Oxidation Behaviors and Mechanisms of C/Si–C–N with a Mullite Interlayer

Abstract

Carbon fiber reinforced Si-C-N matrix composite with a mullite interlayer (C/Mullite/Si-C-N) was fabricated via CVI and PIP process. The oxidation behaviors of C/Mullite/Si-C-N were investigated in air by comparison with that of C/PyC/Si-C-N using thermogravimetry and SEM technique. The results indicate that the weight loss of the C/Mullite/Si-C-N increases with the increase of the temperature below 800 • C, decreases in the range of 800-1000 • C, and increases again above 1000 • C. The increase of the weight loss below 800 • C results from the increase of the oxidation temperature. The closure of the matrix microcracks at 800-1000 • C leads to the decrease of the weight loss, and the reopening of microcracks results in the increase above 1000 • C. The oxidation resistance of C/Mullite/Si-C-N was improved greatly as compared to C/PyC/Si-C-N within the temperature range from 600 • C to 1200 • C. The mullite interphase changes the oxidation mode of C/Si-C-N and leads to the non-uniform oxidation of carbon fibers in the composite. At 600 • C, the oxidation curve of C/Mullite/Si-C-N shows obvious non-linearity, which mainly results from the non-uniform oxidation of carbon fibers.

Influence of the Si–O–C interlayer on the oxidation behaviors and mechanisms of composites

“…The increase in this temperature possibly results from the volume shrinkage of Si-C-N matrix which has been described in Lu et al [7]. When the temperature is above 1000°C, the shrinkage resulting from the densification processes speeds up and the rate of shrinkage becomes higher than the rate of thermal expansion.…”

“…It is known that low crack density is the main reason for C/Mullite/Si-C-N having excellent oxidation resistance in Lu et al [7]. Then, is C/Si-O-C/Si-C-N also the case?…”

“…It can be seen that the weight loss of the C/Si-O-C/Si-C-N increases with the increase in the temperature below 900°C, decreases in the range of 900-1000°C, and increases again above 1000°C, which is different from that of uncoated C/C, [11] but similar to that of C/Mullite/Si-C-N [7] and uncoated C/SiC. [12] …”

“…[5] Our previous results show that the mullite interphase could also enhance the oxidation resistance of the composite, strong interface bonding, lower microcrack density, higher coefficient of thermal expansion (CTE) of interphase, and thicker interlayer contributing to the improvement of oxidation resistance. [6,7] Si-O-C is another interphase material that could improve the oxidation resistance of the composite. But from Lu and Jiao [8], we can see that the Si-O-C interphase does not exhibit the high CTE, and the interface bonding of the composite is also weak.…”