23rd International Colloquium Tribology - January 2022 153 Understanding the friction and wear behavior of in-service lubricants Angela Tortora Ducom Instruments Europe B.V., The Netherlands Deepak H. Veeregowda Ducom Instruments Europe B.V., The Netherlands Simon Regauer OELCHECK GmbH, Germany Christoph Rohbogner OELCHECK GmbH, Germany The lifetime of in-service lubricants is specified by the manufacturers usually on time-frame operating hours or driven distance. Only if the used oils are controlled by trend analysis, experienced tribologists will use contamination, oxidation or additive depletion for a decision, whether the oil can remain in service. It is a general understanding, that depletion of friction reducing and EP-additives over time will lead to higher wear and friction on the lubricated components. However beside changes in the additive composition measured by the ICP or IR-spectra there is no relationship what kind of laboratory test will show the depletion in the application. In this study, we have developed a rapid test method to screen the additive depletion of in-service lubricants using a four-ball tester or FBT-3 (see Figure 1). The trend-oil samples used for this study were ISO 320 wind-mill gear oils, used in an FZG drive gear, and 5W-30 engine oils out truck-engines in regular kilometer intervals. Trend analysis by ICP and other instruments of these oils showed only small changes in the concentration (ppm) of key additive elements like Zn, P and S. It lets assume that there is periodic depletion of additives with a potential effect on corrosion inhibition, friction reduction and antiwear resistance. To verify its influence on antiwear and antifriction behavior samples of these oils were tested for 60 sec at 740 MPa in FBT-3. In situ friction and ball wear scar diameter were measured for each oil. Test results showed an increase in friction coefficient or wear (see Figure 2) due to the small changes in additive concentration (ICP) for in service gear oils. However, in service engine oils showed a decrease in wear and no relationship between friction coefficient and additive concentration. This preliminary study shows that a 60 sec four ball test method can detect tribological effect of in-service additive changes. However, future work that includes tests on more in service trend oil samples is recommended. This big data will be used to determine measurement precision for this test method that can add significant value in maintenance of critical components and allows comments on the sustainability of the lubricants during its lifetime. Figure 1: Description of the Ducom Four Ball Tester (FBT-3) with Image Acquisition System (IAS). ISO 320 wind-mill gear oils 5W-30 engine oils Figure 2: Ball mean wear scar diameters obtained from FBT-3.