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Microstructural characteristics on the surface and subsurface of semimetallic automotive friction materials during braking process
Journal of Materials Processing Technology 140 (2003) 694–699
Microstructural characteristics on the surface and subsurface of semimetallic automotive friction materials during braking process R.J. Talib a,∗ , A. Muchtar b , C.H. Azhari b a
AMREC, SIRIM Bhd, Lot 34, Jalan Hi-Tech 2/4, Kulim Hi-Tech Park, 09000 Kulim, Malaysia b Faculty of Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia
Abstract In this study, a series of friction tests on semimetallic automotive friction materials were conducted on a friction test machine by pressing test samples against a rotating cast iron brake disc, thus simulating actual braking. After each friction test, the morphological changes of the wear surface and subsurface were investigated using scanning electron microscopy. Microstructural examinations showed that the major wear mechanisms in operation during braking are comprised of the following: (i) abrasive, (ii) adhesive, (iii) fatigue, (iv) delamination, and (v) thermal. The wear mechanism and wear transition are found to be influenced by the applied loads and braking times. In the study of the subsurface morphology, microcracks generated in the subsurface are thought to be due to the following phenomena; (i) growth of microvoids, (ii) coalescence of microvoids, (iii) coalescence of second phase particles, and (iv) coalescence of microvoids and second phase particles. The microcracks generated in the subsurface grew and propagated parallel to the sliding surface as the braking times as well as applied loads are increased. Finally, the microcracks grew and joined each other producing wear particles on subsequent braking. These mechanical and thermal failures manifested a complex wear mechanism, causing plastic collapse in the local region which subsequently produced wear particles in different shapes depending on the modes of failure. © 2003 Elsevier B.V. All rights reserved. Keywords: Friction materials; Microstructure; Microcrack; Microvoid; Wear mechanism
1. Introduction Previous study carried out by earlier researchers have found that there are four failure modes operative during braking: (i) chemical changes [1], (ii) thermoinstability [2], (iii) wear mechanism [3,4], and (iv) microcracks [5,6]. These phenomena result in the changes of physical properties and microstructure of friction materials with increased braking times and applied loads. The microstructural changes phenomena such as wear mechanism and microcracks can be elucidated by studying the surface morphology of the wear surface and subsurfaces. Heat generated during braking caused the surface temperature to increase with braking time and load. The onset of degradation of friction material starts at 230 ◦ C, and the degree of degradation increases with temperature within the range of 269–400 ◦ C [7]. Above the degradation temperature, the binding properties of resin become weak. When this happens the wear rate increases exponentially as observed by Rhee [8] and Talib and Azhari [9]. High temper∗ Corresponding author. Tel.: +60-3401-7166; fax: +60-3403-3224. E-mail address: [email protected] (R.J. Talib).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00769-6
atures also decrease the yield strength of the materials and lead to changes in the wear mechanism and the real contact configuration [10]. Wear mechanisms operating in brake lining materials were found to be a complex combination of abrasion, adhesion fatigue, delamination and thermal. Rhee [8] concluded that automotive asbestos-reinforced friction materials wear by the abrasive and adhesive mechanisms at low temperature (below 230 ◦ C) and by the pyrolysis mechanism at high temperatures. On the other hand, the wear of semimetallic friction materials appears to be a complex mixture of the adhesive, oxidation and surface fatigue types of wear [3]. In another study, Scieszka [11] reported that the predominant wear mechanisms operating during the braking process were abrasion, adhesion and thermal. In the early stage of sliding, plastic deformation is generated in the subsurface and results in the generation of dislocation in the subsurface. Dislocation pile-up is generated as the movement of dislocations are blocked by the presence of inclusions and hard particles in the brake pad materials. As a result of dislocation pile-up, the microvoids are formed in the subsurface [12]. On subsequent sliding, the microcracks may be generated due to coalescence or shearing of
R.J. Talib et al. / Journal of Materials Processing Technology 140 (2003) 694–699
these voids. As sliding progresses, the microcracks propagate parallel to the sliding direction and are finally plucked off from the wear surface. This delamination theory has been strengthened by other researchers such as Jahanmir [13], Alpas et al. [14], and Talib et al. [15]. In this study, the microstructural changes on the surface and subsurface of brake pads were observed using scanning electron microscopy (SEM). Based on this observation, a postulate on the wear mechanisms involved during braking processes has been obtained.
2. Materials and experimental methods In this study, a semimetallic friction material for passenger vehicles were subjected to five different applied loads (100, 200, 400, 600 and 800 N) and braking times (3, 5, 9, 12 and 15 min), while the rotating velocity were kept constant at 750 rpm. Test samples were investigated for mechanical wear by pressing them against a rotating pearlitic gray cast iron rotor. The tests were conducted using a Schenck friction test machine (Table 1 and Fig. 1). Test samples were cut from the brake pad backing plate with dimensions of 20±1 mm × 30 ± 1 mm × 15 ± 1 mm. The samples were glued to the backing plate and placed in the oven at 180 ◦ C for 1 h. This backing plate was then attached to brake callipers on both sides of the brake disc. After each test, the samples were then subjected to microstructural examination using scanning electron microscopy model PHILIP SL 300. Samples for surface examination were cut from the backing plate, cleaned with compressed air and then coated with gold. Samples for subsurface examination were further cut parallel and perpendicular to the sliding surface using a fine cutter. These were
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mounted and polished to a surface finish of 1 m. Samples were then coated with gold using a Polaron sputter coater.
3. Results and discussion 3.1. Adhesive wear In the early stage of braking, the morphology of the wear surfaces showed the peak asperities were deformed and have started to form metallic junctions as well as to start growing (Fig. 2). This process is a manifestation of adhesion wear mechanisms. On the second braking, the peak asperities were sheared and became blunt. The two-way transfer during sliding caused the formation of transfer layers on both sides of the sliding surfaces as observed elsewhere [16,17]. With subsequent braking, the wear surfaces were covered with multi layers of transfer materials, which had been compacted, smeared, sheared and flattened (Fig. 3). When the load was increased to 600 N, the plastic flow of the transfer layer was first observed after a braking time of 12 min (Fig. 4). As the load was increased to 800 N, this phenomenon occurred at an earlier braking time of 9 min. In this process, the bulk temperature recorded was 540 ◦ C, but the flash temperature during contact could increase between 1000 and 1125 ◦ C [18]. As a result of the high temperatures, the transfer layers were melted and plastic flow occurred
Table 1 Schenck friction test machine specifications Items
Specifications
Machine type Power Disc diameter Disc material Disc hardness
Schenck friction test machine 75 kW 300 mm Gray cast iron with pearlitic microstructure 190–220 HB
Speed sensor
Sample
Electric Motor 75 kW
Fig. 2. Formation of transfer patches on the wear surface. Applied load 400 N and braking time 3 min.
Applied Force Load cell
Brake disc Temperature sensor
Fig. 1. Friction test machine layout.
Fig. 3. On subsequent braking, the multiple transfer layers were formed on the wear surface. Applied load 200 N and braking time 12 min.
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Fig. 4. Plastic flow formation on the wear surface. Applied load 800 N and braking time 9 min.
Fig. 6. The wear surface became rougher as the applied load was increased. Applied load 800 N and braking time 3 min.
in the direction of sliding as braking process progressed. Thus, it is concluded from the microstructural examinations that the adhesion mechanism is a continuous process during braking under all load conditions and the degree of adhesion increases with increased braking times and applied loads. 3.2. Abrasive wear On set of braking, the harder peak asperities were ploughed into the wear surface (Fig. 5). This process is a manifestation of abrasion wear mechanisms. The abrasion mechanism was not observed on the second braking and this phenomenon was taught to be due to blunting of the peak asperities during the braking processes. The abrasion mechanism was also not observed at the lower applied load of 100 N and this may due to the fact that the low load was not enough for the harder asperities to plough into the surface of brake pad. As the applied load was increased, the surface morphology showed that the surface became rougher and this showed that the peak asperities have penetrated deeper into wear surfaces (Fig. 6). 3.3. Fatigue wear The process of fatigue wear generation began with the formation of plastic deformation. As the braking process proceeded, the cyclic-plastic deformation resulted in the formation of striations. Striations were first observed dur-
Fig. 5. The peak asperities ploughing on the surface. Applied load 400 N and braking time 3 min.
Fig. 7. Striations appear on the second braking. Applied load 200 N and braking time 6 min.
ing braking time of 6 min with applied loads of 100 and 200 N. At the beginning of the striation formation, the distance between the striations was rather small (Fig. 7). With subsequent braking, the striation spacing became larger. The striations spacing also became larger with increased applied load (Fig. 8). As braking proceeded, the fatigue microcrack nucleate on the wear surface of the brake pad where pits and spallation were found (Fig. 9). Above the applied load of 200 N, no striations and symptom of fatigue mechanism were found occurring on the wear surfaces of the brake pads. From the above observations, it may be concluded that, the fatigue mechanism was observed only under applied loads of 200 N and below.
Fig. 8. Distance between striations increases with increased in applied load. Applied load 400 N and braking time 3 min.
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Fig. 9. Fatigue microcrack nucleated at the place where pits and spalls were found. Applied load 100 N and braking time 12 min.
Fig. 12. Subsurface microcrack due to coalescence of microvoids. Applied load 400 N and braking time 6 min.
Fig. 10. Second phase particle nucleated in the subsurface. Applied load 100 N and braking time 12 min.
3.4. Delamination wear Delamination wear starts with the generation of plastic deformation. As the braking proceeded, dislocations were generated due to the accumulation of plastic deformation and finally developed dislocation pile-up in the subsurface as it movement was blocked by the present of inclusions and hard particles in the composition of brake pad materials (Fig. 10). As a result of dislocation pile-up, the microvoids the will be formed in the subsurface. The microvoids in the subsurface will be elongated due to the plastic shear deformation as shown in Fig. 11, and then generate subsurface microcrack as the braking proceed.
Fig. 11. Subsurface microcrack due to shearing of microviods. Applied load 100 N and braking time 12 min.
The generation of the subsurface microcrack was also observed due to the coalescence of microvoids (Fig. 12). Coalescence of hard particles was observed in this study and this phenomena result in the generation of the subsurface microcracks (Fig. 13). The subsurface microcrack was also observed as a result of coalescence of microvoid and second phase particle. Fig. 14 shows that the surface microcraks grew and propagated parallel to the wear surface as the applied loads or braking times increased. Finally, the wear particles flake off from the wear surface when reaching the critical length, which subsequently produced wear particles in the shapes of delaminated sheets. From the microstructural examination, it was observed that the wear particle delaminated on the third braking (12 min) under the applied load of 100 and 200 N (Fig. 15). When the applied load increased to 400 N and above, the wear particle delaminated at earlier braking time of 9 min. It was also found that the microvoids nucleated deeper below the surface as the applied loads and braking times
Fig. 13. Generation of subsurface microcrack due to coalescence of second phase particles. Applied load 200 N and braking time 15 min.
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Fig. 14. The propagation of a microcrack parallel to the wear surface. Applied load 600 N and braking time 9 min.
Fig. 16. Formation of thermo granules. Applied load 600 N and braking time 3 min.
increased. This phenomenon was thought to be the reason why the delaminating process starts earlier when the applied load increased. The above observation shows that the delamination operates under all load conditions and the threshold value was 200 N. Above this threshold load value, the delamination generated earlier. The sequence of the delamination wear process could be concluded as follows: (i) generation of microvoids as a result of dislocation pile-up; (ii) generation of subsurface microcracks as a result of microvoids growth, coalescence of microvoids, coalescence of second phase particles and coalescence of microvoid and second phase particle; (iii) growth and propagation of microcracks parallel to the sliding surface; and finally (iv) thin plates of wear particles were disposed from the surface after reaching a critical length. From the micrograph, it was observed that thermal granules were located at higher position on the wear surfaces and thus became the contact area as the sliding progress (Fig. 16). As the applied loads or braking times increased, the temperature rise at the contact area introduced thermal stresses, which could superimpose onto the mechanical stresses [19]. This phenomena result in increased in total contact stress, thus results in the generation of thermomicrocrack (Fig. 17). Thermal granules were first seen during braking time of 9 min with the applied load of 200 N and the maximum bulk temperature recorded under this applied load was 349 ◦ C. As the load increased to 400 and 600 N, thermal granules
were generated earlier at braking time of 6 and 3 min, respectively. The maximum bulk temperature recorded under applied loads of 400 and 600 N were 471 and 518 ◦ C, respectively. Thermomicrocracks were first seen during the braking time of 12 min with the applied load of 200 N. As the load increased, thermomicrocracks were generated earlier at braking time of 9 min. With subsequent braking, the microcracks grew, propagated and finally joined each other to form multiple microcracks. Thermal granules and thermomicrocracks were not observed under the applied load of 100 N. From microstructural examinations, it was observed that the wear mechanism transisted from one combination to one combination as braking time increased. The transition period and combination of wear was found depending on the load applied during braking. Under the load of 100 N, it was observed that the wear mechanism transisted from adhesion to a complex mixture of adhesion, fatigue and delamination. When the applied load increased to 200 N, the wear mechanism transisted from a complex mixture of adhesion and abrasion to a complex mixture of adhesion, fatigue, delamination and thermal. Under the applied load of 400 N, the wear mechanism transisted from a complex mixture of adhesion and abrasion to a complex mixture of adhesion, delamination and thermal. Where as under the applied load of 600 N and above, the wear mechanism transisted from a complex mixture of adhesion, abrasion and thermal to a complex mixture of adhesion, thermal and delamination.
Fig. 15. Micrograph shows the wear particle to be delaminated from the surface. Applied load 200 N and braking time 12 min.
Fig. 17. Thermomicrocraks generated at the top most of the contact area. Applied load 400 N and braking time 9 min.
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4. Conclusions The major wear phenomena observed in operation during braking were: (i) abrasive, (ii) adhesive, (iii) fatigue, (iv) delamination, and (v) thermal. It was observed that the wear mechanism to be a complex one and no one mechanism to be fully operating. Besides, the wear mechanism and wear transition from one combination to another combination were influenced by braking times and applied loads. The study of the subsurface microstructures further strengthened the wear mechanism postulated. Subsurface mechanism observed further strengthened the wear mechanism observed in braking. The microstructural studies showed that the microcracks generated in the subsurface were thought due to: (i) the growth of microvoids, (ii) coalescence of microvoids, (iii) coalescence of second phase particles, and (iv) coalescence of microvoids and second phase particles. Acknowledgements The authors would like to thank the staff of Mechanical and Automotive Engineering Testing Unit and Metal Performance Technical Centre, SIRIM Bhd for their assistance in this study. References [1] M.G. Jacko, in: Proceedings of the International Conference on Wear of Materials 1977, St. Louis, MO, 1977, pp. 541–546.
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[2] J.R. Barber, Proc. Roy. Soc. A 312 (1969) 381394. [3] T.A. Libsch, S.K. Rhee, in: Proceedings of the International Conference on Wear of Materials 1977, St. Louis, MO, 1977, pp. 555–549. [4] C.H. Azhari, R.J. Talib, J. Phys. Sci. 9 (1998) 51–78. [5] T.L. Ho, in: Proceedings of the International Conference on Wear of Materials 1977, St. Louis, MO, 1977, pp. 70–76. [6] R.J. Talib, A. Muchtar, C.H. Azhari, in: Proceedings of the 10th Scientific Conference Electron Microscopy Conference, Malaysia, Kuala Lumpur, 2001, pp. 181–184. [7] X. Zhigao, L. Xiaofei, A research for the friction and wear properties of a metal-fiber-reinforced composite material. in: B. Wu (Ed.), Mechanical Properties Materials Design, vol. 5, Elsevier, Amsterdam, 1991, pp. 611–615. [8] S.K. Rhee, Wear 29 (1974) 371–393. [9] R.J. Talib, C.H. Azhari, in: Proceedings of the Second International Conference on Advances in Strategic Technologies, Putra Jaya, 2000, pp. 859–868. [10] H. So, Tribol. Int. 29 (5) (1996) 415–423. [11] S.F. Scieszka, Wear 64 (1980) 367–378. [12] N.P. Suh, Wear 25 (1973) 111–124. [13] S. Jahanmir, On mechanics and mechanism of laminar wear particle, in: R.M. Latanision, T.E. Fischer (Eds.), Advances in the Mechanics and Physics of Surfaces, vol. 3, Hardwood Academic Publishers, Switzerland, 1986, pp. 261–332. [14] A.T. Alpas, H. Hu, J. Zhang, Wear 162–164 (1993) 88–195. [15] R.J. Talib, A. Mucthar, C.H. Azhari, J. Asean Microsc. Soc. Thailand 18 (1) (2002) 64–65. [16] S.K. Rhee, M.G. Jacko, P.H.S. Tsang, Wear 146 (1991) 89–97. [17] Z. Siwei, Studies in non-asbestos friction materials: a brief review mechanical properties materials design, in: B. Wu (Ed.), Mechanical Properties Materials Design, vol. 5, Elsevier, Amsterdam, 1991, pp. 403–411. [18] A.E. Anderson, Wear of brake material, in: M. Peterson, W.O. Winer (Eds.), Wear Control Handbook 1980, ASME, New York, 1980, pp. 843–858. [19] B.Y. Ting, Ph.D. Dissertation, Georgia Institute of Technology, 1988.
Microstructural characteristics on the surface and subsurface of semimetallic automotive friction materials during braking process R.J. Talib a,∗ , A. Muchtar b , C.H. Azhari b a
AMREC, SIRIM Bhd, Lot 34, Jalan Hi-Tech 2/4, Kulim Hi-Tech Park, 09000 Kulim, Malaysia b Faculty of Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia
Abstract In this study, a series of friction tests on semimetallic automotive friction materials were conducted on a friction test machine by pressing test samples against a rotating cast iron brake disc, thus simulating actual braking. After each friction test, the morphological changes of the wear surface and subsurface were investigated using scanning electron microscopy. Microstructural examinations showed that the major wear mechanisms in operation during braking are comprised of the following: (i) abrasive, (ii) adhesive, (iii) fatigue, (iv) delamination, and (v) thermal. The wear mechanism and wear transition are found to be influenced by the applied loads and braking times. In the study of the subsurface morphology, microcracks generated in the subsurface are thought to be due to the following phenomena; (i) growth of microvoids, (ii) coalescence of microvoids, (iii) coalescence of second phase particles, and (iv) coalescence of microvoids and second phase particles. The microcracks generated in the subsurface grew and propagated parallel to the sliding surface as the braking times as well as applied loads are increased. Finally, the microcracks grew and joined each other producing wear particles on subsequent braking. These mechanical and thermal failures manifested a complex wear mechanism, causing plastic collapse in the local region which subsequently produced wear particles in different shapes depending on the modes of failure. © 2003 Elsevier B.V. All rights reserved. Keywords: Friction materials; Microstructure; Microcrack; Microvoid; Wear mechanism
1. Introduction Previous study carried out by earlier researchers have found that there are four failure modes operative during braking: (i) chemical changes [1], (ii) thermoinstability [2], (iii) wear mechanism [3,4], and (iv) microcracks [5,6]. These phenomena result in the changes of physical properties and microstructure of friction materials with increased braking times and applied loads. The microstructural changes phenomena such as wear mechanism and microcracks can be elucidated by studying the surface morphology of the wear surface and subsurfaces. Heat generated during braking caused the surface temperature to increase with braking time and load. The onset of degradation of friction material starts at 230 ◦ C, and the degree of degradation increases with temperature within the range of 269–400 ◦ C [7]. Above the degradation temperature, the binding properties of resin become weak. When this happens the wear rate increases exponentially as observed by Rhee [8] and Talib and Azhari [9]. High temper∗ Corresponding author. Tel.: +60-3401-7166; fax: +60-3403-3224. E-mail address: [email protected] (R.J. Talib).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00769-6
atures also decrease the yield strength of the materials and lead to changes in the wear mechanism and the real contact configuration [10]. Wear mechanisms operating in brake lining materials were found to be a complex combination of abrasion, adhesion fatigue, delamination and thermal. Rhee [8] concluded that automotive asbestos-reinforced friction materials wear by the abrasive and adhesive mechanisms at low temperature (below 230 ◦ C) and by the pyrolysis mechanism at high temperatures. On the other hand, the wear of semimetallic friction materials appears to be a complex mixture of the adhesive, oxidation and surface fatigue types of wear [3]. In another study, Scieszka [11] reported that the predominant wear mechanisms operating during the braking process were abrasion, adhesion and thermal. In the early stage of sliding, plastic deformation is generated in the subsurface and results in the generation of dislocation in the subsurface. Dislocation pile-up is generated as the movement of dislocations are blocked by the presence of inclusions and hard particles in the brake pad materials. As a result of dislocation pile-up, the microvoids are formed in the subsurface [12]. On subsequent sliding, the microcracks may be generated due to coalescence or shearing of
R.J. Talib et al. / Journal of Materials Processing Technology 140 (2003) 694–699
these voids. As sliding progresses, the microcracks propagate parallel to the sliding direction and are finally plucked off from the wear surface. This delamination theory has been strengthened by other researchers such as Jahanmir [13], Alpas et al. [14], and Talib et al. [15]. In this study, the microstructural changes on the surface and subsurface of brake pads were observed using scanning electron microscopy (SEM). Based on this observation, a postulate on the wear mechanisms involved during braking processes has been obtained.
2. Materials and experimental methods In this study, a semimetallic friction material for passenger vehicles were subjected to five different applied loads (100, 200, 400, 600 and 800 N) and braking times (3, 5, 9, 12 and 15 min), while the rotating velocity were kept constant at 750 rpm. Test samples were investigated for mechanical wear by pressing them against a rotating pearlitic gray cast iron rotor. The tests were conducted using a Schenck friction test machine (Table 1 and Fig. 1). Test samples were cut from the brake pad backing plate with dimensions of 20±1 mm × 30 ± 1 mm × 15 ± 1 mm. The samples were glued to the backing plate and placed in the oven at 180 ◦ C for 1 h. This backing plate was then attached to brake callipers on both sides of the brake disc. After each test, the samples were then subjected to microstructural examination using scanning electron microscopy model PHILIP SL 300. Samples for surface examination were cut from the backing plate, cleaned with compressed air and then coated with gold. Samples for subsurface examination were further cut parallel and perpendicular to the sliding surface using a fine cutter. These were
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mounted and polished to a surface finish of 1 m. Samples were then coated with gold using a Polaron sputter coater.
3. Results and discussion 3.1. Adhesive wear In the early stage of braking, the morphology of the wear surfaces showed the peak asperities were deformed and have started to form metallic junctions as well as to start growing (Fig. 2). This process is a manifestation of adhesion wear mechanisms. On the second braking, the peak asperities were sheared and became blunt. The two-way transfer during sliding caused the formation of transfer layers on both sides of the sliding surfaces as observed elsewhere [16,17]. With subsequent braking, the wear surfaces were covered with multi layers of transfer materials, which had been compacted, smeared, sheared and flattened (Fig. 3). When the load was increased to 600 N, the plastic flow of the transfer layer was first observed after a braking time of 12 min (Fig. 4). As the load was increased to 800 N, this phenomenon occurred at an earlier braking time of 9 min. In this process, the bulk temperature recorded was 540 ◦ C, but the flash temperature during contact could increase between 1000 and 1125 ◦ C [18]. As a result of the high temperatures, the transfer layers were melted and plastic flow occurred
Table 1 Schenck friction test machine specifications Items
Specifications
Machine type Power Disc diameter Disc material Disc hardness
Schenck friction test machine 75 kW 300 mm Gray cast iron with pearlitic microstructure 190–220 HB
Speed sensor
Sample
Electric Motor 75 kW
Fig. 2. Formation of transfer patches on the wear surface. Applied load 400 N and braking time 3 min.
Applied Force Load cell
Brake disc Temperature sensor
Fig. 1. Friction test machine layout.
Fig. 3. On subsequent braking, the multiple transfer layers were formed on the wear surface. Applied load 200 N and braking time 12 min.
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Fig. 4. Plastic flow formation on the wear surface. Applied load 800 N and braking time 9 min.
Fig. 6. The wear surface became rougher as the applied load was increased. Applied load 800 N and braking time 3 min.
in the direction of sliding as braking process progressed. Thus, it is concluded from the microstructural examinations that the adhesion mechanism is a continuous process during braking under all load conditions and the degree of adhesion increases with increased braking times and applied loads. 3.2. Abrasive wear On set of braking, the harder peak asperities were ploughed into the wear surface (Fig. 5). This process is a manifestation of abrasion wear mechanisms. The abrasion mechanism was not observed on the second braking and this phenomenon was taught to be due to blunting of the peak asperities during the braking processes. The abrasion mechanism was also not observed at the lower applied load of 100 N and this may due to the fact that the low load was not enough for the harder asperities to plough into the surface of brake pad. As the applied load was increased, the surface morphology showed that the surface became rougher and this showed that the peak asperities have penetrated deeper into wear surfaces (Fig. 6). 3.3. Fatigue wear The process of fatigue wear generation began with the formation of plastic deformation. As the braking process proceeded, the cyclic-plastic deformation resulted in the formation of striations. Striations were first observed dur-
Fig. 5. The peak asperities ploughing on the surface. Applied load 400 N and braking time 3 min.
Fig. 7. Striations appear on the second braking. Applied load 200 N and braking time 6 min.
ing braking time of 6 min with applied loads of 100 and 200 N. At the beginning of the striation formation, the distance between the striations was rather small (Fig. 7). With subsequent braking, the striation spacing became larger. The striations spacing also became larger with increased applied load (Fig. 8). As braking proceeded, the fatigue microcrack nucleate on the wear surface of the brake pad where pits and spallation were found (Fig. 9). Above the applied load of 200 N, no striations and symptom of fatigue mechanism were found occurring on the wear surfaces of the brake pads. From the above observations, it may be concluded that, the fatigue mechanism was observed only under applied loads of 200 N and below.
Fig. 8. Distance between striations increases with increased in applied load. Applied load 400 N and braking time 3 min.
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Fig. 9. Fatigue microcrack nucleated at the place where pits and spalls were found. Applied load 100 N and braking time 12 min.
Fig. 12. Subsurface microcrack due to coalescence of microvoids. Applied load 400 N and braking time 6 min.
Fig. 10. Second phase particle nucleated in the subsurface. Applied load 100 N and braking time 12 min.
3.4. Delamination wear Delamination wear starts with the generation of plastic deformation. As the braking proceeded, dislocations were generated due to the accumulation of plastic deformation and finally developed dislocation pile-up in the subsurface as it movement was blocked by the present of inclusions and hard particles in the composition of brake pad materials (Fig. 10). As a result of dislocation pile-up, the microvoids the will be formed in the subsurface. The microvoids in the subsurface will be elongated due to the plastic shear deformation as shown in Fig. 11, and then generate subsurface microcrack as the braking proceed.
Fig. 11. Subsurface microcrack due to shearing of microviods. Applied load 100 N and braking time 12 min.
The generation of the subsurface microcrack was also observed due to the coalescence of microvoids (Fig. 12). Coalescence of hard particles was observed in this study and this phenomena result in the generation of the subsurface microcracks (Fig. 13). The subsurface microcrack was also observed as a result of coalescence of microvoid and second phase particle. Fig. 14 shows that the surface microcraks grew and propagated parallel to the wear surface as the applied loads or braking times increased. Finally, the wear particles flake off from the wear surface when reaching the critical length, which subsequently produced wear particles in the shapes of delaminated sheets. From the microstructural examination, it was observed that the wear particle delaminated on the third braking (12 min) under the applied load of 100 and 200 N (Fig. 15). When the applied load increased to 400 N and above, the wear particle delaminated at earlier braking time of 9 min. It was also found that the microvoids nucleated deeper below the surface as the applied loads and braking times
Fig. 13. Generation of subsurface microcrack due to coalescence of second phase particles. Applied load 200 N and braking time 15 min.
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Fig. 14. The propagation of a microcrack parallel to the wear surface. Applied load 600 N and braking time 9 min.
Fig. 16. Formation of thermo granules. Applied load 600 N and braking time 3 min.
increased. This phenomenon was thought to be the reason why the delaminating process starts earlier when the applied load increased. The above observation shows that the delamination operates under all load conditions and the threshold value was 200 N. Above this threshold load value, the delamination generated earlier. The sequence of the delamination wear process could be concluded as follows: (i) generation of microvoids as a result of dislocation pile-up; (ii) generation of subsurface microcracks as a result of microvoids growth, coalescence of microvoids, coalescence of second phase particles and coalescence of microvoid and second phase particle; (iii) growth and propagation of microcracks parallel to the sliding surface; and finally (iv) thin plates of wear particles were disposed from the surface after reaching a critical length. From the micrograph, it was observed that thermal granules were located at higher position on the wear surfaces and thus became the contact area as the sliding progress (Fig. 16). As the applied loads or braking times increased, the temperature rise at the contact area introduced thermal stresses, which could superimpose onto the mechanical stresses [19]. This phenomena result in increased in total contact stress, thus results in the generation of thermomicrocrack (Fig. 17). Thermal granules were first seen during braking time of 9 min with the applied load of 200 N and the maximum bulk temperature recorded under this applied load was 349 ◦ C. As the load increased to 400 and 600 N, thermal granules
were generated earlier at braking time of 6 and 3 min, respectively. The maximum bulk temperature recorded under applied loads of 400 and 600 N were 471 and 518 ◦ C, respectively. Thermomicrocracks were first seen during the braking time of 12 min with the applied load of 200 N. As the load increased, thermomicrocracks were generated earlier at braking time of 9 min. With subsequent braking, the microcracks grew, propagated and finally joined each other to form multiple microcracks. Thermal granules and thermomicrocracks were not observed under the applied load of 100 N. From microstructural examinations, it was observed that the wear mechanism transisted from one combination to one combination as braking time increased. The transition period and combination of wear was found depending on the load applied during braking. Under the load of 100 N, it was observed that the wear mechanism transisted from adhesion to a complex mixture of adhesion, fatigue and delamination. When the applied load increased to 200 N, the wear mechanism transisted from a complex mixture of adhesion and abrasion to a complex mixture of adhesion, fatigue, delamination and thermal. Under the applied load of 400 N, the wear mechanism transisted from a complex mixture of adhesion and abrasion to a complex mixture of adhesion, delamination and thermal. Where as under the applied load of 600 N and above, the wear mechanism transisted from a complex mixture of adhesion, abrasion and thermal to a complex mixture of adhesion, thermal and delamination.
Fig. 15. Micrograph shows the wear particle to be delaminated from the surface. Applied load 200 N and braking time 12 min.
Fig. 17. Thermomicrocraks generated at the top most of the contact area. Applied load 400 N and braking time 9 min.
R.J. Talib et al. / Journal of Materials Processing Technology 140 (2003) 694–699
4. Conclusions The major wear phenomena observed in operation during braking were: (i) abrasive, (ii) adhesive, (iii) fatigue, (iv) delamination, and (v) thermal. It was observed that the wear mechanism to be a complex one and no one mechanism to be fully operating. Besides, the wear mechanism and wear transition from one combination to another combination were influenced by braking times and applied loads. The study of the subsurface microstructures further strengthened the wear mechanism postulated. Subsurface mechanism observed further strengthened the wear mechanism observed in braking. The microstructural studies showed that the microcracks generated in the subsurface were thought due to: (i) the growth of microvoids, (ii) coalescence of microvoids, (iii) coalescence of second phase particles, and (iv) coalescence of microvoids and second phase particles. Acknowledgements The authors would like to thank the staff of Mechanical and Automotive Engineering Testing Unit and Metal Performance Technical Centre, SIRIM Bhd for their assistance in this study. References [1] M.G. Jacko, in: Proceedings of the International Conference on Wear of Materials 1977, St. Louis, MO, 1977, pp. 541–546.
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