Tribofilms: Aspects of Formation, Stability and Removal

[ZeeOSix]

This paper seems to support that introducing an oil with a different formulation to an already formed tribofilm can have an almost instant effect on the already formed tribofilm - seems to be due to the partial removal/stripping of the existing tribofim, then a rebuilding of a new tribofilm (and a corresponding new friction level) depending on the new oil formulation. Formed tribofilms have a stability factor, and seems both mechanical and chemical means can remove them. A tribofilm is constantly being built and removed, and it's important that the build-up rate is higher than the removal rate. This paper specifically discusses ZDDP and MoDTC tribofilm formations, stability and removal. Figures 11 and 14 are interesting.

They didn't get into how an existing tribofilm built-up with long used oil from a fired engine could be effected by introducing new oil - looks like they only used new oils in this tests. There are other studies that show that an existing tribofilm can be partially stripped with the introduction of new oil.

The AW/AF formulation of different motor oils could have an over-all effect on the wear rate in the parts of an engine where boundary and mix lubrication is happening, depending on their level of formulation difference (which seems to be shrinking in today's oils). As we know, both "film thickness" (MOFT from the oil viscosity) and "film strength" (from the oil AF/AW package) are both factors that determine the over-all wear rate. Oil viscosity is the prime wear mitigator, and film strength is the backup when the viscosity can't provide adequate wear protection.

PDF download: https://www.mgexp.com/phile/1/291112/ZDDP_Paper_Leeds.pdf

 

[paulri]

I do have a question on one of the charts (Figure 14, from page 5484/9).

Now does this mean that at 240 minutes, they inserted new oils with different additive packs (or a base oil, with no additives)?

Also, lower friction coefficient means less friction, which is better, correct?

 

[ZeeOSix]

I do have a question on one of the charts (Figure 14, from page 5484/9). Now does this mean that at 240 minutes, they inserted new oils with different additive packs (or a base oil, with no additives)?

Yes, the way I read it is they established a steady friction level with the ZDDP/MoDTC formulated oil, then changed the oil to the BO, ZDDP only and MoDTC only formulations, and the friction level changed and re-stabilized as shown. Did the same thing with starting with a steady friction level with only a MoDTC formation (Fig 11).

What I noticed between Figures 11 and 14 is that the resulting stable state friction levels ended up at a different value for the BO depending on if the baseline tribofillm was due to just MoDTC (Fig 11) or ZDDP/MoDTC mix (Fig 14).

In Fig 11 (MoDTC tribofilm):
BO stabilized at 0.13
ZDDP stabilized at 0.11
ZDDP/MoDTC stabilized at 0.10

In Fig 14 (ZDDP/MoDTC tribofilm):
BO stabilized at 0.09 (maybe lower than Fig 11 due to robustness of ZDDP?).
ZDDP stabilized at 0.11 (same as Fig 11).
MoDTC stabilized at 0.04 (pretty close to Fig 11 baseline).

What these graphs also seem to show is that it doesn't take very long for the tribofilms to change and be built back up, or build up from scratch at time zero. The graphs show it took around about 60 minutes or less from time zero to create a steady friction level from the tribofilm.