KEY CONCEPTS
• The increasing adoption of electric vehicle technology and its dependence on lithium for batteries has disrupted the lithium market—limiting availability of lithium-based greases.
• Because of limited availability and increasing demand for new properties, such as ecofriendliness, researchers are looking for alternatives to lithium in grease formulations. 
• Today’s grease-based lubricants are essential for a wide variety of applications outside of electric vehicles. Through advanced development and test capabilities, the range of applications will continue to expand.

While lubricants for electric powertrains are a major topic of discussion right now, lubricants, and more specifically lubricating greases, are used in a multitude of other applications. Because of this, major developments in grease formulation, tests and applications are happening now. 

Those in the lubricants industry continue to see great potential for lubricating greases—especially as they become more efficient and morph into more specialized products. The demand for effective niche products also means more research, development and testing—all of this while taking into account material availability, raw material costs and environmental/ legal constraints. The new National Lubricating Grease Institute (NLGI) specifications for high-performance greases adds to this mix.¹ 

Challenges for greases in electric vehicle applications 
Because of design differences between internal combustion engine (ICE) vehicles and electric vehicles (EVs), all lubricants for EVs, not just greases, need to increase attention to electrical conductivity, efficiency, ambient noise and material compatibility. This means greases for EVs require new or modified properties. 

According to STLE member Gustavo Sabogal, product manager lubricants, SKF, the e-mobility industry is facing multiple technological challenges. The first is related to the increasing higher power density of the machines, which demands higher performance under increasingly frequent circumstances. One example is the need to lubricate larger bearings at higher speeds and loads. 

“Historically, large and heavily loaded bearings were associated with low speeds, but more and more, designers are pushing this limit,” he explains. “Normally you would think about oil lubrication, but then you face a new challenge in sealing and increasing the cost of the design. All these factors create a wonderful background for innovation. Bearing lubrication in EVs poses a significant challenge due to the expected grease life under the given speed and temperature conditions. Technologies available now can achieve significantly longer life when compared with traditional ones, but there’s still a gap versus the desired life.” To address this, Sabogal observes that it’s best to work on the lubricant and bearing/ sealing technology in tandem. 

Dr. Markus Urban, head of R&D lubricating greases (projects) for Fuchs Lubricants Germany GmbH in Mannheim, Germany, explains, “Greases for electric drivetrain applications have additional requirements when compared with other applications. Examples include how the passage of current through the bearing effects performance and the lifetime of the grease and bearing and whether insulating or conductive grease or something in between is needed to obtain the best performance. Solutions that are correct for a specific use case can be totally wrong with slightly altered parameters. There won’t be a ‘one-grease-fits-all’ solution. Customized solutions also will be needed in the future.” 

Other important properties for electric drivetrain greases, Urban cites, are oxidation stability, material compatibility, low noise and low-temperature behavior. 

“Electrification also will change the requirements for greases outside the powertrain,” Urban adds. “Many noises are currently masked by the noise of the combustion engine. With elimination of this noise source, passengers will perceive completely different sounds. Therefore, noise, vibration and harshness (NVH) behavior of greases will become even more important in the future.” 

Frank Reichmann, Dipl.-Ing,² Carl Bechem GmbH, business unit special lubricants, also notes that greases for EVs may have additional high-speed requirements. 

Sustainability 
Reichmann considers grease a design element that is one of the most important contributors to sustainability. 

“Grease is a contributor to product sustainability,” he says. “Over its complete lifecycle, this product should be as sustainable as possible. This includes a low product carbon footprint (PCF): cradle-to-gate, cradle-to-grave or even cradle-to-cradle. A production process with low energy demand based on carbon-free resources is one aspect. Even more important are biobased raw materials since raw materials are generally the major part of the PCF of cradle-to-gate lubricants. Currently a lot of new technologies have been introduced to provide biobased raw materials for lubricants. Additionally, there is currently a drive to introduce water-based lubricants as a sustainable solution. Finally, aside from PCF, there are other sustainability aspects such as water demand, the availability of arable land and social aspects that need to be considered.” (See How UEIL Defines Sustainability Contributions to Product Lifecycles.) 

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How UEIL defines sustainability contributions to product lifecycles^A
• Cradle-to-cradle: A complete cycle where at the end of its life, the product is reconstituted into the original raw material and repurposed. This means there is zero waste.
• Cradle-to-grave: A linear lifecycle, beginning with resource extraction and culminating in end-of-life disposal.
• Cradle-to-gate: The lifecycle extends from the original resource extraction to the factory entrance.
• Gate-to-gate: An example would be from a supplier’s gate to the exit gate of a company that imparts additional value to the materials.
A. www.ueil.org/sustainability/faq/

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Urban agrees that sustainability issues include calculating and reducing the PCF of greases. “Customers need to know which base oils and additives are sustainable and cause as little environmental impact as possible,” he says. “In the future, customers will not only request performance data of greases; they also will evaluate sustainability data.” 

Reichmann clarifies that the PCF cradle- to-gate aspect plays only a small role in the PCF cradle-to-grave totality, since the energy demand from friction losses during the lifetime of the drivetrain results in even higher carbon emissions. Thus, he predicts reduction of friction will be the most important requirement in the coming years. He also notes that lubricant sustainability is a general requirement that applies to any industry, not just EVs. 

New requirements for non-EV greases 
Kirsten Schwörer, Dipl.-Ing, global OEM liaison manager driveline and EV fluids for Castrol Germany GmbH, and chair of FVA (Forschungsvereinigung Antribstechnik, a German/European research association) PA Tribology & Lubrication (the working/ project group), explains that more stringent sustainability requirements for greases extend beyond EV applications, with considerations that include: 
• Increasing the lifetime of the grease 
• Effects of the grease on efficiency 
• Improving component protection to extend the lifetime 
• The impact of formulation components of the grease on CO₂ emissions 
• Ethical aspects (water, waste, usage of land for production of raw materials). 

“Some of the previously mentioned have been trends for a couple of years regardless of the sustainability aspect,” she says. “The drive to decrease the number of products used leads to requirements for greases to be suitable for various temperature ranges, over various NLGI grades and for different applications. This also includes the compatibility and suitability for different process fluids, or other media, elastomers, etc.”  

Schwörer adds, “The general trend to build a more compact design often leads to higher temperatures and/or higher loads and torques in the final application; the design becomes more compact and energy dense and, as such, is much more demanding. Also, noise-reducing/silent greases will play a stronger role in light of e-mobility.” 

STLE member Josef Barreto-Pohlen, director of the Centre of Innovation – Grease (COI) for Quaker Houghton, supports a drive toward alternatives to the still-dominant lithium soap grease thickeners. “The costs for lithium skyrocketed, and now no one can foresee if this is going to reverse,” he says. 

As Sabogal mentions, in addition to sustainability, another important requirement for today’s greases is availability. 

“During and after the COVID-19 pandemic, some obstacles were found in the global supply chain that resulted in shortages of several raw materials,” Reichmann concurs. “Additionally, conflicts like the war in Ukraine and the resulting trade restraints have intensified the problem. Therefore, reliable delivery over many years is definitely another challenge.” 

Regulation and other challenges 
Barreto-Pohlen is awaiting the outcome in Europe of the proposal from the French scientific assessment agency for food and nutrition (ANSES) to the EU’s European Chemicals Agency (ECHA) regarding lithium salts harmonized classification and labelling— if it leads to a classification. There also are other regulations under consideration, i.e., fully fluorinated products. 

Sabogal explains, “The implementation of regulations on chemical substances limits the options you have to formulate a product. An additional complication is the lack of a real global harmonization. This leads toward different classifications for a given substance in different geographical regions. To me the cornerstone of environmental responsibility is the consistency of rules worldwide. Unfortunately, the conscience and creativity of humankind sometimes has to be stimulated by rules and regulations. So far these are not yet motivating a massive change toward cleaner technologies.” 

Urban notes an increase in label detail for many raw materials. “This leads to severe labeling of the final grease and refusal by the customer,” he observes. “As a result, many raw materials are no longer available for grease development. Also, some countries have started their own registration and evaluation program for chemicals, which increases the degree of complexity.” 

Digitalization is another challenge Urban cites—speeding up development and calculating properties outside of the lab using simulation and modeling. 

He also points to price increases and limited availability of lithium hydroxide as issues. “We will explore suitable grease thickeners to replace lithium, but improving efficiency will remain one of the main goals for the future,” he says. 



Properties of the ideal grease of the future 
Sabogal doesn’t think there will ever be an ideal grease because of the range of applications. “However, our developments should certainly lead toward cleaner technologies, renewable sources, less hazardous substances and formulas covering a wider range of applications,” he concludes. 

Schwörer explains, “The traditional idea would be one grease for each application— suitable for the broadest temperature ranges, speeds and loads and optimum media compatibility. The various grease applications vary so much from very low to highest speeds, very low to extremely high loads, shock loads, contamination with fluids, dust, etc. Greases for plain bearings or constant velocity joints have different requirements than greases for roller bearings. Also, open gear greases are very different from smaller-sized grease lubricated gearsets, e.g., for robots. So, there is not a universal grease for all applications. However, I do believe that reducing complexity is certainly possible to some extent.” 

Reichman sees greases becoming more specialized. “As technical equipment is becoming more and more effective, the performance requirements of the applied greases also are increasing,” he says. “This means the lubricants need to be optimized to fulfill the specific requirements of one application. This is a trend that will continue. Therefore, for each and every application, specific and unique qualities will be required. Lubricants will become more and more specialized. We are expecting the numbers of different lubricants to increase while the total amount will stagnate or even decrease. This requires a deeper involvement of the lubricant suppliers in the technology and processes of their customers. The grease is a design element of the machine. Tailormade solutions will prevail over commodities. In a nutshell, the most needed feature of the ideal future greases will be fit-for-purpose.” 

Sabogal doesn’t think there will ever be an ideal grease because of the range of applications. “However, our developments should certainly lead toward cleaner technologies, renewable sources, less hazardous substances and formulas covering a wider range of applications,” he concludes. 



Lab methods for predicting grease performance 
According to Schwörer, the following list of tests and lab methods may not be fully complete, but describe most methods for developing and testing greases: 
• Model tests and bearing component tests: SRV (oscillating, friction and wear) mini traction machine (MTM) testing, oil release, walked penetration, four ball tests, etc. 
• Full bearing tests: standardized or internal methods including FE8 and FE9 tests, constant velocity joint testing, gear testing for open gear/small gear lifetime greases, etc. 
• Up to the level of the full-size application: constant velocity joint (CVJ) test rigs in full scale, efficiency testing of drivetrains and field trials in close collaboration with OEMs and customers. 

These tests differentiate performance by: 
• Oil-releasing properties 
• Relubrication and grease service time 
• Wear protection • Efficiency optimization 
• Lifetime and endurance testing 
• Noise, etc. 

Urban uses standard grease industry tests to determine mechanical stability, oil release, rheological properties and influence of electric currents or low temperature behavior. “We also have developed our own test machines and procedures,” he adds, “for example, to measure hoot noise (the whining sound produced during engine start up, most often in cold conditions), oil release or oxidation stability.” 

To predict the performance of a lubricating grease in practice, a grease expert from an additive developing and producing company who prefers anonymity uses test methods FE8 and FE9. 

Sabogal says that, while standard ASTM/DIN/ISO methods are still the basis for testing, his organization has its own testing centers, where they can set up rigs to simulate real applications. “In this way we get very useful insight about, for example, the way the grease flows, the level of friction and wear we can expect under specific conditions, etc.,” he adds. 

Test conditions are adapting more and more to the conditions of the technical applications of the customers. Reichmann observes, “Therefore, for each and every application, an intelligent combination of tribological tests is required. There also is a growing understanding of the importance of the microstructures and rheology of greases through microscopy and rheological analyses. Some new rheological methods are partially substituting for established test methods.” 

Barreto-Pohlen explains that testing and statistical analyses are instrumental to the preselection/screening of a lubricant before bench testing or field testing is initiated. 

New test rigs and procedures for characterizing greases 
The analysis of grease requires a test rig that can be adapted to multiple conditions. According to the anonymous grease expert, these conditions include geometry, temperature, humidity, salinity, electric current, load, speed and oscillation versus translation. The rig also would need to deliver online data, such as temperature, friction, conductivity and surface data of lubricated parts. 

Reichmann explains that grease test rigs are usually closer to the technical application since the performance of the grease is typically tested inside of machine elements such as bearings. “All of the most common test rigs are undoubtedly very important for assessing the performance of greases in bearings now and into the future,” he says. 

According to Reichmann, a bearing test rig that is currently not very common but might gain more attention is the FAG WS22 spindle bearing test rig. It allows the operation of grease in a spindle bearing at very high speeds (up to n = 60,000 min^-1), resulting in a speed factor of n x dm = 2,550,000.  By controlling the bearing temperature, the friction losses of different greases may be assessed. Given all this, the FAG WS22 has the potential to be a very helpful tool for developing low-friction greases and highspeed greases. 

For many parameters, such as electrical or thermal properties, endurance testing, etc., standardized test methods are available. The question, Schwörer observes, is whether these tests are suitable for the design of new applications. “Many of the above-mentioned tests have a limited scope,” she says. “So, this is one area for tribology to focus on in order to develop test methods that better consider the application parameters. Joint research and development in research groups helps to define the scope for new test procedures.” 

Schwörer adds that there is currently a need to develop methods to differentiate performance regarding new failure criteria and/or energy efficiency for respective applications. “Some of the tests are available; however, test criteria and parameters need to be adopted to the new requirements, e.g., temperatures, pressures, loads, load cycles, etc.,” she says. “In these cases, group research is a big help in enabling new innovative technologies to move ahead and succeed.” 

Sabogal points out that test rigs have an inherent limitation in size and cost. “I think the real challenge is to learn more about how applications work in the field and utilize that knowledge to close a feedback loop and refine our testing programs,” he says. “The evolution in sensors and artificial intelligence (AI) will be key to closing that gap. Areas like friction, wear, fretting, grease flow and pumpability still have sufficient room for improvement regarding test methods.” 

Much of the testing and acceptable parameters depends on the intended use and whether it is for first-fill or maintenance lubrication. Barreto-Pohlen explains, “For the first group, stringent specifications with plenty of in-house test methods must be met, whereas for the second group adjusted specifications, like the new High-Performance Multiuse Grease specification from NLGI, need to be considered. We can already see a trend for improved versions of actual test benches, preferably to provide broader conditions in terms of temperature, speed, load and impact of electric fields.” 

Urban adds that for e-mobility applications, new test rigs are needed to analyze the effect of electric currents on greases and components. 



The future of lithium soap thickeners 
Although lithium soap thickeners have exhibited clear performance advantages for years, they are getting more expensive to produce, and there are questions about sustainability. 

“The price of lithium is constantly increasing, driven by the trend toward e-mobility,” Reichmann explains. “The cost of lithium for standard mineral-based grease is already a considerable part of the total cost of the grease’s production. This will continue to grow in importance. That said, lithium soap thickeners are still excellent multipurpose greases. However, with the ongoing trend from multipurpose to fit-for-purpose, the demand for other thickeners with specific properties will increase, and they will be substitutes for lithium soap. Lithium also has a large PCF. This is another reason why lithium soaps are being questioned and will be questioned more in the future.” 

Because of this, Barreto-Pohlen points to a shift to alternative thickener systems, such as mixed polymers. “I expect a growing portfolio of alternatives because lithium soaps have been so versatile. The other actual known thickeners provide a more limited range in performance,” he says. “Parallel to that, I expect new thickener systems to be established supported by the need to find better solutions.” 

Schwörer observes that alternatives to lithium greases have been under discussion for at least the last 10 years. Alternative thickeners are available, but there is usually a downside in terms of the temperature application range, cost, compatibility and/ or availability. “In some areas, where rising operating temperatures are a factor, lithium thickener-based greases will be limited due to their limited operating temperature range. In those areas, polyurea-based greases, a technology that is well established, may play a major role,” she says. 

Sabogal doesn’t see the price of lithium trending downward, but he does think the escalating cost creates an opportunity. “We’ve relied on lithium for long enough and did not dedicate enough effort to strengthen other technologies,” he explains. “But as we learned with the COVID-19 pandemic, with the right focus, great breakthroughs can be achieved in a short time. At this very moment, there are already suitable alternatives for simple lithium-based greases, and it will be very interesting to see how fast the adoption rate for such technologies will be.” 

Sabogal continues, “In a market as conservative as the lubricants market, it is difficult to predict how users will react to replacing formulations based on lithium. This also will have a direct impact on other systems like polyurea or calcium sulphonate complexes, since eliminating the price gap will open new doors to implementing these products in suitable applications. An additional part of the challenge is to extend our knowledge in grease lubrication, that, to a large extent, also has been developed based on lithium-mineral greases. How our methods and formulas will need to be adjusted is still a very interesting question and also will be a challenge for future generations.” 

Advancements in grease development 
Aside from the key areas Reichmann mentioned earlier (sustainability, availability and custom solutions), he notes that another key area is knowledge-based engineering. “This allows a more target-oriented and highly efficient/faster development of tailormade products,” he says. “Knowledge- based engineering is the precondition for an intelligent combination of tribological tests. AI will play a role in product development rather soon. It will become a common tool to support experts in the development of products.” 

Barreto-Pohlen is seeing more and more joint development projects with customers, who are recognizing the lubricating grease as part of the design right from the beginning. Sabogal envisions stronger use of rheology and tribological tests to predict the behavior of a given grease. 

Summary 
For Sabogal, grease (and any lubricant) needs to be thought of as an asset, not a commodity. “Since the lubricant plays an active role in securing reliable operation, the lubricant is part of a system that also includes, in the case of bearings, the seal, housing and lubrication system. If you take that approach, new aspects like elastomer compatibility, contamination ingress control, pumpability and grease flow should be taken into account. A simple example is how you can easily extend relubrication intervals by investing a little bit extra in a proper seal plus a lubrication system or, put in a different way, how much damage a bit of contamination or the lack of properly executed relubrication can infringe on an application.” 

Schwörer’s final advice for furthering the field of grease development is to take advantage of networking and research capabilities offered by industry-related organizations such as STLE.

REFERENCES
1. More information about the new NLGI specification may be available here.
2. Dipl.-Ing is the German equivalent of a master of science degree in engineering in the U.S.

Jeanna Van Rensselar heads her own communication/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at jeanna@smartprcommunications.com.