Friction-Enhancing Properties of ZDDP: Part I

[SuperSalad]

Friction-Enhancing Properties of ZDDP An... Reaction Films

Quote:
Abstract: Many phosphorus-base antiwear films, including those formed by zinc dialkyl dithiophosphates (ZDDP), cause significant increase in friction in thin film, high-pressure lubricated contacts. This can have a deleterious effect on engine oil fuel efficiency. Previous work has shown than friction in increased not under boundary, but under mixed lubrication conditions and it has been suggested that this phenomenon results from an effective roughening of the rubbing surfaces by the formation of unevenly-distributed reaction films.
In the current paper it is shown that, when other additives commonly used in engine oils are added to ZDDP solutions, quite smooth ZDDP reaction films can result. Despite this, the ZDDP still produces a marked increase in friction in mixed lubrication conditions, which suggests that surface roughening is not the main origin of friction enhancement by ZDDP reaction films. In a companion paper, Part II, it is shown that ZDDP reaction films, whether rough or smooth, enhance friction by inhibiting the entrainment of liquid lubricant into rubbing contacts, thereby reducing the elastohydrodynamic oil film thickness.

An interesting article pointing out a lesser known property of ZDDP that I think some people may misunderstand. I think a lot of people have the false belief that ZDDP is a friction reducing additive when the opposite is usually true.

It also validates the reductions in ZDDP over the years beyond just for the purpose of not poisoning the catalysts. Since fuel efficiency is not only an environmental concern but a performance concern as well it makes more sense to advance beyond ZDDP focused formulations.

 

[PolarisX]

Interesting, I too was under the impression ZDDP reduced friction and acting as a sacrificial coating (film).

Is this saying it reduces oil film thickness by creating peaks and valleys?

 

[Tegger]

I was under the impression that ZDDP's primary purpose was to reduce metal-to-metal contact, not to reduce friction.

 

[Doublehaul]

I don't get hung up zddp. Engines are lasting longer and longer while zddp keeps getting reduced. Its old school tech and the modern organic compounds seem to be working as well or better in modern engines.

 

[TTK]

It is interesting. But, reducing friction is not the same as wear reduction.

 

[FCD]

ZDDP was used first as an Anti Oxidation agent in the 50s when it's AW properties were found, in the 70s they found that a level of about 1200PPM reduced lifter wear in the typical OHV Flat tappet engines of the era, it reduces wear, not friction.
Friction modifiers on the other hand can reduce friction, but not wear, they don't have a significant effect on reducing engine wear AFAIK.

 

[SuperSalad]

Originally Posted By: Tegger
I was under the impression that ZDDP's primary purpose was to reduce metal-to-metal contact, not to reduce friction.

Exactly, but my point and one reason why I thought this article was worth sharing, is that many people don't seem to grasp that difference. So hopefully this helps educate those who didn't already know that, and helps us who did know it to understand it more fully.

Friction modifiers and antiwear additives are not the same. Some additives work as a multi-functional additive and provide both of those properties but you can have a friction modifier that doesn't effect wear and an antiwear additive that doesn't effect friction. Or in this case ones that increase friction while reducing wear.

In my experience of formulating motorcycle oils, that friction increasing property can be beneficial for maintaining wet clutch performance so it is a give and take. Even properties that seem like a detriment in most cases can be used for improvements in certain applications.

 

[CT8]

Originally Posted By: Doublehaul
I don't get hung up zddp. Engines are lasting longer and longer while zddp keeps getting reduced. Its old school tech and the modern organic compounds seem to be working as well or better in modern engines.
The need for engines last longer because of the emission warranties

 

[Buhwheat]

Originally Posted By: PolarisX
Interesting, I too was under the impression ZDDP reduced friction and acting as a sacrificial coating (film).


Originally Posted By: Tegger
I was under the impression that ZDDP's primary purpose was to reduce metal-to-metal contact, not to reduce friction.


Isn't that the function of Molybdenium?

 

[userfriendly]

A little friction helps spin lifters, reducing wear.

 

[Shannow]

Some ZDDP stuff that I put up in 2014...apologies for the formatting, it upset some players.

https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3422774/Re:_Chemical_analysis_of_tribo

and another paper.

http://isi-dl.com/downloadpdf/41937

have always been of the understanding that they are self limiting.

 

[Jetronic]

I've known for a few years that ZDDP layers have high friction. But around that time I also learned that friction and wear are not related except in the total absence of friction of course.

Still, ZDDP is very good at what it does.

 

[SuperSalad]

Rather than post a new topic I'll just post Part II Here

Quote:
ABSTRACT
In a companion paper, Part I, it was shown that the friction enhancement produced in
mixed lubrication conditions by ZDDP reaction films is not due to surface roughening
as has previously been supposed. Instead, optional measurement of elastohydrodynamic
film thickness shows that the presence of a ZDDP reaction layer inhibits the entrainment
of liquid lubricant into rolling/sliding contacts. As a result, these contacts operate in
boundary lubrication, with consequently high friction, up to higher speeds than would
otherwise be the case and the attainment of full film elastohydrodynamic lubrication is
also postponed. Possible mechanisms by which ZDDP reaction films might inhibit fluid
entrainment are discussed.

Quote:
CONCLUSIONS
ZDDP reaction films formed in rubbing contacts are known to
produce enhanced friction in mixed lubrication conditions. In
previous work this has been ascribed to a roughening of the
surfaces due to the reaction film. However the current work
shows that this is not the case; instead the ZDDP reaction
film appears to inhibit lubricant entrainment into the contact,
thereby leading to a reduced EHD film thickness compared to
ZDDP-free lubricants. A number of possible reasons for this
behaviour have been discussed but none has been verified.


I think this was the most interesting part for me with the explanation in the article following the list.
Quote:
The question of interest is why a smooth ZDDP reaction
film on the ball surface inhibits fluid entrainment into the
contact and thus reduced the EHD film thickness? A number
of mechanisms, of various degrees of likelihood, can be conceived
for this behavior. as listed below.
1. Starvation of the contact due to non-wetting of the ZDDP reaction film-coated ball.
2. Starvation of the contact due to inlet blocking by ZDDP reaction film material.
3. Presence of a localized film of fluid next to the ZDDP reaction film with much lower viscosity than the bulk solution; (since EHD film thickness is proportional to (viscosity)0.7 (4)).
4. Presence of a localized film of fluid next to the ZDDP reaction film with much lower pressure viscosity coeffi-cient than the bulk solution;(since EHD film thickness is proportional to (pressure viscosity coefficient)0.5 (4)).
5. Modification of contact inlet geometry due to ZDDP reaction film.
6. Slip at the fluid/ZDDP reaction film boundary.