I've heard that moly dust can act as a great lube, but at some point large molybdenum particles in grease start to act like a rubbing compound and actually become abrasive rather than lubricate.

Is this true? And what if it is - what particle size does this occur at?

Here is a tech service bulletin "Correlation between particle size and engine wear." for engine oil that might help answer your question. I imagine it may hold some truth for grease as well.

http://www.amsoil.com/techserv...0Wear.pdf?zo=1181889

Moly can be found in many forms. Some are more abrasive than others.

This message has been edited. Last edited by: Tim Vipond,

Thanks for the response but this article is on contaminants, whereas Moly is an additive or I guess cn be used as a straight lubricant. Fine Moly dust is good - not so sure about Moly granules...

You are welcome.

It depends on the form the moly is in and its hardness. If it is as hard as the metal and its size exceeds the film thickness, it would abrade the metal. If not, it won't.

Good moly will be 1 micron or less. This should not be abrasive so long as the product is properly formulated.

Industrial grade moly is at 7 microns. There are some companies that use this in their greases. Technical fine grade is at .7 microns. All of the high end greases use this. I don't know at what size moly becomes abrasive, but it will be larger than 7 microns.

Thanks everyone for the info...

I've been told that MoS2 is abrasive when it is broken down into SMALL particles. It is the crystal structure that gives MoS2 its "slip' and makes it a solid lubricant. Once it becomes nano-sized, it is ineffective as a solid lubricant and acts as an abrasive.

MoS2
http://en.wikipedia.org/wiki/Molybdenum_disulfide
The info on the wiki site explains how the MoS2 is arranged, a bit like a pack of cards, the weak molecular forces between the MoS2 sheets/"Cards" allows slippage and the MoS2 will perform very well as a solid boundary additive.
But
If the Moly particle is larger than the lubrication film and travels through the loaded bearings or gears on its edge where no slippage is possible then the particle is very robust and will cause abrasive wear. (The reason that MoS2 reinforced gear lubricant usually run @ 45-70 ppm Fe on spectrographic analysis from light abrasive wear.)
The best boundary additive form for MoS2 is fine platelet sized less than 3-4 µm which here was the Acheson Moly Dag 407 an extremely good boundary additive 3-4 years back.
Acheson was purchased 2-3 years ago and we noted that since that purchase there appears to be no MoS2 of the correct size spec available and the MoS2 offered is large, up to 60 µm per MoS2 particle and the planes that are meant to shear are convoluted and intertwined making it impossible to shear resulting in the incorrect MoS2 causing abrasive wear and damage.
We noted that our customers are filtering this incorrect sized MoS2 Gear Lubricants at 5 µm and once the larger MoS2 particles are removed the applications settle down to normal wear.
For MoS2 to be effective the particles must be platelet in shape and less than 3-4 µm in size
Regards
Rob S

The particle size is important since very fine abrasives are used to polish lenses of all types so there are no scratches. If the polish is too coarse the lens will have scratches, and if it is too fine, processing time will increase. In the polishing industry, it is very important to have a very narrow particle size distribution for maximum productivity. The disks used on computer hard drives are polished with very precisely milled abrasives. Abrasives in toothpaste are another example; they must be aggressive but not enough to remove enamel.

Thanks for everyone's post, i have learnt more on moly dust, wonderful coming.