Formation of Oxide Scales on Zirconium Diboride-Silicon Carbide Composites During Oxidation: Relation of Subscale Recession to Liquid Oxide Flow

The formation of oxide scales on zirconium diboride (ZrB2)-silicon carbide (SiC) composites oxidized at high temperatures (> 1500 degrees C) is studied. Subscale recession found in oxidized ZrB2 composites is proposed to form due to flow of boron oxide (boria) (B2O3)-rich borosilicate liquid through convection cells that form upon oxidation of the composite at high temperatures. The flow of the B2O3-rich liquid to the surface, with the subsequent loss of B2O3 to evaporation, explains the formation of a glassy silica-rich layer on the surface commonly reported in the literature. Also the outward flow of the liquid creates a localized inward path for oxygen due to lower viscosity that allows faster oxidation under the convection cells which creates the subscale recession. Optical and electron micrographs of a ZrB2-15%SiC composite oxidized at 1550 degrees C are presented as evidence of flowing liquids. Micrographs of oxide scale deformations are also presented, which are proposed to be related to the formation of oxide scale features called convection cells. The subscale recession and oxide scale deformations of ZrB2-15%SiC composites oxidized for 3 and 4 h at 1550 degrees C were studied with microstructure and chemical composition analysis.