Water can affect the performance of lubricants as well as the metal surfaces directly. Modification of the viscosity behaviour and reactions with additives are just two examples of the effects of water on lubricants, both having a negative impact on their required functionality. Together with other phenomena like cavitation or hydrogen embrittlement, they represent different ways of how water contamination can lead to corrosion, wear and eventually to machine malfunction or even failure. [1] During different lubricant degradation processes like oxidation or thermal degradation, precursors are formed, eventually leading to the formation of sludge and varnish. With their potential to form agglomerates and deposits they are a common cause of machine failures by inducing plugging, wear, corrosion, and other damaging mechanisms. As operators of turbine systems and hydraulic equipment demand high reliability and low Aus Wissenschaft und Forschung 22 Tribologie + Schmierungstechnik · 68. Jahrgang · 3-4/ 2021 DOI 10.24053/ TuS-2021-0016 Introduction Solid particles and water are not only the two most common types of contamination in lubricants, but also inhibit a high damage potential. Solid contamination can occur in many forms, sizes, and textures, like soot, abrasive wear, dust, sludge, or varnish, while water can be present in lubricating oils mainly in three states: dissolved, emulsified, or free. [1][2] A novel method for the evaluation of the contamination dispersing ability of lubricants (CONTA-DISP) Bettina Ronai, Rainer Franz, Franz Novotny-Farkas, Marcella Frauscher* Wasser und Feststoffe stellen die beiden häufigsten Kontaminationen von geschmierten Systemen dar und können überaus problematisch für diese Systeme sein. Um Stillstandszeiten zu reduzieren sowie Ausfälle zu verhindern, enthalten Schmierstoffformulierungen detergierende und dispergierende Additive, die eine wichtige Rolle im Umgang mit Verunreinigungen und der Toleranz dieser spielen. In Ermangelung eines praktikablen Verfahrens zur Bestimmung der diesbezüglich relevanten Eigenschaften wird eine neue Methode zur Bewertung des Dispergiervermögens von Schmierölen vorgestellt, die durch Kombination etablierter Methoden der Schmierstoffanalyse und einer Parameteroptimierung erarbeitet wurde. Eine Bewertung der Methode mit frischen und künstlich gealterten Schmierölen erlaubte eine Differenzierung hinsichtlich des Dispergiervermögens. Schlüsselwörter Geschmierte Systeme, Schmierstoffe, Wasser, Kontamination, Dispergierfähigkeit, Dispersants, ASTM D1401 Water and solid particulate contamination are the two most common contaminants of lubricated systems and may be highly problematic for these systems. To reduce downtime and prevent failure, lubricant formulations contain detergent and dispersant additives that play an important role in terms of contamination tolerance. In lack of a practical procedure for the determination of the relevant properties, a novel method for the evaluation of the dispersing ability of lubricating oils is introduced. Following and combining established lubricant analysis methods, a procedure with optimum parameters was found. An assessment of the method using fresh and artificially altered lubricating oils allowed a differentiation concerning their dispersing ability. Keywords Lubricated systems, lubricants, water, contamination, dispersing ability, dispersants, ASTM D1401 Kurzfassung Abstract * Bettina Ronai, BSc (corresponding author) Orcid-ID: https: / / orcid.org/ 0000-0003-0559-9270 Ing. Rainer Franz AC2T research GmbH, 2700 Wiener Neustadt, Austria Dr. Franz Novotny-Farkas Ingenieurbüro für Erdölwesen, 2320 Schwechat, Austria Dr. Marcella Frauscher, MSc Orcid-ID: https: / / orcid.org/ 0000-0001-7939-9656 AC2T research GmbH, 2700 Wiener Neustadt, Austria TuS_3_4_2021.qxp_TuS_Muster_2021 03.09.21 13: 27 Seite 22 downtime to avoid economic losses, it is important to develop lubricant formulations that reduce or prevent the formation of sludge and varnish. [3][4] Detergents and dispersants are lubricant additives (also referred to as DD additives) that keep oil-insoluble compounds in suspension and avoid degradation products from forming agglomerates, preventing the deposition of sludge and varnish. They generally have an oil-soluble (= apolar) hydrocarbon tail, which enables solubility in the lubricant and a polar head group that interacts with polar contaminants. Sulfonates and salicylates are typical representants of detergents, while succinimides are the most common dispersants. [5] Detergents and dispersants are an important part of the additives used in engine oils (internal combustion engines or gas engines), hydraulic fluids, transmission fluids and compressor oils. [5][6][7] Hydraulic oils containing detergents and dispersants are marked with the letter “D”, like HLPD and HVLPD. These lubricants cannot fulfil the requirements regarding water separability according to DIN ISO 6614 [8] or ASTM D1401 [9], as the DD additives hinder a complete separation of lubricant oil and water. [5] Some rather specific methods exist for the evaluation of the dispersing ability of certain lubricants. ASTM D7899 [10] (“blotter spot method”) describes a simple method for the condition monitoring of the dispersing ability of in-service lubricants, while ASTM D7843 [11] (“membrane patch colorimetry”) presents a method for the evaluation of the “varnish potential” in in-service lubricants. These two methods are generally not applicable for the testing and comparison of fresh or artificially altered lubricating oils, because they are based on the detection of real-life contaminants. Furthermore, there are different laboratory engine tests to evaluate the effectiveness of detergents and dispersants in engine oils for gasoline and diesel engines, e.g., CEC L-106-14 [12]. [13] These engine tests are not only specific for engine oils of internal combustion engines but are also rather time-consuming and expensive. A development towards a broader application range is an in-house method [14] applied by DaimlerChrysler that uses a graphite dispersion to evaluate the detergent/ dispersant properties of hydraulic oils. [5] Nevertheless, no suitable test method for the evaluation of the dispersing ability has been defined that can be applied to any type of lubricant oil at reasonable costs and efforts so far. This publication introduces a novel method for the determination of the water dispersing ability that provides a reasonable indication for the contamination carrying ability in general. This method suggests water as a representative for all types of polar contamination due to its character and as it addresses the same group of additives, i.e. dispersants and detergents. Furthermore, water features advantages like good definability, wide availability, and absolute harmlessness in handling. The presented method is easy to apply, as the needed equipment for the sample preparation is the same as for the determination of the water separability according to ASTM D1401 [9]. For simplicity, the result of the analysis can be obtained using an infrared spectrometer, after performing a calibration by means of water determination according to the Karl Fischer method. Alternatively, the test can be analysed using a Karl Fischer titrator only. Materials and Methods Samples Distilled deionized water (Merck Millipore EMSURE ® grade, USA) was used for all experiments. Table 1 shows an overview of the model lubricants used for the experiments. These model lubricants are fully formulated stationary gas engine oils (abbreviated as “SGEO”) that were blended for the purpose of testing the DD additive package and are not available on the market. Aus Wissenschaft und Forschung 23 Tribologie + Schmierungstechnik · 68. Jahrgang · 3-4/ 2021 DOI 10.24053/ TuS-2021-0016 Oil SGEO-1 SGEO-2 SGEO-3 SGEO-4 SGEO-5 SGEO-6 Performance of DDpackage [15] high efficiency medium efficiency moderate efficiency SGEO DD Package 1 X X - - - - SGEO DD Package 2 - - X X - - SGEO DD Package 3 - - - - X X Group I base oil - X - X - X Group III base oil X X X X X X Viscosity correc on at 100°C with viscosity modifier X X X X X X Table 1: Information about the composition of the model gas engine oil formulations used for the experiments. TuS_3_4_2021.qxp_TuS_Muster_2021 03.09.21 13: 27 Seite 23 Apparatus Prestudy In the prestudy the samples were homogenized at 13000 rpm for 5 minutes with a homogenizer (IKA Ultra-Turrax T 25 basic, Germany). FTIR spectra were recorded using a laboratory FTIR spectrometer (Bruker Tensor 27, USA) in the range of 4000-400 cm -1 . For the evaluation of the water content according to ASTM E2412 the software OPUS 5.0 (Bruker, USA) was used. Additional determination of water content was done with a Karl Fischer titrator (Metrohm 831 KF Coulometer, Switzerland), applying standard DIN 51777-2 [18]. Parameter variation and application of the method For the mixing of the samples in the tests according to the presented method, a semi-automatic demulsibility tester (Greenlab Adem, Hungary) with standard equipment for ASTM D1401 [9] was used. The recording of FTIR spectra and an automatic evaluation of the water content according to ASTM E2412 [16] was carried out with a portable FTIR spectrometer (Eralytics Eraspec Oil, Austria). The determination of water content by titration was carried out with a Karl Fischer titrator (Metrohm 831 KF Coulometer, Switzerland), analogously to the prestudy. Results and Discussion Prestudy Figure 1 shows the water content of the mixtures evaluated by FTIR spectra plotted against their respective water content determined by Karl Fischer titration. The Aus Wissenschaft und Forschung 24 Tribologie + Schmierungstechnik · 68. Jahrgang · 3-4/ 2021 DOI 10.24053/ TuS-2021-0016 Prestudy A prestudy was carried out to investigate the correlation of the water content determined by Karl Fischer titration and by Fourier-transform infrared (FTIR) spectroscopy. Mixtures of five different concentrations of water in a fresh gas engine oil (SGEO-2) in the range of ~ 0.1-10 wt% were prepared. Each target concentration was prepared twice. All mixtures were analysed by means of Karl Fischer titration and FTIR spectroscopy and a double determination was done for both analyses. The water content based on the FTIR spectra was evaluated according to the direct trending method of ASTM E2412 [16], considering the water peak at around 3400 cm -1 . Method, parameter variation & application of the method The basic procedure consisted of weighing in lubricant and water in a graduated glass cylinder, placing it in a pre-heated heating bath and letting it temper for about 30 minutes. Then the oil-water-mixture was stirred with a stirring paddle and left in the heating bath for another 30 minutes before being covered with aluminium foil and stored in a dark, dry place at room temperature. Sampling was carried out after a defined time. The equipment used (glass cylinder, heating bath, stirring paddle) complies with the standard for the determination of the water separability ASTM D1401 [9]. For each test, 70 g of lubricant and 7 g of water were used, resulting in an initial water concentration of 9 wt%. All experiments were carried out at a heating bath temperature of 82 °C. In order to find the optimum parameters for the presented method, a variation of some test parameters was carried out. Two gas engine oils (SGEO-2 and SGEO-5) in fresh condition were used for the experiments determining the influence of the variation of test parameters on the results. Following test parameters were varied: • Stirring time (5, 15, 30 min) • Stirring rate (500, 1000, 1500 rpm) • Sampling position (top, middle, bottom) • Sampling time (after 2, 6, 24, 336 h) The method with the optimum parameters was subsequently applied to a set of model lubricants. Six gas engine oils (see Table 1) in fresh and artificially altered condition were used for the overall evaluation of the presented method. The thermo-oxidative artificial alteration based on CEC L-48-A00 [17] was carried out for eight days with 100 g of oil at a heating bath temperature of 160 °C and an air flow of 10 L/ h. Figure 1: The water content of the mixtures evaluated by FTIR spectra plotted against their respective water content determined by Karl Fischer titration. A linear regression line is calculated based on the first eight values (illustrated in blue), excluding the mixtures in the range of around 10 wt% (illustrated in red). TuS_3_4_2021.qxp_TuS_Muster_2021 03.09.21 13: 27 Seite 24 calculation of the linear regression line is based on the first eight values, excluding the mixtures in the range of around 10 wt%, as they clearly deviate from a linear relationship. The evaluation of the water content by the FTIR spectra is based on the determination of a specific absorbance area in the region of the OH stretching vibrations around 3400 cm -1 . A limitation of this evaluation method is reached when the addressed absorption band arrives in the intensity of total absorption. According to these experiments, this seems to happen in the region between 5 and 10 wt% of water in the lubricant. Therefore, the application range for the determination of the water content by FTIR spectroscopy is estimated to be around 0.2-5 wt%. Parameter-variation experiments Table 2 shows an overview of the parameter-variation experiments. One parameter was varied at a time, keeping the other parameters constant. As the method should be easily adapted being as close as possible to currently used standards, some of the initial parameters derived from the standard parameters of ASTM D1401 [9] (5 minutes stirring time, 1500 rpm stirring rate). Stirring time Stirring time was varied at 5, 15 and 30 minutes. A visual inspection of the mixtures suggested that they are comparable and that it is not necessary to stir for more than 5 minutes in order to achieve sufficient homogeneity. Stirring rate A variation of the stirring rate was carried out with following levels: 500, 1000 and 1500 rpm. Based on a visual inspection it was concluded for the mixture prepared with 500 rpm homogeneity was not given. The observation that at 1500 rpm a higher water content was Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 68. Jahrgang · 3-4/ 2021 DOI 10.24053/ TuS-2021-0016 Varied parameter Oil S rring me (min) S rring rate (rpm) Sampling posi on Sampling me (h) Op mum parameter S rring me SGEO-2 5 1500 middle 24 5 min SGEO-2 10 1500 middle 24 SGEO-2 15 1500 middle 24 SGEO-5 5 1500 middle 24 SGEO-5 10 1500 middle 24 SGEO-5 15 1500 middle 24 S rring rate SGEO-2 5 500 middle 24 1500 rpm SGEO-2 5 1000 middle 24 SGEO-2 5 1500 middle 24 SGEO-5 5 500 middle 24 SGEO-5 5 1000 middle 24 SGEO-5 5 1500 middle 24 Sampling posi on SGEO-2 5 1500 top 24 middle SGEO-2 5 1500 middle 24 SGEO-2 5 1500 bo om 24 SGEO-2 5 1500 top 24 SGEO-2 5 1500 middle 24 Sampling me SGEO-2 5 1500 middle 2 24 h SGEO-2 5 1500 middle 6 SGEO-2 5 1500 middle 24 SGEO-2 5 1500 middle 336 SGEO-5 5 1500 middle 2 SGEO-5 5 1500 middle 6 SGEO-5 5 1500 middle 24 SGEO-5 5 1500 middle 336 Table 2: Overview of parameter variation experiments. TuS_3_4_2021.qxp_TuS_Muster_2021 03.09.21 13: 27 Seite 25 Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 68. Jahrgang · 3-4/ 2021 DOI 10.24053/ TuS-2021-0016 found for both oils (see Table 3), suggests that a higher homogeneity is ensured at this level. Sampling position Three different sampling positions were examined: top, middle, and bottom of the emulsion phase. After the first experiment with SGEO-2, it was observed that sampling at the bottom of the emulsion phase resulted in a significantly higher water content, while samples from the other two positions resulted in rather similar water contents (see Table 4). The experiment repeated with the same oil (SGEO-2) revealed that while the variation between the top and middle position is generally small within one experiment, a more representative sampling can be carried out at the middle of the emulsion phase (see Table 4). Sampling time A sample was taken 2, 6, 24 and 336 hours after homogenization. Table 5 shows the respective water contents for SGEO-2 and SGEO-5. For SGEO-5 the water content fluctuates within a quite narrow range. In contrast, the water content of SGEO-2 seems to change within the first 24 hours of storage before reaching an approximately constant value. In order to ensure sampling at a stage where a stable mixture has been formed and avoid unnecessarily long waiting times, sampling should be carried out after 24 h. Optimum parameters Table 6 summarizes the optimum parameters found in the parameter variation experiments. The stirring time of 5 min and stirring rate of 1500 rpm ensure a homogeneous mixing of the sample while staying true to the standard ASTM D1401 [9] while the sampling from the middle of the emulsion phase after 24 h provides a representative sampling method while being a laboratoryfriendly practice. Repeatability Repeatability experiments were carried out with SGEO- 2 and SGEO-5, using the final parameters (see Table 6). The water contents determined by FTIR spectroscopy are presented in Table 7, including the respective mean and standard deviation for each oil. The relative standard deviation for SGEO-2 is 11 %, which can be considered acceptable. At a relative standard deviation of 3 %, SGEO-5 shows a good repeatability. Typically, petrochemical standards show a worse repeatability. Application of the method An assessment of the dispersing ability of lubricants was carried out with six model gas engine oils (see Table 1) in fresh and artificially altered condition, respectively. The tests were carried out using the final parameters dis- Water content FTIR (A/ cm) Oil 1500 rpm 1000 rpm 500 rpm SGEO-2 615 303 not homogeneous SGEO-5 178 137 Sampling position Sampling time Optimum parameters Table 3: Water contents at different stirring rates. S rring me (min) S rring rate (rpm) Sampling posi on Sampling me (h) Op mum parameter 5 1500 middle 24 Table 6: Overview of optimum parameters. Water content FTIR (A/ cm) SGEO-2 SGEO-5 615 178 477 168 498 174 486 - 486 - Mean 512 173 Standard deviation 58 5 Rela ve standard devia on 0.11 0.03 Table 7: Repeated water contents, their mean and standard deviation. Water content FTIR (A/ cm) Oil top middle bo om SGEO-2 544 507 624 SGEO-2 464 477 - Table 4: Water contents at different sampling positions. Water content FTIR (A/ cm) Oil 2 h 6 h 24h 336 h SGEO-2 390 452 615 622 SGEO-5 170 197 178 201 Table 5: Water contents at different sampling times. TuS_3_4_2021.qxp_TuS_Muster_2021 03.09.21 13: 27 Seite 26 played in Table 6, with an additional sampling after 336 hours. FTIR spectra were recorded of all samples. The samples taken after 336 hours were additionally analysed by Karl Fischer titration (see Table 8). A new calibration line pictured in Figure 2 was established for the FTIR spectrometer used for these evaluations (see Materials and Methods) based on the fresh oil data in Table 8. Based on this calibration, the water content of the 24-hour emulsions was calculated (see Table 9). Images of the fresh and artificially altered gas engine oils 336 h after the test can be seen in Figure 3 and Figure 4, respectively. SGEO-1 and SGEO-2, the oils that contain the DD additive package marked as “highly efficient” (see Table 1), hold a significantly higher water content in fresh condition than the four other lubricants, both in between a range of 4-6 wt%. SGEO-3 to SGEO-6 all hold a lower water content of up to 2 wt% in fresh condition. Despite different additive packages with slightly different DD descriptions (“medium” and “moderate”, see Table 1) being used for the formulations, no significant differences in the dispersing ability of these four oils can be observed in fresh condition. The water content of all lubricants in altered condition lies within a range of 1-2 wt% after the test and therefore cannot be distinguished. While the dispersing ability of the four altered samples of SGEO-3 to SGEO-6 does not differ significantly from that of the respective fresh oil, SGEO-1 and SGEO-2 show a significant loss compared to the fresh oil. It appears that the additives of the DD package used in SGEO-1 and SGEO-2 undergo changes during the artificial alteration that strongly influence the dispersing ability of the respective lubricants. Not many differences in the behaviour of the respective DD additive packages in combination with different base oils can be observed regarding the dispersing ability. A differentiation within a specific additive package can only be detected for the fresh oils containing the “highly efficient” DD package where the formulation containing both group I and group III base oils shows an increased water content after the test in comparison to the formulation containing group III base oil only. Based on the turbid appearance of the emulsions visible in Figure 3 and Figure 4 it can be assumed that besides the probable existence of dissolved water, dispersed water is present and also determined by the described method. Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 68. Jahrgang · 3-4/ 2021 FTIR Absorption (A/ cm) Karl Fischer water content (wt%) Oil fresh altered fresh altered SGEO-1 517 260 4.5 1.6 SGEO-2 622 267 5.8 1.5 SGEO-3 242 279 1.6 1.9 SGEO-4 244 271 1.7 2.0 SGEO-5 201 188 1.1 1.2 SGEO-6 211 174 1.2 1.3 FTIR Absorption (A/ cm) Calculated water content (wt%) Oil fresh altered fresh altered SGEO-1 503 181 4.4 0.9 SGEO-2 615 237 5.7 1.5 SGEO-3 228 220 1.4 1.3 SGEO-4 214 220 1.2 1.3 SGEO-5 178 173 0.8 0.8 SGEO-6 171 190 0.8 1.0 Table 8: Water content of emulsions of gas engine oils in fresh and artificially altered condition determined by FTIR spectroscopy and Karl Fischer titration (sampling after 336 h). Table 9: Water content of emulsions of gas engine oils in fresh and artificially altered condition determined by FTIR spectroscopy (sampling after 24 h). Figure 2: The water content of the emulsions evaluated by FTIR spectra plotted against their respective water content determined by Karl Fischer titration incl. a linear regression line. TuS_3_4_2021.qxp_TuS_Muster_2021 03.09.21 13: 27 Seite 27 Literature [1] A. C. Eachus, “The trouble with water,” Tribology & Lubrication Technology, pp. 32-38, Oct. 2005. [2] T. Mang, Ed., Encyclopedia of Lubricants and Lubrication. Berlin Heidelberg, Germany: Springer, 2014. [3] J. Fitch, “Sludge and Varnish in Turbine Systems,” Practicing Oil Analysis, May 2006. [4] F. Alomar Belmonte, “Clean Solutions for Hydraulic Oil Technology Needs,” Machinery Lubrication, Jun. 2013. [5] T. Mang and W. Dresel, Eds., Lubricants and lubrication, Second edition. Weinheim, Germany: Wiley-VCH, 2007. [6] C.-H. Kuo, Tribology - Lubricants and Lubrication. Rijeka: InTech, 2011. [7] D. M. Pirro, A. A. Wessol, and J. George. Wills, Lubrication fundamentals. Marcel Dekker, 2001. [8] DIN (Deutsches Institut für Normung), “DIN ISO 6614: Mineralölerzeugnisse - Bestimmung des Wasserabscheidevermögens von Mineralölen und synthetischen Flüssigkeiten”, 2002. [9] “ASTM D1401: Standard Test Method for Water Separability of Petroleum Oils and Synthetic Fluids”, 2019. [10] ASTM International, “ASTM D7899: Standard Test Method for Measuring the Merit of Dispersancy of In-Service Engine Oils with Blotter Spot Method”, 2019. [11] ASTM International, “ASTM D7843: Standard Test Method for Measurement of Lubricant Generated Insoluble Color Bodies in In-Service Turbine Oils using Membrane Patch Colorimetry”, 2018. [12] CEC (Co-ordinating European Council for the Development of Performance Tests for Fuels Lubricants and Other Fluids), “CEC L-106-14: Oil Dispersion Test at Medium Temperature for Passenger Car Direct Injection Diesel Engines”, 2018. [13] L. R. Rudnick, Ed., Lubricant Additives: Chemistry and Applications. New York, NY: Marcel Dekker, 2003. [14] DaimlerChrysler, “DBL 6571-4: Determination of Dirt Carrying Behaviour”, 1998. [15] Assessment of DD package performance based on field observations made by Franz Novotny-Farkas due to lack of distinguishability of packages based on physical/ chemical characteristics specified in data sheet. [16] ASTM International, “ASTM E2412: Standard Practice for Condition Monitoring of In-Service Lubricants by Trend Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry”, 2010. [17] CEC (Co-ordinating European Council for the Development of Performance Tests for Fuels Lubricants and Other Fluids), “CEC L-48-A00: Oxidation stability of lubricating oils used in automotive transmissions by artificial ageing”, 2007. [18] “DIN 51777: Mineralölerzeugnisse - Bestimmung des Wassergehaltes durch Titration nach Karl Fischer”, 2020. Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 68. Jahrgang · 3-4/ 2021 DOI 10.24053/ TuS-2021-0016 Conclusions A novel method for the evaluation of the contamination dispersing ability of lubricant oils was introduced. It suggests water as a representative for polar contaminants as it addresses the same classes of additives, i.e., detergents and dispersants. A simple implementation of the method is given as the needed equipment comprises the apparatus used for ASTM D1401, a Karl Fischer titrator, and an FTIR spectrometer. Karl Fischer titration is only necessary for the calibration of the water content for the respective FTIR spectrometer and lubricant. Once a calibration line is established, the determination of the water content can be done by the evaluation of the FTIR spectra. It was shown that the method is suitable for the detection of dissolved and dispersed water in oil in the range of 0.2-5 wt% and achieves repeatable results. An application of the method to fresh and altered model lubricants demonstrated a successful differentiation of the dispersing ability of oils with different formulations. Further experiments with different types of contamination are needed to confirm the validity of water as a model substance representing all types of polar contamination. The conducting of additional experiments with used lubricants from the field could aid a deeper investigation of the correlation between lubricant performance regarding cleanliness in the proposed method and in real-life applications, such as gas engine oils, passenger car engine oils, heavy-duty diesel oils, or hydraulic oils (especially HLPD). Acknowledgements This work was funded by the “Austrian COMET-Program” (project InTribology, no. 872176) via the Austrian Research Promotion Agency (FFG) and the Provinces of Niederösterreich and Vorarlberg and was carried out within the “Excellence Centre of Tribology” (AC2T research GmbH). Figure 3: Fresh oils 336 h after the test. Figure 4: Artificially altered oils 336 h after the test. TuS_3_4_2021.qxp_TuS_Muster_2021 03.09.21 13: 27 Seite 28