Non-galling bearing materials choice?

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Unless you get lucky and someone here happens to know how materials behave in that extreme oxidizing environment, I think you're going to have to talk to a professor of materials science at some university.

I hope you find an expert here but I think you're going to have to find a specialist. Whenever I've had to deal with tough materials questions in the past I've gone to one or two university profs at places like Ohio State or Purdue, who usually know who the expert is. Sometimes I used to start with the ASM. Then I track down the expert they send me to.

Other things you might try are, first, Google's new Scholar function. Obscure engineering papers sometimes show up there. The other one, if I were going to a commercial source, would be Balzers, who make ceramic coatings for demanding applications as well as for cutting tools; finally, I'd try someone who supplies Haynes 25 and other Haynes alloys for technical applications.

Good luck. My guess is that an oxide- or nitride-type ceramic is a likely candidate (silicon nitride, IIRC, is used in high-pressure oxygen environments, but I don't know how it works out as a bearing surface), but that's only a guess. If any of the metals can tolerate it, it probably would be one of the superalloys, like Haynes or Inconel.

-- Ed Huntress

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You miss the point. I have already done the oxygen-compatibility research (or G4/NASA/ASTM did it for me), with the possible exception of the oxygen suitability of Macor which needs an emails' more investigation by me.

I have lots of paper on that, including friction ignition test data, but I want to use materials which will not sustain (rather that initiate) combustion in this particular part of the system, and the materials in the lists above are the only suitable ones I know of.

What I want to know is is a pair of materials from the above lists, one of which will form a strong and tough (but not outrageously-so, about like mild steel) 6mm shaft, and the other will form an unlubricated bearing, with the pair not galling. That's all.

NASA lists Copper Beryllium / Monel 400 as the best pair overall for instance, which would be doable, but any pair would do, and really I am looking for the cheapest and least-galling shaft partner for a leaded tin-bronze bearing, which would be readily available - Monel K-500 would do as a shaft, it is hard and just-about-dissimilar enough, but is there a cheaper and more available choice, from the lists given?

Or does anyone know where to get a foot or two of 6 mm Monel K-500 rod, not-to-expensive?? I've never come across it before. I think it has to be hardened somehow too. Centreless ground accuracy would be really nice ...

No expertise on oxygen-compatibility of materials is sought however, although it would be very much appreciated if available. I shall look into your suggestions about that, thanks.

A note about compatibility:

Designing for oxygen service is a bit of a black art, but the basic principles are either (or all three!):

1) avoid sources of ignition - sounds a bit iffy, but often you have little choice, as almost _everything_ will react with high-pressure oxygen, especially when hot, even teflon will burn 2) use some of the very few materials, like oxide ceramics - sapphire/alumina/silica/zirconia (but not silicon nitride, which burns) that won't react, 3) use materials that won't sustain combustion in the sizes and shapes involved.

It's not really possible to reliably avoid ignition sources in my application, I don't know of suitable unreactive materials - any ceramic experts here? - so I have to go with the last option, materials that will not sustain combustion.

If galling occurs then fresh surfaces will be exposed and/or small particles will be produced, and the material will likely ignite and the parts will likely be ruined, as will some downstream parts; but the materials on the lists won't sustain combustion in these shapes, even if they ignite, and if the parts are made from them the flame will go out, and the overall fire will be containable.

Thanks,

I think you've missed Ed's point. Picking a pair of materials that perform together well in a normal atmosphere at a given temp will not insure that they'll give acceptable performance in a strongly oxidizing environment. I've got some experience at the other end of the spectrum - hydrogen atmospheres at high temperatures - and can tell you that the behavior of bearing materials in air is not a good predictor of their performance in a reducing atmosphere. I'd expect the same to be true for extrapolating from air to oxidizing environments.

Ned Simmons

Good luck, it sounds like a challenge.

-- Ed Huntress

Yeah, but high temperatures tend to make things stick together (diffusion for instance) while the hydrogen fluxes any oxides. Heck, again on the opposite side of things, the bearings may even work better! ;)

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

Better in hydrogen or oxygen? You're right about the reducing atmosphere stripping the surface of metals. This, combined with the heat, makes metals in close contact prone to seizing.

Ned Simmons

From: "Ed Huntress" ..I've gone to one or two university profs at places like Ohio State or Purdue...

Forget OSU! :)

Randy

Ohio State has become a refuge for Indian postgrads in manufacturing engineering and related fields. They're very good.

Purdue has a longer tradition of research in the field, and there are a several cow colleges, including my alma mater, Michigan State, where you'll find some really good people.

-- Ed Huntress

Seriously look up the Steel number - 8620. It is a sliding, non-galling metal. Used as a base metal and slide metal of the M14 as an example.

Mart>

If this is running in oxygen then 8620 would be a baad choice. Carbon steels are dodgy in oxygen. (Yes I know medium pressure oxygen tanks and pipelines are made in carbon steel).

I would consider inconel / tin bronze.