Carbon/carbon composites (C/C composites) possess superior characteristics of low density, high strength, extremely low coefficient of thermal expansion, and high fatigue resistance. In carbonization process, the high-temperature pyrolysis made of carbon, hydrogen, oxygen, and other elements results in a lot of voids and cavities generated in the interior of C/C composites. Therefore, the C/C composites are densified to fill the voids by using repeated impregnation. But densification is a time-wasting and complex process, which increases production costs in the manufacturing process. In this study, the multiwall carbon nanotubes (MWNTs) were adopted as a reinforcement material for C/C composites to reduce the existence of voids or cavities and enhance the mechanical properties of C/C composites. According to the experimental results, the CNT-added C/C composite containing 1.2 wt% CNT possesses the greatest flexure strength, flexure modulus, and interlaminar shearing strength. Plus, the above-mentioned strength and modulus are increased by 23%, 19.2%, and 30%, respectively.
Carbon/carbon composite (C/C composite) is an abbreviation of the carbon fiber-reinforced carbon matrix composite which is reinforced by fiber. No matter fiber or matrix, both of them in composite are composed of single elemental carbon.
In 1993, Buckley and Edie [
In 1993, Savage [
In 1987, Fitzer [
As we previously mentioned the properties of C/C composite, the advantages of carbon/carbon composite made of carbon fiber-reinforced carbon matrix include high-temperature and high elastic coefficient, low density, low thermal-expansion coefficient and high fatigue resistance.
Because of the high aspect ratio [
In this study, the various content and amount of CNTs were tried to be evenly disperse among phenolic resin [
The above-mentioned process not only can prevent porous generation among the matrix during carbonization process but also can make CNTs effectively deliver stress to properly reinforce the mechanical properties in the thickness direction of laminate.
In manufacturing process, carbon fiber cloth was adopted as a major material, which was impregnated into phenolic resin. Heat treatment was then carried out. Next, the following treatments including carbonization, reimpregnation, and graphitization must be done to manufacture C/C composite finally. The entire manufacturing process of C/C composite is shown in Figure
The entire manufacturing process of C/C composite.
First, the CNTs/isopropanol solution was stirred for one hour using homogenizer, then the solution was vibrated using ultrasonication for another two hours to enable CNTs to evenly disperse among the isopropanol solution. Furthermore, the CNTs/isopropanol solution was mixed with phenolic resin for one hour using mechanical mixer.
The CNTs/phenolic resin solution was placed into a vacuum heating oven and carried out the vacuum pumping for five minutes to restrain air bubble existence. The desired dimension carbon fiber cloth was placed on a release paper, and the CNTs/phenolic resin solution was evenly permeated on the carbon fiber cloth. Finally, the carbon fiber cloth with great dispersed CNTs/phenolic resin was placed into a heating oven to be exposed to a temperature at 83°C for 4 hours to evaporate extra solution.
13 pieces of prepregs were piled up into mold and placed on hot press machine to make a laminate (press at 1500 psi, temperature at 175°C). Then the laminate was placed into heating oven in a temperature at 140°C for 3 hours to eliminate internal stress of the laminate [
Hot pressing process of CNTs-added C/C composite.
The desired dimensional specimen was placed in a muffle furnace. The temperature in the muffle furnace increases at an increasing rate of 5°C per minute from room temperature up to 600°C, then keep this temperature for 30 minutes and then the specimen naturally cooled down to room temperature. The carbonization process and carbonized specimen are shown in Figures
Carbonization process of CNTs-added C/C composite.
Carbonized specimen.
The CNTs-reinforced carbon/carbon composites with 4 different proportions of CNT at 0.5 wt%, 1.0 wt%, 1.2 wt%, and 1.5 wt% were fabricated and investigated in this study. The flexure strength test, interlaminar shearing strength test, and impact test were carried out, and the results were compared with those of CNTs-unadded carbon/carbon composites.
Furthermore, the fracture surface of specimen was investigated utilizing SEM image to figure out the dispersion status of CNTs among composites.
The results shown in Figure
Flexure strength of C/C composites and CNTs C/C composites.
Type of C/C | Mean strength (MPa) | Maximum strength (MPa) | Minimum strength (MPa) |
---|---|---|---|
C/C | 55.15 | 57.93 | 52.78 |
C/C/0.5 wt% CNT | 62.56 | 65.19 | 59.65 |
C/C/1.0 wt% CNT | 66.78 | 68.75 | 64.31 |
C/C/1.2 wt% CNT | 67.78 | 70.84 | 65.62 |
C/C/1.5 wt% CNT | 66.48 | 66.68 | 65.91 |
Flexure modulus of C/C composites and CNTs C/C composites.
Type of C/C | Mean modulus (GPa) | Maximum modulus (GPa) | Minimum modulus (GPa) |
---|---|---|---|
C/C | 18.82 | 20.68 | 17.62 |
C/C/0.5 wt% CNT | 19.85 | 20.53 | 18.86 |
C/C/1.0 wt% CNT | 22.20 | 23.05 | 21.28 |
C/C/1.2 wt% CNT | 23.29 | 23.82 | 22.64 |
C/C/1.5 wt% CNT | 20.95 | 22.43 | 19.37 |
Flexure strength of CNTs C/C composite.
Flexure modulus of CNTs C/C composite.
Interlaminar shearing strength is the most important property in the thickness direction of C/C composite laminate. In this study, short beam test was adopted to examine the interlaminar shearing strength. The results shown in Figure
Interlaminar shearing strength of C/C composites and CNTs C/C composites.
Type of C/C | Mean strength (MPa) | Maximum strength (MPa) | Minimum strength (MPa) |
---|---|---|---|
C/C | 4.83 | 4.98 | 4.70 |
C/C/0.5 wt% CNT | 5.15 | 5.40 | 4.57 |
C/C/1.0 wt% CNT | 5.38 | 5.63 | 5.01 |
C/C/1.2 wt% CNT | 6.27 | 6.55 | 6.00 |
C/C/1.5 wt% CNT | 5.16 | 5.28 | 5.04 |
Interlaminar shearing strength of CNTs C/C composite.
Fracture surface of interlaminar shearing strength of CNTs C/C composite. (a) 5000× and (b) 20000×.
Fracture surface of interlaminar shearing strength of 1.2 wt% CNTs C/C composite. (a) 5000× and (b) 20000×.
Fracture surface of interlaminar shearing strength of 1.5 wt% CNTs C/C composite. (a) 5000× and (b) 20000×.
From the results shown in Figure
Impacting energy of C/C composites and CNTs C/C composites.
Type of C/C | Mean energy (J/m) | Maximum energy (J/m) | Minimum energy (J/m) |
---|---|---|---|
C/C | 25.17 | 25.68 | 24.73 |
C/C/0.5 wt% CNT | 24.74 | 27.57 | 22.38 |
C/C/1.0 wt% CNT | 24.29 | 27.83 | 22.44 |
C/C/1.2 wt% CNT | 24.76 | 25.63 | 23.55 |
C/C/1.5 wt% CNT | 26.74 | 31.52 | 23.94 |
Impacting energy of CNTs C/C composite.
Impact testing machine and system.
The flexure strength, flexure modulus, and interlaminar shearing strength of CNTs-added C/C composites are higher than those of CNTs-unadded C/C composites. Because the cross-link of high strength CNTs among microcrevices can stop creviced growth and make crevices tortuous to effectively restrain the creviced growth, the above-mentioned function of CNTs can increase the flexure strength, flexure modulus, and interlaminar shearing strength. Furthermore, as CNTs content increases up to 1.2 wt%, the highest flexure strength, flexure modulus, and interlaminar shearing strength were increased.
Impact test is the fiber breakage dominated by fiber. CNTs do not notably influence resistance to impact whether CNTs were added into matrix or not.
The authors are grateful for the help of the Center for Nanotechnology, Materials Science, and Microsystems at the National Tsing Hua University, Taiwan. Ted Knoy is appreciated for his editorial assistance.