*2.2.1.3 Anti-wear agent*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

*2.2.1 Types of lubricant additives*

these additives, have pourpoints as low as −32°C.

*2.2.1.2 Viscosity index improvers*

*2.2.1.1 Pour point depressants*

fier, biocides may be added depending on the intended applications.

of lubricant additives are discussed. Other additives such as demulsifiers, emulsi-

As the name indicates, these are additives that reduce the pour point of the lubricant, i.e. the lubricant remains in liquid state and maintains its fluidity (pourability) at lower temperatures than without these additives. Usually as temperature decreases, paraffin molecules in the oil start to crystallize as wax (below 50°C) and the oil loses its ability to flow by gravity or to be pumped under pressure. This also affects the viscosity of the oil. Additives such as alkylaromatic polymers and polymethacrylates prevent wax crystal growth by modifying the interface between the wax and the oil molecules, to a certain extent thus lowering the pour point by about 20–30°F (11–17°C). These are present up to a fraction of a percent in all paraffinbased lubricants that lubricate machine elements such as bearings, gears exposed to cold start and cold (winter) operating temperatures. Modern multi-grade engine oils/motor oils composed of partly synthetic oil and partly mineral oil along with

Viscosity index improvers (VII) also known as viscosity modifiers are additives that prevent the oil from losing its viscosity at high temperatures which is a natural tendency of any liquid. These additives are available in all shapes and sizes and quality [8]. Polymethylmethacrylates, olefin copolymers, hydrogenated poly(styrene-co-butadiene or isoprene), esterified polystyrene-co-maleic anhydride are commonly used VIIs. The large oil soluble flexible polymer molecules uncoil and spread out as temperature increases thereby increasing the viscosity as shown in **Figure 2**. Furthermore, their numerous branches entangle with those of other neighboring molecules. By doing this, these macromolecular structures can trap and

control smaller oil molecules, thus increasing the viscosity of the lubricant.

Permanent and temporary shear thinning of VII-thickened formulations can also occur depending upon the quality of the VII. In heavy duty application, due to the large compressive pressure between the two mating surfaces, VII polymer molecules, tend to align with each other and get "squashed" or even get chopped to small pieces under high shear conditions. When the polymer coils elongate and become aligned in the direction of the flow the viscosity temporarily drops resulting in reduced oil film thickness. After the lubricant leaves the contact between

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**Figure 2.** *Mechanism of VII.*

Anti-wear additives are additives that prevent two-body wear of the metallic countersurfaces in the boundary lubrication regime where the film thickness is small and there is asperity - asperity contact. These additives are polar in nature which enables them to attach to the metallic surfaces followed by tribochemical or mechanochemical reactions to form an anti-wear film. This newly formed film undergoes wear and formation at the top layers thus protecting the underlying metallic surface. As these additives form films by chemical reactions, they get used up and the amount of antiwear additives present in the lubricant reduces with time. These are typically phosphorous compounds. Zinc dialkyldithiophosphate (ZDDP) is the most common, the most researched and has been used since the 1940s [9]. Its use has been reduced in passenger vehicles in the last decade due to zinc metal causing poisoning of the catalyst in the exhaust gas catalytic convertor. ZDDP also provide antioxidant and corrosion-inhibition properties to the lubricant. Owing to the multi functionality of ZDDP, finding its replacement has been challenging because molybdenum-based additive such MoDTC (molybdenum dithiocarbamate) or MoDDP (molybdenum dithiophosphate) molecules cannot work as antioxidant. On the other hand, ash-less antiwear additives such as hindered phenols and amines are very expensive and are required in larger quantities. Till date, ZDDP is considered as the most cost-effective antioxidant and antiwear additive available, and the alternatives are currently very expensive.
