Case I : An experiment was conducted using some additives. The additives formed a protective layer on the steel surfaces which reduced the roughness and hence h/Ra increased.
Case II: When the concentration of the additives were increased the h/Ra decreased.
(Ra increased in the second case)
As per literature Case I : Boundary regime and Case II: Mixed regime consisting of dry contact and boundary regime.
Which kind of addtive do you use? Which technique do you use to measure the surface roughness?
All are carbonaceous based additives. We use 3D non contact type of profilometer for surface measurement.
Dear Shubrajit:
From the question you asked; which's more preferable mixed lubrication regime or boundary lubrication regime? I believe there is no crystal clear answer for your question but; I believe it all depends on your application and your test conditions such as contact stresses and your temperature. Remember, your film thickness will deteriorate at higher temperature and higher contact stresses. So if you assume that test conditions are the same then you can talk about the type of the application that you are dealing with. For example for ball bearings, you want maintain mixed lubrication and/or full hydrodynamic lubrication to minimize friction, minimize wear and extend bearings for many life cycles. However; for other applications you might need your lubrication to be boundary lubrication, this's could be valid for any application when lowering friction coefficient is not the main goal. But remember, when adding additives, traction will be reduced for some applications and you might need to keep it high for some application, and that is why you need boundary lubrication if the keeping the traction high is a primary target! So in summary; it is totally depends on your specific applications and ultimate goals.
Now for the two cases you mentioned above; so for case I; I agree with you that by having some additives; a protective layers will form on steel surfaces; therefore h/Ra increased, but I don't think you can judge and say that Ra gets reduced, because you are not sure how much your film thickness h and your roughness Ra will be. If you assume that your film thickness h > than Ra, then there is no asperity to asperity contact, thus your Ra will be fixed. However if your h is smaller than your Ra, then you will have asperity to asperity contact, and then your h will be reduced.
Now for case II; if additives increased, this should increase the film thickness h, thus Ra again is not changing as long as your h > Ra and there is no asperity to asperity contact. Therefore in order to have a legitimate answer, you need to measure or calculate your film thickness as well as your Ra, and there you will be able to say if your Ra will be reduced or increased. So there is no clear legitimate correlation between adding more additives and the value of your Ra.
I hope I didn’t understand your question wrong, otherwise feel free to extend the discussion. Hope, this will help somewhat!
All best Regards,
Mohamed F. Alzoubi
Dear Mr.Alzoubi thankx a lot for your wonderful explanation. I too had some confusion after reading a paper...hence your explanation has cleared my confusion...
It should put it this way.
In the boundary lubrication, the addatives can form a protective layer on the surface such that Ra decrease.
On the other hand, in the mixed lubrication, adding the addatives will increase the viscosity so that h increase.
A constant contact between the friction surfaces at high surface points (microasperities) occurs at boundary lubrication. So this regime is the most undesirable since it is characterized by high coefficient of friction (energy loss), increased wear, possibility of seizure between the bearing and journal materials ,
non-uniform distribution of the bearing load (localized pressure peaks). Very severe
engine bearing failuresare caused by boundary lubrication.Conditions for boundary lubrication are realized mainly at low speed friction (engine start and shutdown) and high loads. So, mixed lubrication regime is more preferable than boundary lubrication regime.
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