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Glassy build-up on heating elements during nitriding

I have a client that has been fighting a glassy build up on Silicon Carbide heating elements during the nitriding of screw machine parts. He suspects a silicone component in his coolant contributes to the problem. Anyone familiar with this?

Wayne W. Althen
- St. Louis, Missouri
2002

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Re: Silicone fluid residue - it's possible, but there are more likely suspects:

Alkali metal compound residues (sodium- and potassium-containing materials that are converted ultimately to the respective oxide, which are in turn volatile at high temperatures). By causing vitrification of the silicon dioxide layer on the element surface (which protects the SiC as Zn is protected by ZnO), the protective layer is attacked, with subsequent attack on the unprotected SiC.

Furnace atmosphere components - lots of things from ammonia ⇦this on eBay or Amazon [affil links] to water vapor to methane - but again, don't forget that the attack via alkali metal oxides can be considered an uninvited atmosphere component.

An "unbalanced" loading of the heating elements (power, size of furnace, temperatures, product loading) can cause the op temps of the elements to be driven up, making them more susceptible to attack by factors mentioned above. Proper sizing of the furnace minimizes this.

None of the client's experience will be foreign to the manufacturer of the heating elements. A call to the client's furnace component supplier will likely lead to the fastest solution.

The major vendors of SiC heating elements often have technical guides and troubleshooting guides available through their respective web sites.

Terry Heller
- Saint Louis, Missouri
September 21, 2009