Surface metrology

The surface of a solid is a two-dimensional domain where the interactions of the solid with the outside world take place. Surface physics has made great strides in the second half of the twentieth century, and many areas of industrial activity are directly concerned by this discipline. It is at the surface of a solid that the chemical reactions that make it evolve occur, and that friction, wear and adsorptions of various contaminants manifest themselves. From mass metrology to bearing operation, from the ground condition of mechanical surfaces to the superpoli of optical surfaces, surface metrology plays an essential role in the control of mechanical, optical or electronic components.

The properties of a surface are extraordinarily numerous and complex. We try to characterize them using simple parameters, which will obviously never give a complete representation of these properties. This is why experimentation is essential if we are to deduce from observations the answer to the question: will the surface perform its functions correctly?

In most standards dealing with surfaces, visual and tactile examination is often the first to be mentioned. Although qualitative, it often represents a synthesis of parameters that are difficult to quantify by other means: texture, hue, appearance in different lights, mechanical and thermal sensations. Just as a doctor can see in an X-ray or a CT scan the elements that will enable him to determine his diagnosis, experience alone enables an engineer or a technician to draw conclusions from the direct or indirect images made available to us by modern instrumentation, as to whether a surface conforms to a particular specification.

In this article, we analyze various methods for characterizing surfaces by their macroscopic (shape: straightness, flatness or circularity) and microscopic (roughness) geometric properties. We describe some of the instruments used to access these properties, and mention others such as near-field microscopes and atomic force microscopes.