KEY CONCEPTS
• The electrification of motor vehicle drivetrains is one driver of the need for better lubricating greases.
• The technical understanding of how grease lubricates machine parts must be further developed to meet market needs for greases that perform in extreme environments. 
• Now, more than ever, greases need to be human- and ecofriendly, with a low carbon footprint and no critical supply chain issues.
• Decreasing frictional losses through improved lubricants has the potential to reduce carbon dioxide (CO2) emissions and reduce climate change.

While the mass market for lubricating oils is decreasing, the importance and use of lubricating greases is increasing. A primary reason for this change is the electrification of motor vehicle drivetrains. Electrification reduces the need for engine oils, reduces the volume of gear oil needed and requires less cooling lubricant for machining the engine components. Many lubricant manufacturers see the future shifting to lubricating greases as more and more powerful and specialized products are required to meet industry demands.

Despite this predicted shift to lubricating greases, the technical understanding of how grease lubricates machine parts is limited. It has been assumed that the oil contributes to lubrication, and the thickener only provides mechanical stability. However, today the thickener components are known to also make a significant contribution to lubrication. Questions remain regarding how a lubricating grease changes over its service life, which parameters have significance for a particular application and what influence temperature has on lubrication. Compared to the testing of oils, there are very few special test methods that allow statements to be made about the suitability of greases in practical applications and not just in rolling bearings.

Grease changes that limit service life
Thomas Litters, senior expert lubricating greases corporate fellow with Fuchs Schmierstoffe GmbH in Mannheim, Germany, identifies rheological properties, chemical properties and mechanical impurities as impacting grease service life. 

Litters says rheological properties include the grease’s consistency, yield point and shear viscosity, as well as oil bleeding. Yield point and, to a lesser extent, shear viscosity refer to the balance between protecting a greased tribosystem from leakage and the need to relubricate it as “very often, greases start first to soften and later to harden.” Oil bleeding refers to the separation of the oil and thickener that can occur as a grease ages. Litters notes: “Oil bleeding is needed to provide all kinds of tribocontacts with a sufficient amount of lubricant and so to avoid starvation as much as possible.”

In contrast to rheological properties, Litters lists oxidation stability, thermal stability, tribochemical additives and, in special cases, (i.e., esters) hydrolytic decomposition as chemical properties important to service life. Oxidation stability refers to the “degradation of thickener and oil, sometimes polymerization to too high viscosities.” Thermal stability includes “oil loss due to oxidative base oil evaporation.” Tribochemical additives can degrade to the point where they have no positive effect. In some cases, additive reaction products can become aggressive to grease thickeners. This also can shorten grease life if additives were not properly selected. Finally, in special cases, hydrolytic decomposition can occur in a grease when water is present in a system.

Litters lists mechanical impurities that are introduced to the grease during use as another way to limit the grease service life. These can include wear debris, pollution or environmental impacts to an operating system.

Dr. Erik Kuhn is a professor at the Tribology Research Center (TREC) in the Institute of Engineering Design and Product Development at the Hamburg University of Applied Sciences in Hamburg, Germany. Kuhn notes that a number of mechanisms are responsible for limiting the service life of grease. One important mechanism he identifies “is the change of the grease structure or structural degradation.”

Upcoming challenges for greases
Litters notes that new types of greases are absolutely needed: “The main challenge in the grease industry is to replace lithium-based thickeners, which still represent more than 75% of the entire global grease market.” He says, “There is a current global trend to replace lithium-based thickeners with calcium sulfonate and polyurea thickeners; however, these alternatives have a limited sustainability as long as they rely on the petrochemical industry (i.e., sulfonates) and energy intensive chemical routes (i.e., urea).”

Dr. Markus Matzke, senior expert for lubrication technology at Robert Bosch GmbH in Stuttgart, Germany, says, “Increasing customer demand for higher power density and longer durability will further increase the loads on grease.” Matzke says, “The high power density of electric motors in modern electric vehicles leads to a challenging load collective of high temperatures, high-speed factors, electric voltage and long service life without any chance of renewing the grease.” Few greases currently on the market can meet all of those requirements.

Matzke says that these customer demands will require “even more sophisticated high-performance greases and a detailed understanding of lubrication and degradation mechanisms of rolling bearing greases.”

Litters notes, “Many additives we use today will be taken out from the market due to environmental challenges or disturbances in the supply chain.” These changes will increase “the pressure to develop more functional thickeners, which can take over typical additive functions (e.g., high extreme pressure [EP] performance).”

More sophisticated knowledge needed 
Matzke notes that while “there is already quite a good level of understanding about the lubrication mechanisms of greases, there is still demand for a more detailed understanding of the mechanisms of grease degradation, as well as the presence and distribution of greases.” More severe application conditions will “require an increasingly profound understanding in order to design the reliable lubrication of the tribological machine element.” There also is an increasing demand for quantitative models of both lubrication and of aging mechanisms to enable a simulation-based design of new components. He finds that currently available qualitative models and equations are not yet sufficient to apply numerical computer-aided engineering-based design tools.

Kuhn says, “There is a lack of knowledge, especially in understanding the structural degradation of greases caused by mechanical shear stress.” When a shear stress is applied, the different reactions of greases are not well understood, which is related to the grease thixotropy phenomenon. Thixotropy is the progressive decrease in the viscosity of a grease over time under a constant applied shear stress. There can be a gradual recovery of viscosity once the stress is removed.

Needed innovations
Matzke notes: “One main function of lubrication is the reduction of friction in moving components, which reduces the energy consumption for operation of any tribological machine.” A 2019 German study found that “23% of the global primary energy consumption can be attributed to frictional losses.”¹ The study concluded that there is a “realistic long-term potential for reduction of global primary energy use by reduction of frictional losses of 8.6 %.”1 Matzke believes that innovative lubricants and tribological systems will offer this increase in efficiency and the corresponding reduction in anthropogenic climate impacts through reduced energy consumption.

Highly sophisticated fully synthetic lubricants have a higher carbon footprint during production than mineral oil-based lubricants. However, the efficiency advantages fully synthetic lubricants offer during the product use phase through frictional reduction may counteract that higher production carbon footprint. Matzke says, “To quantify the benefits of such technological advances, we need to apply the method of life cycle assessment (LCA) according to ISO 14040 broadly.” This means that precise LCA data on the components of lubricants are needed. Matzke predicts: “This will be an important field of innovation for the next five to 10 years.”

New test rigs and procedures needed
Methods are currently available for testing greases to permit direct comparisons of different grease formulations. Matzke notes that this type of benchmarking of greases “is necessary to optimize the formulation and to identify the most promising candidates for an application.” However, these current methods are not capable of linking the measured values with actual product application requirements.

Matzke offers examples of the existing methods for oil separation of ASTM D6184 or DIN 51817. He finds that “both methods are valuable for the comparison of different grease formulations and for the quality control of the grease manufacturing process.” However, he notes that these test results do not correlate with the determination of whether a specific value of oil separation meets the requirements of a given application. This lack translates into selecting greases with the best test values but still requires running component endurance tests that have long durations and high costs.

Another example where existing methods provide limited practical data is in the evaluation of the thermo-oxidative resistance of greases. Matzke notes that this “is a prerequisite for a grease in high-temperature applications to achieve the requested grease service life.” Currently available methods (e.g., DIN 51808, ASTM D8206) assess the thermo-oxidative stability after a fixed duration or a fixed oxygen pressure decrease. However, Matzke says, “These criteria do not provide any information on whether the protecting antioxidant additives are already fully depleted or still available.” Without this information on the additives, it is impossible to determine if the “grease can still meet its requirements or has already failed.”

Matzke’s team and the DIN grease aging working group are working on a method that would identify the missing link between thermo-oxidative benchtop testing and product applications. This method “evaluates the time of final depletion of antioxidants, which corresponds to the moment of thermo-oxidative grease failure.” Matzke notes: “The method has been presented at several conferences, the latest at the German Society of Tribology (GfT) 2021 Conference.”²

Another area where more research would be helpful is the speed factor of greases. The term speed factor is used to indicate the permissible bearing speed for a lubricant. Matzke notes: “There is no international standard defined for the assessment of the speed factor of greases, so it is almost impossible to compare different greases in advance of component endurance tests.” Standardizing such a method would be helpful.



Soap thickeners
Litters finds that the main driver for innovations in soap thickeners are grease-lubricated roller bearings. “Bearings were and will be the most relevant grease application independent from what will happen in automotive and industrial transmissions and in energy generation.” This means that he believes that bearing industry requirements will be the primary driver of grease innovations. Litters says, “Bearing manufacturers want to see grease with better reliability (i.e., today L10 life, tomorrow L1 life) and, as always, better efficiency.”

L10 life is defined as the number of either revolutions or hours that 90% of identical bearings will complete before fatigue is expected to occur. This means that 10% of the bearings are not expected to achieve the L10 life. In contrast to current standards, with a future goal of L1 life, only 1% of bearings would not meet the L1 life goal.

Litters notes: “Other metallic soaps do not show the ‘multipurpose’ properties of lithium-12-Hydroxystearic acid (HSA) soaps.” This has been proven over decades of grease research; however, Litters says that other metallic soaps “can become closer to lithium soaps when additives (e.g., polymers) or advanced production techniques are applied.” By mixing different thickeners, different properties can be developed in the grease that can be helpful to specific applications. Additionally, Litters notes that biopolymers show some promise as possible alternatives to lithium greases or as additives to non-lithium greases, but there is still a long way to go to establish them on the market as long as their costs remain significantly higher.

The market importance of lithium soap thickeners will change under pressure from both raw material prices and increased performance requirements. Litters expects the market share of lithium soap thickeners “will decrease within the next decade by 10% and more” because of environmental concerns. He anticipates that they will be replaced by “polyurea greases for automotive applications (e.g., wheel bearings, driveshafts) and by calcium sulfonates in industrial fields (e.g., mining, steel industry).”

New testing approaches
Matzke notes: “A central feature for the lubrication behavior of a grease is its ability to release its base oil at an adequate rate at all operating conditions.” The rate of oil separation is strongly affected by temperature. Despite that reality, the common methods for evaluating oil separation or oil bleed are carried out at constant temperatures (e.g., 100 C for ASTM D1478-20, and 40 C for DIN 51817).

When temperatures decrease, oil separation rates are dramatically reduced until there is no oil separation. Matzke says, “This needs to be considered in low-temperature applications when there is not sufficient frictional heating of the grease” to avoid starved lubrication and component failure. Although these results have not yet been published, Matzke’s team has found “the temperature at which oil separation is no longer possible is much higher than the typically specified lower operating temperature according to the flow pressure method (i.e., DIN 51805).”

Kuhn suggests focusing on test rigs that permit “an optical device to observe the shearing process in a grease film.” He notes that more theoretical investigations are needed to permit the understanding “of the energetic situation of a stressed grease” as studies of self-optimization phenomena in greases are just beginning. Kuhn says that there is “the need to experimentally observe the degradation process in situ” using different techniques (e.g., atomic force microscopy [AFM], scanning electron microscope [SEM] and rheometer).

Conclusions
The technical understanding of how grease lubricates machine parts must be further developed to meet market needs for greases that perform in extreme environments. Now, more than ever, greases need better sustainability, which Litters defines as being human- and ecofriendly, with a low carbon footprint and no critical supply chain issues. By decreasing frictional losses through improved lubricants, there is the potential to reduce both carbon dioxide emissions and impacts from climate change caused by energy usage.

REFERENCES
1. Gesellschaft für Tribologie e.V. (2022), “Tribology in Germany: Interdisciplinary technology for the reduction of CO₂-emissions and the conservation of resources.” Available here.
2. Matzke, M., Beyer-Faiss, S., Grebe, M. and Hoeger, O. (2022), “Thermo-oxidative grease service life evaluation – laboratory study with the catalytically-accelerated method using RapidOxy,” Tribologie und Schmierungstechnik, 69 (1), pp. 41-49.

Andrea R. Aikin is a freelance science writer and editor based in the Denver area. You can contact her at pivoaiki@sprynet.com.