- freely available
Appl. Sci. 2017, 7(5), 445; https://doi.org/10.3390/app7050445
1.1. Tribology and Lubrication Engineering
1.2. The Functions of Lubricants
- Controlling friction—This is the primary role of lubricants. Reducing friction and preventing wear and seizure (or failure) are necessary in most applications. Well-controlled, high friction is required for clutches, breaks, etc. Since this is the central objective of the lubricant, we discuss the phenomena with a lubrication model (Stribeck curve, see Figure 1). Optimized performances with minimized side effects could be achieved by proper selection of lubricants for specific machine elements. As modern machines are required to work in a more energy efficient way, the roles of lubricants are increasing. The importance of additives is emphasized (see Section 2.1 tribo-improvers).
- Cooling the contact—Heat generated by rubbing motion can have many negative influences on surface materials, such as transformation of microstructure or thermal failure. Ageing of lubricants is accelerated at higher temperatures. Heat accumulation could be prevented by circulating a liquid lubricant. The heat capacity of a lubricant is the controlling factor for this function. This function is mainly supported by the properties of base fluids. Some additives contribute to the heat conductivity of primary (see Section 2.5.2) or secondary (see Section 2.3.5) base fluids.
- Cleaning the contact—Wear particles, external dusts, or deposits by aged lubricants could appear during machine operation. These contaminants negatively influence lubrication performances. Circulating a lubricant can wash out these nuisances physically. Advanced lubricants contain some substances to help the cleaning process (see Section 2.3.2 and Section 2.3.3).
1.3. The Stribeck Lubrication Model
1.4. Tribo-Chemical Reactions
- Heat induces various chemical reactions. It was estimated that rubbing surface could reach as high as 250–450 °C . Friction generates heat and this is considered as the major cause of tribo-chemical reactions.
- Nascent surfaces could be exposed by wear of solid surfaces under mixed and boundary conditions. Similarly, a mechanical stress to crystalline materials can sometimes cause lattice defects. These produce a chemically active site on the rubbing surfaces . The chemical activity of transition metals is induced by vacant d-atomic orbital.
- Exo-electrons could be emitted from rubbing surfaces. Those electrons have low energy to promote chemical reactions but can produce radical intermediates for further chemical reactions .
- Elevated pressure up to Giga-Pascal (>104 bar) could be generated at tribo-contact . Since chemical reactions are initiated by a collision of molecules, compression of reactant raises the probability of the collision. Chemical reactions at high pressure under static conditions are well known .
- Orientation of molecules may occur if they were flown through a narrow area . This means the reacting functional groups are close to each other and increases the probability of reaction. This, together with elevated pressure, contributes to the entropy factor of the reaction.
- Shearing can dissociate chemical bonds in a molecule directly. It is a well-known phenomenon that molecular mass of polymers can decrease under shearing conditions. The formation of radical species by the dissociation of a carbon–carbon bond had been reported . Typical shear rate at tribological contact is in the range of 105–107 s−1 .
1.5. Components in Liquid Lubricants
1.6. Category of Lubricant Additives
1.6.1. Working Function Category
- Tribo-improvers (tribology-improving additives) directly contribute to improve the tribological performances of the lubricant. In short, they are responsible for the primary role of lubricants (see Section 1.2). Friction modifiers (FM), anti-wear agents (AW), extreme pressure additives (EP) are representative ones. They stand at a central position in the technology of lubricant additives.
- Rheo-improvers (rheology-improving additives) concern the fluidity of the base oil. Viscosity modifiers (VM) are the main additives in this group. Pour point depressants (PPD) make the base oil applicable in a chilly environment. They indirectly contribute to the lubrication performances, mainly in hydrodynamic regime.
- Maintainers help to keep the substances (both lubricant and materials of machine elements) in good condition through preventing the degradation of substances participating in the lubrication system. They mainly contribute to prolonging the lifetime of the lubrication system and partly contribute to the lubrication performances in some cases. Antioxidants (AO) play a decisive role in preventing the ageing process of lubricants. Detergents and dispersants can mitigate the negative influences of contaminants on lubrication. Corrosion inhibitors (rust preventives) can protect the tribo-materials from corrosion. Air bubbles may be incorporated in lubricants during machine operation. They cause lubricant starvation at the contact and promote the autoxidation processes. Anti-foam agents (foam breaking agents, defoamers) can break bubbles. Water is a ubiquitous contaminant in most applications. It drops the viscosity of the lubricant and causes ageing of both lubricants and materials. Demulsifiers (emulsion breaking agents) are beneficial for separating contaminated water in a lubricant.
- Auxiliaries are sometimes incorporated with specific purpose in addition to above additives.
1.6.2. Working Site Category
- Interface agents work at the interfaces between different phases. Lubricated contacts involve solid (tribological material)—liquid (lubricant) interfaces. Examples are tribo-improvers as the main actors in this category. Corrosion inhibitors deactivate the surfaces to be attacked by corrosive matters. A lubricant may intake air bubbles while circulating, and thereby develops various forms. The interaction of foam decomposing additives at the border between gas–liquid phases can destroy the bubbles.
- Bulk agents concern the properties and/or stability of liquid as uni-phase matter. Rheo-improvers are representative examples of this group. Antioxidants (except metal deactivators), and other auxiliaries are in this group.
1.6.3. Working Mechanism Category
- Chemical additives are those additives that undergo chemical reaction(s) while working. A chemical reaction is defined as a rearrangement of electrons that bind atoms in a molecule. Chemical changes at interfacial phenomena in tribology are mostly irreversible, although there is a certain possibility of reversible reactions in the liquid phase. The initiation of chemical reaction needs much more energy than physical phenomena. Examples are anti-wear agents that provide boundary film on rubbing surfaces through tribo-chemical reactions. Antioxidants also belong to this group.
- Physical additives are those substances that work without any chemical changes. Examples are nano-particle additives such as tribo-improvers and viscosity modifiers. Physical changes need less activation energy than chemical changes and are usually reversible processes. They last longer than the chemical ones.
2. Individual Lubricant Additives and Their Working Mechanism
2.1.1. Friction Modifiers (FM)
2.1.2. Anti-Wear Agents (AW)
2.1.3. Extreme Pressure Additives (EP)
2.2. Rheo-Improvers (Rheological Properties Improvers)
2.2.1. Viscosity Modifiers (VM)
2.2.2. Pour Point Depressant (PPD)
2.3.1. Antioxidants (AO)
- Thermal dissociation of a carbon–hydrogen bond to yield hydrogen radical and carbon radical.
- Shear stress can dissociate a carbon–carbon bond though direct mechanical forces, yielding two carbon radical intermediates. Polymers can decompose through this mechanism.
- Wear of tribo-materials exposes nascent surfaces that may catalyze the dissociation of a carbon–hydrogen bond.
2.3.4. Corrosion Inhibitor, Rust Preventive
2.3.5. Anti-Foam Agent (Defoamer)
2.3.6. Demulsifiers (Emulsion Breaking Agents)
2.4. Multi-Functional Additives
2.5.2. Conductivity Improver
3. Evaluation of Tribo-Improvers
- Configuration of tribo-contact: a point contact generates high contact stress while line and square contact generate low contact stress. A careful setup of the test specimen is needed to ensure the alignment of contact. This influences the repeatability of the test considerably. Ball-on-flat type point contact provides a comparatively easy setup.
- Type of relative motion: Reciprocation sliding, unidirectional sliding, rolling, and a combination of rolling and sliding are most commonly employed.
- Tribo-materials: Various materials are used in different machine elements. Simple tribo-tests are beneficial for evaluating the compatibility of tribo-improvers with specific materials in machine elements. Surface roughness of the materials should be considered.
- Operating conditions: Load, velocity, and temperature of lubricant should be controlled in a proper way.
- Testing environment: Should be kept away from contaminants as much as possible to obtain the results with good repeatability. Humidity of room air can influence the tribological properties.
4. Discussion as Multi-Component Systems
4.1. Formulation of a Lubricant
4.2. The Quality of Each Ingredient
4.3. The Interaction of Ingredients
- Synergy is defined as the combination improves BOTH friction and wear compared to each ingredient alone.
- Enhanced performance of one while scarifying the other seems to have an improved effect but should not be defined as synergism.
- Antagonism is any other phenomena except the above two.
4.4. Additive Technology for Synthetic Fluids
4.5. Tribo-Improvers for Non-Ferrous Materials
4.6. Additive Technology for Environmentally Acceptable Lubricants
Conflicts of Interest
|API||The American petroleum Institute, http://www.api.org/|
|ASTM||The American Society for Testing and Materials, http://www.astm.org/|
|AW||Anti-wear, anti-wear additive|
|BF||Base fluid (base oil)|
|EP||Experme pressure, extreme pressure additive|
|FM||Friction modifying, fricition modifier|
|ICE||Internal conbustion engine|
|PPD||Pour pont depressant|
|STLE||The Society of Tribologists and lubricationEngineers, http://www.stle.org/|
|VM||Viscosity modifying, Viscosity modifier|
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|Saturates, Mass%||Aromatics, Mass%||Sulfur, ppm||Examples|
|Group I||Solvent-refined mineral oil||65–85||15–35||300–3000||80–119||Extraction of impurities by solvents|
|Group II||Hydro-processed mineral oil||≧93||<7||5–300||80–119||Decomposition of organic sulfides by catalytic hydrogenolysis|
|Group III||Hydro-cracken mineral oil||≧95||<5||0–30||≧120||Isomerization of hydrcarbons|
|Group IV||Oligomers of 1-alkene||-||-||-||-||Synthetic hydrocarbons,so-called PAO(poly alpha-olefin) in the market|
|Group V||All fluids not included in groups I–IV||-||-||-||-||Non-PAO synthetics, plant oils, and some low quality mineral oils|
|Working Mechanism||Working Site|
|R&D Phase||Focus||Test Category||Sample Lubricant||Test Equipment||Outcome||Test Periods||Cost of Evaluation|
|Incubate-break through||Possibility||Laboratory test||Specific components||Universal tribo-tester||The role component|
|Improve||Applicability||Prototype||Tribo-tester according to industrial standards||Technical benefit|
|Optimize||Feasibility||Bench test||Full formulated||Machine component||Engineering benefit|
|Verity& tune||Productivity||Field test||Real machine||Industrial benefit|
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