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POL 5. Part 2. Slide 11. Friction modifiers. These surface protecting additives are polar additives used to control friction, particularly sliding and rolling. As you can see from the illustration, they are attracted to the metal surfaces and work together to alter the friction characteristics of a lubricant. They smooth the transition from static to dynamic motion – easing the transition through boundary and mixed, to the hydrodynamic regime. If the pressure is light, then they produce a cushioning effect, but once the pressure increases, they are swept away, and the effects of the additives are no longer seen. Slide 12. Antioxidants. These types of additives, control and reduce the rate of Oxidation, or what is known as thermal degradation. They are sacrificial additives that delay the onset of oxidation and allow lubricants to operate at higher temperatures. In a machines hydraulic system or gearbox for example, a build-up of heat will accelerate oxidation. For every 10oC rise over 60oC, the oxidation rate doubles, and this increase will half the oils expected sump life. It can cause acids and sludge build-up, resulting in sticky surface deposits and increased viscosity of the oil. Varnish is also formed on machine components caused by oxidation and air entrapment. This can have an effect on the efficiency of the machines components and overall operation. Slide 13. Metal deactivators. These types of additives form protective, inactive layers on the metals surfaces to prevent corrosion. They react with the lubricant to reduce the catalytic effect, or passivate chemicals that could otherwise damage the metals surface layer. Additional protection is given to yellow metals (copper, zinc, lead alloys) and copper. Slide 14. Anti-foam additives. These additives are added to the lubricant to prevent the formation of a stable foam layer. The Presence of foam can be problematic. Instead of an oil layer lubricating the parts, you end up with a layer of air, which can cause cavitation, eventually leading to heat build-up and wear. The additives simply attach, or stick to the bubbles, then as the bubbles reach the surface they reduce the surface tension. The bubble then pops and the additive stays in the lubricant to continue work on other bubbles that are forming. A word of caution, over concentration of the additive can have an adverse effect, and can cause the build-up of a stable layer of foam. Typically, organically modified Silicone compounds are used. Slide 15. Tacky or adhesive additives. These additives are added to lubricants to prevent the lubricant from flinging or dropping off the surface being lubricated. They are called, Tackifiers. Lubricants with low “tackiness” would typically be used on flat / horizontal surfaces, for example, Lathe beds. Lubricants with high “tackiness” would be used on rotational or vertical surfaces, for example, chains or machine tool guides. Slide 16. Solid lubricants. These types of solid additives are introduced as a film between sliding and / or rolling surfaces for extreme operating conditions, such as very high or very low temperatures – over a wide range, for example –200 to 850◦C, and corrosive atmospheres. Such materials normally have a layered crystalline structure which ensures low shear strength, thereby minimising friction. Typical examples of these are graphite, molybdenum disulphide and PTFE. They can be used in lubricating oils and greases. Slide 17. Thank you. You have completed this module.