The microstructure and mechanical behaviors of carbon/carbon (C/C) composites prepared by thermal-gradient chemical vapor deposition (CVD) technique are studied. The fibrous substrates used in this study are carbon mats and carbon cloths.
The growth morphologies of pyrolytic carbon matrices within carbon mat substrates when preparing C/C composites by thermal-gradient CVD technique are investigated, and then a deposition mechanism of pyrolytic carbons on carbon fibers is proposed to explain the phenomena observed. The growth morphologies are a clear indication that pyrolytic carbon matrices mostly arise from the deposition of planar carbon molecules, and that the deposition mechanism of so-called laminar carbon matrix is similar to that of a massive carbon deposit with columnar structure. The variation in the preferred orientation of carbon matrix, revealed by optical reflectivity under polarized light, is believed to occur due to changes in the arrangement of planar carbon molecules.
The variations in microstructure and structural parameters of pyrolytic carbon matrices are systematically investigated as a function of deposition condition. It is shown that the matrices, which were deposited at temperatures of 1100-1400℃ and from 10-70% propane concentrations, have wide ranges of density ($1.55-2.10 g/cm^3$) and crystallite size (23-35 Å ) as well as various microstructures with a strong dependence of the deposition condition. The matrices have either a columnar, smooth laminar, isotropic, or transition structure. The isotropic structure formed in the high temperature and low propane concentration region has lower density and smaller crystallite size, compared with the smooth laminar structure or the columnar structure. The transition structures are formed between the isotropic region and the smooth laminar region, and have an intermediate character in structure. The density and the crystallite size have a proportional relationship with each other. The structural changes are discussed in view of the arrangement of planar carbon molecules.
The mechanical properties of C/C composites are investigated by three-point flexural tests. The mechanical properties are strongly influenced by the matrix structure which in turn depends on the deposition condition employed. The fracture strengths and elastic moduli of the composites are in the range of $13-22×10^3$ psi and $2.7-4.9×10^6$ psi, respectively. The lower fracture strength and elastic modulus of isotropic structure and columnar structure are due to the low matrix density and to the presence of matrix cracks, respectively. The fracture strain remains almost constant (~0.5%) over the whole ranges investigated. The role of the matrix crack during the fracture is discussed.
The fracture behaviors of two-dimensional carbon/carbon composites during flexural tests are investigated, and failure modes are also examined in view of the fiber/matrix interface. The type of the interface is dependent on the deposition condition. Only physical interlockings seem to be present at the interface established between carbon fiber and pyrolytic carbon. A tightly bound interface causes a catastrophic failure. Otherwise, a stepwise failure occurs resulting from a delamination of cloth layers, a pullout of fibers, and a crack propagation through the gap between fiber bundles.
화학증착법으로 제조한 탄소/탄소 복합재료의 미세구조와 기계적 거동을 연구하였다. 연구에 사용한 섬유substrate는 탄소mat와 탄소cloth였으며, 열분해탄소matrix는 thermal-gradient CVD technique으로 증착시켰다.
먼저 탄소mat substrate를 사용한 복합재료에서 열분해탄소의 성장 형태를 관찰하고 이로부터 열분해탄소의 증착기구를 제시하였다. 열분해탄소의 증착은 기상에서 형성된 탄소평면분자의 증착으로부터 일어나며, 증착층 내의 crystallite의 우선방향성은 탄소평면분자의 배열상태에 의해 결정된다.
증착온도가 1100-1400℃, 프로판농도가 10-70% 인 증착조건 범위내에서 열분해탄소 matrix는 편광하에서의 미세구조뿐 아니라 밀도 ($1.55-2.10 g/cm^3$)와 crystallite size (23-35Å)도 폭넓게 변화하며, 이들 변화는 증착조건에 크게 의존한다. 미세구조는 columnar, smooth laminar, isotropic, 또는 transition 구조를 나타낸다. isotropic 구조가 밀도와 crystallite size가 가장 작으며, columnar 구조가 두값 모두 가장 크다. 증착조건의 변화에 기인한 구조의 변화를 탄소평면분자의 배열의 관점에서 고찰하였다.
본 연구에서 3점 굽힘시험으로 측정된 탄소/탄소 복합재료의 기계적 성질은 파괴강도가 $13 - 22×10^3psi$, 탄성계수가 $2.7 - 4.9×10^6psi$ 였으며, 탄소 matrix의 구조에 큰 영향을 받는 것으로 나타난다. isotropic 구조와 columnar 구조의 경우 두 값 모두 낮은데, 이는 각각 낮은 matrix밀도와 matrix균열의 존재에 기인하는 것으로 생각된다. 파괴변형은 모든 증착조건에 걸쳐 약 0.5 % 로 일정하다. 파괴시 matrix균열의 역할을 고찰하였다.
탄소cloth를 substrate로 사용한 탄소/탄소 복합재료의 기계적 거동은 섬유와 matrix간의 계면유형에 따라 달라지며 이는 또 증착조건에 의존한다. 탄소섬유와 열분해탄소matrix 사이에는 physical interlocking만이 존재하는 것으로 보인다. 계면이 강하게 결합된 경우 catastrophic failure를 보이고, 그렇지 않은 경우에는 cloth층간의 분리, 섬유의 뽑힘, 또는 섬유다발 사이로의 균열전파 등에 기인한 stepwise failure가 일어난다.
