Behavior of pure and modified carbon/carbon ... - Springer Link

International Journal of Minerals, Metallurgy and Materials Volume 21, Number 2, February 2014, Page 190 DOI: 10.1007/s12613-014-0884-y

Behavior of pure and modified carbon/carbon composites in atomic oxygen environment Xiao-chong Liu, Lai-fei Cheng, Li-tong Zhang, Xin-gang Luan, and Hui Mei Science and Technology on Thermostructural Composite Materials Laboratory, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China (Received: 10 August 2013; revised: 11 October 2013; accepted: 20 October 2013)

Abstract: Atomic oxygen (AO) is considered the most erosive particle to spacecraft materials in low earth orbit (LEO). Carbon fiber, carbon/carbon (C/C), and some modified C/C composites were exposed to a simulated AO environment to investigate their behaviors in LEO. Scanning electron microscopy (SEM), AO erosion rate calculation, and mechanical property testing were used to characterize the material properties. Results show that the carbon fiber and C/C specimens undergo significant degradation under the AO bombing. According to the effects of AO on C/C-SiC and CVD-SiC-coated C/C, a condensed CVD-SiC coat is a feasible approach to protect C/C composites from AO degradation. Keywords: atomic oxygen; oxidation; carbon/carbon composites; ceramic matrix composites

1. Introduction Due to the inherent properties of the low coefficient of thermal expansion (CTE), good resistance to thermal shock, excellent inertness in cosmic radiation environment, and high mechanical performance at the high temperature of 2000°C, carbon/carbon (C/C) composites have become one of the most potential candidate materials used for aeronautic engine nozzles and ablation thermal-proof structure in the aerospace applications [1-2]. In space missions, spacecraft would enter into or move out low earth orbit (LEO) more than once or stay in their aerospace orbits for a long time [3]. Therefore, spacecraft materials should be considered and systematically investigated under the service environments to characterize the material stability and reusability [4]. Atomic oxygen (AO), the predominant atmosphere in LEO, is generated by the dissociation of oxygen molecules subject to the solar ultraviolet radiation, which is about 1015 atoms/(cm2·s) at an altitude of 200 km above the ground [5-6]. From previous reports, the extreme chemical activity of AO originating from its single-atom structure, as well as its high energy (5 eV) induced by the spacecraft orbit speed Corresponding author: Lai-fei Cheng

of 7-8 km/s confers to AO particles a severe erosion ability against aerospace materials [7-8]. However, pure C/C and modified C/C composites, such as C/C-SiC and SiC-coatedC/C, have not yet been systematically investigated in the AO environment.

2. Experimental 2.1. Materials Needle-punched integrated felts prepared by using the three-dimensional needling technology were used to fabricate the C/C composite. The carbon fiber used is PAN-based T300TM (12K, Toray, Japan). Nonwoven cloth and short-cut fiber web were alternately laminated and integrated by needle-punching step by step. The density of the prepared 3D needled carbon preform is about 0.7 g⋅cm−3. The carbon matrix of the C/C composite was manufactured by chemical vapor infiltration (CVI) at 800-1100°C, and propylene gas was used as precursor [9]. The density of the C/C composite used is 1.60 g⋅cm−3. The C/C-SiC composite was prepared by reactive melt infiltration (RMI) based on the C/C composite and its density is 2.1 g⋅cm−3. Moreover, the chemical vapor deposition (CVD) technology was used

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© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2014

X.C. Liu et al., Behavior of pure and modified carbon/carbon composites in atomic oxygen environment

to produce a dense SiC layer (20-100 μm) directly on the C/C substrate for preparing a SiC-coated C/C composite. Methyltrichlorosilane (MTS, CH3SiCl3) is used as the gaseous precursor in this process, and the density of the prepared SiC-coated C/C is 1.75 g⋅cm−3 [10-11]. 2.2. AO environment simulator The AO atmosphere simulation facility was designed by Beihang University. In the AO experiment, the as-received specimens were fixed on the experimental plate. When the pressure of the facility chamber was reduced to 4.0 × 10−3 Pa by means of a vacuum system, the oxygen stream was introduced into the chamber through a flow controller. Simultaneously, some electrons were discharged by a heated cathode filament and were accelerated by using a directed magnetic field to strike and debone O−O for producing the desired AO particles [12]. 2.3. Analysis and observation To characterize the AO erosive effect on different materials, previous researchers defined a parameter named AO erosion rate, which represents the volume loss of the target material per AO particle [13]:

ΔM s (1) F ρs As where Er is the AO erosion rate of the sample, cm3·AO−1; ΔM s is the mass loss of the sample, g; As is the surface area of the sample exposed to AO, cm2; ρs is the density of Er =

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the experimental sample g·cm−3; and F is the fluence of AO, AOs·cm−2. To characterize the microscopic evolution of the specimens before and after AO treatment, a scanning electron microscope (S-2700, Hitachi High-Tech Corporation, Japan) was used in this study.

3. Results and discussion 3.1. Carbon fiber Carbon fiber usually acts as reinforcing material in C/C composites; therefore, the effect of AO on the carbon fiber needs to be considered to characterize the effect of AO on the C/C. The T300TM carbon fiber (3K) was used for this experiment. The carbon filament diameter was 7 μm. The superficial morphology of the T300TM carbon fiber (3K) is shown in Fig. 1 before and after AO treatment. It can be found that some grooves parallel to the fiber axis exist around the circle of the original filament (Fig. 1(a)). After 2 h of AO treatment, the outline of the grooves became obscure and the filament adopted a corduroy-like appearance (Fig. 1(b)). After 10 h of AO treatment, the side of the fiber confronting the AO flow sharply shrunk, and its microstructure showed some deep erosive ditches on the carbon filament (Fig. 1(c)). These results revealed that the carbon fiber is extremely vulnerable to the AO oxidation. Moreover, the micromorphology showed that the AO particles have high-speed impact on the target, subsequently forming a cotton-like outline on the carbon filament.

Fig. 1. Micromorphology of the T300TM carbon filament: (a) original; (b) AO treatment (2 h); (c) AO treatment (10 h).

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In this study, we developed a method to calculate the AO erosion rate of the T300TM carbon fiber (3K). Before the AO test, the carbon filaments were hackled and arranged side by side, as shown in Fig. 2. The direction of the AO flow was perpendicular to the upper area of all the filament lines. After AO treatment, the weight change was measured, and the

actual area ( As ) of the AO bombardment on the filaments was determined by Eq. (2). πd π×7 As = 3000 × × L = 3000 × × L ×10−6 m 2 (2) 2 2 where d is the diameter of the T300TM carbon fiber (3K), and L is the length of the tested fiber.

Fig. 2. Schematic diagram of the calculation of AO erosion rate on the fiber.

Fig. 3 shows the curve of the AO erosion rate of T300TM depending on the AO treatment time. At the initial AO bombing, the AO erosion rate is about 3.21 × 10−24 cm3⋅AO−1. With increasing treatment time, the AO erosion rate of the carbon fiber decreased, but remained in the 10−24 range, which is equal to that of the mostly polymer materials [14-15]. This result indicated that the carbon fiber belongs to AO-sensitive materials.

carbon, induced by the multiple-CVI process, also became more obscure (Fig. 4(b)). Therefore, it can be concluded that AO seriously corrodes the C/C composite. In this study, the chemical reaction between the carbon and the AO particle can be described as follows: vacuum C + O(5 eV) ⎯⎯⎯ → CO ↑

(3)

C + 2O(5 eV) ⎯⎯⎯→ CO2 ↑

(4)

vacuum

The AO erosion rate of the C/C specimens is shown in Fig. 5. Initially, the AO erosion rate is about 4.63 × 10−20 cm3⋅AO−1, which is four orders of magnitude larger than that of the carbon fiber, meaning that the C/C composite is more sensitive to the AO than the carbon fiber. With increasing AO treatment time, the AO erosion rate decreased gradually because the probability of valid AO collision points bombed on the C/C target would decline with the increase of the AO erosive cavities. 3.3. Modified C/C composite

Fig. 3. AO erosion rate of the T300TM carbon filament.

3.2. C/C composite Fig. 4 shows the surface morphology of the C/C composite before and after the AO treatment. Before the AO treatment, the carbon fibers and the multilaminated pyrolytic-carbon matrix were regularly displayed within the cross section of the C/C composite (Fig. 4(a)). After 10 h of AO treatment, the C/C specimen had approximately the same AO corrosion morphology as that of the carbon fiber on the cross section of the specimen, in which many uniformly distributed erosive cavities were present. Some laminated or deposited traces of the multilayer pyrolytic

In previous studies, researchers traditionally incorporated the SiC phase into the constitution of the C/C composite to improve durability in some complex oxidation environments. In this study, C/C-SiC and CVD-SiC-coated C/SiC specimens were fabricated and studied in the AO environment to find an effective way to protect the C/C material from the AO degradation. The morphology of the original C/C-SiC specimen is shown in Fig. 6(a), in which some cracks due to the nonuniform reaction between the pyrolytic carbon and the post-infiltrated molten silicon are evident. After 10 h of AO treatment (Fig. 6(b)), the laminated traces of the composite matrix became clear again. Moreover, some erosion cavities appeared on the C/C-SiC specimen, which distributed not only on the matrix but also on the fiber zone. This indicated


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Fig. 4. Micromorphology of the carbon/carbon (C/C) composite: (a) original C/C; (b) after 10 h of AO treatment.

SiC-coated C/C is about 3.3 × 10−26 cm3⋅AO−1, which is the lowest AO erosion rate found in this study (Fig. 7(a)). According to their AO erosion rates, the anti-AO erosion ability can be listed in a descending sequence as SiC-coated C/C, C/C-SiC, carbon fiber, and C/C. 3.4. Flexure strength test

Fig. 5. AO erosion rate of the C/C composite depending on AO treatment time.

that C/C-SiC is somewhat sensitive to AO. Fig. 6(c) shows the surface morphology of the original SiC-coated C/C specimen, which exhibits some cauliflower-like SiC crystals on the coat. After 10 h of AO treatment (Fig. 6(d)), some erosive marks appeared on the specimen surface, in which the top of the SiC crystal was darker than the non-treated samples. Therefore, by comparing their micromorphologies, it can be concluded that the SiC-coated C/C composite has a higher AO-resistant ability than C/C-SiC. Fig. 7 shows the AO erosion rate of the C/C-SiC and the SiC-coated C/C. The average AO erosion rate of the C/C-SiC is about 1.95 × 10−25 cm3⋅AO−1, and that of the

In order to characterize the strength evolutions of the C/C, C/C-SiC, and CVD-SiC-coated C/C specimens in the AO environment, a three-point bending test was performed, and the flexural data are presented in Fig. 8. The C/C specimen has the lowest virgin flexural strength among the tested materials. Moreover, the C/C composite also has the largest strength drop after 10 h of AO bombing, with a 57.7% decrease from 85 to 36 MPa. This result indicates that AO greatly erodes the C/C composite. The C/C-SiC, which has the highest original strength that dropped by 23.36% from 110 to 84 MPa, displayed a better AO-resistant ability than the C/C composite. The virgin CVD-SiC-coated C/C has a medium flexural strength among the tested materials; however, its strength-time curve kept a steady state with little fluctuation, which proved that the CVD-SiC-coated C/C has the best anti-AO performance. This result indicated that the CVDSiC coating is a good approach to protect C/C from AO bombing.

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Fig. 6. Micromorphology of the composite surface: (a) original C/C-SiC; (b) C/C-SiC after 10 h of AO treatment; (c) original SiC-coated C/C; (d) SiC-coated C/C after 10 h of AO treatment.

Fig. 7. AO erosion rate of the modified C/C composite: (a) C/C-SiC; (b) SiC-coated C/C.

4. Conclusions

Fig. 8. Average flexural strength of the specimens after AO treatment.

In order to find an effective way to protect C/C composites from AO degradation, some C/C-based composites were investigated to characterize their behaviors in a simulated AO environment. Some interesting results were obtained: (1) A significant degradation was detected for carbon fiber and C/C composites upon AO bombing. The carbon matrix of the C/C composite is more sensitive to the AO than that of the carbon fiber. (2) The approach of infiltrating silicon into the C/C composite cannot protect C/C from AO degradation. (3) Based on the AO erosion rate calculations, the


AO-resistant ability of the materials can be listed in an increasing order as C/C, carbon fiber, C/C-SiC, and CVDSiC-coated C/C. (4) The CVD-SiC coating is a feasible way to enhance the AO resistance of C/C composites.

Acknowledgements This work was financially supported by the Major International (Regional) Joint Research Project under the National Natural Science Foundation of China (No. 50820145202) and the Major State Basic Research Development Program of China (No. 2011CB605806).

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