Silicon oxycarbide coatings on graphite fibers: chemistry, processing, and oxidation resistance

Silicon oxycarbide coatings are applied to graphite fibers by dip coating tows of fibers into hexane solutions of the silsesquioxane terpolymer -[MeHSiO]0.1[MeSi(OnPr)O]0.2[MeSi(O)1.5]0.7- to form silsesquioxane coatings typically 0.1 μm thick. The coatings are then pretreated by heating in N2 to selected temperatures in 200 °C increments to 1000 °C. The amorphous polymeric coatings were shown by (FTIR) analysis to convert to amorphous silicon oxycarbide coatings on heating to temperatures above 600°C. The oxidation resistance of the pretreated coated fibers is compared with the oxidation resistance of uncoated fibers by heating tows of both types of fibers in static air for 3 h at selected temperatures to 800 °C, where the coated fibers barely survive heating for 1 h. The uncoated fibers do not survive heating in air at 700 °C even for only 1 h. Thus, the coatings increase fiber stabilities towards oxidation by 75–100 °C. All materials were characterized by FTIR anaylsis, X-ray photoelectron spectroscopy, scanning electron microscopy and nuclear magnetic resonance where possible.