High temperature crack sealant based on SiO2-B2O3 for SiC coating on carbon–carbon composites

Boria Effects on the High Temperature Oxidation of Silicon Carbide

“…Boria volatilization reactions, given earlier by Reactions 6 -8 16 , are also known to occur in borosilicate glasses 57,60 . The occurrence of these reactions in a thermally grown borosilicate glass can alter its surface composition at the gas-oxide interface.…”

“…Several studies found that formation of a low viscosity thermally grown borosilicate glass protected carbon (C) fibers or C-coated SiC fibers from rapid oxidation [56][57][58][59][60] . The fibers were infiltrated with a SiC matrix, which was prone to crack, thus exposing the fibers to O2 by ingress through the cracks [56][57][58][59][60] . Boron-based interlayers were incorporated in the SiC matrix [56][57][58][59][60] .…”

“…The fibers were infiltrated with a SiC matrix, which was prone to crack, thus exposing the fibers to O2 by ingress through the cracks [56][57][58][59][60] . Boron-based interlayers were incorporated in the SiC matrix [56][57][58][59][60] . Similar to the oxidation of BN to boria, shown by Reactions 3 -4, the oxidation of the boron-based interlayers produced boria.…”

“…determined that boria coatings were better in sealing SiC cracks and less prone to dewetting than borosilicate glass coatings, both of which were synthesized by a sol-gel method 60 . Oxidation of boria-coated SiC plates in dry air from 1200 -1600˚C resulted in the unexpected development of a much thicker SiO2 scale than had formed an uncoated SiC plate exposed to the same oxidation conditions 60 .…”

“…Oxidation of boria-coated SiC plates in dry air from 1200 -1600˚C resulted in the unexpected development of a much thicker SiO2 scale than had formed an uncoated SiC plate exposed to the same oxidation conditions 60 . The increased SiO2 growth was attributed to a reaction between SiC and B2O3 to form SiO2 and to greater O2 diffusivity through the borosilicate scale 60 . These findings are in accord with Schlichting who showed orders of magnitude increases in Si oxidation rates to occur in dry O2…”