Materials Chemistry and Physics 101 (2007) 7–11

A rapid fabrication of C/C composites by a thermal gradient chemical vapor infiltration method with vaporized kerosene as a precursor Jiping Wang ∗ , Junmin Qian, Guanjun Qiao, Zhihao Jin State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China Received 8 September 2005; received in revised form 23 January 2006; accepted 4 February 2006

Abstract A thermal gradient, atmospheric pressure chemical vapor infiltration method with simultaneous vaporized kerosene as a precursor for rapid fabrication of C/C composites was studied. By this method, carbon felts (bulk density ∼0.2 g cm−3 ) were densified to C/C composites with density of 1.67 and 1.71 g cm−3 when prepared at 1050 and 1150 ◦ C for 6 h, respectively. X-ray diffraction result indicates that the composites have a strong ability to graphitize and the higher deposition temperature leads to the increased graphitization degree. Polarized light microscope and scanning electron microscope images reveal that fibers of the composites prepared for 6 h are surrounded by ring-shaped pyrocarbon matrix with a thickness of ∼20 ␮m, and that the matrix is delaminated to 4–6 layer-like regions. The deposition process is analyzed by dividing the reactor into four regions associated with specific functions and the reasons for the rapid fabrication are proposed as the short convection and diffusion path for the precursor and the existing of thermal gradient across the preform. © 2006 Published by Elsevier B.V. Keywords: Carbon/carbon composite; Chemical vapor infiltration; Rapid densification; Microstructure

1. Introduction Carbon/carbon (C/C) composites, possessing noteworthy thermal and mechanical characteristics, have been extensively applied in the aerospace industry mainly as protection for atmosphere re-entry and disc brakes of aircraft [1]. One of the most common fabrication methods of C/C composites in the industry is the isothermal, isobaric chemical vapor infiltration (CVI) technique, which has been developed for a long time. However, the main drawback of this method is the very long process time with rather low overall precursor efficiency. Therefore, many derivative techniques, such as CVI with thermal gradient, forced flow-CVI or pulse-CVI, etc., have been developed to reduce the process time. Among them, the last method, being called as film boiling chemical vapor infiltration, has attracted much interest for its short process time and higher carbon yield [2,3]. The high efficiency of this film boiling CVI method stems from of a strong thermal gradient inside a porous preform, which is completely immersed into a liquid precursor. The infiltration

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0254-0584/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2006.02.006

and deposition mechanism, as well as the influence of the deposition parameters (temperature, pressure, precursors, etc.) on the deposition rate and the type of pyrocarbon have been thoroughly investigated in literatures [4–9]. Nevertheless, limitation of the method still exists. Firstly, the preform is immersed in a liquid where an interaction of chemistry and hydrodynamics takes place; this interaction is complex, on which little is known from a theoretical viewpoint [10]. Therefore the correlation between the deposition parameters and the properties of the prepared C/C composites is likely unstable. Secondly, the inability to alter precursor concentration is also a potential disadvantage [11]. These limitations, together with another disadvantage that the efficient energy used for the deposition has been evaluated only around 10% of the total output energy [10], have prevented the regular film boiling CVI method from being widely utilized. To overcome the above-mentioned shortcomings, the present paper provides a new method, which combines the advantages of the regular film boiling CVI and the classical thermal gradient CVI. Our method, separating the liquid precursor with the preform and using two heat sources in the reactor, enables us to get a rapid deposition of C/C composites with using less inlet power and adjustable process conditions. The density and porosity, X-ray diffraction and microstructure of the composites are

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J. Wang et al. / Materials Chemistry and Physics 101 (2007) 7–11

studied. The densification process is analyzed and the reason for the rapid deposition is proposed. 2. Experimental procedure 2.1. Material preparation 2.1.1. Preform and precursor A PAN-based carbon felt (size: Ø 80 mm × 10 mm, bulk density: ∼0.2 g cm−3 , fiber diameter: 9–13 ␮m) was used as a preform. A liquid hydrocarbon mixture, kerosene (molecular formula: C9 Hn –C16 Hm ) with a boiling temperature range of about 180–260 ◦ C, was chosen as a precursor. 2.1.2. Experimental set-up The experimental device is schematically shown in Fig. 1. In the center of a cylindrical quartz glass reactor, there are two graphite susceptors (upper susceptor and lower susceptor) which have same diameter (Ø 100 mm) but with different thickness. The lower susceptor is separated with the bottom of the reactor by an adiabatic material, which can protect the glass reactor from overheating. The upper susceptor is supported by four sticks above the lower susceptor. The preform is affixed on the lower surface of the upper susceptor by means of carbon filament. The residual surfaces of the upper susceptor are wrapped with adiabatic material, which can reduce the heat loss to the reactor. The kerosene is filled in the reactor and the liquid surface is above the lower susceptor but below the preform. During the deposition, the two susceptors are inductively heated by the inductor around the reactor. The lower susceptor is used to boil and vaporize the liquid kerosene, which provides gaseous hydrocarbon for deposition. On the other hand, the upper susceptor is used to heat the surface of the preform. A thermocouple is located at the upper surface of the preform to measure the deposition temperature. N2 gas is introduced through the inlet for a safe consideration. Above the reactor is a water-cooled condenser, which permitted recovery of unconsumed hydrocarbon. The N2 gas and other by-products is exhausted from the top outlet. During the process, the heating rate is controlled by adjusting the power of an inductive generator. The precursor is added from the precursor inlet to the rector for continuous infiltration. Based on above description, we characterize this process as a liquidvaporized, cold wall, inductive heated, thermal-gradient, atmospheric pressure, and isobaric chemical vapor infiltration method.

2.2. Characterization The samples used for the following tests were cut from the center of the prepared C/C composites discs. The densities and porosities of the samples were measured using Archimedes principle. The ground samples (∼0.3 mm diameter grains) of the as-prepared and the heat-treated composites were examined via X-ray diffraction (XRD, D/MAX-RA X-ray diffractometer) between 20 and 70◦ (2θ) to determine crystal structure, d-spacing, and crystallite size. According to Bragg’s law, the d-spacing of the carbon (0 0 2) plane was determined using the following equation: d=

λ 2 sin θ

(1)

where, λ is the wavelength of Cu K␣ radiation (0.1541 nm) and θ is the diffraction angle in radians. Crystallite size Lc was obtained from the Scherrer equation: Lc =

0.9λ B cos θ

(2)

where B is the half maximum intensity in radians of the (0 0 2) peak. Polished surfaces of the C/C composites were studied by a polarized light microscope (PLM, Reichert, MeF3) while fracture surfaces were studied by a scanning electron microscope (SEM, Hitachi, S-2700) operated at 25 kV and 20 mA.

3. Results and discussion 3.1. Density and porosity of the C/C composites

2.1.3. Process The densification of the preform was performed at 1050 and 1150 ◦ C, with the deposition times chosen as 2, 4 and 6 h, respectively. The composites prepared for 6 h were heat-treated at 2200 ◦ C for 2 h under argon with a heating rate of 5 ◦ C min−1 whose purpose is to analysis the graphitization of the composites.

The density and porosity of the C/C composites prepared either at 1050 or at 1150 ◦ C as a function of the deposition time are shown in Fig. 2. It shows that with the increasing deposition time, the density of the both kind composites increases, while the porosity decreases. For the sample prepared at 1050 ◦ C for 6 h, the density increased from ∼0.2 to 1.67 g cm−3 . This represents an average increase rate of 0.245 g cm−3 h−1 , which is much higher than the result, 0.044 g cm−3 h−1 , of the inductively hearted thermal-gradient CVI method [2,12,13]. The slightly higher density of samples prepared at 1150 ◦ C indicates that higher deposition temperature is more preferable for the rapid deposition than the lower temperature. It also can be noticed that the density increments decline with the deposition time increasing. For example, the increment of the densities between the composites prepared at 1050 ◦ C

Fig. 1. Sketch of experimental device for preparing C/C composites.

Fig. 2. Density and porosity of the C/C composites prepared at 1050 and 1150 ◦ C vs. deposition time.

J. Wang et al. / Materials Chemistry and Physics 101 (2007) 7–11

for 2 h and for 4 h is 0.57 g cm−3 , which is much larger than the increment (0.18 g cm−3 ) between that for 4 and 6 h. The decline is due to the changes of carbon preform structure with progressive deposition. As the porosity decrease, the transport of hydrocarbons from the cool side of the preform to the hot deposition zone becomes difficult [3,14], thus making the reaction rate decreased.

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Table 1 Value of d002 -spacing and crystallite size Lc of the as-prepared and the heattreated C/C composites Preparing temperature (◦ C) 1050 1150

As-prepared

Heat-treated

d002 (nm)

Lc (nm)

d002 (nm)

Lc (nm)

0.3436 0.3410

4.9 5.5

0.3393 0.3382

16.5 20.8

3.2. X-ray diffraction analysis of the C/C composites The X-ray diffraction patterns of the C/C composites are shown in Fig. 3. It shows that the as-prepared (Fig. 3(a)) and

the heat-treated (Fig. 3(b)) samples are all turbostratic carbon [1,11], but the relative intensity and the shape of peaks (0 0 2), (0 0 4) and (1 0 0) are different. The d-spacing of the (0 0 2) peaks and the crystallite sizes are calculated (Table 1) on the purposes to measure degree of graphitization. The results show that after heat treatment, the value of d002 -spacing decreases and the crystallite size Lc increases for the samples prepared at 1050 and 1150 ◦ C, which indicate their strong abilities to graphitize [1,5]. A lower d-spacing value and a larger crystal size Lc of the composites prepared at 1150 ◦ C compared with that of prepared at 1050 ◦ C, reveal that higher deposition temperature leads to the increased graphitization degree.

3.3. Microstructure of the C/C composites

Fig. 3. XRD patterns of (a) the as-prepared and (b) the heat-treated C/C composites.

PLM images of the C/C composites prepared at 1050 and 1150 ◦ C for 6 h are shown in Fig. 4. It is observed that fibers are surrounded by concentric pyrocarbons matrix. The pyrocarbons matrixes, which have thicknesses about 20 ␮m in both samples, are composed of 4–6 layer-like regions and each layer may have different optical textures. The forming of the different texture is related to the local conditions during the deposition, such as temperatures, gas concentration, as well as ratio of surface area to pore volume and especially the type of the reacting molecule or free radical [8,15–17]. However, the optical properties (Ae values) were not analyzed in the present paper. A further investigation can be done in later work. It can also be seen that small cracks located in pyrocarbon matrix, especially in interface of the layers. This can be attributed to the thermal coefficient mismatch between the fibers and the pyrocarbon, and the damage from specimen preparing. SEM observations revealed no influence of the CVI temperature and post heat-treatment on matrix morphology. Fig. 5 shows typical SEM images of fracture surfaces of the studied composites. The same concentric layers surrounding fibers as in Fig. 4 can be recognized (see Fig. 5(a)). A single fiber and its surrounding pyrocarbon matrix in a cross-section view shown in Fig. 5(b) reveals that fiber pullout occurs and the adhesion between them is not strong. The surface of pyrocarbon matrix is of convex sphere shape, which is considered as the result of the growth patterns of pyrocarbon matrix [3,18]. The laminar-type morphology of the pyrocarbon matrix is resolved in more detail at a higher magnification in Fig. 5(c). It can be seen that the pyrocarbon matrix consists of several thick carbon layers and each layer consists of numerous sublayers.

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J. Wang et al. / Materials Chemistry and Physics 101 (2007) 7–11

Fig. 4. PLM images of the polished surface of the C/C composites prepared (a) at 1050 ◦ C and (b) 1150 ◦ C for 6 h.

3.4. Process analysis The process from the liquid precursor to the solid pyrocarbon matrix is generally quite complex. It includes vaporization of the liquid precursor, homogeneous gas-phase reaction and heterogeneous surface reaction. Actually, these processes occur simultaneously [2,3,5,10]. To get a clear process analysis of our method, we divide the reactor into four parts with each part corresponding to a different temperature and characterizing a specific function (as shown in Fig. 6). Part 1, the boiling liquid precursor (including the low susceptor): Its temperature maintains the boiling point, which is about 160 ◦ C measured in our experiment. This part is a mass reservoir for the vaporization of the liquid precursor. The vaporization rate is dependent on the enthalpy of vaporization of the liquid precursor at the boiling point and the heat power of the low susceptor. Part 2, the vaporized hydrocarbon: Its temperature is a little above the boiling point. In this part, the vaporized hydrocarbon is transported from the liquid surface to the reaction zone. The distance for the transport is very short, which is below 50 mm under our experimental condition. The precursor vapor density is related with the flow rate of the N2 and the temperature in this

Fig. 5. SEM images of the fracture surface of the C/C composites prepared at 1150 ◦ C for 6 h with (a) low, (b) middle and (c) high magnification.

part. Therefore it is possible to control the gaseous hydrocarbon concentration in the reactor by adjusting the heat power and the flow rate of the N2 entry and exhaust gas. Part 3, reaction zone (including the upper susceptor and the preform): The upper surface of the preform is at a higher temperature (>1000 ◦ C) than that of the lower surface, which can be considered as the temperature of the vaporized hydrocarbon (∼170 ◦ C measured in our experimental conditions). Therefore, an average temperature gradient, which is greater than 80 ◦ C mm−1 is formed in the thickness direction of the preform.

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sities of the C/C composites prepared at 1050 and 1150 ◦ C for 6 h reach 1.67 and 1.71 g cm−3 , respectively. The composites show a strong ability to graphitize and the higher deposition temperature (1150 ◦ C) leads to a higher graphitization degree. The fibers of the composite are surrounded by ring-shaped pyrocarbons matrix with a thickness of ∼20 ␮m. The reasons for the rapid fabrication of C/C composites are proposed as the short transport path for vaporized hydrocarbon and the thermal gradient across the preform. Acknowledgement This study was supported by the National Natural Science Foundation of China (no. 50272051). References

Fig. 6. Different part in the reactor, which is divided according to different temperature, each part characterizes a specific function.

The homogeneous gas-phase reaction and heterogeneous surface reaction occur in this part. Part 4, condensation zone; where the temperature is below the boiling point and some vapor condensed and returned to the reactor. Compared with common-used CVI method, the reasons for the rapid fabrication of C/C composites of the present method are proposed as follows: (1) the vaporized precursor can transport quickly from the liquid surface to the preform because of a short convection and diffusion path; (2) the thermal gradient exists across the preform; therefore the deposition can be achieved in one single cycle. 4. Conclusions C/C composites were fabricated rapidly by a thermal gradient, atmospheric pressure chemical vapor infiltration method with simultaneous vaporized kerosene as a precursor. The den-

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