International Journal of Minerals, Metallurgy and Materials Volume 17, Number 4, August 2010, Page 500
DOI:10.1007/s12613-010-0348-y
Corresponding author: Zhuan Li E-mail: li_
© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2010
Preparation and properties of C/C-SiC brake composites
fabricated by warm compacted-in situ reaction
Zhuan Li, Peng Xiao, and Xiang Xiongplatform
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China
(Received: 12 August 2009; revid: 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 compod 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. The 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 Rearch and Development Program of China (No.2006AA03Z560) and the Graduate Degree Thesis Innovation Foundation of Central South University (No.2008yb019).]
1. Introduction
Recently, powder metallurgy (PM) and carbon/carbon (C/C) friction pads or linings are two kinds of materials ud 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 caud 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 u as brake composites in pasnger 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 rvice life, especially lower nsibility 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 u as the friction material in brake pads and d
isks [8-10]. For example, the rearchers 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 prent, the main preparation methods of C/C-SiC composites are as follows: (1) a gas pha route, also re-ferred to as the chemical vapor infiltration (CVI); (2) a liq-uid pha 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 501
tion/pyrolysis (PIP) and the liquid silicon infiltration (LSI), which are also called (reactive) melt infiltration (RMI or MI) process; (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 ud 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.
中文译英文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 propod for fab-ricating the composites. The microstructure, mechanical property, and tribological characterization of the C/C-SiC brake composites were investigated.
2. Experimental procedure
2.1. Fabrication of the composites
The 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 subquent 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 den matrix with a minimal amount of clod porosity. The average amount of clo po
rosity in the C/C-Si green body after curing was measured to be about 5vol%. The cond 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. Conquently, the resin was slowly heated from 200 to 650°C during pyrolysis to allow volatile decomposition products to diffu 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 fabrication
cost.
Fig. 1. Schematic reprentation of the warm compacted-in situ reaction.
2.2. Testing methods
考研大趋势The 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 was
502 Int. J. Miner. Metall. Mater ., Vol.17, No.4, Aug 2010
transformed 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 obrvation and analysis
The 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 phas as well as the SiC-polytypes were in-vestigated by X-ray diffraction (XRD, Rigaku-3014).
3. Results and discussion
3.1. Phas and microstructure
The XRD pattern of C/C-SiC composites is shown in Fig. 2. The pha 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 were
Fig. 2. XRD pattern of the C/C-SiC composite.
compod of 58wt% C, 37wt% SiC, and 5wt% Si.
The microstructures of C/C-SiC composites are shown in Fig. 3. It can be en 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 decompod products of phenolic resin dif-fu 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 resin
carbon.
Fig. 3. Microstructures of C/C-SiC composites: (a) cross ction; (b) optical micrograph.
3.2. Mechanical properties
The C/C-SiC brake composites fabricated by the warm compresd-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 respon at the initial stage, followed by inelastic behavior as the stress increas. After the stress in-creas to its maximum value, it then decreas rapidly. This is responsible for brittle failure within the specimens. A fracture surface with the matrix sheared is obrved 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 503
Fig. 4. Typical load-displacement curves for C/C-SiC under
合格证英文
the compressive load.
Fig. 5. Typical SEM micrographs of the samples after the compressive test: (a) vertical compression; (b) parallel compression.
bakatparin
3.3. Tribological properties
The 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 increa of brake speed, which afterward decreas as far as to 0.61 at 24 m/s. The wear rate increas at first and then decreas with the increa 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-
cusd 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 ca
n be en that the shape of the curves is basically similar, and it becomes steadier with the increa of brake speed. At the prior stage of braking, the friction coefficient rapidly ris with a slight ‘prepeak’ in the curve. After the ‘prepeak’, it slowly de-
考研课程creas and tends to be relatively smooth when the brake
Fig. 7. Brake curves of C/C-SiC composites at different brake speeds.
504 Int. J. Miner. Metall. Mater ., Vol.17, No.4, Aug 2010
condition is stabilized. The cau will be discusd in the following ction of worn surface and worn debris analys. 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.
fray>springboardThe C/C-SiC composites have amounts of micro-peaks on the surfaces of the brake samples, including the hard SiC pha, resin carbon, and the cond hard pha Si. The 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 tho 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 becau of the debris ploughing. As the brake speed increas, 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 rate
increasing accordingly.
女强人英文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.21世纪英语
