Effects of density and heat treatment of C/C preforms on microstructure and mechanical properties of C/C–SiC composites

Abstract

Twill multidirectional carbon-fiber-reinforced carbon and silicon carbide composites (i.e., C/C-SiC) were prepared via chemical vapor infiltration combined with reactive melt infiltration process. The effect of heat treatment (HT) on the microstructure and mechanical properties of C/C-SiC composites obtained by C/C preforms with different densities was thoroughly investigated. The results show that as the bulk density of C/C preforms increases, the thickness of the pyrolytic carbon (PyC) layer increases and open pore size distribution narrows, making the bulk density and residual silicon content of C/C-SiC composites decrease. Moreover, the flexural strength and tensile strength of the C/C-SiC composites were improved, which can be attributed to the increased thickness of the PyC layer. The compressive strength reduces due to the decrease of the ceramic phase content. HT improves the graphitization degree of PyC, which reduces the silicon-carbon reaction rate and thereby the content of the SiC phase. HT induces microcracks and porosity but not obviously affects the mechanical properties of C/C-SiC composites. However, the negative impact of HT can be compensated by the increased density of the C/C preforms.

Effects of preform structure on microstructure and mechanical properties of long/short fiber‐coupled C/C–SiC composites

“…The volume fraction of each component of the C/C–SiC composite suggests that the CS1 sample contains a higher volume fraction of residual silicon and a lower volume fraction of carbon. This is due to the low density of the C1 sample, which is consistent with previous research results 13 . Furthermore, the closed porosity of CS2 and CS3 samples is slightly higher, possibly due to overlapping pores between the continuous carbon‐fiber cloth layers.…”

“…The true density ( ρ true ) of powdered C/C–SiC composites was determined using a gas pycnometer (Ultrapyc 1200e, Quantachrome Instruments). Consequently, the closed porosity ( φ closed ) was determined based on the bulk density, true density, and skeletal density 13 . The calculation formula is as follows: $$$$\begin{equation}{\rho }_{bulk} = \frac{{{m}_1{{\rho}}}}{{{m}_2 - {m}_3}}\end{equation}$$$$ $$$$\begin{equation}{\rho }_{skel} = \frac{{{m}_1{{\rho}}}}{{{m}_1 - {m}_3}}\end{equation}$$$$ $$$$\begin{equation}{\varphi }_{open} = \frac{{{m}_2 - {m}_1}}{{{m}_2 - {m}_3}} \times 100{\mathrm{\% }}\end{equation}$$$$ $$$$\begin{equation}{\varphi }_{closed} = \frac{{1/{\rho }_{skel} - 1/{\rho }_{true}}}{{1/{\rho }_{bulk}}} \times 100{\mathrm{\% }}\end{equation}$$$$where m 1 , m 2 , and m 3 are dry weight, soaked weight, and suspended weight, respectively.…”

“…Consequently, the closed porosity (φ closed ) was determined based on the bulk density, true density, and skeletal density. 13 The calculation formula is as follows: 𝜌 𝑠𝑘𝑒𝑙 = 𝑚 1 𝜌 𝑚 1 − 𝑚 3…”

“…The mechanical properties of C/C–SiC composites are influenced by various factors, including processing temperature, holding time, the density of porous C/C composite, and structure of the carbon‐fiber preform 11–17 . Among these factors, the structure of the carbon‐fiber preform significantly impacts both the mechanical properties and the densification process of C/C–SiC composites 17,18 .…”

“…[8][9][10] The mechanical properties of C/C-SiC composites are influenced by various factors, including processing temperature, holding time, the density of porous C/C composite, and structure of the carbon-fiber preform. [11][12][13][14][15][16][17] Among these factors, the structure of the carbon-fiber preform significantly impacts both the mechanical properties and the densification process of C/C-SiC composites. 17,18 The orientation and volume fraction of carbon fiber directly affects the mechanical properties of the composite, with the content of long fiber bundles in the axial direction enhancing the tensile properties of C/C-SiC composites in that direction.…”