Impact of Liquid Lubricant on the Flattening Behaviour of Single Asperities

S. Weidel, U.Engel
Chair of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstr. 11, 91058 Erlangen, Germany

Abstract

Scaled friction tests show that in micro forming applications friction is increasing significantly when forming processes are scaled down. This phenomenon can be explained by the model of open and closed lubricant pockets characterising the surface topography: the forming load is transmitted from the tool to a lubricated workpiece by three different bearing ratios. These are the real contact area (RCA) as well as open (OLP) and closed lubricant pockets (CLP). The developing hydrostatic pressure which is built up in CLPs takes a part of the external forming load, thus reducing the normal pressure on the asperities leading to a decrease in friction.

With miniaturisation in micro forming applications, the ratio of CLPs is reduced drastically as the surface topographies are mainly invariant to scaling. Thus, the forming load is mainly transmitted by the remaining RCA leading to an increase in friction. Hence, the interface between tool and RCA has to be investigated in more detail for the characterisation of the tribological conditions in microforming. In contrast to the macroscopic approach, where the RCA is assumed is assumed to be flattened completely, in microforming submicron effects within the RCA have to be considered.

In order to investigate the contact state in the RCA a novel, high-resolution experimental set-up has been developed which enables the measurement of the force-displacement characteristics during flattening the surface topography, and Simultaneously, the in-situ observation of the developing real contact area by using a translucent tool. Thus, the deformation behaviour of idealised asperities represented by pyramids with a base area of 120 x120 μm2 and a height of 32 μm can be examined. In-process measurement is complemented by post-process topography analysis.

The present paper will present recent results and experiences obtained by the investigations described above. The detailed knowledge about the evolution of surface topography is relevant in particular to microforming but also for an improved understanding of tribological phenomena in general.