Cailiao Baohu (Nov 2023)
Preparation of C/C-SiC Composites by Chemical Vapor Infiltration and Its Impact Resistance Properties
Abstract
In order to investigate the effect of different counts of carbon fiber protofilaments on the impact resistance of C/C-SiC composites, two types of C/C-SiC composites (3K-C/C-SiC and 12K-C/C-SiC) were produced through the ceramic substrate densification of 2.5D carbon fiber needled preforms with different counts of carbon fiber protofilaments (K) via a two-step chemical vapor infiltration (CVI) method. The density and open porosity of two kinds of C/C-SiC omposites were determined by the volumetric method and mercury porosimeter. The structure and morphology of C/C-SiC composites were characterized through characterization methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). Results showed that the counts of carbon fiber protofilaments were found to have a significant impact on the structure and properties of C/C-SiC composites. Pyrolytic carbon (PyC) and SiC were uniformly deposited on the surfaces of two kinds of carbon fibers for C/C-SiC composites prepared by two-step CVI. The weight gain for the 3K carbon fiber 2.5D needled preform was approximately 30%, while it was about 25% for the 12K carbon fiber 2.5D needled preform. The 3K-C/C-SiC composites had a density of approximately 0.89 g/cm3 and an open porosity of about 52.87%. The 12K-C/C-SiC composites had a density of approximately 0.74 g/cm3 and an open porosity of about 56.53%. These differences were attributed to the lower fiber content, lower density, and greater porosity of the 3K carbon fiber 2.5D needled preform, which facilitated greater deposition of PyC and SiC on the surfaces of fibers. At the same time, the density of SiC was greater than that of C, resulting in a higher density and lower porosity of 3K-C/C-SiC composites. By characterizing the structure and morphology of two types of C/C-SiC composites, it was found that the SiC contents in 3K-C/C-SiC and 12K-C/C-SiC composites were about 27.2% and 11.1%, respectively, and the C contents were about 72.8% and 88.9%, respectively. The thicknesses of PyC layer in a single fiber bundle were about 2.162 and 2.145 μm, respectively. The thicknesses of SiC layers were approximately 1.890 and 1.284 μm, respectively. 3K-C/C-SiC composite material had a higher SiC content and lower C content. This was due to the lower carbon fiber content and higher porosity of the 3K carbon fiber 2.5D needle preform, which could cause more PyC and SiC to deposit on the surfaces of the carbon fibers. Through the impact resistance test, it was determined that the impact toughness of 3K-C/C-SiC and 12K-C/C-SiC composites was approximately 25.624 kJ/m2 and 14.310 kJ/m2, respectively, with absorbed energy values of about 1.52 J and 1.09 J, respectively. Results of pendulum impact tests showed that the 3K-C/C-SiC composites had superior impact resistance compared to the 12K-C/C-SiC composites. This enhanced impact resistance was mainly due to the thicker SiC layer on the surface of 3K-C/C-SiC composites, which required more fracture work, and PyC possessed excellent toughness, which enhanced the fibers’ impact resistance. Furthermore, the toughness of the 3K carbon fiber preform was superior to that of the 12K carbon fiber preform. As a result, fracturing the fiber bundles in the 3K-C/C-SiC required more fracture work.
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