The Influence of Friction Blocks Connection Configuration on High-Speed Railway Brake Systems Performance

The influence of friction blocks connection configuration on the interfacial tribology behavior and FIVN (friction-induced vibration and noise) of the high-speed railway braking system is systematically investigated with a scaled brake test bench. The potential relationship among interface contact status, friction, wear, pressure distribution, thermal response, and vibration noise of the system is studied under dragging test conditions. The results indicate that the connection configuration of the friction blocks has a significant impact on systematic interfacial tribology behavior, thermal response, and vibration noise. A floating connection mode can suppress the vibration noise of the brake system. The interfacial thermal response and systematic vibration noise are quite relevant with the contact status, interfacial wear, and pressure distribution. The increase of interfacial wear will lead to an expansion of pressure concentration area and an aggravation of vibration noise. This research is helpful for further design optimization and noise reduction of the railway brake system.


Introduction
High-speed railways are the trend of rail transport development in the world. The last few decades have witnessed the rapid development of high-speed railways [1]. The statistics from UIC Passenger Department show that the global highspeed railways operating mileage is over 50,000 km, and the number will be 60,000 km before 2030. Braking systems are essential for the safe operation of high-speed railways. A friction sub-structure consisting of brake discs and brake pads is used as the basic braking device in high-speed trains. The kinetic energy of the train is mainly converted into heat by drag friction [2] at the brake interface in the process of braking, during which brake discs and brake pads are subject to excessive wear [3] due to the strong friction [4,5], leading to uneven thermal response [6][7][8], thus considerably affecting the safety of train operation. Meanwhile, friction-induced vibration and noise (FIVN) significantly disrupt passengers' riding comfort. Therefore, it is of great importance to comprehensively study the interfacial tribology behavior [9][10][11][12][13][14] and FIVN [15][16][17][18][19][20] of the brake system of high-speed trains.
The contact interface undergoes severer drag friction during the brake process, leading to complex tribology behavior and thermal response at the brake interface. Many researchers have made great efforts to study wear behaviors and thermal response characteristics of brake pads in recent years, and many meaningful achievements are obtained. Zhang et al. [21] studied the effects of brake friction materials on the interfacial wear characteristics. They found that the formed oxide particles of Cr in the friction material can enhance the stability of the third body friction layer during braking. Vladescu et al. [22] and Segu and Hwang [23] investigated the effect of surface groove texture of braking materials on interfacial frictional wear and temperature rise variation. It was observed that the grooves had a suppressive effect on interfacial temperature rise, and they could collect abrasive debris to reduce the accumulation of abrasive debris, thus diminishing interfacial wear. Xing et al. [24] carried out an experimental study on brake pads' friction and wear characteristics by processing two groove surface fabrics with different arrangements on the surface of brake pads, indicating that the wavy groove fabric had a better wear reduction effect. Wu et al. [25] introduced brake pads consisting of multiple friction blocks into the brake system and studied the tribological properties of the braking material at different braking speeds and different braking pressures. Xiao et al. [26] tested the friction and wear properties of copper-based powder metallurgy brake materials. It was discovered that cracks would occur on the pad's surface under the effect of thermo-mechanical coupling and expand to the superficial layer of the material. This phenomenon results in a material spalling phenomenon when the interfacial temperature is high. The frictional wear characteristics at the brake interface under two environmental conditions (dry/wet) were studied by Wang et al. [27] and Shi et al. [28]. Results showed that there was a period of weak braking during the braking process in the wet condition. Within this period, the transition of the front part of the brake interface from wet to dry led to asymmetric wear of the brake interface.
The generation and evolution of FIVN are closely related to the friction at the brake interface. The relationship between the interface behaviors and the FIVN has been studied, and numerous significant research results have been obtained. Hoffmann and Gaul [29] pointed out that characteristics of interfacial wear (surface topography, surface roughness, etc.) played a crucial role in the generation of FIVN. Experimental methods were used to study the relevant properties of the friction system with microscopic observations. It was found that there was a considerable accumulation of abrasive debris on the brake interface when the brake squeal occurs [30]. Majcherczak et al. [31] and Graf and Ostermeyer [32] investigated the interrelationship among tribological features, other interfacial factors, and unstable vibration response on the braking interface. The results showed that the third body formed by the agglomeration and compaction of abrasive particles at the brake interface caused changes in the friction coefficient, which in turn significantly influences the unstable vibration propensity of the braking system. Tonazzi et al. [33] investigated the bi-stable phenomenon of the frictional continuum system and discovered that in a specific range of additional sliding velocities, the oscillator has two steady states, namely steady sliding, and stick-slip oscillations. Lee et al. [34] found that the occurrence trend and intensity of unstable stick-slip motion were related to the damage strength and stiffness of the contact zone, the size of which affected the instability of the friction system. Stender et al. [35] applied a machine learning approach to detect and characterize vibrations and predict brake squeal. The approach could predict the occurrence and onset of brake squeal with high accuracy.
Massi et al. [36] and Tison et al. [37] revealed that the size of the contact zone on the brake interface and the contact pressure distribution determined the frequency and amplitude of stick-slip by friction tests. Sinou et al. [38] studied the possible link between the surface thermal impacts and structural modifications in the brake system and found that at temperatures below 100 °C, this might result in a modest modification of the nonlinear squeal signal. The influence of the contact area on the friction characteristics and vibration noise propensity of the brake system was studied by Bergman et al. [39] and Seol et al. [40]. They found that the contact area on the brake interface consisted of a stationary primary contact area and a transient secondary contact area. The formation of the secondary contact plateaus triggered the instability of the brake system.
Normally, there are two available ways to fix the friction blocks on the brake pad, i.e., fixed (friction blocks are bolted or welded to the brake disk carrier) and floating (friction blocks are linked with the brake disk carrier by spring), as shown in Fig. 1. The studies discussed above have focused on the fixed joint, while the floating joint has rarely been paid attention. The floating mode friction block was designed to adjust the friction block's contact angle, improve the tribological behavior, and suppress the friction-induced vibration. However, it would lead to a relatively higher cost of production than the fixed mode due to its complex structure and high quality of manufacturing process. It may even cause problems such as reduced structural reliability and falling blocks. Therefore, the influence of different fix modes of friction blocks on the tribology behavior at the brake interface and FIVN, which has been scarcely studied, is worthwhile to give an in-depth discussion.
Two ways to fix the friction block, namely fixed joint and floating joint, are employed in this work, under which the tribology behavior and FIVN at the brake interface are measured and compared to evaluate the influence of friction block fix mode on the tribology behavior and dynamic response of the brake interface. The relationship among friction block fix mode, contact status, interfacial wear, thermal response, contact stress distribution, and FIVN are established, which is of great significance in guiding the design and optimization of friction block fix mode.

Presentation of the Brake Dynamometer and Sample Preparation
The test is conducted on a self-developed brake test bench, as shown in Fig. 2a. The test bench consists of four systems: the control system, drive system, loading system, and signal analysis system. The main components include servomotors, reducers, couplings, brake discs, friction blocks, clamps, 3-D accelerometers, force sensors, actuators, cylinders, support structures, and microphones. During the test, the control system regulates the rotation speed of the servo motor and the load pressure, enabling braking tests to be performed at different brake speeds and loading forces. The acceleration sensor (range ± 500 g, frequency response 0.5 Hz-40 kHz, sensitivity 10 mV/g), force sensor (sensitivity: 5 mV/N, Measurement range: ± 2000 N) and the microphone (MTG MK250, sensitivity 50 mV/g, dynamic range 15-146 dB, frequency response 3.5-20 kHz) are used to collect the vibration acceleration, interfacial force, and sound pressure signals during the brake process in real-time (with sampling frequency set to 50 kHz).
The force sensor is positioned between the brake pad fixture and the push rod, the 3-D acceleration sensor is fixed above the fixture, and the microphone is disposed at 15 cm of frictional contact between the brake disc and the brake block.
A hand-held thermal imager (FLIR E40, with an accuracy of ± 2 °C) is used to record the post-test interface temperature on the brake interface.
Simultaneously, the evolution of the contact status, i.e., the contact profile and the contact pressure distribution between the brake disc and friction block, are measured using a pressure distribution measuring instrument (Tactilus from the company of Sensor Prod). Notably, the above measurement is done when the brake disc is stationary.
As shown in Fig. 2b, fixed joint block and floating joint block are, respectively, introduced into the brake system to conduct friction tests. The corresponding contact position and the detailed dimension of the block specimens are shown here. The fixed joint block is directly linked with the fixture by bolts, and the floating joint block is connected to the fixture by two elastic elements, namely disc spring and spring clamp. The specimens of friction block and brake disc are made from powder metallurgy and forged steel, respectively.

Experimental Work
The experimental parameters are as follows: brake speed ω = 100 r/min, brake force F = 500 N, friction radius R = 120 mm. Before the tests, the contact pressure distribution on the interface between the brake disc and the friction block is measured by pressure film. During the following formal test, the contact area and the contact pressure are measured every 5 min. When measuring the contact area and the contact pressure, the friction block is first separated from the brake disc, and the rotating disc is stopped. After that, a pressure film sensor is put on the disc surface, and the brake force is loaded to establish the contact. Then the static contact pressure and the contact area can be obtained. The measured data is processed by analysis software to illustrate the evolution of the contact status at the brake interface. The vibration and noise signals are collected every 30 s for 20 min after the test begins. After the test, a final pressure distribution measurement between the brake disc and the friction block is conducted. Afterward, an optical microscope, white light interferometer, and scanning electron microscope are used to observe and analyze the wear patterns of friction blocks. To ensure the repeatability and reliability of the test results, each installation configuration (fixed connection and floating connection) is tested repeatedly ten times, and each time is conducted with a new friction block. In addition, the next test will not be carried out until the average surface temperature of the disc drops to 30 °C. The ambient temperature during the test is 23 ~ 26 °C, and relative humidity is controlled within 60 ± 10%.

Contact Status
As shown in Fig. 3, the contact pressure distribution is measured for the two friction blocks before the test, and then the pressure is measured every five minutes during the first 20 min of the experiment. All the friction blocks are taken from the real brake pads of high-speed trains, which contain almost identical surfaces morphology. It can be observed from Fig. 3 that the initial contact status (0 min) on the interface of the two connection modes is diverse, with obviously different contact pressure distributions on the contact area. In the case of the floating joint block, its initial contact area symmetrically distributes on both sides of the interface. At the same time, the evolution of the contact area mainly occurs in the medium part of the interface during the test. Thus, the contact pressure distribution on this contact interface becomes more evenly with time. However, the initial contact area of the fixed joint block concentrates at the leading and left edges of the friction block (near disc center). Meanwhile, the newborn contact area still occurs around these two regions in the test, which aggravates the To quantitatively characterize the contact status for the two different friction blocks during the test, the grid number on the pressure film where pressure is detected is calculated every time to obtain the varying total contact area of two friction blocks, as given in Fig. 4. For both friction blocks, the contact area increases as the testing time went on. The contact area of the floating joint block is always larger than that of the fixed joint block in the experiment. Therefore, the above results demonstrate the superiority of the floating joint mode in modifying the contact behavior of the brake interface.  Figure 5 shows the average coefficient of friction (COF) for the friction blocks of the two connection methods during the test. Obviously, because the applied braking force and relative translation velocity have constant values, the COFs for the two friction blocks are almost constant. Moreover, the COF of the fixed joint block is always larger than the floating joint block during the test. The reasons are discussed in the following. The differences in connection structures (the fixed joint and the floating joint) used for the two friction blocks lead to the differences in the structure stiffnesses of the two friction systems, which results in dissimilar contact behavior (such as contact status). The difference in the contact behavior affects the degree of surface wear degradation. The tribological behaviors and interfacial characters (such as third body, surface roughness, wear debris) during the wear degradation will further affect the COF [41][42][43]. Therefore, the differences in COF for the two friction blocks (the fixed joint and the floating joint) could be the reflection of the combination of the above factors. The COF is a crucial factor determining the stability of the brake system, and relatively greater COF indicates more severe wear and more substantial vibration noise in turn [44,45]. These phenomena will be discussed in later sections.

Interfacial Wear Behaviors
To observe the wear situation on the surface of the friction block, macroscopic analysis and microscopic analysis are carried out for the worn surface of the two friction blocks after the test, as shown in Fig. 6. It can be observed that the overall distribution of abrasive debris on the surfaces of the two friction blocks is significantly different (Fig. 6I). According to the difference, the wear interface is divided into two emblematic regions and labeled as region A and region B. Large amounts of abrasive debris are accumulated in region A, while a relatively small amount of wear debris can be found in region B. For the fixed joint block, there is a large region A. It mainly appears in the lower-left part of the interface. Oppositely, in the floating joint block, region A takes up a relatively smaller proportion on the interface. It dominantly concentrates on the lower-right part of the interface. The macroscopic analysis of the surfaces of the two friction blocks illustrates that the friction block interfaces of the two connection methods exhibit completely divergent wear characteristics. During the test, the floating connection mode can reduce the interfacial wear to produce relatively small amounts of abrasive debris.
Optical microscopy is used to observe the micrographs of regions A and B on the surface of the two friction blocks with different connection modes, as shown in Fig. 6. The bright areas identified in both region A and region B stand for the substrate exposed on the material surface and the contact platforms formed during the brake process [9]. Some black areas can also be found near the bright areas, representing the ploughings resulting from material removal. The 2D contour of region A of the fixed joint block is presented more clearly in Fig. 6e. There seem to be secondary contact plateaus forming near the leading edge of the primary contact plateaus. This phenomenon will be further discussed in the SEM analysis. It is noticeable that region A on the interface of the floating joint block shows more bright areas compared with the fixed joint block.
Furthermore, the dark black areas are significantly smaller than region A on the interface of the fixed joint block. Further comparative observations of the regions B of the two friction interfaces yield similar results compared with the above findings. The above phenomenon indicates that the interface of the floating joint block undergoes relatively mild wear during the brake process.
A white light interferometer is also used to observe the morphological features of the friction surface of the two friction blocks. The observed zone a, b, c, and d are highlighted in Fig. 7, respectively, representing regions B and A of the fixed joint block and the floating joint block. As illustrated in Fig. 7a and b, no virtually large flat areas exist in zone-a and zone-b of the fixed joint block. Moreover, a large amount of abrasive debris scatters on the contact plateaus and in the ploughings. Meanwhile, more ploughings and exfoliations are being observed in zone-b compared with zone-a. It means region A of the fixed joint block experiences wilder wear than region B. Figure 7c and d depict typical topographic features of regions A and B of the floating joint block. Many scratch marks, ploughings, and a slight accumulation of abrasive debris can be found on the contact platform at zone-d. While the contact platform at zone-c is much smoother, suggesting that region B on the floating joint block surface shows relatively mild wear. In summary, interfacial wear is more severe in region A for both friction blocks. To evaluate the wear behavior of regions A and B of the two friction blocks with different connection modes, zonea and zone-c will be a control group, zone-b, and zone-d will be another one. Compared with the floating joint block, a coarser surface is identified in zone-a and zone-b of the fixed joint block. A considerable amount of abrasives are accumulated on the contact plateaus. Due to the continuous grinding action of the hard particles in wear debris, evident ploughings and exfoliations are formed on the interface. The results indicate that the fixed joint block surface is more severely worn than the floating joint block. The 2-D profile curves for the four regions are provided in Fig. 7e and f. Significant fluctuations can be seen in the 2-D contours of the fixed joint friction block surface, which stand for deep ploughings and exfoliations on the worn surface. The above observations further reveal that the floating joint mode can improve the wear behavior of the brake interface by modifying the contact status.
The softer materials are preferentially worn away, leaving harder materials (steel fibers, abrasive particles, and others) exposing on the block surface, which forms the primary contact plateaus. Rigid particles in the wear debris abrade the primary contact plateaus and result in scratches along the friction direction. The abrasive debris gathered near the primary contact plateaus falls into low spots or ploughings due to gravity and interfacial vibration. The accumulated particles are constantly compacted and eventually become the secondary contact plateaus. The observation presented in Fig. 7f is in line with the above speculation. Besides, the secondary contact plateau is relatively higher than the primary contact plateau [46], leading to local contact stress concentration and, consequently, systemic instability. According to the above analysis, the formation of secondary contact plateaus is somehow facilitated by the generation of abrasive particles. Therefore, the continuous accumulation of abrasive particles on the contact interface is helpful to enlarge the actual contact area bearing high pressure, which aggravates the instability of the friction system and produces a negative impact on interface temperature and vibration response.
To explore further the topography of the worn surface and the formation mechanism of the secondary contact  (F-B), large-scale light grey platforms (F-C), and black-grey platforms in the region near its leading edge (F-D). Energy-dispersive x-ray spectroscopy (EDX) analysis of the four typical topographies is also performed to determine the material composition, and the results are shown in Fig. 8b. It is observed that the blackgrey platform (point 1) and the white agglomerated particles (point 3) have almost the same proportion of element ingredients. The white agglomerated particles representing abrasive debris can be seen scattering in the spalling pits and furrows, indicating that the secondary contact plateaus are formed by the accumulation and compaction of abrasive debris. The energy spectral of the light grey (point 2) shows that the main elements in which are steel and copper fibers, suggesting that this platform is primary contact plateaus. The existence of steel increases the strength of the composite material due to its low solid solubility in copper, suggesting that contact platforms are easily formed during the friction process. In addition, the energy spectrum of point 4 exhibits a high peak of carbon, indicating that the material in the dark region is mainly graphite which can lubricate the friction surface to improve wear resistance [47].

Contact Pressure Distribution
The contact pressure distribution on the brake interface is displayed in Fig. 9. It can be seen that, for the fixed joint block, the contact pressure is mainly concentrated in the lower-left portion of its surface. While for the floating block, the contact pressure concentrates on the lower-right part of  (Figs. 6, 7). Due to the wear degradation of the friction block, the actual contact area is different from the nominal one. In this paper, the actual average contact pressure is defined as the ratio between the braking force and the actual contact area. Thus, the actual average contact pressures of the two connection configurations are different, see Fig. 9. The results show that the fixed-connection block has a relatively higher contact pressure on its interface, where the average contact pressure is 48 Psi, approximately 14.2% higher than that of the floating-connection block. The results are similar to the wear behavior of the two friction blocks. The accumulation and compaction of abrasive debris on primary contact plateaus can result in secondary contact plateaus. As a result, contact areas with high local pressure, which bear most of the contact load, are observed.
The grid number on the pressure film of different pressure values is counted for the two friction blocks, as shown in Fig. 10. A contact area with a pressure value of more than 100 is defined as the high-pressure area (Psi > 100). Obviously, the high-pressure area of the fixed joint block is much larger than the floating joint block. The reason may lie in the fact that more abrasive particles are produced on the brake interface in the fixed joint mode, which is constantly agglomerated and compacted during the brake process and eventually become the secondary contact plateaus located on the primary contact plateaus. These asperities bear the most contact pressure, and thus the regions where the contact plateaus locate become the high-pressure areas. This observation is consistent with the report of Kim [46]. Therefore, the friction block fix mode significantly influences the contact pressure distribution at the brake interface.

Effects of Friction Blocks Connection
Configuration on Thermal Response of the Brake Interface Figure 11 depicts the average, maximum, and minimum temperatures on the surface of the two friction blocks at the end  The results indicate that the interfacial temperature of the fixed joint block increases at a faster rate than that of the floating joint block. The accumulation of abrasive debris on the block surface affects the interfacial thermal response significantly [48]. Associate with the previous wear analysis about the friction interface (Figs. 6, 7), it is concluded that the contact status of the fixed joint block is inferior to that of the floating joint block during the test. Much abrasive debris is produced on the severely worn surface of the fixed joint block, which is repeatedly ground by the friction force, causing much heat and raising the temperature on the interface. In contrast, the floating joint block shows a better contact status, which results in mild surface wear and less abrasive particles accumulations. Therefore, the pad surface of the floating joint block is smoother than that of the fixed joint block, and contact pressure concentration is alleviated, so is the surface temperature.
To obtain the radial thermal response on the brake interface, temperature values of twenty points evenly distributed on line AB (the leading edge) are extracted, as presented in Fig. 12. The temperature on the fixed joint interface fluctuates considerably in the radial direction. The highest temperature appears at the central measuring point, while the lowest temperature occurs at the left point of the line. The phenomenon is the same as the floating block. However, it is observed that the temperature is nearly symmetrically distributed on the floating block surface. Moreover, the temperature of the floating joint block is lower than that of the fixed connection block, showing that the floating friction block has a better thermal performance.

Effects of Friction Blocks Connection Configuration on the System Vibration Noise
The equivalent sound pressure level (ESPL) analysis of the two friction systems with different connection modes is conducted to investigate the effect of block fix mode on the friction-induced noise, as presented in Fig. 13. The ESPL of the brake system with the fixed joint block exceeds 100 dB throughout the test. In contrast, the ESPL of the braking system with the floating friction block is consistently below 90 dB during the entire experiment. Therefore, both brake systems generate high-intensity friction noise, and the brake system with the fixed joint block produced stronger noise than the system with the floating joint block. Besides, the frictional noise intensity concerning the floating connection mode is relatively weak at the initial phase of the test, only about 84 dB. However, the strength of the frictional noise at the start gradually increases as time went on and eventually remains at a stable level of about 89 dB. Hence, the floating connection mode chosen in this study can effectively reduce the noise intensity. Braking vibration noise is chiefly caused by the selfexcited vibration of the friction system [49,50]. The comparison of the vibration noise, tangential vibration acceleration, and normal vibration acceleration signals of the two friction systems is provided in Fig. 14 for the initial phase of the experiment (5-10 s) and the stable phase (25-35 s). Firstly, compared with the initial phase, there is a relative increase in the amplitudes of the vibration acceleration and sound pressure signals during the stabilization phase for both brake systems. Secondly, the tangential acceleration and normal acceleration signals of the fixed connection mode show persistent high oscillations in both the initial and stable stages of the experiment. Consequently, high-intensity friction noise is produced in the meantime. Compared with the fixed joint system, the vibration acceleration signal of the floating joint system exhibits a smaller amplitude and less intensity, which coincides with the result of the sound pressure signals. Therefore, the vibration and noise performance of the friction system can be significantly improved by fixing the friction block in the floating mode.
The tangential vibration of the friction block can cause localized impact and relative slip at high speed and frequency, increasing the normal and shear stress and affecting the braking system's interfacial characteristics [10,38]. Therefore, to reveal the inherent relationship between the vibration response and interfacial characteristics, an analysis of the tangential vibration acceleration signal throughout the test time of the two brake systems is conducted. The Root Means Square (RMS) of tangential vibration acceleration of both systems is calculated every 2.5 s, as shown in Fig. 15. RMS of tangential vibration of the fixed joint system is always greater than that of the floating joint system during the whole test. In addition, the RMS of the tangential  vibration of the two brake systems exhibits different evolution trends. The RMS curve of the floating system shows an increase at the start and then a decrease, while the curve of the fixed system shows a constant increase in the whole test process. These two curves have the same evolutionary tendency as the ESPL change of vibration noise signal (Fig. 13). The tangential vibration acceleration of the fixed joint system is always more significant than the floating joint system. It indicates that the local normal stress and shear stress of the floating friction block are less than that of the fixed joint block. Namely, the interfacial characteristics of the floating joint system are better, equipped with the ability to adjust the interface contact.
The Power Spectral Density (PSD) analysis of the vibration noise and acceleration is conducted during the initial phase (5-10 s) and stabilization phase (25-35 s) of the test, as shown in Fig. 16. For the fixed joint system (Fig. 16a, c, e), the frequency components of vibration noise pressure and acceleration signals, as well as the energy level maintains at the same level throughout the whole process. Five distinct high-energy frequency components can be seen in the vibration acceleration signals in these stages of the brake process, which accounts for the high-frequency vibration noise in the friction system. For the system with floating joint block (Fig. 16b, d, f), in the initial and stable phase of the test, a similar frequency distribution is also found in the frequency spectrum of the noise and vibration accelerations. However, the energy level at the dominant frequency in the initial phase is relatively lower than that in the steady phase. The result indicates that the floating connection mode can improve the vibration state on the interface during the experiment and suppress self-excited vibration level to some extent, thus postponing the generation of unstable vibration squeal.
From another perspective, Fig. 16a and b show that during the stable phase, the dominant frequency of the noise signal of the fixed joint system is 3631 Hz (109.7 dB). Meanwhile, the vibration noise signal in the floating joint system possesses less energy (97.5 dB) at the highest energy major frequency. It is worth noting that the energy levels of the primary frequency for vibration noise of the floating joint system are 73.4 dB at the initial stage and 97.5 dB at the final stage, respectively. Friction-induced noise is primarily generated by the friction system's selfexcited vibration. Normal vibration has a lower energy intensity (140.4 dB) in the early stage than it does in the stable stage (148.9 dB). Furthermore, tangential vibration acceleration drops from 158.2 dB (stable) to 153.5 dB (early). This occurrence also demonstrates that the floating connection method can reduce the energy of the frictioninduced vibration at the prime frequency.
As shown in Fig. 17, one cycle of sound pressure signal of two systems during the stable stage is extracted to study further how the connection mode of friction block affects the vibrational noise of the friction system. The signals are divided into two phases according to the evolution trend. In phase 1, the two systems exhibit different trends in the growing stage. The vibration noise signal of the fixed brake system shows an exponential growth at the start [51] and reaches its peak with a complicated evolution manner. In contrast, the growth trend of the vibration noise signal of the floating brake system is gentler and milder. In phase 2, there is a declining trend in the vibration noise signals of both systems. Vibration noise in the fixed joint system registers a compound downward trend after reaching the peak, with three high-amplitude fluctuations in the downward process.
Nevertheless, the vibration noise signal of the floating brake system gradually and gently decreases after reaching the peak. In general, the vibration noise signal of the fixed braking system has a higher peak and larger overall amplitude than that of the floating brake system. Hence, the floating connection mode can suppress the vibration noise of the friction system due to the anti-vibration property of the elastic elements.
The frequency domain of the sound pressure is divided into three regions in Fig. 18, which are labeled as low-frequency chattering region (100-1000 Hz), low-frequency squeal region (1000 Hz-5000 Hz), and high-frequency squealing region (> 5000 Hz). In this study, the sound pressure with intensity larger than 70 dB is defined as highintensity noise. For the fixed brake system, only one main frequency (f 1 ) can be found in the low-frequency region, while four visible frequencies can be seen in the high-frequency region, multiples of the fundamental frequency f 1 . This phenomenon suggests that the unstable vibration of the friction system is aroused from model coupling [52]. Meanwhile, the results reveal there are three frequencies with their energy values exceeding 70 dB (f 1 , 2f 1 , 3f 1 ). While for the floating system, in the high-frequency squeal region, there is only one high-energy principal frequency (f 6 ). However, in the low-frequency squeal region, four main frequencies are identified. Moreover, only one frequency component (f 6 ), the energy value of which exceeds 70 dB. At the same time, the energy of the fundamental frequency in the floating system (f 6 ) is visibly lower than that of the fixed system (f 1 ). Therefore, it can be inferred that the floating connection mode can limit the friction-induced vibration and thus suppressing the high-frequency components in the noise spectrum. Table 1 registers the frequency components of the noise signal during the stable phase of the two systems, in which "×" represents the presence of the frequency. The frequency components of the two systems differ significantly. There are four frequency components in the highfrequency squeal region (> 5000 Hz) of the fixed friction Fig. 15 Comparison of RMS of tangential vibration acceleration between two friction systems system: 7263 ± 50 Hz, 10,890 ± 50 Hz, 14,500 ± 50 Hz, and 18,145 ± 50 Hz, while correspondingly the floating friction system has only one frequency of 17,065 Hz. It indicates that the block connection mode significantly affects the frequency of friction-induced vibration and noise. The natural frequency of the friction system is closely related to the structure of the system [52]. The elastic elements used in the floating connection system will change its structure, thus altering the natural frequency component of the system and decoupling the unstable FIVN.

Discussion
Finally, a discussion summarises the results to illustrate the relationship among the friction block fixation mode, interfacial tribology behavior, thermal response, and FIVN of the brake system is described in Fig. 19, highlighting the importance of friction blocks connection configuration. The fixation mode can influence the interfacial properties by changing the system structure, thus improving the thermal distribution and tribology behavior of the brake system. Before the experiment, the initial contact status of the floating joint block shows that the actual contact zone symmetrically distributes on the pad surface. As the test went on, the actual contact zone mainly evolves around the middle part of the pad surface, which produces a uniform distribution of contact stress. In contrast, the actual contact area of the fixed joint block is always concentrated on the leading and left edge of its surface during the whole braking process. Therefore, an uneven distribution of contact pressure is observed (see Fig. 3). In addition, Fig. 4 shows that the contact area of the floating friction block is always greater than that of the fixed type. Therefore, the contact status of the floating joint block is better than that of the fixed joint block.
The disparity in the contact status affects the degree of wear on the interface. Compared with the surface of the floating friction block, the surface of the fixed joint block is rougher, with massive abrasive particles being accumulated on the contact platforms. The results show that the surface of the fixed joint block is more severely worn than that of the floating joint block (Figs. 6, 7). The degree of interfacial wear affects the quantity and distribution of abrasive debris, which is repeatedly ground by the friction force, which ultimately affects the thermal response on the interface (Figs. 11, 12).
The interfacial wear features are closely related to the contact pressure distribution. The abrasive debris collected in the vicinity of the primary contact plateaus falls into the low spots or scratches of these plateaus due to gravity and interfacial vibration, gradually being accumulated, compacted, and eventually forming a new third body, i.e., the secondary contact plateaus. This result was verified by scanning electron microscopy observations (Fig. 8). The secondary contact plateaus are relatively higher than other contact platforms, which are the main contact pressure carriers. Both fixed and floating joint interfaces contain heavily worn regions with much abrasive debris. The results from the pressure distribution measurement (Figs. 9, 10) are consistent with the wear analysis (Figs. 6, 7). Moreover, it can further affect the vibration and noise characteristics of the friction system (Figs. 13,14). It is worth noting that the braking system with the fixed joint block produces a higher intensity of vibration (Fig. 15). It leads to partial impact and relative slip with high velocity and frequency at the interface. Also, it increases local normal and shear stress, thus directly affecting contact pressure distribution at the brake interface.
In addition, connection modes of friction block can directly impact vibration noise features of the system. Systematic natural frequency is closely related to the system   structure. The floating connection mode changes the structure of the brake system by introducing elastic elements and thus alerts the frequency of its vibration response. As shown in Fig. 18 and Table 1, the systematic vibration and noise frequency are significantly changed. The frequency components in the high-frequency squeal region are reduced with the floating connection mode. Besides, elastic elements in the floating joint mode can reduce the vibration and noise to a lower level. Notably, as documented in the literature, braking noise is determined by various parameters, and it is generally difficult to fully control them. The unstable excitation patterns recovered in the experiments are related to the setup mode or/and are partially influenced by the boundary conditions, which may differ from the real braking system. Therefore, the vibration noise results obtained from the experiments may have limitations. However, this work still has some referential value since the results indicate that the connection mode of the friction block has a significant influence on FIVN. From the perspective of qualitative research, the friction-induced vibration and noise performance of the floating block is much better than the fixed block. In summary, by changing the structure of the brake system, connection modes of the friction block have considerable effects on the interfacial characteristics, thermal response, and FIVN. The floating connection mode effectively suppresses the vibration noise, improves the interfacial contact status, and decreases the wear degree, significantly affecting the systematic vibration noise features. Contact stress concentration expands as the degree of interfacial wear grows, thus increasing the intensity of the unstable friction-induced vibration and noise generated at the brake interface. Interfacial characteristics, thermal response, and vibration noise properties of the brake system affect the safety of railway operations and passenger comfort significantly. The connection mode of friction block is a critical factor affecting such properties of brake systems of the high-speed train. Therefore, it is possible to improve the interface characteristics, thermal response, and vibration noise performance of the brake interface by changing the connection mode of the friction block.

Conclusions
The experimental method is adopted to systematically study the relationship between connection configuration and system performance (interface characteristics, thermal response, and vibration noise behavior). The conclusions with the parameters in this study are summarized as follows: 1. The friction blocks connection configuration has significant influences on interface characteristics. Compared with the fixed connection mode, a more even contact status, larger contact area, and more minor contact stress concentration phenomenon can be obtained for the floating joint block. The worn degree is improved, and only a relatively small build-up of abrasive debris is observed. More contact platforms become the predominant bearing regions. At the same time, the accumulated and compacted abrasive debris near the primary contact plateaus cause the formation of a new third body, i.e., the secondary contact plateau, which leads to pressure concentration. 2. The friction blocks connection configuration has significant influences on frictional interface thermal response. Contact status of the fixed joint block is inferior to that of the floating joint block during the test. Much abrasive debris is produced on the severely worn surface of the fixed joint block, which is then repeatedly ground by the friction force, causing much heat and raising the temperature on the interface during the test. However, thermal distribution characteristics are improved by the floating joint block. 3. The friction blocks connection configuration has significant influences on vibration noise behavior. Compared with the fixed connection mode, the floating friction block can adjust the pressure distribution at the interface, and the systematic vibration noise behavior can be improved. In addition, the floating connection mode of friction blocks can suppress the vibration noise intensity through the structural characteristics of elastic elements. The vibration noise in the high-frequency squeal region is significantly reduced.