From "The Oil Analysis Handbook"

Soot is abrasive. Iintuitively one might think of carbon black as being relatively 'soft'. However, soot particles, in any form, are harder than steel. Motor oils are formulated with additives that keep soot in suspension and stop it from agglomerating but, as with other additives, they are sacrificial. Once a certain level of soot loading is reached (typically around 3 percent by mass for most common motor oils), the soot particles will no longer be able to be carried by the oil and sludgy deposits will form. Soot is not only abrasive but it also causes the viscosity of the oil to increase.

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"What is a friction modifier? How do these modifiers help in industrial gear oil?"

Friction modifiers and mild antiwear agents are polar molecules added to lubricants for the purpose of minimizing light surface contacts (sliding and rolling) that may occur in a given machine design. These are also called boundary lubrication additives. Esters and natural and synthetic fatty acids, and some solid materials such as graphite and molybdenum disulfide, are used for these purposes.

These molecules have a polar end (head) and an oil-soluble end (tail). Once placed into service, the polar end of the molecule finds a metal surface and attaches itself. If one could 'see' the orientation of the molecules on the surface, it would appear something like the fibers of a carpet, with each molecule stacked vertically beside the others.

As long at the frictional contact is light, these molecules provide a cushioning effect when one of the coated surfaces connects with another coated surface. If the contact is heavy, then the molecules are brushed off, eliminating any potential benefit of the additive.

When the machine designer anticipates more than light surface contact (from shock loading for instance) then the designer would select a stronger type of friction modifier characterized as an antiwear additive. Zinc dialkyldithiophosphate is a common antiwear agent. This type of additive literally reacts with the metal surface when the reaction energy (temperature) is high enough. The reaction layer provides sacrificial surface protection.

As the loading and metallic contact increase, the strength of additive and the strength of the reaction process increases. This leads to the use of sulphur-phosphorus based EP chemicals. The EP additives form organo-metallic salts on the loaded surfaces that serve as sacrificial films to protect against aggressive surface damage.

Mike Johnson, Noria Corporation

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