Has anyone read a clear and understandable explanation of why ZDDP is favored in oils for high contact pressures, but MoS2 is favored in greases?
Sample quotes
MoS2 with particle sizes in the range of 1–100 µm is a common dry lubricant. Few alternatives exist that can confer the high lubricity and stability up to 350 °C in oxidizing environments. Sliding friction tests of MoS2 using a pin on disc tester at low loads (0.1–2 N) give friction coefficient values of <0.1.
Molybdenum disulfide is often a component of blends and composites where low friction is sought. A variety of oils and greases are used, because they retain their lubricity even in cases of almost complete oil loss, thus finding a use in critical applications such as aircraft engines.
Examples of some diverse applications of MoS2-based lubricants include two-stroke engines (e.g., motorcycle engines), automotive CV and universal joints, ski waxes,[10] and even some bullets.
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The main use of ZDDP is in anti-wear additives to lubricants such as greases, gear oils, and motor oils, which often contain less than 1% of this additive. It has been reported that zinc and phosphorus emissions may damage catalytic converters and standard formulations of lubricating oils for gasoline engines now have reduced amounts of the additive, though diesel engine oils remain at higher levels. Crankcase oils with reduced ZDDP have been cited as causing damage to, or failure of, classic/collector car flat tappet camshafts and lifters which undergo very high boundary layer pressures and/or shear forces at their contact faces, and in other regions such as big-end/main bearings, and piston rings and pins. Roller camshafts are more commonly used to reduce camshaft lobe friction in modern engines.
From
http://en.wikipedia.org/wiki/Molybdenum_disulfide#Petroleum_refininghttp://en.wikipedia.org/wiki/Zinc_dithiophosphate
