24th International Colloquium Tribology - January 2024 207 Analysis of Tribo-Films in Industrial Applications Joerg W. H. Franke 1* , Janine Fritz 1 , Daniel Merk 2 1 Schaeffler Technologies AG & Co. KG, Herzogenaurach, Germany 2 Schaeffler Technologies AG & Co. KG, Schweinfurt, Germany * Corresponding author: frankjer@schaeffler.com 1. Introduction The performance of rolling bearings in industrial applications is strongly dependent on lubrication, including formation of tribo-films in the mechanical contact. As a result of the research a detailed description of these tribo-films is available. These analysis methods are rarely available in the industry, but mainly at research institutes and universities. In industrial practice it is necessary to use less time consuming, flexible, widely available, and in the best case, non-destructive techniques. In [1] a selection of specimens from FE8 WEC tests were investigated by “micro-X-ray Fluorescence Spectroscopy” (m-XRF) and “Attenuated Total Reflection Fourier Transform Infrared Spectroscopy” (ATR-FTIR). The presentation at 24 th International Colloquium on Tribology is focused on following questions: a. How repeatable are tribo-films? b. Are the methods of analysis sufficiently discriminating? c. How robust are tribo-films? 2. Methods 2.1 Rolling bearing testing The FE8 test rig [2] is usually used to test standardized test setups according to the DIN standards DIN 51819-02 [3] or DIN 51819-03 [4]. The aim is to test lubricants (oils and greases) according to their general behavior, i.e., the anti-wear behavior of specific additives. In this context, a test procedure was established to investigate the mixed friction behavior of point and line contacts. The FE8-25 tests with the cylindrical roller thrust bearing 81212 exhibit different failure modes depending on the lubricant chemistry. In addition to the harsh test conditions of mixed friction at high speeds and slippage, this bearing type exhibits special kinematic conditions that lead to different values of frictional energy across the raceway. The main failure mechanisms of this test are sub-surface fatigue damage due to White Etching Cracks (WECs) and/ or surface-initiated fatigue damage (SIF). The various stress zones with different energy inputs perpendicular to the raceway of a washer after a FE8-25 test run can often be easily recognized visually. Figure-1: Different stress zones and the friction energy density along the raceway of an FE8 axial washer 2.2 Surface analysis The selected methods, m-XRF and ATR-FTIR microscopy, allow the characterization of areas of interest via mapping in their entirety. Both methods are non-destructive, so it was possible to analyze the specimen generated on FE8 test without any mechanical sample preparation. Before the analysis, it was only necessary to clean the sample by rinsing off lubricant residues on the surface with a suitable organic solvent, e.g., n-heptane (CAS No. 142-82-5). In µ-XRF spectroscopy a high energetic radiation is used to excite atoms of the specimen. In these atoms electrons from inner shells are removed from the atom. In a very short time these vacancies are filled with electrons from outer shells. The free energy can be emitted as an AUGER-electron or as an X-ray photon. The energy of the emitted X-ray photon depends on the difference of binding energies of both involved electron levels - the vacancy and the level from which the electron jumps into the vacancy. Because this difference is characteristic for every element, the excited specimen emits a characteristic fluorescence radiation. This can be used to analyze both qualitative and quantitative composition of specimens. The m-XRF offers the possibility of a position sensitive elemental analysis of non-homogeneous material. ATR-FTIR microscopy was used to determine the chemical structure of the generated tribo-films on the washer surface. FTIR spectroscopy exploits the fact that most molecules absorb light in the infrared region of the electromagnetic spectrum. The absorbed energy leads to a change in the molecular dipole moment. These molecular vibrations generate an IR spectrum that serves as a characteristic “molecular fingerprint” from which the determination of the specimen’s chemical structure can be made. In addition, the ATR technique uses the effect that optical absorption spectra can be easily obtained by observing the interaction of the totally reflected light emerging from the optically dense medium with the optically thin medium. The ATR-FTIR microscopy on the raceway side of the washer was analyzed by mapping a grid of 13 × 13 measurement points both in the circumferential and transverse to the direction of the raceway of the thrust bearing surface. The 13 measurement points of each row (identical tribological conditions) were used to calculate a sum spectrum. The detailed equipment parameters were identical as described in [1]. 3. Results 3.1 Similar oil compositions More than 10 years of FE8-25 testing with a variety of different types of lubricants allows the comparison of tribo-film 208 24th International Colloquium Tribology - January 2024 Analysis of Tribo-Films in Industrial Applications formation with several similar but not identical oil formulations. As an example, the resulting tribo-film of five oil formulations with at least ZDDP and calcium sulfonate was compared. In 3 out of 5 specimens the typical amorphous calcium carbonate bands are observed in the infrared spectrum. In addition, an absorption band at 1006 cm -1 (i.e. P-O typical) is visible. In the other two specimens, however, this band is shifted to 1110-1130-cm -1 and no carbonate typical bands are detectable. This also correlates with the results of the element analysis. The phosphorous signal is strongly dominant when no carbonate peaks are detectable. In the other three cases the sulfur signal is on a higher level. So similar formulations usually result in similar tribo-films, but the exact oil formulation determines the true result. 3.2 Different additive formulations A less additivated ester oil was used to produce a different tribo-film. As expected, the specimen failed early after 21 hours with surface-initiated failure. The element analysis shows only a low occupancy of the surface. Traces of sulfur and nearly no identification of other elements. The IR-spectrum is dominated by an iron oxide typical band. Unfortunately, oxygen is not detectable by the selected surface analysis method. Therefore, only the infrared information is available. Nevertheless, the result is a clear differentiation compared to the phosphorus or calcium based tribo-films on most other specimens. Figure-2: Surface analysis - samples run with different chemistries 3.3 Stability of tribo-film A common doubt regarding tribo-films is their stability after preparation procedures, e.g., cutting of the specimen by using cooling fluids etc. Thus, two specimens from one test were used. The first specimen was analyzed directly after the test run. The second one was first cut to prepare a sample for structure analysis. The remaining piece was analyzed exactly as the first part. The analysis shows a similar if not identical tribo-film. The films seem robust enough and an analysis after the cutting procedure is also promising. Figure-3: Surface analysis of cut and uncut part from same test run. 4. Conclusion By combining the two surface analysis methods developed (m-XRF and ATR-FTIR microscopy), it was possible-based on the samples available-to differentiate tribo-films on used parts. These correlative spectroscopy techniques are suitable to describe the tribo-film in an adequate precision for industrial application. In addition, compared to externally available complex methods, significantly larger areas can be characterized in a spatially resolved manner via mapping. This is a smart way to determine the actual respective chemical composition of the generated tribo-films in a sufficient resolution. It supports the objective to characterize tribo-films by using comparatively inexpensive, fast, and non-destructive analysis methods, which are more commonly widespread in the industry. References [1] Franke, J.W.H.; Fritz, J.; Koenig, T.; Merk, D. Influence of Tribolayer on Rolling Bearing Fatigue Performed on an FE8 Test Rig - A Follow-up. Lubricants 2023, 11, 123. https: / / doi.org/ 10.3390/ lubricants11030123 [2] DIN 51819-1; Testing of Lubricants-Mechanical-Dynamic Testing in the Roller Bearing Test Apparatus FE8-Part 1: General Working Principles 2016-12. Beuth Verlag: Berlin, Germany, 2016. [3] DIN 51819-2; Testing of Lubricants-Mechanical-Dynamic Testing in the Roller Bearing Test Apparatus FE8-Part 2: Test Method for Lubricating Greases-Applied Test Bearing: Oblique Ball Bearing or Tapered Roller Bearing. Beuth Verlag: Berlin, Germany, 2016. [4] DIN 51819-3; Testing of Lubricants-Mechanical-Dynamic Testing in the Roller Bearing Test Apparatus FE8-Part 3: Test Method for Lubricating Oils-Applied Test Bearing: Axial Cylindrical Roller Bearing, 2016- 12. Beuth Verlag: Berlin, Germany, 2016.