- Email: [email protected]
SiC brake materials
Tribology International 44 (2011) 25–28
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Wear mechanisms of the C/SiC brake materials Shangwu Fan n, Litong Zhang, Laifei Cheng, Jianxin Zhang, Shangjie Yang, Heyi Liu 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 in fo
abstract
Article history: Received 24 July 2009 Received in revised form 28 August 2010 Accepted 20 September 2010 Available online 29 September 2010
The C/SiC brake materials were fabricated by chemical vapor infiltration combined with liquid melt infiltration. The wear mechanisms of C/SiC brake materials were investigated. The main wear mechanisms were grain-abrasion, oxidation-abrasion, fatigue wear, and adhesive wear. These wear mechanisms always occurred simultaneously , and showed mutual enhancing effects between them. Grain-abrasion mainly was the result of hard SiC grain action. Adhesive wear could cause high wear rates and a large unstable friction coefficient. Si was the significant factor on the adhesive wear, so Si in the C/SiC brake materials must be removed. Crown Copyright & 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: C/SiC Wear mechanisms Brake materials
1. Introduction C/SiC composite is a new type of high performance brake material developed following powder metallurgy materials and C/C composites. C/SiC composites possess a series of outstanding advantages such as low density, good high-temperature resistance, high strength, excellent frictional properties, low wear rate, and long life [1–9]. The German Aerospace Center in Stuttgart (DLR) takes the lead in investigating the C/SiC brake materials in high performance automobile applications [3]. At present, the C/SiC brakes have been successfully applied to Porsche, Ferrari, Daimler Chrysler, and other high-end performance cars [2,10]. For high braking performance, the C/SiC brake materials are potential candidates for high speed trains, aircraft, and emergency brakes for elevators and cranes [2,4,8,11]. In 2008, the C/SiC aircraft brakes were first installed on a certain airplane for a trial flight and achieved success; these were prepared by Northwestern Polytechnical University and Xi’an Aviation Braking Science and Technology Co., Ltd., China [12]. The wear mechanisms of the materials are very important for the brake materials’ design. At present, research on wear mechanisms of C/SiC brake materials has seldom been reported. In the present communication, the wear mechanisms of the C/SiC brake materials were investigated.
2. Experiments The C/SiC brake materials were fabricated by chemical vapor infiltration combined with liquid melt infiltration (LMI). The C/SiC n
Corresponding author. Tel.: + 86 29 8849 4622; fax: + 86 29 8849 4620. E-mail addresses: [email protected], [email protected] (S. Fan).
brake materials were composed of 65 wt% C, 27 wt% SiC, and 8 wt% Si. The density and porosity were 2.1 g cm 3 and 4.4%, respectively [9]. The preparation process of the test samples is described in Ref. [9]. A part of friction tests was conducted on a disk-on-disk type laboratory scale dynamometeras described in Ref. [9], and the other part of friction tests was done on a full-scale dynamometer. The full-scale dynamometer was made by the Vickers company in the United Kingdom. Debris and friction surface were observed by optical microscopy (Olympus PM-T3, Olympus LEXT-OLS3000) and SEM (Hitachi S-4700). The phases were analyzed by XRD (XRD, PANALYTIAL, X’PERT, Netherlands). 3. Results and discussion 3.1. Grain-abrasion Brake wear debris and friction surface can help to understand the wear processes. Fig. 1 is the typical optical micrograph of the ploughing action of the hard debris on the friction surface of the C/SiC brake materials. The C/SiC brake materials are composed of C (carbon fiber and pyrolytic carbon), SiC, and Si. SiC particles, which acted as hard points, ploughed on the friction surface and then removed the material by micro-cutting. Fig. 2(a) shows the SiC morphology of the brake materials after removing silicon by etching the C/SiC specimen with a mixture of hydrofluoric and nitric acid (HNO3:HF¼ 4:1) at 40 1C for 48 h and burning carbon off at 700 1C for 20 h in air. The initial SiC grains in C/SiC brake materials presented a tetrahedron structure, while the SiC grains in the debris were smooth (Fig. 2(b)). The main reason is that the SiC grains in debris acted as hard points, ploughed, and micro-cut on the friction surface; the edge angle of the SiC grains
0301-679X/$ - see front matter Crown Copyright & 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2010.09.003
26
S. Fan et al. / Tribology International 44 (2011) 25–28
was worn blunt under friction fatigue stress, which indicated a grain-abrasion mechanism. 3.2. Fatigue wear During braking, cracks were generated and they propagated on the friction surface of the C/SiC brake materials (Fig. 3). The cracks were generated by high local compressive stress, frictional traction forces repeated for a large number of times on the friction surface in the course of braking, and then the matrix and fibers were peeled off. Under the cyclic stress, cracks were generated and propagated, which resulted in the formation of wear particles, which was fatigue wear.
3.4. Adhesive wear The C/SiC brake materials contained the Si phase. Near the friction surface, the Si phase may be softened by braking, which could generate adhesion and lead to material transfer (Fig. 5(a)). The materials transfer on the friction surface could result in severe wear and grooves on the surface (Fig. 5(b)). Since the temperature near the friction surface was high towards the end of the braking, the adhesive wear was prone to occur at the end of braking, which could have negative influence on the frictional property. Fig. 6 shows the typical braking curve under the
3.3. Oxidation-abrasion Fig. 4 shows the XRD patterns of the debris and the C/SiC brake materials. It is seen that the C/SiC brake materials were composed of C, Si, and SiC, while the debris contained not only C, Si, and SiC but also SiO2. It could be concluded that the Si or SiC in the brake materials was oxidized to form SiO2 in the braking process. During braking, the temperature of the friction surface could increase from 600 to 800 1C [13], which could lead to the oxidation of Si or SiC. At 600–800 1C, the C phase (carbon fibers and pyrolytic carbon) could be oxidized more easily rather than Si and SiC. It means that the C/SiC brake materials could be oxidized at 600–800 1C. Therefore, an oxidation-abrasion mechanism could occur for the C/SiC brake materials during braking. pyrolytic carbon
Fig. 3. Typical optical micrograph of crack propagation on the friction surface of the C/SiC brake material. (crack propagations are marked by arrows).
Carbon fiber
Debris embedded into the friction surface Grooves left by debris ploughing
10µm Fig. 1. Optical micrograph of ploughing action of the hard debris on the friction surface.
Fig. 4. XRD patterns of the debris and the C/SiC brake materials.
Fig. 2. Micrographs of the SiC grain in (a) the C/SiC brake materials and (b) the debris.
S. Fan et al. / Tribology International 44 (2011) 25–28
27
Fig. 5. Typical macrostructures of the friction surface formed by adhesive wear of the C/SiC brake materials; (a) transfer particles on friction surface and (b) grooves on the friction surface.
Fig. 6. Typical braking curve under the adhesive wear condition.
adhesive wear condition. The frictional moment was abruptly enhanced at the end of braking, which led to unstable frictional properties. After Si in the C/SiC brake materials was removed, the friction surface was smooth. There was no materials transfer and no grooves on the friction surface. The braking moment was not much enhanced at the end of braking (Fig. 7). It is indicated that Si is a significant factor in adhesive wear. Since adhesive wear could lead to high wear rate and an unstable friction coefficient, it is very important to remove Si from C/SiC brake materials. In C/SiC brake materials, grain-abrasion, oxidation-abrasion, fatigue wear, and adhesive wear always occur simultaneously, and there were mutual enhancing effects between them. Grainabrasion was mainly the result of the hard SiC grain action. At the same time, it could cause the local high temperature to speed up the oxidation-abrasion and lead to adhesive wear. Grain-abrasion could also cause the fatigue wear; oxidation-abrasion could weaken the strengthening effects of the carbon fiber, thereby aggravating grain-abrasion, and fatigue wear.
Fig. 7. Macrostructure of friction surface and braking curve for C/SiC brake materials (Si removed); (a) macrostructure of friction surface and (b) braking curve.
4. Conclusions The main wear mechanisms of the C/SiC brake materials were grain-abrasion, oxidation-abrasion, fatigue wear, and adhesive wear. Grain-abrasion mainly was the result of the hard SiC grain action; at the same time it could cause the local high temperature to speed up the oxidation-abrasion and lead to adhesive wear. Grain-abrasion could cause the fatigue wear and oxidationabrasion could weaken the carbon fiber strengthening, thereby aggravating grain-abrasion and fatigue wear. Si was the significant factor in adhesive wear. Adhesive wear can cause high wear rates and a large unstable friction coefficient; so Si in the C/SiC brake materials must be removed.
Acknowledgement The authors acknowledge the financial support of Natural Science Foundation of China (Contract no. 90405015 and 50672076) and
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S. Fan et al. / Tribology International 44 (2011) 25–28
Program for Changjiang Scholars and Innovative Research Team in University. References [1] Haug T, Rebstock K. New material technologies for brakes. In: Breuer B, Dausend U, editors. Advanced Brake Technology. USA: SAE International; 2003. p. 37–47. [2] Krenkel W, Berndt F. C/C–SiC composites for space applications and advanced friction systems. Mater Sci Eng A 2005;412:177–81. [3] Krenkel W, Heidenreich B, Renz R. C/C-SiC composites for advanced friction systems. Adv Eng Mater 2002;4:427–36. [4] Vaidyaraman S, Purdy M, Walker T, Horst S. C/SiC material evaluation for aircraft brake applications. In: Krenkel W, Naslain R, Schneider H, editors. Proceedings of the fourth international conference on high temperature ceramic matrix composites HT-CMC4, 2001 October 01–03, Munich, Germany. Berlin: Wiley-VCH, Weinheim; 2001. p. 802–8. [5] Krenkel W, Henke T. Design of high performance CMC brake disks. Key Eng Mater 1999;164–165:421–4. [6] Krenkel W. C/C-SiC composites for hot structures and advanced friction systems. Ceram Eng Sci Proc 2003;24:583–92.
¨ [7] Muhlratzer A., Leuchs M. Application of non-oxide CMCs. In: Krenkel W, Naslain R, Schneider H, editors. Proceedings of the fourth international conference on high temperature ceramic matrix composites HT-CMC4 , 2001 October 01–03, Munich, Germany. Berlin: Wiley-VCH, Weinheim; 2001. p. 288–98. [8] Fan S, Zhang L, Xu Y, Cheng L, Tian G, Ke S, et al. Microstructure and tribological properties of advanced carbon/silicon carbide aircraft brake materials. Compos Sci Technol 2008;68:3002–9. [9] Fan S, Zhang L, Xu Y, Cheng L, Lou J, Zhang J, et al. Microstructure and properties of 3D needle-punched carbon/silicon carbide brake materials. Compos Sci Technol 2007;67:2390–8. [10] Pak ZS. Cf/SiC/C composites for frictional application. In: Krenkel W, Naslain R, Schneider H, editors. Proceedings of the fourth international conference on high temperature ceramic matrix composites HT-CMC4 , 2001 October 01–03, Munich, Germany. Berlin: Wiley-VCH, Weinheim; 2001. p. 820–5. [11] El-Hija HA, Krenkel W, Hugel S. Development of CC–SiC brake pads for highperformance elevators. Int J Appl Ceram Technol 2005;2:105–13. [12] /http://www.avbuyer.com.cn/c/2009/209985.htmlS. [13] Liu B, Yi MZ, Xiong X, Jiang JC, Huang BY. Preparation of oxidation resistant coatings for carbon/carbon composites aircraft brake pairs. J Nonferr Metal 2000;10:864–7. [in Chinese].
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Wear mechanisms of the C/SiC brake materials Shangwu Fan n, Litong Zhang, Laifei Cheng, Jianxin Zhang, Shangjie Yang, Heyi Liu 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 in fo
abstract
Article history: Received 24 July 2009 Received in revised form 28 August 2010 Accepted 20 September 2010 Available online 29 September 2010
The C/SiC brake materials were fabricated by chemical vapor infiltration combined with liquid melt infiltration. The wear mechanisms of C/SiC brake materials were investigated. The main wear mechanisms were grain-abrasion, oxidation-abrasion, fatigue wear, and adhesive wear. These wear mechanisms always occurred simultaneously , and showed mutual enhancing effects between them. Grain-abrasion mainly was the result of hard SiC grain action. Adhesive wear could cause high wear rates and a large unstable friction coefficient. Si was the significant factor on the adhesive wear, so Si in the C/SiC brake materials must be removed. Crown Copyright & 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: C/SiC Wear mechanisms Brake materials
1. Introduction C/SiC composite is a new type of high performance brake material developed following powder metallurgy materials and C/C composites. C/SiC composites possess a series of outstanding advantages such as low density, good high-temperature resistance, high strength, excellent frictional properties, low wear rate, and long life [1–9]. The German Aerospace Center in Stuttgart (DLR) takes the lead in investigating the C/SiC brake materials in high performance automobile applications [3]. At present, the C/SiC brakes have been successfully applied to Porsche, Ferrari, Daimler Chrysler, and other high-end performance cars [2,10]. For high braking performance, the C/SiC brake materials are potential candidates for high speed trains, aircraft, and emergency brakes for elevators and cranes [2,4,8,11]. In 2008, the C/SiC aircraft brakes were first installed on a certain airplane for a trial flight and achieved success; these were prepared by Northwestern Polytechnical University and Xi’an Aviation Braking Science and Technology Co., Ltd., China [12]. The wear mechanisms of the materials are very important for the brake materials’ design. At present, research on wear mechanisms of C/SiC brake materials has seldom been reported. In the present communication, the wear mechanisms of the C/SiC brake materials were investigated.
2. Experiments The C/SiC brake materials were fabricated by chemical vapor infiltration combined with liquid melt infiltration (LMI). The C/SiC n
Corresponding author. Tel.: + 86 29 8849 4622; fax: + 86 29 8849 4620. E-mail addresses: [email protected], [email protected] (S. Fan).
brake materials were composed of 65 wt% C, 27 wt% SiC, and 8 wt% Si. The density and porosity were 2.1 g cm 3 and 4.4%, respectively [9]. The preparation process of the test samples is described in Ref. [9]. A part of friction tests was conducted on a disk-on-disk type laboratory scale dynamometeras described in Ref. [9], and the other part of friction tests was done on a full-scale dynamometer. The full-scale dynamometer was made by the Vickers company in the United Kingdom. Debris and friction surface were observed by optical microscopy (Olympus PM-T3, Olympus LEXT-OLS3000) and SEM (Hitachi S-4700). The phases were analyzed by XRD (XRD, PANALYTIAL, X’PERT, Netherlands). 3. Results and discussion 3.1. Grain-abrasion Brake wear debris and friction surface can help to understand the wear processes. Fig. 1 is the typical optical micrograph of the ploughing action of the hard debris on the friction surface of the C/SiC brake materials. The C/SiC brake materials are composed of C (carbon fiber and pyrolytic carbon), SiC, and Si. SiC particles, which acted as hard points, ploughed on the friction surface and then removed the material by micro-cutting. Fig. 2(a) shows the SiC morphology of the brake materials after removing silicon by etching the C/SiC specimen with a mixture of hydrofluoric and nitric acid (HNO3:HF¼ 4:1) at 40 1C for 48 h and burning carbon off at 700 1C for 20 h in air. The initial SiC grains in C/SiC brake materials presented a tetrahedron structure, while the SiC grains in the debris were smooth (Fig. 2(b)). The main reason is that the SiC grains in debris acted as hard points, ploughed, and micro-cut on the friction surface; the edge angle of the SiC grains
0301-679X/$ - see front matter Crown Copyright & 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2010.09.003
26
S. Fan et al. / Tribology International 44 (2011) 25–28
was worn blunt under friction fatigue stress, which indicated a grain-abrasion mechanism. 3.2. Fatigue wear During braking, cracks were generated and they propagated on the friction surface of the C/SiC brake materials (Fig. 3). The cracks were generated by high local compressive stress, frictional traction forces repeated for a large number of times on the friction surface in the course of braking, and then the matrix and fibers were peeled off. Under the cyclic stress, cracks were generated and propagated, which resulted in the formation of wear particles, which was fatigue wear.
3.4. Adhesive wear The C/SiC brake materials contained the Si phase. Near the friction surface, the Si phase may be softened by braking, which could generate adhesion and lead to material transfer (Fig. 5(a)). The materials transfer on the friction surface could result in severe wear and grooves on the surface (Fig. 5(b)). Since the temperature near the friction surface was high towards the end of the braking, the adhesive wear was prone to occur at the end of braking, which could have negative influence on the frictional property. Fig. 6 shows the typical braking curve under the
3.3. Oxidation-abrasion Fig. 4 shows the XRD patterns of the debris and the C/SiC brake materials. It is seen that the C/SiC brake materials were composed of C, Si, and SiC, while the debris contained not only C, Si, and SiC but also SiO2. It could be concluded that the Si or SiC in the brake materials was oxidized to form SiO2 in the braking process. During braking, the temperature of the friction surface could increase from 600 to 800 1C [13], which could lead to the oxidation of Si or SiC. At 600–800 1C, the C phase (carbon fibers and pyrolytic carbon) could be oxidized more easily rather than Si and SiC. It means that the C/SiC brake materials could be oxidized at 600–800 1C. Therefore, an oxidation-abrasion mechanism could occur for the C/SiC brake materials during braking. pyrolytic carbon
Fig. 3. Typical optical micrograph of crack propagation on the friction surface of the C/SiC brake material. (crack propagations are marked by arrows).
Carbon fiber
Debris embedded into the friction surface Grooves left by debris ploughing
10µm Fig. 1. Optical micrograph of ploughing action of the hard debris on the friction surface.
Fig. 4. XRD patterns of the debris and the C/SiC brake materials.
Fig. 2. Micrographs of the SiC grain in (a) the C/SiC brake materials and (b) the debris.
S. Fan et al. / Tribology International 44 (2011) 25–28
27
Fig. 5. Typical macrostructures of the friction surface formed by adhesive wear of the C/SiC brake materials; (a) transfer particles on friction surface and (b) grooves on the friction surface.
Fig. 6. Typical braking curve under the adhesive wear condition.
adhesive wear condition. The frictional moment was abruptly enhanced at the end of braking, which led to unstable frictional properties. After Si in the C/SiC brake materials was removed, the friction surface was smooth. There was no materials transfer and no grooves on the friction surface. The braking moment was not much enhanced at the end of braking (Fig. 7). It is indicated that Si is a significant factor in adhesive wear. Since adhesive wear could lead to high wear rate and an unstable friction coefficient, it is very important to remove Si from C/SiC brake materials. In C/SiC brake materials, grain-abrasion, oxidation-abrasion, fatigue wear, and adhesive wear always occur simultaneously, and there were mutual enhancing effects between them. Grainabrasion was mainly the result of the hard SiC grain action. At the same time, it could cause the local high temperature to speed up the oxidation-abrasion and lead to adhesive wear. Grain-abrasion could also cause the fatigue wear; oxidation-abrasion could weaken the strengthening effects of the carbon fiber, thereby aggravating grain-abrasion, and fatigue wear.
Fig. 7. Macrostructure of friction surface and braking curve for C/SiC brake materials (Si removed); (a) macrostructure of friction surface and (b) braking curve.
4. Conclusions The main wear mechanisms of the C/SiC brake materials were grain-abrasion, oxidation-abrasion, fatigue wear, and adhesive wear. Grain-abrasion mainly was the result of the hard SiC grain action; at the same time it could cause the local high temperature to speed up the oxidation-abrasion and lead to adhesive wear. Grain-abrasion could cause the fatigue wear and oxidationabrasion could weaken the carbon fiber strengthening, thereby aggravating grain-abrasion and fatigue wear. Si was the significant factor in adhesive wear. Adhesive wear can cause high wear rates and a large unstable friction coefficient; so Si in the C/SiC brake materials must be removed.
Acknowledgement The authors acknowledge the financial support of Natural Science Foundation of China (Contract no. 90405015 and 50672076) and
28
S. Fan et al. / Tribology International 44 (2011) 25–28
Program for Changjiang Scholars and Innovative Research Team in University. References [1] Haug T, Rebstock K. New material technologies for brakes. In: Breuer B, Dausend U, editors. Advanced Brake Technology. USA: SAE International; 2003. p. 37–47. [2] Krenkel W, Berndt F. C/C–SiC composites for space applications and advanced friction systems. Mater Sci Eng A 2005;412:177–81. [3] Krenkel W, Heidenreich B, Renz R. C/C-SiC composites for advanced friction systems. Adv Eng Mater 2002;4:427–36. [4] Vaidyaraman S, Purdy M, Walker T, Horst S. C/SiC material evaluation for aircraft brake applications. In: Krenkel W, Naslain R, Schneider H, editors. Proceedings of the fourth international conference on high temperature ceramic matrix composites HT-CMC4, 2001 October 01–03, Munich, Germany. Berlin: Wiley-VCH, Weinheim; 2001. p. 802–8. [5] Krenkel W, Henke T. Design of high performance CMC brake disks. Key Eng Mater 1999;164–165:421–4. [6] Krenkel W. C/C-SiC composites for hot structures and advanced friction systems. Ceram Eng Sci Proc 2003;24:583–92.
¨ [7] Muhlratzer A., Leuchs M. Application of non-oxide CMCs. In: Krenkel W, Naslain R, Schneider H, editors. Proceedings of the fourth international conference on high temperature ceramic matrix composites HT-CMC4 , 2001 October 01–03, Munich, Germany. Berlin: Wiley-VCH, Weinheim; 2001. p. 288–98. [8] Fan S, Zhang L, Xu Y, Cheng L, Tian G, Ke S, et al. Microstructure and tribological properties of advanced carbon/silicon carbide aircraft brake materials. Compos Sci Technol 2008;68:3002–9. [9] Fan S, Zhang L, Xu Y, Cheng L, Lou J, Zhang J, et al. Microstructure and properties of 3D needle-punched carbon/silicon carbide brake materials. Compos Sci Technol 2007;67:2390–8. [10] Pak ZS. Cf/SiC/C composites for frictional application. In: Krenkel W, Naslain R, Schneider H, editors. Proceedings of the fourth international conference on high temperature ceramic matrix composites HT-CMC4 , 2001 October 01–03, Munich, Germany. Berlin: Wiley-VCH, Weinheim; 2001. p. 820–5. [11] El-Hija HA, Krenkel W, Hugel S. Development of CC–SiC brake pads for highperformance elevators. Int J Appl Ceram Technol 2005;2:105–13. [12] /http://www.avbuyer.com.cn/c/2009/209985.htmlS. [13] Liu B, Yi MZ, Xiong X, Jiang JC, Huang BY. Preparation of oxidation resistant coatings for carbon/carbon composites aircraft brake pairs. J Nonferr Metal 2000;10:864–7. [in Chinese].