Lubrication

component to break quickly with a few cycles before failure. In this condition, plasticity is induced after every cycle, and the wear particle develops after many cycles have been accumulated. In the initial cycles, only shallow grooves caused by plastic deformation are created rather than wear particles.

(d) Corrosive wear

Both wear and corrosion wear mechanisms are involved when a material deteriorates due to corrosion wear. Both wear and corrosion actions can potentially cause significant material loss or damage. When facing both systems simultaneously, the consequences could be more severe. When sliding occurs, especially in corrosive liquids or gases, reaction products are typically generated on the surface through chemical or electrochemical exchanges. The wear process is virtually comparable to the specimen if these reaction products have a high interfacial bonding and behave like bulk material. The reaction products produced when solid materials interact with the corrosive medium cause wear, which typically differs substantially from the composite [13]. The periodic film formation and elimination by corrosive and abrasive action, respectively, are the foundation of the corrosive wear model. The pattern of material loss resembles that of corrosive wear. The tribo-chemical process at the contact surface is sped up when two objects come into touch with corrosive media. The primary corrosive substance in the atmosphere is oxygen, and oxidative wear of metallic materials in the air is the word most usually used to describe this process. The presence of oxygen in the oil is a common cause of corrosive wear. It has been demonstrated that iron oxide makes up most of the wear debris for steel lubricated contacts and that removing oxygen from the oil eliminates wear.

2.3. Lubrication

lubricant's primary function is to prevent contact between the moving or sliding surfaces, which reduces friction and the material damage it causes - adding specific additives, which may affect either some physical characteristics or some elemental composition, enhance the quality of the lubricant.

There are primarily two types of fluid films: the first occurs when fluid is injected between mating surfaces, and enough pressure is generated to separate them. Because the two surfaces in this type of lubrication are apart, wear and friction are often relatively minimal. In the other type of lubrication, the lubricant forms a weak layer that may support the load by adhering to the surface.

The distance between bodies is known as the layer width h, and it helps to lessen friction by minimising early mechanical breakdown of the materials. The hmin and the equivalent objects' degree of roughness (σ), which is the geometric mean of the roughness Ra of the two surfaces, are what determine if a film is present [1]:

𝜎 = [𝑅 + 𝑅 ] ( 2.13 )

Specific thickness, often known as the lambda ratio λ, is a dimensionless parameter determined from hmin and σ. It is described as the ratio [1]:

𝜆 =ℎ

𝜎 ( 2.14 )

Here Rqi is the standard error of the i-th layer in contact's surface heights. Typically, the lubrication schedules are chosen based on λ as follows:

1. 𝜆 ≥ 3: Hydrodynamic lubrication 2. 0.1 < 𝜆 < 3: Mixed lubrication 3. 𝜆 ≤ 0.1: Boundary lubrication

All the different lubrication regimes are explained in section 4.2 related to the Stribeck curve.

2.3.1 Types of lubricants

Based on their state, lubricants may be divided up into the following categories [15]:

 All liquid lubricants, such as mineral, natural, synthetic, and emulsion oils, are referred to as lubricating oils

 Grease, a form of semi-solid solid lubricant, is frequently comprised of a detergent emulsion with natural resource

 All lubricants in solid forms, such as powders, coatings, and composites, are referred to as solid lubricants (graphite, graphene, molybdenum disulfide)

 Air is the most common type of gaseous lubricant, but any gas can be used.

Liquid lubricant is the most popular form. Lubricating films need to sustain the pressure across the contact surface, keep them apart, and lower the opposition to moving or rolling at the junction. This approach is based on fluid mechanics, such as viscosity connection to temperature and pressure.

2.3.2 Properties of liquid lubricants

o Viscosity: Viscosity is a characteristic of liquids that allows them to oppose their flow.

o Density: a way to gauge how tightly packed the bulk of an object or material is.

o Flash and Fire points: the coldest value at which lubricating oil produces sufficient vapours to burn up when a small flame is introduced close to it is known as the flash point. The fire point, on the other hand, is the coldest value that allows the vapours of the lubricating oil to ignite for a minimum of five seconds when a small flame is present. A suitable lubricant must have a flash point higher than the operating temperature, and the latter prevents the possibility of fire during lubricant use.

o Cloud and Pour point: the cloud point refers to the temperature at which, after being gradually cooled, lubricating oil starts to appear cloudy or hazy. The pour point, instead, is described in section 4.7.3. Low pour points are necessary for lubrication oils used in machinery that operates at lower temperatures; otherwise, the machine will jam up as the lubricant oil solidifies.

o Aniline point: It is described as the lowest equilibrium solution temperature for samples of equal quantities of lubricating oil and aniline. It suggests that the lubricant oil in touch with the rubber sealing package may be degrading.

o Corrosion stability: It is calculated by performing a corrosion test in which a polished copper strip is submerged in lubricating oil for a predetermined time at a predetermined temperature. A good lubricant should not impact the corrosion strip.