What is it about?
This work aims to find a chemical process that modifies the surface finish of precision metal spheres to enable their use as reference elements in optical metrology. The chemical process should not substantially alter the original quality or dimensional accuracy, but it should give the spheres a matte finish, eliminating reflections. The spheres used are AISI 316L stainless steel bearing spheres, which are of low cost, high availability and great dimensional accuracy, making them suitable as reference elements if their high gloss is removed. Two procedures are tested in the research. On the one hand, different passivation acids are tested, and on the other, a chemical attack with a much more aggressive acid, aqua regia (hydrochloric acid, HCl, and nitric acid, HNO3, in a 1:3 ratio). Tests showed that none of the passivation methods sufficiently eliminated glare and reflections. However, chemical etching by immersion in aqua regia did produce a matte and homogeneous surface finish, reducing reflectivity and promoting the diffusion of incident light without loss of precision. The paper presents the tests to find the optimal exposure time to aqua regia as well as the influence of chemical etching from a dimensional and geometrical point of view, both in contact and laser sensor optical measurement. The research has considered a representative series of the chemical attack procedure to validate the repeatability of the method Finally, it has been verified that the method is repeatable and that improvements (close to 45%) in the metrological accuracy of the laser sensor measurements are achieved when using spheres treated with aqua regia compared to original spheres.
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Why is it important?
it has been demonstrated that the chemical attack with aqua regia is not only a valid method for generating matte surfaces suitable for optical metrology, but a process that can also be implemented at low cost and with high reproducibility
Perspectives
The proposed "etching protocol" provides a simple, reproducible, and low-cost route for preparing metallic reference spheres for optical metrology. Its combination of accessibility, mechanical robustness, and improved measurement performance makes it an attractive solution for calibration and verification procedures in both research and industrial environments.
Eduardo Cuesta
University of Oviedo
Read the Original
This page is a summary of: Validation of a Surface Chemical Attack Process on Precision Metal Spheres for Use in Non-Contact Metrology, Metals, March 2026, MDPI AG,
DOI: 10.3390/met16040381.
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