Oxidation Behavior of Zirconium Diboride-Silicon Carbide Composites at High Temperatures.

Abstract

The ZrB2-SiC composite is a prominent member of Ultra-High Temperature Ceramics (UHTCs). Here the oxidation behavior of ZrB2-SiC composites at temperatures between 1500-1900ºC is studied. The structure and composition of complex oxide scales, formed at these temperatures, are characterized using microstructural and elemental analysis. A novel method, called the Ribbon Method, was developed for testing UHTCs at high temperatures, rapidly at low cost. Self-supported UHTC ribbon specimens are resistively heated with a table-top apparatus to achieve temperatures from 900-2000ºC. The Ribbon Method is a novel method for rapid oxidation characterization of UHTC at xxi high temperatures and a valuable alternative to the current high temperature facilities for UHTCs. Oxidation studies with the Ribbon Method showed that a SiO2 rich borosilicate surface layer forms during the oxidation of the ZrB2-SiC composite and acts as a protective barrier at lower temperatures by hindering oxygen diffusion through the surface layer. The SiO2-rich surface layer starts to volatilize extensively at temperatures above 1700ºC resulting in a decreases in the oxidation resistance of the composite. A novel mechanism is proposed for the high temperature oxidation of ZrB2-SiC based composites. This mechanism is based on liquid transport of oxide liquid solution formed during oxidation at temperatures around 1550ºC. Patterns in borosilicate surface layer of oxidized ZrB2-SiC composites were discovered, showing evidence of liquid flow in the oxide film. These patterns, called here convection cells, are formed when a fluid B2O3-rich borosilicate liquid containing dissolved ZrO2 is transported to the surface where the B2O3 is lost by evaporation, depositing ZrO2 in a viscous SiO2-rich liquid. The driving force for the liquid transport is proposed to be the large volume increase upon oxidation. Liquid transport of the oxide liquid solution is claimed to play a significant role in the formation of oxide scales of ZrB2-SiC composites and other boride-SiC based UTHC composites. Mass transport by liquid flow has not yet been reported for the high temperature oxidation of ZrB2-SiC composites thus a novel mechanism for the high temperature oxidation of these materials is presented.