유-무기 하이브리드형 일방향 탄소/탄소 복합재료의 물성

조민석, 박수진, 이재락

한국화학연구소, 화학소재연구단

Characteristics of Unidirectional Carbon-Carbon Composites

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) has been widely used in many fields due to their excellent mechanical properties, chemical stability and thermal stability. So they are developed due to the demands of the aircraft and aerospace industries. Their use as rocket propulsion components, reentry tips, exhaust cones and friction pads leads to the evaluation of their ablation performance[1].

Phenolic resin is a versatile group of materials that meet extended requirements both in properties and processing. They are commonly used as starting materials in the resin-based process, C/C composites preparation by a procedure employing pyrolysis and carbonization of resin matrix and subsequent densification. since they are easy to handle and give considerable carbon char yield[2,3].

Although carbon materials have excellent thermal properties and stability, the disadvantage of oxidation loss in air restricts their high temperature applications to those in oxidation atmosphere.

In their real applications, the study on oxidation reaction inhibition for the prediction of C/C composite working life in oxidizing environments is one of the primary concerns for the prediction of composite behaviors, together with the study of thermal and mechanical properties of C/C composites. On the other hand, silicides like 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 metal-like ductility[4,5]. So as these materials will be lead to the high mechanical performance at high application temperatures.

The objective of the present work is to provide understanding of the effects of the ceramic powders on the oxidation reaction inhibition and mechanical properties at room and high temperatures. The three-point bending test for flexural strength and modulus properties, the interlaminar shear strength (ILSS) by the shot-beam test, thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM), porosity and density changes of the C/C composites with and without ceramic powders are discussed.

Experimental

PAN-based high strength carbon fiber (Taekwang Ind., TZ-307, 12K) with a  3.5GPa. tensile strength and a 245GPa. tensile modulus were used in the form of plain weave from the fibers of which were treated through commercial standard surface oxidation. The resole type of phenolic resin (Kangnam Chemical) was used as the matrix precursors of C/C composites to be studied. The MoSi2 powders (<2□, supplied from Aldrich Co.) were used to pulverize as freezer/mill (Spexcertiprep Co., spex6700). This powders were used as the oxidation inhibitor of C/C composites and prepared by taking 0, 2, 4, 6, 8, 12 and 20wt.% of MoSi2 on the basis of C/C composites.

Composite plates 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 greenbody composites from the eight and thirty plies of resin impregnated carbon fibers were prepared in a hot-press at 10.5MPa. and 220℃ for 120min.  After the curing of the resin, the plates were subjected to the carbonization process at 1100℃ at a heating rate of 2℃/min.

The oxidation inhibition properties of C/C composites were characterized from the weight loss changes of specimens by using TGA of up to 1000℃ in oxidizing environments.

The mechanical behaviors of C/C composites were characterized by the three-point bending test for flexural strength and modulus properties (s/d=16, c.h.s=2.0mm/min). The interlaminar shear strength (ILSS) of the composites between fiber and matrix and/or ceramic powder were characterized by the shot-beam test for the degree of adhesion at the interfaces (s/d=4, c.h.s=2.0mm/min).

Also, the high temperature mechanical behaviors of C/C composites were measured by the three-point bending test for flexural strength and modulus properties (s/d=32:1, c.h.s=0.5mm/min) at high temperature (1000℃). The interlaminar shear strength (ILSS) of the composites at high temperature (1000℃) between fiber and matrix and/or ceramic powder were measured by the shot-beam test for the degree of adhesion at the interfaces (s/d=6, c.h.s=0.5mm/min).

Results and Discussion

Density and Porosity

In the unidirectional C/C composite carbonization process at temperatures up to 1100℃, bulk density of greenbodys and C/C composites as content of MoSi2 shown in Fig. 1. As expected, the bulk density increases in increasing MoSi2. The behaviors of both composites studied change in the content of MoSi2, which may be shown to be a result of the volume change of the degrading matrix. This is explained by the high isotropic shrinkage of the resin matrix which causes voids, pyrolysis cracks and, in the worst case, delamination.

According to the results of the cured phenolic resin investigated by Lum et al. [6], the outgassing of free phenol and the formation of water, CO and CO2 takes place at a low temperature (below 350℃), and low-weight molecular compounds such as aromatic species are released by thermal fragmentation of the polymer structure between 350 and 500℃. It is noted that the behaviours of both composites studied are similar to those of the cured phenolic resin. As a consequence, apparent porosity of greenbody composites are higher than C/C composites. As expected, the bulk densities for the composites made with MoSi2 are higher than those unmade. And the increasing of the amount of MoSi2 in C/C composites shows the decreasing of apparent porosities of the composites. This is due to the presence of MoSi2 in open and closed pores of the composites.

Mechanical Properties

Fig. 2 shows the elastic modulus in flexure of the C/C composites as contents of MoSi2. These results indicate that, as increasing MoSi2, the composites made with MoSi2 have relatively higher mechanical properties than do the composites made without MoSi2.

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 the interface between fiber and matrix may be measured by the short beam test for ILSS of the mechanical behaviors.

From the ILSS results as shown in Fig. 3, the presence and the increasing of the amount of MoSi2 lead to the increase of ILSS of C/C composites. 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-ceramic powders (or it decreases the porosities between fibers and matrix) and it allow the BDT behavior which is an intrinsic properties of MoSi2.

References

1. J. B. Donnet and R. C. Bansal, "Carbon Fibers", 2ed., Marcel Dekker,

New York (1990).

2. A. Knop and L. A. Pilato, "Phenolic Resins", Springer-Verlag, Berlin (1986).

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

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

5. B. R. Zhang and F. Marino, J. Am. Ceram. Soc., 80, 269 (1996).

6. R. Lum, C. W. Wilkins, M. Robbins, A. M. Lyons and R. P. Jones, Carbon, 21,    111 (1983).

Figure 1. Bulk density of Greenbody composites and C/C composites as

content of MoSi2.

Figure 2. Elastic modulus in flexure of the C/C composites as content of MoSi2 at room and high temperature.

Figure 3. Interlaminar shear strength of the C/C composites as content of MoSi2 at room and high temperature.