Tagged: strategic reliability solutions

The Influence of Lubricant Selection on Degradation

Guidelines should always be followed when selecting a lubricant for a particular application. OEMs will have specific criteria ranges for specialty applications that must be satisfied. Some general guidelines which should be considered can be summarized in the table below based on the listed mechanisms above.

Based on the three listed mechanisms above, one can identify that choosing a lubricant can impact the type of degradation which occurs during its lifetime. As such, when selecting lubricants, it is critical to note their applications and the conditions they will endure.

Having a history of lubricant failures for particular equipment can also assist in this regard by informing users of past failure trends. Therefore, when selecting a lubricant, operators can be more mindful of the properties which should not be compromised during the selection process.

The process of troubleshooting degradation in lubricants has been covered in detail in the book, “Lubrication Degradation – Getting Into the Root Causes” by Bob Latino and myself, published by CRC Press, Taylor and Francis.

Want to read the entire article? Find it here in Precision Lubrication Magazine!

Which Degradation Mechanism Is Affected?

My previous article published in Precision Lubrication covered six degradation mechanisms: oxidation, thermal degradation, microdieseling, electrostatic spark discharge, additive depletion, and contamination.

Upon further investigation, there are only three mechanisms where selecting the correct lubricant will impact the degradation mode. These are; oxidation, microdieseling, and electrostatic spark discharge. The properties of the lubricant can easily influence each of these degradation mechanisms.

When selecting a lubricant, especially for rotating equipment, one of the critical areas of importance is the performance of the antioxidants. When formulated, oils must be balanced to protect the components in various aspects.

Thus, some oils that boast a high level of antioxidants may suffer from low levels of antiwear, or these increased levels can react with other components to reduce the performance of the oil. During oxidation, antioxidants are depleted at an accelerated rate which can lead to lube oil varnish. Hence, the choice of lubricant can influence this degradation mechanism.

A good trending test, in this case, would be the RULER test to accurately quantify and trend the remaining useful antioxidants for the oil. This test can easily distinguish and quantify the type of antioxidant rather than providing an estimate of the oxidation, as with the RPVOT test.

It has been noted that oils with an RPVOT of more than 1000 mins have a low reproducibility value which can mislead users during trending of lubricant degradation. Corrosion inhibitors, not just antioxidants, have also influenced the RPVOT values. Thus, there are better tests for monitoring the presence of antioxidants and helping operators to detect the onset of possible lube oil varnish.

On the other hand, during microdieseling, entrained air can lead to pitting the equipment’s internals and eventually the production of sludge or tars depending on whether the entrained air experiences a high or low implosion pressure.

If bubbles become entrained in the lubricant and do not rise to the surface, this can directly result from the lubricant’s antifoaming property. The antifoaming property is essential when selecting an oil, especially for gearboxes. Typically, OEMs will have recommendations for their components that should be followed.

Another degradation mechanism that can be influenced by lubricant selection is electrostatic spark discharge. This mechanism occurs when the lubricant accumulates static electricity after passing through tight clearances. These then discharge at the filters or other components inside the equipment, providing sharp points or ideal areas to allow static discharge.

This is frequently seen in hydraulic oils due to the very tight clearances within the equipment. If fluid conductivity is above 100 pS/m, the risk of static being produced is reduced. Some OEMs also provide particular values the lubricant should meet for this property.

 

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Has the Lubricant Failed the Equipment, or Has the Equipment Failed the Lubricant?

Many lubrication engineers are faced with finding the most appropriate lubricant for an application. Therefore, they are tasked with selecting the “right” lubricant; subsequently, their decision can influence several outcomes.

A lot of the positive results are in the realm of extending the life of the oil, providing better energy efficiency, and even saving costs associated with downtime. However, can the choice of an “incorrect” lubricant impact its degradation process or lead to the presence of lube oil varnish?

Has the Lubricant Failed the Equipment, or Has the Equipment Failed the Lubricant?

Lubricants provide many different functions. These can range from moving heat or contaminants away from the components, minimizing wear and friction, improving efficiency, providing information about the status of the lubricant, or even transmitting power, as is the case with hydraulic oils.

There has been the time aged question of whether a lubricant fails the equipment or the equipment has failed the lubricant. If a deeper dive is performed into this question, one can deduce that lubricants are engineered to withstand particular conditions.

Once those conditions are met, lubricants can perform their intended functions. However, if the conditions exceed the tolerances of the lubricant, then one will notice a faster degradation. In this case, the environment and its conditions have failed the lubricant.

On the other hand, lubricants are designed to be sacrificial and are used up while in service. Hence, it is normal to see additives’ values deplete when trending oil analysis values, especially for turbine oils. Quite notably, additives responsible for antiwear or extreme pressure will decrease over time as they protect the components.

For this instance, the lubricant would have been performing its function until it could no longer do so or has reached its end of life. The conditions in the environment cannot be blamed for the lubricant failing. This is the nature of the lubricant.

Lubricant condition monitoring lets analysts detect whether a lubricant is undergoing degradation and can even help determine some areas where it has begun to fail. For instance, if the RULER® test can quantify the remaining antioxidants in an oil. Analysts can easily interpret its results to determine if the process of oxidation is occurring within that lubricant.

Similarly, an FTIR test can detect whether contaminants are present in the lubricant or if the additive packages have become severely depleted. These tests all aid in allowing analysts to successfully determine whether or not a lubricant is performing at its full functional capacity.

 

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How Do Lubricant Additives Work?

Each additive works differently to produce its function on the base oil and the overall finished lubricant. This section will explore how each of the lubricant additives works and some of the challenges they may experience.

Pour Point Depressants

As noted above, the pour point depressants help control the flow of the lubricant. This is achieved by modifying the wax crystals present in the lubricant’s base oil. At lower temperatures, the liquid usually has trouble being poured due to the presence of wax molecules in the base oil1.

There are two main types of pour point depressants, namely;

  • Alkylaromatic polymers adsorb on the wax crystals as they form, thus preventing them from growing and adhering to each other. This effectively controls the crystallization process and ensures the lubricant can be poured.
  • Polymethacrylates co-crystallize with wax to prevent crystal growth.

While these additives do not entirely prevent wax crystal growth, they lower the temperature at which these rigid structures are formed. These additives can achieve a pour point depression of up to 28°C (50°F); however, the common range is typically between 11-17°C (20-30°F).

Solubility thresholds may limit the use of this type of additive to achieve the desired effect on the base oil.

VI Improvers

These additives are typically long-chain, high-molecular-weight polymers that change their configuration in the lubricant based on temperature4. When the lubricant is in a cold environment, these polymers adopt a coiled form to minimize the effect on viscosity. On the other hand, in a hot environment, they will straighten out, allowing the oil to produce a thickening effect.

While it is more desirable to use high molecular weight polymers (since they provide a better thickening effect), these long-chain molecules are also subject to degradation due to mechanical shearing. Therefore, a balance must be reached between the molecular weight and shear stable service condition.

Another challenge for formulators is to balance the polymer’s tendency to shear with the expected viscosity thickening due to oxidative processes and the viscosity thinning due to the dilution of fuel1.

Friction Modifiers

These usually compete with the antiwear and extreme pressure additives (and other polar compounds) for surface room. However, they become activated at temperatures when the AW and EP additives are not yet active. Thus, they form thin mono-molecular layers of physically adsorbed polar soluble products or tribochemical friction-reducing carbon layers, which exhibit a lower friction behavior than AW and EP additives2.

There are different groups of friction modifiers based on their function. Some are mechanically working FMs (solid lubricating compounds, e.g., Molybdenum disulfide, graphite, PTFE, etc.), adsorption layers forming FMs (e.g., fatty acid ester, etc.), tribochemical reaction layers forming FMs, friction polymer forming FMs and organometallic compounds.

Defoamants (Antifoam)

When foam forms in the lubricant, tiny air bubbles become trapped either at the surface or on the inside (called inner foam). Defoamants work by adsorbing on the foam bubble and affecting the bubble surface tension. This causes coalescence and breaks the bubble on the lubricant’s surface1.

For the foam that forms at the surface, called surface foam, defoamants with a lower surface tension are used. They are usually not soluble in base oil and must be finely dispersed to be sufficiently stable even after long-term storage or use.

On the other hand, inner foam, which is finely dispersed air bubbles in the lubricant, can form stable dispersions. Common defoamants are designed to control surface foam but stabilize inner foam2.

Oxidation Inhibitors

As noted above, antioxidants are usually deployed during the propagation phase to neutralize the scavenging radicals or decompose the hydroperoxides3. There are two main forms of antioxidants: primary and secondary antioxidants.

Primary antioxidants, also known as radical scavengers, remove radicals from oil. The most common types are amines and phenols.

Secondary antioxidants are designed to eliminate peroxides and form non-reactive products in the lubricant. Some examples include zinc dithiophosphate (ZDDP) and sulphurized phenols.

Mixed antioxidant systems also exist where two antioxidants have a synergistic relationship. One example is the relationship between phenols and amines, where phenols deplete early during oxidation while amines deplete later. Another example is using primary and secondary antioxidants to remove radicals and hydroperoxides.

Rust and Corrosion Inhibitors

Rust and Corrosion inhibitors are usually long alkyl chains and polar groups that can be adsorbed on the metal surface in a densely packed formation of hydrophobic layers.

However, this is a surface-active additive, and as such, it competes with other surface-active additives (such as antiwear or extreme pressure additives) for the metal surface. There are two main groups for corrosion additives: antirust additives (to protect ferrous metals) and metal passivators (for non-ferrous metals2).

Rus inhibitors have a high polar attraction to metal surfaces. They form a tenacious, continuous film that prevents water from reaching the metal surface. It must also be noted that contaminants can introduce corrosion into an oil, just as organic acids are produced.

Detergents and Dispersants

Detergents are polar molecules that remove substances from the metal surface, similar to a cleaning action. However, some detergents also provide antioxidant properties. The nature of a detergent is particularly important as metal-containing detergents produce ash (typically calcium, lithium, potassium, and sodium)1.

On the other hand, dispersants are also polar, and they keep contaminants and insoluble oil components suspended in the lubricant. They minimize particle agglomeration, which in turn maintains the oil’s viscosity (compared to particle coalescing, which leads to thickening). Unlike detergents, dispersants are considered ashless. They typically work at low operating temperatures.

Antiwear Additives

These are typically polar with long chain molecules that adsorb onto the metal surfaces to form a protective layer. This can reduce friction and wear under mild sliding conditions. Usually, these additives are formed from esters, fatty oils, or acids, which can only work at low or moderate levels of stress within the system.

The most common form of antiwear is ZDDP, which is used in engine or hydraulic oils. On the other hand, an ashless phosphorus type of antiwear also exists for systems that require that characteristic, and tricreysl phosphate is the usual choice.

Extreme Pressure Additives

Since extreme pressure additives only become active when higher temperatures or heavier loads are on a system, they have earned the name “Anti-scuffing additives.”

Unlike antiwear additives, extreme pressure additives react chemically with the sliding metal surfaces to form relatively insoluble surface films. This reaction only occurs at higher temperatures, sometimes between 180-1000°C, depending on the type of EP additive used1.

It must be noted that even with the presence of EP additives in a lubricant, there will still be some wear during the break-in period as the additives have yet to form their protective layers on the surfaces.

EP additives must also be designed for the system they protect as different metals have varying reactivity (EP additives designed for steel-on-steel systems may not be appropriate for bronze systems as they are not as reactive with bronze).

EP additives also contribute to polishing the sliding surfaces as they experience the most significant chemical reaction when the asperities are in contact and the localized temperatures are at their highest. They tend to be created from compounds containing sulphur, phosphorus, borate, chlorine, or other metals4.

Do Lubricant Additives Degrade Over Time?

As noted earlier, most additives can deplete over time as they get used up in their various functions. Antiwear and rust protection additives continuously coat the surfaces of the interfacing metals.

This can cause their initial concentrations to decrease over time until it reaches a point where the concentration of the additive is too low to offer any protection. In this case, it has not degraded but depleted.

In earlier years, there used to be prevalent issues with the separation of additives from the finished lubricant due to filtration. However, with the evolution of technology and better practices, this is no longer a common problem operators face.

In the past, operators would notice frequent clogging of their filters and subsequent reduction of additive concentrations, rendering the oil unprotected. It was common to notice additives settling to the bottom of a drum of oil after standing still for some time.

In essence, lubricant additives do not really degrade over time; rather, their concentrations get depleted, which assists in the lubricant degrading faster than a finished lubricant with higher additive concentrations.

Innovation and Future Trends for Additives

What does the future look like for additives within our industry? Will they go away completely?

From my estimations, we’re a long way from that happening. The lubricant industry has evolved over the years, with many advances from the chemical side, which has developed better-suited additives, and the OEM side, which has pushed the chemists to develop lubricant additives that can adapt to equipment changes.

OEMs are creating more components that can withstand higher temperatures, increased pressures, and more demanding environments. Lubricants must also be developed for this specific use, and additive technology will continue to evolve as these boundaries are pushed.

We are also being driven towards more environmentally friendly products, and additives are also on that list. Most of the metals used in the production of additives (such as EP or AW additives) are toxic to the environment, and alternatives are being discovered.

In the field of tribology, there has also been continued research into ways of reducing friction and wear. This is coupled with research into the interaction of varying surfaces and ways lubricants can effectively reduce the coefficient of friction, leading to increased energy efficiency and fuel efficiency in some cases.

Lubricant additives will be around for some time as everything that moves needs to be lubricated, and base oils do not have all the required properties to handle varying temperatures and other conditions that the machine encounters.

While their structure will change to adapt to provide a more environmentally friendly impact, their functions will also evolve based on their future requirements.

References

1 Bruce, R. W. (2012). Handbook of Lubrication and Tribology, Volume II Theory and Design, Second Edition. Boca Raton: CRC Press.

2 Mang, T., & Dresel, W. (2007). Lubricants and Lubrication – Second Completely Revised and Extended Edition. Weinheim: WILEY-VCH GmbH.

3 Livingstone, G., Wooton, D., & Ameye, J. (2015). Antioxidant Monitoring as Part of Lubricant Diagnostics – A Luxury or a Necessity?

4 Pirro, D. M., Webster, M., & Daschner, E. (2016). Lubrication Fundamentals – Third Edition Revised and Explained. Boca Raton: CRC Press.

Want to read the entire article? Find it here in Precision Lubrication Magazine!

What are the types of Lubricant Additives?

There are many types of lubricant additives, and various formulations exist from different suppliers. In this section, we will cover the most common additives found in finished lubricants.

Pour Point Depressants

All liquids have a particular temperature at which they can effectively flow. The liquid’s viscosity and current temperature determine how quickly it moves. As the name implies, this type of additive can assist in lowering the temperature at which the lubricant flows1.

VI Improvers

This should not be confused with Pour Point Depressants. Viscosity Index Improvers are also known as Viscosity Modifiers2. They assist the lubricant in increasing its viscosity at higher temperatures, allowing lubricants to operate in wider temperature ranges.

Friction Modifiers

When two surfaces rub against each other, friction is formed. Depending on the type and extent of friction, some surfaces can experience welding and even adhesive wear. This is where friction modifiers can help by reducing frictional forces associated with stick-slip oscillations and noises.

Defoamants (Antifoam)

Some lubricants succumb to foam being created in their systems. When foam is made, it significantly impacts the functions of the lubricant and can lead to excessive wear due to lack of lubrication (they disrupt the surface of the lubricant), cavitation (due to the presence of air bubbles), and even increased oxidation (due to presence of air trapped in the system). Foam can also affect the ability of a liquid to transfer heat or cool. Defoamants or antifoam additives reduce the amount of foam being produced.

Oxidation Inhibitors (Antioxidants)

Oxidation occurs in most lubricants. During the oxidation process, free radicals emerge, propagating to form alkyl or peroxy-radicals and hydroperoxides, which eventually react with others to form oxidation by-products. During the propagation phase, antioxidants are usually deployed to neutralize the free radicals or decompose the hydroperoxides3. As such, these additives are sacrificial in nature, as they protect the base oil from oxidation by being depleted.

There are many types of antioxidants, including phenolics and aromatic nitrogen compounds, hindered phenols, aromatic amines, zinc dithiophosphates, and a couple of others.

Rust and Corrosion Inhibitors

If oxygen and water are present at a location containing iron, then rust can be formed. Corrosion affects the non-ferrous metals in the presence of acids in the lubricant1. Most pieces of equipment succumb to rust and corrosion quite easily, so these inhibitors were developed to mitigate these effects by forming protective layers on the surfaces of the equipment.

Detergents and Dispersants

These two often get confused as they usually work together to prevent deposits from accumulating in the oils. Detergents neutralize deposit precursors (especially in engine oils), while dispersants suspend the potential sludge or varnish-forming materials4.

Antiwear Additives

Antiwear additives reduce friction and wear, especially during boundary lubrication conditions. They are designed to reduce wear when the system is exposed to moderate stress2.

Extreme Pressure Additives

Extreme Pressure additives are usually confused with antiwear additives, or the names are used interchangeably. However, extreme pressure additives begin to work when the system experiences high stress and try to prevent the welding of moving parts, unlike antiwear additives, which work when the system experiences moderate stress.

 

Why Do We Need Lubricant Additives?

Lubricants keep the world turning. Once something moves, a lubricant should be present to reduce friction or wear between the surfaces. But what makes lubricants so unique in our industry? Is it just the base oil?

No, this is where the power of lubricant additives truly shines, an area many overlook.

Why Do We Need Lubricant Additives?

Before getting into the world of additives, let’s step back to the basics: why are they needed? A lubricant is composed of base oil and additives. Depending on the type of oil, different ratios of additives will be used for the various applications. Additionally, each Lubricant OEM will have its unique formula for its lubricant.

To simplify this, we can think of making a cup of tea. The first thing we need is some hot water in a cup. This can be our base oil. It can be used on its own (some people drink hot water or use it for other purposes), but if we want to make a cup of tea, we must add stuff.

Depending on the purpose for which you’re drinking the tea, you may choose a particular flavor. Perhaps peppermint for improved digestion or to help improve your concentration or chamomile to keep you calm.

These flavors can represent the various types of oils: gear oils, turbine oils, or motor oils. Different blends are suited for different applications.

Now, while we’ve added the tea bag to the hot water (and some people can drink tea like this), others need to add sweetener or milk. These are the additives to the base oil (hot water).

Depending on the preference of the person drinking the tea, there will be varying amounts of sweetener (honey, stevia, or sugar) and varying amounts of milk (regular, low-fat, oat, dairy-free). The combinations are endless!

The same can be said of additives in finished lubricants. Depending on the type of oil (tea flavor, think gear or turbine oil) and its application (the person drinking the tea, with dietary preferences of being dairy-free or sugar-free), the combination of lubricant additives and their ratios will differ. The percentage of additives can vary from 0.001 to 30% based on the type of oil.

Additives have three main functions in a finished lubricant. They can;

  • Enhance – improve some of the properties of the base oil
  • Suppress – reduce some of the characteristics of the base oil
  • Add new properties – introduce new features to the base oil

The finished lubricant will have properties from the base oil and additives combined.

Want to read the entire article? Find it here in Precision Lubrication Magazine!

The Difference Between Antiwear and Extreme Pressure Additives

The terms antiwear additives and extreme pressure additives are often used interchangeably, suggesting that they provide the same functions in a lubricant. This is not exactly true. While there are many similarities in how they function, both additives have distinct functions in protecting lubricants.

Both are film-forming additives (Bruce, 2012). Their functions are to reduce wear between two contacting surfaces or reduce friction to lower the heat produced between the two rubbing surfaces.

They can also be classified as boundary additives that can be temperature-dependent (EP additives) or non-temperature-dependent (Antiwear additives). They both function to mitigate against wear, which is usually caused during boundary lubrication where higher speeds, loads, or temperatures can cause contact with the asperities.

One of the significant differences, as noted by Mang & Dresel, 2007 is that antiwear additives are designed to reduce wear when the system is exposed to moderate stress. On the other hand, EP additives are much more reactive. These are used when the system’s stress is very high to prevent the welding of moving parts.

According to (Bruce, 2012), there are four main groups of commercially available EP additives based on the structures containing phosphorus, sulphur, chlorine, and overbased sulfonates. He explains that the phosphorus, sulphur, and chlorine-containing EP additives are activated by heat over a range of temperatures.

For instance, chlorine-containing EP additives are usually activated between 180-240°C, phosphorus-containing additives are activated at higher temperatures, and sulphur-containing additives operate at 600-1,000°C.

On the other hand, overbased sulfonates contain a colloidal carbonate that reacts with iron to form a thin-film barrier layer between tribocontacts. This protects the surface from direct contact and welding.

As we can see, antiwear and EP additives protect the surfaces between which the lubricant exists. However, they are activated differently and subsequently perform two different functions.

Antiwear additives protect against wear and are not temperature dependent, while EP additives are activated by high stress to prevent the welding of moving parts.

Both functions are essential to protecting the system from additional wear and ensuring it remains operational.

 

Want to read the entire article? Find it here in Precision Lubrication Magazine!

 

References

Bloch, H. (2009). Practical lubrication for industrial facilities, Second edition. Lilburn: Fairmont Press Inc.

Bruce, R. W. (2012). Handbook of Lubrication and Tribology, Volume II, Theory and Design, Second Edition. Boca Raton: CRC Press Taylor and Francis Group.

Coyle, C. L., Greaney, M. A., Stiefel, E. I., Francis, J. N., & Beltzer, M. (1991, Feb 26). United States of America Patent No. 4,995,996.

Mang, T., & Dresel, W. (2007). Lubricants and Lubrication, Second, Completely Revised and Extended Edition. Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA.

Mortier, R. M., Fox, M. F., & Orszulik, S. T. (2010). Chemistry and Technology of Lubricants, Third Edition. (C. Bovington, Ed.) Dordrecht Heidelberg: Springer Science+Business Media B.V. doi:10.1023/b105569_3

Pirro, D. M., Webster, M., & Daschner, E. (2016). ExxonMobil, Lubrication Fundamentals, Third Edition, Revised and Explained. USA: CRC Press Taylor and Francis Group.

Zhang, J., & Spikes, H. (2016). On the Mechanism of ZDDP Antiwear Film Formation. Tribol Lett, pp. 1–2.

Types Of Antiwear Additives and How They Work

There are many types of antiwear additives, but they typically all fall under the category of polar materials such as fatty oils, acids, and esters, according to Pirro, Webster & Daschner, 2016. According to Mortier, Fox, & Orszulik, 2010, several compounds can form surface films to help protect against friction and wear.

These include:

  • Oxygen-containing organic compounds (with a polar head that can adsorb to surfaces). These can include alcohols, esters, and carboxylic acids.
  • Organic compounds containing nitrogen groups
  • Organic sulphur compounds which can form reacted films at surfaces
  • Organic phosphorus compounds which can form reacted films at surfaces
  • Organic boron compounds which may form reacted films at surfaces
  • Organic molybdenum compounds which can form MoS2 film on surfaces
  • ZDDPs, which can form polymeric films on surfaces

While this is an exhaustive list, the more popular ones are listed below. In this next part of the article, we will also dive into how they function.

Organic Oxygen Compounds

According to Mortier, Fox, & Orszulik, 2010, these compounds usually include esters, alcohols and acids. These are generally responsible for improving the “oiliness” or reducing the friction for most lubricants. However, how does this work?

Carboxylic acids form metallic soaps with the contacting surfaces. According to Mortier, Fox, & Orszulik, 2010, some evidence suggests that the upper limit of friction coincides with the melting point of the metal soap. As such, when the upper limit of friction is reached, the metallic soap melts, protecting the surface and performing its antiwear function.

Interestingly, there has been a debate concerning whether these long-chain surfactant friction modifiers reduce friction by forming adsorbed films of monolayer thickness or if they form thick films equivalent to several or many multilayers.

Again, as per Mortier, Fox, & Orszulik, 2010, after experimenting, it was concluded that some of these types of additives form thick boundary films while others do not.

The thick boundary films result from the formation of insoluble iron (II) oleate on the rubbing surfaces. For metal oleates, this will only occur for metals lower than iron in the electrochemical series.

Thus, when speaking about organic oxygen compounds, they help to reduce the friction in lubricants by forming layers on the contacting surfaces.

Organophosphorus Esters

These types of esters have long been used as antiwear additives, according to Mortier, Fox, & Orszulik, 2010. There are two different types of reaction films which are typically formed:

  • Films derived from tricresyl phosphate which form thin films (0.1-2nm) consisting of low shear strength FePO4 and FePo4.2H2O
  • Films consisting of iron (III) monoalkyl/aryl phosphate oligomers are thicker (approximately 100-300nm) and polymeric.

It is important to note that for the tricresyl phosphate (TCP) to be effective, the presence of oxygen, water, and other polar impurities is necessary to form the reaction film. Typically, the hydrolysis of the ester occurs initially, which releases phosphoric acid. This is then critical in the formation of the surface oxide film.

Another noteworthy function of the ester of phosphoric acid is that it helps ensure the solubility of the product in the oil. It can also aid in rust protection by hydrolysis to the phosphoric acid.

During the formation of the film, there is a loss of an alkyl group by hydrolysis, which generates two P-O ligands for coordination. This phosphate anion, which was formed, has reduced oil solubility, which allows for the boundary layer of oil covering the metal surface.

Eventually, as the polymer continues growing, the film moves from a soft, viscous liquid to that of a glass-like solid. This glass-like solid allows the surfaces to stay separated, thus reducing wear.

Essentially, organophosphorus esters form films that can either be very thin or thicker and glass-like, depending on their nature. While they act as antiwear additives, they can also perform the function of rust inhibition in the appropriate environments.

Molybdenum Sulfur

Coyle et al., Patent No. 4,995,996, 1991 recognize Molybdenum disulphide as a lubricant additive and discuss its origins. They mention that molybdic xanthine typically decomposes under particular conditions to form the molybdenum sulfide on protected materials. The use of thiosulfenyl xanthates has also been formulated for particular ashless lubricants.

As per Mortier, Fox, & Orszulik, 2010, compounds such as MoDTC (molybdenum dithiocarbamate) or MoDDP (molybdenum dithiophosphate) typically react with the surfaces to produce the famous molybdenum disulphide. In this compound, there is an ease of shearing, which leads to unusually low coefficients of friction.

A synergistic relationship exists between MoDTC and ZDDP. While MoDTC does not form low friction layers independently, these layers are only formed when ZDDP is present. The layer of MoS2 is only formed on top of the glass of ZDDP reaction products. The ZDDP layer acts as a source of sulphur, reduces the oxidation of MoS2 and limits the diffusion of sulphur from MoS2 into the ferrous substrate.

Interestingly, Molybdenum disulphide (also commonly known as “Moly”) is extremely popular in grease applications especially in the mining industry. “Moly” is known for being a solid additive to grease thickeners for specific applications.

As seen above, it may not exactly be “Moly” added to the lubricant, but rather, it is only created when its parent compound decomposes and is formed.

Zinc Dialkyldithiophosphates (ZDDP)

These are the most commonly used antiwear additives on the market and are known by their chemical abbreviation ZDDP. Originally, ZDDP was developed as an antioxidant additive. However, it has been used in many applications, such as engine, hydraulic, and even circulating oils, as both an antiwear and antioxidant additive.

According to Bruce, 2012, The Ecole Centrale de Lyon / Shell Corporation collaboration made significant conclusions on ZDDP performance. This study shows that ZDDP produces a thin film of iron sulfide and zinc sulfide nearest to the metal surface. Next, there is a zinc polyphosphate layer, made up of long-chain zinc polyphosphates and then soluble alkylphosphates, closest to the oil layer.

According to Zhang & Spikes, 2016, at very high temperatures (above 150°C), ZDDP reacts slowly to form films on solid surfaces. This occurs despite the absence of rubbing and is called “thermal films .” However, at lower temperatures (below 25°C) in the presence of rubbing films in a ZDDP lubricant, these ZDDP films are generated more rapidly. These are called “tribofilms”. Based on analysis, it is suggested that both films have similar structures.

It has also been shown (through inelastic electron tunneling spectroscopy, IETS with Yamaguchi and Ryason) that secondary ZDDP is adsorbed much more readily than primary ZDDP. On the other hand, alkaryl ZDDP is hydrolyzed on adsorption onto aluminum oxide surfaces.

According to Mortier, Fox, & Orszulik, 2010, ZDDP reduces wear by forming relatively thick boundary lubrication films. These are usually 50-150nm thick and are based on a complex glass-like structure (as mentioned earlier). The figure below, taken from Mortier, Fox, & Orszulik, 2010, shows the structure of this ZDDP glass film.

Structure and composition of a ZDDP glass film (taken from Mortier, Fox, & Orszulik, 2010)

The strength of the ZDDP’s antiwear function lies in the structure of the alkyl groups. Chain branching and chain length have critical roles in this determination. Short-chain primary alkyl groups are more reactive than long primary alkyl groups.

As Mortier, Fox, & Orszulik, 2010, explain, the ZDDPs most efficient at antiwear film formation typically suffer depletion due to thermal effects. Under very high temperatures and/or long drain service, the most active ZDDP may not provide the best wear protection.

 

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What Are Antiwear Additives?

As the name suggests, antiwear additives help to prevent wear in one way or another. However, what makes them unique compared to other additives in lubricants? Why are they used more predominantly in specific applications than other applications? This article explores antiwear additives, why they are needed, and how they work.

What Are Antiwear Additives?

According to (Bloch, 2009), antiwear agents can also be called mild EP (Extreme Pressure) additives. In some cases, they may also act as antioxidant additives (depending on their chemical structure). In essence, antiwear additives protect against friction and wear when the surfaces experience moderate boundary conditions.

During moderate boundary conditions, the full film of the lubricant has not yet formed, and asperities on both surfaces can come into contact with each other. As such, antiwear additives can also be called boundary lubrication additives.

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Usually, these antiwear additives react chemically with the metal to form a protective layer. This layer or coating will allow the two surfaces to slide over each other with low friction and minimal metal loss. As such, antiwear additives have also adopted the nickname “anti-scuff” additives.

According to Pirro, Webster, & Daschner, 2016, the adsorbed film on metal surfaces is formed from long-chain materials. In these cases, the polar ends of the molecules attach to the metal while the projecting ends of the molecules remain between the surfaces.

Under mild sliding conditions, wear is reduced; however, under severe conditions, molecules can be rubbed off such that the wear-reducing effect is lost. When this happens, it is evident in the oil analysis data with the presence of wear metals in large quantities.

In essence, antiwear additives help protect the oil while reducing friction, protecting the surfaces, and, in some cases, enhancing the oil to be more resistant to oxidation. While they can perform these functions, it must be noted that there are many different types of antiwear additives.

 

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Measuring Oil Viscosity

The viscosity of oil is one of its most essential characteristics. Thus, it is important to understand how this is measured and quantified. There are two main types of viscosity, dynamic (or absolute) and kinematic viscosity.

The dynamic viscosity measures the force required to overcome fluid friction in a film and is reported in centipoise (cP) or in SI units Pascal Seconds (Pa s) where 1 Pa s = 10 P (Poise). It can also be considered the internal friction of a fluid. This is usually used for calculating elastohydrodynamic lubrication related to rolling element bearings and gears.

On the other hand, the kinematic viscosity of a fluid is the relative flow of a fluid under the influence of gravity. Its unit of measure is centistokes (cSt) which in SI units is mm2/s, 1cSt = 1 mm2/s. (Mang & Dresel, 2007).

Kinematic Viscosity = Dynamic Viscosity / Density

Other units of measure for viscosity include; Saybolt, Redwood, and Engler, but these are less widely used than the cSt or cP, especially for lubricants.

Oil Viscosity Grades and Standards

One of the best-kept secrets about viscosity is that a particular grade often represents a range. When oils are classified, one may see an ISO 32 or ISO 220 and believe that the oil will have this exact viscosity (32 cSt or 220 cSt). However, this is not the case.

There are three general classifications where viscosity grades have particular ranges based on the fluid type. The fluid may behave differently in each application, hence the need for these three scales. However, there is a chart that allows users to convert the various scales into the one needed.

Engine Oil Classification (SAE J300)

As per the Society of Automotive Engineers (SAE), the SAE J300 standard classifies oils for use in automotive engines by viscosities determined at low shear rates and high temperature (100°C), high shear rate and high temperature (150°C) and both low and high shear rates at low temperature (-5°C to -40°C) (Pirro, Webster, & Daschner, 2016).

Engine manufacturers have widely used this system to aid in designing lubricants suited for these applications. As such, oil formulators also adhere to these classifications when engineering lubricants.

One will note the use of the suffix letter “W” in some of the grades below. These oils are intended for low ambient conditions, whereas those without the “W” are intended for oils that will not encounter low ambient conditions.

These are commonly described as multigrade (where the “W” is found between two numbers) and monograde oils (where the “W” is at the end or the grade is identified by a number only) in the table below.

The table shows that the viscosities must fall within a particular range to be classified. For instance, a 5W30 oil should meet the specifications of:

  • Low temperature, Cranking viscosity of 6600 cP at -30°C
  • Low temperature, Pumping Viscosity Max with No Yield Stress of 60,000 cP at -35°C
  • Low shear rate Kinematic viscosity at 100°C should be between 9.3 -12.5 cSt
  • High Shear rate viscosity at 150°C Max at 2.9 cP

One will notice the range of 9.3 to 12.5 cSt (at 100°C). This is where oils can be blended to either end of this scale but still achieve the classification of a 5w30 oil.

Axle and Manual Transmission Lubricant Viscosity Classification (SAE J306)

As per the SAE recommended practice J306, automotive manual transmissions and drive axles are classified by viscosity, measured at 100°C (212°F), and by the maximum temperature at which they reach the viscosity of 150,000 cP (150 Pa s) when cooled and measured in accordance to ASTM D2983 (Method of Test for Apparent Viscosity at Low Temperature Using Brookfield Viscometer). (Pirro, Webster, & Daschner, 2016).

The table below shows that for an SAE grade of 190, the kinematic viscosity must fall within the range of 13.5 to 18.5 cSt at 100°C. While most viscosities tend to fall mid-range of these values, it also indicates that if the lubricant achieves 18 cSt at 100°C, it can still be classified at an SAE grade 90.

The most common multigrade lubricants within this grade fall within the 80w90 or 75w140 classifications.

Another factor for these types of lubricants is API GL4 or GL5 ratings. It must be noted that a GL5 lubricant is recommended for hypoid gears operating under high-speed, high-load conditions.

On the other hand, a GL4 lubricant is usually recommended for the helical and spur gears in manual transmissions and transaxles operating under moderate speeds or loads. These should not be used interchangeably as the GL5 lubricants tend to adhere to the surfaces and may cause more damage in a GL4 application.

Figure 5: Automotive Gear Lubricant Viscosity Classification. Source: Lubrication Fundamentals, Third Edition Revised and Expanded by Pirro D. M., Webster M., and Daschner E. pg 52

Viscosity System for Industrial Fluid Lubricants

This classification was jointly developed by the ASTM and STLE (Society for Tribologists and Lubrication Engineers). Initially, the system was based on viscosities measured at 100°F but converted to viscosities measured at 40°C.

ASTM D2422 and ISO 3448 are the references for this system. In this system, it is clearer to see the variances in the ranges of viscosities. In this case, the mid-point of the range is used as the ISO viscosity. To determine the range of any ISO viscosity, one can calculate ±10% of the mid-point value to get the minimum and maximum values of the range.

Figure 6: Viscosity System for Industrial Fluid Lubricants. Source: Lubrication Fundamentals, Third Edition Revised and Expanded by Pirro D. M., Webster M., and Daschner E. pg 52

All of these systems can be represented in the figure below, where it is easy to calculate the oil viscosity using another system:

Figure 7: Various viscosity systems in one chart.