Aus Wissenschaft und Forschung 41 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005 Thermo-oxidative grease service life evaluation - laboratory study with the catalytically accelerated method using the RapidOxy Markus Matzke, Susanne Beyer-Faiß, Markus Grebe, Olav Höger* Dieser Beitrag wurde im Rahmen der 62. Tribologie-Fachtagung 2021 der Gesellschaft für Tribologie (GfT) eingereicht. Die zunehmenden Anforderungen an die Produktlebensdauer, einschließlich geringerer Ausfallraten und höherer Belastungen, stellen eine Herausforderung für Schmierfette dar und es sind geeignete Methoden zur Abschätzung der Fettgebrauchsdauer erforderlich. Thermo-Oxidation ist ein dominanter Degradationsmechanismus in vielen Automobilanwendungen bei erhöhten Betriebstemperaturen. Bei der Tribologie- Fachtagung der Gesellschaft für Tribologie (GfT) 2019 wurde die Bosch-Methode zur quantitativen beschleunigten Prüfung der thermo-oxidativen Stabilität von Fetten mit dem RapidOxy Gerät vorgestellt. Diese Methode unterscheidet sich von der ASTM D8206 und der DIN 51830-1, da sie das Fett in Kontakt mit den Oxidationskatalysatoren Stahl oder Messing untersucht und zusätzlich zur Berechnung der Arrhenius- Aktivierungsenergie des Fettes verwendet wird. Die Oxidationsinduktionszeit, die durch eine signifikante Abnahme des Druckgefälles im Autoklaven angezeigt wird, wird als Kriterium für die Lebensdauer des Fettes verwendet. Die Methode wurde in der DIN-Arbeitsgruppe „Fettalterung“ vorgestellt und eine Laborstudie mit Beteiligung von sieben Mitgliedslaboratorien wurde durchgeführt. Ziel dieser Studie war es, die Wiederholbarkeit und Reproduzierbarkeit dieser Methode zu bewerten und die Anwendung des Arrhenius-Gesetzes für diesen Mechanismus zu überprüfen. Die Studie wurde mit einem handelsüblichen Schmierfett der NLGI-Klasse 2 durchgeführt, das aus einem Lithium- 12-Hydroxystearat-Verdicker, Mineralöl und einem Additivpaket besteht. Das Fett wurde in dünnen Schichten entweder auf Stahl- oder Messingplatten bei With increasing product lifetime requirements including reduced failure rates and higher loads greases are challenged and appropriate methods for grease service life estimation are necessary. Thermo-oxidation is one dominant degradation mechanism in many automotive applications at elevated operating temperatures. Kurzfassung Abstract * Dr. Markus Matzke Orcid-ID: https: / / orcid.org/ 0000-0002-1832-3946 Robert Bosch GmbH, 71272 Renningen Susanne Beyer-Faiß, Geschäftsführerin Orcid-ID: https: / / orcid.org/ 0000-0001-9650-4876 Dr. Tillwich GmbH Werner Stehr, 72160 Horb-Ahldorf Dr. Markus Grebe, Kompetenzzentrum Tribologie Mannheim (KTM), Hochschule Mannheim, 68163 Mannheim Olav Höger, Shell Global Solutions (Deutschland) GmbH Shell Technology Center Hamburg PTX/ L/ TI, 21107 Hamburg 130 °C, 145 °C und 160 °C aufgetragen und die Oxidationszeit gemessen. Die Oxidationsinduktionszeiten bei verschiedenen Temperaturen wurden zur Ableitung der Aktivierungsenergien der Arrhenius-Gleichung für beide Plattenmaterialien verwendet. Die Anwendbarkeit des Arrhenius-Gesetzes wurde durch R 2 -Werte über 0,998 für eine lineare Regression im Arrhenius- Diagramm verifiziert. Die Ergebnisse zeigten eine bemerkenswert geringe Streuung der verschiedenen Aktivierungsenergiebeträge. Die Werte der Aktivierungsenergie des untersuchten Fettes lagen im Bereich von 126 - 134 kJ/ mol bei Stahl- und 99 - 113 kJ/ mol bei Messingkontakt. Die Druckkurve als Bewertungskriterium wurde mit der Sichtprüfung und rheologischen Fließkurven verglichen. Basierend auf den Ergebnissen dieser Gemeinschaftsstudie und auf internen Erfahrungen bei Bosch wurde diese Methode dem DIN im Jahr 2020 zur Definition als neue Industrienorm vorgeschlagen. Der Entwurf „DIN 51830-2 Prüfung von Schmierfetten - Bestimmung der Oxidationsbeständigkeit von Schmierfetten - Teil 2: Bestimmung der Arrhenius-Aktivierungsenergie des thermo-oxidativen Abbaus“ befindet sich derzeit in der Begutachtung des DIN-Arbeitskreises und ein offizieller Ringversuch mit mehreren Schmierfetten und weiteren Laboren ist in Vorbereitung. Schlüsselwörter Fettgebrauchsdauer, thermo-oxidative Degradation, Oxidationsinduktionszeit, RapidOxy, beschleunigtes Schmierfetttestverfahren nigtes Verfahren” [4]. Both standards apply the same criterion as established for oils, namely the time until the pressure has decreased by 10 % of its initial maximum value. From this criterion, the depletion of the antioxidant and the corresponding end of grease service life cannot be determined. Furthermore, both standards do not take the contact with steel or brass as used in applications into account. Therefore, at the GfT conference in 2019, the Bosch method for quantitatively accelerated testing of thermo-oxidative stability of greases with the RapidOxy was presented [5]. This method differs from ASTM D8206 and DIN 51830-1 because it measures the grease in contact with the oxidation catalysts steel and brass and is used to calculate the Arrhenius activation energy of the grease. The oxidation induction time which is indicated by a significant decrease of the autoclave pressure gradient is applied as criterion for grease service life. 2 Scope of investigation The first objective of this study was to evaluate this new method in an increased number of established grease laboratories. Furthermore, this should provide a first indication of method repeatability and reproducibility. The approach was an experimental study with participation of seven laboratories which are working on lubricating greases and members of the DIN working group “grease ageing”. Aus Wissenschaft und Forschung 42 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005 1 Background and motivation Grease-lubricated automotive components with long lifetimes at elevated temperatures rely on proper lifetime lubrication. Therefore, the service life of the lubricating grease must be known and meet the lifetime requirements of the machine element. With increasing product lifetime requirements including reduced failure rates and higher loads greases are challenged and appropriate methods for grease service life estimation are necessary. Thermo-oxidation is one dominant degradation mechanism in many automotive applications at elevated operating temperatures. Since grease testing under field-typical conditions is not efficient, accelerated endurance tests are needed. Dornhöfer presented a Bosch internal method for accelerated estimation of thermo-oxidation of greases at the GfT conference 2016 [1]. In his test method he already applied the contact of grease with catalytically active materials steel or brass. Beyer-Faiss also reported from tests for thermo-oxidation and clearly pointed out the relevance of contact with brass alloys as accelerator for lubricant degradation [2]. This knowledge was considered when the new method with the RapidOxy as used in our study was developed. The RapidOxy autoclave tests has already be standardized as ASTM D8206 (2018) “Oxidation Stability of Lubricating Greases - Rapid Small Scale Oxidation Test (RSSOT)” [3] and the corresponding DIN 51830-1 “Prüfung von Schmierstoffen - Bestimmung der Oxidationsbeständigkeit von Schmierfetten - Teil 1: Beschleu- At 60 th German Tribology Conference 2019, the Bosch method for quantitatively accelerated testing of thermo-oxidative stability of greases with the RapidOxy test apparatus was presented. This method differs from ASTM D8206 and DIN 51830-1 because it measures the grease in contact with the oxidation catalysts steel or brass and it is additionally used to calculate the Arrhenius activation energy of the grease. The oxidation induction time which is indicated by a significant decrease of the autoclave pressure gradient is applied as criterion for grease service life. The method was introduced to the DIN working group “grease ageing” and a laboratory study with participation of seven member laboratories was carried out. The objective of this study was to evaluate repeatability and reproducibility of this method and to verify the application of Arrhenius law for this mechanism. The study was carried out with a commercially available lubricating grease of NLGI-class 2 which consists of a lithium 12-hydroxystearate thikkener, mineral oil and an additive package. The grease was spread in thin layers on either steel or brass plates and the activation energy was measured at 130 °C, 145 °C and 160 °C. The oxidation induction times at different temperatures were used to derive the activation energies of the Arrhenius equation on both plate materials. The applicability of Arrhenius law was verified by R 2 -values above 0,998 for a linear regression in the Arrhenius plot. Results indicated remarkably low scattering of values. The values of activation energies of this specific grease were in the range of 126 - 134 kJ/ mol on steel and 99 - 113 kJ/ mol on brass. The pressure curve as evaluation criterion was correlated with visual inspection and rheological flow curves. Based on the results of this collaborative study and on internal experiences at Bosch, this method was proposed to DIN in 2020 for definition as a new industry standard. The draft “mDIN 51830-2 Testing of greases - Determination of oxidation resistance of greases - Part 2: Determination of the Arrhenius activation energy of thermo-oxidative degradation” is currently in review of the DIN working group and an official round robin test with several greases and more laboratories is currently in preparation. Keywords Grease service life, thermo-oxidative degradation, oxidation induction time, RapidOxy, accelerated grease test method 3 Method description 3.1 Test preparation and parameters All measurements were carried out with the RapidOxy tester from Anton Paar GmbH. The grease sample is applied to a cleaned steel or brass sheet of 26 mm x 26 mm (Figure 1, left). The metal sheet is cleaned by thoroughly wiping it first with acetone, then with isopropanol and last with petroleum ether. Steel or brass is used as a carrier for the grease in order to take into account its catalyzing properties for the oxidation of the grease and to design test conditions which are similar to real use conditions where greases are in contact with steel surfaces. The amount of grease is weighed to 0.5 g and spread with a spatula to an even layer on the metal plate. The sample is then placed in the cleaned glass petri dish that is provided with the device and a fresh FKM O-ring is placed in the groove (Figure 1, middle). Finally, the lid is screwed on the chamber and the safety hood is placed and locked over the autoclave. After that, the chamber is flushed with pure oxygen gas and then pressure is increased to the defined initial value of 700 kPa. The gas valve is closed and the heating to the desired temperature starts. During and after heating the internal chamber pressure is recorded. The instrument stops the test as soon as the termination criterion is reached. Typically, the termination criterion is set to the pressure decrease of 30 % from the initial maximum value. The value of 30 % has been defined to fully achieve the significant drop of pressure. In some cases depending on the grease type a higher value is necessary. Test parameters: Grease sample amount: 0.5 g Plate materials: Non-alloyed quality steel (1.0330); brass (CW508L, formerly 2.0321 / CuZn37) Initial oxygen pressure: 700 kPa Temperatures: 130 °C, 145 °C; 160 °C Grease sample: The study was carried out with a commercially available lubricating grease of NLGI class 2, which consists of a Lithium 12-hydroxystearate thickener, a mineral oil of a kinematic viscosity of 100 mm 2 / s at 40 °C and an additive package that includes an antioxidant additive. The grease was provided to all seven laboratories from the same manufacturing batch. 3.2 Estimation of oxidation induction time from pressure curve results A typical test protocol for the grease is displayed in Figure 2. After the test start, the sample and the gas volume in the autoclave is heated up. Since the volume is closed the internal pressure increases. After that, the pressure steadily decreases with an almost linear and comparably flat slope. Pressure is decreasing because of oxidation of grease components. It is assumed that the initial oxygen pressure decrease is caused by the oxidation of protecting antioxidant additive, which is supposed to prevent degradation of the base oil and thickener. After a certain duration, the pressure sharply decreases. This point corresponds to the nearly full depletion of the antioxidant and the beginning oxidation of base oil and thickener. This duration shall be called the oxidation induction time (OIT) which is a function of grease composition and test conditions. The full depletion of the antioxidant additive is the end of useful service life of the grease since it is not recommended to operate components with degraded base oil or thickener. The desired lubrication properties would be affected and proper component operation and reliability could not be guaranteed. 3.2 Calculation of Arrhenius activation energy As described by Dornhöfer in 2016 [1] the service life of a grease must be estimated and meet the requirements of the temperature load collective over product lifetime. For this, the Arrhenius equation (equation 1) is applied and the activation energy E A must be evaluated. Aus Wissenschaft und Forschung 43 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005 Figure 1: Application of grease sample on plate (left), open pressure chamber with grease sample on steel plate in petri dishes (middle) and whole test instrument with close safety hood (right) ed at 130 °C, 145 °C and 160 °C and oxidation induction times were determined as described in Chapter 3.2. Each temperature and plate material was measured twice in each laboratory. After a series of three temperatures, the Arrhenius activation energy was calculated from the gradient of the linear regression of oxidation induction times. 4.1 Oxidation induction times and Arrhenius activation energy on steel Results of the logarithmic values of oxidation induction times on steel are plotted over the inverse absolute temperature in Figure 3 in order to linearize the exponential Arrhenius law and enable a linear regression with a gradient that can be converted to the activation energy. The six measurement values of each laboratory were fitted with a linear regression and values for their coefficients of determination R 2 were calculated. Each laboratory achieved an excellent R 2 of at least 0,998 which means that the assumption of the Arrhenius equation is valid. Furthermore, the high values of R 2 are a clear indicator of the repeatability of measurements within each laboratory. The regression of all data points from all laboratories together provided a R 2 of 0,996 demonstrates the reproducibility of results between different laboratories. The Arrhenius activation energies were calculated by simple multiplication of the indicated slopes with the universal gas constant R. The results from the seven la- Aus Wissenschaft und Forschung 44 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005 (equation 1) with: k is the rate constant for the chemical reaction, in this case the oxidation rate T is the absolute temperature (in Kelvin) A is the pre-exponential factor, a constant for each chemical reaction. According to collision theory, A is the frequency of collisions in the correct orientation E A is the activation energy for the reaction (in the same units as R ·T) R is the universal gas constant (8,314 J·K -1 ·mol -1 ) The activation energy can be calculated when oxidation induction times as expression for oxidation rate at different test temperatures are determined. With application of the natural logarithm to equation 1 the equation can be expressed as a linear equation (equation 2): (equation 2) When the logarithmic values for OIT (equals “ln k”) are plotted versus the inverse absolute temperature 1/ T the results should follow a linear trend and the slope of the linear graph is equivalent to E A / R. 4 Results For the estimation of the activation energies of this grease in contact with steel and brass tests were conduct- = ln = + . Figure 2: Typical pressure curve in autoclave during typical single experiment and evaluation of oxidation induction time by crossing of two fitted tangents boratories as well the arithmetic mean value are summarized in Table 1. It must be emphasized that this was the first time for several laboratories to apply this evaluation routine of oxidation induction time. Furthermore, it needs to be pointed out that for this study there was no software routine available to estimate the oxidation induction time. Instead, this was carried out by the crossing of two manually fitted tangents. Values range from 126 kJ/ mol to 134 kJ/ mol which is a surprisingly low relative span of 6,2 % of the arithmetic mean value despite the fact that the manual tangent fitting was done by seven different operators. 4.2 Oxidation induction times and Arrhenius activation energy on brass Results of the oxidation induction times on brass are plotted in Figure 4. The coefficients of determination of the linear regression are above 0,99 for each laboratory series and are almost as high as on steel and verify the applicability of Arrhenius law also on brass. The results of the activation energies on brass are summarized in Table 2. Values range from 98 kJ/ mol to 113 kJ/ mol which is a relative span of 13,9 % of the arithmetic mean value of 10 kJ/ mol. It needs to be pointed out that the lower activation energy on brass does not indicate the lower resistance against thermo-oxidation. Instead, as the gradient of the linearized Arrhenius regression, the activation energy expresses the sensitivity against temperature variation. A lower activation energy means that the oxidation induction time is not as strongly affected by temperature increase than a grease with higher activation energy. In our case, the lower activation energy of 108 kJ/ mol on brass (steel 129 kJ/ mol) indicates that the oxidation induction time varies stronger by temperature variation in contact with steel than with brass. However, this does not express the overall stability. The total stability is expressed in the oxidation induction time at a defined temperature. In Figures 3 and 4 the natural logarithm of the oxidation induction time is applied to linearize the exponential Arrhenius equation and is plotted on the y-axis as “ln OIT”. The higher values of ln OIT on steel (10,5 - 13) express the longer service life of this grease on steel than on brass with ln OIT of 8,8 - 11,4. It can be concluded Aus Wissenschaft und Forschung 45 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005 Figure 3: Arrhenius plot of logarithmic oxidation induction times at three temperatures on steel (zero offset for improved visibility of single data points) E A =m*R/ 1000 E A [kJ/ mol] BOSCH 126 15162 HS Mannheim KTM 128 15437 FUCHS 131 15808 Dr. Tillwich 126 15121 SHELL 134 16072 SKF 127 15288 Anton Paar 132 15867 a arithmeticc meann value 129 slope m of linear regression [ln(min)*K] Table 1: Arrhenius activation energies on steel decrease is only few hours after the oxidation induction time and after 20 % the thermo-oxidation has clearly reached the second regime. After these interrupted tests, greases were visually inspected and flow curves were measured. Photographs of the grease sample serve as a very first rough qualitative indicator of the state of degradation (Figure 5). After 5 % and 10 % the initial beige color has changed to a homogenous amber color and the initial contours of the spatula application are still visible as an indicator of residual structural stability. After 15 % the color is significantly darker in the center of the sample and oil can be found beside the plate. After 20 % the sample has a shiny, liquid-like appearance and homogenous brown color. The spatula contour cannot be found any longer but the sample shows a liquid meniscus at the plate edges. Finally, after 40 % the sample has become a dark sticky resin that has nothing in common with the original grease. Aus Wissenschaft und Forschung 46 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005 that the contact with brass, which is often used in machine elements like journal bearings or rolling bearing cages, catalyzes the thermo-oxidative degradation of this grease stronger than the contact with steel. Nevertheless, this finding cannot be generalized to all greases because different types grease components respond differently to contact with steel and brass. 4.3 Correlation of pressure decrease with rheological properties In order to emphasize the high relevance of the chosen test criterion of oxidation induction time for the application in machine elements the following additional test series was carried out in the BOSCH grease laboratory in Renningen. 0,5 g of the same grease as in the previous measurements was tested in the RapidOxy at 150 °C on steel. The tests were stopped after 5 %, 10 %, 15 %, 20 % and 40 % decrease of initial relative pressure in order to evaluate the rheology of the grease after variable thermo-oxidative stress before and after the oxidation induction time. The pressure curves with 5 % and 10 % pressure decrease indicate that the oxidation induction time was not reached, yet (Figure 5). The test stop after 15 % pressure Figure 4: Arrhenius plot of logarithmic oxidation induction times at three temperatures on brass (zero offset for improved visibility of single data points) E A =m*R/ 1000 E A [kJ/ mol] BOSCH 106 12757 HS Mannheim KTM 109 13118 FUCHS 98 11747 Dr. Tillwich 109 13119 SHELL 112 13458 SKF 113 13569 Anton Paar 110 13248 a arithmetic mean value 108 slope m of linear regression [ln(min)*K] Table 2: Arrhenius activation energies on brass The overview of flow curves is plotted in Figure 6. In order to be able to display the wide range of values, a logarithmic scale of the y-axis is used in the overview of all curves. The more common linear scale of the y-axis was applied on a smaller range in Figure 7 to achieve better discrimination of the curves. After the first 5 % pressure decrease, the initial flow curve with an initial flow limit around 1000 Pa has shifted to a higher shear stress around 2000 Pa, which expresses a slightly stiffer consistency. After 10 %, there is a significant change compared to 5 % and the smooth flow curve does not indicate any inhomogeneity or partial degradation. After 15 % and 20 %, the flow curve exhibits a significant shift to lower values and there is almost no initial flow limit which would have to be expected for a grease. These flow curves resemble rather a Newtonian behavior of Aus Wissenschaft und Forschung 47 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005 Figure 5: Pressure curves with stops after 5 %, 10 %, 15 %, 20 % and 40 % decrease of relative pressure Figure 6: Rheological flow curves in cone-on-plate rheometer of grease after variable test duration in RapidOxy at 150 °C; logarithmic scale of shear stress 5 Conclusion • The proposed test method has been positively evaluated by the DIN working group “grease ageing” for the determination of thermo-oxidative stability of the chosen lubricating grease and for calculation of the Arrhenius activation energy of its degradation kinetics. • The new method takes the contact of grease with the catalytically active materials steel and brass, which are used in many machine elements, into account and simulates conditions which are more closely related to real application conditions. • The oxidation induction time (OIT) as evaluation criterion, which is indicated by the significant oxygen pressure drop, corresponds with the structural degradation and failure of grease properties. Since the pressure decrease at oxidation induction time varies strongly with different grease formulations an arbitrary value of 10 % as in ASTM D8206 or DIN 51830-1 cannot reliably detect the end of grease service life. Therefore, the criterion of oxidation induction time identification by the change of pressure gradient serves as a better indicator for grease service life in applications than an arbitrary value of pressure decrease of 10 %. • This study of seven laboratories indicated an acceptable repeatability and reproducibility and limited effort for the estimation of activation energies. • Due to these positive results, the working group decided to standardize this method as DIN 51830-2. The draft version of this method has been submitted to the Aus Wissenschaft und Forschung 48 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005 an oil than a Bingham behavior of a grease. This provides another evidence of the structural breakdown of this thickener right after the oxidation induction time. Such a grease without initial flow limit cannot remain in the desired position of the lubricated machine element. In such a condition, this grease, which is formulated as a rolling bearing grease (NLGI 2), would be thrown off by centrifugal forces even at very low rotational speeds. It is more than obvious that such a degraded grease after passing the oxidation induction time cannot meet its required specifications and must be considered as structurally failed. After passing the oxidation induction time the gradient of pressure increases which expresses that the rate of grease oxidation is significantly increased. After 40 % pressure decrease the flow curve is shifted to extremely high values around 50000 Pa which can be interpreted as a resinification of the base oil. In some cases, this might provide a certain emergency lubrication but cannot be considered as engineering lubrication of highperformance machine elements. The oxidation induction time, as it was identified in this study, is indicated by the transition from the first regime of linear and slow consumption of oxygen to the second regime of significantly faster decrease of oxygen pressure. During the first regime the grease is protected against thermo-oxidative degradation by its antioxidant additives. During this period, the grease is supposed to sustain its desired structural and rheological properties. When the antioxidant content is fully consumed, there is no more protection of the thickener and base oil against thermooxidation. Thermo-oxidation of these main grease components leads to significant degradation, which is reflected in its rheological properties and visual appearance. Figure 7: Rheological flow curves in cone-on-plate rheometer of grease after variable test duration in RapidOxy at 150 °C; detailed range of shear stress in linear scale working group and is currently in review. In order to release this method as an official standard, values of repeatability and reproducibility need to be determined by an official round robin test with at least 30 degrees of freedom. This round robin test is currently in preparation and laboratory measurements are expected to begin by the end of 2021. • Additionally, measurement data with various grease compositions are being collected from several laboratories to evaluate for which grease formulation this test method and the criterion of sharp pressure drop can be applied. Acknowledgements Thank shall also be addressed to the DIN working group grease ageing for participation in this laboratory study and discussion of the method. Gratitude needs to be expressed to the people who carried out the measurements. References [1] Dornhöfer, G.: Ermittlung der Schmierfettgebrauchsdauer mit zeitraffender Prüfmethode und Übertragbarkeit auf reales Temperaturkollektiv; presented at GfT-Fachtagung 2016, Göttingen [2] Beyer-Faiss, S.: Prüfmethode zur Beurteilung der Alterungs- und Oxidationsstabilität von Schmierstoffen; presented at 14. International Colloquium Tribology 2004, Stuttgart/ Ostfildern [3] ASTM D8206 (2018) Oxidation Stability of Lubricating Greases - Rapid Small Scale Oxidation Test (RSSOT); 2018; Beuth-Verlag [4] DIN 51830-1 Prüfung von Schmierstoffen - Bestimmung der Oxidationsbeständigkeit von Schmierfetten - Teil 1: Beschleunigtes Verfahren; 2020; Beuth-Verlag [5] Matzke, M.; Dornhöfer, G.; Schöfer, J.: Quantitativelyaccelerated testing of grease oxidation - a parameter study with the RapidOxy; presented at GfT-Fachtagung 2019, Göttingen Aus Wissenschaft und Forschung 49 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0005