Fundamental Studies of Tribological Contacts with Minimally Filled Gaps Based on Modeling and Experimentation

• Applicants: Professor Dr.-Ing. Michael Müller; Professor Dr.-Ing. Georg-Peter Ostermeyer
• Subject Area: Mechanics
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 390252106

There is still no fundamental understanding of contacts under fluid influence in the case where the gap of a tribological system is only partially filled with a fluid. This applies in particular to systems in which the ratio of fluid volume to gap volume ("filling ratio") is significantly smaller than 1. Within the framework of the DFG project "Fundamental Studies of Tribological Contacts with Partially Filled Gaps Based on Modeling and Experimentation", the influence of the fluid volume on the contact mechanical properties was explicitly investigated experimentally and on the basis of models. The results are very promising. For example, the expected very strong changes in friction and wear behavior are shown under variation of the fluid quantity for filling levels from 0 (dry) to 1 (fully filled). The fluid adopts very specific distributions in the gap under partial filling and, depending on the load condition and viscosity, can separate the opposing surfaces. A particularly interesting phenomenon was shown by measurement in the range of minimal lubrication, i.e. a filling level of 0.1 and less. Here it has been demonstrated that even the smallest amounts of fluid are sufficient to significantly reduce the coefficient of friction. A detailed investigation of this special regime of minimal lubrication requires further demands on measurement technology and numerics, which forms the core of this follow-up application. Against this background, novel numerical and metrological investigations for the identification of the effect of smallest fluid quantities on the contact mechanics are to be applied within the scope of this project.On the model part, the necessary extension is carried out on the one hand via a coupling of the structural mechanics, which is to be newly integrated into the model, with the solid friction via the contact forces as well as the contact kinematics. On the other hand, the structural mechanics is linked to the fluid dynamics via the gap topography and the hydrodynamic forces. NURBS are used to describe the solid-state structural mechanics, also with coupling to the fluid dynamics. To elaborate the influence of elasticity, studies are also carried out with highly elastic materials. The studies are validated experimentally. For this purpose, on the one hand, studies are carried out on a macro / meso scale. Here, systematic experiments will be carried out with the specially developed WDI test rig and, in particular, the influence of the fluid quantity on the fluid transport processes and the correlated frictional behavior will be investigated. On the other hand, this effect is to be made verifiable on the tribologically relevant microscale using a pin-on-disc tribometer. The main aim is to apply defined minimum quantities of lubricant and to detect the change in gap heights (which is quite small in microscopic terms) and to identify and evaluate suitable wear measures.

• DFG Programme: Research Grants