Quantitative characterisation of surface finishes on stainless steel sheet using 3D surface topography analysis

The main aim of this project was to quantitatively characterise the developed surface topography of finishes on stainless steel sheet using three-dimensional surface analysis techniques. At present surface topography is measured using (mainly) stylus profilometry and analysed with 2D parameters, such as Ra, Rq and Rz. These 2D measurements are not only unreliable due to a lack of standardised measurement methodology, but are also difficult to relate directly to the actual shape of the topography in 3 dimensions. They bear little direct relation to the functional properties of the surface of stainless steel, making them less useful than their 3D counterparts. Initially it is crucial to ensure that the surface topography data collected is correct, accurate and relevant, by defining a measurement strategy. Models of the surface topography are developed encompassing the usual features of the topography and variations in the topography caused by production or 'defects'. The functional features are discussed and predicted relevant parameters are presented. The protocol covers the selection of the correct measuring instrument based on the surface model and the size of the relevant functional features so that the desired lateral and vertical resolution and range is achievable. Measurement data is then analysed using Fast Fourier Transforms (FFTs) to separate the different frequencies within the spatial frequencies detected on the surface. The frequency of the important features shows up dominantly on a Power Spectral Density (PSD) plot and this is used to find the correct sampling interval to accurately reconstruct the 3D surface data. The correct instrument for further measurements is then selected using a Steadman diagram. Operational details of the measuring instruments available for this project are given and variables for these instruments are discussed. Finally, measurement method recommendations are made for each of the four finishes modelled. Based on this surface characterisation an attempt is made to identify the 3D parameters that give a quantitative description of common stainless steel sheet finishes with respect to some aspects of their production and functional performance. An investigation of the differences in manufacturing processes, gauge and grade of material is presented, providing an insight into the effect on topography of such divergences. The standardised 3D parameter set is examined to determine its sensitivity to common variations in the topography of the 2B finish and therefore their potential relevance. A new data separation technique of the material probability curve for use on the 3D datasets establishes a cut-off (transition point) between the two main functionally relevant features of the 2B surface (plateaus and valleys) by finding the intersection of the asymptotes of a fitted conic section, giving a non subjective methodology to establish the section height. The standardised 3D parameters are then used on the separated data, with the aim of being more functionally relevant to the main surface studied. Functional tests to rate capability of these parameters in the areas of optical appearance, lubricant retention and corrosion are carried out and the appropriate topography parameters are related to their performance.