Surface characterization of carbon fiber reinforced polymers by picosecond laser induced breakdown spectroscopy

Abstract

Adhesive bonding of composite materials requires reliable monitoring and detection of surface contaminants as part of a vigorous quality control process to assure robust and durable bonded structures. Surface treatment and effective monitoring prior to bonding is essential in order to obtain a surface which is free from contaminants that may lead to inferior bond quality. In this study, the focus is to advance the laser induced breakdown spectroscopy (LIBS) technique by using pulse energies below 100 μJ (μLIBS) for the detection of low levels of silicone contaminants in carbon fiber reinforced polymer (CFRP) composites. Various CFRP surface conditions were investigated by LIBS using ~10 ps, 355 nm laser pulses with pulse energies below 30 μJ. Timeresolved analysis was conducted to optimize the gate delay and gate width for the detection of the C I emission line at 247.9 nm to monitor the epoxy resin matrix of CFRP composites and the Si I emission line at 288.2 nm for detection of silicone contaminants in CFRP. To study the surface sensitivity to silicone contamination, CFRP surfaces were coated with polydimethylsiloxane (PDMS), the active ingredient in many mold release agents. The presence of PDMS was studied by inspecting the Si I emission lines at 251.6 nm and 288.2 nm. The measured PDMS areal densities ranged from 0.15 to 2 μg/cm 2. LIBS measurements were performed before and after laser surface ablation. The results demonstrate the successful detection of PDMS thin layers on CFRP using picosecond μLIBS.

Correlation of Trace Silicone Contamination Analyses on Epoxy Composites Using X-ray Photoelectron Spectroscopy (XPS) and Laser-Induced Breakdown Spectroscopy (LIBS)

“…Within this task, team members from The Boeing Company, United Technologies–Pratt and Whitney, UTC Aerospace Systems, and NASA Langley Research Center (LaRC) have been evaluating various surface treatment methods, effects of various contaminants on adhesive bonding, and characterization tools for surface analysis. 3–7 Laser ablation is an attractive means of surface preparation of carbon fiber reinforced polymer (CFRP) composites and metal alloys because it provides a clean, high precision, reproducible surface that enables high quality adhesive bonds. 4,8–11…”

“…As part of this work, a tool was developed utilizing laser-induced breakdown spectroscopy (LIBS) for rapid determination of low levels of problematic contaminants. 7 Prior work has identified silicones as significant threats to adhesive bonding even at exceptionally low concentrations (<1 µg/cm 2 ). Silicone contamination can be introduced into the CFRP laminate from the fabrication process.…”

“…Laser surface treatment enables high precision processing that lends itself to automation and reproducibility for surface activation and removal of surface contaminants on CFRP surfaces. 3–14 By adjusting the laser parameters, it is possible to control the laser–solid interactions to remove surface contaminants and create texture on the surface without exposing or breaking the carbon fibers underneath the resin surface layer. This technique can also be used for other surfaces such as metal alloys and can be used to selectively remove surface layers such as paint or coatings without affecting the underlying substrate.…”

“…15–19 The rapid ejection of the laser-excited material from the solid reduces the heat transfer to the substrate outside the excited volume, producing minimal thermal damage. 7,16,19–22 For the case of CFRP composites, picosecond, and ultrashort pulses enable the ablation with low average laser power, which mitigates subsurface interlaminar damage.…”

“…In this work, laser pulse energies < 100 μJ were used for LIBS surface analysis, referred to as µLIBS. 7,23–27 The laser system used for LIBS surface characterization was the same as that for laser surface treatment, providing the opportunity for in situ surface inspection before and after laser ablation. The implemented LIBS system in the LaRC laboratory enabled the nondestructive surface characterization of CFRP surfaces in a rapid manner without any sample preparation or special atmospheric conditions.…”