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Ceramics

Some members asked me to post anything I learned about the ceramic materials I work with, so: One sample was the coating of Zircar, Inc. ceramic fiberboard with their rigidizer and firing it. Four soaking coats of rigidizer were applied to a square of 3200 deg. F. compatible fiberboard furnace liner and fired to 2500 deg. F. The predicted rigidizing ceramic surface layer was produced to a measured thickness of just over 3/16" thick within the surface of the fiberboard. The sample was then brought up to 2800-3000 deg F and held for 125 seconds. The back side of the fiberboard went from an original 82 deg. F. to 103 deg F. during this time. The surface attained a white-hot condition and radiated such an aura that we had to use welders goggles to view it.

A piece of steel 1.8" thick and 1" wide by 4" long was laid onto this fired surface and melted with a welders cutting torch without appreciable damage to the surface. (This entailed a temperature of the metal strip and the surrounding surface of approximately 1800 deg. F.) This took 2 1/2 minutes and the back surface reached 118 deg. F. The front surface retained a temperature of over 250 deg. F for over 3 minutes while the back surface began to cool almost immediately.

The Cotronics ceramic adhesive was coated onto a square of 1" thick fiberboard and allowed to cure for 48 hours. Another was adhered to a square of unprepared (raw) steel and allowed to cure for the same period of 48 hours. The square of adhered fiberboard was pulled off the steel plate leaving a 3/16" thick layer of fiberboard and adhesive which could not be scraped off except with a sharpened gasket scraper and much effort.

The surface-coated adhesive was fired to 2,300 deg. F. It formed a hard surface coating measured in several places at an average of 5/32" thick. This layer was hard, rough and relatively brittle but very resistant to breakage. It was heated to 3,200 deg. F. (400 deg. F. over its design operational temperature) and formed scattered surface cracks but remained adhered to the fiberboard substrate. Thermal transfer through the fiberboardwas comparable to the previous model. A piece of 1/8" steel bar stock was sandblasted to a pure white profile on Wednesday and coated with an 80 mil coating of the Contronics adhesive and cured until today. A welder's cutting torch with a neutral flame was used (in the hands of a 35-year experienced welder) to heat the steel bar. The steel around and up to the edges of the coating were carefully melted away by playing the flame across the width of the bar, i.e., across bare metal, cured adhesive and then bare metal on the other side in order to attempt to expose the metal and the coating to the same conditions. The cured ceramic adhesive was slightly browned to a light tan color but did not crack of detach from the metal. It could not be scraped from the metal with a knife before or after heating/curing/firing.

The ceramic material is cast using some stuff a friend uses in the ceramics for their plasma arc furnace. You cast it and then fire it...no machining. The main problem is thermal shock causing cracks. If you can get it to survive the first half-second it will last for about as long as you have fuel remaining. I re-use them until they crack.

I found out the other day how NASA makes the heat tiles for the shuttle and want to try that. A good alternative for a short-duration burn is graphite which is messy but can be machined on a lathe. If you try this use a vacuum cleaner to suck up the carbon as it is cut unless you want to try a good dose of Black Lung Disease. A woodworking shop vacuum system with a heppa filter is far better if the cost is justified.