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Friction instability induced by iron and iron oxides on friction material surface
Author’s Accepted Manuscript Friction instability induced by iron and iron oxides on friction material surface H.J. Noh, H. Jang
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To appear in: Wear Received date: 20 September 2017 Revised date: 20 December 2017 Accepted date: 30 December 2017 Cite this article as: H.J. Noh and H. Jang, Friction instability induced by iron and iron oxides on friction material surface, Wear, https://doi.org/10.1016/j.wear.2017.12.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Friction instability induced by iron and iron oxides on friction material surface H. J. Noh, H. Jang* (*[email protected]) Department of Materials Science and Engineering, Korea University, Seoul, South Korea 02841 Abstract This study analyzed the effect of ferrous wear particles, which can be transferred from gray iron brake discs to the friction material surface during brake applications, on friction instability. To simulate the chemical change of the sliding surfaces, ferrous particles, such as pure iron, Fe2O3, and Fe3O4 particles, were embedded on the sliding surface of a friction material, and their friction characteristics were investigated using a scale dynamometer. The results showed that the ferrous particles aggravated friction instability, showing larger stickslip amplitudes and wider velocity ranges for friction instability than the bare specimen. The embedded ferrous particles increased the static coefficient of friction and produced larger stick-slip amplitudes in the order of Fe2O3, iron, and Fe3O4, while the friction material without ferrous particles showed the least instability. The surface energy and amount of highpressure contact plateaus on the sliding surface were attributed to the high friction instability caused by the interfacial adhesion increased by ferrous particles at the sliding interface. This explained the frequent friction-induced noise and vibrations found with a corroded or relatively soft gray iron disc. Keywords: Gray iron, Iron oxide, Friction instability, Adhesion, Surface energy
1. Introduction Brake induced noise and vibrations have long been considered major problems by the automotive community. Much effort has been made to remove friction instability by minimizing the friction force oscillation at the sliding interface [1-4] and by controlling the transfer function of the brake system [5-7]. This is because the friction oscillation triggered at the sliding interface can be intensified or diminished by the brake system. However, a robust design of the brake system without friction-induced instability is still considered a challenging task owing to the lack of fundamental understanding of the triggering mechanism. In order to remove the brake noise and vibrations, recent studies have focused on the sliding interface with particular attention to the mechanical [8-10], morphological [11-13], and chemical properties [14, 15] of the friction material surface based on the knowledge of tribology. They have shown that the composition, strength, and size distribution of the contact plateaus on the sliding surface play crucial roles for friction instability of brake friction materials. This occurs because the contact plateaus are produced by wear debris compaction on the friction material surface and their chemical composition is closely related to the mechanical properties and size distribution of the plateaus [16-21]. In particular, the compositional change of the friction material surface during brake applications has attracted increased attention because a friction material surface with high iron content shows high propensity to friction instability. High noise occurrence from the friction materials after
extended usage or after being rubbed by corroded discs has supported the strong effect of iron contents on the friction material surface. Recently, Lee et al. [22] discovered that a considerable amount of disc wear debris was transferred to the sliding surface of the friction material. The authors showed that wear-resistant discs minimized propensity of brake squeal occurrence. Sriwibbon et al. [23] demonstrated that disc wear affected the noise occurrence owing to high iron content in the friction material surface. Park et al. [18] also noted that the corroded gray iron discs increased the amplitude of stick-slip by increasing the static coefficient of friction owing to oxides produced on the gray iron disc. It is well known that the amount of disc wear is determined by the aggressiveness of the brake friction material and hardness of gray iron. Jang et al. [24] showed that the gray iron microstructure plays considerable influence on the preferential wear of brake discs. Cho et al. [25] also found that the wear amount of gray iron changes the friction coefficient of brake friction material owing to transferred ferrous wear particles from gray iron discs. The disc wear debris can exist on the friction material surface in different phases. Typical ferrous wear particles produced from a gray iron disc are composed of iron from the pearlitic matrix of gray iron, hematite induced by corrosion (Fe2O3), and magnetite (Fe3O4) by chemical oxidation at elevated temperatures. The corrosion-induced Fe2O3 is well known to promote brake-induced noise, which often lasts until the oxide layer is completely removed from the sliding interface. Shin et al. [19] studied friction instability caused by corroded gray iron discs, which was attributed to the removed oxide partially transferred to the surface of the brake friction material. Park et al [18] also investigated the influence of a corroded brake disc on friction induced stick-slip. They reported that the transferred oxide particles increased the static friction coefficient and intensified the amplitude of stick-slip, suggesting that the oxide particles assisted the formation of the contact plateaus. Osterle et al. [26] studied the effect of Fe3O4 on the friction characteristics of brake-friction materials, focusing on the effect of Fe3O4 on the third-body formation. However, the systematic investigation about the role of ferrous wear debris transferred to the surface of the friction material on friction instability is not found in the literature, although the ferrous debris transfer is expected to play an important role in the formation and destruction of secondary contact plateaus on the friction material surface. In this study, the effect of disc wear debris on friction instability is investigated. Assuming that ferrous wear debris such as iron, Fe2O3, and Fe3O4 are transferred to and affect the production of contact plateaus on the friction material surface during brake applications, their effect on stick-slip amplitude was investigated. Results indicated that the ferrous wear particles increased the stick-slip amplitude by increasing the surface adhesion, which was determined by the change of the surface energy and contact area. 2. Experiments 2.1 Materials The friction material specimens used in this study were commercial non-steel-type products designed for mid-size passenger cars. The friction material specimen contained typical ingredients such as phenolic resin as a binder, organic and inorganic reinforcing fibers, inert space fillers, and friction modifiers [27], although its exact composition was classified as proprietary information. To simulate the transferred ferrous wear debris from gray iron discs, fine particles of iron (Strem Chemicals, USA), Fe2O3 (Sigma Aldrich, USA), and Fe3O4
(Kojundo Chemical Laboratory, Japan) were pressed at 3 MPa onto the friction material surface. To provide the same volume of ferrous particles dispersed on the surface, the amount of the particles was set to 0.3 g, 0.29 g and 0.45 g for Fe2O3, Fe3O4, and iron, respectively, considering the specific gravity. To simulate the in situ surface condition during brake applications, the ferrous particles (50 vol.%) were mixed with ground wear debris (50 vol.%) of the same friction material. The ferrous particles were screened to prepare within a similar size range, from 0.5 to 5 μm, to reduce possible size effects [16]. A friction material specimen without ferrous particles was also prepared as a reference. In this case, the same amount of ground friction-material particles was pressed to the bare specimen surface. The friction material specimens were burnished before the friction tests to remove loose particles on the sliding surface. As a counter surface, gray cast iron discs were used. The gray iron discs showed typical graphite flakes (type A) in the pearlitic matrix. Contact stiffness of the friction material specimen was measured following the ASTM D695 procedure using a zirconia hemisphere (radius 10 mm) that maintained the contact area during compression tests using a universal testing machine (3367, Instron, USA) [17]. 2.2 Friction test The friction material specimens were burnished to make uniform contact layers with stable friction films using a Krauss-type tribometer (MoinSys, Korea) [28]. The detailed burnish procedure can be found in Table 1. The surface images of the friction material specimen after burnish are shown in Fig. 1. After burnish, a 1/5-scale brake dynamometer (NeoPlus, Korea) was used to examine friction instability [29]. The size of the friction materials was 45×18×6 mm3 and the gray iron disc was 160 mm in diameter and 8 mm in thickness. The gray cast iron discs showed the typical microstructure of type A graphite morphology. Stick-slip tests were conducted to compare friction instability of the friction materials in the velocity range of 0.02–3 mm/s under hydraulic pressure of 3 MPa. The absolute humidity was maintained during the friction tests at 5.93–7.74 g/m3 (RH 23–30%). The detailed conditions for the stick-slip tests are listed in Table 1. The stop-and-go tests to measure the static coefficient of friction were carried out at 0.01 mm/s after maintaining the static contact for 60 s at 2 MPa.
Fig. 1. Surface images of the friction materials after burnish. Ferrous particles mixed with friction material wear debris were embedded on the friction material surface before burnish. 2.3 Surface analysis A surface analysis was performed after the friction test. The stiffness of the friction material was measured using a universal testing machine (UTM-3367, Instron Inc.). The
topography of the disc and friction material surfaces was examined using a laser confocal microscope (VK-8710, Keyence). The contact area at the interface between the friction material and counter disc was measured using pressure sensitive films (Fuji Inc., Japan). The surface energy was calculated based on the contact angle data obtained using an apparatus for contact angle measurement (MCIK, OCA-15EC). Three reference liquids (distilled water, Glycerol, and diiodomethane) with known surface energy were used to estimate the surface energy of the friction material. Table 1 Test mode and conditions. Test mode
Test condition Speed: 1.5 m/sec, Pressure: 7 bar IBT: 100 ◦C Dragged 150 sec (12 cycles in a constant pressure) Humidity: AH 5.93-7.74 g/m3 (RH 23-30%)
Burnish 1
Krauss-type tribometer
Burnish 2
1/5 scale brake dynamometer
Speed: 50 mm/sec, Pressure: 30 bar IBT: 25-27 ◦C Dragged 10 min (3 cycles) Humidity: AH 5.93-7.74 g/m3 (RH 23-30%)
Stick-slip test
1/5 scale brake dynamometer
Speed: 0.02 - 3 mm/sec, Pressure: 30 bar IBT: 25-27 ◦C Humidity: AH 5.93-7.74 g/m3 (RH 23-30%)
3. Results and discussion The change in friction oscillation at different sliding velocities was first examined to investigate the effect of the ferrous wear debris on friction instability. Figure 2 shows typical friction oscillations at low sliding velocities (v) obtained during dynamometer tests using the friction materials embedded with and without ferrous particles. It shows that the friction induced intermittent motion changes from the saw-tooth type stick-slip to harmonic oscillation and its amplitude is diminished until it becomes indiscernible beyond the critical velocity (vc). The test results also indicate that the oscillation amplitude, average friction level, and critical velocity for smooth sliding are different according to the embedded ferrous particles. This demonstrates that the type of transferred ferrous wear debris from the gray iron disc changes the excitation at the sliding interface. The amplitude of stick-slip was the largest in the case of embedding Fe2O3 on the friction material surface followed by that of iron and Fe3O4, while the fluctuation was relatively small in the case of the bare friction material. The critical velocities for smooth sliding were approximately 2.6, 2.2, 1.5, and 0.05 mm/s for the specimens with Fe2O3, iron, Fe3O4, and for the bare specimen, respectively, indicating that the velocity range for friction instability expanded owing to the ferrous particles. It is noteworthy to observe the larger amplitude and higher critical velocity for the friction material with Fe2O3, because it supports the increased noise propensity of the brake friction materials when they are slid with the corroded gray iron disc [18, 22, 23].
To understand the effects of the ferrous particles on the sliding surface in more detail, the oscillation amplitude was plotted as a function of stick time (or dwell time), as shown in Fig. 3. This figure provides information about the critical velocity for smooth sliding when tangential contact stiffness (kt) of the friction material is known, because the slope of the stick episode of the stick-slip profile in Fig. 2 is proportional to ktv/L [30, 31], where L is the normal load and k is tangential stiffness. With known tangential stiffness, the critical velocity can be determined from the initial slope from the origin of the plot [18, 32]. On the other hand, the height of the curve in Fig. 3 is dependent on the failure strength of the sliding interface at the moment of slip events, and it plays a crucial role in determining the amplitude of stick-slip. Therefore, the profile of oscillation amplitude as a function of stick time can be considered a fingerprint for stick-slip behavior of a sliding couple. Other studies based on model experiment using PMMA and transient finite element simulation showed similar results [33, 34].
Fig. 2. Friction coefficient as a function of sliding distance at different velocities using (a) bare specimen and specimens with (b) Fe2O3, (c) Fe3O4, and (d) Iron particles. The friction materials with ferrous particles showed higher friction instability than the bare specimen.
Fig. 3. Oscillation amplitude of the friction coefficient plotted as a function of stick time. Friction material specimens with ferrous particles showed larger amplitudes of friction oscillation than the bare specimen. The figure indicates that the specimens with ferrous particles show larger amplitudes of friction oscillation than the bare specimen. In particular, the specimen with Fe2O3 particles showed a steep tangent at the origin of the regression curve, indicating a wider velocity range for friction instability, while the specimens with iron and Fe3O4 particles indicate narrower velocity ranges owing to relatively gentler slopes at the origin. The substantial stick-slip amplitude from the specimen with Fe2O3 particles indicates a high static coefficient of friction, which is known to be strongly dependent on the size of the contact plateaus and shear strength at the friction interface. Tangential stiffness (kt) of the sliding interface has also been considered an important material property to interpret stick-slip phenomena because the critical velocity for steady sliding can be influenced by tangential stiffness [8, 18]. Assuming that tangential stiffness can be calculated from normal stiffness (kn) using the relation kt = 2 kn ((1-ν1)/E1+(1- ν2)/E2) ⁄ ((2ν1)/E1+(1- ν2)/E2) [35], where the subscripts indicate friction material and gray iron for Poisson’s ratio (ν) and elastic modulus (E), tangential stiffness was estimated from the measured normal stiffness of the friction material specimen [36]. Fig. 4 shows stiffness as a function of the pressure (or load) for the specimens with different ferrous particles, which was
converted from the load–displacement curve obtained from compression tests. At 3 MPa, normal stiffness was 22.04, 21.25, 16.44, and 18.95, for the specimens embedded with Fe2O3, iron particles, Fe3O4, and for the bare specimen, respectively. However, the difference in the estimated tangential stiffness appeared to show minimal effect on the friction instability, compared to the strong influence of the friction oscillation in Fig. 3, owing to the different ferrous particles embedded at the sliding interface. Based on the equation of motion of a sliding body, the mathematical solution for high friction instability can also be explained using the stiffness–load curves in Fig. 4. This is because the stiffness can be dissociated into surface stiffness (kt (s)) and matrix stiffness (kt (m)), and its ratio (kt (s)/kt (m)) has been used to estimate the amount of friction oscillation [37] so that the high stiffness ratio produces bigger friction fluctuations during sliding. Assuming that the matrix stiffness of the friction material is equal, then the difference in normal contact stiffness is mainly determined by the surface stiffness that is affected by the ferrous particles on the sliding surface. This suggests that the ferrous particles transferred to the friction material surface produced friction films with different mechanical properties owing to different chemical nature of the ferrous particles. The strong influence of the ferrous particles on surface stiffness suggests that the stick-slip profiles shown in Fig. 2 and their critical velocity were also determined by surface stiffness of the friction material specimen.
Fig. 4. Normal stiffness as a function of pressure (and load) measured from the friction material surfaces with and without ferrous particles after burnish. The large amplitudes of stick-slip and expanded velocity range for friction instability found in the specimens embedded with Fe2O3 and iron particles are in line with the high surface stiffness and the bigger forces required for interfacial failure shown in Fig. 3. Because the size of the friction amplitude in Fig. 3 is determined by the static and kinetic coefficients
of friction, their values were measured by stop-and-go friction tests to confirm the effect of ferrous particles. Fig. 5 shows the time domain change of the friction coefficient obtained from the specimens with and without ferrous particles. Incipient large amplitudes of stick-slip appeared when the specimen surfaces were embedded with ferrous particles, while the bare specimen showed small amplitudes without exhibiting the initial large fluctuation. In order to find the governing mechanism for large friction oscillation from the specimens with ferrous particles at the sliding interface, the surface energy of the friction material specimens was measured, because it is known that friction is mainly determined by chemical adhesion, mechanical interlocking, and abrasion [38]. The effect of surface adhesion on friction characteristics was investigated by comparing the surface energy that was obtained from contact angle measurements following the Young’s theory [39]. Based on the measured wetting angles of reference liquids on the friction material surface in Table 2, the surface energy was calculated using the Owens–Wendt–Rabel–Kaelble (OWRK) method [40]. The effect of surface roughness on surface energy, suggested by Wenzel [41], was less than 2% in these cases.
Fig. 5. Change in the friction coefficient found during stop-and-go friction tests. The initial stick episode was pronounced in the case of ferrous particles embedded on the friction material surface. The test was carried out at 0.1 mm/s after rested for 60 sec at 2 MPa.
Table 2 The results of contact angle measurement. Distilled water
Diiodomethane
Glycerol
Bare
69.89 (±1.65)
34.72 (±0.88)
71.76 (±1.57)
Fe2O3
56.34 (±3.56)
28.43 (±1.26)
62.35 (±1.91)
Fe3O4
77.53 (±5.71)
32.57 (±1.07)
69.15 (±1.51)
Iron powder
60.74 (±2.93)
31.65 (±1.44)
71.05 (±1.47)
Table 3 gives the surface energy of the friction material embedded with and without ferrous particles. With ferrous particles, the friction material showed high surface energies, indicating a larger influence on interfacial adhesion than that of the bare specimen. To examine the relationship between surface energy and friction, the static coefficient of friction was plotted as a function of total surface energy of the friction material specimen by considering the loaded surface area. The loaded surface area was obtained by using pressure sensitive films. Fig. 6 shows the pressure distribution on the contact surface of the friction material after burnish. This figure indicates that the pressure distribution is broad, while the specimens embedded with ferrous particles show pronounced high-pressure contact plateaus on the sliding surface. The contact area was plotted as a function of pressure in Fig. 7. This figure shows that the friction materials embedded with ferrous particles exhibit high pressure contact plateaus, above 3 MPa, which is the pressure level given during compression for the tests using pressure sensitive films. Table 3 Average values of surface energy calculated from contact angle measurements, loaded surface area, and surface energy of friction materials with different ferrous particles.
γSG
γSG (= γSGA)
Loaded surface area: B (P > 3MPa) (mm2)
γSG (= γSGB)
(mJ/m2)
Loaded surface area: A (mm2)
Bare
41.37
497
20.4
48
1.97
Fe2O3
47.77
558
26.4
155
7.32
Fe3O4
42.82
554
23.2
75
3.14
Iron powder
43.47
540
23.1
127
5.42
3
(x 10 mJ)
(x 103 mJ)
Fig. 6. Contact area with pressure distribution on the friction material surface specimens after burnish. The compression test was carried out at 3 MPa. To find the role of the ferrous particles for the pronounced stick-slip, the static coefficient of friction obtained from the stop-and-go tests was plotted as a function of surface energy. Fig. 8 shows the static coefficient of friction plotted with surface energy, which was calculated by multiplying the average surface energy (γSG) by the loaded contact area, with surface pressure over 3 MPa. The loaded surface area above 3 MPa is listed in Table 3. Fig. 8 indicates that static friction, which is determined by shear failure at the interface, is well correlated with high-pressure contact area. On the other hand, the correlation between static friction and total contact area was poor, suggesting a dominant role of the high-pressure plateaus for determining the friction characteristics. The high surface energy of the specimen embedded with Fe2O3 particles is supported by the high contact stiffness in Fig. 4 owing to the hard surface and pronounced stick-slip during the sliding tests at slow velocities. This is consistent to the high noise propensity of the commercial friction materials exhibiting pronounced contact plateaus on the sliding surface [42].
Fig. 7. Contact area of friction material, which is measured using pressure sensitivity films, as a function of contact pressure on the specimen surface.
Fig. 8. Static friction coefficient plotted as a function of surface energy. The surface energy was calculated by multiplying the average surface energy by the area loaded higher than 3 MPa. Another interesting feature of Fig. 8 is the relative amounts of contribution from surface adhesion and other sources such as mechanical interlocking and abrasion to the static friction level since the static friction coefficient is approximately 0.23 from linear regression when surface energy is negligible. The figure demonstrates that the contribution from surface energy changes according to the type of ferrous particles on the surface, indicating bigger contribution in the case of friction material specimens with ferrous particles than the bare specimen, while the relative contribution from the mechanical action is bigger in the case of bare specimens. This suggests that shear failure at the sliding interface can be determined not only by surface adhesion but also by other mechanical contributions, such as asperity deformation and plowing by hard asperities at the sliding interface. Based on the results from surface analysis and stick-slip tests, this study suggests that the friction material surface with Fe2O3 was most susceptible to friction instability during sliding. This was consistent with the high propensity to brake noise and vibrations when the gray iron disc was corroded in a humid environment. The friction material with iron particles also showed a large stick-slip amplitude, with a relatively large velocity range for friction instability, which is consistent with the fact that a soft gray iron disc with extensive wear tends to produce noise and vibrations owing to iron transfer to the friction material surface. However, it was not possible to correlate the stick-slip behavior of a friction material surface embedded with Fe3O4 particles with any specific friction induced phenomena, owing to very limited information in the literature. The detailed investigation about the contribution of
ferrous particles due to mechanical actions at the sliding surface also remains a subject for future research.
4. Conclusions The influence of ferrous wear debris on friction instability was studied by embedding iron, Fe2O3, and Fe3O4 on a friction material surface, in order to simulate wear debris transfer from a gray iron disc to the friction material surface. Based on the stick-slip tests and surface analyses, the mechanism for friction instability was investigated and the results of this study can be summarized as follows. 1. The friction material surfaces embedded with ferrous wear debris were expected to increase stick-slip amplitude and velocity range for friction instability. This was consistent with the high noise propensity of the brake friction material surface with high iron contents. 2. The friction material surface containing Fe2O3 or iron particles showed a large stick-slip amplitude owing to high surface energy, which is consistent with the high noise propensity of friction material slid with corroded or soft gray iron discs. 3. Stick-slip amplitude and static coefficient of friction were well correlated with surface energy of the friction material specimen, which was calculated considering a highpressure contact area. 4. The critical velocity for steady sliding was determined by the interfacial failure force and stiffness in combination. Acknowledgements This study was partially supported by Korea University and Hyundai MOBIS Co.
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Ferrous particles on the friction material surface aggravates friction instability.
Stick-slip is prominent when ferrous particles are present on the sliding surface
High surface energy increases interfacial adhesion and static friction.
Brake noise can be affected by the ferrous particles on the sliding interface
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PII: DOI: Reference:
S0043-1648(17)31405-9 https://doi.org/10.1016/j.wear.2017.12.025 WEA102332
To appear in: Wear Received date: 20 September 2017 Revised date: 20 December 2017 Accepted date: 30 December 2017 Cite this article as: H.J. Noh and H. Jang, Friction instability induced by iron and iron oxides on friction material surface, Wear, https://doi.org/10.1016/j.wear.2017.12.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Friction instability induced by iron and iron oxides on friction material surface H. J. Noh, H. Jang* (*[email protected]) Department of Materials Science and Engineering, Korea University, Seoul, South Korea 02841 Abstract This study analyzed the effect of ferrous wear particles, which can be transferred from gray iron brake discs to the friction material surface during brake applications, on friction instability. To simulate the chemical change of the sliding surfaces, ferrous particles, such as pure iron, Fe2O3, and Fe3O4 particles, were embedded on the sliding surface of a friction material, and their friction characteristics were investigated using a scale dynamometer. The results showed that the ferrous particles aggravated friction instability, showing larger stickslip amplitudes and wider velocity ranges for friction instability than the bare specimen. The embedded ferrous particles increased the static coefficient of friction and produced larger stick-slip amplitudes in the order of Fe2O3, iron, and Fe3O4, while the friction material without ferrous particles showed the least instability. The surface energy and amount of highpressure contact plateaus on the sliding surface were attributed to the high friction instability caused by the interfacial adhesion increased by ferrous particles at the sliding interface. This explained the frequent friction-induced noise and vibrations found with a corroded or relatively soft gray iron disc. Keywords: Gray iron, Iron oxide, Friction instability, Adhesion, Surface energy
1. Introduction Brake induced noise and vibrations have long been considered major problems by the automotive community. Much effort has been made to remove friction instability by minimizing the friction force oscillation at the sliding interface [1-4] and by controlling the transfer function of the brake system [5-7]. This is because the friction oscillation triggered at the sliding interface can be intensified or diminished by the brake system. However, a robust design of the brake system without friction-induced instability is still considered a challenging task owing to the lack of fundamental understanding of the triggering mechanism. In order to remove the brake noise and vibrations, recent studies have focused on the sliding interface with particular attention to the mechanical [8-10], morphological [11-13], and chemical properties [14, 15] of the friction material surface based on the knowledge of tribology. They have shown that the composition, strength, and size distribution of the contact plateaus on the sliding surface play crucial roles for friction instability of brake friction materials. This occurs because the contact plateaus are produced by wear debris compaction on the friction material surface and their chemical composition is closely related to the mechanical properties and size distribution of the plateaus [16-21]. In particular, the compositional change of the friction material surface during brake applications has attracted increased attention because a friction material surface with high iron content shows high propensity to friction instability. High noise occurrence from the friction materials after
extended usage or after being rubbed by corroded discs has supported the strong effect of iron contents on the friction material surface. Recently, Lee et al. [22] discovered that a considerable amount of disc wear debris was transferred to the sliding surface of the friction material. The authors showed that wear-resistant discs minimized propensity of brake squeal occurrence. Sriwibbon et al. [23] demonstrated that disc wear affected the noise occurrence owing to high iron content in the friction material surface. Park et al. [18] also noted that the corroded gray iron discs increased the amplitude of stick-slip by increasing the static coefficient of friction owing to oxides produced on the gray iron disc. It is well known that the amount of disc wear is determined by the aggressiveness of the brake friction material and hardness of gray iron. Jang et al. [24] showed that the gray iron microstructure plays considerable influence on the preferential wear of brake discs. Cho et al. [25] also found that the wear amount of gray iron changes the friction coefficient of brake friction material owing to transferred ferrous wear particles from gray iron discs. The disc wear debris can exist on the friction material surface in different phases. Typical ferrous wear particles produced from a gray iron disc are composed of iron from the pearlitic matrix of gray iron, hematite induced by corrosion (Fe2O3), and magnetite (Fe3O4) by chemical oxidation at elevated temperatures. The corrosion-induced Fe2O3 is well known to promote brake-induced noise, which often lasts until the oxide layer is completely removed from the sliding interface. Shin et al. [19] studied friction instability caused by corroded gray iron discs, which was attributed to the removed oxide partially transferred to the surface of the brake friction material. Park et al [18] also investigated the influence of a corroded brake disc on friction induced stick-slip. They reported that the transferred oxide particles increased the static friction coefficient and intensified the amplitude of stick-slip, suggesting that the oxide particles assisted the formation of the contact plateaus. Osterle et al. [26] studied the effect of Fe3O4 on the friction characteristics of brake-friction materials, focusing on the effect of Fe3O4 on the third-body formation. However, the systematic investigation about the role of ferrous wear debris transferred to the surface of the friction material on friction instability is not found in the literature, although the ferrous debris transfer is expected to play an important role in the formation and destruction of secondary contact plateaus on the friction material surface. In this study, the effect of disc wear debris on friction instability is investigated. Assuming that ferrous wear debris such as iron, Fe2O3, and Fe3O4 are transferred to and affect the production of contact plateaus on the friction material surface during brake applications, their effect on stick-slip amplitude was investigated. Results indicated that the ferrous wear particles increased the stick-slip amplitude by increasing the surface adhesion, which was determined by the change of the surface energy and contact area. 2. Experiments 2.1 Materials The friction material specimens used in this study were commercial non-steel-type products designed for mid-size passenger cars. The friction material specimen contained typical ingredients such as phenolic resin as a binder, organic and inorganic reinforcing fibers, inert space fillers, and friction modifiers [27], although its exact composition was classified as proprietary information. To simulate the transferred ferrous wear debris from gray iron discs, fine particles of iron (Strem Chemicals, USA), Fe2O3 (Sigma Aldrich, USA), and Fe3O4
(Kojundo Chemical Laboratory, Japan) were pressed at 3 MPa onto the friction material surface. To provide the same volume of ferrous particles dispersed on the surface, the amount of the particles was set to 0.3 g, 0.29 g and 0.45 g for Fe2O3, Fe3O4, and iron, respectively, considering the specific gravity. To simulate the in situ surface condition during brake applications, the ferrous particles (50 vol.%) were mixed with ground wear debris (50 vol.%) of the same friction material. The ferrous particles were screened to prepare within a similar size range, from 0.5 to 5 μm, to reduce possible size effects [16]. A friction material specimen without ferrous particles was also prepared as a reference. In this case, the same amount of ground friction-material particles was pressed to the bare specimen surface. The friction material specimens were burnished before the friction tests to remove loose particles on the sliding surface. As a counter surface, gray cast iron discs were used. The gray iron discs showed typical graphite flakes (type A) in the pearlitic matrix. Contact stiffness of the friction material specimen was measured following the ASTM D695 procedure using a zirconia hemisphere (radius 10 mm) that maintained the contact area during compression tests using a universal testing machine (3367, Instron, USA) [17]. 2.2 Friction test The friction material specimens were burnished to make uniform contact layers with stable friction films using a Krauss-type tribometer (MoinSys, Korea) [28]. The detailed burnish procedure can be found in Table 1. The surface images of the friction material specimen after burnish are shown in Fig. 1. After burnish, a 1/5-scale brake dynamometer (NeoPlus, Korea) was used to examine friction instability [29]. The size of the friction materials was 45×18×6 mm3 and the gray iron disc was 160 mm in diameter and 8 mm in thickness. The gray cast iron discs showed the typical microstructure of type A graphite morphology. Stick-slip tests were conducted to compare friction instability of the friction materials in the velocity range of 0.02–3 mm/s under hydraulic pressure of 3 MPa. The absolute humidity was maintained during the friction tests at 5.93–7.74 g/m3 (RH 23–30%). The detailed conditions for the stick-slip tests are listed in Table 1. The stop-and-go tests to measure the static coefficient of friction were carried out at 0.01 mm/s after maintaining the static contact for 60 s at 2 MPa.
Fig. 1. Surface images of the friction materials after burnish. Ferrous particles mixed with friction material wear debris were embedded on the friction material surface before burnish. 2.3 Surface analysis A surface analysis was performed after the friction test. The stiffness of the friction material was measured using a universal testing machine (UTM-3367, Instron Inc.). The
topography of the disc and friction material surfaces was examined using a laser confocal microscope (VK-8710, Keyence). The contact area at the interface between the friction material and counter disc was measured using pressure sensitive films (Fuji Inc., Japan). The surface energy was calculated based on the contact angle data obtained using an apparatus for contact angle measurement (MCIK, OCA-15EC). Three reference liquids (distilled water, Glycerol, and diiodomethane) with known surface energy were used to estimate the surface energy of the friction material. Table 1 Test mode and conditions. Test mode
Test condition Speed: 1.5 m/sec, Pressure: 7 bar IBT: 100 ◦C Dragged 150 sec (12 cycles in a constant pressure) Humidity: AH 5.93-7.74 g/m3 (RH 23-30%)
Burnish 1
Krauss-type tribometer
Burnish 2
1/5 scale brake dynamometer
Speed: 50 mm/sec, Pressure: 30 bar IBT: 25-27 ◦C Dragged 10 min (3 cycles) Humidity: AH 5.93-7.74 g/m3 (RH 23-30%)
Stick-slip test
1/5 scale brake dynamometer
Speed: 0.02 - 3 mm/sec, Pressure: 30 bar IBT: 25-27 ◦C Humidity: AH 5.93-7.74 g/m3 (RH 23-30%)
3. Results and discussion The change in friction oscillation at different sliding velocities was first examined to investigate the effect of the ferrous wear debris on friction instability. Figure 2 shows typical friction oscillations at low sliding velocities (v) obtained during dynamometer tests using the friction materials embedded with and without ferrous particles. It shows that the friction induced intermittent motion changes from the saw-tooth type stick-slip to harmonic oscillation and its amplitude is diminished until it becomes indiscernible beyond the critical velocity (vc). The test results also indicate that the oscillation amplitude, average friction level, and critical velocity for smooth sliding are different according to the embedded ferrous particles. This demonstrates that the type of transferred ferrous wear debris from the gray iron disc changes the excitation at the sliding interface. The amplitude of stick-slip was the largest in the case of embedding Fe2O3 on the friction material surface followed by that of iron and Fe3O4, while the fluctuation was relatively small in the case of the bare friction material. The critical velocities for smooth sliding were approximately 2.6, 2.2, 1.5, and 0.05 mm/s for the specimens with Fe2O3, iron, Fe3O4, and for the bare specimen, respectively, indicating that the velocity range for friction instability expanded owing to the ferrous particles. It is noteworthy to observe the larger amplitude and higher critical velocity for the friction material with Fe2O3, because it supports the increased noise propensity of the brake friction materials when they are slid with the corroded gray iron disc [18, 22, 23].
To understand the effects of the ferrous particles on the sliding surface in more detail, the oscillation amplitude was plotted as a function of stick time (or dwell time), as shown in Fig. 3. This figure provides information about the critical velocity for smooth sliding when tangential contact stiffness (kt) of the friction material is known, because the slope of the stick episode of the stick-slip profile in Fig. 2 is proportional to ktv/L [30, 31], where L is the normal load and k is tangential stiffness. With known tangential stiffness, the critical velocity can be determined from the initial slope from the origin of the plot [18, 32]. On the other hand, the height of the curve in Fig. 3 is dependent on the failure strength of the sliding interface at the moment of slip events, and it plays a crucial role in determining the amplitude of stick-slip. Therefore, the profile of oscillation amplitude as a function of stick time can be considered a fingerprint for stick-slip behavior of a sliding couple. Other studies based on model experiment using PMMA and transient finite element simulation showed similar results [33, 34].
Fig. 2. Friction coefficient as a function of sliding distance at different velocities using (a) bare specimen and specimens with (b) Fe2O3, (c) Fe3O4, and (d) Iron particles. The friction materials with ferrous particles showed higher friction instability than the bare specimen.
Fig. 3. Oscillation amplitude of the friction coefficient plotted as a function of stick time. Friction material specimens with ferrous particles showed larger amplitudes of friction oscillation than the bare specimen. The figure indicates that the specimens with ferrous particles show larger amplitudes of friction oscillation than the bare specimen. In particular, the specimen with Fe2O3 particles showed a steep tangent at the origin of the regression curve, indicating a wider velocity range for friction instability, while the specimens with iron and Fe3O4 particles indicate narrower velocity ranges owing to relatively gentler slopes at the origin. The substantial stick-slip amplitude from the specimen with Fe2O3 particles indicates a high static coefficient of friction, which is known to be strongly dependent on the size of the contact plateaus and shear strength at the friction interface. Tangential stiffness (kt) of the sliding interface has also been considered an important material property to interpret stick-slip phenomena because the critical velocity for steady sliding can be influenced by tangential stiffness [8, 18]. Assuming that tangential stiffness can be calculated from normal stiffness (kn) using the relation kt = 2 kn ((1-ν1)/E1+(1- ν2)/E2) ⁄ ((2ν1)/E1+(1- ν2)/E2) [35], where the subscripts indicate friction material and gray iron for Poisson’s ratio (ν) and elastic modulus (E), tangential stiffness was estimated from the measured normal stiffness of the friction material specimen [36]. Fig. 4 shows stiffness as a function of the pressure (or load) for the specimens with different ferrous particles, which was
converted from the load–displacement curve obtained from compression tests. At 3 MPa, normal stiffness was 22.04, 21.25, 16.44, and 18.95, for the specimens embedded with Fe2O3, iron particles, Fe3O4, and for the bare specimen, respectively. However, the difference in the estimated tangential stiffness appeared to show minimal effect on the friction instability, compared to the strong influence of the friction oscillation in Fig. 3, owing to the different ferrous particles embedded at the sliding interface. Based on the equation of motion of a sliding body, the mathematical solution for high friction instability can also be explained using the stiffness–load curves in Fig. 4. This is because the stiffness can be dissociated into surface stiffness (kt (s)) and matrix stiffness (kt (m)), and its ratio (kt (s)/kt (m)) has been used to estimate the amount of friction oscillation [37] so that the high stiffness ratio produces bigger friction fluctuations during sliding. Assuming that the matrix stiffness of the friction material is equal, then the difference in normal contact stiffness is mainly determined by the surface stiffness that is affected by the ferrous particles on the sliding surface. This suggests that the ferrous particles transferred to the friction material surface produced friction films with different mechanical properties owing to different chemical nature of the ferrous particles. The strong influence of the ferrous particles on surface stiffness suggests that the stick-slip profiles shown in Fig. 2 and their critical velocity were also determined by surface stiffness of the friction material specimen.
Fig. 4. Normal stiffness as a function of pressure (and load) measured from the friction material surfaces with and without ferrous particles after burnish. The large amplitudes of stick-slip and expanded velocity range for friction instability found in the specimens embedded with Fe2O3 and iron particles are in line with the high surface stiffness and the bigger forces required for interfacial failure shown in Fig. 3. Because the size of the friction amplitude in Fig. 3 is determined by the static and kinetic coefficients
of friction, their values were measured by stop-and-go friction tests to confirm the effect of ferrous particles. Fig. 5 shows the time domain change of the friction coefficient obtained from the specimens with and without ferrous particles. Incipient large amplitudes of stick-slip appeared when the specimen surfaces were embedded with ferrous particles, while the bare specimen showed small amplitudes without exhibiting the initial large fluctuation. In order to find the governing mechanism for large friction oscillation from the specimens with ferrous particles at the sliding interface, the surface energy of the friction material specimens was measured, because it is known that friction is mainly determined by chemical adhesion, mechanical interlocking, and abrasion [38]. The effect of surface adhesion on friction characteristics was investigated by comparing the surface energy that was obtained from contact angle measurements following the Young’s theory [39]. Based on the measured wetting angles of reference liquids on the friction material surface in Table 2, the surface energy was calculated using the Owens–Wendt–Rabel–Kaelble (OWRK) method [40]. The effect of surface roughness on surface energy, suggested by Wenzel [41], was less than 2% in these cases.
Fig. 5. Change in the friction coefficient found during stop-and-go friction tests. The initial stick episode was pronounced in the case of ferrous particles embedded on the friction material surface. The test was carried out at 0.1 mm/s after rested for 60 sec at 2 MPa.
Table 2 The results of contact angle measurement. Distilled water
Diiodomethane
Glycerol
Bare
69.89 (±1.65)
34.72 (±0.88)
71.76 (±1.57)
Fe2O3
56.34 (±3.56)
28.43 (±1.26)
62.35 (±1.91)
Fe3O4
77.53 (±5.71)
32.57 (±1.07)
69.15 (±1.51)
Iron powder
60.74 (±2.93)
31.65 (±1.44)
71.05 (±1.47)
Table 3 gives the surface energy of the friction material embedded with and without ferrous particles. With ferrous particles, the friction material showed high surface energies, indicating a larger influence on interfacial adhesion than that of the bare specimen. To examine the relationship between surface energy and friction, the static coefficient of friction was plotted as a function of total surface energy of the friction material specimen by considering the loaded surface area. The loaded surface area was obtained by using pressure sensitive films. Fig. 6 shows the pressure distribution on the contact surface of the friction material after burnish. This figure indicates that the pressure distribution is broad, while the specimens embedded with ferrous particles show pronounced high-pressure contact plateaus on the sliding surface. The contact area was plotted as a function of pressure in Fig. 7. This figure shows that the friction materials embedded with ferrous particles exhibit high pressure contact plateaus, above 3 MPa, which is the pressure level given during compression for the tests using pressure sensitive films. Table 3 Average values of surface energy calculated from contact angle measurements, loaded surface area, and surface energy of friction materials with different ferrous particles.
γSG
γSG (= γSGA)
Loaded surface area: B (P > 3MPa) (mm2)
γSG (= γSGB)
(mJ/m2)
Loaded surface area: A (mm2)
Bare
41.37
497
20.4
48
1.97
Fe2O3
47.77
558
26.4
155
7.32
Fe3O4
42.82
554
23.2
75
3.14
Iron powder
43.47
540
23.1
127
5.42
3
(x 10 mJ)
(x 103 mJ)
Fig. 6. Contact area with pressure distribution on the friction material surface specimens after burnish. The compression test was carried out at 3 MPa. To find the role of the ferrous particles for the pronounced stick-slip, the static coefficient of friction obtained from the stop-and-go tests was plotted as a function of surface energy. Fig. 8 shows the static coefficient of friction plotted with surface energy, which was calculated by multiplying the average surface energy (γSG) by the loaded contact area, with surface pressure over 3 MPa. The loaded surface area above 3 MPa is listed in Table 3. Fig. 8 indicates that static friction, which is determined by shear failure at the interface, is well correlated with high-pressure contact area. On the other hand, the correlation between static friction and total contact area was poor, suggesting a dominant role of the high-pressure plateaus for determining the friction characteristics. The high surface energy of the specimen embedded with Fe2O3 particles is supported by the high contact stiffness in Fig. 4 owing to the hard surface and pronounced stick-slip during the sliding tests at slow velocities. This is consistent to the high noise propensity of the commercial friction materials exhibiting pronounced contact plateaus on the sliding surface [42].
Fig. 7. Contact area of friction material, which is measured using pressure sensitivity films, as a function of contact pressure on the specimen surface.
Fig. 8. Static friction coefficient plotted as a function of surface energy. The surface energy was calculated by multiplying the average surface energy by the area loaded higher than 3 MPa. Another interesting feature of Fig. 8 is the relative amounts of contribution from surface adhesion and other sources such as mechanical interlocking and abrasion to the static friction level since the static friction coefficient is approximately 0.23 from linear regression when surface energy is negligible. The figure demonstrates that the contribution from surface energy changes according to the type of ferrous particles on the surface, indicating bigger contribution in the case of friction material specimens with ferrous particles than the bare specimen, while the relative contribution from the mechanical action is bigger in the case of bare specimens. This suggests that shear failure at the sliding interface can be determined not only by surface adhesion but also by other mechanical contributions, such as asperity deformation and plowing by hard asperities at the sliding interface. Based on the results from surface analysis and stick-slip tests, this study suggests that the friction material surface with Fe2O3 was most susceptible to friction instability during sliding. This was consistent with the high propensity to brake noise and vibrations when the gray iron disc was corroded in a humid environment. The friction material with iron particles also showed a large stick-slip amplitude, with a relatively large velocity range for friction instability, which is consistent with the fact that a soft gray iron disc with extensive wear tends to produce noise and vibrations owing to iron transfer to the friction material surface. However, it was not possible to correlate the stick-slip behavior of a friction material surface embedded with Fe3O4 particles with any specific friction induced phenomena, owing to very limited information in the literature. The detailed investigation about the contribution of
ferrous particles due to mechanical actions at the sliding surface also remains a subject for future research.
4. Conclusions The influence of ferrous wear debris on friction instability was studied by embedding iron, Fe2O3, and Fe3O4 on a friction material surface, in order to simulate wear debris transfer from a gray iron disc to the friction material surface. Based on the stick-slip tests and surface analyses, the mechanism for friction instability was investigated and the results of this study can be summarized as follows. 1. The friction material surfaces embedded with ferrous wear debris were expected to increase stick-slip amplitude and velocity range for friction instability. This was consistent with the high noise propensity of the brake friction material surface with high iron contents. 2. The friction material surface containing Fe2O3 or iron particles showed a large stick-slip amplitude owing to high surface energy, which is consistent with the high noise propensity of friction material slid with corroded or soft gray iron discs. 3. Stick-slip amplitude and static coefficient of friction were well correlated with surface energy of the friction material specimen, which was calculated considering a highpressure contact area. 4. The critical velocity for steady sliding was determined by the interfacial failure force and stiffness in combination. Acknowledgements This study was partially supported by Korea University and Hyundai MOBIS Co.
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Ferrous particles on the friction material surface aggravates friction instability.
Stick-slip is prominent when ferrous particles are present on the sliding surface
High surface energy increases interfacial adhesion and static friction.
Brake noise can be affected by the ferrous particles on the sliding interface