Antiwear additives boundary lubrication

Antiwear Compounds. Additives are used in many lubricating oils to reduce friction, wear, and scuffing and scoring under boundary lubrication conditions, ie, when fuU lubricating films cannot be maintained. Two general classes of materials are used to prevent metallic contact. 

Zinc dialkyldithiophosphates (ZDDPs) function mainly as antioxidants and antiwear additives. Molecules of ZDDPs adsorb on metal surface to participate in surface tribofilm formation under conditions of boundary lubrication. The solid tribofilms are formed at the metal surface to protect even under conditions of coarse contact under load (Bom et al., 1992). 

The crown-ether compounds as boundary lubricants and antioxidation additives. On the sliding surface, bromobenzo-15-crown-5 coordinates with ferrous ions and forms a strong reaction layer which protects the underlying metal surface. In the base stock solution, the crown ring can capture the metal ions which catalyze the oxidation of oil formulation (Brois and Gutierrez, 1987, 1989, 1992 and 1994 Le Suer and Norman, 1965 and 1966 Moreton, 1998). Bromobenzo-15-crown has excellent antiwear, antifriction and antioxidation properties, better than the ZDDP tested. 

A reciprocating wear tester was used to investigate the nature of antiwear boundary lubrication films formed by several ZDDP additives in mineral oil. The surface films were analyzed by SEM-EDX, EPMA and scanning Auger. The antiwear film contained S, O, Zn, and P elements with trace amounts of Fe and C. Once antiwear films are formed they can be removed by enhanced roughness and the presence of hydroperoxides (Sheasby et al., 1990). 

An important aspect of the function of compounded lubricants is to increase the load that can be carried by machinery without catastrophic damage to the rubbing components. Since the typical antiwear additives affect the viscosity of the carrier oil very little, it is not a fluid film effect that is responsible for the load-carrying augmentation. Examination of the various basic wear processes leads to the choice of the adhesive mechanism as the one most likely to respond to the action of boundary or extreme-pressure additives. The type of macroscopically observed severe wear which has this mechanistic process as its primary cause is generally designated as icu i ng (c(S. Chapter 13, Sections 13.4 and 13.6), and it is in this sense, as a description rather than a definition, that the term scuffing is used in the discussion to follow. 

The other surface-active chemical components include antiwear additives, friction modifiers, and antifoam additives [15]. The functions of these other lubricant additives, as their names imply, are quite different from these two surfactants. For example, antiwear additives, such as zinc dial-kyldithiophosphates, are used to prevent wear on engine parts friction modifiers, such as glycerol monooleate, are used to reduce boundary friction in engine operation to improve fuel economy antifoam additives such as polymethylsiloxane are used to inhibit foam formation. Excessive foam formation in lubricants can cause inadequate oil supply to certain parts of the engine and a high rate of oil loss. 

In the boundary lubrication regime, the antiwear and Mction-reduction processes are associated to the build-up of a tribologic film resulting from the chemical reaction between oil additives and sliding surfaces in contact [1-3]. The case of zinc dialkyl dithiophosphate (ZDDP) additive has been extensively studied by analytical transmission electron microscopy (ATEM) and the morphology, nature, structure and tribologic properties of the tribochemical film were obtained [4-6], The Figures 4.1 and 4.2 summarize the main results. 

Li et al. [6] have discussed the mechanism of DDP-capped copper and silver nanoparticles as oil additives. In their studies, the traditional electrical contact resistance (ECR) measuring technique, which has been applied to various studies of the contact of tribo-pairs in mixed lubrication or in boundary lubrication, has been used to investigate the in situ formation of the boundary film of metal nanoparticles. It is easy to understand that the ECR value between metallic tribo-pairs is very high under hydrodynamic lubrication, but very low (<0.1 ohm) when metallic contact occurs. Of course, the ECR value is larger than several thousands of ohms when a nonmetallic layer exists on top of specimens. For boundary lubrication of a metallic interface with AW/EP additives such as ZDDP blended in lubricating oils, antiwear efficiency relies on the reaction film formation, which acts also as an insulating barrier for electrical current. The ECR can therefore be used to monitor the formation process of the reaction film. 

The role of antiwear additives in boundary lubrication has been mentioned briefly in the general presentation (Section 1). The way in which zinc dithio-phosphate (ZDDP) acts as an antiwear additive is almost certainly that of the elimination of the abrasive wear contribution of crystallized iron oxide species as a result of their tribochemical reactions with the additive molecules and/or various degradation products. The use of XAS and TEM/EELS together with complementary CEMS and AES represents a powerful analytical combination for the elucidation of the tribochemical reactions responsible for antiwear mechanisms. The first part of this section summarizes analytical results obtained with friction tests carried out in the ambient atmosphere. The second part describes some original results obtained with friction tests and in situ post mortem AES analysis in ultrahigh vacuum. In this way the intrinsic frictional properties of the solid reaction film itself, without the effect of the environment, and the ability of friction to modify the nature and the structure of the surface films are investigated. 

It is worth stressing that the evolutions of the friction coefficient under mixed and boundary conditions can be quite different when different lubricants are used [16-17] the presence of antifriction and antiwear additives in particular can alter the shape of these curves [18-21]. In particular the value of the friction coefficient under boundary lubrication conditions can vary depending on many factors, such as materials, roughness and boundary layer properties [22-26]. 

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