Micro-Pitting and Wear Assessment of PAO vs Mineral-Based Engine Oil Operating under Mixed Lubrication Conditions

[naav]

I found a really interesting open access article you guys might find interesting: https://www.mdpi.com/2075-4442/7/5/42

There's a lot going on in this article but I think the main takeaways for passenger car motor oils are:

1. Mineral oils containing ZDDP anti-wear additives can promote micro-pitting under mixed lubrication conditions. ZDDP additives, while beneficial for reducing wear in sliding contacts, can lead to surface-initiated rolling contact fatigue in rolling element bearings. This can result in micro-pitting, where small pits form on the surface of metal components, potentially leading to more severe wear over time
2. PAO-Based Oil (Oil B):
   • PAO-based oil tends to have lower wear due to its lower pressure-viscosity coefficient, which reduces contact severity and decreases the wear-promoting action of ZDDP additives. PAO-based oils, with their lower pressure-viscosity coefficient, generate lower pressure gradients and thus lower localized stresses. This reduces the wear-promoting action of ZDDP additives and the tendency for surface fatigue, making PAO oils generally more effective in reducing wear under similar lubrication conditions.
   • Local micro-pitting damage is higher for PAO oil at low and intermediate lambda ratios but is completely eliminated at higher lambda ratios. This is because PAO oil's lower pressure-viscosity coefficient and lower elastohydrodynamic lubrication (EHL) friction reduce surface stresses, thereby mitigating micro-pitting. Since PAO oil causes less wear, the surface of the engine parts is not being worn away quickly. This means that any micro-pits that form are not removed by wear and continue to grow and become more pronounced.
3. Mineral-Based Oil (Oil A):
   • Mineral oil exhibits higher wear due to its higher pressure-viscosity coefficient, leading to higher pressure gradients and enhanced wear-promoting action of ZDDP additives. This effect becomes more pronounced at higher lambda ratios, where lubrication is better. Mineral oils typically have a higher pressure-viscosity coefficient compared to PAO-based oils. This means that the viscosity of mineral oils increases more significantly under pressure. While this can sometimes be beneficial for forming a thicker lubricating film, it can also lead to higher pressure gradients within the contact area. The higher pressure gradients associated with mineral oils can result in increased localized stresses on the contacting surfaces. These increased stresses can enhance the action of ZDDP (zinc dialkyldithiophosphate) additives. ZDDP is known for forming a protective anti-wear film on metal surfaces under pressure and heat, but under higher stress conditions, it can also promote surface fatigue and wear mechanisms such as micro-pitting.
   • While wear damage is higher for mineral oil, resulting in lower micro-pitting damage due to the competitive nature of these two damage mechanisms, the effect of base oil type on wear is less pronounced during low lambda conditions and more significant in better lubrication conditions. The high wear caused by mineral oil conceals past micro-pitting damage.

 

[cheesepuffs2]

Does that mean that an oil like Valvoline VR1 conventional might be best to pass on?

 

[naav]

Does that mean that an oil like Valvoline VR1 conventional might be best to pass on?

Dunno; oil is so complicated and I haven't wrapped my head around it. Based on this however your chances of survival might be improved with PAO compared to mineral-based oil but the main thing is this completely destroys the "higher ZDDP = always better" theories floated around here sometimes. I'm specifically referring to those that add ZDDP additives to their motor oils. ZDDP concentration needs to be matched against the base oils and expected lambda ratio distribution at a minimum.

 

[naav]

The main thing I kept tripping up on while reading the article is transverse vs longitudinal roughness lay. I don't think there is ever a transverse roughness lay in mass produced engine bearing surfaces.

 

[SR5]

It's a deep paper, thanks for posting, and I've only managed to have a quick look at it. But at first glance something stands out to me.

"Both oils are based on the base oil mixture of Group III and polyalphaolefins (PAO) and contain the same standard additive package, including an anti-wear additive known as the ZDDP additive. The prototype engine oil B is optimized for rolling contacts without viscosity modifiers and a much higher PAO base oil blend viscosity to have similar viscosity as oil A. By doing so, oil B has a higher ratio of PAO to mineral base oil compared to the standard production oil A "

Note that Oil-B with more PAO also doesn't contain any viscosity modifiers (VM), I assume in the form of polymer viscosity index improvers (VII). To me this sounds like Oil-A does contain a commercial VM / VII package. It also sounds like oil-A gets some of its viscosity from it's VII, while oil-B gets all of its viscosity from the base oils.

Another way of reading this, consistent with other papers people here posted ages ago, is that you get less wear with a monograde over a multigrade.

Unfortunately it looks like two variables, but one measurement, which confuses the situation (for me at least). Obviously they need more funding to stay employed and keep going with more tests and more controls.

 

[MolaKule]

An interesting paper for sure.

"Where the lambda ratio[n] (Λ) is defined as the ratio of the minimum EHL film thickness and the composite surface roughness. They suggest that in the presence of wear, micro-pitting damage is the highest at an intermediate value of the lambda ratio, i.e., micro-pitting is less at low or high lambda. However, no actual experimental verification was provided. "

So this experiment's hypothesis was to determine micropitting for mainly sliding bearings with the ZDDP anti-wear/antioxidant present in various base oils, as micropitting was previously observed for roller bearings with ZDDP? Just trying to wrap my brain around this paper.

One question I had to ask is, since it seems micropitting has been with us forever in sliding bearings, how does this correlate to premature wear and engine failure?

It seems to me that most engine failures are due to the lack of proper lubrication such as insufficient MOFT, improper maintenance schedules, engine design flaws, and unpredicted material failures, etc.

 

[naav]

An interesting paper for sure.

"Where the lambda ratio[n] (Λ) is defined as the ratio of the minimum EHL film thickness and the composite surface roughness. They suggest that in the presence of wear, micro-pitting damage is the highest at an intermediate value of the lambda ratio, i.e., micro-pitting is less at low or high lambda. However, no actual experimental verification was provided. "

So this experiment's hypothesis was to determine micropitting for mainly sliding bearings with the ZDDP anti-wear/antioxidant present in various base oils, as micropitting was previously observed for roller bearings with ZDDP? Just trying to wrap my brain around this paper.

One question I had to ask is, since it seems micropitting has been with us forever in sliding bearings, how does this correlate to premature wear and engine failure?

It seems to me that most engine failures are due to the lack of proper lubrication such as insufficient MOFT, improper maintenance schedules, engine design flaws, and unpredicted material failures, etc.

I researched cavitation in depth and posted my summary of a paper which discusses the physics of it in a new post https://bobistheoilguy.com/forums/t...boration-with-renault-lubricants-2015.383730/

 

[MolaKule]

I researched cavitation in depth and posted my summary of a paper which discusses the physics of it in a new post https://bobistheoilguy.com/forums/t...boration-with-renault-lubricants-2015.383730/

Right, we know what cavitation and wear mechanisms are as it has been discussed here before many times.

But that does not answer the question: What is the major engine failure mechanism or mechanisms since say APL SL and later oils were introduced?

 

[chris719]

What is the major engine failure mechanism or mechanisms since say APL SL and later oils were introduced?

I'm not sure there is a dominant one across the industry. Some brands have valvetrain issues, some cylinder bore / piston skirt, and some with rod or main bearings.

 

[ripcord]

Kind of a wiseacre response, but the major failure mechanisms seem to be rust, accidents, overheating and transmission failure.