Oxidation Behavior of Zirconium Diboride-Silicon Carbide Composites at HighTemperatures.

[摘要] The ZrB2-SiC composite is a prominent member of Ultra-High TemperatureCeramics (UHTCs). Here the oxidation behavior of ZrB2-SiC composites at temperaturesbetween 1500-1900ºC is studied. The structure and composition of complex oxide scales,formed at these temperatures, are characterized using microstructural and elementalanalysis.A novel method, called the Ribbon Method, was developed for testing UHTCs athigh temperatures, rapidly at low cost. Self-supported UHTC ribbon specimens areresistively 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 atxxihigh temperatures and a valuable alternative to the current high temperature facilities forUHTCs. Oxidation studies with the Ribbon Method showed that a SiO2 rich borosilicatesurface layer forms during the oxidation of the ZrB2-SiC composite and acts as aprotective barrier at lower temperatures by hindering oxygen diffusion through thesurface layer. The SiO2-rich surface layer starts to volatilize extensively at temperaturesabove 1700ºC resulting in a decreases in the oxidation resistance of the composite.A novel mechanism is proposed for the high temperature oxidation of ZrB2-SiCbased composites. This mechanism is based on liquid transport of oxide liquid solutionformed during oxidation at temperatures around 1550ºC. Patterns in borosilicate surfacelayer of oxidized ZrB2-SiC composites were discovered, showing evidence of liquid flowin the oxide film. These patterns, called here convection cells, are formed when a fluidB2O3-rich borosilicate liquid containing dissolved ZrO2 is transported to the surfacewhere the B2O3 is lost by evaporation, depositing ZrO2 in a viscous SiO2-rich liquid. Thedriving force for the liquid transport is proposed to be the large volume increase uponoxidation. Liquid transport of the oxide liquid solution is claimed to play a significantrole in the formation of oxide scales of ZrB2-SiC composites and other boride-SiC basedUTHC composites. Mass transport by liquid flow has not yet been reported for the hightemperature oxidation of ZrB2-SiC composites thus a novel mechanism for the hightemperature oxidation of these materials is presented.

[发布机构] University of Michigan

[效力级别] Materials Science and Engineering

[关键词] Oxidation Behavior of Zirconium Diboride-Silicon Carbide;Materials Science and Engineering;Engineering;Materials Science and Engineering