*2.2.1.10 Extreme pressure (EP) additives*

Extreme Pressure additives are required to reduce friction, control wear and prevent severe surface damage in heavy duty application of gears and bearings at high temperatures and pressures. They are also known as antiscuffing additives. They react chemically with metallic surfaces to form a sacrificial surface film that prevents the welding and subsequent seizure of asperities at the metal-to-metal contact. Additionally, they contribute to smoothing of the surfaces as these are formed at contact asperities and the load is then distributed uniformly over a greater contact area, thus reducing the severity of wear and ensuring effective lubrication. Effectiveness of EP additives relies on their reactivity and their ability to readily form thick surface films at high loads and high contact temperatures that are created at the mechanical

contacts. These additives usually contain sulfur and phosphorus compounds and chlorine or boron compounds. Ashless EP additives such as dithiocarbamates, dithiophospates, thiolesters, phosphorothioates, thiadiazoles, aminephosphates, phosphites may be preferred in some applications where chlorine may cause corrosion.

Other additives such as demulsifiers, emulsifier, biocides are added to meet specific requirements. Emulsifiers are used as a binder between oil and water molecules in oil-water-based metal-working fluids to help create a stable oil-water emulsion. Without the emulsifier, oil and water will separate out from each other due to differences in specific gravity and interfacial tension. On the other hand, demulsifiers are used to separate oil-water emulsions. Demulsification and removal of the aqueous phase from the oil-based lubricant minimizes harmful effects such as corrosion, foaming and cavitation from occurring. Biocides may be added to waterbased lubricants to control the bacterial growth.

#### **2.3 Greases**

Lubricating grease are a class of lubricant that do not flow like a fluid but bleed (release oil) when squeezed between contacting surfaces. They have a gel like consistency and can be described as a solid or semifluid like material and in some cases can be used in vertical or overhead applications because they can have good drip resistance due to their Non-Newtonian rheological properties. They are particularly useful in applications that are sealed for life; for example bearings and remote gearboxes. Functionality of greases include sealing out contamination and water ingress, prevent corrosion, compatible with polymers and elastomers, provide antiwear and extreme pressure load protection while reducing friction. Greases have three main components: fluid, thickener and additives [13].

A typical grease consists of 75-95% fluid base stock, 2-25% thickener and 0-25% additives. The base stock is chosen based on the required applications. Hence, the fluid can be petroleum based for most automotive and industrial application, synthetic based for low and high temperature application or may be wax based (no flow) for high load caring capacity. The thickeners are metal soaps which are created using the fundamental reaction of an acid and a base. Calcium based soaps have been the simplest and earliest used thickeners with a maximum operating temperature between 60 and 70°C. In the last decade, calcium sulfonate greases, polyurea greases, aluminum complex greases, lithium complex greases, sodium complex greases, and clay-based greases have received general acceptance due to their higher service temperatures of more than 150°C. However, each having their own set of pros and cons. For example, calcium-based greases do not perform well over a wide range of temperature, sodium based have deteriorated performance in the presence of water. Clay and polyurea based greases are used for high temperature (service temperatures of 190-220°C) and application that have limited relubrication access. The third component of greases are additives. Commonly used additives are listed below.

**179**

*2.4.1 PTFE*

*2.4.2 Carbon based materials*

*Lubricant and Lubricant Additives*

• Odorants (perfumes)

• Dyes

**2.4 Solid lubricants**

*DOI: http://dx.doi.org/10.5772/intechopen.93830*

Tackifiers are additives that increase the adhesive property of the grease or the lubricant. They prevent the lubricant from flinging off the metal surface during rotational movement. To be acceptable to the manufacturers and the end users the greases must be free of offensive odor and have a desirable color. Therefore, odorants and dyes are added to the grease. Although these have little effect on the grease

Greases have the unique ability to incorporate liquid as well solid additives. Solid

Solid lubricants on their own are vital to niche applications such as space missions, satellites release and deployment mechanisms. Vacuum and microgravity of space eliminates the use of liquid lubricants. The importance of a thin film of solid lubricant can be emphasized on the fact that the success of an entire mission can be compromised if the, receiver and transmitter antennas or solar arrays packed securely during launch fail to release smoothly with precision due to extreme friction of a deployment mechanism. Solid lubricants can be used at low temperatures as well as at high temperatures where the liquid lubricant may solidify or vaporize respectively. Even at extreme pressures where liquid lubricant will be squeezed out, solid lubricants are used. However, solid lubricants are not limited to extreme

conditions [14]. A wide variety of low friction coatings are used in various engineering applications that require high electrical and thermal conductivities, low wear rates and high lubricity at all operating temperatures. Newer engineered coatings have increased complexity and have transitioned from single or multi component structures to nanostructured and functional gradient structures.

capacity. Such materials are called composite materials.

The most recognized polytetrafluoroethylene (PTFE) coating is Teflon® discovered in 1938 at DuPont. These are highly linear fluorocarbon molecules. They offer a low friction surface with moderate wear. They have low chemical reactivity and low surface energy. PTFE on its own performs best at low loads and wears rapidly at higher loads; hence they need reinforcements to increase strength and load bearing

Carbon based materials such as graphite in both micro and nano forms is a popular solid lubricant additive whereas diamondlike carbon (DLC) makes excellent low friction coatings. However, they have different mechanisms of

performance, their appeal to senses has an impact on the product selection.

additives such as molybdenum disulphide (MoS2), graphite, hexaboron nitride and polytetrafluoroethylene (PTFE) in the form of fine dispersed powder (nano and micro particles) have been used in lubricants and greases to provide ultralow friction and wear protection. Solid additives provide a physical separation between two contacting surfaces when fluid is unable to provide load support. The lattice structure of these solid lubricants plays an important role in transferring a thin low shear layer on the metal surfaces especially where load is high, and speed is low.

• Tackifiers (adhesive agents)


*Lubricant and Lubricant Additives DOI: http://dx.doi.org/10.5772/intechopen.93830*


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

based lubricants to control the bacterial growth.

have three main components: fluid, thickener and additives [13].

**2.3 Greases**

contacts. These additives usually contain sulfur and phosphorus compounds and chlorine or boron compounds. Ashless EP additives such as dithiocarbamates, dithiophospates, thiolesters, phosphorothioates, thiadiazoles, aminephosphates, phosphites

Other additives such as demulsifiers, emulsifier, biocides are added to meet specific requirements. Emulsifiers are used as a binder between oil and water molecules in oil-water-based metal-working fluids to help create a stable oil-water emulsion. Without the emulsifier, oil and water will separate out from each other due to differences in specific gravity and interfacial tension. On the other hand, demulsifiers are used to separate oil-water emulsions. Demulsification and removal of the aqueous phase from the oil-based lubricant minimizes harmful effects such as corrosion, foaming and cavitation from occurring. Biocides may be added to water-

Lubricating grease are a class of lubricant that do not flow like a fluid but bleed (release oil) when squeezed between contacting surfaces. They have a gel like consistency and can be described as a solid or semifluid like material and in some cases can be used in vertical or overhead applications because they can have good drip resistance due to their Non-Newtonian rheological properties. They are particularly useful in applications that are sealed for life; for example bearings and remote gearboxes. Functionality of greases include sealing out contamination and water ingress, prevent corrosion, compatible with polymers and elastomers, provide antiwear and extreme pressure load protection while reducing friction. Greases

A typical grease consists of 75-95% fluid base stock, 2-25% thickener and 0-25%

additives. The base stock is chosen based on the required applications. Hence, the fluid can be petroleum based for most automotive and industrial application, synthetic based for low and high temperature application or may be wax based (no flow) for high load caring capacity. The thickeners are metal soaps which are created using the fundamental reaction of an acid and a base. Calcium based soaps have been the simplest and earliest used thickeners with a maximum operating temperature between 60 and 70°C. In the last decade, calcium sulfonate greases, polyurea greases, aluminum complex greases, lithium complex greases, sodium complex greases, and clay-based greases have received general acceptance due to their higher service temperatures of more than 150°C. However, each having their own set of pros and cons. For example, calcium-based greases do not perform well over a wide range of temperature, sodium based have deteriorated performance in the presence of water. Clay and polyurea based greases are used for high temperature (service temperatures of 190-220°C) and application that have limited relubrication access. The third component of greases are additives. Commonly used additives are

may be preferred in some applications where chlorine may cause corrosion.

**178**

listed below.

• Antiwear additives

• Friction Modifier

• Extreme Pressure additives

• Rust and corrosion inhibitor

• Antioxidants

Tackifiers are additives that increase the adhesive property of the grease or the lubricant. They prevent the lubricant from flinging off the metal surface during rotational movement. To be acceptable to the manufacturers and the end users the greases must be free of offensive odor and have a desirable color. Therefore, odorants and dyes are added to the grease. Although these have little effect on the grease performance, their appeal to senses has an impact on the product selection.

Greases have the unique ability to incorporate liquid as well solid additives. Solid additives such as molybdenum disulphide (MoS2), graphite, hexaboron nitride and polytetrafluoroethylene (PTFE) in the form of fine dispersed powder (nano and micro particles) have been used in lubricants and greases to provide ultralow friction and wear protection. Solid additives provide a physical separation between two contacting surfaces when fluid is unable to provide load support. The lattice structure of these solid lubricants plays an important role in transferring a thin low shear layer on the metal surfaces especially where load is high, and speed is low.

### **2.4 Solid lubricants**

Solid lubricants on their own are vital to niche applications such as space missions, satellites release and deployment mechanisms. Vacuum and microgravity of space eliminates the use of liquid lubricants. The importance of a thin film of solid lubricant can be emphasized on the fact that the success of an entire mission can be compromised if the, receiver and transmitter antennas or solar arrays packed securely during launch fail to release smoothly with precision due to extreme friction of a deployment mechanism. Solid lubricants can be used at low temperatures as well as at high temperatures where the liquid lubricant may solidify or vaporize respectively. Even at extreme pressures where liquid lubricant will be squeezed out, solid lubricants are used. However, solid lubricants are not limited to extreme conditions [14]. A wide variety of low friction coatings are used in various engineering applications that require high electrical and thermal conductivities, low wear rates and high lubricity at all operating temperatures. Newer engineered coatings have increased complexity and have transitioned from single or multi component structures to nanostructured and functional gradient structures.

#### *2.4.1 PTFE*

The most recognized polytetrafluoroethylene (PTFE) coating is Teflon® discovered in 1938 at DuPont. These are highly linear fluorocarbon molecules. They offer a low friction surface with moderate wear. They have low chemical reactivity and low surface energy. PTFE on its own performs best at low loads and wears rapidly at higher loads; hence they need reinforcements to increase strength and load bearing capacity. Such materials are called composite materials.

#### *2.4.2 Carbon based materials*

Carbon based materials such as graphite in both micro and nano forms is a popular solid lubricant additive whereas diamondlike carbon (DLC) makes excellent low friction coatings. However, they have different mechanisms of

friction reduction. Graphite has hexagonal crystal structure which has the intrinsic property of easy shear. DLC exhibits high hardness and low friction due to an amorphous structure that combines graphitic and diamond phases. They can be doped with hydrogen or nitrogen for achieving desirable properties. Recently, a series of patents on superlubricity of nano-diamonds and graphene films have been filled [15].
