How to Characterize Surfaces with Different Functions

In many cases, functions of engineered surfaces concern only a portion of the surface and not the whole surface. For instance, when two surfaces are in contact, such as in the case of mechanical seals, only some of the portions of the surfaces (the highest hills and peaks) interact with one another. Here, deep valleys or dales do not play a part.

In contrast, the potential to hold lubricant in a cylinder bore’s internal surface (to guarantee accurate sliding of the piston) is only reliant on pits and grooves.

In spite of this, the majority of the specifications on drawings specify field parameters such as Wa or Ra, which are computed by using all points of a surface or a profile. Conventional surface texture parameters are generally based on sums of heights. However, another category of parameters, called feature parameters, focuses only on specific portions or points of the surface.

French Motifs

Depending on the functional role of an engineered surface, all or part of the surface may be taken into consideration when calculating parameters.

François Blateyron, ISO surface metrology expert, Digital Surf

Three decades ago, a graphical method called “R&W motifs” was developed by a French consortium of automotive manufacturers, with the intention of determining structures, or motifs, on profiles that have an active role in the functions of the surfaces.

This method, which was later standardized under the reference ISO 12085, specifies motifs as the triplet “peak-valley-peak.” The algorithm reported in the standard is based on the identification of local extrema followed by the selection of important motifs.

A motif, if found to be insignificant, is combined with one of the neighboring motifs to create a larger one. At the end of the selection, only important motifs are left. Then, the parameters are computed to find out the mean width and mean height of the motifs, and they can be used in specifications.

This method was found to be very effective, specifically following a large-scale effort to characterize over 20,000 mechanical components by using these parameters in relation to various functions. Subsequently, this enabled the derivation of guidelines for designers to assist them in accurately specifying the surface texture of components on drawings based on the expected function.

Areal Features

More recently, an innovative feature detection method was established in the ISO 25178-2 standard. The method is based on watershed segmentation of the surface after a pruning method for the selection of important features on the surface.

Areal features can be areas (dales or hills), lines (course or ridge), or points (saddle points, pits, or peaks) which can be subsequently characterized using numerical values such as form factor, orientation, volumes, areas, heights, and so on. They are known as feature parameters.

During the detection of dales — the micro-scale equivalent of catchment basins — the ridge lines surrounding them correspond to contours of the texture cells. Contours surrounding hills are known as course lines. This segmentation and the parameters related to it enable characterization of the way the surface interacts with its surroundings using its features. Most importantly, only specific parts of the surface are taken into account.

Watershed segmentation can also be adopted for the detection of shapes. Pre-filtering points out the steep slopes, and the outcome is a type of contour detection enabling automatic partition of the surface.

Several applications can be benefited if feature parameters are used. Metrologists and designers now have to collaborate to determine the functional correlation between these parameters and their process parameters. Similar to the profiles with French motifs, it is expected that through feature parameters to control functions might result in better correlations than the extensively used, yet slightly meaningless, Ra.

What is Functional Correlation?

This term relates to the intended function of a surface on a component as conceived by the designer. For instance, a cylinder block’s top surface is intended to have a sealing function with respect to the opposite surface formed by the cylinder head through a gasket. The sealing efficiency is associated with the depth of large dales, the height of large hills, and the deviation of flatness of these surfaces.

Identification of texture cups and qualification of:

• Mean size
• Mean volume
• Density etc.

Checking the sealing efficiency of a cylinder block

A function on steel sheets is their potential to support paint adhesion while staying adequately smooth to enable the sliding of press forming tools on the surface without leading to the formation of cracks. This function is accomplished by texturing small-scale patterns onto the surface through rollers (thereby developing the needed roughness) and also by leaving adequate flat plateaus such that the surface looks smooth at a larger scale.

• Automatic segmentation of texture cells on the structured surface
• Detection of missing cells

Steel sheets: obtaining optimum paint adhesion and large-scale surface smoothness

Resources

• ISO 16610-85: GPS—Filtration—Areal morphological—Segmentation
• Feature parameters, chapter 3 in “Characterisation of areal surface texture”, published by Springer
• Feature parameters, in the Digital Surf Surface Metrology Guide: www.digitalsurf.com/en/guidearealfeatureparameters.html
• Segmentation and feature parameters—A corrected definition of watershed segmentation and feature parameters defined in ISO 25178-2, F. Blateyron, Conference proceedings of the International Conference on Surface Metrology, April 2016, www.researchgate.net/publication/303390269

This information has been sourced, reviewed and adapted from materials provided by Digital Surf.

For more information on this source, please visit Digital Surf.