Sol–gel derived borosilicate glasses and thin film coatings on SiC substrates: Boron loss and carbon retention due to processing and heat treatment

• 1 μm thick sol–gel derived borosilicate coatings on SiC substrates were optimized.
• Boron loss occurred in sol–gel synthesis and heat treatment.
• Residual carbon was evident in the glass structure.
• Ar–H2 heat treatment atmosphere enhanced borosilicate bond formation.

Synthesis of freestanding borosilicate glass and borosilicate thin films (< 1 μm thick) applied on silicon carbide substrates was carried out by a sol–gel method for the purpose of determining boria effects on silicon carbide oxidation. Extensive characterization of boron loss, compositional reproducibility, and film quality was carried out through every stage in the gel to glass conversion. Boron loss in the gel synthesis and during heat treatment ranged from 30 to 68 mol% and increased with solvent loading. Hydrolyzed products were evident in glasses heat treated at lower temperatures, but were not present as crystalline phases in the amorphous glass network. Reproducibility of boron concentration in the glass was relatively unaffected by heat treatment conditions, but formation of the borosilicate bond was enhanced in certain gas environments. Carbon retention was characterized for total concentration. Carbon bonding both within the glass network and as a free carbon phase was evident by spectroscopic analysis.