Thermo-oxidative grease service life evaluation – laboratory study with the catalytically accelerated method using the RapidOxy

Abstract

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. At 60th 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 thickener, 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 R2-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.