KEY CONCEPTS
• Issues include how well testing correlates to field performance, the availability and choice of relevant reference oils and the baked-in bias of some standardized tests.
• Much of the disconnect between the lab and real-world conditions can be traced back to the results of the test and the way formulators use those results. 
• It is in everyone’s best interest to develop more tribotests that are repeatable, consistent and correlate to specific field performance.

The standardization of tribotesting has led many OEMs to demand that tests be completed on all lubricants. However, some of this testing may not represent real- world conditions and may specify outdated reference oils. The question then becomes whether there needs to be a slightly different approach to tribotesting when it comes to qualifying new lubricants. 

There is currently a plethora of chemistries that do an outstanding job of reducing friction and wear, along with other benefits. The perpetual problem is passing the daunting volume of standardized tests, i.e., four-ball weld, low temperature storage, high temperature volatility, dynamic seals, pin and vee block, etc. 

Some of these tests are very difficult to pass and based on equipment developed with limited technology. This restricts new methods to being very similar to older chemistries and limits the opportunity for change. In addition, lubricant standards are becoming tighter, further restricting the types of instruments approved for use and standardized tests are sometimes not germane to the lubricants’ real-world duty cycles—a cause of significant frustration for formulators. 

Two immediate issues are how well testing correlates to field performance and the availability and choice of relevant reference oils. 

Tests and the correlation to field performance 
STLE member Dr. Marc Ingram, engineer and owner, Ingram Tribology, Carmarthen, Wales, UK, explains, “It is very difficult to find good correlations between most mechanical rig tests and field performance. What the current standardized tests do is ensure a minimum level of performance from the lubricant—in a few key areas— wear, scuffing (load carrying capacity) and micropitting. The machine designer will then build in a safety factor above this minimum level of performance, and a good level of reliability would be expected in the field. More advanced tribology studies and correlation work is usually done on more general and not yet standardized rigs like the Mini Traction Machine (MTM) or custom instruments.” 

It makes sense that tribotests reproducing operating conditions similar to those in the actual end-use machinery will be most likely to correlate with field performance. Because of this, it is a best practice, when testing, to use materials and surface finish/ surface roughnesses that are the same as, or very similar to, those in the machine. 

Dr. Ian Taylor, visiting professor at the Jost Institute for Tribotechnology, University of Central Lancashire, UK, and STLE Fellow, notes that there are tests that correlate well to field performance. Taylor also is former technology manager for Lubrication Science at Shell. He uses the example of reciprocating rigs. “They are useful for evaluating friction and wear in valve trains and close to top dead center of piston assemblies (since these machine elements operate in a reciprocating mode),” he says. “More general rigs, such as the MTM, are useful since they operate over a very wide speed range (from a few mm/second to a few thousand mm/sec) and so can be used to measure the friction performance of a lubricant from boundary lubrication, through mixed and almost to elastohydrodynamic (EHD) lubrication. This can give a general ranking of the friction performance of a series of candidate oils. The test also is useful for evaluating antiwear additives.” 

The MTM is a ball-on-disc instrument that measures the frictional properties of lubricated and unlubricated contacts under a wide range of rolling and sliding conditions. 

Taylor adds that since the MTM test also can be used with different slide-roll ratios, it also is useful for measuring traction curves of different fluids. “Wear testing is usually more difficult to get right when compared to friction testing since it is crucial to match materials, surface finish, speeds and loads if the tribotest has any chance of generating wear results that correlate with those in the real machine,” he notes. In addition to tribotests, viscometric tests (kinematic viscosities, high temperature/high sheer [HTHS] and cold cranking simulator [CCS] viscosities) also are performed and the results published in lubricant data sheets. Taylor says that in many cases, high temperature/high shear and cold crank viscosities have been found to correlate with the fuel economy performance of a lubricant. 

STLE member Rafe Britton, technical director at Lubrication Expert, points out that whether or not a test might correlate to field performance is often a question of how the end-user is integrating the results into their analysis or selection of product. “Although it’s not a ‘tribotest’ specifically, the Rotating Pressure Vessel Oxidation Test (RPVOT) is a great example of this,” he says. “The ASTM standards and most lubricants industry professionals will agree that the reduction of oxidation resistance in RPVOT over time is an indicator of oil degradation and remaining useful life, but many end-users and OEMs use the starting RPVOT as a proxy for the oxidation stability of the oil. I see many end-users incorrectly basing their selection of a turbine oil on the principle that higher RPVOT is better.” 

Britton continues, “Four-ball testing is another example of this. As a useful tool for formulators, I think it’s a great test. The problem is that end-users (and OEMs indirectly through their approval criteria) look at four-ball values as a proxy for ‘oil performance.’ I generally advise my end-user clients that the closer the tribotest is to a real-world scenario, the more weight it should be given in selection.” 

The example he uses is the FZG scuffing test. FZG is the Technical Institute for the Study of Gears and Drive Mechanisms (Forschungsstelle für Zahnräder und Getriebebau) of the Technical University in Munich, where the test rig was developed. Although the gear set geometry and test conditions may be more rigorous than those in real-world applications, because it uses real gears for testing, it correlates well to real-world performance. This contrasts with the four-ball test, where the point-onpoint sliding contact isn’t relevant to many industrial lubrication regimes. 

STLE member Weixue Tian, applied research department manager, Exxon- Mobil Lubricants Technology, agrees with the assumption that whether a tribotest correlates to field performance depends on comparison of the tribotest conditions (temperature, contact type, material pair, pressure, shear stress, etc.) with field conditions, and what the test is designed for versus relevant field performance. 

To illustrate the lack of correlation that can occur between testing and end use, Tian brings up the example of a traction test using an MTM. “It is a powerful tool to correlate frictional performance of gear oil,” he says. “While great correlation can be observed between the MTM test and many gear oil applications, a few years ago I came across an article that used the MTM traction test to correlate the gas turbine oil frictional performance of a Group V base oil. The traction in MTM is determined by dynamic viscosity of the oil at close to 1 GPa (gigapascal) of oil film pressure and high shear stress, while in gas turbines with tilting-pad bearings, the friction is determined by dynamic viscosity of film pressure of 10 MPa (megapascal) or less— this would be about 100 times less pressure than the MTM test—and with relatively low shear stress. The lubricant viscosity behavior at those conditions is significantly different, and the MTM traction test does not correlate to tilting-pad bearing power loss in gas turbines at all.” 

The choice of reference oils 
The use of reference oils for testing makes sense: testing the same oil, in the same tribotest, with the same operating conditions. This would lead to the conclusion that the friction and wear results should be the same. However, Taylor points out that usually there are slight differences that can be used to estimate the error bars on the test results. He adds that it also can be useful to compare what appear to be the same tests using the same reference oils but performed in different labs, since the same results should be produced in separate laboratories. 

The selection of reference oils is critical when designing a tribotest. In fact, Ingram says this choice is one of the most difficult decisions, and it also can be charged with emotion, or have some commercial bias. He uses the following examples: 

1. A company’s old product versus its new product 
2. The company’s product versus a competitor’s product 
3. A fully formulated oil versus a base oil mix 
4. A fully formulated oil versus an oil with a lower percentage of additives 

So even though the test has been established, these issues in choosing reference oils can cloud the testing, benchmarking and comparison of technologies. 

Ingram believes that the best reference oils are those known to have (depending on the application) good performance in the field and bad performance in the field. Another approach he suggests would be to formulate custom reference oils, i.e., one that causes failure and one that doesn’t— this would require a thorough knowledge of the failure mechanism and how it is affected by additive chemistry. 

“Establishing these reference oils is difficult—but is key in the design of good tribotests which correlate to field performance,” Ingram concludes.

The effect of standardized tribotesting on the range of chemistries 
Ingram observes that standardized mechanical tests can be biased toward certain types of chemistries. He uses the example of the four-ball weld test (ASTM D2783), which is biased toward the presence of extreme pressure (EP) (usually polysulfide) chemistries. “The FZG scuffing test (ASTM D5182) also can be passed with clever thought behind the formulation,” he says. “It’s common to allow some wear of the gears to reduce roughness and contact pressure. The reference oils selected during the development of the test will usually bias the test toward certain formulation styles. Sometimes making it difficult to formulate using a different style, thus limiting the range of possible lubricants and innovation.”  

Taylor cites other issues with standardized tests. “Most tribotests use engineering steels for the test components, since most chemistries will need to perform well in machines that are predominantly made of steel,” he explains. “If some chemistries are designed to perform well on diamond-like carbon (DLC) or ceramics, then test specimens will need to be made from these materials, and that can slow down laboratory testing.” 

In the case of DLC/ceramics, Taylor says it is essential to make sure the test specimens are exactly the same as the DLC/ceramic coatings used in the machine. “In Japan, some OEMs specify that the friction coefficient of fuel economy candidate oils (as tested in reciprocating rigs) should be below a certain amount, which can only be achieved (at the moment) by the use of molybdenum-based friction modifiers, and so such test limits will place restrictions on the chemistries used in those instances,” he advises. 

Britton makes the comparison between tribotesting with chemical testing. “With chemical testing, both the American Petroleum Institute (API) and European Automotive Manufacturers’ Association (ACEA) require a minimum phosphorous value to qualify for a license,” he explains. “This operates under the assumption that only phosphorous-based antiwear additives are capable of affording the protection required for modern engines. That might well currently be the case, but it makes any future revolutionary additive technologies based on novel chemistries dead-on-arrival.” 

The merit of specific tribotests 
Generally the most expensive and time-consuming tests in the formulation process are full-level rig tests and field tests, which can take months or even years to plan, execute and complete. When it comes to the merits of specific tests, opinions are mixed. 

“The four-ball wear test is widely used but is generally completely useless,” Taylor explains. “It certainly has zero relevance for engine lubricants and is not used for evaluating such lubricants. It is used in some gear and industrial lubricant specifications, but it can have unwanted side effects.” He uses the example of an OEM that wants a higher four-ball weld load. While this can be achieved by using a more chemically active EP additive, the lubricant could potentially have more corrosive wear. “This would need to be tested separately,” he says, concluding, “In terms of giving insight into the performance of lubricants and lubricant additives, the fourball test can usually be omitted in most cases without any consequences.” 

In Taylor’s experience, general purpose tests such as the MTM machine and reciprocating tests are usually the most useful tribotests for laboratory screening. 

Ingram advises maintaining most of the tribotests in the short term, while looking for better alternatives. Tian agrees, saying that because there is strong interest in reducing the amount of field testing and full-scale rig testing for formulation development, it is in everyone’s best interest to develop more tribotests that are repeatable, consistent and correlate to a specific field performance. “The important thing to keep in mind is that we are always giving up something when we run bench scale tribotests compared to actual application in the field,” he says. “Being able to identify, simulate and balance the primary factors affecting field performance is one of the critical factors for success for a tribotest campaign. Sometimes a certain tribotest becomes ‘standard’ and is used for situations that the test is not designed for, that’s when correlation tends to fail.” 

To sum up, Taylor says that since engine tests and field tests are very expensive, standardized tribotests can be useful for highlighting the most likely candidate chemistries to select for engine and field tests. It is far more economical to run, for example, 10 candidate chemistries, through laboratory tests, and then select the top two or three from the laboratory tests to go forward with the much more expensive actual engine or field tests. 

Conclusions 
Britton believes that much of the disconnect between the lab and real-world use can be traced back to the results of the test and the way formulators use these results. “The fourball test is extremely useful for formulators, but when we publish it on the product data sheet as an indication of product quality, we invite our customers to base their selection on these criteria,” he observes. “This bleeds over into OEM approvals too, where I see a lot of tests being misused.” 

The ability to create more relevant tests that correlate well to field performance is there, but success relies on close collaboration among stakeholders. “We now have vastly superior knowledge about how to design great tribotests—simulating the speeds, contact pressures, EHD film thicknesses, contact temperatures etc.,” Ingram concludes. “So, we are able to emulate different machine elements in the lab and accurately predict things like wear rate, scuffing point, micropitting and friction. The wear measures can be used to predict machine longevity, and the friction measures can be used to predict efficiency. These will give OEMs much greater confidence in the performance of the lubricants—as these tribotests would be much more relevant and have much better correlation to the field. A collaboration between OEMs, additive companies, oil companies, instrument manufacturers and test houses could reveal some fantastic alternative tribotests.” 

He adds, “I would encourage any OEM to be hesitant before simply loading lubricant specifications with standardized mechanical tests—and to really think about how relevant that test is to their product and how useful the data is. A poorly considered tribotest may limit the formulation style of the lubricant unnecessarily. A really well-thought- out tribotest—with techniques like using the correct metallurgies, pressures, temperatures, etc.—would make a much better addition to the lubricant specification. There are lots of lubricant and tribotests specialists available to help.”

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