Oxidation studies of SiC-coated 2.5D carbon fibre preforms

Applying SiC coatings to carbon fibre (C-f) preforms can be very helpful for controlling the fibre/matrix interface characteristics and hence for the successful development and application of continuous Cf-based ceramic matrix composites. The present work focuses on two aspects of this, viz. i) the chemical vapour infiltration (CVI) coating of SiC onto 2.5D C-f preforms and ii) the subsequent oxidation studies of the resulting preforms in the temperature range 1300-1700 degrees C. With respect to the latter, thermogravimetric analysis (TGA) was used to study the continuous oxidation behaviour up to 1400 degrees C, with the results being compared to those for uncoated samples, whilst isothermal oxidation studies were also carried out in the temperature range from 1300 to 1700 degrees C for 4 h to understand the transition from passive to active oxidation of the SiC. Phase identification and detailed microstructural characterisation of the coated preforms and oxidised samples were carried out using XRD, SEM-EDS and TEM-SAED-EDS in order to understand the oxidation mechanism. Improved oxidation resistance was observed compared to other coating methods from the literature; this is believed to be due to the 20 nm particle size of the SiC deposited and the thicker coating at the surface compared to the interior tows of the carbon preforms. A transition from passive to active oxidation of the SiC was seen in the temperature range of 1400-1600 degrees C, whilst the glassy layer formed cracked, presumably on cooling due to thermal expansion dif-ferences, at temperatures >1500 degrees C.