Ultra-short Pulse Laser Ablation for Cross-section of Auto Body Paints

Ultra-short pulse duration lasers (USP) ablation materials have received much attention from industry and microscopy groups for sample preparation because of their straightforward application. Ultrashort pulse laser-prepared specimen surface has heat affected zone often below a few tens of nanometers, artefacts may be presented such as LIPSS (laser-induced periodic surface structures), redeposition debris on the cut, and shockwave damage. Minimizing these surface artefacts for a USP laser not inside of electron microscope chamber can be tedious due to a wide range of parameter space available for laser processing e.g., wavelengths, repetition rates, power, pulse energy, scan speed, scan strategies, shape dimensions, x, y pitch, and the number of passes. Multi-layered coating applied on the automotive body has two main functions, producing durable surfaces to protect the vehicle body against corrosion and decorating to give a visual aspect of the car. Automotive painting is the most expensive operational aspect of an automobile assembly plant. Industrial practitioners and researchers are greatly interested in controlling and monitoring the painting process (e.g., layer thickness, chemical composition, etc.) to improve the quality of coatings and their mass production. In this paper, we demonstrate a novel approach for high-throughput automated cross-sectioning of car paint coatings using a Laser Plasma FIB. Traditionally the car paint sample is embedded in polymer resin as a block. These blocks are then polished mechanically in multiple steps to make the cross-section of paint on the steel body substrate. This approach is, however, time-consuming and makes it difficult to produce the extremely smooth surfaces needed for high-quality analysis. Gallium-focused ion beam (FIB) milling can only reasonably produce 20 µm wide and deep cross-sections of samples, making it unsuitable for generating accessing paint samples, as the paint layer thickness can be more than 100 µm. Creating pristine cross-sections wider and deeper than a few hundred microns is time-consuming for a high-current xenon plasma FIB as it requires low-current polishing and the application of a thick protective layer before cross-sectioning. The Helios 5 Laser Plasma FIB combines a fully integrated femtosecond laser, scanning electron microscope (SEM), and FIB. The laser enables fast, millimeter-scale material removal without the need for a protective layer, while combined SEM and EDX (energy-dispersive X-ray spectroscopy) can be used to reveal structural and chemical details of the paint layers. This allows for fast feedback during auto-body coating quality control. In conclusion, car body paint chemical information was not affected by femtosecond laser ablation sample preparation. The in-chamber laser ablation processing monitored by SEM images allows tuning laser parameters quickly to generate a high-quality cross-section without applying PFIB cleaning. Schematic of automotive paint structure modified with permission from reference [1]. a) SEM backscattered electron image of an auto body coating cross-section. (b) Interested-element mapping correlates to the paint structure. The cross-section was prepared only with the integrated in-chamber laser without applying a post-laser FPIB clean-up step. The images demonstrate a high quality of femtosecond laser processing and enables the instrument to maintain high-throughput characterization. Laser only – time to result for the cross-section is less than 5 mins for an 800×150 µm area in the depth of more than 100 µm.