- Email: [email protected]
carbon composites by an electrified preform heating CVI method
Carbon 40 (2002) 1957–1963
Fabrication of carbon / carbon composites by an electrified preform heating CVI method Ruiying Luo* College of Science, Beijing University of Aeronautics and Astronautics, 37 Xueyuan Rd., Haidian District, Beijing, 100083, PR China Received 8 October 2001; accepted 5 January 2002
Abstract Carbon / carbon composites are manufactured using the electrified preform producing directly heat CVI process. The preforms are prepared by laminating the carbon fiber felts with crossply reinforcement, and infiltrated with carbon using natural gas or propylene as a reactant, with nitrogen as diluent at atmospheric pressure. The relations between the resistivity of samples and infiltration time are determined under the operating conditions. The results indicate that the preforms have gained a high infiltration rate by this technology, and the samples have higher densities using natural gas rather than propylene. Their highest average bulk densities are up to 1.71 g / cm 3 after the preforms of 11003500335 mm size have been densified for 80 h using natural gas. The carbon fibres in the preforms have not been damaged by this technology as yet, and the composites prepared have sufficiently high flexural properties. As the brake angular velocity is increased with the constant brake moment inertia and specific pressure, the average coefficient of friction for the composites prepared using natural gas is linearly and greatly decreased, but the variations of the brake moment inertia have a slight influence on the average coefficient of the friction when the brake angular velocity and specific pressure are kept constant. Their average thickness wear is 13310 24 mm / surface per stop. 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon / carbon composites; B. Chemical vapor infiltration; D. Frictional properties; Mechanical properties
1. Introduction Carbon / carbon (C / C) composites have low densities, good high-temperature structural properties, high thermal stabilities and heat capacities, especially their acceptable coefficients of friction, over a wide range of temperatures [1]. They are considered as the best brake materials, jet engine component materials, and the most promising candidate materials for high-temperature structural applications [2,3]. The C / C composites are at present prepared using many cycles of resin or pitch infiltration followed by carbonization and graphitization, or by using the isothermal chemical vapor infiltration (CVI) process [4,5]. The cycles of these methods sometimes have to be repeated many times and, as a result, processing can often take 1 month or longer. The thermal gradient method allows considerably higher infiltration rates to be attained than those with conventional isothermal technology, but *Corresponding author. Fax: 186-10-8232-8283. E-mail address: [email protected] (R. Luo).
the disadvantages of the thermal gradient method are the limitations to geometry, dimension of parts, single item processing, and difficulties in scaling up this process. A great deal of work has been carried out on thermal gradient CVI processing of C / C composites [6]. Vaidyaraman et al. have fabricated C / C composites by forced flow-thermal gradient CVI [7], but some shortcomings of this method have not been overcome yet. In electrified preform heating CVI (ECVI) technology, preforms are used as the resistance and produce heat, consequently the temperature is naturally the highest in the central parts of the preform thickness which cannot be easily densified using the isothermal method. On the other hand, fibre preforms possess low thermal conductivities to permit the thermal gradient to be established. Also some high gas flow rates are used to cool the exterior surface of the preforms. These factors ensure that a large thermal gradient occurs across the fibre preform thickness. In the ECVI process, the rate of increase in temperature for the preforms is very rapid, and full-size or a greater number samples can be manufactured simultaneously in an
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00027-1
1958
R. Luo / Carbon 40 (2002) 1957 – 1963
identical furnace. A temperature gradient on the order of 150–300 8C is applied across the preform thickness. Other advantages include zero incidence of overcrusting and ability of the process to run at atmospheric pressure. Machining of the samples and the need for low pressure vessels and pumping apparatus commonly associated with the isothermal method can thus be eliminated. In this paper, the ECVI processes for the preparation of C / C composites are proposed.
2. Experimental
2.1. Preparation of preforms The size of preforms is 11003500335 mm. Their volume fraction of carbon fibres is 40%. Fibres containing 12 000 filaments per tow obtained from the Lanzhou carbon fiber plant, China, have been used to prepare the preforms. The preforms have been manufactured according to the following steps: the carbon fiber felts have been laminated (Fig. 1), and two layers have been oriented at an angle of 908, and then needled progressively till the thickness of preforms is reached.
2.2. Densification of preforms The ECVI equipment is shown in Fig. 2. The ECVI processes have been performed using natural gas or a flowing mixture of propylene and nitrogen. The ratio of propylene to nitrogen is about 1:5. The flow rate of natural gas or mixture is less than 40 l / min. The first step of the test is to evacuate the furnace and check for leaks; upon verification that the leak rate is below 0.13 Pa / min, nitrogen is used to purge the furnace for about 30 min, and then the furnace is filled with nitrogen to atmospheric pressure. Following this step, the preforms are heated to the required temperature with flowing nitrogen, which protects the preforms from oxidation. The temperatures at the outside surface and the central part of the preforms (called the infiltration tempera-
Fig. 2. A schematic of the equipment for the ECVI method.
ture) are measured throughout the run using two thermocouples, one of which is fixed near the surface of the preform, while the other is fixed at the center of the preform thickness. After the infiltration temperature has stabilized for 30 min, natural gas or a flowing mixture of propylene and nitrogen is sent into the furnace. As the infiltration proceeds, the thermocouple at the center of the preforms is progressively moved towards the surface of the preform, but the value of the infiltration temperature has been kept constant under a single infiltration process. After the densification of the preforms is finished, the samples are cut into pieces and heated to 2500 8C.
2.3. Temperature profiling To develop the appropriate temperature gradient, it is necessary for us to know the actual temperature on the surface of the preforms and at the center of the preform thickness, the effect of the cooler environment and gas flow rate on the temperature. Therefore a temperature calibration experiment is conducted. Table 1 gives the temperature on the surface of the preforms and at the center of the preform thickness. It is shown that the cooler environment, gas flow rates and types, etc. have a strong influence on the temperature.
2.4. Measurement of bulk density of the samples Specimens of 10310310 mm are cut from each sample, and the number is not less than seven for each test point. Then they are weighed in an analytical balance. The bulk density of the samples can be calculated.
2.5. Mechanical properties Fig. 1. The architecture of carbon fibre felts used to prepare preforms.
The specimens used for the mechanical-property tests
R. Luo / Carbon 40 (2002) 1957 – 1963
1959
Table 1 The temperature on the surface of preforms and in the center of the preform thickness Thermal insulation thickness (mm)
Reagent gas type
Reagent flow rate (l / min)
10 10 10 5 10 10 10 10 10
Natural gas Natural gas Natural gas Natural gas Natural gas
35 40 35 35 35
Propylene Propylene Propylene
5 5 6
are cut out from the full-size samples with the fabric plane perpendicular to the bend, and the number is still not less than seven for each test point. The mechanical properties are determined using the three-point bending test on the electron universal testing machine model CSS-1110. The cross-section of the specimen bar is 1036 mm, the support span is 40 mm, and the crosshead speed is 1 mm / min.
Nitrogen flow rate (l / min)
Surface temperature (8C)
Center (infiltration) temperature (8C)
35 25 25 30
780 760 890 710 680 860 730 820 725
1000 1000 1100 1000 900 1000 820 900 820
Flexural strength (sf ) and flexural modulus (Ef ) are calculated with the following equations: 3PL sf 5 ]]2 2bh DPL 3 Ef 5 ]] 4bh 3 Df where L is the span of the bending test, h is the thickness of the specimen, and b is the width. DP/Df is the slope of the straight line of the load–displacement curve. The fracture of the specimens is analyzed by SEM model AMRAY-1000B.
2.6. Frictional and wear properties The samples are prepared using natural gas at 1000 8C for the frictional and wear property testing. The specimens are cut out from the same samples, treated at high temperature, then machined into annular rings (Fig. 3), protected from oxidation, and mounted on the friction test machine model MM-1000 to perform the frictional and wear property testing. The test machine holds a rotor disc and an identical stator disc. The dimensions of the disc are: internal diameter of 53 mm, external diameter of 75 mm, and initial thickness of 10 mm; 100 stops are carried out in all.
3. Results and discussion
3.1. Effect of infiltration time on the resistivity of the preforms
Fig. 3. The rotors and stators used in the laboratory scale friction test machine.
In this technology, since the preforms produce heat directly after being electrified, the temperature value of the preforms depends mainly on the power of the furnace and the value of their resistivity, but the resistivity value of the preforms varies with infiltration time. Therefore it is indispensable that the relations between the resistivity of
1960
R. Luo / Carbon 40 (2002) 1957 – 1963
Fig. 4. The variations of preform resistivity with infiltration time using natural gas at 1000 8C.
the preforms and the infiltration time be known to design better this kind of set-up and control effectively the infiltration process. Fig. 4 gives the variations of the preform resistivity with infiltration time under the constant infiltration temperature. Fig. 4 shows that as the infiltration proceeds, the resistivity of the preforms is greatly reduced at the beginning of the infiltration, and then slowly decreases. Their resistivity is reduced more rapidly using natural gas than propylene at the same infiltration time, as a result of the different infiltration rates of natural gas and propylene.
Fig. 5. The relationship between average density and infiltration time.
can diffuse more easily into the interiors of the preforms at atmospheric pressure. These results are somewhat different from that reported by Vaidyaraman et al. using FCVI technology [7], because in the FCVI technology, no matter how much the molecular weight of reagents is, they all can be forced through the preforms, and the carbon is infiltrated there. However, for the ECVI processes, the reagents go into the preforms only by free diffusion, the larger the molecular weight of the reagents, the slower the reagents diffuse into the performs, so in contrast, the infiltration rate of carbon becomes small using propylene.
3.3. Mechanical properties 3.2. Effect of processing parameters and reagent types on the infiltration rate The influence of the infiltration temperature on the densification rate is given in Fig. 5. After the preforms are densified for 80 h using natural gas at 900, 1000, 1100 8C, respectively, the average bulk density of the samples prepared at 1000 8C is the highest, up to 1.71 g / cm 3 . If the infiltration temperature is too slow (i.e. less than 900 8C), the infiltration rate of carbon is very slow, as a result of smaller average bulk density of the samples. Though the infiltration of carbon can be speeded up by increasing the temperature if the infiltration temperature is too high (i.e. more than 1100 8C), the coolest portion of the preforms, or their outer surface, also has very high temperature, and it receives the first deposit. Therefore the surface crusting is formed. This is why the infiltration rate of the preforms slows down at 1100 8C again. Fig. 5 also shows that a higher infiltration rate can be gained using natural gas rather than propylene. This can be attributed to the smaller molecular weight of methane which is the main component in natural gas, for methane
The mechanical-property test data of C / C composites Table 2 Mechanical properties of ECVI C / C composite Properties 3
Density (g / cm ) Flexural strength (MPa) Standard difference Dispersed coefficient (%) Flexural modulus (GPa) Standard difference Dispersed coefficient (%) Displacement at failure (mm) Standard difference Dispersed coefficient (%)
Natural gas
Propylene
1.71 101.3
1.69 99.7
8.7 8.59
6.43 6.44
28.0
21.2
2.17 7.75
2.08 9.81
0.57
0.59
0.12 21.05
0.13 22.03
The specimens were densified for 360 h using propylene.
R. Luo / Carbon 40 (2002) 1957 – 1963
1961
carbon is formed [8]. Therefore C / C composites fail in a stepwise process with fibre pull-out and delamination (Fig. 7b).
3.4. Frictional and wear properties
Fig. 6. The load–displacement curves of C / C composites prepared by the ECVI method.
prepared using natural gas and propylene are depicted in Table 2. The corresponding load–displacement curves in three-point flexural testing are shown in Fig. 6. The C / C composites manufactured from natural gas exhibit similar strength, and displacement at failure compared to those made using propylene, but the modulus of the former is higher than that of the latter. Fig. 7a shows that C / C composites prepared using propylene possess a smooth fracture surface, and display a catastrophic failure mode, which is possible owing to the strong interface between fibres and infiltration carbon in this category of composite. This being the case, fibres may fracture in the primary crack plane, leading to a catastrophic failure mode devoid of delamination or fibre pull-out. While for C / C composites made from natural gas, a higher degree of the preferred orientation of carbon is probably present on the surface of fibres, and a weaker bond between fibres and deposition
3.4.1. Effect of moment inertia and brake angular velocity Fig. 8 shows the effect of moment inertia and brake angular velocity on the variations of brake torque with brake time. Table 3 shows the brake conditions and the required brake time. Fig. 9a shows the coefficient of friction under different testing conditions. It is obvious that the average coefficient of friction for C / C composites is slightly decreased with the moment inertia increasing under constant brake angular velocity and specific pressure conditions. From Fig. 9b, it is shown that as the brake angular velocity is increased with the constant moment inertia and specific pressure exerted on the brake discs, the coefficient of friction for C / C composites is greatly and linearly decreased, which results possibly from the effect of the brake angular velocity on the state of friction surface and the energy on the discs. When the velocity is increased to 5800 rpm, the coefficient of friction becomes moderate (Fig. 9b); nevertheless, as the angular velocity is equal to 7000 rpm, the brake torque is very low at the last stage of braking (Fig. 8b). Therefore C / C composites prepared using natural gas possess good frictional properties with 2 moment inertia of 0.53 kg m , brake angular velocity of 5800 rpm, and brake specific pressure of 0.8 MPa. 3.4.2. Wear After 100 the average mm / surface
performance of ECVI C /C composites brakings are performed, it can be found that thickness wear of discs is only 13310 24 per stop. The specific wear reported here is
Fig. 7. SEM fractographs of ECVI C / C composites; (a) C / C composites made using propylene; (b) C / C composites made using natural gas.
R. Luo / Carbon 40 (2002) 1957 – 1963
1962
Fig. 8. The effect of angular velocity and moment inertia on torque–time curves for ECVI C / C composites (specific pressure, 0.8 MPa).
comparable to that of C / C composites prepared using the isothermal method [9], and the friction surface of discs is smooth.
Fig. 9. The average coefficient of friction for ECVI C / C composites under different braking conditions (specific pressure, 0.8 MPa).
Table 3 Brake testing conditions Test no.
Moment inertia (kg m 2 )
Angular velocity (rpm)
Energy (kJ)
Brake time (s)
1 2 3 4 5
0.32 0.53 0.73 0.53 0.53
5800 5800 5800 3500 7000
58.96 97.66 134.5 35.56 142.24
9.5 13.4 14.5 7.6 15.7
Specific pressure, 0.8 MPa.
R. Luo / Carbon 40 (2002) 1957 – 1963
4. Conclusion Full-size and dense C / C composites can be manufactured in a short time using the electrified preform heating CVI method. The rate of infiltration is higher for natural gas than for propylene. The C / C composites prepared using natural gas possess high flexural properties. The coefficient of friction is linearly and greatly decreased as the brake angular velocity is increased with the constant moment inertia and specific pressure, and average thickness wear is 13310 24 mm / surface per stop.
References [1] Schmidt DL, Davidson KE, Theibert LS. Unique applications of carbon–carbon composite materials. SAMPE J 1999;35(3):27–39. [2] Luo RY. Friction performance of C / C composites prepared using rapid directional diffused chemical vapor infiltration processes. Carbon 2001, in press.
1963
[3] Zhu YC, Ohtani S, Sato Y, Iwamoto N. Influence of boron ion implantation on the oxidation behavior of CVD-SiC coated carbon–carbon composites. Carbon 2000;38:501–7. [4] Emig G, Popovska N, Schoch G, Stumm T. The coating of continuous carbon fiber bundles with SiC by chemical vapor deposition: a mathematical model for the CVD-process. Carbon 1998;36(4):407–15. [5] Bruggert M, Hu Z, Huttinger KJ. Chemistry and kinetics of chemical vapor deposition of pyrocarbon. Carbon 1999;37:2021–30. [6] Savage G. In: Carbon–carbon composites, London: Chapman & Hall, 1993, pp. 95R–7R. [7] Vaidyaraman S, Lackey WJ, Freeman GB, Agrwal PK, Langman MD. Fabrication of carbon–carbon composites by forced flow-thermal gradient chemical vapor infiltration. Carbon 1995;33(6):1469–70. [8] Min S, Lee JY. Fracture behavior of two-dimensional carbon / carbon composites. Carbon 1989;27(6):423–30. [9] Luo RY. Research of technology, property, and microstructure for carbon / carbon composites made by rapid CVD method. Xi’an, People’s Republic of China: Northwestern Polytechnical University, Ph.D. Thesis, 1995:55–58.
Fabrication of carbon / carbon composites by an electrified preform heating CVI method Ruiying Luo* College of Science, Beijing University of Aeronautics and Astronautics, 37 Xueyuan Rd., Haidian District, Beijing, 100083, PR China Received 8 October 2001; accepted 5 January 2002
Abstract Carbon / carbon composites are manufactured using the electrified preform producing directly heat CVI process. The preforms are prepared by laminating the carbon fiber felts with crossply reinforcement, and infiltrated with carbon using natural gas or propylene as a reactant, with nitrogen as diluent at atmospheric pressure. The relations between the resistivity of samples and infiltration time are determined under the operating conditions. The results indicate that the preforms have gained a high infiltration rate by this technology, and the samples have higher densities using natural gas rather than propylene. Their highest average bulk densities are up to 1.71 g / cm 3 after the preforms of 11003500335 mm size have been densified for 80 h using natural gas. The carbon fibres in the preforms have not been damaged by this technology as yet, and the composites prepared have sufficiently high flexural properties. As the brake angular velocity is increased with the constant brake moment inertia and specific pressure, the average coefficient of friction for the composites prepared using natural gas is linearly and greatly decreased, but the variations of the brake moment inertia have a slight influence on the average coefficient of the friction when the brake angular velocity and specific pressure are kept constant. Their average thickness wear is 13310 24 mm / surface per stop. 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon / carbon composites; B. Chemical vapor infiltration; D. Frictional properties; Mechanical properties
1. Introduction Carbon / carbon (C / C) composites have low densities, good high-temperature structural properties, high thermal stabilities and heat capacities, especially their acceptable coefficients of friction, over a wide range of temperatures [1]. They are considered as the best brake materials, jet engine component materials, and the most promising candidate materials for high-temperature structural applications [2,3]. The C / C composites are at present prepared using many cycles of resin or pitch infiltration followed by carbonization and graphitization, or by using the isothermal chemical vapor infiltration (CVI) process [4,5]. The cycles of these methods sometimes have to be repeated many times and, as a result, processing can often take 1 month or longer. The thermal gradient method allows considerably higher infiltration rates to be attained than those with conventional isothermal technology, but *Corresponding author. Fax: 186-10-8232-8283. E-mail address: [email protected] (R. Luo).
the disadvantages of the thermal gradient method are the limitations to geometry, dimension of parts, single item processing, and difficulties in scaling up this process. A great deal of work has been carried out on thermal gradient CVI processing of C / C composites [6]. Vaidyaraman et al. have fabricated C / C composites by forced flow-thermal gradient CVI [7], but some shortcomings of this method have not been overcome yet. In electrified preform heating CVI (ECVI) technology, preforms are used as the resistance and produce heat, consequently the temperature is naturally the highest in the central parts of the preform thickness which cannot be easily densified using the isothermal method. On the other hand, fibre preforms possess low thermal conductivities to permit the thermal gradient to be established. Also some high gas flow rates are used to cool the exterior surface of the preforms. These factors ensure that a large thermal gradient occurs across the fibre preform thickness. In the ECVI process, the rate of increase in temperature for the preforms is very rapid, and full-size or a greater number samples can be manufactured simultaneously in an
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00027-1
1958
R. Luo / Carbon 40 (2002) 1957 – 1963
identical furnace. A temperature gradient on the order of 150–300 8C is applied across the preform thickness. Other advantages include zero incidence of overcrusting and ability of the process to run at atmospheric pressure. Machining of the samples and the need for low pressure vessels and pumping apparatus commonly associated with the isothermal method can thus be eliminated. In this paper, the ECVI processes for the preparation of C / C composites are proposed.
2. Experimental
2.1. Preparation of preforms The size of preforms is 11003500335 mm. Their volume fraction of carbon fibres is 40%. Fibres containing 12 000 filaments per tow obtained from the Lanzhou carbon fiber plant, China, have been used to prepare the preforms. The preforms have been manufactured according to the following steps: the carbon fiber felts have been laminated (Fig. 1), and two layers have been oriented at an angle of 908, and then needled progressively till the thickness of preforms is reached.
2.2. Densification of preforms The ECVI equipment is shown in Fig. 2. The ECVI processes have been performed using natural gas or a flowing mixture of propylene and nitrogen. The ratio of propylene to nitrogen is about 1:5. The flow rate of natural gas or mixture is less than 40 l / min. The first step of the test is to evacuate the furnace and check for leaks; upon verification that the leak rate is below 0.13 Pa / min, nitrogen is used to purge the furnace for about 30 min, and then the furnace is filled with nitrogen to atmospheric pressure. Following this step, the preforms are heated to the required temperature with flowing nitrogen, which protects the preforms from oxidation. The temperatures at the outside surface and the central part of the preforms (called the infiltration tempera-
Fig. 2. A schematic of the equipment for the ECVI method.
ture) are measured throughout the run using two thermocouples, one of which is fixed near the surface of the preform, while the other is fixed at the center of the preform thickness. After the infiltration temperature has stabilized for 30 min, natural gas or a flowing mixture of propylene and nitrogen is sent into the furnace. As the infiltration proceeds, the thermocouple at the center of the preforms is progressively moved towards the surface of the preform, but the value of the infiltration temperature has been kept constant under a single infiltration process. After the densification of the preforms is finished, the samples are cut into pieces and heated to 2500 8C.
2.3. Temperature profiling To develop the appropriate temperature gradient, it is necessary for us to know the actual temperature on the surface of the preforms and at the center of the preform thickness, the effect of the cooler environment and gas flow rate on the temperature. Therefore a temperature calibration experiment is conducted. Table 1 gives the temperature on the surface of the preforms and at the center of the preform thickness. It is shown that the cooler environment, gas flow rates and types, etc. have a strong influence on the temperature.
2.4. Measurement of bulk density of the samples Specimens of 10310310 mm are cut from each sample, and the number is not less than seven for each test point. Then they are weighed in an analytical balance. The bulk density of the samples can be calculated.
2.5. Mechanical properties Fig. 1. The architecture of carbon fibre felts used to prepare preforms.
The specimens used for the mechanical-property tests
R. Luo / Carbon 40 (2002) 1957 – 1963
1959
Table 1 The temperature on the surface of preforms and in the center of the preform thickness Thermal insulation thickness (mm)
Reagent gas type
Reagent flow rate (l / min)
10 10 10 5 10 10 10 10 10
Natural gas Natural gas Natural gas Natural gas Natural gas
35 40 35 35 35
Propylene Propylene Propylene
5 5 6
are cut out from the full-size samples with the fabric plane perpendicular to the bend, and the number is still not less than seven for each test point. The mechanical properties are determined using the three-point bending test on the electron universal testing machine model CSS-1110. The cross-section of the specimen bar is 1036 mm, the support span is 40 mm, and the crosshead speed is 1 mm / min.
Nitrogen flow rate (l / min)
Surface temperature (8C)
Center (infiltration) temperature (8C)
35 25 25 30
780 760 890 710 680 860 730 820 725
1000 1000 1100 1000 900 1000 820 900 820
Flexural strength (sf ) and flexural modulus (Ef ) are calculated with the following equations: 3PL sf 5 ]]2 2bh DPL 3 Ef 5 ]] 4bh 3 Df where L is the span of the bending test, h is the thickness of the specimen, and b is the width. DP/Df is the slope of the straight line of the load–displacement curve. The fracture of the specimens is analyzed by SEM model AMRAY-1000B.
2.6. Frictional and wear properties The samples are prepared using natural gas at 1000 8C for the frictional and wear property testing. The specimens are cut out from the same samples, treated at high temperature, then machined into annular rings (Fig. 3), protected from oxidation, and mounted on the friction test machine model MM-1000 to perform the frictional and wear property testing. The test machine holds a rotor disc and an identical stator disc. The dimensions of the disc are: internal diameter of 53 mm, external diameter of 75 mm, and initial thickness of 10 mm; 100 stops are carried out in all.
3. Results and discussion
3.1. Effect of infiltration time on the resistivity of the preforms
Fig. 3. The rotors and stators used in the laboratory scale friction test machine.
In this technology, since the preforms produce heat directly after being electrified, the temperature value of the preforms depends mainly on the power of the furnace and the value of their resistivity, but the resistivity value of the preforms varies with infiltration time. Therefore it is indispensable that the relations between the resistivity of
1960
R. Luo / Carbon 40 (2002) 1957 – 1963
Fig. 4. The variations of preform resistivity with infiltration time using natural gas at 1000 8C.
the preforms and the infiltration time be known to design better this kind of set-up and control effectively the infiltration process. Fig. 4 gives the variations of the preform resistivity with infiltration time under the constant infiltration temperature. Fig. 4 shows that as the infiltration proceeds, the resistivity of the preforms is greatly reduced at the beginning of the infiltration, and then slowly decreases. Their resistivity is reduced more rapidly using natural gas than propylene at the same infiltration time, as a result of the different infiltration rates of natural gas and propylene.
Fig. 5. The relationship between average density and infiltration time.
can diffuse more easily into the interiors of the preforms at atmospheric pressure. These results are somewhat different from that reported by Vaidyaraman et al. using FCVI technology [7], because in the FCVI technology, no matter how much the molecular weight of reagents is, they all can be forced through the preforms, and the carbon is infiltrated there. However, for the ECVI processes, the reagents go into the preforms only by free diffusion, the larger the molecular weight of the reagents, the slower the reagents diffuse into the performs, so in contrast, the infiltration rate of carbon becomes small using propylene.
3.3. Mechanical properties 3.2. Effect of processing parameters and reagent types on the infiltration rate The influence of the infiltration temperature on the densification rate is given in Fig. 5. After the preforms are densified for 80 h using natural gas at 900, 1000, 1100 8C, respectively, the average bulk density of the samples prepared at 1000 8C is the highest, up to 1.71 g / cm 3 . If the infiltration temperature is too slow (i.e. less than 900 8C), the infiltration rate of carbon is very slow, as a result of smaller average bulk density of the samples. Though the infiltration of carbon can be speeded up by increasing the temperature if the infiltration temperature is too high (i.e. more than 1100 8C), the coolest portion of the preforms, or their outer surface, also has very high temperature, and it receives the first deposit. Therefore the surface crusting is formed. This is why the infiltration rate of the preforms slows down at 1100 8C again. Fig. 5 also shows that a higher infiltration rate can be gained using natural gas rather than propylene. This can be attributed to the smaller molecular weight of methane which is the main component in natural gas, for methane
The mechanical-property test data of C / C composites Table 2 Mechanical properties of ECVI C / C composite Properties 3
Density (g / cm ) Flexural strength (MPa) Standard difference Dispersed coefficient (%) Flexural modulus (GPa) Standard difference Dispersed coefficient (%) Displacement at failure (mm) Standard difference Dispersed coefficient (%)
Natural gas
Propylene
1.71 101.3
1.69 99.7
8.7 8.59
6.43 6.44
28.0
21.2
2.17 7.75
2.08 9.81
0.57
0.59
0.12 21.05
0.13 22.03
The specimens were densified for 360 h using propylene.
R. Luo / Carbon 40 (2002) 1957 – 1963
1961
carbon is formed [8]. Therefore C / C composites fail in a stepwise process with fibre pull-out and delamination (Fig. 7b).
3.4. Frictional and wear properties
Fig. 6. The load–displacement curves of C / C composites prepared by the ECVI method.
prepared using natural gas and propylene are depicted in Table 2. The corresponding load–displacement curves in three-point flexural testing are shown in Fig. 6. The C / C composites manufactured from natural gas exhibit similar strength, and displacement at failure compared to those made using propylene, but the modulus of the former is higher than that of the latter. Fig. 7a shows that C / C composites prepared using propylene possess a smooth fracture surface, and display a catastrophic failure mode, which is possible owing to the strong interface between fibres and infiltration carbon in this category of composite. This being the case, fibres may fracture in the primary crack plane, leading to a catastrophic failure mode devoid of delamination or fibre pull-out. While for C / C composites made from natural gas, a higher degree of the preferred orientation of carbon is probably present on the surface of fibres, and a weaker bond between fibres and deposition
3.4.1. Effect of moment inertia and brake angular velocity Fig. 8 shows the effect of moment inertia and brake angular velocity on the variations of brake torque with brake time. Table 3 shows the brake conditions and the required brake time. Fig. 9a shows the coefficient of friction under different testing conditions. It is obvious that the average coefficient of friction for C / C composites is slightly decreased with the moment inertia increasing under constant brake angular velocity and specific pressure conditions. From Fig. 9b, it is shown that as the brake angular velocity is increased with the constant moment inertia and specific pressure exerted on the brake discs, the coefficient of friction for C / C composites is greatly and linearly decreased, which results possibly from the effect of the brake angular velocity on the state of friction surface and the energy on the discs. When the velocity is increased to 5800 rpm, the coefficient of friction becomes moderate (Fig. 9b); nevertheless, as the angular velocity is equal to 7000 rpm, the brake torque is very low at the last stage of braking (Fig. 8b). Therefore C / C composites prepared using natural gas possess good frictional properties with 2 moment inertia of 0.53 kg m , brake angular velocity of 5800 rpm, and brake specific pressure of 0.8 MPa. 3.4.2. Wear After 100 the average mm / surface
performance of ECVI C /C composites brakings are performed, it can be found that thickness wear of discs is only 13310 24 per stop. The specific wear reported here is
Fig. 7. SEM fractographs of ECVI C / C composites; (a) C / C composites made using propylene; (b) C / C composites made using natural gas.
R. Luo / Carbon 40 (2002) 1957 – 1963
1962
Fig. 8. The effect of angular velocity and moment inertia on torque–time curves for ECVI C / C composites (specific pressure, 0.8 MPa).
comparable to that of C / C composites prepared using the isothermal method [9], and the friction surface of discs is smooth.
Fig. 9. The average coefficient of friction for ECVI C / C composites under different braking conditions (specific pressure, 0.8 MPa).
Table 3 Brake testing conditions Test no.
Moment inertia (kg m 2 )
Angular velocity (rpm)
Energy (kJ)
Brake time (s)
1 2 3 4 5
0.32 0.53 0.73 0.53 0.53
5800 5800 5800 3500 7000
58.96 97.66 134.5 35.56 142.24
9.5 13.4 14.5 7.6 15.7
Specific pressure, 0.8 MPa.
R. Luo / Carbon 40 (2002) 1957 – 1963
4. Conclusion Full-size and dense C / C composites can be manufactured in a short time using the electrified preform heating CVI method. The rate of infiltration is higher for natural gas than for propylene. The C / C composites prepared using natural gas possess high flexural properties. The coefficient of friction is linearly and greatly decreased as the brake angular velocity is increased with the constant moment inertia and specific pressure, and average thickness wear is 13310 24 mm / surface per stop.
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1963
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