단일 공정으로 제조한 유-무기 하이브리드형 탄소/탄소 복합재료의 특성

조민석, 박수진, 이재락

한국화학연구소, 화학소재연구단

Characteristics of Carbon-Carbon Composites throughout One Step Manufacturing Process in Organic-Inorganic Hybrid System

Min-Seok CHO, Soo-Jin PARK, Jae-Rock LEE

Advanced Materials Division, Korea Research Institute of Chemical Technology

Introduction

Carbon fiber-reinforced carbon matrix composites (C/C composites) are new emerging materials with potential applications as high temperature structural materials for the space and aeronautic industries. They are now quite highly developed materials and most successful high temperature composites available for use in inert atmospheres or for applications of short duration in oxidizing environments. This is because they possess not only light weight but also high thermal conductivity and dimensional stability up to temperatures of 3000℃ in a protective atmosphere [1,2].

C/C composites can be prepared by using carbon fibers as reinforcement and thermosetting or thermoplastic resins as matrix precursors. On pyrolysis, the thermosetting resins yield a non-graphitizable matrix, whereas the use of thermoplastic resins leads to a graphitizable carbon. Matrix microstructure plays an important role in deciding the overall performance and particularly the mechanical properties of C/C composites. In view of this, numerous studies have been carried out to control the microstructure and the degree of graphitization of the matrix to obtain the desired properties [3]. In addition to matrix type, the heat treatment temperature (HTT) also influences the degree of graphitization. To improve the degree of graphitization of the matrix different methods, in addition to the final HTT, are employed graphitization under catalytic graphitization. Catalytic graphitization is brought about by addition of certain inorganic and organic additives to the carbonaceous matrix. The desired effect is to lower the temperature needed to achieve a certain degree of graphitization [4]. Improvement in the mechanical properties of the C/C composites has also been reported earlier by using thermosetting resins like phenolic resin with graphite powder. However, no such study is available on the C/C composites based on thermosetting resin matrices mixed by oxidation inhibitors throughout one step manufacturing process.

In the inorganic powders, molybdenum disilicide (MoSi2) may be more suitable for this application, as these materials have high melting point (2030℃) and excellent resistance to high-temperature oxidation. Especially these materials exhibits a brittle-to-ductile transition (BDT) in the vicinity of 900℃, and above this temperature shows significant metallic ductility [5].

The present study was therefore undertaken to understand graphitizability of the thermosetting matrix, i.e., phenolic resin mixed with MoSi2 as inorganic powders during the development of C/C composites throughout one step manufacturing process and its influence on the mechanical properties of the resulting composites.

Experimental

Polyacrylonitrile (PAN) based high-strength carbon fibers (Taekwang Ind. Co., TZ-307, 12K) with a 3.5GPa tensile strength and a 245GPa tensile modulus were used in the filament fibers of which were no sizing and no surface treatments. The resole type of phenolic resin (Kangnam Chemical Co.) was used as the matrix precursors of C/C composites to be studied. The MoSi2 powders (<2□, supplied from Aldrich Co.) were used as an oxidation inhibitor after pulverization by freezer/mill (Spexcertiprep Co., spex6700) in reduce their particle size (about 0.3□). This powders were systematically prepared by 0, 4, 12 and 20wt.% of MoSi2 on the basis of resin matrix.

Composite laminates were obtained by the prepreg method. Individual prepregs were first made by uniformly mixing fibers and the phenolic resin of a controlled composition by a drum-winding machine. The green body composites were prepared in a hot-press at 10.5MPa. and 220℃ for 120min as vacuum bagging method. After the curing of the resin, the laminates were subjected to the carbonization process at 1100℃ at a heating rate of 10℃/hr and the graphitization process at 2300℃ at a heating rate of 400℃/hr. C/C composites made out of these matrices were labelled as A, B, C and D for 0, 4, 12 and 20% MoSi2 by weight, respectively.

The bulk density and open porosity of the composites throughout one step manufacturing process characterized by the water displacement technique were investigated as contents of MoSi2. A wide angle X-ray diffractogram of the composites was recorded on a Rigaku Model D/MAX-III B using Cu K□ radiation as the source for measuring the crystalline parameter, d002. Flexural strength of the composites was measured on an Instron model 1125 universal testing machine using the three-point bending technique. Interlaminar shear strength (ILSS) of the composites was characterized by the shot-beam test for the degree of adhesion at interfaces. And, the overall microstructural feature of the composites was examined using scanning electron microscopy to detailed interfacial morphology between carbon fibers and carbon matrix and/or inorganic powders.

Results and Discussion

Table 1 exhibits the results of bulk density (Db) and open porosity (Po) measured as contents of MoSi2 for C/C composites throughout one step manufacturing process (HTT 2300℃). As expected, the bulk densities increase according to the decrease of the open porosities. Also it is seen that at the composites made with MoSi2, the bulk densities are higher than those of the composites made without MoSi2 at 2300℃. These results indicate that the Po decreases, at first, as the MoSi2 content increases and attains a maximum at 12wt.%, and thereafter, simply increases with the content. This is due to the presence of MoSi2 in open and closed pores of the composites, whereas an excess of MoSi2 makes another interphase between fiber and matrix studied by SEM.

Table 2 shows the effect of MoSi2 addition to the matrix on the interlayer spacing of C/C composites (HTT 2300℃). As seen from the table, the interlayer spacing of composite A was 3.432Å which was reduced to 3.372Å for composite D, showing an improvement in the degree of graphitization with addition of MoSi2. Whereas, the crystallite height (Lc) increases with increasing the contents of MoSi2. It is clear that inorganic powders influence the graphitization of the matrix and fibers, resulting in increasings the crystallite size and the concentration of crystallite boundaries. In this work, similar result between the Lc and the d-spasing of the graphite structure was shown by Mizushima [6].

Table 1. Physical properties as contents of MoSi2

for C/C composites (up to HTT 2300℃).

Content	Db (g/cm3)	Po (%)
0wt.% (A)	1.42	14.45
4wt.% (B)	1.47	10.98
12wt.% (C)	1.66	9.77
20wt.% (D)	1.67	10.81

Table 2. The effect of MoSi2 addition to the matrix on the interlayer spacing (d002)

of C/C composites (up to HTT 2300℃).

Content	d002 (Å)	Lc*
0wt.% (A)	3.432	0.088
4wt.% (B)	3.424	0.092
12wt.% (C)	3.414	0.109
20wt.% (D)	3.372	0.205

*Lc	: the crystallite height (nm)

Fig. 1 shows the effect of MoSi2 contents to the matrix on the X-ray diffraction pattern of C/C composites (HTT 2300℃). It shows a well-defined (002) peak and well-resolved (100) and (101) peaks at the composites made with MoSi2. These results would indicate that the MoSi2 powders produces a progressive graphitization of the carbon fiber/thermoset resin derived matrix studied by SEM.

Fig. 1 X-ray diffraction pattern of the C/C composites

as contents of MoSi2 on HTT (2300℃).

Fig. 2 shows that the flexural strength (□f) of the C/C composites increased with increasing contents of MoSi2 at 2300℃. Up to the graphitization stage (2300℃) the □f improved from 200MPa for composite A to 360MPa for composite D. It is suggested that up to 2300℃ the MoSi2 promoted crack branching at the fiber matrix interface and avoided a catastrophic brittle failure of the composites because of BDT properties.

While it is generally accepted that good mechanical properties and long durability of the composites largely depend on fiber-matrix and/or fiber-matrix-filler interfacial adhesion in the case of the fiber reinforced matrix composites, since the transferring load stress from the matrix to the fiber would require the strong adhesion at the interface of the constitutive elements rather than a high cohesive density. The degree of adhesion at interfaces between fiber and matrix may be measured by the short beam test for ILSS of the mechanical behaviors.

Fig. 2 Flexural strength of the C/C composites as content

of MoSi2 on HTT (2300℃).

Fig. 3 ILSS of the C/C composites as content

of MoSi2 on HTT (2300℃).

From the ILSS results as shown in Fig. 3, the presence and the increasing of the amount of MoSi2 leads to the increase of ILSS of C/C composites. As expected, the increasing of bulk density (or decreasing of open porosity) is closely related to the growing of ILSS of the composites, as well □f. From which, it can be concluded that the effects of the amount of MoSi2 on the composite properties are twofold: it increases the adherent surface areas between fibers-matrix-in-

organic powders (or it decreases the porosities between fibers and matrix) and it allows the BDT behavior which is an intrinsic properties of MoSi2.

References

1. S. J. Park and J. R. Lee, J. Mater. Sci., 33, 647 (1998).

2. C. R. Thomas, "Essentials of Carbon-Carbon Composites", The Royal Society of

Chemistry, Cambridge (1993).

3. R. J. Zaldivar, R. W. Kobayashiand and G. S. Rellick, Carbon, 29, 1145 (1991).

4. S. R. Dhakate, R. B. Mathur and O. P. Bahl, Carbon, 30, 1753 (1997).

5. S. Jandhyala, J. Am. Ceram. Soc., 76, 226 (1993).

6. S. Mizushima, Fifth Carbon Conference, Vol. 2, (1961) p 439.