Effect of braking pressure and braking speed on the ...

Composites Science and Technology 70 (2010) 959–965

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Effect of braking pressure and braking speed on the tribological properties of C/SiC aircraft brake materials Shangwu Fan *, Litong Zhang, Laifei Cheng, Guanglai Tian, Shangjie Yang National Key Laboratory of Thermostructure Composite Materials, P.O. Box 547, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China

a r t i c l e

i n f o

Article history: Received 4 December 2009 Received in revised form 31 January 2010 Accepted 12 February 2010 Available online 19 February 2010 Keywords: A. Ceramic–matrix composites B. Friction/wear E. Liquid melt infiltration

a b s t r a c t Effects of braking pressure and braking speed on the tribological properties of C/SiC aircraft brake materials has been studied using a disk-on-disk type laboratory scale dynamometer. The braking temperature increased with increasing braking speed and was less affected by changes in braking pressure. The friction coefficient increased to the maximum value at 10 m/s and then fell with the increase of braking speed at the same braking pressure. The friction coefficient decreased with the increase of braking pressure at the same braking speed. The wear rate increased with braking speed increasing at the same braking pressure. The wear rate was little at braking speed below 20 m/s, and rapidly increased when the braking speed exceeded 20 m/s. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction C/SiC composites are new type of high performance brake materials developed after powder metallurgy (PM) and C/C composites [1]. The advantages of PM brakes are the maturity in material development and low cost, while the application of PM brakes is limited by their major disadvantages such as high weight, poor performance at high temperature, and prone to corrosion [2,3]. C/C brakes were developed to overcome the disadvantages of the PM brakes, exhibiting excellent thermal and mechanical properties with lower weight. However, the C/C brakes suffer from insufficient stability of friction coefficient caused by humidity [4–6]. Combining the advantages of PM and C/C brakes, and overcoming most of the disadvantages of PM and C/C brakes, C/SiC composites exhibit some other superior performances such as high and stable friction coefficient, long life, low wear rate, and lower sensibility to surroundings and oxidation [3,7–10]. In the early 1990s, Krenkel et al. at the German Aerospace Center (DLR) in Stuttgart started investigations of C/SiC composites for high performance automobile applications [11]. Up to now, the C/ SiC brakes have been successfully applied to Porsche, Ferrari and Daimler Chrysler [12,13]. Investigations by the Ceramic Composite Aircraft Brake Consortium of USA indicated that C/SiC materials may be feasible as a next-generation aircraft brake material [7]. Nowadays, plenty of work have been done for developing C/SiC aircraft brake materials [7,14–19]. In 2008, the C/SiC aircraft brakes were installed on a certain airplane for trial flight and achieved

* Corresponding author. Tel.: +86 29 8849 4622; fax: +86 29 8849 4620. E-mail addresses: [email protected], [email protected] (S. Fan). 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.02.012

success which were prepared by Northwestern Polytechnical University and Xi’an Aviation Braking Science and Technology Co., Ltd. in China [20]. However, the systematical research for the effects of braking parameters on the tribological properties of C/SiC aircraft brake materials has seldom been reported. In the present paper, effects of braking parameters on the tribological properties of C/SiC aircraft brake materials are systematically investigated. 2. Experiments 2.1. Fabrication The C/SiC aircraft brake materials were fabricated by chemical vapor infiltration combined with liquid melt infiltration (LMI). The C/SiC were composed of 65 wt.% C, 27 wt.% SiC, and 8 wt.% Si. The density and porosity were 2.1 g cm3 and 4.4%, respectively [18]. 2.2. Testing methods The tribological properties were tested on a disk-on-disk type laboratory scale dynamometer (Fig. 1) by reference to [18]. The kinetic energy absorbed by braking was supplied by the inertia wheels, which were driven by a DC motor. The tested specimens acted as both rotor and stator. When the inertial wheel, which rotated with the rotor specimen simultaneously, was accelerated to a certain rotational velocity, braking was achieved through the friction between the rotor and stator under a certain braking pressure. Rotating velocity, braking moment, and braking time were

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S. Fan et al. / Composites Science and Technology 70 (2010) 959–965

Fig. 1. Schematic diagram of the dynamometer. (1) clamp nut, (2) inertial wheel, (3) bearing, (4) clutch, (5) rotor holder, (6) rotor, (7) stator, (8) stator holder, (9) pressing cylinder, (10) strap, (11) motor, and (12) lathe-bed.

recorded by computer. The friction coefficient can be calculated from the following equation:

M ¼ lðr 1 þ r 2 ÞP=2

ð1Þ

where M is moment, l friction coefficient, P braking pressure, r1 inner radius, and r2 outer radius. The temperature near the friction surface (braking temperature) was measured during friction tests with a thermocouple inserted in a hole. The hole for measuring temperature was close to the friction surface in circumference surface of the stator. The tested braking temperature was the temperature closed to the friction surface. The friction test conditions are listed in Table 1. The tests under each testing condition were repeated for 20 times. The microstructures were observed by optical microscopy (OLYMPUS PM-T3, OLYMPUS LEXT-OLS3000), and SEM (S-4700). The debris granularity was analyzed by the particle size analyzer (Malvern MasterSizer S-version). 3. Results and discussion 3.1. Effect of braking pressure and braking speed on braking temperature The effect of braking pressure and braking speed on braking temperature is shown in Fig. 2. It indicated that the braking temperature was increased with the braking speed increasing, and was less influenced by braking pressure. During braking process, the brake disk translated the kinetic energy into heat energy through friction acting. The total braking energy was increased with the braking speed increasing when the inertia kept constant, and was not related with the braking pressure. Therefore, the braking temperature was increased with the braking speed increasing, and was less affected by braking pressure.

Fig. 2. The effect of braking pressure and braking speed on braking temperature.

shown in Fig. 3. The friction coefficient increased to the maximum value at 10 m/s and then fell afterwards with the increase of braking speed at the same braking pressure. The friction coefficient decreased with the increase of braking pressure at the same braking speed. 3.2.1. Effect of braking speed on frictional properties Friction force is the result from the mechanical action and intermolecular force between the two friction surfaces. A large amount of micro peaks or valleys on the friction surface are unavoidable. The micro peaks are generally referred to as asperities. The mechanical force included actions, such as the micro peaks and valleys meshed with each other, leading to deformation, shearing, breaking, and the asperities embedded into the dual surface as a result of ploughing on the friction surfaces, and so on. The carbon fiber, pyrolytic carbon, SiC and Si are all brittle phase. At the same braking pressure, the areal braking energy was increased with braking speed increasing. When the braking speed was lower than 10 m/s, the impact and shear force between asperities in the dual friction surface were small, so that the asperities could not be completely sheared. With braking speed increasing, the areal braking energy gradually increased, so that many asperities created brittle fracture. Many new asperities and little debris were generated with the original ones rupture. As the number of the asperities increase, the asperities ploughing, meshing, deforming, shearing and breaking were increased. And the results were that the resistance continuously increased and the friction coefficient increased. The schematic illustration of the action of asperities is shown in Fig. 4. Fig. 5 is the optical micrograph of friction surface under braking speed at 10 m/s and pressure at 0.9 MPa for C/SiC brake materials. In Fig. 5, some grooves left on the friction surfaces caused by the asperities ploughing. Therefore, the friction coefficient increased with increasing braking speed when the braking speed was lower than 10 m/s.

3.2. Effect of braking pressure and braking speed on frictional properties The relationships among the braking speed, braking pressure and the average friction coefficient of C/SiC brake materials were

Table 1 The parameters of friction testing condition. Testing condition Inertia (kg m2) Braking speed (m s1) Braking pressure (MPa)

A 5

B

C

10

0.235 15 20 0.5, 0.7, 0.9

D

E

F

25

28

Fig. 3. The effect of braking pressure and braking speed on the average friction coefficient of C/SiC brake materials.


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rotor new asperities

asperity

asperity

new asperities debris

stator

(a) Asperities meshing

(b) Asperities brittle fracture

(c) New asperities ploughing Fig. 4. Schematic illustration of the action of asperities in friction surface of C/SiC brake materials.

Fig. 5. The optical micrograph of friction surface under braking speed at 10 m/s and pressure at 0.9 MPa for C/SiC brake materials.

When the braking speed exceeded 10 m/s, the impact and shear force between asperities in the dual friction surface became high enough to cut the asperities. With braking speed increasing, the impact and shear force between asperities in the dual friction surface were rapidly increased, so that many asperities were sheared and pulverized to form a lot of debris. Under the effect of braking

pressure, the debris filled the gap between peaks and valleys, thereby, the ploughing effects of the asperities was reduced. With braking speed increasing, more and more debris was produced, increasing covered areas of friction surface by the debris. Finally, a continuous friction film formed on the friction surface, resulting in the friction resistance and the friction coefficient decrease. The schematic illustration of the debris filled and the friction film formed is shown in Fig. 6 [21]. Fig. 7 is the optical micrographs of the typical friction surface at 0.9 MPa and different braking speed. Fig. 7 shows that with braking speed increasing, part of region in the friction surface was covered by debris (as shown in Fig. 7c), the non-continuous friction film was formed gradually (as shown in Fig. 7d and e), and finally the continuous friction film was formed (as shown in Fig. 7f). The main reason was analyzed as follows. First of all, with braking speed increasing, the braking energy was increased. As a result, the great impact and shear force was generated between asperities in the friction surface. Under the impact and shear force, a great deal of debris was produced for the asperities in the friction surface meshing, deforming, shearing and breaking. With the increasing of braking energy, the debris was grinded to be super fine powder under compressive and shearing stress. At

debris

(a) Debris filled in the gap between peaks and valleys friction film

(b) Friction film formed between the friction surfaces Fig. 6. Schematic illustration of the debris filled and the friction film formed.

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(a) 5 m/s

(b) 10 m/s

(c)15 m/s

(d) 20 m/s

(e) 25 m/s

(f) 28 m/s

Fig. 7. The optical micrographs of the typical friction surface under 0.9 MPa pressure at different braking speed.

5 m/s, the braking energy was low, and the braking time was short. The debris between dual friction surface underwent small impact and shear force, and the debris was grinded short time between dual friction surface. Thereby, the debris was big and the size was up to 60 lm at 5 m/s (As shown in Fig. 8). When braking speed was higher than 10 m/s, the braking energy was great, braking time was extended, and the force between asperities in the friction surface was strong. The debris was ruptured and grinded under impact, shear and compressive stress. And the time the debris grinded in dual friction surface was extended. As a result, the number of the big debris dropped off with the braking speed increasing, when the speed was higher than 10 m/s. As shown in Fig. 8, when the speed was higher than 25 m/s, most of the debris were super fine, and with sizes less than 3 lm. Fig. 9 is granularity analysis of debris. It shows that the size of debris gradually decreases with the increasing of braking speed. The results were consistent with Fig. 8. The debris mainly distributed from 1.4 to 88 lm at 5 m/s, from 1 to 52 lm at 10 m/s, from 0.42 to 42 lm at 20 m/s, and from 0.15 to 13 lm at 28 m/s.

With the braking speed increasing, the temperature of the friction surface rose gradually. The debris was inclined to deform at high temperature. The applied force between super fine debris was great because of the high surface free energy of them. Consequently, the debris tended to form friction film under high temperature and braking pressure, which lowered the contact area of friction surface and resulted in the decreasing of the action of asperities in the friction surface, the friction resistance, and the friction coefficient. Fig. 10 presents the macrostructures of the friction surfaces at different braking speed under 0.9 MPa pressure. Fig. 10 also shows as above results. With the increasing of the braking speed, the macro worn trace continuously decreased and formed slippy friction film, which lowered the action of the asperities in the friction surface, and hence decreased the friction resistance. 3.2.2. Effect of braking pressure on frictional properties At the same braking speed, the interacting compressive stress between the asperities was increased with braking pressure increasing. Under considerable compressive stress, cracks were


(a) 5 m/s

(b) 10 m/s

(c) 15 m/s

(d) 20 m/s

(e) 25 m/s

(f) 28 m/s

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Fig. 8. SEM photos of typical micrographs of the debris under 0.9 MPa pressure at different braking speed.

to deform and be milled fine debris under considerable compressive stress, finally formed friction film in the friction surface. Therefore, the friction coefficient decreased with the increase of braking pressure at the same braking speed. 3.3. Effect of braking pressure and braking speed on wear properties

Fig. 9. The size distribution of the debris at different braking speed under 0.9 MPa pressure.

emerged and extended in the interacting asperities, and caused debris. The debris filled in the gap between peaks and valleys, thereby, the ploughing action of the asperities was reduced (as shown in Fig. 11). Moreover, during braking, the debris was easier

The relationships among braking pressure, braking speed and wear rate were shown in Fig. 12. It shows that the wear rate increased with braking speed increasing at the same braking pressure. The wear rate is small when the braking speed is lower than 20 m/s. However, the wear rate is rapidly increased with the braking speed increasing when the braking speed is higher than 20 m/s. With braking speed increasing, the impact and shear force between asperities in the dual friction surface, and the ploughing force on the dual friction surface by asperities were continuously increased, which made that the wear rate increased with the braking speed increasing. When the braking speed were higher than 20 m/s, the temperature on the friction surface was likely to be

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5 m/s

10 m/s

20 m/s

15 m/s

25 m/s

28 m/s

Fig. 10. The macrostructures of the friction surface at different braking speed under 0.9 MPa pressure.

Cracks

asperity

(a) cracks generated in asperites

debris

(b) debris formed

Fig. 11. Schematic illustration of cracks and debris formed under compression stress.

Fig. 12. The effect of braking pressure and braking speed on the wear rate of C/SiC brake materials.

higher than 450 °C. It induced the carbon matrix and carbon fibers closed to the friction surface to be oxidized, the compressive and inter-laminar shear strength closed to the friction surface to be lowered, which made the wear be deteriorate, and the wear be aggravated with the pressure increasing. When the braking speed was less than 20 m/s, the effect of braking pressure on the wear rate was less. When the braking speed was low, the temperature on the friction surface was low, which led the oxidation to be less, and the supporting capacity closed to the friction surface to be reduced very little. Consequently, the effect of braking pressure on the wear rate was less, when the braking speed was lower than 20 m/s.

4. Conclusions

(1) The braking temperature increased with increasing braking speed and was less affected by changes in braking pressure. (2) The braking speed and braking pressure significantly affected the friction coefficient. The friction coefficient increased to the maximum value at 10 m/s and then fell afterwards with the increase of braking speed at the same braking pressure. The friction coefficient decreased with the increase of braking pressure at the same braking speed.


(3) The wear rate increased with braking speed increasing at the same braking pressure. The wear rate was small and the effect of braking pressure on the wear rate was less, when the braking speed was lower than 20 m/s. However, the wear rate was rapidly increased with the braking speed increasing when the braking speed was higher than 20 m/s. Acknowledgements The authors acknowledge the financial support of Natural Science Foundation of China (Contract Nos. 90405015 and 50672076), Program for Changjiang Scholars and Innovative Research Team in University, and project supported by the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 46-QP-2009). References [1] Krenkel W. Design of ceramic brake pads and disks. Ceram Eng Sci Proc 2002;23(3):319–29. [2] Haug T, Rebstock K. New material technologies for brakes. In: Breuer B, Dausend U, editors. Advanced brake technology, vol. 4. USA: Advance; 2003. [3] Krenkel W, Heidenreich B, Renz R. C/C–SiC composites for advanced friction systems. Adv Eng Mater 2002;4(7):427–36. [4] Chen JD, Chern Lin JH, Ju CP. Effect of humidity on the frictional behavior of carbon–carbon composites. Wear 1996;193:38–47. [5] Yen BK. Influence of water vapor and oxygen on the tribology of carbon materials with sp2 valence configuration. Wear 1996;192:208–15. [6] Blanco C, Bermejo J, Marsh H, Menendez R. Chemical and physical properties of carbon as related to brake performance. Wear 1997;213:1–12.

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