Surface finishes — Characterization

The properties of solids and their behavior often depend more on the characteristics of their surfaces than on their mass or volume properties. The surface of a solid body is the part of the solid that limits it in space and separates it from the surrounding environment. It is essential for functions such as friction, wear and corrosion resistance, thermal and electrical conduction, resistance to mechanical stress, static or dynamic sealing, appearance, etc.

Technically produced industrial surfaces always show irregularities in relation to the ideal surface. To describe them accurately, we would need to use a continuous function z (x, y) giving the altitude of each point relative to the ideal surface.

To understand the influence of a surface's condition on the functioning of a part and its behavior over time, we need to be able to characterize its geometry, whether macro, micro or, in some cases, even nano-geometry.

Only the characterization of surface conditions on a profile is standardized. The development of technologies and equipment has made it possible to obtain a three-dimensional representation of the surface (topography). Although this approach is currently the most accurate way of characterizing a surface, it is not yet recognized, and its application is not foreseeable in production.

Problems relating to surface geometry arise on three levels:

• design: what surface finish is best suited to the function to be performed, and how should it be specified on a technical drawing?

• surface finishing: what means should be used to obtain the desired surface finish?

• surface measurement: what measurement methods and conditions are needed to check that the surface meets specifications? This aspect will be covered in the .

This file and the next focus on the industrial aspects of characterizing and measuring surface finishes, based on current standards and work in progress.

Elements concerning imperfections (shrinkage, scratches, pores, etc.) and the appearance (color, gloss) of surfaces are not covered in this dossier.

Characterizing surface conditions on a profile is currently the only standardized method.

Current standardization is a real step forward, with the GPS (Geometric Product Specification) concept recognized and adopted by the main industrial countries. It should therefore be used to specify and measure surface conditions, particularly in trade.

While this system is very well suited to surfaces obtained using methods that...