Fast and reliable analysis of the topology of surfaces, as well as the quality of coatings, is in demand from manufacturers of high-quality products. Both when it comes to decorative and functional applications. If a surface cannot be characterized using standard measuring devices or visually assessed by an experienced quality inspector, the only option is often to use complex and sometimes destructive testing methods such as cross-sections or scanning electron microscope examinations. Laser-induced surface wave spectroscopy offers an exciting alternative here.
How laser-induced surface wave spectroscopy works
A specially designed laser excites the component surface to be examined to produce inaudible sound waves. Depending on the objective of the investigation, specific frequency spectra are generated. Depending on the frequency, these sound waves propagate at different speeds in different depths and layers of the material. For example, the frequency spectrum can be used to determine whether the sound propagates predominantly through an organic paint layer. At the other end of the measuring section, sensors on the surface record which waves arrive and when. After the analytical evaluation of the measured values for the different sound frequencies, an individual fingerprint of the examined material, its surface and coatings is obtained. In particular, conclusions can be drawn about the effective mechanical properties and defects of the analyzed workpiece. This is because every crack, every pore or accumulation of foreign atoms in the material influences the path of the sound waves.
Non-destructive and fast testing
"This technology enables us to examine coatings and surfaces non-destructively, quickly and very precisely," explains project manager Dr. Stefan Makowski, who heads the Coating Characterization Group at the Fraunhofer IWS and was involved in the development of the LAwave technology. "With LAwave, we are now taking the step towards application in industry." Specific fields of application can be found, for example, in automotive engineering, surface coating and microelectronics. For example, surface acoustic wave spectroscopy can evaluate cracks and pores on thermally sprayed surfaces without destroying the component, as is usually the case with conventional cross-sectional examinations. In the semiconductor industry, the removal of impurity layers on silicon surfaces can be examined. LAwave technology is also suitable for the quality control of PVD coatings, such as wear-resistant and friction-reducing coatings made of diamond-like carbon on motorcycle chains and engine components. In this case too, non-destructive quality control is not possible using conventional methods due to the low layer thickness of just a few nanometers.
Potential for the environment and health
LAwave-supported analysis of the latest generations of brake discs offers great potential for protecting the environment and health: The vehicle industry is gradually moving towards coating steel discs with special layers of hard metal, ceramic or other materials to reduce abrasion and corrosion. On the one hand, this is intended to ensure that cars and motorcycles meet the increasingly stringent particulate matter limits in the EU. On the other hand, manufacturers are thus preventing an unintended consequence of the switch to electric drives: Electric vehicles often only use the engine brake to recharge their batteries via recuperation. They use the conventional wheel brakes less often - and these rust more quickly as a result. Both problems can be greatly reduced by the aforementioned additional layers, although they cannot yet be tested non-destructively.
In research and in some industrial laboratories, this laser-induced surface wave spectroscopy is already a tried and tested measurement technology, but its practical application in industry has failed in particular due to the complex operation and evaluation of the systems, which required a great deal of scientific expertise, so that this useful method has mainly been used for research projects in universities and non-university institutes over the last 20 years. Since then, however, the Fraunhofer IWS in Dresden has continuously developed and improved the technology and software and, together with partners, made the design more user-friendly. A cooperation between the Fraunhofer IWS and the Chair of Technical Design at the Technical University of Dresden (TUD) contributed to this. The specially developed software with its intuitive user interface controls the measurement and enables reproducible and automated evaluation based on a definable measurement recipe. In this version, the LAwave measuring system enables the fast and non-destructive characterization of small and medium-sized components.
More user-friendliness, also through AI
The Fraunhofer IWS is currently developing further technological improvements that will open up new fields of application for LAwave. For example, artificial intelligence can significantly improve the analysis quality even further. A mobile LAwave measuring head is also on the agenda. It is designed to enable the surface analysis of internally coated tubes or rollers as well as other particularly heavy, large or complex-shaped machine components that cannot be clamped in a stationary device.
"LAwave" will be on display for the first time at a trade fair from April 23 to 26, 2024 at the international quality assurance trade fair "Control" at stand 8201 in hall 8 at Messe Stuttgart.