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The effect of groove-textured surface on friction and wear and friction-induced vibration and noise
Wear 301 (2013) 671–681
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The effect of groove-textured surface on friction and wear and friction-induced vibration and noise J.L. Mo a,n, Z.G. Wang a, G.X. Chen a, T.M. Shao b, M.H. Zhu a, Z.R. Zhou a a b
Tribology Research Institute, Traction Power State Key Laboratory, Southwest Jiaotong University, Chengdu 610031, China State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
abstract
Article history: Received 1 September 2012 Received in revised form 23 January 2013 Accepted 26 January 2013 Available online 5 February 2013
This paper presents an experimental study on the influence of groove-textured surface on tribological behaviors and friction-induced vibration and noise properties. Groove-textured surfaces with different widths and pitches were manufactured on compacted graphite iron materials (brake disc material) by electromachining. The difference between the groove-textured and original smooth surfaces in friction and wear and vibration and noise properties was studied, by using a developed device which is able to synchronously measure and analyze the friction force, vibration acceleration and noise signals in a ballon-flat reciprocating sliding configuration. It is shown that the squeal generated from the groovetextured surface was more influenced by the dimensional proportion of groove width to pitch, instead independently by groove width or pitch. Groove-textured surfaces with a specific dimensional proportion of groove width to pitch of 1/2, i.e., the width of groove equal to the width of ridge, showed good potential in reducing and suppressing squeal. The groove was the dominant surface component of contact surface topography affecting the generation of squeal compared to the microscopic irregularities of the worn surface. The wave-fluctuations of the friction force caused by counterface ball sliding across the grooves were found to play a crucial role in the squeal generation, which can effectively disturb the self-excited vibration of the friction system and consequently reduce the tendency to squeal. & 2013 Elsevier B.V. All rights reserved.
Keywords: Surface texturing Friction noise Friction vibration Friction and wear
1. Introduction Friction-induced noise is usually observed when one metal counterface slides over another under certain conditions which can be generally classified into two categories: (a) low frequency noise (about 10–500 Hz), termed chatter, moan or groan; (b) medium and high frequency noise (around 500–18000 Hz), called squeal or squeak. No precise definition of chatter and squeal has gained complete acceptance until now; however, it is generally agreed that squeal is a sustained, high frequency noise [1–5]. Compared to chatter, squeal is considered as a more serious problem and pollution for modern industry and living. Squeal noise is a widespread phenomenon in engineering systems with a sliding contact. Train-wheels on a curved track and brake systems are two typical tribo-systems that frequently generate squeal. Although many studies over the years have been conducted on the mechanisms causing squeal and large efforts have been made to reduce the squeal tendencies, the phenomenon is not yet fully understood and these efforts still no general solutions exist
n
Corresponding author. Tel.: þ86 28 87600601; fax: þ 86 28 87603142. E-mail addresses: [email protected], [email protected] (J.L. Mo).
0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.082
[1–3,6–14]. Therefore, a deeper understanding of the actual source of excitation of the squeal in the friction interface is needed. It is reported that squeal noise induced by friction is influenced by the normal load, relative sliding speed, nature of the surfaces in contact and environment conditions, etc. Among them, surface topography is considered to play a key role in the squeal generation [1,6–14]. Dry contact sliding between two structures is frequently found to generate squeal in a certain topography condition of contact surface. Furthermore, different surface topographies may generate squeal at different frequencies. By means of experiment, many researchers studied the influence of surface topography on the characteristics of squeal noise. Chen et al. [6] investigated the correlation between squeal and topography characteristics of wear scar in reciprocating sliding. Researchers from Uppsala University in Sweden [7–9] studied the connection between brake pad surface topography and the occurrence of squeal noise, by comparing the pad surfaces after interrupting the testing under silent and squealing conditions. They also carried out brake test on brake discs which were shot-blasted to produce small pits on the disc surfaces, to investigate the influence of the disc topography on the generation of automotive disc brake squeal. Jibiki et al. [10] investigated the relation between friction
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noise and fretting behavior in a wide range of fretting conditions, and accordingly the mechanism of the friction noise is discussed. Sherif [11] studied the mechanism of squeal triggering or vanishing according to the measurement of the surface topography of a squealing pad/disc assembly. Eadie et al. [12] explored how to reduce the squeal tendencies in the case of wheel in curve track by the change of friction characteristics using a ‘third body’ and the modification of contact surface between wheel and rail. Rusli et al. [13] studied the effect of surface topography on the normal and tangential contact stiffness and the friction coefficient, which were considered to be the dominant factors of the squeal generation by means of mode-coupling analysis. Massi et al. [14–16] developed a simplified experimental brake set-up and conducted an experimental tribological analysis to investigate the role of the contact problems in the squeal phenomena. The above-mentioned studies showed that the microscopic topography of the contact surface has a strong influence on the generation of squeal. Massi et al. [14] found that the features of the third body and the surface topography were completely different in the cases of with and without squeal: several cracks and material exfoliations were observed on the surface layer of the pad material after squeal events, while the contact surfaces after braking without squeal were found to be smooth and compact. Rhee et al. [17] hypothesized that squeal can be due to an effect of ‘‘local hammering’’ between the contact surfaces which excites a mode of the structure. The ‘‘local hammering’’ was explained as material detachments or asperity formations and deformations by Chen et al. [6,18]. However, there still remains question about why squeal noise is generated in certain topography conditions but vanished in other conditions, and how the surface topography affects the generation of squeal, considering the random distribution of asperities and wear debris. There is very limited information in the literature on its physical background. Moreover, the complicated frictional contact surface with many uncertain factors made it difficult to ensure the repeatability of experiments. Therefore, there is a need to use a specific surface with good geometric repeatability to study the effect of surface topography on the characteristics of squeal noise. Researchers from Uppsala University in Sweden [9] had tried to use grit blasting as a tool to investigate fundamentals of the contact conditions leading to squeal. Spiral shaped patterns were grit blasted on both sides of the disc, which could turn out to disturb any self-excited disc vibrations and consequently reduce the tendency to squeal. The spiral shaped patterns they used can be classified as one kind of surface texturing, which has emerged in the last two decades as a viable option of surface engineering resulting in significant improvement in load capacity, wear resistance, friction coefficient etc. of tribological components [19]. Fundamental research works on different forms and shapes of surface texturing for tribological applications had been reported [20–29]. However, the interest of these studies is mainly in the effect of surface texturing on the friction and wear properties at dry and lubricated conditions. There are very few reports on the correlation between texturedsurface and friction-induced noise. Understanding the relationship between tribological behaviors and friction-induced noise properties of surface texturing can further investigate the generation mechanism of squeal and lead to the specification of optimized surface texturing for reducing squeal. In this work, groove-textured surfaces with different widths and pitches were manufactured on compacted graphite iron samples. A special device which is able to synchronously measure and analyze the friction force, vibration acceleration and noise properties in a ball-on-flat reciprocating sliding configuration was developed. An experimental study on the influence of groove-textured surface on friction and wear and friction-induced vibration and
noise properties was performed, and the mechanism of squeal generation and the possibility of reducing squeal by using grooved surface texturing were discussed.
2. Experimental procedure 2.1. Samples preparation Compacted graphite iron ( 3.5 wt% C, 2.5 wt% Si and 1.5 wt% Mn) with microhardness of HV0.03 240 kg/mm2 and elastic modulus (E) of 158 GPa was used as the experimental material. All the samples were cut from the brake discs of train to size of 10 mm 10 mm 20 mm. The samples were polished using 800-grit and 1200-grit silicon carbide abrasive paper and polishing cloth under a water stream, to a surface roughness of approximately 0.04 mm Ra. The surface roughness of the samples was measured by using stylus profilometry. Four measurements (at random position and across orientation) were recorded to calculate an average Ra value. Groove-textured surfaces with different widths and pitches were manufactured on the compacted graphite iron samples by electromachining. The geometrical sketch of groove-textured surface is shown in Fig. 1, and the geometrical parameters of the samples are shown in Table 1. The ball counterface was a 10 mm diameter chromium bearing steel ball (AISI 52100, HV0.05 510 kg/mm2, E¼210 GPa, 0.02 mm Ra). 2.2. Experimental test set-up A special device was developed to synchronously measure and analyze the friction force, vibration acceleration and noise signals during reciprocating sliding wear test in a ball-on-flat configuration, which mainly consists of tribological testing system, signal acquisition and analysis system and fixture system. A schematic view of the experimental set-up is shown in Fig. 2. Flat specimen (1) was fixed on the lower holder (2) which was mounted on the reciprocating sliding device (3). Ball specimen (4) was fixed on the upper holder (5) which mounted on a 2-D (X–Y) strain–gauge force sensor (6). This sensor was mounted on the two-dimensional (Y–Z) moving stage (7), and consequently the ball specimen can be motorized by a lateral positioning system with position encoder and a vertical positioning system with position encoder. To start the test, the moving stage moves down slowly to allow the upper holder carrying ball specimen to go through the horizontal bracket (8), and then make the ball and flat specimen contact with applying a constant normal load which is servo-controlled during the test. The upper holder is close sliding fit with the horizontal bracket which connects to the mounting frame (9) through two piezoelectric force sensors (10). Then, when flat specimen is driven to reciprocating sliding against counterface ball, a part of the friction force between the contact surfaces will be transmitted to the two piezoelectric force sensors. Another part of the friction force will be transmitted to the upper strain–gauge force sensor. The real value of friction force between the contact surfaces is the sum of the outputs of the strain–gauge and piezoelectric force sensors. It is noting that actual dynamic friction force is truly able to be recorded during the test by both the strain–gauge and piezoelectric force sensors; however, only the signal recorded by
Fig. 1. Geometrical sketch of the groove-textured surface.
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Table 1 Geometrical parameters of the groove-textured surfaces. Samples (T-x-y)
Pitch d (mm),
Width w (mm),
Depth h (mm)
T-300-150 T-1000-150 T-2000-150 T-500-125 T-500-150 T-500-250 T-1000-500
3007 10 10007 10 20007 10 5007 10 5007 10 5007 10 10007 10
150 7 10 150 7 10 150 7 10 125 7 10 150 7 10 250 7 10 5007 10
1007 10 1007 10 1007 10 1007 10 1007 10 1007 10 1007 10
Note: T-x-y represents groove-textured surface with pitch of x mm and groove width of y mm.
Fig. 2. Schematic of the test apparatus: (1) flat specimen, (2) lower holder for flat specimen, (3) reciprocating sliding device, (4) ball specimen, (5) upper holder for ball specimen, (6) strain–gauge force sensor, (7) two-dimensional moving stage, (8) horizontal bracket, (9) mounting frame, (10) piezoelectric force sensor, (11) three-dimensional acceleration sensor and (12) microphone.
the piezoelectric force sensor with high frequency response can be used to the analysis of high-frequency squeal noise. A threedimensional acceleration sensor (11) was mounted on the upper holder to analyze the vibration of the contact surface. A microphone (12) was located nearby the friction interface to measure the noise signal.
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a MUELLER-BBM 32-channels vibration and sound measurement and analysis system. The sampling frequency of the acquisition system was set at 12.8 kHz, considering a frequency measurement range of 0–5000 kHz is sufficient to measure all the dominant frequencies of squeal in the present experimental conditions. The dominant frequencies of the friction-induced noise in this study are above 500 Hz, which are mainly referred to squeal, since the system stiffness of the experimental set-up is large. The background noise was measured and analyzed, which has dominant frequency of about 331 Hz. It is significantly different from the frequency of the squeal we concerned and has no influence on the squeal analysis. Impact hammer test was performed to measure the natural frequency of the tribo-system at the position of the counterface ball in contact with the flat sample surface. The upper holder was stroked impulsively by a hammer (KISLTLER 9724A5000) along both the friction (X) and normal (Y) directions and the impulse response (activated vibrations) of the system were measured and analyzed. The tribo-system was identified by the state-space method using the hammering impulse force as input and the acceleration in X or Y direction as an output. The results of the tribo-system identification are presented in Table 2. All the tests in this work were conducted under atmospheric conditions with controlled relative humidity of 60 710% RH and at room temperature of around 25 1C. Each test was repeated at least four times to ensure good testing repeatability, considering that the generation of squeal is somewhat random. The tribological testing parameters are as following: normal load of 20 N, sliding displacement of 4 mm at frequency of 1 Hz, testing time of 1500 s corresponding to number of cycles of 1500. The maximum (initial) Hertzian contact stress was calculated to be 1.15 GPa for the applied normal load of 20 N. The surfaces of the ball and
Table 2 Natural frequencies and damping ratios of the tribo-system. In friction (X) direction
In normal (Y) direction
Frequency (Hz)
Damping ratio
Frequency (Hz)
Damping ratio
645 1662 2368
0.0176 0.0180 0.0317
646 1666 2381
0.0170 0.0150 0.0433
2.3. Instrumentation, acquisition and experimental parameters The measurement range and resolution of the strain–gauge force sensor (CETR DFH-10) is 1–100 N and 0.005 N, respectively. The two-dimensional (Y–Z) moving stage has position resolution of 2 mm at vertical positioning direction (Y axis) and 1 mm at lateral positioning direction (Z axis). The reciprocating sliding device (CETR R35HE) can provide linear motion with adjustable stroke length and variable frequency up to 60 Hz. The piezoelectric force sensor (KISTLER 9712B500) has measurement range up to 2225 N, sensitivity of 2.23 mV/N and natural frequency of 70 kHz. The vibration of the friction system is measured by a three-dimensional acceleration sensor (KISTLER 8688A50). The accelerometer measurement range is 750 g, frequency range is 0.5 Hz to 5 kHz, sensitivity is 100 mV/g, and mass is 6.5 g. The friction noise is recorded by a 1/200 electret condenser microphone (MTG MK250) which was placed at about 40 mm away from the friction interface. The microphone dynamic range is 15–146 dB, frequency range is 3.5–20 kHz and sensitivity is 50 mV/Pa. All the signals from piezoelectric force sensor, acceleration sensor and microphone are acquired and analyzed synchronously by
Fig. 3. Equivalent noise pressure level of the smooth surface and different groovetextured surfaces.
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Fig. 4. Power spectrum density analysis of friction force (a), vibration accelerations in friction (X) direction (b) and in normal (Y) direction (c), and sound pressure (d).
flat specimens were cleaned with alcohol and acetone before testing, and were always changed for each test. After the tests, the wear scars were studied using stylus profilometry (Aep NanoMap-D), scanning electron microscopy (SEM, JEOL JSM-6610LV)
and energy dispersive X-ray spectroscopy (EDX, OXFORD X-MAX50 INCA-250), and the vibration and noise properties of the samples were considered in relation to their tribological behaviors.
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to significantly reduce squeal level and even suppress squeal generation compared to the smooth surface. Among them, T-500250 showed the most potential in squeal suppression, as no presence of squeal can be measured throughout the whole test. The squeal level was found to increase with the increase of pitch when comparing the cases of T-300-150, T-1000-150 and T-2000500 groove-textured surfaces which have the same groove width. Similarly, groove-textured surface with a larger groove width was found to show more potential in squeal reduction by comparison of the T-500-125, T-500-150 and T-500-250 groove-textured surfaces having the same pitch. Interestingly, all the three groove-textured surfaces (T-300-150, T-500-250 and T-1000500) had a specific dimensional proportion of groove width to pitch of 1/2, i.e., the width of groove equal to the width of ridge, showed good potential in squeal reduction and suppression. Therefore, the squeal generated from the groove-textured surface seems to more influenced by the dimensional proportion of groove width to pitch, instead independently by groove width or pitch. The groove-textured surface with a certain dimensional proportion of groove width to pitch could significantly reduce squeal. 3.2. Frequency spectrum analysis of friction force and frictioninduced vibration and noise
Fig. 5. Coherence analysis of friction force, vibration accelerations and sound pressure of the T-500-125 (a) and T-500-250 (b) groove-textured surfaces.
There are close correlation among microscopic topography of contact surface, friction force and friction-induced vibration and noise [6–14,30–34]. To further investigate the influence of surface topography on the generation of squeal, spectrum analysis of friction force and friction-induced vibration and noise signals was conducted and the inherent correlation between these signals was discussed. Fig. 4 shows the power spectrum density (PSD) of friction force, vibration accelerations and noise pressure of smooth surface and different groove-textured surfaces, which indicated how the powers of these signals were distributed in the frequency domain. All the signals of the surfaces which generated squeal showed the same dominant frequency of about 1500 Hz, though these surfaces are quite different in topography. The signals of the T-300-150 and T-1000-500 groove-textured surfaces showed lower power density level at the dominant frequency. In contrast, no dominant frequency at about 1500 Hz can be observed for all the signals of the T-500-250 groovetextured surface, on which no presence of squeal can be measured throughout the whole test. It has been reported that whichever of tribogical configurations, such as pin-disc, reciprocating sliding or pad-disc is used, there is always a strong coupling between the normal and tangential vibrations of friction systems when squeal occurs [35–38]. Coherence analysis of friction force, vibration accelerations in friction (X) and normal (Y) directions, and sound pressure signals were performed to further investigate their inherent correlation. The coherence function is expressed by
3. Results and discussion
2
C xy ðf Þ ¼ 3.1. The effect on groove-textured surface on noise pressure Equivalent continuous A-weighted sound pressure level of each 100 s test duration was evaluated. The equivalent noise pressure level of smooth surface and different groove-textured surfaces as a function of testing time is shown in Fig. 3. The friction-induced noise in this work was classified as squeal according to the frequency analysis which is shown in the following section. During testing, the background sound pressure level without squeal was about 63–65 dB (Leq). The T-300-150, T-500-250 and T-1000-500 groove-textured surfaces were found
9Pxy ðf Þ9 Pxx ðf ÞPyy ðf Þ
ð1Þ
where Cxy( f ) is the coherence function, Pxy(f ) is the cross spectrum of signals x and y, Pxx( f ) and Pyy( f ) is the power spectra of signals x and y, respectively, and f is the frequency. If Cxy( f ) is more than 0.6, signal x is considered to have a good coherence with signal y [18]. The coherence analysis results of these signals are shown in Fig. 5. For the T-500-125 groove-textured surface with significant squeal generation, the correlation coefficients between friction force and vibration accelerations and sound pressure were nearly 1 at the dominant frequency of about 1500 Hz (Fig. 5a). Similar
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Fig. 6. Time history records of friction force (a), vibration acceleration in friction (X) direction (b) and (Y) direction (c), and sound pressure (d).
coherence analysis results were observed for all the surfaces with significant presence of squeal. In contrast, the correlation coefficients evaluated for the squeal-free T-500-250 groove-textured
surface was very low at the dominant frequency of about 1500 Hz (Fig. 5b). The results presented above showed that the squeal is very closely correlated with the friction force and the vibrations
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Fig. 7. Friction force curves at the testing time of about 100 s (a) and at the end of the test (b), measured by strain–gauge force sensor.
in both friction (X) and normal (Y) directions at the dominant frequency of about 1500 Hz. This dominant frequency is very close to the natural frequency of the tribo-system shown in Table 2. Therefore, squeal was found to originate from the unstable vibratory response due to friction-induced vibration corresponding to mainly one natural frequency of the tribosystem. The squeal phenomenon in this work was mainly caused by the local dynamics at the contact surface. 3.3. Time history records of friction force and friction-induced vibration and noise The time history records of friction force, vibration accelerations and sound pressure signals are shown in Fig. 6. The testing time from 1499 s to 1500 s was studied considering that the squeal generation had become steady in this period for all the surfaces. For both the smooth surface and the T-500-125 groovetextured surface, squeal was found to mainly occur within half cycle, where significant continuous high-frequency fluctuations of friction force and vibration accelerations in both friction (X) and normal (Y) directions were observed. Minor and discontinuous
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high-frequency fluctuations were observed for the T-1000-500 groove-textured surface, which were responsible for the generation of the squeal with lower level. In contrast, nearly no highfrequency fluctuations of friction force and vibration accelerations were observed for the T-500-250 groove-textured surface without squeal. In the cases of both the T-500-250 and T-1000-500 groovetextured surfaces, the friction forces changed significantly when counterface ball slid across the grooves, as can be seen in Fig. 6(a). There was about 8 and 4 wave-fluctuations can be found in the friction force curve of T-500-250 and T-1000-500 groove-textured surface within half cycle (one stroke), respectively, which was corresponding to the number of grooves within the sliding displacement of 4 mm. This change of friction force is thought to be correlated with the change of friction contact conditions when counterface ball slides across the grooves [39,40]. There are also about 8 grooves on the T-500-125 groove-textured surface within the sliding displacement of 4 mm. However, no wavefluctuation can be observed in the friction force curve of this surface throughout this cycle. To further investigate this phenomenon, the output signals of the strain–gauge force sensor were used. Fig. 7 shows the friction force curves recorded by the strain–gauge force sensor. Visible wave-fluctuations in the friction force curves were observed for all the three groove-textured surfaces at the testing time of about 100 s; however, no presence of wave-fluctuations can be found in the friction force curve of the T-500-125 groove-textured surface anymore at the end of the test. This may suggests that the wavefluctuations of the friction force caused by counterface ball sliding across the grooves did not cause continuous unstable vibrations and consequently the generation of squeal. Instead, the wavefluctuations could help to disturb and suppress the generation of continuous high-frequency fluctuations of friction force and vibration accelerations which was considered as the major cause of squeal [9]. Furthermore, time-frequency analysis of friction force and vibration acceleration in friction (X) direction was performed to investigate the frequency change within one cycle. As can be seen in Fig. 8, the change of friction contact conditions caused by the counterface ball sliding across the grooves did not change the dominant frequency of the friction force and vibration. However, significant suppression of energy distribution at the dominant frequency can be observed for both the T-500-250 and T-1000500 groove-textured surfaces which still had wave-fluctuations in their friction force curves throughout this cycle. The time– frequency spectra of the T-500-125 groove-textured surface were almost the same as those of the smooth surface, showing dominant frequencies of about 1500 Hz, because there are no wave-fluctuations in the friction force curve of the T-500-125 groove-textured surface anymore throughout this cycle.
3.4. Friction and wear analysis The real value of friction force between the contact surfaces is the sum of the outputs of the strain–gauge and piezoelectric force sensors, considering that the friction force was transmitted to both these sensors when flat specimen is driven to reciprocating sliding against counterface ball. Fig. 9 shows the steady friction coefficients of the smooth and groove-textured surfaces. The groove-textured surfaces exhibited higher friction coefficients compared to the smooth surface. However, no inherent correlation between the value of friction coefficient and the generation of squeal can be found in this work. The T-500-250 groove-textured surface without squeal showed higher friction coefficient compared to the smooth surface as well as the T-500-125 groove-textured
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Fig. 8. Time–frequency analysis of friction force (a) and vibration acceleration in friction (X) direction (b) corresponding to the time history records of these signals shown in Fig. 6.
Fig. 9. Steady friction coefficients of the smooth and groove-textured surfaces.
surface with significant presence of squeal. Besides, the friction coefficient of the groove-textured surface was found to increase with the increase of groove width and pitch.
Fig. 10 shows the SEM images and EDX spectra of the smooth and groove-textured surfaces. Observation of the worn surface morphologies indicates that the smooth surface underwent significant abrasive wear under reciprocating sliding condition. Features of ploughing, wear debris layer and detachment can be observed in the middle of the wear scar (Fig. 10a). The worn surface on the ridge of the T-500-125 groove-textured surface exhibited similar wear features (Fig. 10b). The morphology of the wear debris layer indicates that the worn surface suffered progressive surface degradation, experiencing continuous detachment of particles from its surface. EDX spectra of the wear debris layer and the underlying worn surface show a significant presence of oxygen and element from the ball material (Cr) in the wear debris layer, due to tribo-oxidation and transfer of the ball material occurring during the sliding wear test; however, no presence of elements from the ball material can be detected in the underlying worn surface (Fig. 10e). Eventually, the wear debris layer became detached after a higher number of sliding cycles. The exfoliation of the wear debris layer leads to the introduction of loose particles to the contact surface, which tends to induce the high-frequency fluctuation of friction force and consequently the generation of squeal [10,18]. It is noting that accumulation and exfoliation of the wear debris layer can also observed in the wear scars of the T-500-250 and T-1000-500
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Fig. 10. SEM images of the worn surfaces of the smooth surface sample (a), and the T-500-125 (b), T-500-250 (c) and T-1000-500 (d) groove-textured surfaces samples; and EDX spectra (e) of wear debris layer and the undamaged surface (each sampled as shown in b).
groove-textured surfaces (Fig. 10c and d). However, this phenomenon was relatively slighter compared to the situations with the smooth surface and T-500-125 groove-textured surface. The profiles of the wear scars were obtained by measuring across the sliding direction, as shown in Fig. 11. The worn surfaces on the ridges of the groove-textured surfaces were measured. The T-500-125 groove-textured surface exhibited smaller wear depth while the T-500-250 and T-1000-500 groove-textured surfaces showed higher wear depth, compared to the smooth surface. Fundamental experimental research work on various forms and shapes of surface texturing for tribological applications showed
that the surface texturing provides micro-traps to capture wear debris [19]. In this work, the T-500-250 and T-1000-500 groovetextured surfaces were found to allow an easier wear debris escape from the contact zones into the grooves compared to the T-500-125 groove-textured surface. The easier removal of wear debris from the contact surface will accelerate the wear and result in a higher wear depth due to continuously exposing fresh material surface, which also cause an increase of friction coefficient [30]. Accordingly, this can explain the relatively higher friction coefficients of the T-500-250 and T-1000-500 groovetextured surfaces as shown in Fig. 9.
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Fig. 11. Profiles of the wear scars.
3.5. Influence of groove-textured surface on the generation mechanisms and tendencies of squeal There is correlation between tribological behaviors and squeal generation on both macroscopic and microscopic level. The worn surface topography of groove-textured surface comprises two main components: macroscopic groove and microscopic features. Macroscopic groove consists of the widely spaced regularities that are produced by machining process. Microscopic features consists of the irregularities characteristics of the worn surface on ridge, such as asperities, ploughing, wear debris layer and detachment, etc. In this work, the groove was found to be the dominant surface component of contact surface topography affecting the generation of squeal compared to the microscopic irregularities of the worn surface on ridge. The high-frequency fluctuations of friction force mainly result from the irregularities characteristics of the worn surface, which is normally considered as the actual source of excitation of the squeal instability in the friction interface [6,13,17,18]. Groove-textured surface with specific geometry was found to allow an easier wear debris escape from the contact zones into the grooves, which would help to reduce the irregularities and consequently the squeal tendencies. However, most important of all, the groove-textured surface could effectively interrupt the continuous contact of the friction surfaces when the counterface ball slides across the grooves, which would suppress the generation of high-frequency fluctuations of the friction force, disturb the self-excited vibration of the friction system, and consequently significantly reduce squeal.
4. Conclusions Groove-textured surfaces with different widths and pitches were manufactured on compacted graphite iron materials. An experimental study was performed to investigate the influence of groove-textured surface on tribological behaviors and frictioninduced vibration and noise properties, by using a developed device which is able to synchronously measure and analyze the friction force, vibration acceleration and noise signals in a ball-onflat reciprocating sliding configuration. From the experimental results, it can be concluded as follows:
(1) The squeal generated from the groove-textured surface was mainly influenced by the dimensional proportion of groove
width to pitch, instead independently by groove width or pitch. In this work, groove-textured surfaces with a specific dimensional proportion of groove width to pitch of 1/2 can significantly reduce squeal. (2) All the friction force, vibration acceleration and noise pressure signals of the surfaces with significant presence of squeal showed the same dominant frequency of about 1500 Hz, though they are quite different in topography. Squeal is very closely correlated with the friction force and the vibrations in both friction and normal directions at the dominant frequency which is very close to the natural frequency of the tribo-system. The squeal in this work was mainly caused by the local dynamics at the contact surface which excited a mode of the tribo-system. (3) No inherent correlation between the value of friction coefficient and the generation of squeal can be observed in this work. Squeal was found to occur associated with significant continuous high-frequency fluctuations of friction force and vibration accelerations, which were mainly caused by the microscopic irregularities of the worn surface, such as ploughing, wear debris layer and detachment. Groove-textured surface with specific geometry allowed an easier wear debris escape from the contact zones into the grooves, which would help to reduce the irregularities and consequently the squeal tendencies. Moreover, wave-fluctuations of the friction force caused by counterface ball sliding across the grooves can effectively disturb the self-excited vibration of the friction system and consequently reduce the tendency to squeal. For the groove-textured surface in this study, the groove was found to be the dominant surface component of contact surface topography affecting the generation of squeal compared to the microscopic irregularities of the worn surface on ridge.
Acknowledgements The authors would like to thank senior engineer X.Y. Shi, Southwest Jiaotong University, for helpful discussions. The authors are grateful for the financial support of the National Scientific Foundation of China (No. 51005191 and No. U1134103), and the National Outstanding Young Scientists Foundation of China (51025519).
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Wear journal homepage: www.elsevier.com/locate/wear
The effect of groove-textured surface on friction and wear and friction-induced vibration and noise J.L. Mo a,n, Z.G. Wang a, G.X. Chen a, T.M. Shao b, M.H. Zhu a, Z.R. Zhou a a b
Tribology Research Institute, Traction Power State Key Laboratory, Southwest Jiaotong University, Chengdu 610031, China State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
abstract
Article history: Received 1 September 2012 Received in revised form 23 January 2013 Accepted 26 January 2013 Available online 5 February 2013
This paper presents an experimental study on the influence of groove-textured surface on tribological behaviors and friction-induced vibration and noise properties. Groove-textured surfaces with different widths and pitches were manufactured on compacted graphite iron materials (brake disc material) by electromachining. The difference between the groove-textured and original smooth surfaces in friction and wear and vibration and noise properties was studied, by using a developed device which is able to synchronously measure and analyze the friction force, vibration acceleration and noise signals in a ballon-flat reciprocating sliding configuration. It is shown that the squeal generated from the groovetextured surface was more influenced by the dimensional proportion of groove width to pitch, instead independently by groove width or pitch. Groove-textured surfaces with a specific dimensional proportion of groove width to pitch of 1/2, i.e., the width of groove equal to the width of ridge, showed good potential in reducing and suppressing squeal. The groove was the dominant surface component of contact surface topography affecting the generation of squeal compared to the microscopic irregularities of the worn surface. The wave-fluctuations of the friction force caused by counterface ball sliding across the grooves were found to play a crucial role in the squeal generation, which can effectively disturb the self-excited vibration of the friction system and consequently reduce the tendency to squeal. & 2013 Elsevier B.V. All rights reserved.
Keywords: Surface texturing Friction noise Friction vibration Friction and wear
1. Introduction Friction-induced noise is usually observed when one metal counterface slides over another under certain conditions which can be generally classified into two categories: (a) low frequency noise (about 10–500 Hz), termed chatter, moan or groan; (b) medium and high frequency noise (around 500–18000 Hz), called squeal or squeak. No precise definition of chatter and squeal has gained complete acceptance until now; however, it is generally agreed that squeal is a sustained, high frequency noise [1–5]. Compared to chatter, squeal is considered as a more serious problem and pollution for modern industry and living. Squeal noise is a widespread phenomenon in engineering systems with a sliding contact. Train-wheels on a curved track and brake systems are two typical tribo-systems that frequently generate squeal. Although many studies over the years have been conducted on the mechanisms causing squeal and large efforts have been made to reduce the squeal tendencies, the phenomenon is not yet fully understood and these efforts still no general solutions exist
n
Corresponding author. Tel.: þ86 28 87600601; fax: þ 86 28 87603142. E-mail addresses: [email protected], [email protected] (J.L. Mo).
0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.082
[1–3,6–14]. Therefore, a deeper understanding of the actual source of excitation of the squeal in the friction interface is needed. It is reported that squeal noise induced by friction is influenced by the normal load, relative sliding speed, nature of the surfaces in contact and environment conditions, etc. Among them, surface topography is considered to play a key role in the squeal generation [1,6–14]. Dry contact sliding between two structures is frequently found to generate squeal in a certain topography condition of contact surface. Furthermore, different surface topographies may generate squeal at different frequencies. By means of experiment, many researchers studied the influence of surface topography on the characteristics of squeal noise. Chen et al. [6] investigated the correlation between squeal and topography characteristics of wear scar in reciprocating sliding. Researchers from Uppsala University in Sweden [7–9] studied the connection between brake pad surface topography and the occurrence of squeal noise, by comparing the pad surfaces after interrupting the testing under silent and squealing conditions. They also carried out brake test on brake discs which were shot-blasted to produce small pits on the disc surfaces, to investigate the influence of the disc topography on the generation of automotive disc brake squeal. Jibiki et al. [10] investigated the relation between friction
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noise and fretting behavior in a wide range of fretting conditions, and accordingly the mechanism of the friction noise is discussed. Sherif [11] studied the mechanism of squeal triggering or vanishing according to the measurement of the surface topography of a squealing pad/disc assembly. Eadie et al. [12] explored how to reduce the squeal tendencies in the case of wheel in curve track by the change of friction characteristics using a ‘third body’ and the modification of contact surface between wheel and rail. Rusli et al. [13] studied the effect of surface topography on the normal and tangential contact stiffness and the friction coefficient, which were considered to be the dominant factors of the squeal generation by means of mode-coupling analysis. Massi et al. [14–16] developed a simplified experimental brake set-up and conducted an experimental tribological analysis to investigate the role of the contact problems in the squeal phenomena. The above-mentioned studies showed that the microscopic topography of the contact surface has a strong influence on the generation of squeal. Massi et al. [14] found that the features of the third body and the surface topography were completely different in the cases of with and without squeal: several cracks and material exfoliations were observed on the surface layer of the pad material after squeal events, while the contact surfaces after braking without squeal were found to be smooth and compact. Rhee et al. [17] hypothesized that squeal can be due to an effect of ‘‘local hammering’’ between the contact surfaces which excites a mode of the structure. The ‘‘local hammering’’ was explained as material detachments or asperity formations and deformations by Chen et al. [6,18]. However, there still remains question about why squeal noise is generated in certain topography conditions but vanished in other conditions, and how the surface topography affects the generation of squeal, considering the random distribution of asperities and wear debris. There is very limited information in the literature on its physical background. Moreover, the complicated frictional contact surface with many uncertain factors made it difficult to ensure the repeatability of experiments. Therefore, there is a need to use a specific surface with good geometric repeatability to study the effect of surface topography on the characteristics of squeal noise. Researchers from Uppsala University in Sweden [9] had tried to use grit blasting as a tool to investigate fundamentals of the contact conditions leading to squeal. Spiral shaped patterns were grit blasted on both sides of the disc, which could turn out to disturb any self-excited disc vibrations and consequently reduce the tendency to squeal. The spiral shaped patterns they used can be classified as one kind of surface texturing, which has emerged in the last two decades as a viable option of surface engineering resulting in significant improvement in load capacity, wear resistance, friction coefficient etc. of tribological components [19]. Fundamental research works on different forms and shapes of surface texturing for tribological applications had been reported [20–29]. However, the interest of these studies is mainly in the effect of surface texturing on the friction and wear properties at dry and lubricated conditions. There are very few reports on the correlation between texturedsurface and friction-induced noise. Understanding the relationship between tribological behaviors and friction-induced noise properties of surface texturing can further investigate the generation mechanism of squeal and lead to the specification of optimized surface texturing for reducing squeal. In this work, groove-textured surfaces with different widths and pitches were manufactured on compacted graphite iron samples. A special device which is able to synchronously measure and analyze the friction force, vibration acceleration and noise properties in a ball-on-flat reciprocating sliding configuration was developed. An experimental study on the influence of groove-textured surface on friction and wear and friction-induced vibration and
noise properties was performed, and the mechanism of squeal generation and the possibility of reducing squeal by using grooved surface texturing were discussed.
2. Experimental procedure 2.1. Samples preparation Compacted graphite iron ( 3.5 wt% C, 2.5 wt% Si and 1.5 wt% Mn) with microhardness of HV0.03 240 kg/mm2 and elastic modulus (E) of 158 GPa was used as the experimental material. All the samples were cut from the brake discs of train to size of 10 mm 10 mm 20 mm. The samples were polished using 800-grit and 1200-grit silicon carbide abrasive paper and polishing cloth under a water stream, to a surface roughness of approximately 0.04 mm Ra. The surface roughness of the samples was measured by using stylus profilometry. Four measurements (at random position and across orientation) were recorded to calculate an average Ra value. Groove-textured surfaces with different widths and pitches were manufactured on the compacted graphite iron samples by electromachining. The geometrical sketch of groove-textured surface is shown in Fig. 1, and the geometrical parameters of the samples are shown in Table 1. The ball counterface was a 10 mm diameter chromium bearing steel ball (AISI 52100, HV0.05 510 kg/mm2, E¼210 GPa, 0.02 mm Ra). 2.2. Experimental test set-up A special device was developed to synchronously measure and analyze the friction force, vibration acceleration and noise signals during reciprocating sliding wear test in a ball-on-flat configuration, which mainly consists of tribological testing system, signal acquisition and analysis system and fixture system. A schematic view of the experimental set-up is shown in Fig. 2. Flat specimen (1) was fixed on the lower holder (2) which was mounted on the reciprocating sliding device (3). Ball specimen (4) was fixed on the upper holder (5) which mounted on a 2-D (X–Y) strain–gauge force sensor (6). This sensor was mounted on the two-dimensional (Y–Z) moving stage (7), and consequently the ball specimen can be motorized by a lateral positioning system with position encoder and a vertical positioning system with position encoder. To start the test, the moving stage moves down slowly to allow the upper holder carrying ball specimen to go through the horizontal bracket (8), and then make the ball and flat specimen contact with applying a constant normal load which is servo-controlled during the test. The upper holder is close sliding fit with the horizontal bracket which connects to the mounting frame (9) through two piezoelectric force sensors (10). Then, when flat specimen is driven to reciprocating sliding against counterface ball, a part of the friction force between the contact surfaces will be transmitted to the two piezoelectric force sensors. Another part of the friction force will be transmitted to the upper strain–gauge force sensor. The real value of friction force between the contact surfaces is the sum of the outputs of the strain–gauge and piezoelectric force sensors. It is noting that actual dynamic friction force is truly able to be recorded during the test by both the strain–gauge and piezoelectric force sensors; however, only the signal recorded by
Fig. 1. Geometrical sketch of the groove-textured surface.
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Table 1 Geometrical parameters of the groove-textured surfaces. Samples (T-x-y)
Pitch d (mm),
Width w (mm),
Depth h (mm)
T-300-150 T-1000-150 T-2000-150 T-500-125 T-500-150 T-500-250 T-1000-500
3007 10 10007 10 20007 10 5007 10 5007 10 5007 10 10007 10
150 7 10 150 7 10 150 7 10 125 7 10 150 7 10 250 7 10 5007 10
1007 10 1007 10 1007 10 1007 10 1007 10 1007 10 1007 10
Note: T-x-y represents groove-textured surface with pitch of x mm and groove width of y mm.
Fig. 2. Schematic of the test apparatus: (1) flat specimen, (2) lower holder for flat specimen, (3) reciprocating sliding device, (4) ball specimen, (5) upper holder for ball specimen, (6) strain–gauge force sensor, (7) two-dimensional moving stage, (8) horizontal bracket, (9) mounting frame, (10) piezoelectric force sensor, (11) three-dimensional acceleration sensor and (12) microphone.
the piezoelectric force sensor with high frequency response can be used to the analysis of high-frequency squeal noise. A threedimensional acceleration sensor (11) was mounted on the upper holder to analyze the vibration of the contact surface. A microphone (12) was located nearby the friction interface to measure the noise signal.
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a MUELLER-BBM 32-channels vibration and sound measurement and analysis system. The sampling frequency of the acquisition system was set at 12.8 kHz, considering a frequency measurement range of 0–5000 kHz is sufficient to measure all the dominant frequencies of squeal in the present experimental conditions. The dominant frequencies of the friction-induced noise in this study are above 500 Hz, which are mainly referred to squeal, since the system stiffness of the experimental set-up is large. The background noise was measured and analyzed, which has dominant frequency of about 331 Hz. It is significantly different from the frequency of the squeal we concerned and has no influence on the squeal analysis. Impact hammer test was performed to measure the natural frequency of the tribo-system at the position of the counterface ball in contact with the flat sample surface. The upper holder was stroked impulsively by a hammer (KISLTLER 9724A5000) along both the friction (X) and normal (Y) directions and the impulse response (activated vibrations) of the system were measured and analyzed. The tribo-system was identified by the state-space method using the hammering impulse force as input and the acceleration in X or Y direction as an output. The results of the tribo-system identification are presented in Table 2. All the tests in this work were conducted under atmospheric conditions with controlled relative humidity of 60 710% RH and at room temperature of around 25 1C. Each test was repeated at least four times to ensure good testing repeatability, considering that the generation of squeal is somewhat random. The tribological testing parameters are as following: normal load of 20 N, sliding displacement of 4 mm at frequency of 1 Hz, testing time of 1500 s corresponding to number of cycles of 1500. The maximum (initial) Hertzian contact stress was calculated to be 1.15 GPa for the applied normal load of 20 N. The surfaces of the ball and
Table 2 Natural frequencies and damping ratios of the tribo-system. In friction (X) direction
In normal (Y) direction
Frequency (Hz)
Damping ratio
Frequency (Hz)
Damping ratio
645 1662 2368
0.0176 0.0180 0.0317
646 1666 2381
0.0170 0.0150 0.0433
2.3. Instrumentation, acquisition and experimental parameters The measurement range and resolution of the strain–gauge force sensor (CETR DFH-10) is 1–100 N and 0.005 N, respectively. The two-dimensional (Y–Z) moving stage has position resolution of 2 mm at vertical positioning direction (Y axis) and 1 mm at lateral positioning direction (Z axis). The reciprocating sliding device (CETR R35HE) can provide linear motion with adjustable stroke length and variable frequency up to 60 Hz. The piezoelectric force sensor (KISTLER 9712B500) has measurement range up to 2225 N, sensitivity of 2.23 mV/N and natural frequency of 70 kHz. The vibration of the friction system is measured by a three-dimensional acceleration sensor (KISTLER 8688A50). The accelerometer measurement range is 750 g, frequency range is 0.5 Hz to 5 kHz, sensitivity is 100 mV/g, and mass is 6.5 g. The friction noise is recorded by a 1/200 electret condenser microphone (MTG MK250) which was placed at about 40 mm away from the friction interface. The microphone dynamic range is 15–146 dB, frequency range is 3.5–20 kHz and sensitivity is 50 mV/Pa. All the signals from piezoelectric force sensor, acceleration sensor and microphone are acquired and analyzed synchronously by
Fig. 3. Equivalent noise pressure level of the smooth surface and different groovetextured surfaces.
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Fig. 4. Power spectrum density analysis of friction force (a), vibration accelerations in friction (X) direction (b) and in normal (Y) direction (c), and sound pressure (d).
flat specimens were cleaned with alcohol and acetone before testing, and were always changed for each test. After the tests, the wear scars were studied using stylus profilometry (Aep NanoMap-D), scanning electron microscopy (SEM, JEOL JSM-6610LV)
and energy dispersive X-ray spectroscopy (EDX, OXFORD X-MAX50 INCA-250), and the vibration and noise properties of the samples were considered in relation to their tribological behaviors.
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to significantly reduce squeal level and even suppress squeal generation compared to the smooth surface. Among them, T-500250 showed the most potential in squeal suppression, as no presence of squeal can be measured throughout the whole test. The squeal level was found to increase with the increase of pitch when comparing the cases of T-300-150, T-1000-150 and T-2000500 groove-textured surfaces which have the same groove width. Similarly, groove-textured surface with a larger groove width was found to show more potential in squeal reduction by comparison of the T-500-125, T-500-150 and T-500-250 groove-textured surfaces having the same pitch. Interestingly, all the three groove-textured surfaces (T-300-150, T-500-250 and T-1000500) had a specific dimensional proportion of groove width to pitch of 1/2, i.e., the width of groove equal to the width of ridge, showed good potential in squeal reduction and suppression. Therefore, the squeal generated from the groove-textured surface seems to more influenced by the dimensional proportion of groove width to pitch, instead independently by groove width or pitch. The groove-textured surface with a certain dimensional proportion of groove width to pitch could significantly reduce squeal. 3.2. Frequency spectrum analysis of friction force and frictioninduced vibration and noise
Fig. 5. Coherence analysis of friction force, vibration accelerations and sound pressure of the T-500-125 (a) and T-500-250 (b) groove-textured surfaces.
There are close correlation among microscopic topography of contact surface, friction force and friction-induced vibration and noise [6–14,30–34]. To further investigate the influence of surface topography on the generation of squeal, spectrum analysis of friction force and friction-induced vibration and noise signals was conducted and the inherent correlation between these signals was discussed. Fig. 4 shows the power spectrum density (PSD) of friction force, vibration accelerations and noise pressure of smooth surface and different groove-textured surfaces, which indicated how the powers of these signals were distributed in the frequency domain. All the signals of the surfaces which generated squeal showed the same dominant frequency of about 1500 Hz, though these surfaces are quite different in topography. The signals of the T-300-150 and T-1000-500 groove-textured surfaces showed lower power density level at the dominant frequency. In contrast, no dominant frequency at about 1500 Hz can be observed for all the signals of the T-500-250 groovetextured surface, on which no presence of squeal can be measured throughout the whole test. It has been reported that whichever of tribogical configurations, such as pin-disc, reciprocating sliding or pad-disc is used, there is always a strong coupling between the normal and tangential vibrations of friction systems when squeal occurs [35–38]. Coherence analysis of friction force, vibration accelerations in friction (X) and normal (Y) directions, and sound pressure signals were performed to further investigate their inherent correlation. The coherence function is expressed by
3. Results and discussion
2
C xy ðf Þ ¼ 3.1. The effect on groove-textured surface on noise pressure Equivalent continuous A-weighted sound pressure level of each 100 s test duration was evaluated. The equivalent noise pressure level of smooth surface and different groove-textured surfaces as a function of testing time is shown in Fig. 3. The friction-induced noise in this work was classified as squeal according to the frequency analysis which is shown in the following section. During testing, the background sound pressure level without squeal was about 63–65 dB (Leq). The T-300-150, T-500-250 and T-1000-500 groove-textured surfaces were found
9Pxy ðf Þ9 Pxx ðf ÞPyy ðf Þ
ð1Þ
where Cxy( f ) is the coherence function, Pxy(f ) is the cross spectrum of signals x and y, Pxx( f ) and Pyy( f ) is the power spectra of signals x and y, respectively, and f is the frequency. If Cxy( f ) is more than 0.6, signal x is considered to have a good coherence with signal y [18]. The coherence analysis results of these signals are shown in Fig. 5. For the T-500-125 groove-textured surface with significant squeal generation, the correlation coefficients between friction force and vibration accelerations and sound pressure were nearly 1 at the dominant frequency of about 1500 Hz (Fig. 5a). Similar
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Fig. 6. Time history records of friction force (a), vibration acceleration in friction (X) direction (b) and (Y) direction (c), and sound pressure (d).
coherence analysis results were observed for all the surfaces with significant presence of squeal. In contrast, the correlation coefficients evaluated for the squeal-free T-500-250 groove-textured
surface was very low at the dominant frequency of about 1500 Hz (Fig. 5b). The results presented above showed that the squeal is very closely correlated with the friction force and the vibrations
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Fig. 7. Friction force curves at the testing time of about 100 s (a) and at the end of the test (b), measured by strain–gauge force sensor.
in both friction (X) and normal (Y) directions at the dominant frequency of about 1500 Hz. This dominant frequency is very close to the natural frequency of the tribo-system shown in Table 2. Therefore, squeal was found to originate from the unstable vibratory response due to friction-induced vibration corresponding to mainly one natural frequency of the tribosystem. The squeal phenomenon in this work was mainly caused by the local dynamics at the contact surface. 3.3. Time history records of friction force and friction-induced vibration and noise The time history records of friction force, vibration accelerations and sound pressure signals are shown in Fig. 6. The testing time from 1499 s to 1500 s was studied considering that the squeal generation had become steady in this period for all the surfaces. For both the smooth surface and the T-500-125 groovetextured surface, squeal was found to mainly occur within half cycle, where significant continuous high-frequency fluctuations of friction force and vibration accelerations in both friction (X) and normal (Y) directions were observed. Minor and discontinuous
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high-frequency fluctuations were observed for the T-1000-500 groove-textured surface, which were responsible for the generation of the squeal with lower level. In contrast, nearly no highfrequency fluctuations of friction force and vibration accelerations were observed for the T-500-250 groove-textured surface without squeal. In the cases of both the T-500-250 and T-1000-500 groovetextured surfaces, the friction forces changed significantly when counterface ball slid across the grooves, as can be seen in Fig. 6(a). There was about 8 and 4 wave-fluctuations can be found in the friction force curve of T-500-250 and T-1000-500 groove-textured surface within half cycle (one stroke), respectively, which was corresponding to the number of grooves within the sliding displacement of 4 mm. This change of friction force is thought to be correlated with the change of friction contact conditions when counterface ball slides across the grooves [39,40]. There are also about 8 grooves on the T-500-125 groove-textured surface within the sliding displacement of 4 mm. However, no wavefluctuation can be observed in the friction force curve of this surface throughout this cycle. To further investigate this phenomenon, the output signals of the strain–gauge force sensor were used. Fig. 7 shows the friction force curves recorded by the strain–gauge force sensor. Visible wave-fluctuations in the friction force curves were observed for all the three groove-textured surfaces at the testing time of about 100 s; however, no presence of wave-fluctuations can be found in the friction force curve of the T-500-125 groove-textured surface anymore at the end of the test. This may suggests that the wavefluctuations of the friction force caused by counterface ball sliding across the grooves did not cause continuous unstable vibrations and consequently the generation of squeal. Instead, the wavefluctuations could help to disturb and suppress the generation of continuous high-frequency fluctuations of friction force and vibration accelerations which was considered as the major cause of squeal [9]. Furthermore, time-frequency analysis of friction force and vibration acceleration in friction (X) direction was performed to investigate the frequency change within one cycle. As can be seen in Fig. 8, the change of friction contact conditions caused by the counterface ball sliding across the grooves did not change the dominant frequency of the friction force and vibration. However, significant suppression of energy distribution at the dominant frequency can be observed for both the T-500-250 and T-1000500 groove-textured surfaces which still had wave-fluctuations in their friction force curves throughout this cycle. The time– frequency spectra of the T-500-125 groove-textured surface were almost the same as those of the smooth surface, showing dominant frequencies of about 1500 Hz, because there are no wave-fluctuations in the friction force curve of the T-500-125 groove-textured surface anymore throughout this cycle.
3.4. Friction and wear analysis The real value of friction force between the contact surfaces is the sum of the outputs of the strain–gauge and piezoelectric force sensors, considering that the friction force was transmitted to both these sensors when flat specimen is driven to reciprocating sliding against counterface ball. Fig. 9 shows the steady friction coefficients of the smooth and groove-textured surfaces. The groove-textured surfaces exhibited higher friction coefficients compared to the smooth surface. However, no inherent correlation between the value of friction coefficient and the generation of squeal can be found in this work. The T-500-250 groove-textured surface without squeal showed higher friction coefficient compared to the smooth surface as well as the T-500-125 groove-textured
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Fig. 8. Time–frequency analysis of friction force (a) and vibration acceleration in friction (X) direction (b) corresponding to the time history records of these signals shown in Fig. 6.
Fig. 9. Steady friction coefficients of the smooth and groove-textured surfaces.
surface with significant presence of squeal. Besides, the friction coefficient of the groove-textured surface was found to increase with the increase of groove width and pitch.
Fig. 10 shows the SEM images and EDX spectra of the smooth and groove-textured surfaces. Observation of the worn surface morphologies indicates that the smooth surface underwent significant abrasive wear under reciprocating sliding condition. Features of ploughing, wear debris layer and detachment can be observed in the middle of the wear scar (Fig. 10a). The worn surface on the ridge of the T-500-125 groove-textured surface exhibited similar wear features (Fig. 10b). The morphology of the wear debris layer indicates that the worn surface suffered progressive surface degradation, experiencing continuous detachment of particles from its surface. EDX spectra of the wear debris layer and the underlying worn surface show a significant presence of oxygen and element from the ball material (Cr) in the wear debris layer, due to tribo-oxidation and transfer of the ball material occurring during the sliding wear test; however, no presence of elements from the ball material can be detected in the underlying worn surface (Fig. 10e). Eventually, the wear debris layer became detached after a higher number of sliding cycles. The exfoliation of the wear debris layer leads to the introduction of loose particles to the contact surface, which tends to induce the high-frequency fluctuation of friction force and consequently the generation of squeal [10,18]. It is noting that accumulation and exfoliation of the wear debris layer can also observed in the wear scars of the T-500-250 and T-1000-500
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Fig. 10. SEM images of the worn surfaces of the smooth surface sample (a), and the T-500-125 (b), T-500-250 (c) and T-1000-500 (d) groove-textured surfaces samples; and EDX spectra (e) of wear debris layer and the undamaged surface (each sampled as shown in b).
groove-textured surfaces (Fig. 10c and d). However, this phenomenon was relatively slighter compared to the situations with the smooth surface and T-500-125 groove-textured surface. The profiles of the wear scars were obtained by measuring across the sliding direction, as shown in Fig. 11. The worn surfaces on the ridges of the groove-textured surfaces were measured. The T-500-125 groove-textured surface exhibited smaller wear depth while the T-500-250 and T-1000-500 groove-textured surfaces showed higher wear depth, compared to the smooth surface. Fundamental experimental research work on various forms and shapes of surface texturing for tribological applications showed
that the surface texturing provides micro-traps to capture wear debris [19]. In this work, the T-500-250 and T-1000-500 groovetextured surfaces were found to allow an easier wear debris escape from the contact zones into the grooves compared to the T-500-125 groove-textured surface. The easier removal of wear debris from the contact surface will accelerate the wear and result in a higher wear depth due to continuously exposing fresh material surface, which also cause an increase of friction coefficient [30]. Accordingly, this can explain the relatively higher friction coefficients of the T-500-250 and T-1000-500 groovetextured surfaces as shown in Fig. 9.
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Fig. 11. Profiles of the wear scars.
3.5. Influence of groove-textured surface on the generation mechanisms and tendencies of squeal There is correlation between tribological behaviors and squeal generation on both macroscopic and microscopic level. The worn surface topography of groove-textured surface comprises two main components: macroscopic groove and microscopic features. Macroscopic groove consists of the widely spaced regularities that are produced by machining process. Microscopic features consists of the irregularities characteristics of the worn surface on ridge, such as asperities, ploughing, wear debris layer and detachment, etc. In this work, the groove was found to be the dominant surface component of contact surface topography affecting the generation of squeal compared to the microscopic irregularities of the worn surface on ridge. The high-frequency fluctuations of friction force mainly result from the irregularities characteristics of the worn surface, which is normally considered as the actual source of excitation of the squeal instability in the friction interface [6,13,17,18]. Groove-textured surface with specific geometry was found to allow an easier wear debris escape from the contact zones into the grooves, which would help to reduce the irregularities and consequently the squeal tendencies. However, most important of all, the groove-textured surface could effectively interrupt the continuous contact of the friction surfaces when the counterface ball slides across the grooves, which would suppress the generation of high-frequency fluctuations of the friction force, disturb the self-excited vibration of the friction system, and consequently significantly reduce squeal.
4. Conclusions Groove-textured surfaces with different widths and pitches were manufactured on compacted graphite iron materials. An experimental study was performed to investigate the influence of groove-textured surface on tribological behaviors and frictioninduced vibration and noise properties, by using a developed device which is able to synchronously measure and analyze the friction force, vibration acceleration and noise signals in a ball-onflat reciprocating sliding configuration. From the experimental results, it can be concluded as follows:
(1) The squeal generated from the groove-textured surface was mainly influenced by the dimensional proportion of groove
width to pitch, instead independently by groove width or pitch. In this work, groove-textured surfaces with a specific dimensional proportion of groove width to pitch of 1/2 can significantly reduce squeal. (2) All the friction force, vibration acceleration and noise pressure signals of the surfaces with significant presence of squeal showed the same dominant frequency of about 1500 Hz, though they are quite different in topography. Squeal is very closely correlated with the friction force and the vibrations in both friction and normal directions at the dominant frequency which is very close to the natural frequency of the tribo-system. The squeal in this work was mainly caused by the local dynamics at the contact surface which excited a mode of the tribo-system. (3) No inherent correlation between the value of friction coefficient and the generation of squeal can be observed in this work. Squeal was found to occur associated with significant continuous high-frequency fluctuations of friction force and vibration accelerations, which were mainly caused by the microscopic irregularities of the worn surface, such as ploughing, wear debris layer and detachment. Groove-textured surface with specific geometry allowed an easier wear debris escape from the contact zones into the grooves, which would help to reduce the irregularities and consequently the squeal tendencies. Moreover, wave-fluctuations of the friction force caused by counterface ball sliding across the grooves can effectively disturb the self-excited vibration of the friction system and consequently reduce the tendency to squeal. For the groove-textured surface in this study, the groove was found to be the dominant surface component of contact surface topography affecting the generation of squeal compared to the microscopic irregularities of the worn surface on ridge.
Acknowledgements The authors would like to thank senior engineer X.Y. Shi, Southwest Jiaotong University, for helpful discussions. The authors are grateful for the financial support of the National Scientific Foundation of China (No. 51005191 and No. U1134103), and the National Outstanding Young Scientists Foundation of China (51025519).
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