Microstructural and Chemical Characterization of the Tribolayer Formation in Highly Loaded Cylindrical Roller Thrust Bearings

Carsten Gachot,Philipp Grützmacher,Andreas Stratmann,Sebastián Suárez,Georg Jacobs,Chiajui Hsu,Andreas Rosenkranz

doi:10.3390/lubricants4020019

Abstract

Zinc dithiophosphates (ZDDP) have been widely applied in automobile industry for over 70 years as a lubricant additive for wear protection. Tribolayers have been described as blue- and brown-colored layers on surfaces observed by microscopical observation or even bare eye presumably as a consequence of layer thickness or chemical composition. However, the reaction pathways of ZDDP tribolayers are still not yet fully understood. In the present study, the difference between the blue- and brown-colored tribolayers has been revealed by high resolution methods in cylindrical roller thrust bearings at relatively high contact pressures of around 1.92 GPa. After running a FE8 standard bearing test with a normal load of 80 kN and a temperature of 60 °C, said tribolayers could be identified on the bearing surfaces. By using Raman spectroscopy, it could be shown that the blue-colored layers are enriched by FeS and ZnS whereas the brown-colored layers show a significant amount of Fe3O4. This is an interesting finding as it clearly shows a correlation between the color appearance of the films and the chemical composition besides potential film thickness variations. Finally, transmission electron microscopy verified the amorphous nature of the formed tribolayer which is in a good agreement with literature.

Highlights

Nowadays, there is pushing demand in fuel economy for passenger vehicles in order to fulfill legislative requirements for CO2 emissions
There is pushing demand in fuel economy for passenger vehicles in order to fulfill legislative requirements for CO2 emissions. This in turn leads to the introduction of ultra-low viscosity lubricants which could be one efficient way for lubricants to contribute to the fuel economy performance of passenger cars by reducing shear forces
They could show that pronounced tribolayers under thermal activation are only possible at temperatures higher than 150 ̋ C, whereas loaded conditions already yield in an efficient tribolayer at room temperature

Summary

Introduction

There is pushing demand in fuel economy for passenger vehicles in order to fulfill legislative requirements for CO2 emissions. Fujita and Spikes studied the morphology of the tribolayers for different temperatures and contact pressures up to 950 MPa [6] They could show that pronounced tribolayers under thermal activation are only possible at temperatures higher than 150 ̋ C, whereas loaded conditions already yield in an efficient tribolayer at room temperature. As the appearance of blue and brown regions on the loaded surfaces usually indicates the successful formation of the wear protective tribolayer and is most pronounced for the highest applied speed at 20 rpm, a stronger emphasis is put on analyzing said colored regions for this particular speed within this research work

Materials and FE8 Testing

Chemical Analysis

Topography and Microstructural Analysis

Results and Discussion

Cylindrical roller

Conclusions