LIBS system as a central element of surface analysis at Fraunhofer IFAM

Traces of latex gloves or even hand sweat on cleaned electronic components or an oil film on a tire sheet: manufacturing processes, machining and the use of release agents can contaminate surfaces. To ensure stable bonding, however, surfaces must be enormously clean. Bonding is a so-called special process. This means that bonding processes cannot be 100% non-destructively inspected, therefore in-process quality assurance is essential. An important aspect here - in addition to the analysis of the adhesives - is the detection of surface contamination so that the adhesive forces are guaranteed.  

Quality control with LIBS

Surface analysis, especially with regard to contamination detection to safeguard bonding processes, for example, is one of the core disciplines at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials. At the heart of this is laser-induced breakdown spectroscopy (LIBS), which can be used to analyze the elemental composition of surfaces in milliseconds. "Quality assurance, and therefore surface analysis, is enormously important. But it must be possible to integrate quality control into operational processes in the best possible way. With our research on the LIBS system, we at Fraunhofer IFAM are constantly developing a correspondingly simple and adaptable system," says Dr. René Neuholz. Without any further sample preparation, the LIBS system reliably and quickly indicates contamination - while providing a high level of information about the type and extent of contamination based on the elements measured.

The areas of application for the LIBS system are as diverse as the causes of the contamination. One example is the manufacturing process of fiber-reinforced composites: To ensure that fiber-reinforced composites can be released from the mold after curing, release agents containing silicon are used. The LIBS system is also used to prevent so called paint wetting disorders: in order to apply an even, glossy paint finish to a car door, the door must be clean. In addition to release agents, the LIBS system can also detect impurities, such as hand cream residues and oil films, among others.

LIBS for robot-controlled quality assurance

Together with LTB Lasertechnik Berlin GmbH , Fraunhofer IFAM has now designed a compact LIBS measuring head for mounting on existing robot systems. Until now, this was not possible due to the weight, size, and technology of the LIBS system, and samples had to be moved in front of the system. Now researchers and developers have optimized the LIBS technology so that the laser can variably approach the surface via the robot-controlled measuring head. This marks another milestone in the continuous development of LIBS technology, which incidentally evolved from space research.

Analyzing surfaces with laser-induced plasma spectroscopy

What does the LIBS process look like in practice? An appropriately pulsed laser uses mirrors and lenses to focus on the surface to be examined. The high energy density of the laser generates strongly excited atoms in the form of a local plasma. When the atoms cool down –i. e. when they return to their ground state – element-specific light radiation is emitted. A light guide picks up this radiation and directs it into a spectrometer. This analyzes the wavelength of the light in the range 200-780nm, registers its intensity and thus recognizes the elemental composition of the surface.

Virtually non-destructive testing: Adapting the laser parameters to the material

The great advantage of the method: the settings of the laser intensity of the LIBS system are specially adapted to the nature of the material in order to keep the impairments as low as possible. After all, less laser energy is needed for contamination analysis of a metallic car door than for plastic components. In addition, the selection of the appropriate laser wavelength can further reduce the minimally invasive impairment. This is achieved with a 266nm laser, which reduces the interventions with the surface to 5% of the conventional impairments of a 1064nm laser, while not affecting the sensitivity of the system. The system is thus a virtually non-destructive detection method. This sets laser-induced breakdown spectroscopy apart from other related spectroscopic techniques, such as mass spectroscopy. At the Fraunhofer Institute for Manufacturing Technology and Advanced Materials, all measurement systems for investigations as well as experienced contact persons are available. The experts at Fraunhofer IFAM support companies in the inline integration of the LIBS system into production processes.

The LIBS system applied in various industries and fields

"Laser-induced breakdown spectroscopy can add value in different industries. Any manufacturing process where surface and material properties are highly relevant will benefit from the LIBS system," says René Neuholz. The recycling sector of the metal industry is already benefiting from the technology. Fraunhofer IFAM is conducting intensive research into the possibilities for fiber-reinforced composites, which are often used in aviation due to their low weight. The automotive industry, particularly in the area of preventing paint wetting disorders, is also a major area of research and application for the institute. In the future, the food industry will also play a greater role. Here, the LIBS system can ensure more sustainable processes. One example: In a dairy processing plant, water use and production time can be reduced when cleaning pipes by providing accurate information about the cleaning condition. In addition, no chemical agents are applied when using the LIBS system. This underlines the environmental friendliness of the process.

The LIBS system is a forward-looking method for contamination analysis of almost all surfaces and materials with a wide range of properties (solid, liquid, pasty). Quick and easy to use and with high information value, the LIBS system can be integrated into inline processes. LIBS plays an important role in the continuous automation of processes. Thus, the use of cyber-physical systems in maritime technology is also on the agenda of Fraunhofer IFAM's visionary projects. For example, highly automated drones may soon be independently examining the harbor walls on Helgoland, trained by experts in surface analysis.

Dr. René Neuholz is a research associate at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM in the Quality Assurance and Cyber-Physical Systems department, where he is responsible for surface analysis and spectroscopic methods.