Strange phenomenon observed, friction and wear

[Q_Goest]

There's a machine that uses a type of plastic material (reinforced Teflon) as a wear ring. The plastic rides on a highly polished metal surface and eventually wears out at which point the machine must be rebuilt.

If the plastic and metal sliding surfaces are exposed to bone dry (ie: no water vapor whatsoever) hydrogen, the plastic will wear out after roughly 10,000 hours of use.

If the plastic and metal sliding surfaces are exposed to bone dry helium, the plastic will wear out after only 40 hours of use. Obviously, this material is no good for this service and another material must be used.

So wear rate seems to be highly dependent on the gas involved. Note that all other parameters are identical, including pressure, temperature, contact stress, velocity, etc…

I've also heard circumstantial evidence that coefficient of friction may also depend on the gas involved. I don't believe the conventional view (of friction coefficient or wear rate) acknowledges any sort of difference based on what gas materials are exposed to, but at least in the case of wear rate, I have overwhelming evidence that wear rate is strongly related to which gas is involved. Now I'm wondering if coefficient of friction might also be related.

Does anyone know if or how coefficient of friction might vary depending on the gas involved?

 

[sid_galt]

Just a wild speculation, maybe hydrogen forms some bonds with the metal surface reducing wear?

edit: Another wild speculation: there is a tendency of metal to form metal hydrides because of a potential difference. Now a plastic contains hydrogen atoms. Although the H in plastic can't bond with metal, maybe it is attracted to the metal causing friction. Now in hydrogen environment, a fraction of the metal surface reacts with hydrogen to form hydrides. The hydrogen in these hydrides is not attracted or maybe repels the H in the plastic reducing friction?

 

[Q_Goest]

Hi Sid. Teflon is a essentially a carbon (think of a hydrocarbon) chain with florine where hydrogen would otherwise be. So there's no hydrogen in Teflon (the plastic being used). In fact, Teflon can have a slightly exothermic reaction with hydrogen under the right circumstances. I believe it's the florine that bonds to hydrogen forming HF and a second hydrogen takes the place of the F on the carbon chain.

I've talked to some chemists in my company about this and didn't come up with much. Not sure why the hydrogen should affect the metal and form any kind of layer. I don't think that happens. The use of Teflon in wearing parts like this is so that a microscopically thin layer of Teflon coats the metal. The Teflon part is then riding on this thin film. One theory holds that the film may be swept off more readily by helium for some reason.

 

[FredGarvin]

Q,
I wonder if this may be of some help:
http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=4287575

While not quite the same materials, it may point you in a direction you haven't looked. Could there be an adsorption issue when going from the hydrogen to the helium?

 

[Q_Goest]

Thanks Fred. I can open the link, but the paper must be some place else and I can't seem to find the link.

Could there be an adsorption issue when going from the hydrogen to the helium?

ummmmm, not sure what you mean. Just to clarify, the machine never sees both fluids. There are numerous machines, some are started up and run with hydrogen and some start up and run with helium but they are otherwise identical. The machines never see both services in their lifetime.

You did however give me some more ideas for searches and I'm looking a bit more now. Still haven't found much of interest but thanks for the ideas.