SILICON OXYCARBIDE COATINGS ON GRAPHITE FIBERS .2. ADHESION, PROCESSING, AND INTERFACIAL PROPERTIES

Silicon oxycarbide coatings are applied to graphite fibers by dip coat ing tows of fibers into hexane solutions of the silsesquioxane terpoly mer MeHSiO](0.1)[MeSi(OnPr)O](0.2)[MeSi(O)(1.5)](0.7)- to form silsesq uioxane coatings typically 0.1 mu m thick. The coatings are then pretr eated by heating in N-2 to selected temperatures (200 degrees C increm ents to 1000 degrees C). The amorphous polymeric coatings were shown t o convert to amorphous silicon oxycarbide coatings on heating to tempe ratures above 600 degrees C. Single carbon fibers coated with silicon oxycarbide coatings were pyrolyzed and then encapsulated in a transpar ent epoxy matrix so that stress could be applied and the adhesion and fracture behavior of the coatings could be observed. The properties an d adhesion of the coatings to the carbon fibers change with the degree of pyrolysis. Initially the adhesion is low and interfacial failure i s observed. After thermal processing to 800 degrees C and above a sign ificant increase in adhesion is detected but the failure mode remains interfacial and circumferential cracks in the coating are detected. A non-linear axisymmetric finite element model of this single fiber coat ed with material having properties varying in a manner similar to the coating was used to gain insight into observed changes.