Friction modifiers

R, mst inhibitor O, oxidation inhibitor D, detergent—dispersant VI, viscosity-index improver P, pour-point depressant W, antiwear EP, extreme pressure F, antifoam and M, friction modifier. 

Metals such as copper, iron, or a combination of the two, usually modified with tin, bismuth, and/or lead ate used as binders of sintered friction materials where deformation under the high forming pressure is requited to lock together the property modifiers within a matrix. Metals such as copper, iron, zinc, aluminum, and occasionally lead are also used as friction modifiers. 

A liquid oil, an emulsifier, and a friction modifier, which includes certain polyether compounds, can be added to a drilling fluid consisting of a water-in-oil emulsion formed from a brine [1155]. The friction modifier serves to decrease the coefficient of friction of the well drilling fluid. Decreasing the coefficient of friction lowers the force required to turn the drill bit in the hole. Gravitational forces increase the coefficient of friction in deviated-, horizontal-, and extended-reach wells. 

G. A. Malchow, Jr. Friction modifier for oil-based (invert) well drilling fluids and methods of using the same. Patent US 5593953, 1997. 

Friction modifiers Compounds like fatty acids form physisorbed layers on the metal surfaces. They reduce friction under conditions of mixed lubrication and help to avoid stick-slip motion. 

A) Surface protective additives (i) antiwear (AW)/extreme-pressure (or temperature) improver (EP), (ii) anticorrosion and rust inhibitor, (iii) detergent and dispersant, and (iv) friction modifier. Examples of protective additives are listed in Table 2.4. 

One oil contained a soluble molybdenum friction modifier additive while the other did not. The IR bands in the 1200 to 1100 cm 1 and 650 to 600 cm 1 regions have been assigned to the symmetric PO 2 stretching and symmetric bending modes, respectively, observed in divalent cations in metaphosphate glasses. 

Both XPS and Auger analyses were performed for tribofilm. The Auger results revealed higher concentration of Fe, Zn, Mg and P, but lower levels of carbon relative to XPS. Phosphorus is present in phosphate form and sulfur as sulfide in unmodified oil. The oil containing a soluble molybdenum friction modifier additive formed films which were thinner and less continuous than films formed from the unmodified oil. The film was dominated by magnesium phosphate which was identified by AES, and by the combination of XPS and IR. Sulfur was present as sulfate and sulfide while molybdenum was present as Mo+4 and Mo+6, as shown in Fig. 4.8 (Lindsay et al., 1993). 

Fig. 4.8. XPS Mo3d spectra from the film produced from engine oil containing a soluble molybdenum friction modifier additive from the outer ring region (A), the middle ring region (B), and the center ring region (C), as well as the spectrum of the agent, M0S2 (D), (Lindsay et al., 1993)...

The lubrication system is extremely complex. The mechanism of lubrication is partly dictated by the nature of interactions between the lubricant and the solid surface. Additives blended into lubricating oil formulations either adsorb onto the sliding surfaces, eg., fatty alcohols, fatty amines, amides, phosphoric acid esters (friction modifiers), or react with the surface, eg., ZDDP, MoDTC, MoDDP organic phosphates (extreme pressure). Some interactions affecting the surfaces of metals include adsorption, chemisorption, and tribochemical reactions-these form new compounds on the surface and lubrication by reaction products (Bhushan and Gupta, 1991 Briscoe et al., 1973 Briscoe and Evens, 1982 Heinicke, 1984 Hsu and Klaus, 1978 and 1979 Klaus and Tewksbury, 1987 Lansdown, 1990 Liston, 1993 McFadden et al., 1998 Studt, 1989). 

Phosphorus is present in zinc dialkyldithiophosphate and deactivate platinum catalysts in engine exhaust systems. Using zinc, antimony and oxothiomolybdate dialkyl-dithiocarbamate (MoDTC) complexes alone or in combination with other lubricating oil additives appears to solve this problem. Antimony complexes are used also as extreme pressure agents while molybdenum ones as friction modifier additives (Hill et al., 1994). Analyses of Zn(dtc)2 decomposition using the thermogravimetric method, flash vacuum pyrolysis, and four-ball machine... 

Sensitivity of exhaust - after treatment devices. It is clear that excessive deposition of phosphorus and sulfur on the catalyst can cause the reduction in system efficiency. Oil phosphorus contaminant comes from the oil additive ZDDP. The reduction in its use adversely affects both antiwear and antioxidation performance. Sulfur comes from the base oil, antiwear additives, detergents, organomolybdenum friction modifiers, and from the fuel. There is strong pressure from OEMs to reduce the sulfur level of the fuel, and to reduce the sulfur contamination of the catalyst, which results from presence of sulfur in oil. 

These organo-molybdenum compounds are in commercial use, for example as friction-modifiers. Their mechanism of action is considered in Chapter 9, in connection with the in situ production of molybdenum disulphide films. 

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