Post coating treatment of silicon carbide coated carbon-carbon substrates

A method for post-treating of a carbonaceous substrate having a primary protective coating of silicon carbide to seal the silicon carbide coating and substrate comprising:

• (a) forming a first slurry of from about 22.3% to about 27.2% monoaluminum phosphate, from about 18.23% to about 22.27% water, from about 12.6% to about 15.4% particulate silicon carbide, from about 12.67% to about 15.4% silicon carbide felt, from about 15.2% to about 18.6% alumina, and from about 9.0% to about 11.0% of a material from the group consisting of boron and zirconia hydride;
• (b) coating the primary coating of the carbonaceous substrate with a thin film of said first slurry,
• (c) curing the substrate by progressive heating to about 1500.degree. F. in an inert atmosphere at a rate which prevents bubbling or blistering of said film of said first slurry, and maintaining the substrate at about 1500.degree. F. for a sufficient period of time to complete the curing process;
• (d) forming a second slurry of from about 11.6% to about 14.2% water, from about 31.1% to about 38.1% of a 38% aqueous solution sodium silicate, from about 2.2% to about 2.8% sodium borate, from about 22.5% to about 27.5% particulate silicon carbide, and from about 22.5% to about 27.5% silicon carbide felt;
• (e) coating the substrate produced in step (c) with a thin film of said second slurry;
• and
• (f) curing said thin film of said second slurry by progressive heating of the substrate to about 600.degree. F. at a rate sufficient to prevent blistering or bubbling of said thin film of said second slurry.