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I have a dry film moly-graphite lubricant (CoF=.075) and a lubricious CVD coating (CoF=.05).

The CoF of each of these lubricious materials was determined using the same ASME standardized test.

The standardized test consists of: a plain steel object is coated in the lubricant to be tested; it is placed on a plain steel plate; the plate is slowly tilted until the object starts to slide; the angle at which sliding occurs determines the CoF (a lower angle correpsonds to a lower CoF).

If I used both the coating and the lubricant together, would that lower the CoF further?

I.E. If I took the steel test object, and I coated it with the CVD coating, then on top of that I applied the dry-film moly-graphite lubricant, would that combination have a lower CoF than either material alone?

If yes, How much? If no, Why not?

This seems like a relatively straight forward question, but I've yet to get a clear answer from anyone. Most people seem to drag other variables into the problem statement, but I'd like to know, all other variables remaining equal, can we answer this question - or is it truly impossible?


The following assumptions apply:

  • The only variable that changes between tests is the lubricant used. All other factors remain exactly the same in each test. E.g. surface finish, temperature, interface load, materials being tested, etc. all remain exactly the same between tests.
  • The CVD coating is permanent and non-removable. The moly-lubricant can be applied over the CVD coating and/or to the other part; either way is perfectly acceptable, so whichever scenario allows you to answer the question is fine by me.

PS:

This question was triggered by a problem I ran into at work.

It's not a homework assignment - I've been out of school for almost a decade now :)

It started off as a simple question that no one in the office could answer, so I turned to the internet, because I'm a curious george.

So far, my attempts for help on other internet sites have been met mostly with hostility and wild assumptions about the problem that were not stated in the get go.

I got one answer here so far, and I realized some assumptions regarding outside factors were being made that I didn't anticipate, so I revised the question to clarify what assumptions should apply in any attempt to answer the question.

I just want to know, if all other factors remain exactly the same, what effect to expect from adding a lubricant to an already extremely lubricious material.

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You can't calculate friction coefficients this way. At the atomic-scale level, the friction coefficient depends on the interactions of the two materials across the interface. If you measure the behaviour of the interface between materials A and B, and A and C, you can't say anything about the interface between B and C.

In any case, friction coefficients are dependent on the surface finish, temperature, relative velocity, interface pressure, etc., etc. The only way to measure an "accurate" value is to do an experiment that matches the "real life" conditions as closely as possible.

(Note: The claim that friction depends on the interface pressure refers to the (very simple) Coulomb model of friction - which has the advantage that it is simple enough to use in high-school-level mechanics questions, but is only "accurate" for a very limited range of materials and operating conditions.)

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  • $\begingroup$ Thanks for the answer! I see some assumptions being made about changing other variables between tests. The tests are standardized, and for this question we assume everything remains the same between them (surface finish, temperature, load, etc.) I revised the question to state this. I do understand many of the factors which affect friction coefficients. What I want to know is, if all other factors remain the same, what should I expect by adding a lubricant to an already lubricious coating? $\endgroup$ – CBRF23 Apr 26 '16 at 13:30
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    $\begingroup$ I think the answer is you shouldn't necessarily expect anything. I am hardly an expert in friction, tribology or rheology, but as a materials scientist I have never heard of any sort of rule-of-mixtures or min-max type rules for coefficient of friction. Why not run a design of experiments with appropriate equipment? $\endgroup$ – wwarriner Apr 26 '16 at 14:20
  • $\begingroup$ @starrise - Why not?: time and money. But if I had unlimited funds and unlimited time to pursue all the studies that tickled my fancy you can bet this would be on the top of the list :) $\endgroup$ – CBRF23 Apr 26 '16 at 15:51
  • $\begingroup$ @starrise - Also, I'm not looking for any type of rules persay, I doubt there would be any universal rules that would apply. I just figured there has to be someone out there with the knowledge and experience to chime in with "typically you would get X, except for when you get Y." $\endgroup$ – CBRF23 Apr 26 '16 at 15:54
  • $\begingroup$ Fair enough. :) It would certainly be an interesting study to attempt, but I believe by now there would probably be some well known textbook result in general. $\endgroup$ – wwarriner Apr 26 '16 at 16:47
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The CDV surface may be more slippery than the untreated block, but the ramp surface is the same. Adding lube to the interface will make the tilting ramp more slippery, reducing the overall coefficient of friction. Unless both surfaces are already super slippery, then the dry powder lube could conceivably cake up and increase the CoF. I think if the CoF of the dry surfaces is considerably less than the lube, the lube may increase the CoF. Ie, Mud makes it easier to drag your canoe onshore, but it plays havoc with your trailer wheel bearings. I am a Mech, so no math out of me.

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