Characterization of a duplex precursor-derived and pyrocarbon coating for needled carbon fiber felts

A polycarbosilane (PCS) blended with tetraethyl orthosilicate (TEOS) in xylene solvent was used as a precursor for developing anti-oxidation coatings on a needle-punched carbon fiber felt with pyrolytic carbon deposition. Elemental analysis, Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TG) were employed to characterize curing products of the precursor. The converted ceramic products of the precursor under Ar flow and in vacuum to a maximum temperature of 1000 °C and 1200 °C, respectively, were investigated by FT-IR, X-ray diffraction (XRD), TG and transmission electron microscope (TEM) with energy-dispersive X-ray spectroscopy (EDX). All pyrolysis products were found to contain β-SiC and amorphous SiO2 with more acute β-SiC XRD peaks obtained for the higher pyrolysis temperature. Two representative TEM regions were shown to have different empirical chemical compositions. Needled carbon fiber felts with a pyrolytic carbon inner deposition layer impregnated in the precursors were cured in air and then pyrolyzed at high temperatures to form duplex SiC-based outer coating layers. Comparative study of a series of processing conditions through scanning electron microscope (SEM) observation indicated that, carbon fiber preforms, which were coated by using the 10% PCS and 5 wt% TEOS precursor in xylene and repeating the impregnation–pyrolysis cycle twice, witnessed relatively complete coverage of the coating in the thickness range of 35–120 nm with a small amount of pyrolysis product lumping.