Property enhancements via matrix microstructure modification of carbon–carbon composites prepared by CVI processing

Abstract

Carbon-carbon (CC) composites have received considerable interest for aerospace applications due to their superior strength retention at high temperatures and high heats of ablation etc. [1]. In the manufacturing of CC composites, carbon fiber preforms can be categorized dependent upon their final applications, e.g. UD, 2-D, 3-D and n-D etc. [2, 3]. Among them, even though they are high labor-dependent, carbon fiber rodnetwork n-D preforms have been used for thick (larger than 100 mm thickness) and high performance CC composites, especially for aerospace applications. In general, rod-network n-D CC composites are employing three routes for densification (1) chemical vapor infiltration (CVI) of hydrocarbon gases (2) liquid process with impregnation and pyrolysis of high carbon yield polymers or pitches (liquid process) (3) combination of CVI and liquid process [4]. In CVI process, under suitable conditions, pyrolytic carbon (PC) matrix has a preferred crystalline orientation which results in considerable anisotropy. In pyrolytic graphite (PG) which is more anisotropic than PC, for instance, the thermal expansion in the thickness direction (c direction in graphite structure) is approximately 20 times that in the plane of deposition (ab plane) [5]. Similarly, mechanical properties measured parallel to the ab plane are considerably higher than those measured perpendicular to it. Even though the degree of anisotropy is lower than that of PG, PC anisotropy creates considerable thermal stresses during CC processing, which lead to delaminations between PC matrix and the carbon fiber rods and micro-cracks in PC matrix itself. These delamination defects may be eliminated or at least greatly reduced by the modification of PC microstructure resulting in increase of CC composites performance [6, 7]. High-strength PAN-based carbon fiber (ACELAN TZ-307, Korea) with 12 K manufactured by Taekwang Industries Co., (Korea) was used in the present study. The fiber has tensile strength of 3800 MPa, tensile modulus 260 GPa, elongation 1.3%, and density of 1800 kg/m3 [8]. Polyvinyl alcohol (PVA 110, Kuraray, Japan) was used as a binder for carbon fiber rods preparation. Carbon fiber rods with 1.0 mm diameter were prepared using a typical pultrusion process. Three-D orthogonal rod-network preforms (W 150 × D150 × H200 mm) having 59% fiber volume percent