Silicon oxycarbide coatings on graphite fibers II. Adhesion, processing, and interfacial properties

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 (200 °C increments to 1000 °C). The amorphous polymeric coatings were shown to convert to amorphous silicon oxycarbide coatings on heating to temperatures above 600 °C. Single carbon fibers coated with silicon oxycarbide coatings were pyrolyzed and then encapsulated in a transparent epoxy matrix so that stress could be applied and the adhesion and fracture behavior of the coatings could be observed. The properties and adhesion of the coatings to the carbon fibers change with the degree of pyrolysis. Initially the adhesion is low and interfacial failure is observed. After thermal processing to 800 °C and above a significant 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 coated with material having properties varying in a manner similar to the coating was used to gain insight into observed changes.