201004 International Journal of Minerals Metallurgy and Materials

International Journal of Minerals, Metallurgy and Materials Volume 17, Number 4, August 2010, Page 500DOI:10.1007/s12613-010-0348-yCorresponding author: Zhuan Li E-mail: li_zhuan@© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2010Preparation and properties of C/C-SiC brake compositesfabricated by warm compacted-in situ reactionZhuan Li, Peng Xiao, and Xiang XiongState Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China(Received: 12 August 2009; revised: 28 August 2009; accepted: 2 September 2009)Abstract: Carbon fibre reinforced carbon and silicon carbide dual matrix composites (C/C-SiC) were fabricated by the warm compacted-in situ reaction. The microstructure, mechanical properties, tribological properties, and wear mechanism of C/C-SiC composites at different brake speeds were investigated. The results indicate that the composites are composed of 58wt% C, 37wt% SiC, and 5wt% Si. The density and open porosity are 2.0 g·cm–3 and 10%, respectively. The C/C-SiC brake composites exhibit good mechanical properties. The flexural strength can reach up to 160 MPa, and the impact strength can reach 2.5 kJ·m–2. The C/C-SiC brake composites show excellent tribological performances. The friction coefficient is between 0.57 and 0.67 at the brake speeds from 8 to 24 m·s−1. The brake is stable, and the wear rate is less than 2.02×10−6 cm3·J−1. These results show that the C/C-SiC brake composites are the promising candidates for advanced brake and clutch systems.Keywords: C/C-SiC; ceramic matrix composites; tribological properties; microstructure[This work was financially supported by the National High-Tech Research and Development Program of China (No.2006AA03Z560) and the Graduate Degree Thesis Innovation Foundation of Central South University (No.2008yb019).]1. IntroductionRecently, powder metallurgy (PM) and carbon/carbon (C/C) friction pads or linings are two kinds of materials used in the advanced brake system. The advantages of PM brakes are the maturity in material development and low cost, while the main disadvantages are the high density (7.8 g·cm–3) and poor performance at high temperatures [1-2]. C/C brakes are developed to overcome the disadvantages of PM brakes. Compared with PM brakes, C/C brakes have distinct advan-tages, such as excellent thermal and mechanical properties with a lower weight. However, C/C brakes suffer from the insufficient stability of friction coefficient caused by humid-ity. Moreover, C/C composites are prone to oxidation be-yond the temperature of 400°C and high cost, which prevent their wide use as brake composites in passenger cars and trains or emergency brakes of lifts and cranes [3-5].The C/C-SiC composite is a new type of high perform-ance brake material developed following the PM material and C/C composite material. In combination with lower density (about 2.0 g·cm–3), high thermal shock resistance, longer service life, especially lower sensibility to surround-ings and temperature for a silicon carbide share of at least 20wt%, C/C-SiC composites are promising candidates for the advanced friction material in the future [2, 6-7]. Several institutes and industries have been investigating C/C-SiC composites for their use as the friction material in brake pads and disks [8-10]. For example, the researchers of Stuttgart University and German Aerospace Center have applied C/C-SiC composites to the friction domain since the middle of 1990s and developed C/C-SiC brake linings ap-plied in 911 Turbo of Porsche and Audi A8 automobiles.At present, the main preparation methods of C/C-SiC composites are as follows: (1) a gas phase route, also re-ferred to as the chemical vapor infiltration (CVI); (2) a liq-uid phase route including the polymer impregna-Z. Li et al.,Preparation and properties of C/C-SiC brake composites fabricated by warm compacted-in situ reaction 501tion/pyrolysis (PIP) and the liquid silicon infiltration (LSI), which are also called (reactive) melt infiltration (RMI or MI) processes; (3) a ceramic route, i.e., a technique combining the impregnation of the reinforcement with a slurry and a sintering step at high temperatures and high pressures, also referred to as the high-pressure-sinter process (HP-Sinter). Each of the former routes displays both advantages and drawbacks. CVI and PIP with a long preparation process are mainly used to manufacture hot structural C/C-SiC compos-ites for spaceflight [11-13]; RMI has the best figuration and a short fabrication period, but the cost of preforms is high [14-15]. HP-Sinter does harm to carbon fibre and further impacts the composite’s performance.The aim of the current work was to develop an improved technique to reduce the processing and make the C/C-SiC brake composites practical for more industrial applications. The warm compacted-in situ reaction was proposed for fab-ricating the composites. The microstructure, mechanical property, and tribological characterization of the C/C-SiC brake composites were investigated.2. Experimental procedure2.1. Fabrication of the compositesThe short-cut carbon fibre (PAN, T700) with length be-tween 3 and 10 mm was employed, which was supplied by Toray, Japan. The carbon matrix in C/C-SiC composites was phenolic resin and graphite powder in origin. The di-ameter and purity of the silicon powder were 30-50 μm and 99.0%, respectively, according to the information supplied from the manufacturer of Da Zelin-silicon Co. Ltd, Beijing, China. The processing of warm compacted-in situ reaction consists of three main steps, as shown in Fig. 1. The first step involves the mix of C/C-Si green preforms at moderate temperature and pressure and followed by subsequent curing at 160 and 200°C. Curing was done under normal pressure to facilitate the escape of volatile matter by-products of cur-ing reactions and thereby generated a dense matrix with a minimal amount of closed porosity. The average amount of close porosity in the C/C-Si green body after curing was measured to be about 5vol%. The second step involves the pyrolysis of C/C-Si green preforms to convert phenolic resin into resin carbon as parts of carbon matrices in the tempera-ture range of 200-650°C. Consequently, the resin was slowly heated from 200 to 650°C during pyrolysis to allow volatile decomposition products to diffuse through the ma-trix without disrupting the integrity of the composites. Fi-nally, C/C-Si green preforms have the in-situ reaction be-tween 1500 and 1750°C during high temperature treatment (HTT). Thereby, Si reacted with the carbon matrices and then formed the SiC matrix. The density of the resulting C/C-SiC composites was about 2.0 g·cm–3, and the open porosity was about 10%. It is concluded that the warm compacted-in situ reaction exhibits many advantages in comparison to traditional routes, such as simple technology, the large range choice of raw materials, and low fabricationcost.Fig. 1. Schematic representation of the warm compacted-in situ reaction.2.2. Testing methodsThe density and the open porosity of the samples were measured by the Archimedes’ method. The flexural strength and flexural elastic modulus were measured using the three-point-bending method with the samples of 4 mm×10 mm×55 mm, and the loading rate was 0.5 mm·min–1. The compressive strength and compressive elastic modulus were measured with the samples of 10 mm×10 mm×10 mm, and the loading rate was 0.5 mm·min–1. All the tests were con-ducted on a CSS-44100 device at room temperature.The friction and wear properties of the C/C-SiC brake material were tested on a QDM150 friction testing machine with C/C-SiC composites as the static plate and corre-sponding steel disks (HRC 50, 30CrMoSiVA) as the mov-ing plate. The sizes of the test specimens and corresponding steel disks were 25 mm×25 mm×10 mm and φ300 mm, re-spectively. The friction testing required a dry condition, the brake pressure was 1.0 MPa, and the brake for 2000 circles (namely, 1884 m) was at the constant brake speeds of 8, 12, 16, 20, and 24 m·s–1, respectively.The brake speed, brake moment, and brake distance were recorded by computer, and the friction coefficient was502 Int. J. Miner. Metall. Mater ., Vol.17, No.4, Aug 2010transformed from the brake moment. The wear performance was evaluated with the volume wear rate. The volume wear rate was calculated with the weight difference of the sample before and after brake tests divided by the brake speed. 2.3. Microstructure observation and analysisThe microstructure of C/C-SiC brake composites, the morphology of worn surfaces, and wear debris were ana-lyzed by scanning electron microscopy (SEM, JSM- 6360LV). The phases as well as the SiC-polytypes were in-vestigated by X-ray diffraction (XRD, Rigaku-3014).3. Results and discussion3.1. Phases and microstructureThe XRD pattern of C/C-SiC composites is shown in Fig. 2. The phase analysis reveals that the C/C-SiC composites include residual silicon, carbon, and silicon carbide. The gravimetric analysis was employed to determine the content of carbon, residual silicon, and SiC in the composites. By etching the C/C-SiC specimen with the aqueous solution of 90vol% nitric acid and 10vol% hydrofluoric acid at room temperature for 48 h to remove the residual silicon, whereas the contents of carbon fibre and resin carbon matrix were measured by burning it off at 700°C for 10 h in air. There-fore, the content of each component could be calculated.The results indicated that the C/C-SiC composites wereFig. 2. XRD pattern of the C/C-SiC composite.composed of 58wt% C, 37wt% SiC, and 5wt% Si.The microstructures of C/C-SiC composites are shown in Fig. 3. It can be seen that the carbon fibre and matrix dis-tribute alternately in the parallel direction, which is perpen-dicular to the direction of warm compaction (as shown in Fig. 3 (a)). The decomposed products of phenolic resin dif-fuse through the matrix, so there are some microcracks and holes in the matrix during carbonization. With the help of X-ray energy dispersive analysis (EDAX), it is obvious that the round and filiform components are short carbon fibres. The bright white matters are SiC and residual Si. The gray regions are carbon matrices including graphite and resincarbon.Fig. 3. Microstructures of C/C-SiC composites: (a) cross section; (b) optical micrograph.3.2. Mechanical propertiesThe C/C-SiC brake composites fabricated by the warm compressed-in situ reaction show good mechanical proper-ties. The values of flexural strength and impact strength can reach 160 MPa and 2.5 kJ·m –2, respectively, and the values of vertical compression and parallel compression can reach 112 and 84 MPa, respectively.The load-displacement curves and SEM micrographs of fracture surfaces after compressive failure of C/C-SiC com-posites are shown in Figs. 4 and 5, respectively. The vertical compressive failure behavior shows ‘toughness’ and a higher amount of fibre pulled-out on the fracture surface (Fig. 5 (a)), which suggests a weak bonding between the fi-bre and the matrix. The parallel compressive failure behav-ior exhibits elastic response at the initial stage, followed by inelastic behavior as the stress increases. After the stress in-creases to its maximum value, it then decreases rapidly. This is responsible for brittle failure within the specimens. A fracture surface with the matrix sheared is observed after compressive failure (Fig. 5 (b)).Z. Li et al., Preparation and properties of C/C-SiC brake composites fabricated by warm compacted-in situ reaction 503Fig. 4. Typical load-displacement curves for C/C-SiC underthe compressive load.Fig. 5. Typical SEM micrographs of the samples after the compressive test: (a) vertical compression; (b) parallel compression.3.3. Tribological propertiesThe relationships among brake speed, friction coefficient, and wear rate are shown in Fig. 6. Apparently, the friction coefficient is 0.57 at 8 m/s and reaches the maximum value of 0.67 at 16 m/s with the increase of brake speed, which afterward decreases as far as to 0.61 at 24 m/s. The wear rate increases at first and then decreases with the increase of brake speed, but the maximum value of 2.02×10−6 cm 3·J –1 appears at 20 m/s. The tribological phenomena will be dis-cussed later.Fig. 6. Relationship between friction properties and brake speed of C/C-SiC composites.The typical brake curves of C/C-SiC composites at dif-ferent brake speeds are shown in Fig. 7. It can be seen that the shape of the curves is basically similar, and it becomes steadier with the increase of brake speed. At the prior stage of braking, the friction coefficient rapidly rises with a slight ‘prepeak’ in the curve. After the ‘prepeak’, it slowly de-creases and tends to be relatively smooth when the brakeFig. 7. Brake curves of C/C-SiC composites at different brake speeds.504 Int. J. Miner. Metall. Mater ., Vol.17, No.4, Aug 2010condition is stabilized. The cause will be discussed in the following section of worn surface and worn debris analyses. Braking is a mutual affection between two relative mo-tional worn surfaces, so the tribological characteristics are usually determined by brake conditions for certain compos-ites. After the brake test at each speed, the C/C-SiC compos-ites samples showed different friction surfaces. The typical friction surfaces are shown in Fig. 8.The C/C-SiC composites have amounts of micro-peaks on the surfaces of the brake samples, including the hard SiC phase, resin carbon, and the second hard phase Si. These micro-peaks mesh with each other, leading to deformation, shearing, and breaking, which result in the friction coeffi-cient rising at the early stage of each brake and shows the abrasive wear mechanism. At the same time, a large amount of abrasive particles are generated as a result of broken mi-cro-peak ploughing on the friction surfaces (as shown in Fig.8(a)). The higher the brake speed, the more the abrasive par-ticles. Therefore, the friction coefficient and the wear rate at 12 m/s are higher than those at 8 m/s. As shown in Figs. 8(a) and (b), some grooves are left behind on the surfaces of the C/C-SiC brake samples because of the debris ploughing. As the brake speed increases, the brake energy enhances accordingly and reaches the value that makes the wear de-bris deformed, and it fills the micro-valleys in the surfaces of the C/C-SiC brake samples to form friction films covered on the friction surface under the brake pressure. Therefore, when the brake speed reaches 16 and 20 m/s, there are fric-tion films covering the worn surfaces (as shown in Figs. 8(c) and (d)), and there are more at the speed of 20 m/s. The fric-tion films lead to the true friction surface between the C/C-SiC composite samples and the corresponding steel disk enhancing, and the friction coefficient and wear rateincreasing accordingly.Fig. 8. SEM images of the worn surfaces of C/C-SiC brake composites at different brake speeds: (a) 8 m/s; (b) 12 m/s; (c) 16 m/s; (d) 20 m/s; (e) 24 m/s; (f) 24 m/s.Z. Li et al.,Preparation and properties of C/C-SiC brake composites fabricated by warm compacted-in situ reaction 505When the brake speed reaches 24 m/s, the friction film on the friction surface becomes more continuous (as shown in Fig. 8(e)). At the same time, the temperatures of the fric-tional surface of C/C-SiC composites rise fast with the in-crease of brake speed. The friction heat can cause to an un-stable temperature field in the C/C-SiC composites surface for the different coefficients of thermal expansion of various components. This phenomenon causes the thermal expan-sion rates between the frictional surface and the subsurface to become different, as well as the different region of the frictional surface. Meanwhile, the brake pressure leads to an unstable pressure field for the concavo-convex frictional surface of the C/C-SiC composite sample. There are many microcracks on the friction surface because of the unstable temperature and pressure fields (as shown in Fig. 8(f)), they are rubbed repeatedly with the corresponding steel disk and easily fall off, which become flake wear debris.4. Conclusions(1) The warm compacted-in situ reaction offers a poten-tial method for manufacturing C/C-SiC brake composites. The C/C-SiC composites are composed of 58wt% C, 37wt% SiC, and 5wt% Si.(2) The C/C-SiC brake composites exhibit good me-chanical properties, especially excellent toughness, to avoid the catastrophic brittle fracture.(3) The C/C-SiC brake composites show excellent tri-bological performances at different brake speeds, including a good stability of brake and higher friction coefficients between 0.57 and 0.67, in combination with the wear rate less than 2.02×10−6 cm–3·J–1.(4) With a low density of 2.0 g·cm–3, good mechanical properties, outstanding frictional performances, and good abrasive resistance, it is concluded that C/C-SiC brake composites are the promising candidates for advanced brake and clutch systems.References[1] P.P. Yao, H.C. Sheng, X. Xiong, and B.Y. Huang, Worn sur-face characteristics of Cu-based powder metallurgy bake ma-terials for aircraft, Trans. Nonferrous Met. Soc. China,17(2007), No.1, p.99.[2] W. Krenkel, B. Heidenreich, and R. 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