Metal finishing Q&As since 1989
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Looking for high temp. coating/ treatment for graphite crucibles
2004
I'm looking for a high temperature coating, or treatment, to apply onto graphite crucibles to prevent adhesion of SiO, or ZnS materials during 'evaporation' process.
The environment is high vacuum 4x10-4 Torr, temperature range is ambient up to 1800-1900 °C, time in process is 30-60 minutes. I have tried both fine powder and pellet forms of the SiO or ZnS materials. The graphite crucibles have a max. grain size of 0.0004" and withstand temps. up to 5000 deg F in vacuum.
During the process a hard crust of residual material forms on the interior of the custom ($$) made crucibles. The powdered materials are usually more consistent and easier to remove, the pellets last longer in process but leave a more 'lumpy' impenetrable crust. Brushing and spraying on BN (boron nitrate) solution has mixed results, sometimes the crust scrapes off fairly easily using a putty knife and other times it is nearly impossible to remove using a hammer and chisel!
Any suggestions offered will be appreciated.
Ken MartinIndustrial Mfr. - Bloomfield, Connecticut, USA
I may be way off on the simplistic side here, but could you flux the crucible with borax [affil links]? That's what we do with our graphite crucibles for brass casting to keep the melt from sticking and for fluxing.
Guillaume Sudeterre- Missouri
2004
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