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Effect of pressure in the transition between moderate and severe wear regimes in brake friction materials
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Effect of pressure in the transition between moderate and severe wear regimes in brake friction materials L.Y. Barros a, *, J.C. Poletto a, D. Buneder a, P.D. Neis a, N.F. Ferreira a, R.P. Pavlak b, L.T. Matozo b a b
Universidade Federal do Rio Grande do Sul, Laboratory of Tribology, Sarmento Leite 425, Porto Alegre, Brazil Fras-le S.A, Rod RS-122, 10945, Km 66, Caxias do Sul, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Wear regime Brake friction materials Tribofilm Contact plateaus
The transition in wear regime is a tribological phenomenon observed in many materials, where a sudden change in wear regime (from moderate to severe) occurs when operating parameters (e.g. pressure or sliding velocity) are changed. However, this phenomenon is rarely explored in brake friction materials (BFMs). Severe wear regime for BFMs results in an increase in cost and leads to environmental issues due to the high amount of loss in material. This work aims to study the transition in BFM applying different levels of pressure, where a BFM was rubbed against a gray cast iron disc in a tribometer. The tribofilm deposited on the disc surface and the area ratio of contact plateaus of BFMs were analyzed. The coefficient of friction (COF) and the BFM’s wear were also measured. The wear regime changed from moderate to severe at a limit pressure of 90 bar. At the severe wear regime, a sudden increase in the COF was observed, and the wear of the BFM was approximately 13 times higher than in the moderate wear regime. It was also seen a removal of the tribofilm deposited on the disc’s surface and a reduction of contact plateaus over the BFM’s surface.
1. Introduction A good automotive brake friction material (BFM) needs fulfill many performance and quality requirements under several operating condi tions, such as a stable level of coefficient of friction (COF), light weight, low noise and acceptable cost versus performance [1,2]. Besides, it is desirable that the BFM presents low levels of wear in order to decrease economical cost and reduce environmental impact. In the last recent years, the wear of BFMs was addressed in many studies. The crescent concern with the particles that are released to the environment due to the wear of BFMs led to restrictions on the use of some hazardous in gredients in the formulation of friction materials. For example, the use of copper (a common ingredient found in BFM formulations) in brake pads will have to be reduced considerably in the next years due to recent U.S. legislations [3–5]. Environmental concerns about the impact of haz ardous ingredients added to BFMs have been rising in the last years. For this reason, there is a great research effort towards a better under standing of BFM’s wear. Tribological studies on polymer [6,7] and steel [8–10] have reported a transition in the wear regime from moderate to severe when the contact pressure and/or sliding velocity are increased. For BFMs,
however, this transition is rarely reported in the literature. One of the few publications about this subject is found in Ref. [11]. In this study, the authors performed a set of experiments with two types of semi-metallic BFMs. The normal load was varied under a constant sliding velocity, and a low wear rate was observed for low normal load. The authors mentioned that under low wear rate, abrasive wear mech anism was dominant for either the brake pad and disc. At high normal load, however, a sudden transition in wear rate was observed, and a high level of wear was reported for both BFM and disc. As described by the researchers, the high level of wear rate was caused by thermal degra dation of the phenolic binder and tribo-oxidation of the BFM and disc. Fernandes et al. [12] also studied the transition in wear regime for friction materials (used in clutch systems). The authors performed ex periments varying load and sliding velocity aiming to achieve the transition. According to the researchers, when the transition was reached, the thickness and the uniformity of the tribofilm decreased with the increase of the severity of the tests. An important parameter for the wear in BFMs is the tribofilm (fric tion layer or friction film) which is formed on the brake disc’s surface. The friction film is originated from wear particles generated in the friction pad-disc contact. It is mainly constituted by iron oxide [13,14]
* Corresponding author. E-mail address: [email protected] (L.Y. Barros). https://doi.org/10.1016/j.wear.2019.203112 Received 14 August 2019; Received in revised form 30 October 2019; Accepted 31 October 2019 Available online 3 November 2019 0043-1648/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: L.Y. Barros, Wear, https://doi.org/10.1016/j.wear.2019.203112
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and prevents the direct contact between the BFM and the disc [15]. Thus, the tribofilm reduces wear in BFMs, as well as it contributes for the stabilization of the COF [16]. In this sense, it is important to have a better comprehension of tribofilm formation/destruction during braking. However, a limited number of researchers has studied the tri bofilm formed in brake materials. In an investigation of the current research group (Barros et al. [17]), the tribofilm formation was inves tigated for two brake materials: a non-asbestos organic (NAO) and a semi-metallic (SM). It was concluded that the tribofilm is heteroge neously distributed on the disc’s surface and its formation can vary under different normal loads. However, the tribofilm behavior was not correlated with the BFM’s wear. Another important parameter related with the wear of friction ma terials is the contact plateaus, whose theory was firstly proposed by Erikson [18,19]. According to the author, the contact plateaus consists of flat areas formed on the BFM’s surface, which represent the real contact area. The contact plateaus are originated in the tribological contact between brake pad and disc, arising few microns from the sur face of the BFM. Typically, the contact plateaus are classified as primary and secondary plateaus. The primary plateaus are formed by harder materials used in BFM, such as metal fibers, for instance. They contribute the most to the wear of the pad and disc, producing debris on the tribological interface. Some of these debris may form nucleation sites around the primary plateaus, leading to the formation of secondary plateaus [19,20]. An earlier study from the current research group [21], however, reported that the secondary plateaus can be formed without the support of the primary plateaus. According to the literature [17,22], an increase in the load lead to a raise in the area ratio of the contact plateaus. On the other hand, the contact plateaus are destroyed when their size become too large [1]. However, investigations correlating the area ratio of the contact plateaus with the wear regime (moderate and severe) were not found in the literature. The current article aims to achieve a better understanding of the BFM’s transition in wear regime (from moderate to severe) under different operating conditions. Therefore, braking operations under different normal loads were performed on a laboratory-scale tribometer in order to change the wear regime of the BFMs from moderate to severe. The following parameters were monitored during the tests: COF, tribo film deposited on the brake disc’s surface and the area ratio of contact plateaus formed in the BFMs.
tribometer. The composition of the brake friction material used in the experiments is shown in Table 1. Two specimens of the same composition, referred as B.1 and B.2, were employed in the experiments. The discs used in this study are made of gray cast iron, which is a typical material used in commercial brake discs. They have diameter and thickness of 159 mm and 12 mm, respectively. Before each experiment, the surface of the discs was abraded with sandpaper with different grit sizes (240, 300, 400, 500, 600 and 1500, in this sequence), and a roughness Ra lower than 0.20 μm was obtained. A completely new disc was used as counterface for each specimen of BFM. For each disc, a K-type thermocouple was embedded at 2 mm from the disc’s surface, at the same sliding radius of the BFMs, which corresponds to the center of the friction track. 2.3. Setup of experiments The procedure consists of two sections. Firstly, a conditioning sec tion, based on SAE J2522 – AK Master procedure [26], was carried out in the laboratory-scale tribometer in order to avoid running-in effects. Various contact pressures and temperatures were applied in this section. Results of the conditioning are not the main goal of the present work and so they are not discussed in this paper. Then, a characterization section was applied, where six different levels of contact pressure were used. This procedure was performed aiming to reach the limit pressure, which marks the transition between moderate and severe wear regimes. Thus, contact pressures of 1.9, 2.4, 2.9, 3.4, 3.9 and 4.3 MPa were employed, representing a vehicle hydraulic pressure of 40, 50, 60, 70, 80 and 90 bar, respectively. Table 2 shows the operating parameters used in the present study. In the characterization section, the initial-final sliding velocity (7.5–2.8 m/s) represents a light vehicle moving from 80 to 30 km/h. The deceleration varies according to the pressure used, aiming to keep the same amount of frictional work for all operational condi tions. Initial temperature, measured in the disc, was set 100 � C for all experiments. Finally, 50 braking were applied for each hydraulic pres sure. All contact pressures were applied for both specimens (B.1 and B.2). 2.4. Wear measurements An electronic balance (with precision of �0.1 mg) was used for measuring the weight of the specimens (mass loss) during braking ap plications. The mass losses were measured every set of contact pressure, i.e. for each 50 braking applications. By doing so, the amount of wear provided in each condition was monitored. Finally, the mass loss of the specimens was normalized by the sum of the braking energy of the 50 braking applications performed in each pressure. In the literature, many researchers [22,27–29] have normalized the specimen wear rate in a similar way. The energy performed on the laboratory-scale tribometer is given by Eq. (2):
2. Methodology 2.1. The laboratory-scale tribometer The COF of the brake pads selected in this study was measured using a laboratory-scale tribometer. This test rig (pin-on-disc system) was designed according to the Theory of Scale described in Sanders [23]. So, it can reproduce the same contact pressure, sliding velocity and tem perature as those found in automotive brake systems [24]. By consid ering the sliding radius (R) and measuring data of braking torque and normal force, the laboratory-scale tribometer allows to calculate the coefficient of friction (μ), as shown in Eq. (1).
μ¼
T FN R
50 X
Eb ¼
FN μtðvi
(2)
vf Þ
n¼1
where Eb is the energy produced by the braking process for each pres sure [N.m], n is the number of a given braking application, FN is the normal force [N], μ is the average of the COF determined in each braking
(1)
where T is the braking torque [N.m], FN is the normal force [N] exerted by the BFM on the disc’s surface and R is the sliding radius [m]. Details of the machine’s operation and design can be found in Refs. [24,25].
Table 1 Approximate formulation of the brake friction material.
2.2. Specimens and discs preparation The brake pads employed in the experiments, classified as semimetallic, were machined into a cylindrical shape with diameter of 15 mm in order to be used as specimen in the laboratory-scale 2
Categories
Volume [%]
Binder Fibers Abrasives Lubricants Fillers
30 22 5 28 15
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Table 2 Set-up of the tests performed on the laboratory-scale tribometer. Section
Braking applications
Hydraulic pressure [bar]
Contact pressure [MPa]
Initial-final sliding velocity [m/s]
Conditioning Characterization
275 50
In accordance with AK Master procedure (SAE J2522) 40 1.9 7.5–2.8 50 2.4 60 2.9 70 3.4 80 3.9 90 4.3
application [ ], t is the duration of a given braking [s], Vi is the initial sliding velocity between pad and disc [m/s] and Vf is the final sliding velocity between pad and disc [m/s]. Thus, the normalized wear is determined by Eq. (3): ΔWN ¼
1000⋅Δm Eb
Deceleration [m/ s2 ]
Initial temperature of the disc [� C]
3.15 3.93 4.72 5.51 6.28 7.09
100
obtained by the Matlab’s code, the lower is the amount of tribofilm deposited on the disc’s surface. It is worth noting that, in order to avoid border effect (indicated in Fig. 3-a), 1 mm of each side of the friction track was removed, resulting in an analysis over 13 mm of the friction track.
(3)
3. Results and discussion
where ΔWN is the normalized wear [mg/N.m] and Δm is the mass loss of the specimen [g].
3.1. Coefficient of friction and specimen wear results Tribological results (COF and wear of BFM) obtained for the different hydraulic pressures are shown in Fig. 2. A very similar behavior of the COF for specimens B.1 and B.2 is seen, showing a good repeatability between the repetitions. In the pressure ranging from 40 to 60 bar, a decrement of the COF is observed with the increase of the pressure. This behavior is also reported by other authors [17,33,34]. At 70 and 80 bar, the COF curve of the BFMs exhibited a near stable behavior. A limit pressure (LP) was found in 90 bar. At the LP, the BFMs suffered a sudden increase in COF, which is also followed by a considerable increase in wear (Fig. 3-b). In the current work, at the severe wear regime (at 90 bar), the wear of the BFMs was approximately 13 times higher than in the moderate wear regime (at pressures lower than LP). Differently of the results found in the literature [11], which reported a transition in wear rate with the increase in normal loads at high temperatures (above 300 � C), the present study exhibited a transition in wear regimes at low temperatures. The maximum temperature during braking was lower than 130 � C for all the experiments. For this reason, the high level of BFM’s wear rate seems not to be associated only with the thermal degradation of the phenolic binder, as concluded by Ref. [11].
2.5. Micrography and analysis of contact plateaus of the BFMs Several micrographs of the specimen’s surface were captured in the experiments. The micrographs, taken in a central region of the BFMs, were captured by a Carl Zeiss microscope, model Axio Lab. A1, equipped with a digital CMOS camera with 5 megapixels of resolution. Five mi crographs of the BFMs were captured for each pressure condition: at braking 1st, 5th, 10th, 30th and 50th. The micrographs were registered in the exact same region of the specimens, enabling a qualitative anal ysis of the evolution of the contact plateaus over braking applications. Besides, the micrographs were analyzed by a computer code written in Matlab (Mathworks®), by which it is possible to estimate quantitatively the area ratio of contact plateaus (in % of the nominal specimen area). Details about the code’s operation is found in earlier studies of the present research group [30–32]. 2.6. Analysis of the tribofilm on the disc’s surface Micrographs of the disc’s surface were performed with a stereo Carl Zeiss microscope, model Stemi 508, equipped with a digital CMOS camera with 5 megapixels of resolution. A micrography of the disc’s surface was taken at the end of each braking application. The images were taken in 8-bits greyscale mode (Fig. 1-a), where each pixel of the image has an intensity ranging from 0 (black) to 255 (white). A com puter code in Matlab (Mathworks®) was used to analyze the pixels in tensity of the whole image. Then, the code calculates the average intensity of all pixels, which indicates the amount of tribofilm on the disc’s friction track (Fig. 1-b). The higher the average of pixel intensity
3.2. Qualitative analysis of contact plateaus in the BFMs and tribofilm on the disc’s surface The surface microscopy of the disc and BFMs (B.1 and B.2) in the moderate (stop #250) and severe (stop #280) wear regimes are shown in Fig. 3. Those optical images allow to compare and visualize differ ences in the surfaces. It is worth noting that due to the flatness of the contact plateaus, they appear bright (white) in the images of the BFMs
Fig. 1. Disc’s surface analysis: a) micrography, b) friction track to be analyzed, subtracted by 1 mm in both sides. 3
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Fig. 2. Results of (a) coefficient of friction and (b) normalized wear of the specimens B.1 and B.2.
Fig. 3. Images of the BFMs and disc’s surfaces in the transition between moderate and severe wear regimes: (a) B.1 and (b) B.2.
since they have a good reflection of the incident light of the microscope [32]. It is possible to observe a drastic change in both BFMs and disc surfaces when the transition in the wear regime occurs. In case of the BFMs, a clear reduction of the amount of contact plateaus have been
seen when the severe wear regime is reached. In case of the disc, a brighter friction track is observed in the severe wear regime for both specimens B.1 and B.2, which that means the tribofilm deposited on the disc’s surface was greatly removed. The tribofilm is known for playing a lubricity role in the pad-disc systems. In the situation that exist a 4
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tribofilm developed in the pad/disc contact, a predominance of tribooxidative wear occurs, leading to a relatively low and stable COF, as well as moderate wear of the specimen. Although, when the tribofilm is removed from the contact between the tribopair, it leads to a predom inance of abrasive/adhesive wear mechanism, which increases the COF and contributes to the destruction of the contact plateaus. A sequence of images (Fig. 4) shows the destruction of a contact plateau during the transition in the wear regime. In order to make visualization easier, a line is used in the images to highlight the approximate boundaries of the contact plateau. It is observed that for all stops at 80 bar and also for the first braking application at 90 bar (stop #251), the contact plateau under analysis did not suffer a significant change in its structure. However, in the 5th (stop #255) and 10th (stop #260) braking applications at 90 bar a consider able destruction of the contact plateau is seen. In other words, when limit pressure is reached (90 bar in the present study), there is a destruction of the contact plateaus, as well as a removal of tribofilm deposited on the disc, as explained earlier. These two factors explain the sudden increase in the BFM’s wear, as shown in Fig. 2-b. In the next section, a quantitative analysis of the contact plateaus and tribofilm on the disc surface is presented.
applications, while the tribofilm is a slower process, taking some dozens of stops to have a significant effect. The hypothesis sustained by the experimental observations is that the tribofilm is gradually reduced in thickness during the subsequent stops at the limit pressure. The reduc tion is noted once the layer of tribofilm is removed and the disc material (underneath the tribofilm) is exposed. Beyond the reduction of the tri bofilm when the LP is reached, a slight reduction of the tribofilm is observed with the increased of pressure (from 40 to 80 bar). This behavior is supported by the literature [12], in which the thickness and uniformity of the tribofilm decreases with the severity of the test con ditions (pressure and sliding speed). However, opposite results are also reported [35], where an increase in the severity leads to an increase in the formation of tribofilm. At 90 bar, when the LP is reached, a sudden reduction of the area of contact plateaus occurs, as it was observed in qualitative analysis (Fig. 3), which leads to an increase in specimen wear. On the other hand, the area of contact plateaus shows a stable value (around 18 � 2%) in the moderate wear (between 50 and 80 bar), except at 40 bar, where it is possible to observe a considerable variation. During the steps of pressure (stops #50, #100, #150 and #200), a slight variation in area of contact plateaus is observed in the respective first braking application. This phenomenon indicates that the contact plateaus present an immediate response when the pressure is increased, returning to a stable value as more braking applications are performed. According to the literature, the area ratio of contact plateaus increases with normal load and be comes “saturated” at a certain level of load [22]. In the present exper iments, the area ratio of contact plateaus seems to have reached a saturated level at 50 bar, since it did not increase as the pressure is raised. Fig. 6 shows the results of COF, specimen’s wear, area ratio of contact plateaus and average in pixel intensity of the disc’s surface for the experiment B.2. It is possible to observe a very similar behavior between the exper iments with B.2 and B.1. At the LP (90 bar), a sudden increase in COF and a reduction in the tribofilm are observed. While the first increases in few stops, the second takes many braking applications to show a sub stantial change. Compared to B.1 experiments, the area ratio of contact plateaus for the specimen B.2 shows the same stable value (around 18 � 2%) in the moderate wear regime (between 50 and 80 bar), except at the 5th braking application of the pressure with 80 bar. At this stop, a sudden decrease in the area ratio of contact plateaus occurred, where the same level as observed in the severe wear regime (around 10 � 2%) is seen. Then, the value stabilizes by itself in the next stops performed at 80 bar. A rapid and momentary change in the wear regime (moderate to severe) at 80 bar is a hypothesis to explain this phenomenon, since this pressure is close to the LP point, in which the severe wear occurs (90 bar). However, this momentary change to severe wear was not enough to cause a considerable increase in wear, COF or tribofilm on the disc’s surface. This may be an evidence that, among the analyzed pa rameters, the area ratio of contact plateaus is the first parameter to change when severe wear occurs.
3.3. Quantitative results of contact plateaus in the BFMs and tribofilm on the disc’s surface Results of COF, wear, area ratio of contact plateaus and the average pixel intensity obtained for the friction track on the disc for the exper iment performed with specimen B.1 are shown in Fig. 5. It is worth remembering the interpretation of the analysis of the pixel intensity: the darker the image, the lower is the intensity of pixels captured by the camera, meaning that disc’s surface is more covered by tribofilm. Be sides, for a better understanding of the graph, the average pixel intensity was normalized from 0 (darker) to 1 (brighter). It is possible to observe the same processes described earlier, i.e. when the LP is reached (90 bar), a sudden increase in COF is observed, as well as a reduction in the tribofilm. But both processes have different timing: the COF shows a considerable increase in very few braking
4. Conclusions The experiments indicate that there is a limit pressure (LP) over which the wear regime of BFMs changes from moderate to severe. Re sults of COF, tribofilm over the disc’s surface, area ratio of contact plateaus and wear of the specimens seem to be associated with each other when the LP is reached. The main results can be listed as follows: � A limit pressure (LP) at 90 bar was found for the BFM selected in the present study � The wear of the BFMs in the severe wear regime was approximately 13 times higher than in the moderate wear regime � An abrupt reduction of the area ratio of contact plateaus occurs in severe wear regime
Fig. 4. Detail of the destruction of a contact plateau during the transition in the wear regime for specimen B.1: (a) stop #210, b) stop #230, c) stop #250, d) stop #251, e) stop #255 and f) stop #260. 5
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Fig. 5. Coefficient of friction, normalized wear, average in pixel intensity of the disc and area ratio of contact plateaus (specimen B.1).
Fig. 6. Coefficient of friction, normalized wear, average in pixel intensity of the disc and area ratio of contact plateaus (specimen B.2).
� The tribofilm deposited on the disc’s surface was greatly removed when the LP is reached � A sudden increase in COF occurs in the severe regime � The transition in wear regimes can occur at low temperatures, indicating that the severe wear of BFMs is not associated only with the thermal degradation of phenolic binder � Among the analyzed parameters, area ratio of contact plateaus and COF quickly respond to the change in the wear regime, while the tribofilm seems a slower process
the work reported in this paper. Acknowledgements This work was partially supported by the Brazilian research agency CNPq (Conselho Nacional de Desenvolvimento Científico e Tec �gico). The authors also are grateful to the Fras-le S/A company by nolo the support during the realization of this work. References
Declaration of competing interest
[1] G.P. Ostermeyer, L. Wilkening, Experimental investigations of the topography dynamics in brake pads, SAE Int. J. Passeng. Cars - Mech. Syst. 6 (2013).
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 6
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Contents lists available at ScienceDirect
Wear journal homepage: http://www.elsevier.com/locate/wear
Effect of pressure in the transition between moderate and severe wear regimes in brake friction materials L.Y. Barros a, *, J.C. Poletto a, D. Buneder a, P.D. Neis a, N.F. Ferreira a, R.P. Pavlak b, L.T. Matozo b a b
Universidade Federal do Rio Grande do Sul, Laboratory of Tribology, Sarmento Leite 425, Porto Alegre, Brazil Fras-le S.A, Rod RS-122, 10945, Km 66, Caxias do Sul, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords: Wear regime Brake friction materials Tribofilm Contact plateaus
The transition in wear regime is a tribological phenomenon observed in many materials, where a sudden change in wear regime (from moderate to severe) occurs when operating parameters (e.g. pressure or sliding velocity) are changed. However, this phenomenon is rarely explored in brake friction materials (BFMs). Severe wear regime for BFMs results in an increase in cost and leads to environmental issues due to the high amount of loss in material. This work aims to study the transition in BFM applying different levels of pressure, where a BFM was rubbed against a gray cast iron disc in a tribometer. The tribofilm deposited on the disc surface and the area ratio of contact plateaus of BFMs were analyzed. The coefficient of friction (COF) and the BFM’s wear were also measured. The wear regime changed from moderate to severe at a limit pressure of 90 bar. At the severe wear regime, a sudden increase in the COF was observed, and the wear of the BFM was approximately 13 times higher than in the moderate wear regime. It was also seen a removal of the tribofilm deposited on the disc’s surface and a reduction of contact plateaus over the BFM’s surface.
1. Introduction A good automotive brake friction material (BFM) needs fulfill many performance and quality requirements under several operating condi tions, such as a stable level of coefficient of friction (COF), light weight, low noise and acceptable cost versus performance [1,2]. Besides, it is desirable that the BFM presents low levels of wear in order to decrease economical cost and reduce environmental impact. In the last recent years, the wear of BFMs was addressed in many studies. The crescent concern with the particles that are released to the environment due to the wear of BFMs led to restrictions on the use of some hazardous in gredients in the formulation of friction materials. For example, the use of copper (a common ingredient found in BFM formulations) in brake pads will have to be reduced considerably in the next years due to recent U.S. legislations [3–5]. Environmental concerns about the impact of haz ardous ingredients added to BFMs have been rising in the last years. For this reason, there is a great research effort towards a better under standing of BFM’s wear. Tribological studies on polymer [6,7] and steel [8–10] have reported a transition in the wear regime from moderate to severe when the contact pressure and/or sliding velocity are increased. For BFMs,
however, this transition is rarely reported in the literature. One of the few publications about this subject is found in Ref. [11]. In this study, the authors performed a set of experiments with two types of semi-metallic BFMs. The normal load was varied under a constant sliding velocity, and a low wear rate was observed for low normal load. The authors mentioned that under low wear rate, abrasive wear mech anism was dominant for either the brake pad and disc. At high normal load, however, a sudden transition in wear rate was observed, and a high level of wear was reported for both BFM and disc. As described by the researchers, the high level of wear rate was caused by thermal degra dation of the phenolic binder and tribo-oxidation of the BFM and disc. Fernandes et al. [12] also studied the transition in wear regime for friction materials (used in clutch systems). The authors performed ex periments varying load and sliding velocity aiming to achieve the transition. According to the researchers, when the transition was reached, the thickness and the uniformity of the tribofilm decreased with the increase of the severity of the tests. An important parameter for the wear in BFMs is the tribofilm (fric tion layer or friction film) which is formed on the brake disc’s surface. The friction film is originated from wear particles generated in the friction pad-disc contact. It is mainly constituted by iron oxide [13,14]
* Corresponding author. E-mail address: [email protected] (L.Y. Barros). https://doi.org/10.1016/j.wear.2019.203112 Received 14 August 2019; Received in revised form 30 October 2019; Accepted 31 October 2019 Available online 3 November 2019 0043-1648/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: L.Y. Barros, Wear, https://doi.org/10.1016/j.wear.2019.203112
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and prevents the direct contact between the BFM and the disc [15]. Thus, the tribofilm reduces wear in BFMs, as well as it contributes for the stabilization of the COF [16]. In this sense, it is important to have a better comprehension of tribofilm formation/destruction during braking. However, a limited number of researchers has studied the tri bofilm formed in brake materials. In an investigation of the current research group (Barros et al. [17]), the tribofilm formation was inves tigated for two brake materials: a non-asbestos organic (NAO) and a semi-metallic (SM). It was concluded that the tribofilm is heteroge neously distributed on the disc’s surface and its formation can vary under different normal loads. However, the tribofilm behavior was not correlated with the BFM’s wear. Another important parameter related with the wear of friction ma terials is the contact plateaus, whose theory was firstly proposed by Erikson [18,19]. According to the author, the contact plateaus consists of flat areas formed on the BFM’s surface, which represent the real contact area. The contact plateaus are originated in the tribological contact between brake pad and disc, arising few microns from the sur face of the BFM. Typically, the contact plateaus are classified as primary and secondary plateaus. The primary plateaus are formed by harder materials used in BFM, such as metal fibers, for instance. They contribute the most to the wear of the pad and disc, producing debris on the tribological interface. Some of these debris may form nucleation sites around the primary plateaus, leading to the formation of secondary plateaus [19,20]. An earlier study from the current research group [21], however, reported that the secondary plateaus can be formed without the support of the primary plateaus. According to the literature [17,22], an increase in the load lead to a raise in the area ratio of the contact plateaus. On the other hand, the contact plateaus are destroyed when their size become too large [1]. However, investigations correlating the area ratio of the contact plateaus with the wear regime (moderate and severe) were not found in the literature. The current article aims to achieve a better understanding of the BFM’s transition in wear regime (from moderate to severe) under different operating conditions. Therefore, braking operations under different normal loads were performed on a laboratory-scale tribometer in order to change the wear regime of the BFMs from moderate to severe. The following parameters were monitored during the tests: COF, tribo film deposited on the brake disc’s surface and the area ratio of contact plateaus formed in the BFMs.
tribometer. The composition of the brake friction material used in the experiments is shown in Table 1. Two specimens of the same composition, referred as B.1 and B.2, were employed in the experiments. The discs used in this study are made of gray cast iron, which is a typical material used in commercial brake discs. They have diameter and thickness of 159 mm and 12 mm, respectively. Before each experiment, the surface of the discs was abraded with sandpaper with different grit sizes (240, 300, 400, 500, 600 and 1500, in this sequence), and a roughness Ra lower than 0.20 μm was obtained. A completely new disc was used as counterface for each specimen of BFM. For each disc, a K-type thermocouple was embedded at 2 mm from the disc’s surface, at the same sliding radius of the BFMs, which corresponds to the center of the friction track. 2.3. Setup of experiments The procedure consists of two sections. Firstly, a conditioning sec tion, based on SAE J2522 – AK Master procedure [26], was carried out in the laboratory-scale tribometer in order to avoid running-in effects. Various contact pressures and temperatures were applied in this section. Results of the conditioning are not the main goal of the present work and so they are not discussed in this paper. Then, a characterization section was applied, where six different levels of contact pressure were used. This procedure was performed aiming to reach the limit pressure, which marks the transition between moderate and severe wear regimes. Thus, contact pressures of 1.9, 2.4, 2.9, 3.4, 3.9 and 4.3 MPa were employed, representing a vehicle hydraulic pressure of 40, 50, 60, 70, 80 and 90 bar, respectively. Table 2 shows the operating parameters used in the present study. In the characterization section, the initial-final sliding velocity (7.5–2.8 m/s) represents a light vehicle moving from 80 to 30 km/h. The deceleration varies according to the pressure used, aiming to keep the same amount of frictional work for all operational condi tions. Initial temperature, measured in the disc, was set 100 � C for all experiments. Finally, 50 braking were applied for each hydraulic pres sure. All contact pressures were applied for both specimens (B.1 and B.2). 2.4. Wear measurements An electronic balance (with precision of �0.1 mg) was used for measuring the weight of the specimens (mass loss) during braking ap plications. The mass losses were measured every set of contact pressure, i.e. for each 50 braking applications. By doing so, the amount of wear provided in each condition was monitored. Finally, the mass loss of the specimens was normalized by the sum of the braking energy of the 50 braking applications performed in each pressure. In the literature, many researchers [22,27–29] have normalized the specimen wear rate in a similar way. The energy performed on the laboratory-scale tribometer is given by Eq. (2):
2. Methodology 2.1. The laboratory-scale tribometer The COF of the brake pads selected in this study was measured using a laboratory-scale tribometer. This test rig (pin-on-disc system) was designed according to the Theory of Scale described in Sanders [23]. So, it can reproduce the same contact pressure, sliding velocity and tem perature as those found in automotive brake systems [24]. By consid ering the sliding radius (R) and measuring data of braking torque and normal force, the laboratory-scale tribometer allows to calculate the coefficient of friction (μ), as shown in Eq. (1).
μ¼
T FN R
50 X
Eb ¼
FN μtðvi
(2)
vf Þ
n¼1
where Eb is the energy produced by the braking process for each pres sure [N.m], n is the number of a given braking application, FN is the normal force [N], μ is the average of the COF determined in each braking
(1)
where T is the braking torque [N.m], FN is the normal force [N] exerted by the BFM on the disc’s surface and R is the sliding radius [m]. Details of the machine’s operation and design can be found in Refs. [24,25].
Table 1 Approximate formulation of the brake friction material.
2.2. Specimens and discs preparation The brake pads employed in the experiments, classified as semimetallic, were machined into a cylindrical shape with diameter of 15 mm in order to be used as specimen in the laboratory-scale 2
Categories
Volume [%]
Binder Fibers Abrasives Lubricants Fillers
30 22 5 28 15
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Table 2 Set-up of the tests performed on the laboratory-scale tribometer. Section
Braking applications
Hydraulic pressure [bar]
Contact pressure [MPa]
Initial-final sliding velocity [m/s]
Conditioning Characterization
275 50
In accordance with AK Master procedure (SAE J2522) 40 1.9 7.5–2.8 50 2.4 60 2.9 70 3.4 80 3.9 90 4.3
application [ ], t is the duration of a given braking [s], Vi is the initial sliding velocity between pad and disc [m/s] and Vf is the final sliding velocity between pad and disc [m/s]. Thus, the normalized wear is determined by Eq. (3): ΔWN ¼
1000⋅Δm Eb
Deceleration [m/ s2 ]
Initial temperature of the disc [� C]
3.15 3.93 4.72 5.51 6.28 7.09
100
obtained by the Matlab’s code, the lower is the amount of tribofilm deposited on the disc’s surface. It is worth noting that, in order to avoid border effect (indicated in Fig. 3-a), 1 mm of each side of the friction track was removed, resulting in an analysis over 13 mm of the friction track.
(3)
3. Results and discussion
where ΔWN is the normalized wear [mg/N.m] and Δm is the mass loss of the specimen [g].
3.1. Coefficient of friction and specimen wear results Tribological results (COF and wear of BFM) obtained for the different hydraulic pressures are shown in Fig. 2. A very similar behavior of the COF for specimens B.1 and B.2 is seen, showing a good repeatability between the repetitions. In the pressure ranging from 40 to 60 bar, a decrement of the COF is observed with the increase of the pressure. This behavior is also reported by other authors [17,33,34]. At 70 and 80 bar, the COF curve of the BFMs exhibited a near stable behavior. A limit pressure (LP) was found in 90 bar. At the LP, the BFMs suffered a sudden increase in COF, which is also followed by a considerable increase in wear (Fig. 3-b). In the current work, at the severe wear regime (at 90 bar), the wear of the BFMs was approximately 13 times higher than in the moderate wear regime (at pressures lower than LP). Differently of the results found in the literature [11], which reported a transition in wear rate with the increase in normal loads at high temperatures (above 300 � C), the present study exhibited a transition in wear regimes at low temperatures. The maximum temperature during braking was lower than 130 � C for all the experiments. For this reason, the high level of BFM’s wear rate seems not to be associated only with the thermal degradation of the phenolic binder, as concluded by Ref. [11].
2.5. Micrography and analysis of contact plateaus of the BFMs Several micrographs of the specimen’s surface were captured in the experiments. The micrographs, taken in a central region of the BFMs, were captured by a Carl Zeiss microscope, model Axio Lab. A1, equipped with a digital CMOS camera with 5 megapixels of resolution. Five mi crographs of the BFMs were captured for each pressure condition: at braking 1st, 5th, 10th, 30th and 50th. The micrographs were registered in the exact same region of the specimens, enabling a qualitative anal ysis of the evolution of the contact plateaus over braking applications. Besides, the micrographs were analyzed by a computer code written in Matlab (Mathworks®), by which it is possible to estimate quantitatively the area ratio of contact plateaus (in % of the nominal specimen area). Details about the code’s operation is found in earlier studies of the present research group [30–32]. 2.6. Analysis of the tribofilm on the disc’s surface Micrographs of the disc’s surface were performed with a stereo Carl Zeiss microscope, model Stemi 508, equipped with a digital CMOS camera with 5 megapixels of resolution. A micrography of the disc’s surface was taken at the end of each braking application. The images were taken in 8-bits greyscale mode (Fig. 1-a), where each pixel of the image has an intensity ranging from 0 (black) to 255 (white). A com puter code in Matlab (Mathworks®) was used to analyze the pixels in tensity of the whole image. Then, the code calculates the average intensity of all pixels, which indicates the amount of tribofilm on the disc’s friction track (Fig. 1-b). The higher the average of pixel intensity
3.2. Qualitative analysis of contact plateaus in the BFMs and tribofilm on the disc’s surface The surface microscopy of the disc and BFMs (B.1 and B.2) in the moderate (stop #250) and severe (stop #280) wear regimes are shown in Fig. 3. Those optical images allow to compare and visualize differ ences in the surfaces. It is worth noting that due to the flatness of the contact plateaus, they appear bright (white) in the images of the BFMs
Fig. 1. Disc’s surface analysis: a) micrography, b) friction track to be analyzed, subtracted by 1 mm in both sides. 3
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Fig. 2. Results of (a) coefficient of friction and (b) normalized wear of the specimens B.1 and B.2.
Fig. 3. Images of the BFMs and disc’s surfaces in the transition between moderate and severe wear regimes: (a) B.1 and (b) B.2.
since they have a good reflection of the incident light of the microscope [32]. It is possible to observe a drastic change in both BFMs and disc surfaces when the transition in the wear regime occurs. In case of the BFMs, a clear reduction of the amount of contact plateaus have been
seen when the severe wear regime is reached. In case of the disc, a brighter friction track is observed in the severe wear regime for both specimens B.1 and B.2, which that means the tribofilm deposited on the disc’s surface was greatly removed. The tribofilm is known for playing a lubricity role in the pad-disc systems. In the situation that exist a 4
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tribofilm developed in the pad/disc contact, a predominance of tribooxidative wear occurs, leading to a relatively low and stable COF, as well as moderate wear of the specimen. Although, when the tribofilm is removed from the contact between the tribopair, it leads to a predom inance of abrasive/adhesive wear mechanism, which increases the COF and contributes to the destruction of the contact plateaus. A sequence of images (Fig. 4) shows the destruction of a contact plateau during the transition in the wear regime. In order to make visualization easier, a line is used in the images to highlight the approximate boundaries of the contact plateau. It is observed that for all stops at 80 bar and also for the first braking application at 90 bar (stop #251), the contact plateau under analysis did not suffer a significant change in its structure. However, in the 5th (stop #255) and 10th (stop #260) braking applications at 90 bar a consider able destruction of the contact plateau is seen. In other words, when limit pressure is reached (90 bar in the present study), there is a destruction of the contact plateaus, as well as a removal of tribofilm deposited on the disc, as explained earlier. These two factors explain the sudden increase in the BFM’s wear, as shown in Fig. 2-b. In the next section, a quantitative analysis of the contact plateaus and tribofilm on the disc surface is presented.
applications, while the tribofilm is a slower process, taking some dozens of stops to have a significant effect. The hypothesis sustained by the experimental observations is that the tribofilm is gradually reduced in thickness during the subsequent stops at the limit pressure. The reduc tion is noted once the layer of tribofilm is removed and the disc material (underneath the tribofilm) is exposed. Beyond the reduction of the tri bofilm when the LP is reached, a slight reduction of the tribofilm is observed with the increased of pressure (from 40 to 80 bar). This behavior is supported by the literature [12], in which the thickness and uniformity of the tribofilm decreases with the severity of the test con ditions (pressure and sliding speed). However, opposite results are also reported [35], where an increase in the severity leads to an increase in the formation of tribofilm. At 90 bar, when the LP is reached, a sudden reduction of the area of contact plateaus occurs, as it was observed in qualitative analysis (Fig. 3), which leads to an increase in specimen wear. On the other hand, the area of contact plateaus shows a stable value (around 18 � 2%) in the moderate wear (between 50 and 80 bar), except at 40 bar, where it is possible to observe a considerable variation. During the steps of pressure (stops #50, #100, #150 and #200), a slight variation in area of contact plateaus is observed in the respective first braking application. This phenomenon indicates that the contact plateaus present an immediate response when the pressure is increased, returning to a stable value as more braking applications are performed. According to the literature, the area ratio of contact plateaus increases with normal load and be comes “saturated” at a certain level of load [22]. In the present exper iments, the area ratio of contact plateaus seems to have reached a saturated level at 50 bar, since it did not increase as the pressure is raised. Fig. 6 shows the results of COF, specimen’s wear, area ratio of contact plateaus and average in pixel intensity of the disc’s surface for the experiment B.2. It is possible to observe a very similar behavior between the exper iments with B.2 and B.1. At the LP (90 bar), a sudden increase in COF and a reduction in the tribofilm are observed. While the first increases in few stops, the second takes many braking applications to show a sub stantial change. Compared to B.1 experiments, the area ratio of contact plateaus for the specimen B.2 shows the same stable value (around 18 � 2%) in the moderate wear regime (between 50 and 80 bar), except at the 5th braking application of the pressure with 80 bar. At this stop, a sudden decrease in the area ratio of contact plateaus occurred, where the same level as observed in the severe wear regime (around 10 � 2%) is seen. Then, the value stabilizes by itself in the next stops performed at 80 bar. A rapid and momentary change in the wear regime (moderate to severe) at 80 bar is a hypothesis to explain this phenomenon, since this pressure is close to the LP point, in which the severe wear occurs (90 bar). However, this momentary change to severe wear was not enough to cause a considerable increase in wear, COF or tribofilm on the disc’s surface. This may be an evidence that, among the analyzed pa rameters, the area ratio of contact plateaus is the first parameter to change when severe wear occurs.
3.3. Quantitative results of contact plateaus in the BFMs and tribofilm on the disc’s surface Results of COF, wear, area ratio of contact plateaus and the average pixel intensity obtained for the friction track on the disc for the exper iment performed with specimen B.1 are shown in Fig. 5. It is worth remembering the interpretation of the analysis of the pixel intensity: the darker the image, the lower is the intensity of pixels captured by the camera, meaning that disc’s surface is more covered by tribofilm. Be sides, for a better understanding of the graph, the average pixel intensity was normalized from 0 (darker) to 1 (brighter). It is possible to observe the same processes described earlier, i.e. when the LP is reached (90 bar), a sudden increase in COF is observed, as well as a reduction in the tribofilm. But both processes have different timing: the COF shows a considerable increase in very few braking
4. Conclusions The experiments indicate that there is a limit pressure (LP) over which the wear regime of BFMs changes from moderate to severe. Re sults of COF, tribofilm over the disc’s surface, area ratio of contact plateaus and wear of the specimens seem to be associated with each other when the LP is reached. The main results can be listed as follows: � A limit pressure (LP) at 90 bar was found for the BFM selected in the present study � The wear of the BFMs in the severe wear regime was approximately 13 times higher than in the moderate wear regime � An abrupt reduction of the area ratio of contact plateaus occurs in severe wear regime
Fig. 4. Detail of the destruction of a contact plateau during the transition in the wear regime for specimen B.1: (a) stop #210, b) stop #230, c) stop #250, d) stop #251, e) stop #255 and f) stop #260. 5
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Fig. 5. Coefficient of friction, normalized wear, average in pixel intensity of the disc and area ratio of contact plateaus (specimen B.1).
Fig. 6. Coefficient of friction, normalized wear, average in pixel intensity of the disc and area ratio of contact plateaus (specimen B.2).
� The tribofilm deposited on the disc’s surface was greatly removed when the LP is reached � A sudden increase in COF occurs in the severe regime � The transition in wear regimes can occur at low temperatures, indicating that the severe wear of BFMs is not associated only with the thermal degradation of phenolic binder � Among the analyzed parameters, area ratio of contact plateaus and COF quickly respond to the change in the wear regime, while the tribofilm seems a slower process
the work reported in this paper. Acknowledgements This work was partially supported by the Brazilian research agency CNPq (Conselho Nacional de Desenvolvimento Científico e Tec �gico). The authors also are grateful to the Fras-le S/A company by nolo the support during the realization of this work. References
Declaration of competing interest
[1] G.P. Ostermeyer, L. Wilkening, Experimental investigations of the topography dynamics in brake pads, SAE Int. J. Passeng. Cars - Mech. Syst. 6 (2013).
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence 6
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