Surface Indentation Test (SIT) for sheet metal

Abstract

In order to investigate the possibility to predict the frictional response of different surface topographies for steel sheet materials, a Surface Indentation Test (SIT) was used. The SIT was developed at Volvo Technological Development and is used for measuring plastic bearing ability on sheet materials. The results from the SIT were evaluated against results from a Bending-Under-Tension (BUT) friction test. The study included 12 different surface topographies, of which 2 were shotblasted (SB), 9 were electron beam-textured (EBT), and 1 was electro-chromium deposition (ECD), some being uncoated and some zinc-coated. The SIT was performed in a Brinell tester with a ground and slightly crowned Brinell ball. The surface was measured with an interference microscope close to the centre of the indentation mark. The BUT testing was performed in mixed lubrication under equal conditions for all materials used. For surface measurement of the sheet surface topography of the materials from the BUT test, a 3D stylus instrument was used. The surface topography was measured in the contact zone of the test samples. For the surface measurements a ball-filtering technique was applied. The area fractions of contact were evaluated in order to investigate which pressure in the SIT correlated best to the BUT test. A pressure of 88.8 MPa gave a very good correlation. In order to describe the frictional behaviour caused by the two main lubrication mechanisms, Micro Plasto HydroDynamic Lubrication (MPHDL) and Micro Plasto HydroStatic Lubrication (MPHSL), a so-called Wihlborg-Crafoord index (WC index) was determined from the SIT surface measurements. The WC index is the number of isolated oil pockets (NIOPt) multiplied by the border length of the lubricant area at area fraction of contact (BLalfa) and divided by the area fraction of contact (α). This WC index describes how effective the supply of lubricant is at the contact zones and correlates well to the friction coefficient within each coating.