of GSs-SACT. Although the effects of grooves on the friction coefficients are not significant, the noise sound pressures are significantly affected. At a load of 10 N, the friction coefficients and sound pressures of GSs-SAC exhibited lower values than GSs. However, as the applied load increased, the lubricating performance of Sn-Ag-Cu exhibited a limited effect, while the anti-friction and friction-induced noise reduction performances of GSs-SAC exhibited the same trend as those of GSs. Under high loads, GSs-SACT could obtain lower friction coefficients and sound pressures than GSs-SAC.
Analyses on the instantaneous changes in the friction coefficient and force could highlight unique tribological characteristics, and the test results presented in Fig. 4 illustrate the relationship between noise and friction force. The friction coefficients increase with the increase in frictional forces. The inconsistent rules of frictional forces and coefficients are similar. Nonetheless, at the turning positions of the reciprocating process, the curves of frictional forces and friction coefficients indicate a significant step-jumping. Moreover, a higher friction-induced noise is recorded at the large jump region in the amplitude. The step-jumping amplitudes of GSs, GSs-SAC, and GSs-SACT decrease more significantly than SSs. The curves of frictional forces and coefficients reflect tooth-jumping, and the jumping number is typically equal to the groove number. Therefore, the step-jumping amplitudes of frictional forces are weakened by tooth-jumping, thereby suppressing the friction-induced noise. Moreover, with the increase in tooth-jumping amplitudes, the friction-induced noise becomes more significant. The tooth-jumping of curves is triggered by the grooves, and the amplitudes could be reduced by the lubricants in the grooves. Therefore, compared with other samples, the frictional forces, coefficients, and friction-induced noises on GSs-SACT are the least. With the increase in the load, the frictional forces and coefficients, step-jumping amplitudes of curves, and sound pressures of friction-induced noise increase. The noise reduction effects of grooves could be minimized with the increase in the load. At an applied load of 10 N, taking GSs as the reference, the jumping of frictional forces and friction coefficients of GSs-SAC is improved by the grooves and Sn-Ag-Cu. Nevertheless, the inconsistent rules of frictional forces and friction coefficients of GSs-SAC and GSs are similar, and the effects of grooves and Sn-Ag-Cu are reduced at higher load (30 N). However, under the high load (30 N), the jumping amplitudes of frictional forces and friction coefficients could be significantly reduced by the composite lubricants and grooves on GSs-SACT.  Furthermore, a practical engineering application is adopted as an analogy to illustrate the effects of grooves on frictional force, which elucidates the explanation better. Nevertheless, the amplitude reduction mechanisms of surface friction and liquid flow differ. As illustrated in Fig. 5, tanker speed always changes, and the oil in the tank fluctuates constantly. The fluctuation force of oil would make the tanker run in an unstable state, which could trigger accidents. Therefore, "wave damping plates" are installed inside the tank to minimize the large fluctuation of oil, thereby decreasing the fluctuation force. The wave damping effect can be realized on GSs-SACT to reduce the fluctuation of friction force. Moreover, surface profiles at the friction interface are not uniform, which could affect the change in the frictional force. Meanwhile, the counterpart ball sliding speeds are altered, and the frictional forces also fluctuated constantly. Therefore, the friction-induced vibration and noise are induced by the instability of the friction system. The grooves are equivalent to the "wave damping plates", which could reduce the larger fluctuation of friction force. Furthermore, the frictional force fluctuations are weakened by the grooves. In addition, the jumping amplitudes of frictional forces are minimized by SACT. In contrast, the frictioninduced vibration and noise could be suppressed by the "wave damping effects" of grooves and SACT. The synergistic effects of grooves and SACT can improve the inconsistent rules of frictional forces, which enhance the friction-induced noise reduction ability of GSs-SACT.

Friction-induced vibration and noise characteristics of samples
Power spectrum density analyses of noise and vibration signals of the samples are presented in Fig. 6. The inconsistent rules of vibration PSD curves and noise are similar. Meanwhile, the vibration and noise of SSs have high power distributions in the entire spectra, thereby producing the high-frequency noise (high-pitched noise) during the tests. Compared with those of SSs, more peaks in the PSD curves of GSs, GSs-SAC, and GSs-SACT are generated by the "wave damping effect" of grooves and composite lubricants. The power distributions in the entire spectrum and friction-induced noises are significantly reduced. Moreover, the peak spectrum intensities of GSs are significantly reduced, thereby inhibiting the high-frequency noise, and producing low-frequency chattering (low-intensity impact noise accompanied by rapid vibration). The spectrum intensities in the local frequencies of GSs-SAC are lower, which is attributed to the chattering, and hence they generate the low-noise. Furthermore, the spectrum intensities of GSs-SACT are the lowest, and the peak distributions in its PSD curves are the least. In addition, no significant friction-induced noise was observed during the tests. As illustrated in Figs. 7 and 8, the spectrum distributions and compositions of vibration and noise are similar. Meanwhile, the spectra of SSs contain the highest frequencies and the largest intensities, while GSs-SACT spectra contain the least frequencies and the smallest intensities. The spectra of SSs, GSs, and GSs-SAC are distributed in frequencies of 500-3,000, 500-4,000, and 0-1,500 Hz, respectively, whose noises are the high-pitched noise, chattering, and low-noise, respectively. Moreover, the spectra of GSs-SACT without the obvious friction-induced noise are primarily distributed in 0-500 Hz. Therefore, the forms of the friction-induced noises are affected by the surface contact characteristics. The surface contact characteristics are improved by the grooves and lubricants, which could suppress the friction-induced vibration and noise.
The coherence analyses of the vibration accelerations and sound pressures of the samples are presented in Fig. 9. The coherence coefficients of SSs, GSs, and www.Springer.com/journal/40544 | Friction    GSs-SAC with the obvious friction-induced noises are all approximately 0.7, thus indicating that the vibration acceleration and noise signals are highly correlated. The coherence coefficients of GSs-SACT without the significant friction-induced noise are in the range of 0-0.1, thereby indicating that the signals are slightly correlated. Therefore, the friction-induced noise is closely related to the local vibration on surfaces triggered by the sliding friction. Moreover, the high-pitched noise is produced by the frictioninduced vibration on SSs. The friction-induced vibration and noise are effectively reduced by the grooves on GSs. However, the low-frequency chattering is generated by the periodic impacts between counterpart ball and groove edges during the reciprocating sliding. Meanwhile, the synergistic effects of lubricants and grooves suppress the periodic vibration at the friction interface, thus inhibiting the friction-induced noise.

Worn surface analysis
As illustrated in Fig. 10, spalling and larger wear www.Springer.com/journal/40544 | Friction debris are evident on the SSs, which clearly indicate that the dominant wear mechanism is adhesion. Furthermore, narrow furrows and wear debris can be observed on GSs. The spalling phenomena on groove edges are triggered by the impact of the counterpart ball. However, the presence of the micro-grooves on the contact surfaces indicated that abrasion is the wear mechanism. The furrows, smaller wear debris, and Sn-Ag-Cu could be observed on GSs-SAC; however, no obvious furrow and spalling exists on the GSs-SACT. Furthermore, the cracking and deformation of Sn-Ag-Cu-Ti 3 C 2 in the grooves are observed. Consistently, GSs-SACT exhibited the best anti-friction performance, which alters the wear mechanisms and extends the service life of materials.
As presented in Fig. 11 is observed that the cross-sectional profile of SSs exhibits apparent convex edges with parabolic plastic deformation while the plastic deformation of GSs is the inverted triangle shape. The profile of GSs-SAC depicts a clear fluctuation, and the profile of GSs-SACT exhibits a smooth curve. Therefore, the wear debris is trapped by the grooves, thus reducing the damage of wear debris to the surfaces. Sn-Ag-Cu spreads out over the contact surface, and fills the pits on GSs-SAC. Consequently, the low-shear conjunction layer and relative smooth surface trigger a remarkable reduction in the wear rate. The elemental distributions on GSs-SACT before and after friction tests are presented in Fig. 12. Under the influence of frictional force and heat, the composite lubricants with MXene-Ti 3 C 2 are presented and distributed uniformly on the worn surface. Moreover, the oxygen element significantly increases on the worn surface, which indicates that tribo-chemical reactions occur during the friction process. In addition, oxides and intermetallic compounds are produced, which could affect the wear resistance performances of materials.  As presented in Fig. 13, the SSs with noticeable bulges have severe sub-surface defects. In addition, plastic deformations, grain refinements, transverse cracks, and micro-pores can be detected. The plastic deformations, grain refinements, and cracks also appear in the GSs sub-surface. However, the groove edges are broken by impacting, and the wear debris caused by the furrows are collected in the grooves. As Sn-Ag-Cu spreads out and fills the worn pits, it smoothens the worn surface of GSs-SAC. A damaged layer with the cracks and micro-pores is distinguished in the sub-surface close to the worn GSs-SAC. The Sn-Ag-Cu-Ti 3 C 2 alloy with MXene-Ti 3 C 2 diffuses on the worn GSs-SACT, which reduces the frictioninduced damage. Therefore, under the influence of frictional force and heat, Sn-Ag-Cu can cover the surface cracks and fill the surface pits, which facilitates a self-healing effect for the surface damages. Therefore, the "nano-nail effect" of 2D MXene-Ti 3 C 2 improves the anti-friction property of Sn-Ag-Cu, which makes the repairing effect of composite lubricants completely feasible. As illustrated in Fig. 14, XRD patterns of SSs, GSs, and the initial surface (smooth surface before the friction test) are similar, thereby indicating that no significant tribo-chemical reaction occurs on the worn surfaces after the friction tests. Meanwhile, XRD patterns of GSs-SAC and GSs-SACT exhibit new crystal peaks, and the tribo-chemical reactions occur at the interfaces. The intermetallic compounds of Ag 4 Sn, FeSn, and Fe 3 Sn 2 , as well as Ag 2 O, CuO, and SnO 2 oxides are produced on GSs-SAC and GSs-SACT. Moreover, some titanium products such as TiC, SnTi 2 , and TiO 2 could be detected on the GSs-SACT. As presented in Table 5, several hexagonal crystal structures are produced by the tribo-chemical reactions. The hexagonal systems with low shear stress and strong bearing capacities exhibit better tribological properties. Moreover, the hard phases with small diameters and high densities are produced by the tribo-chemical reactions on GSs-SAC and GSs-SACT, which improve the mechanical properties of lubricating films. During the sliding process, the lubricating films with excellent anti-friction properties on the worn surface are formed by the tribo-chemical reactions, which inhibit the frictional damage and enhance the bearing capacity of GSs-SACT.
The crystallinities of sample surfaces significantly decreased, and the average grain diameters increased after the sliding tests. Several damages of the material crystal structure were caused by friction. Moreover, the crystallinity of GSs-SAC exhibits the lowest damage, and the average grain diameter is the largest. Sn-Ag-Cu on GSs-SAC could activate the surface chemical energies of materials and promote the tribo-chemical reactions. However, MXene-Ti 3 C 2 in Sn-Ag-Cu-Ti 3 C 2 exhibits "nano-nail effect", which increases the worn surface crystallinities and average  grain diameters; hence, it improves the lubrication performance. The residual strains are eliminated by Sn-Ag-Cu-Ti 3 C 2 , which improves the stress field distributions on GSs-SACT. The results indicated that the synergistic effects of MXene-Ti 3 C 2 and Sn-Ag-Cu triggered a significant enhancement in the anti-friction property.
SSs with severe material damage are primarily characterized by adhesive wear, and the high-frequency noises are produced during the tests. However, the existence of narrow furrows on GSs verifies that the dominant wear mechanism is abrasion. In addition, the sliding of Si 3 N 4 against GSs exhibited lowfrequency chattering. The wear debris settled in the surface grooves, which minimaxes the wear rate of GSs. Because Sn-Ag-Cu repairs the worn cracks on GSs-SAC, the friction damages are significantly reduced, and the friction processes are accompanied by the lower friction-induced noise. The surface defects (cracks and pores) on SSs produced during the rubbing of the sliding surfaces trigger the spalling and wear debris on the worn surface. Nevertheless, the wear mechanism turned to the tribo-chemical wear by Sn-Ag-Cu, which produces oxides and intermetallic compounds with anti-friction effects to improve the tribological properties. Moreover, MXene-Ti 3 C 2 could enhance the anti-friction property of the Sn-Ag-Cu lubricant on GSs-SACT.

Characteristics of composite lubricant Sn-Ag-Cu-Ti 3 C 2
No adhesive trend exists for tin-based alloy lubricants on the iron-based materials caused by the immiscibility between tin (Sn) and iron (Fe), which could inhibit the adhesive wear on the surfaces. Therefore, tin-based alloys are widely used as lubricants (bearing alloy) on the surface of iron-based materials. The immiscibility between tin (Sn) and iron (Fe) is an important advantage of bearing alloy. The common bearing alloys are Babbitt alloys, such as ZChSnSb11-6 (Sb: 10.0-12.0 wt%, Cu: 5.5-6.5 wt%, Sn: Balance) and ZChSnSb8-4 (Sb: 7.0-9.0 wt%, Cu: 3.5-5.5 wt%, Sn: balance). As illustrated in Fig. 15, the contact angles of Sn-Ag-Cu and Sn-Ag-Cu-Ti 3 C 2 on the smooth and groove surfaces of CSS-42L steel at 500 °C are investigated under vacuum condition using the contact angle measuring instrument. All contact angles are larger than 90°. Therefore, Sn-Ag-Cu and Sn-Ag-Cu-Ti 3 C 2 guarantee the immiscibility properties of the tin-based alloy on iron-based materials, which are beneficial in minimizing the adhesive wear. The worn surface with the higher microhardness would exhibit the best wear resistance. As presented in Fig. 16, the microhardnesses of worn surfaces are larger than those of surfaces before tests. The microhardnesses of worn surfaces are increased with the increase in the loads, thus indicating that the frictional behaviors could increase microhardnesses, as a rational result of the strain hardening, owing to the plastic deformation after the sliding. Moreover, the microhardnesses on worn surfaces could be increased by the composite lubricants of Sn-Ag-Cu-Ti 3 C 2 . Compared with SSs, GSs, and GSs-SAC, the microhardnesses of GSs-SACT under loads of 10, 20, and 30 N are increased by 32.8%, 27.6%, and 30.0%, as well as by 18.3%, 24.2%, and 12.5%, and by 10.7%, 16.0%, and 6.3%, respectively. Furthermore, the worn surfaces are repaired by Sn-Ag-Cu-Ti 3 C 2 , thereby improving the microhardness and wear resistance of worn surfaces. As illustrated in Fig. 17, after the friction tests, clusters of surface cracks could be observed on Sn-Ag-Cu and Sn-Ag-Cu-Ti 3 C 2 surfaces. Sn-Ag-Cu has significantly larger cracks than Sn-Ag-Cu-Ti 3 C 2 . Moreover, with the increase in the load, the cracks on Sn-Ag-Cu are increased, such that the lengths and widths become longer and wider. However, the effects of the applied load on Sn-Ag-Cu-Ti 3 C 2 are limited. With its unique micro-nano size-effects, MXene-Ti 3 C 2 could enhance the integrity and anti-friction property of lubricating films.   of SSs increases, while those of GSs and GSs-SAC decrease, and that of GSs-SACT is maintained at approximately 0.6. However, the skew levels of surface profiles can be characterized by R sk . A larger R sk implies a larger proportion of valleys in the profiles. When R sk is negative, the proportion of peaks in the profiles is larger than that of valleys. Furthermore, the valleys are more than the peaks in SSs profiles, whose proportion in the profiles is increased with the increase in the load. Because the valleys considered the starting points of cracks, the surfaces that have more valleys exhibit a higher tendency to be cracked. Moreover, the peaks are more than the valleys in the profiles of GSs and GSs-SAC, and the proportion of peaks and valleys in the profiles increases with the increase in the load. The peaks are the stress concentration points, and the increase in the peaks would trigger surface damages. Meanwhile, the R sk of GSs-SACT is the smallest, which indicates that the valleys with the shallow depth are uniformly distributed in the profiles. When the loads are increased, it is observed that the profiles on GSs-SACT are smoother and more uniform.
As presented in Figs. 18(d)-18(f), the steep shape of the surface profiles is characterized by the average absolute slope (Δa). The profiles of SSs are the steepest, and those of GSs-SACT exhibit the least steepness. Moreover, with the increase in the load, the steepness of the profiles increases. During the friction, the stress concentrations could be caused by the profiles with the larger steepness. Accordingly, the surface damages increase. Meanwhile, the wrinkle levels of the surface profiles are characterized by the length ratio (l r ). The wrinkle levels of surface profiles increase with the increase in the load, which indicates that the deformations and damages on the surfaces increase. Furthermore, λ a represents the average wavelength of the profile. The profile wavelengths increase with the increase in the load. The smaller wavelength could improve the bearing capacity and wear resistance of profiles. In addition, GSs-SACT exhibited the minimum values of Δa, l r , and λ a .
Correspondingly, GSs-SACT exhibited the highest wear resistance and bearing capacity owing to the uniform profile shape and limited presence of the stress concentration zones.
The amplitude distribution function (ADF) represents the probability density functions of profile amplitudes within the evaluated length. ADF distribution areas of the samples increase with the increase in the load,   Figs. 19(a)-19(c), the distributions of sharp peaks and deep valleys on GSs-SACT with the uniform profile heights are the least, which would reduce the probabilities of damages. Peak count (PC) represents the quantitative distribution of peaks within the evaluated length, which influences the wear resistance performance of surfaces. When the load is increased from 10 to 20 N, the peaks in the profiles increase. Although the low-amplitude peaks decrease as the load is increased from 20 to 30 N, the high-amplitude peaks increase. Therefore, under all loads, GSs-SACT profiles exhibit the lowest number of total and highamplitude peaks, which could improve the wear resistance of the surfaces. Bearing ratio (BR) represents the ratios of profile lengths to the evaluated lengths at the horizontal sections, which reflect the bearing capacity of the surface. Moreover, the sharp peaks are the stress concentration points of interface interaction, and the deep valleys represent the starting points of the cracks. The sharp peaks and deep valleys promote the damages to the material surface. Furthermore, the larger BR curve slopes lead to lower proportions of sharp peaks and deep valleys in the profiles. The sharp peaks and deep valleys increase with the increase in the load, which would reduce the bearing capacity of the profiles. Meanwhile, the effects of loads on the BR curves of GSs-SACT are the minimum, thus indicating that the bearing capacity of GSs-SACT GSs-SACT profiles are described by the amplitude parameters (R a , R ku , and R sk ), spatial parameter (λ a ), hybrid parameters (Δa and l r ), and statistical curves (ADF, PC, and BR), which show the contact characteristics and wear resistance mechanisms of GSs-SACT. Moreover, the profiles with several sharp peaks and deep valleys on the worn SSs are steep and wrinkled, which indicate the poor wear resistance and high-frequency noise during the drying friction. The grooves on GSs could reduce the wear and friction-induced noise. However, abrasive wear is observed on GSs, whose profiles have not been improved. The anti-friction effects of grooves and Sn-Ag-Cu on GSs-SAC could improve the profiles and suppress the friction-induced noise. However, with the increase in the load, the anti-friction effects on GSs-SAC are reduced. The better contact characteristics are produced by the synergistic effects of grooves and Sn-Ag-Cu-Ti 3 C 2 on GSs-SACT, which obtains the best bearing capacity and anti-friction property. Meanwhile, the profile characteristics on GSs-SACT are difficult to induce the self-excited vibrations of friction system; therefore, no obvious Fig. 19 (a, c, e) Amplitude distribution functions, and (b, d, f) bearing ratios and peak counts of the surface profiles.
www.Springer.com/journal/40544 | Friction adhesive wears are steeped and wrinkled, respectively. The surface profiles with the sharp peaks and deep valleys result in the fluctuation of frictional forces, which trigger friction-induced vibration and noise. The GSs-SACT with uniform profiles could effectively suppress the self-excited vibrations and frictioninduced noise. This study could provide the experimental bases for the applications of anti-friction and friction-induced noise reduction performance of CSS-42L in the field of tribology.

Conclusions
In this study, the effects of textured surfaces filled with Sn-Ag-Cu and MXene-Ti 3 C 2 on the friction, wear, and friction-induced noise of CSS-42L steel were investigated. According to the test results, the following conclusions were drawn: 1) GSs-SACT exhibited the lowest friction coefficients and friction-induced noise among SSs, GSs, and GSs-SAC. The synergistic effects of textures and Sn-Ag-Cu-Ti 3 C 2 significantly improve the changing rules of frictional forces, which enhance the noise reduction ability on GSs-SACT.
2) The wear mechanism of SSs is adhesive wear, which produces high-frequency noise during the tests. The abrasive mechanism of GSs triggered lowfrequency chattering. The low-noise induced by GSs-SAC is attributed to the tribo-chemical wear. However, GSs-SACT exhibited the minimum noise and wear.
3) Sn-Ag-Cu-Ti 3 C 2 exhibited immiscibility properties with tin-based alloy on CSS-42L, which could restrict adhesive wear. The tribo-chemical reactions on GSs-SACT produced oxides and intermetallic compounds with strong bearing capacities and excellent wear resistance. The crystallinities on the worn surfaces were improved by the "nano-nail effect" of MXene-Ti 3 C 2 , which enhances the anti-friction performance of composite lubricants. The tribological properties of GSs-SACT were remarkably improved. 4) When the profiles were steeped and wrinkled, the friction-induced noise could be clearly observed. The synergistic effects of grooves and Sn-Ag-Cu-Ti 3 C 2 on GSs-SACT are beneficial in forming uniform profiles with excellent bearing capacities and wear resistance, which prevents the self-excited vibration of friction system, as well as the friction-induced noise.
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