Boundary lubrication, polarity, PAOs and additives

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My understanding is that in normal engine operation the predominant and desirable mode of lubrication is hydrodynamic. That is, a measurably thick film of oil is maintained between components and no contact occurs. In some proportion of normal operation, however, the hydrodynamic capabilities of any oil are exceeded, such as between camshafts and followers/rocker arms and between rings and cylinder walls near the top of the piston stroke, and under certain operating conditions such as start-up.

When the hydrodynamic capabilities of an oil are exceeded, metal-to-metal contact is avoided by (1) boundary lubrication and, finally, by (2) additive chemistries which sacrificially coat the metal surfaces.

I have come to believe that maximizing the oil's hydrodynamic effectiveness is a function of only two qualities: flow and HTHS viscosity. To a great degree those qualities are in opposition, such that increasing HTHS will tend to require greater kinematic viscosity which will tend to impede flow, and increasing flow will tend to result in lower HTHS viscosity.

My questions here have to do with what happens when hydrodynamic breakdown occurs. How exactly does boundary lubrication happen? One aspect of it seems to be related to polarity: that is, a polar oil will remain aggressively attached to metal surfaces, even in a layer that is only a few molecules thick, and will resist being scrubbed away.

I recall a recent post which quoted an industry expert noting that PAO oils are not polar and that, because of that, by themselves they were useless as boundary lubricants; and that consequently their abilities as boundary lubricants relied entirely on additives. If correct, what types of additives would those be, and how do they work?

Could it be that the non-polar nature of PAOs might help to explain why it is that conventional oils, despite many inferior laboratory qualities, are able to equal or exceed PAO oils' performance in terms of wear? In other words, that PAO oils are superior where cold performance and extreme heat are involved, and that they last longer, but that they are inferior in boundary lubrication, and that as a consequence their overall wear performance is only equal to conventionals, or even slightly inferior?

Could it be true, also, that Grp V oils, which are held to be highly polar in nature, can have superior properties when boundary lubrication is encountered, at least partly due to their polarity?

And how does the creation of sacrificial coatings, such as those formed by the breakdown of ZDDP, and perhaps moly for that matter, interact with the oil's boundary lubrication qualities? My current understanding is that boundary lubrication would tend to be breached first, then perhaps whatever molecular moly was coating the surface in question, and that the sulphur compounds formed by ZDDP breakdown actually react chemically with the metals and therefore become the outermost layer of the component, forming only when created by high heat from friction and acting as the final line of defense before metallic wear begins.

Finally, we have seen the replacement of ZDDP to some degree with other additives, and I am curious how these fit into the picture. Some show up in UOAs because they contain Boron, and I believe there may be others. How much is known about how these newer additive chemistries work, and how they interact with the older chemistries, which for now are still present in all motor oils?

It seems to me that the best wear protection would depend on optimizing all three qualities in a motor oil: hydrodynamic capabilities, to reduce the need for boundary lubrication; boundary lubrication itself; and AW/EP chemistries that act when boundary lubrication breaks down.
 
Originally Posted By: glennc
I have come to believe that maximizing the oil's hydrodynamic effectiveness is a function of only two qualities: flow and HTHS viscosity. To a great degree those qualities are in opposition, such that increasing HTHS will tend to require greater kinematic viscosity which will tend to impede flow, and increasing flow will tend to result in lower HTHS viscosity.
Good write up, by the way.

Unfortunately, there are some more base oil properties that come into play, particularly the increase in viscosity due to pressure. This thread touches on them: http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=941621#Post941621 I think some other threads do too but don't have any links.

Well, maybe this one will help too. http://www.bobistheoilguy.com/forums/ubb...true#Post933133
This may also interest you: http://www.bobistheoilguy.com/forums/ubb...true#Post787754

Quote:
My questions here have to do with what happens when hydrodynamic breakdown occurs. How exactly does boundary lubrication happen? One aspect of it seems to be related to polarity: that is, a polar oil will remain aggressively attached to metal surfaces, even in a layer that is only a few molecules thick, and will resist being scrubbed away.
I've seen polarity seem to help and seen it not seem to help, depending on the specifics of what's tested and how it's tested.

Quote:
I recall a recent post which quoted an industry expert noting that PAO oils are not polar and that, because of that, by themselves they were useless as boundary lubricants; and that consequently their abilities as boundary lubricants relied entirely on additives. If correct, what types of additives would those be, and how do they work?
I don't believe PAO's are useless boundary lubricants. If they were, test results of 4-ball wear and other similar tests would show very high wear compared to other un-additized base oils. Results I've seen don't support that view.

Quote:
Could it be that the non-polar nature of PAOs might help to explain why it is that conventional oils, despite many inferior laboratory qualities, are able to equal or exceed PAO oils' performance in terms of wear? In other words, that PAO oils are superior where cold performance and extreme heat are involved, and that they last longer, but that they are inferior in boundary lubrication, and that as a consequence their overall wear performance is only equal to conventionals, or even slightely inferior?
That's a very tough cookie to cut and I won't attempt it now.

Quote:
Finally, we have seen the replacement of ZDDP to some degree with other additives, and I am curious how these fit into the picture. Some show up in UOAs because they contain Boron, and I believe there may be others. How much is known about how these newer additive chemistries work, and how they interact with the older chemistries, which for now are still present in all motor oils?
ZDDP and moly can act synergistically, giving lower wear than either alone. ZDDP forms relatively quickly and faster than moly so it is great to have ZDDP in high concentration during running in of an engine. This is critical in some valvetrain designs but not so in others. Sadly, detergents, dispersants, anti-oxidants, and potentially any other surface-active additive can interfere with anti-wear additives' ability to do their job. It all has to be balanced for the intended application to make it ideal.

Many additives have been borated and I think they are often borated dispersants (particulary with Mobil 1). These don't function as anti-wear additives but by borating them they can reduce the pro-wear effect that otherwise the same NON-borated dispersant has. There are other boron additives that do function as anti-wear additives but it's hard to know what type of boron additive is in any given oil containing boron.

There are many ashless anti-wear additives but since they are undetectable by UOA they are an unknown to us usually. Some mixes of anti-wear additives has shown good results when mixed with the right combination of ash-containing additives. If they take out the ash-containing additives, the goodness goes out the door.

I have hundreds of pages of studies printed out but unfortunately that does not allow me to link you to them!

If you haven't already, read the Interesting Articles forum's threads that seem applicable to these matters.
 
Quote:
My current understanding is that boundary lubrication would tend to be breached first, then perhaps whatever molecular moly was coating the surface in question, and that the sulphur compounds formed by ZDDP breakdown actually react chemically with the metals and therefore become the outermost layer of the component, forming only when created by high heat from friction and acting as the final line of defense before metallic wear begins.


ZDDP films are still being studied and are not fully understood. All the reading I have done (not much compared to the real Lube engineers here) suggest that it forms films ON the surface. Chemicals that form films WITH the metal are known as Extreme Pressure additives which are typically found in gear oils and greases. They tend to not be corrosively stable at the high heats in an engine and require higher heat from asperity contact that does AW to form films.

Moly that is used in an engine is mostly for friction reduction, though it can provide some AW.
 
JAG, I figured you'd get in at some point, but that was quick!

Originally Posted By: JAG
Originally Posted By: glennc
I have come to believe that maximizing the oil's hydrodynamic effectiveness is a function of only two qualities: flow and HTHS viscosity. To a great degree those qualities are in opposition, such that increasing HTHS will tend to require greater kinematic viscosity which will tend to impede flow, and increasing flow will tend to result in lower HTHS viscosity.
Unfortunately, there are some more base oil properties that come into play, particularly the increase in viscosity due to pressure.

Sorry, I should have qualified that more completely by saying something like "depends on only two things: flow and the viscosity characteristics of the oil at the point of wear." In other words, that hydrodynamic lubrication is viscosity-driven but that viscosity is more than a simple measure of kinematic viscosity - indeed, as your links indicate, much more. That would of course be an excellent topic for another thread, or more likely, many other threads. As for this thread...

Originally Posted By: JAG
Originally Posted By: glennc
I recall a recent post which quoted an industry expert noting that PAO oils are not polar and that, because of that, by themselves they were useless as boundary lubricants; and that consequently their abilities as boundary lubricants relied entirely on additives. If correct, what types of additives would those be, and how do they work?

I don't believe PAO's are useless boundary lubricants. If they were, test results of 4-ball wear and other similar tests would show very high wear compared to other un-additized base oils. Results I've seen don't support that view.

The quote I'm referring to was in regards to PAO base stocks. Naturally the finished oil would have anti-wear capabilities such that performance in wear tests like you describe would be acceptable and possibly even superb. Of course, there has been a lot of discussion of the four-ball test's applicability to real-world engine wear. I am specifically curious about the "boundary lubrication" behavior of the PAO oils, rather than the "chemical anti-wear" behavior: hopefully I'm making the distinction clear even if not in the proper terms. If you look at these as two different things (maybe they are not separable), what is it about any oil, other than a polar attraction, that resists boundary lubrication breakdown? If there were no AW/EP additives at all in the oil, how would it perform once hydrodynamic lubrication fails? And is it even relevant (easy to say no, but do we really know one way or another)? I suppose I am specifically most interested in the question of the importance and effects of polarity, and whether there are any other qualities of the oil, aside from viscosity effects and additive effects, that contribute to the oil's qualities as a boundary lubricant.

Originally Posted By: JAG
Originally Posted By: glennc
Could it be that the non-polar nature of PAOs might help to explain why it is that conventional oils, despite many inferior laboratory qualities, are able to equal or exceed PAO oils' performance in terms of wear? In other words, that PAO oils are superior where cold performance and extreme heat are involved, and that they last longer, but that they are inferior in boundary lubrication, and that as a consequence their overall wear performance is only equal to conventionals, or even slightly inferior?
That's a very tough cookie to cut and I won't attempt it now.

Indeed, and quite a can of worms! That is one of the BIG QUESTIONS though, at least for me!

Originally Posted By: JAG
Many additives have been borated and I think they are often borated dispersants (particulary with Mobil 1). These don't function as anti-wear additives but by borating them they can reduce the pro-wear effect that otherwise the same NON-borated dispersant has. There are other boron additives that do function as anti-wear additives but it's hard to know what type of boron additive is in any given oil containing boron.

It sounds as though it is the boron itself that, when separated from the additive which has been "borated," reacts somehow to assist the anti-wear capabilities of the oil. Is that your understanding as well? What is the mechanism? Is it similar to the sulfer reactions that occur on ZDDP breakdown, another sort of "surface active" chemistry?

Originally Posted By: JAG
There are many ashless anti-wear additives but since they are undetectable by UOA they are an unknown to us usually. Some mixes of anti-wear additives has shown good results when mixed with the right combination of ash-containing additives. If they take out the ash-containing additives, the goodness goes out the door.

So I have read, probably in some of your own posts in fact. It seems that for now, anti-wear properties can be optimized by adding other chemistries to oils but ZDDP can not yet be replaced and remains the most important part of the anti-wear package, probably by far.

Originally Posted By: JAG
I have hundreds of pages of studies printed out but unfortunately that does not allow me to link you to them!

I think that would be some very slow but interesting reading.

Thanks for the reply here. These are some of the most fascinating bottom-line questions when it comes to oil performance, in my opinion. Naturally performance in terms of cleanliness is also very important and can't be completely separated from the question of performance as a lubricant, but for educational purposes it seems useful to attempt to separate the two to facilitate understanding and to separate both from the question of service life. As with many things the three tend to be at odds with one another.
 
It seems to me that once the hydrodynamics are lost, it then very quickly relies on the additives in the oil to provide protection for a little while.
 
ZDDP will form a film as well as into the metal it is a "anti wear" additive being kinda weaker than "anti Scuff" or wrongly called EP.
bruce
 
Glenn, I knew you meant pure base oils (no additives) which is why I said "un-additized".

I can't remember whether on borated dispersants and other similar additives whether the boron atoms can separate and do their own thing. There are patents online that go into a lot of details if you care to search for them.

This is always a good read. If you look at the link below and open the PDSs of the different esters and alkylated napthalenes you can compare them.
http://www.exxonmobilsynthetics.com/Publ...erex_Grades.asp

My second link in my first post shows some base oil properties and how they affect lubrication. Polarity is not listed but maybe there is info on that somewhere else.
 
Originally Posted By: bruce381
wrongly called EP

That is an interesting little piece of information right there. "Wrongly called," I assume, because it has nothing to do with pressure but instead has to do with friction (scuff).

Thanks for the reply.
 
Quote:
"Wrongly called," I assume, because it has nothing to do with pressure but instead has to do with friction (scuff).

More like heat, caused by friction. The higher the pressure placed on opposing pieces of metal, the greater the friction and heat. It takes more heat to get EP or Anti-scuff chemistry to react. AW reacts at lower temperatures.

Note that localized asperity contacts can (with enough pressure) generate enough heat to weld metal together (scuffing or "cold" welding) so there is plenty of heat available even if the base oil is not the required temp. overall.

http://www.engineersedge.com/lubrication/extreme_pressure_additives.htm

Terry Dyson has stated that PP's AW is not dependant on ZDDP. ZDDP is also a multi-function additive and can be more focused on AW or Anti-oxidant.
 
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