24th International Colloquium Tribology - January 2024 267 Estimation of Remaining Useful Life of Greases after Thermo- Oxidative Ageing by Application of New Method DIN 51830-2 Markus Matzke 1)* , Gerd Dornhöfer 2 ) 1 Robert Bosch GmbH, Renningen, Germany 2 BMS, Leonberg, Germany * Corresponding author: markus.matzke@de.bosch.com 1. Introduction Thermo-oxidative grease degradation is a dominant mode of lubricating grease failure in high-temperature automotive applications [1]. With the new test method draft E DIN 51830 part 2 a new method was established which can determine the Arrhenius activation energy for thermo-oxidative grease degradation for specific greases in contact with application relevant metal contact materials like steels and brass [2]. Previous talks at TAE, GfT and STLE illustrated the evolution of this method and showed various use cases of this method [3]-[5]. Beside the determination of characteristic thermo-oxidative ageing parameters for the setup of Arrhenius models, quantitative data about the degree of thermo-oxidative ageing after endurance tests or field use can provide helpful insight about the robustness of a grease. 2. Scope This presentation shall focus on how the new method draft E DIN 51830-2 can be applied to evaluate the degree of accumulated thermo-oxidative degradation of a used grease sample or respectively the remaining level of protecting antioxidant additives. For purpose of simplicity the term remaining useful life (RUL) of used grease samples after thermo-oxidative ageing in static conditions or in field use is used here. 3. Experimental 3.1 Test method E DIN 51830-2 Measurements were carried out according to E DIN 51830- 2 [2]. The principle of this method is the static ageing of a grease sample at elevated temperature in a pressurized oxygen autoclave. The decisive difference to earlier methods like DIN 51808 [6] is the contact with a catalytically active material like steel or brass as in real applications. The important difference to other methods applying the Rapid Small Scale Oxidation Tester (RSSOT) is the evaluation criterion. The new method E DIN 51830-2 evaluates the inflection point of the pressure curve (Figure 1) as indicator of antioxidant depletion and consecutive rheological failure [4] which corresponds to grease failure in applications while ASTM D8206 applies an arbitrary criterion of 10% oxygen consumption [7]. All measurements of oxidation induction time were carried out with the following parameters: Contact material: Steel 1.0330 Temperature: 150 °C Sample amount: 0,5 g Initial oxygen pressure: 700 kPa Figure 1: Pressure and temperature curve in oxidation autoclave during typical measurement and evaluation of oxidation induction time 3.2 Concept of RUL determination The concept of this approach is the reduction of the antioxidant content in the grease during exposition to elevated temperatures. This affects the oxidation induction time (OIT) in measurements according to E DIN 51830-2. The characteristic inflection point is shifted to shorter OIT values by exposition to elevated temperatures. 3.3 Thermal pre-ageing To apply a defined thermal load on the greases they were exposed to a constant temperature of 150 °C with variable but measured durations from 0…885 h. The grease samples were applied on 1.0330 steel sheets with a grease thickness of 1 mm as described in [1]. 3.4 Greases under evaluation For the demonstration here three commercial fully formulated greases were selected for this investigation: Table 1: Overview of grease samples for demonstration Sample label Thickener type Base oil type NLGI consistency Li-PAO Lithium soap PAO 2 AlX-PAO Aluminium complex soap PAO 1 Li-MO Lithium soap Mineral 2 Estimation of Remaining Useful Life of Greases after Thermo-Oxidative Ageing by Application of New Method DIN 51830-2 268 24th International Colloquium Tribology - January 2024 4. Results 4.1 Impact of thermal pre-ageing on oxidation induction time Oxygen pressure curves from a series of measurements with variable duration of pre-ageing are displayed in Figure 2. Thermal pre-ageing affects the inflection point in the autoclave pressure curve from 1731 minutes for the fresh reference of grease Li-PAO and results in reduced oxidation induction times. Figure 2: Autoclave pressure curves of grease Li/ PAO after thermal pre-ageing at 150 °C on steel To quantify the correlation between duration of pre-ageing and oxidation induction time all results from grease Li/ PAO are plotted in Figure 3: Figure 3: Oxidation induction times of Li-PAO vs. duration of thermal pre-ageing at 150 °C on steel There is a linear trend for the reduction of the oxidation induction time with increasing duration of thermal pre-ageing. Including the implausible value of 1854 minutes there is a certain but not very precise correlation with an R² of 0.8215. Excluding this maverick would result in an R² of 0.8466. Despite the request for an increased R² there is an obvious reduction of the oxidation induction time which is an indicator of the antioxidant reservoir in a grease. The ratio of OIT for a used grease sample versus the fresh reference can be interpreted as indicator for the relative remaining useful life (RUL) under thermo-oxidative ageing. This concept shall also be demonstrated with a reduced number of repetitions by two further greases, labelled as AlX- PAO and Li-MO. Results of their relative oxidation induction times compared to the fresh reference are plotted in Figure 4. Both greases exhibit a similar behavior as Li-PAO with a linear decrease of their oxidation induction times along increasing duration of thermal pre-ageing with improved values of R² of 0.9388 and 0.9459. Figure 4: Oxidation induction times of Li-MO and AlX- PAO vs. duration of thermal pre-ageing at 150 °C on steel 5. Summary and conclusion The initial hypothesis of reducing oxidation induction time with increased thermal pre-ageing has been verified and measurement series with three commercial soap-thickened greases indicated a linear correlation. The concept is applicable for the evaluation of used grease samples in terms of their residual strength against thermo-oxidative failure when compared to the fresh reference. Due to scattering of results in the current state of method development the precision is still limited but results can already be used for a traffic light type categorization for the state of thermo-oxidative ageing. For an increased precision of RUL evaluation noise factors need to be identified and reduced within future work. References [1] Dornhöfer, G.: Ermittlung der Schmierfettgebrauchsdauer mit zeitraffender Prüfmethode und Übertragbarkeit auf reales Temperaturkollektiv; GfT-Fachtagung 2016 [2] E DIN 51830: 2024-04, Prüfung von Schmierfetten- - Bestimmung der Oxidationsbeständigkeit von Schmierfetten- - Teil- 2: Beschleunigte Ermittlung der Arrhenius-Aktivierungsenergie der thermo-oxidativen Degradation, in revision [3] Matzke, M.; Dornhöfer, G.; Schöfer, J.: Study of thermooxidative grease ageing and proposal of a test method standard, 22nd International Colloquium Tribology, Technische Akademie Esslingen, Ostfildern, 2020 [4] Matzke, M.- ; - Beyer-Faiß, S.; - Grebe, M.; - Höger, O.: Thermo-oxidative grease service life evaluation-- laboratory study with the catalytically accelerated method using the RapidOxy, Tribologie und Schmierungstechnik 2022, Issue 1, pp 41-49 [5] Matzke, M.; Höger, O.; Litters, T.; Fischer, J.: DIN-51830-2-- Evolution of an Advanced Method for Characterization of Thermo-Oxidative Grease Failure, 77th STLE Annual Meeting, Long Beach, CA, 2023 [6] DIN 51808: 2018-02, Prüfung von Schmierstoffen — Bestimmung der Oxidationsbeständigkeit von Schmierstoffen — Sauerstoff-Verfahren [7] ASTM D8206 (2018) Oxidation Stability of Lubricating Greases - Rapid Small Scale Oxidation Test (RS- SOT); 2018; Beuth-Verlag