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.