Picosecond laser surface treatment and analysis of thermoplastic composites for structural adhesive bonding

Abstract

In this work, laser surface treatment and laser induced breakdown spectroscopy (LIBS) were combined to improve process control of surface treatment and pre-bond surface characterization to measure the reduction in silicone content. This approach addresses guidelines provided by the Federal Aviation Administration (FAA) in Advisory Circular 20–107B focused on improved process controls, as well as the goals of the NASA Advanced Composites Project focused on reducing the timeline to certification. A picosecond laser was used to surface treat poly (ether ether ketone) (PEEK)/carbon fiber composites using two different wavelengths (355 and 1064 nm). The same laser was used to characterize the PEEK surfaces for silicone contamination before and after laser treatment via LIBS. Preliminary experiments were conducted using a rapid screening methodology whereby up to 12 laser treatment conditions were compared by failure mode analysis on a single sample. The panel was subsequently cobonded with an adhesive to an epoxy matrix prepreg and mechanically tested. Coupon level tests were then conducted with a down-selected set of laser treatment conditions. Additionally, laser treated panels were characterized using methods including water contact angle measurement, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The results showed that laser treatment parameters strongly influence the adherend surface and subsequent bond performance. XPS and LIBS measurements agreed well for assessment of silicon (Si) concentration. Also, post-ablation silicon-to-carbon (Si/C) ratios from LIBS measurements were correlated with the fracture toughness and observed failure modes from double cantilever beam testing.